Ilham Elaboudi

Ahmed Mdarhri

Christian Brosseau

Ali Nourdine

Mourad Rzaizi

Laurent Servant

Comparing the sorption kinetics of poly-tetrafluoroethylene processed either by extrusion or spark plasma sintering

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction and overview

PTFE is a synthetic fluoropolymer used in numerous industrial applications (it is often referred by its trade name Teflon). The production of PTFE is forecast to continue to increase over the course of the next ten years [1]. The demand arises because PTFE combines various excellent properties: chemical resistance, high temperature stability, favorable mechanical properties and low permittivity [START_REF] Burkarter | Superhydrophobic electrosprayed PTFE[END_REF]. Such unique characteristics made PTFE widely used in automotive, electrical, and electronic industries, aircraft and aerospace, communications, construction, medical devices and implants stents, special packaging, and protective garments [START_REF] Burkarter | Superhydrophobic electrosprayed PTFE[END_REF][START_REF] Dhanumalayan | Performance properties and applications of polytetrafluoroethylene (PTFE) -a review[END_REF]. Additionally, PTFE has outstanding permeability properties making it relevant for sealing and in compounds for transferring aggressive or ultrapure fluids such as pipe liners, fittings, valves, and pumps [START_REF] Dhanumalayan | Performance properties and applications of polytetrafluoroethylene (PTFE) -a review[END_REF][START_REF] Massey | Permeability properties of plastics and elastomers: a guide to packaging and barrier materials[END_REF][START_REF] Troy | The Science and Practice of Pharmacy[END_REF].

However, PTFE is difficult to process by using conventional thermoplastics methods, such as extrusion-molding or injection-molding [START_REF] Dhanumalayan | Performance properties and applications of polytetrafluoroethylene (PTFE) -a review[END_REF][START_REF] Liewellyn | Extrusion of Tetrafluoroethylene polymer[END_REF][START_REF] Hambir | Sintering of ultrahigh molecular weight polyethylene[END_REF]. These methods require heating PTFE powder until complete melting (380°C), and during this stage, PTFE can be subject to oxidation, i.e. degradation. Additionally, its high viscosity in the molten state makes shaping difficult and restricts the use of filler particles which can be embedded into a PTFE matrix [START_REF] Dhanumalayan | Performance properties and applications of polytetrafluoroethylene (PTFE) -a review[END_REF][START_REF] Colin | Polymer degradation during processing[END_REF]. During the cooling phase, volumetric changes associated with crystallization and density difference between the crystalline and amorphous phases produce shrinkage [START_REF] Baker | Effect of composition, processing, and structure on properties of engineering Plastics[END_REF]. Metallurgy techniques such as hot pressing (HP), hot isostatic press (HIP), or high velocity compaction (HVC) have been applied to process PTFE powder but these methods present many drawbacks [START_REF] Suàrez | Challenges and Opportunities for Spark Plasma Sintering: A key Technology for a New Generation of Materials[END_REF][START_REF] Cavaliere | Spark Plasma Sintering: Process Fundamentals[END_REF], e.g. high pressures and high temperatures are needed. Moreover, during the compaction process, the material undergoes very large permanent strains inducing pore heterogeneities and residual stresses within the compact as well as local texture of the nascent crystalline phase [START_REF] Huang | Automated industrial PTFE billet sintering temperature profile optimization for residual stress reduction[END_REF][START_REF] Andena | Simulation of PTFE sintering: Thermal stresses and deformation behavior[END_REF][START_REF] Canto | Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders[END_REF]. After compaction, the sintered material is heated above the melting temperature before being cooled. During this heat treatment, complex mechanisms induce anisotropic deformations by growth and expansion of the polymer micro-particles. Furthermore, rates of sintering are limited (≈10°C/min) and require a long processing time, i.e. typically a few hours. An innovative method was tested recently to prevent such problems and overcome those limitations by using an advanced metallurgic technique named spark plasma sintering (SPS) [START_REF] Andena | Simulation of PTFE sintering: Thermal stresses and deformation behavior[END_REF][START_REF] Canto | Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders[END_REF][START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF].This method is a solid consolidation sintering process and consists in apply simultaneously a low voltage, high density pulse current and uniaxial pressure to the powder particles. This heating mechanism is beneficial to the process since heating takes place at the contact between powder and sintering tool. Thus, it concentrates sintering energy on the contact necks of powder particles and renders the process energy economic and easy to control.

The key advantages offered by this sintering method include high densities achieved in a short time (typically a few minutes), high sintering rate (up to 400°C/min) at low temperatures or below melting temperature for some polymeric materials [START_REF] Sebileau | Consolidation by spark plasma sintering (SPS) of PEEK[END_REF], and decrease of particle growth during processing [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF][START_REF] Sebileau | Consolidation by spark plasma sintering (SPS) of PEEK[END_REF].

Until today, SPS was applied to process ceramic and metallic materials [START_REF] Suàrez | Challenges and Opportunities for Spark Plasma Sintering: A key Technology for a New Generation of Materials[END_REF][START_REF] Cavaliere | Spark Plasma Sintering: Process Fundamentals[END_REF] and few work has been devoted to polymeric materials and composites [START_REF] Schwertz | Consolidation by spark plasma sintering of polyimide and polyetheretherketone[END_REF][START_REF] Schwertz | Optimization of the Spark Plasma Sintering Processing Parameters Affecting the Properties of Polyimide[END_REF][START_REF] Wei | Enhanced thermoelectric properties of polymer/inorganic bulk composites through EG treatment and spark plasma sintering processing[END_REF][START_REF] Lahlali | A high viscous polymer processed by conventional and unconventional techniques: evaluation of mechanical behaviour and physical characteristics[END_REF]. Polyimide and polyetheretherketone (PEEK) powders were sintered using SPS process [START_REF] Schwertz | Consolidation by spark plasma sintering of polyimide and polyetheretherketone[END_REF]. Authors reported that optimizing the sintering conditions (compressive load, heating and cooling rate, pulse current density, dwell time) has a strong impact on the physical properties of the sintered polymers, specifically of the density and mechanical properties (Young's modulus, yield strength, maximum compressive strain). For instance, PEEK with improved mechanical properties was obtained by SPS below the melting temperature, compared to PEEK obtained with the conventional injection molding technique [START_REF] Sebileau | Consolidation by spark plasma sintering (SPS) of PEEK[END_REF].

In previous studies [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF][START_REF] Lahlali | A high viscous polymer processed by conventional and unconventional techniques: evaluation of mechanical behaviour and physical characteristics[END_REF], the mechanical properties of PTFE processed by SPS have been investigated. The results were in agreement with those published in the archival literature for PTFE fabricated by standard method. Given the importance of SPS processing variables (rate of heat, temperature of sintering, pressure, dwell time), SPS could have some advantage to improve polymer quality and reduce the manufacturing time and cost.

Complementary to examining its mechanical characteristics, we believe it is of paramount importance to investigate other physical and chemical properties of SPS-PTFE in order to compare them with PTFE manufactured by ordinary methods. These characteristics include permeability properties, notably for its use in vascular graft applications as well as other surgical procedure requiring a long-term dimensional stability [START_REF] Massey | Permeability properties of plastics and elastomers: a guide to packaging and barrier materials[END_REF]. This manuscript reports and compares the swelling properties and kinetics of solvent diffusion in PTFE elaborated by extrusion (hereafter denoted as Ext-PTFE) and SPS (hereafter denoted as SPS-PTFE samples). It is long established that the transport phenomenon in polymers depends on the nature of the penetrant and on the polymer structure in combination with the processing method [START_REF] Manabe | Supermolecular structure of polymer solids and its effects on penetrant transport[END_REF][START_REF] Chainey | Transport phenomena in polymer films[END_REF]. Our central aim is to bring more detailed understanding of the impact of the processing method on the sorption kinetics of PTFE. For this purpose, different liquids were tested: toluene, n-hexane, tetrahydrofuran (THF) and chloroform. Sorption kinetics was investigated using Berens-Hopfenberg (BH) empirical model [START_REF] Berens | Diffusion and relaxation in glassy polymer powders: 2. Separation of diffusion and relaxation parameters[END_REF].The results are discussed in terms of diffusion and polymer relaxation phenomenon. An attempt was made to correlate the liquid properties, polymer characteristics (interaction free energy, Hansen parameters solubility, crystallinity) and fitting parameters (diffusion coefficient, relaxation or Fickian diffusion contribution) of our modeling method. Furthermore, the swollen polymers were subjected to additional analysis by infrared spectroscopy, X-ray diffraction and uniaxial tensile tests in order to study the effect of sorption on the structure and mechanical properties of the samples depending on the processing method.

Experimental section

Materials

Industrial grade PTFE was used to perform the experiments. On the one hand, Ext-PTFE samples were purchased from Guarniflon Society, of density 2.18 g/cm 3 . On the other hand, SPS samples were obtained by using PTFE powder provided by the Hoechst-Germany company (Hostaflon TF1620) and an HP 25/1 machine from FCT system GmbH (Germany).

The size distribution of the PTFE particles was obtained from SEM observations by dispersing PTFE powder in a suitable solvent (2-propanol). The average value of particle diameter is found to be about 5.44±0.61 µm according to a log-normal distribution [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF]. The crystallinity degree of the as-received PTFE powder is about 65% measured by DSC method.

To distinguish the various contributions to the interfacial wettability, we employed different types of contacting liquids (purchased from Sigma Aldrich): toluene, tetrahydrofuran, nhexane, and chloroform. Several of their physical properties are listed in Table1.

SPS processing

The experimental set-up describing the SPS technique is shown in Fig. 1. It is composed of four parts: the DC generating DC pulsed current via two electrodes, an hydraulic system providing the uniaxial compaction force by means of two counter sliding punches, the electrical resistance heating part under vacuum (< 2mbar) and the water cooling chamber. All elements of the set up (die, punches, electrodes, spacers) are made of conductive graphite.

During processing, 15g of PTFE powder was placed in a die having a cylindrical shape (40 mm and 60 mm as inner and outer diameters respectively). The sintering temperature, heating rate and the compact force were controlled (Fig. 2). The value of the electric current used to activate the micro-powder material was fixed to 2 kA with a low voltage value (2 V) [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF]. At the beginning (phase I in Fig. 2), and at the cooling step (phase IV in Fig. 2), the sample is subjected to an uniaxial compaction force of 30 MPa exerted by counter-sliding punches and through two graphitic spacers. This pressure is sufficient to sinter PTFE powder at room temperature since it exceeds the yield strength of PTFE (typically ranging from 9 MPa and 12

MPa [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF][START_REF] Dupont-Fluoroproducts | properties handbook[END_REF]). The die was heated at 340°C and with heating rate 200°C/min (phase II). The holding time at this temperature was 1min (phase III) and the cooling rate to attain the ambient temperature was 1°C/s (phase IV). To reach the prescribed temperature cycle, the pulse current was automatically adjusted by controlling the sample temperature using a thermocouple (positioned in the graphitic die at 3-4mm from the sample side). Direct temperature measurements at the edge and the center of the sample during heating revealed the presence of a temperature gradient within the polymer [START_REF] Canto | Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders[END_REF]. At the end of the sintering, the dense polymer has a circular disk shape with 40 mm diameter and 4 mm thickness. Tested PTFE samples were cut as small rectangular pieces from the processed polymers. The thickness, length and width of the tested pieces were measured using a high-grade digital caliper. Dimensions of tested samples are 50×10×2 mm for the Ext-PTFE sample and 40×4×1mm for SPS-PTFE sample.

× - = m m m Q t t , b) - (1 100 0 0 × - = m m m Q e e .

Contact angle measurement

The hydrophobicity of Ext-PTFE and SPS-PTFE was carried out in terms of static contact angle of water and several liquids (toluene, n-hexane, THF and chloroform). Contact angles were measured at room temperature 20°C and ambient relative humidity 52%using an optical meter EasyDrop KRUSS and rectangular polymers samples. A water droplet with 1mm diameter was dropped on the surfaces of samples from a distance of 1 cm by vibrating the tip of a micro-syringe. The drop image was taken by an integrated camera and analyzed with the number of three measuring cycles, and the average value was recorded as the final contact angle result.

XRD and FTIR characterizations

Ext-PTFE and SPS-PTFE samples in the dry state or saturated by liquids were analyzed by Xray Diffraction (XRD) and Infrared (IR) spectroscopy. Before measurement, rectangularshaped (5x5x0.5 mm 3 ) samples were conditioned in liquids at room temperature until saturation is occurred. Then, the excess surface solvent was removed by absorbent paper just before characterization by XRD and IR spectroscopy. The crystalline structure of neat and swollen PTFE samples was analyzed by XRD using PHILIPS X'PERT apparatus. The source of irradiation was Cu Kα (λ = 1.5418Å). The XRD patterns of samples were recorded from 10° to 50° (2θ).The ATR spectra of the samples were measured using the PLATINUM Diamond ATR accessory and VERTEX 70 FT-IR spectrometer. 20 scans were made at a resolution of 2cm -1 for spectral accumulation in every measurement.

DSC measurement

The thermal transitions; melting and crystallization of PTFEs were investigated. The experiments were carried out with a PerkinElmer Series 7 differential scanning calorimeter (DSC) at a heating and cooling rate of 10°C/min. The DSC instrument was calibrated with Indium and Zinc as temperature calibration standards. Samples were subjected to a one thermal cycle ranging from 30 °C to 400°C. The first melting event provided the enthalpy of the crystals formed during the cooling after extrusion (Ext-PTFE) or sintering (SPS-PTFE), while the second melting event refers to the crystals formed during the cooling in the DSC.

Melting enthalpies were calculated by integration of the endothermic peaks and were used to estimate the degree of crystallinity.

Tensile measurement

Tensile tests of dry and swollen PTFE samples were performed at room temperature using an Instron 3369 testing machine. Samples had a square cross section and dimensions as follows:

for Ext-PTFE: 10×2×50mm, and SPS-PTEF: 5×2×40mm. All measurements were performed at room temperature and at a fixed cross-head speed displacement (1mmmin -1 ). The mechanical stress history was identical for all dry and swollen samples. Because of the possible evaporation of the solvent when swollen PTFE samples are subjected to high deformations for a long time, the maximal strain ) ( max ε value was fixed to 50%.

Modeling solvent sorption kinetics

The sorption kinetics of polymers in a low molecular weight liquid has been studied extensively. A standard model for mass diffusion kinetics is based on the second Fick's law. It is known in the archival literature as case I or Fickian diffusion and is represented by Eq.( 2)

(2) ² ² x C D t C ∂ ∂ - = ∂ ∂ ,
where C is the solvent concentration, x denotes the distance from the sample center, D is the diffusion coefficient, and t is time. If we assume that the surface concentration attains its equilibrium value C∝ immediately when it is exposed to solvent and remains constant through diffusion process, the initial and boundary conditions are given by

l/2 x C C , 0 2 / x l/2 - , 0 , 0 t - ± = = ≥ ≤ ≤ = = ∞ + t l C
, where l is thehalf-depth ofa sample. If Mt denotes the total amount of diffusing substance which entered the sample at time t and M∝ the corresponding quantity at saturation, the solution of Eq.( 2) using the boundary conditions is given by [START_REF] Cranck | The mathematics of diffusion[END_REF][START_REF] Ritger | A simple equation for description of solute release I. Fickian and non-Fickian release from non-swellable devices in the form of slabs, spheres, cylinders and discs[END_REF] ( )

(3) ² 4 ² )² 1 2 ( exp ² 1 2 1 ² 8 1 0 ∑ ∞ = ∞             + -         +       - = n t t l n D n M M π π .
In Eq.( 3), the thickness of the polymer is 2l, and Di is the diffusion coefficient. At short times, Eq. ( 3) approximates to [START_REF] Ritger | A simple equation for description of solute release I. Fickian and non-Fickian release from non-swellable devices in the form of slabs, spheres, cylinders and discs[END_REF] (4) ² . 2

1/2       = ∞ l t D M M i t π .
Equation 4 allows us estimating the diffusion coefficient Di. For a non-Fickian behavior and Mt/M∞< 0.5, the diffusion coefficient can be calculated using Eq.( 4). In this case Di reflects the diffusion rate at the first stage of sorption (initial diffusion). According to many reports [START_REF] Wind | A capacitance study of pseudo-Fickian diffusion in glassy polymer coatings[END_REF][START_REF]Permeation of Gases and Vapours in Polymers[END_REF][START_REF] Frisch | Sorption and Transport in Glassy Polymers-A Review[END_REF][START_REF] Berens | Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA[END_REF], a considerable deviation from ideal Fickian diffusion has been observed where the diffusion rate changes as function of time. This anomalous behavior arises as a direct consequence of the glass transition temperature depletion in these systems called also plasticization. As was interpreted by Roger et al. [START_REF]Permeation of Gases and Vapours in Polymers[END_REF], plasticization is interrelated to the finite rate by witch changes in polymer structure occur in response to stresses imposed by the medium during sorption. The nature of this glass transition is as yet far from being fully understood and no general theory actually exists dealing with anomalous diffusion in polymers [START_REF] Ritger | A simple equation for description of solute release I. Fickian and non-Fickian release from non-swellable devices in the form of slabs, spheres, cylinders and discs[END_REF][START_REF]Permeation of Gases and Vapours in Polymers[END_REF]. Features of non Fickian diffusion are pointed out by applying a more general empirical law to fit sorption kinetic data

(5) . n t k M M t = ∞ .
In Eq.( 5), k is a constant and depends on the polymer structural properties. The value n=0.5 indicates a Fickian diffusion (Case I) occurring when the polymer matrix is not perturbed by the solvent and the diffusion coefficient of the solvent is independent of the solvent concentration within the duration of the sorption experiment. The value n=1 indicates (case II) diffusion for which the rate of polymer relaxation is faster than the diffusion rate. The polymer is affected and plasticized by the solvent and the diffusion coefficient depends on the local solvent concentration. A value of n ranging between 0.5 and 1 indicates an anomalous transport or a two-stage behavior and occurs when polymer relaxation and diffusion rates are close. If n ranges between 0 and 0.5, the diffusion process is called pseudo-Fickian and is considered as well as an anomalous behavior. As reported by Berens and Hopfenberg [START_REF] Berens | Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA[END_REF] for non-Fickian diffusion, the incremental sorption shows larger relative contributions from the slow relaxation process. This phenomenon is related to the slow redistribution of available free volume through the relatively large scale segmental motions in the relaxing polymer.

Thus, it delays the rate of approach to equilibrium but increases the overall sorption. In order to analyse the pseudo-Fickian diffusion behavior, Berens and Hopfenberg (BH) suggested an empirical model [START_REF] Berens | Diffusion and relaxation in glassy polymer powders: 2. Separation of diffusion and relaxation parameters[END_REF][START_REF] Berens | Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA[END_REF] based on the linear superposition of Fickian diffusion, Eq.( 7), and relaxation processes, Eq.( 8),

( ) [ ] (8) exp 1 (7) ² ² )² 1 2 ( exp ² )² 1 2 ( 8 1 (6) 0 t k M M t L D n n M M M M M M M M R R R n F F R F t - - =               + - + - = + = ∞ ∞ = ∞ ∞ ∞ ∞ ∑ α π π α
, where αF and αR denote the mass fractions of uptake in the overall sorption attributed to Fickian diffusion and relaxation phenomenon, respectively, kR is the first-order time constant associated with the long time drift in mass uptake and controlled by the viscoelastic relaxation of the polymer chains to accommodate the penetrant. Thus, BH model provides as well a means for calculating a meaningful diffusion coefficient from sorption data involving long term relaxations which may overshadow the rapid achievement of the Fickian diffusion [START_REF] Berens | Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA[END_REF][START_REF] Burgess | Water sorption in poly(ethylene furanoate) compared to poly(ethylene terephthalate). Part 2: Kinetic sorption[END_REF]. In order to optimize correctly the three unknown parameters (αF, kR,D) in Eq.( 7), their effect on the sorption kinetic profile needs first to be evaluated, and the initial iteration values need to be carefully selected.

Results and discussion

Crystallinity of PTFE samples

The thermograms depicted in Fig. 3 show the transitions of Ext-PTFE and SPS-PTFE samples in the temperature range 30-400°C. A summary of the melting, crystallization peaks temperatures and heats of fusion are given in Table 2. In the first cycle of heating-cooling, the Ext-PTFE exhibits a peak of melting at 333°C and provides a total heat of fusion 45J/g. In case of SPS-PTFE, double melting peaks are observed at 330°C and 348°C. The corresponding endothermal enthalpies of fusion are about 10.4 and 9.3 J/g respectively. Peaks observed at lower temperatures for both samples are assigned to the melting of folded chain crystals (FCC) [START_REF] Khanna | The melting temperature of polytetrafluoroethylene[END_REF][START_REF] Lui | Understanding chain folding morphology of semicrystalline polymers based on a rod-coil multiblock model[END_REF]. The melting temperature of FCC in EXT-PTFE is the highest, it could be assumed that crystallites in these PTFEs differ by size and probably by density of packing, their increase induce an increase in the melting temperature of FCC [START_REF] Serov | Double melting in polytetrafluoroethylene γ-irradiated above its melting point[END_REF][START_REF] Oshima | Radiation induced crosslinking of Polytetrafluoroethylene[END_REF][START_REF] Wunderlich | Thermal Analysis of Polymeric Materials[END_REF]. The second peak observed in SPS-PTFE at 348°C can be associated to the melting of the extended chain crystals (ECC) which are perfect and well known for their superheatability [START_REF] Khanna | The melting temperature of polytetrafluoroethylene[END_REF][START_REF] Lui | Understanding chain folding morphology of semicrystalline polymers based on a rod-coil multiblock model[END_REF][START_REF] Serov | Double melting in polytetrafluoroethylene γ-irradiated above its melting point[END_REF][START_REF] Oshima | Radiation induced crosslinking of Polytetrafluoroethylene[END_REF]. Multiple melting peaks are observed only after melting-recrystallization-next melting cycles of pristine polymer. Indeed, in [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF], a single melting peak at a high temperature 351°C is observed in the DSC traces of pristine PTFE powder [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF]. At the cooling step of the first cycle, crystallization of melted SPS-PTFE occurs at temperature peak 319°C but a high temperature crystallization peak shoulder is also observed at 349°C. It is possible that a very low fraction of the crystals persists to melting to about 400°C and exists in the liquid crystalline state as it was proved in case of high level orientation of PTFE molecular chains [START_REF] Shimizu | Development of high-modulus and high-strength poly(tetrafluoroethylene) fibers by elongation at a liquid crystalline state[END_REF]. This state reduces the free energy barrier to nucleation and crystallization occurs at lower degree of super-cooling [START_REF] Lui | Understanding chain folding morphology of semicrystalline polymers based on a rod-coil multiblock model[END_REF].

In the second cycle, the high melting peak temperature is absent after recrystallization of SPS-PTFE. It is supposed that after heating up to 400°C (first cycle), all high ordered crystalline regions (ECC) disappear, and crystallization during cooling (1 st cycle) starts with folded chains nucleus. This is followed by the addition of more segments through a chain folding process. Because of kinetic barrier, these chains cannot sufficiently adjust their conformations from folding to straight to form a thermodynamic equilibrium state, i.e., ECC [START_REF] Lui | Understanding chain folding morphology of semicrystalline polymers based on a rod-coil multiblock model[END_REF]. In the 2 nd cycle of heating-cooling, the melting peak temperature of Ext-PTFE shifts from 333 to 330 °C. This indicates that the crystals formed during 1 st cooling still far from equilibrium. It could also signify that crystals formed during extrusion are larger in size compared to that of recrystalized Ext-PTFE (first cycle cooling phase). The crystallinity degree of PTFE samples was calculated using χc=(ΔHm/ΔHm,∝)×100%, where

ΔHm is the measured melting enthalpy and ΔHm,∝ is the melting enthalpy of 100% crystalline PTFE [START_REF] Wunderlich | Thermal Analysis of Polymeric Materials[END_REF][START_REF] Blaine | Thermal applications note polymer heats of fusion[END_REF][START_REF] Mark | Polymer Data Handbook[END_REF]. Assuming that ΔHm,∝ is 82 J/g for PTFE [START_REF] Blaine | Thermal applications note polymer heats of fusion[END_REF][START_REF] Mark | Polymer Data Handbook[END_REF], the degrees of crystallinity are 54.9% and 24% for Ext-PTFE and SPS-PTFE samples, respectively. The low crystallinity of SPS-PTFE was expected because the sample was cooled at a high rate (60°C/min) at the phase IV of the SPS process (Fig. 2).

Wettability of PTFE surfaces

In order to evaluate the interfacial wettability of PTFE samples and to distinguish the various contributions, we employed a series of contacting liquids. The values of the water contact angle for the two kinds of samples are close to each other in agreement with the large hydrophobicity of PTFE [START_REF] Chen | Molecular dynamics simulations of wetting behavior of water droplets on polytetrafluorethylene surfaces[END_REF]. For other used liquids (b,c,d,e), contact angles are less than 90° indicating that wetting of the PTFE surface is favorable [START_REF] Yuan | Contact Angle and Wetting Properties[END_REF],

which can be assigned to the low liquid surface free energy according to Young equation [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF].

To provide an understanding of the interfacial free energy (IFE) of PTFE samples, the Owens-Wendt approach was chosen since it takes into consideration the sorption of liquid on polymer surface which depends on various interfacial interactions between the liquid and the polymer phases. This approach proposes that the IFE can be described by a sum of independent components: dispersive terms (e.g. London forces arising from the electron dipole fluctuations within the molecule) and polar components (e.g.van der Waals forces and hydrogen bonding).Owens and Wendt [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF] have proposed the following expressions Unknowns are the polar and dispersive components of PTFEs. They were calculated from contact angle measurements using more than two liquids with known polar and dispersive components (Table3) [START_REF] Clint | Adhesion under water: surface energy considerations[END_REF][START_REF] Gonzalez-Martin | Determination of the Carbon Black Surface Free Energy Components from the Heat of Immersion Measurements[END_REF]: water, glycerol, ethanol and diiodomethane in addition to toluene, n-hexane, THF and chloroform [START_REF] Żenkiewicz | Methods for the calculation of surface free energy of solids[END_REF][START_REF] Hansen | Polymer additives and solubility parameters[END_REF]. Details of calculations are given in appendix A.

Table 4 shows that the free surface energy (FSE) of different processed PTFE samples are close to each other and similar to those reported in literature [START_REF] Falsafi | Surface and interfacial properties[END_REF]. The polar surface energy of SPS-PTFE is slightly higher. This difference could arise from many factors such as roughness and porosity [START_REF] Yuan | Contact Angle and Wetting Properties[END_REF][START_REF] Gonzalez-Martin | Determination of the Carbon Black Surface Free Energy Components from the Heat of Immersion Measurements[END_REF][START_REF] Katasho | Mechanisms for Enhanced Hydrophobicity by Atomic-Scale Roughness[END_REF]. Fig. 4: Free energy at the interface liquid/polymer.

Hansen solubility parameters

The Hansen solubility parameters (HSP) are physicochemical parameters, and are widely used to estimate the type of interactive forces responsible for compatibility between materials [START_REF] Hansen | Hansen Solubility Parameters: A User's Handbook[END_REF][START_REF] Ebnesajjad | Properties of Neat (Unfilled) and Filled Fluoropolymers[END_REF][START_REF] Hansen | Prediction of environmental stress cracking in Plastics with Hansen solubility parameters[END_REF]. It predicts the affinity between a couple of polymer and solvent on which the swelling degree depends.

Briefly stated, the basis of the HSP is the assumption that the cohesive energy (E) might be divided into three parts corresponding to atomic dispersion (ED), molecular dipolar interactions (EP), and hydrogen-bonding interactions (EH). Similarly, the total Hansen parameterδT 2 can be divided into three components corresponding to the dispersion (δD 2 = ED/V), polar (δP 2 = EP/V) and hydrogen bonding (δH 2 = EH/V) by using δT 2 = E/V =δD 2 +δP 2 +δH 2 , where V denotes the molar volume of liquid. Using the HSP, the compatibility between two materials can be represented as the relative energy density (RED) which is the ratio between the solubility parameter distance (Ra) and the interaction radius (Ro) of the polymer. Ra is defined as

2 1 2 2 1 2 2 1 2 2 ) ( ) ( ) ( 4
H H P P D D a R δ δ δ δ δ δ - + - + - = (11).
Further, a radius of interaction R0 is needed (set to 3.9 for PTFE [START_REF] Clint | Adhesion under water: surface energy considerations[END_REF]).If RED ≤ 1, compatibility between the two materials is highly likely. If RED ≥ 1, the two materials will not be compatible and there cannot be wetting between the two materials. The HSP and Ra values for the four liquids and PTFE [START_REF] Clint | Adhesion under water: surface energy considerations[END_REF] are listed in Table 5. The compatibility between PTFE and the solvent progressively increased from THF, nhexane, chloroform and toluene. It is important to highlight that to achieve effective wetting, the surface tension of the surface to be wetted should be higher than the surface tension of the material that will wet it. The surface tension of a material depends on the secondary bond energy, and there are correlations between the surface tension and the solubility parameters.

These correlations involve molar volume of liquid molecules [START_REF] Storm | Surface protection and coatings for wind turbine rotor blades[END_REF].Thus, the HSP data above are compatible with the wetting observations shown in Fig. 4.

Sorption kinetics

The swelling ratio

100 0 0 × - = m m m Q t t
as function of time is shown in Fig. 5. For all sorption experiments, Qt increases with time until an equilibrium value (equilibrium uptake Qe) is reached. The saturation uptake of Ext-PTFE samples are less than 1%. The rate of sorption and equilibrium uptake are different for each studied liquid but a general trend is observed, i.e. for all PTFE samples investigated, Qe increases in the following order: chloroform>THF and is similar for n-hexane and toluene (right columns of Table 5). The swelling behavior of PTFE is conveniently determined by fitting Mt/M∞ versus t using Eq.( 5) [START_REF]Permeation of Gases and Vapours in Polymers[END_REF][START_REF] Frisch | Sorption and Transport in Glassy Polymers-A Review[END_REF]. The results are shown in Table 6 and indicate that the sorption kinetics of Ext-PTFE and SPS-PTFE samples by the tested liquids are pseudo-Fickian in nature, i.e. n <0.5. The data above indicate that the experimental data cannot be fitted using a Fickian model, i.e.

Eq.(3). To provide a clearer understanding of the trends in sorption kinetics observed for our samples, we examined the behavior of Mt/M versus t 1/2 /l (where l denotes sample thickness) in initial stage of swelling for which polymers are solvent-free [START_REF] Frisch | Sorption and Transport in Glassy Polymers-A Review[END_REF][START_REF] Sabard | Influence of film processing conditions on the morphology of polyamide 6:Consequences on water and ethanol sorption properties[END_REF]. This linearity allows us estimating the value of the initial diffusion coefficient Di by using Eq.( 4). To deal with differences in samples thickness l, an apparent diffusion time KF= (πD/l²) was calculated (Table 7). For toluene, n-hexane, THF and chloroform, it is observed that the values of KF are smaller in SPS-PTFE compared to Ext-PTFE samples indicating a slower liquid diffusion at the beginning of sorption. What is remarkable is that the opposite trend is observed for the final swelling equilibrium Qe values of SPS-PTFE which are found to be the largest. The observed slowing of the initial diffusion for SPS-PTFE compared to Ext-PTFEs originates from its higher IFE and non uniform densification of PTFE powder originating during the sintering process. Indeed, achieving an homogeneous temperature distribution during sintering is of major concern for the SPS process. The main parameters influencing the temperature gradient inside the specimen include the powder's electrical conductivity, the wall thickness of the die, the presence of graphite papers in contact with the powder, and the temperature dependent thermal and electrical properties of the applied tool materials as well as the powder compact [START_REF] Suàrez | Challenges and Opportunities for Spark Plasma Sintering: A key Technology for a New Generation of Materials[END_REF][START_REF] Cavaliere | Spark Plasma Sintering: Process Fundamentals[END_REF]. Many authors describe and demonstrate the benefit of making use of an optimized pressing tool system [START_REF] Cavaliere | Spark Plasma Sintering: Process Fundamentals[END_REF], or the so-called hybrid system to reduce the observed thermal gradient [START_REF] Suàrez | Challenges and Opportunities for Spark Plasma Sintering: A key Technology for a New Generation of Materials[END_REF]. 7.

Overall, these results indicate that the polymer relaxation contribution is dominant for SPS-PTFE samples in contrast to Ext-PTFE samples for which transport is mainly controlled by Fickian diffusion with the exception of chloroform.

4.0

This behavior can be related to the crystallinity of each sample. As mentioned in section 4.1, the crystallinity of Ext-PTFE samples is higher than the SPS-PTFE counterparts. Thus, SPS-PTFE is more subjected to redistribution of its free volume to provide additional sites and accessibility to penetrant molecules, and consequently the relaxation process contribution in sorption kinetics should be substantial. However, chains mobility is restricted in Ext-PTFE due to its high crystallinity. As a result, transport of solvent into pre-existing and available vacancies or sites of Ext-PTFE would be ensured mostly by Fickian diffusion contribution. A direct comparison between time constants of diffusion and first order relaxation is provided via the Deborah number De (= KF/KR) [START_REF] Sk | Diffusion coefficient modeling in polyester barrier materials: applications of infinite series solutions[END_REF][START_REF] Vrentas | A Deborah number for diffusion in polymer solvent systems[END_REF]. In our case, the De values are ranging from 1 and 10 which means that sorption kinetics is limited by polymer relaxation phenomenon.

Additionally, SPS-PTFE samples exhibit the highest characteristic times of diffusion KF reflecting higher diffusion coefficients compared to Ext-PTFE samples. We note that this trend corroborates the high equilibrium uptake of liquid by SPS-PTFE samples. The structural features of each kind of samples can explain the observed differences in the D coefficient values. The larger crystalline domains of Ext-PTFE samples constitute more obstacles to the liquid diffusion paths and create an additional tortuosity that can delay diffusion according to the expression D=D * /(ζ βimm), where D * is the diffusion coefficient in the bulk amorphous polymer, βimm is the chain immobilization factor that relates to the reduced mobility of the polymer chains in the proximity of the crystals and ξ is the tortuosity that accounts for the increased diffusion path in order to bypass crystallites [START_REF]Permeation of Gases and Vapours in Polymers[END_REF][START_REF] Sk | Diffusion coefficient modeling in polyester barrier materials: applications of infinite series solutions[END_REF][START_REF] Michaels | Sorption and flow of gases in polyethylene[END_REF]. Additionally, the possible existence of amorphous segments embedded in the crystals which constitute the rigid amorphous fraction decreases and delays the transport of liquid molecules inside the polymer.

Influence of liquid

Among the prominent factors that can affect the absorbency of liquids by a polymer are the molar volume of solvents, density, and the chemical affinity with the polymer [START_REF]Permeation of Gases and Vapours in Polymers[END_REF][START_REF] Hansen | Hansen Solubility Parameters: A User's Handbook[END_REF][START_REF] Michaels | Sorption and flow of gases in polyethylene[END_REF][START_REF] Cheng | Recent Advances in Polymeric Solvent-Resistant Nanofiltration Membranes[END_REF].

Table 1 outlines the relevant parameters of the liquids investigated. As mentioned above, the equilibrium uptake Qe of Ext-PTFE and SPS-PTFE increases in this order: toluene ≤ n-hexane≤THF<chloroform. Additionally, the molar volume of toluene and n-hexane are higher than those of THF and chloroform, thus the equilibrium uptake decreases as the molar volume of the solvent molecule increases (Fig 8). In many studies dealing with semi-crystalline polymers, it was observed that n-alkane and other elongated or flattened molecules like nhexane are more accessible than spherical molecules of similar volume [START_REF]Permeation of Gases and Vapours in Polymers[END_REF][START_REF] Hansen | Hansen Solubility Parameters: A User's Handbook[END_REF][START_REF] Aitkin | Transport and solubility of isomeric paraffins in rubber[END_REF]]. This observation can provide an explanation why n-hexane is more absorbed than toluene even though its molar volume is slightly larger. Similarly, chloroform which has the highest density and the lowest molar volume can increase the equilibrium uptake by PTFE samples. According to the sorption results of PTFE and HSP approach, no correlation is evidenced between the liquid equilibrium uptake by PTFE and the Hansen parameters for each liquid investigated (Table 5). This conclusion corroborates prior studies [START_REF] Hansen | Hansen Solubility Parameters: A User's Handbook[END_REF][START_REF] Ebnesajjad | Fluoroplastics, Volume 1: Non-Melt Processible Fluoropolymers -The definitive User's Guide and Data book[END_REF] that have shown equilibrium uptake by PTFE less than 1% for most hydrogen containing liquids and which does not depend on the solubility parameters. Furthermore, the same references [START_REF] Hansen | Hansen Solubility Parameters: A User's Handbook[END_REF][START_REF] Ebnesajjad | Fluoroplastics, Volume 1: Non-Melt Processible Fluoropolymers -The definitive User's Guide and Data book[END_REF] reported that halogenated solvents penetrate more PTFE. This is what we observed for chloroform which has more chemical affinity with PTFE.

Structural characterization of swollen samples

The effect of swelling on the polymer structural characteristics can be also inferred from the XRD pattern appearance. Figure 9(a) illustrates XRD patterns of native and swollen Ext-PTFE samples with a sharp peak at 2θ = 18.4° and several weak peaks as well as two broad diffuse regions centered at 16.5° and 39.7°. Indexed peaks are shown in Table 8.

According to the temperature-pressure phase diagram, the phase behavior of PTFE is known to occur in four crystalline forms as reported by many studies [START_REF] Mark | Polymer Data Handbook[END_REF][START_REF] Bunn | Structures of Molecules and Crystals of Fluoro-Carbons[END_REF][START_REF] Brown | The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)[END_REF]. At normal pressure and a temperature of 19 °C, PTFE transforms form-II which is the triclinic structure to form-IV referring to the hexagonal lattice. The last form consists of hexagonally packed zigzag, twisted helices of linear chain polymeric molecules (-CF2-CF2-).The transition form-II to form-IV consists of a decrease in screw frequency from 136 (13 units at 6 full spiral rotations) to 157. At 30°C, form-IV transforms to I giving away to a pseudohexagonal structure, this conversion is considered to be a loss of ordering in the mutual-rotation of separate adjacent spirals. Forms II or I can be transformed to form III at high pressure (0.5 GPa) and high temperature (80°C) giving way to form a planar zigzag structure. 8). These disparities originate from the preferred orientation and growth of crystal planes due to the processing conditions [START_REF] Manabe | Supermolecular structure of polymer solids and its effects on penetrant transport[END_REF][START_REF] Ebnesajjad | Properties of Neat (Unfilled) and Filled Fluoropolymers[END_REF] for Ext-PTFE and the interaction mechanisms between PTFE grains for SPS-PTFE [START_REF] Brown | The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)[END_REF]. The XRD patterns of swollen Ext-PTFE samples keep general features with non-swollen samples, but the (100) peak is slightly broadened and shifted to smaller angular distances. For swollen SPS-PTFE, the three peaks around 37.3, 41.51° and 49.5° corresponding to the (107), ( 108) and (210) peaks disappeared. Moreover, the (100) and (110) peak intensities are weaker. The penetration of liquid into the polymer introduces disorder in its crystalline structure by rotation of molecules segments (CF2-CF2)n relative to each other (twisting and untwisting) around and along the molecular axis. These features are in agreement with those of an early theoretical modeling and experimental study of crystalline ultra-dispersed PTFE powder [START_REF] Bouznik | A crystal structure of ultra-dispersed form of Polytetrafluoroethylene based on X-ray powder diffraction data[END_REF]. In this paper, the authors concluded that a systematic absence of (hkl) reflections with l≠0 accompanied by broadening and intensity decrease of the (100) peak can be related to molecular disorder around and along the molecular axis. The broadening and shifting of the (100) peak to lower frequency was also observed by Liu et al.

[65] for irradiated PTFE with dose up to 4 MGy in oxygen at room temperature. This investigation showed clearly the formation of new oxygen containing groups within the structure of PTFE leading to lattice expansion and hindrance of the regular arrangement of chain atom into the crystallite. Thus, the penetrant molecules change the lattice spacing and induce a microstain in the SPS-PTFE crystallites. Langford et al. [START_REF] Langford | Scherrer after sixty years: A survey and some new results in the determination of crystallite size[END_REF] reported that the XRD line broadening is influenced by the apparent crystallite sized which can be calculated by applying Debye-Scherrer's equation

θ λ cos B k d = (12),
where B (rad) is defined as full width at half-maximum (FWHM) of the (100) peak, k is the Scherrer's constant (close to unity), λ is the wavelength of the irradiation, and θ is the Bragg angle between the incident ray and the diffraction pattern. The XRD data were fitted with a convoluted Gaussian peak shape on a peak-by-peak basis using the software OriginPro 8.5. Figure 10 shows the crystallite size of swollen and dry PTFE samples evaluated from Eq.( 12).

For SPS-PTEF, it can be seen that the apparent crystalline size decreases as the equilibrium uptake is increased. For Ext-PTFE, large error bars indicate that the crystalline domains were weakly influenced by the liquid penetration.

FTIR analysis

The IR spectra of dry and swollen PTFE samples are shown in Fig. 11. The IR spectra of dry and swollen PTFE samples are shown in Fig. 11. In all spectra, the prominent IR peaks at 1150 and 1212 cm -1 are related to the symmetric and asymmetric stretching modes of the CF2 groups [START_REF] Clint | Adhesion under water: surface energy considerations[END_REF][START_REF] Mihaly | FTIR and FT-Raman Spectroscopy Study on polymer based high pressure digestion vessels[END_REF][START_REF] Kang | Chemical band structure of a-C:F films with law dielectric constant deposited by using CH4/CF4 IPCVD[END_REF][START_REF] Sablinskas | Applications[END_REF]. Table 9 summarizes the IR band characteristics of samples and depicts differences between IR spectra of unswollen and swollen PTFEs. For the swollen SPS-PTFE spectra, the appearance of absorption bands related to CH, CH2, CH3 groups, and C-Cl is due to the penetrating liquids in the polymer phase [START_REF] Sablinskas | Applications[END_REF]. For Ext-PTFE, no significant changes are observed in the IR spectra after swelling which is coherent with its small equilibrium uptake. Overall, these IR observations indicate that the pristine SPS-PTFE didn't experience chemical degradation from the SPS process and only the physisorption is involved for swollen polymer witch corroborate the chemical inertia of the SPS-PTFE.

Mechanical properties of swollen PTFEs

Forces like compression or traction can induce a permanent deformation under swelling conditions and eventually provoke mechanical failure of these materials [START_REF] Burkarter | Superhydrophobic electrosprayed PTFE[END_REF][START_REF] Dhanumalayan | Performance properties and applications of polytetrafluoroethylene (PTFE) -a review[END_REF][START_REF] Ebnesajjad | History of Polytetrafluoroethylene and expanded PTFE membrane[END_REF][START_REF] Lampman | Mechanical behavior and Wear[END_REF][START_REF] Daniels | Tensile Properties[END_REF]. Figure 12(a) shows the stress-strain relationships of dry PTFE samples. The main parameters deduced from these experimental results are the Young's modulus, yield stress, tensile strength, and elongation at break (Table 10). For SPS-PTFE samples, these data reveal large deformations at break up to ~390%, high elastic modulus and a better ductility compared to the Ext-PTFE samples. A detailed interpretation of this behavior is beyond the scope of the current study but is likely attributed to the high extent of fibrillation developed under stress for SPS-PTFE samples leading to strong viscoelastic dissipation. Improving mechanical properties of PTFE by SPS processing was reported in an earlier paper [START_REF] Mdarhri | Processing, microstructure and mechanical analysis of Polytetrafluoroethylene sintered by field assisting sintering technique[END_REF]. Besides, the higher crystallinity of Ext-PTFE samples has for effect to increase its brittleness and decrease its elongation at break. These observations are in agreement with those of Ridelli et al. [START_REF] Riddell | Fatigue mechanisms of thermoplastics[END_REF] who reported that high crystallinity reduces damping, however resulting also in an increase of the elastic modulus. Despite the higher crystallinity of Ext-PTFE compared to that of SPS-PTFE, the latter is found to be more rigid and presents higher strength, ductility and toughness. In an attempt to explain this behavior, we want note that similar trends have been reported in Refs. [START_REF] Bedouia | Micromechanical modelling of elastic properties in Polyolefins[END_REF][START_REF] Hu | The effects of molecular weight and cooling rate on fine structure, stress-strain behavior and wear of Polytetrafluoroethylene[END_REF][START_REF] Thomas | Effects of fabrication on the properties of Teflon resins[END_REF]. For instance, F. Bedoui and coworkers [START_REF] Bedouia | Micromechanical modelling of elastic properties in Polyolefins[END_REF] have demonstrated, on the basis on a micromechanical model in which the semi-crystalline polymer material microstructure is represented by crystalline inclusions embedded into an amorphous matrix, the key importance of the inclusion aspect ratio (length over thickness), to explain a lower modulus for HDPE (E=1200 MPa, Xc= 70%) than for PP (E=1400 MPa, Xc= 40%)) despite a higher rigidity for both phases and a higher crystallinity for HDPE. Another report has showed an increase of ultimate strength, elongation and toughness under uniaxial tension when the crystallinity decreases for molded PTFE materials [START_REF] Hu | The effects of molecular weight and cooling rate on fine structure, stress-strain behavior and wear of Polytetrafluoroethylene[END_REF], suggesting that the mechanical properties of semi-crystalline thermoplastic polymers are strongly microstructuredependant [START_REF] Humbert | A re-examination of the elastic modulus dependence on crystallinity in semi-crystalline polymers[END_REF][START_REF] Chazeau | Relationships between Microstructural Aspects and Mechanical Properties[END_REF]. The complexity of this microstructure includes the differences in molecular architecture of these materials, thermal treatment, and crystallization conditions.

For our samples, we believe that the aspect ratio of the crystalline phase for SPS-PTFE plays For swollen PTFE samples, Table 10 shows that the Young's modulus and yield stress of SPS-PTFE decrease. The decreasing trend of these parameters with equilibrium uptake is also observed for toluene, THF and chloroform. This can be explained by a polymer plasticization by the liquid leading to a higher mobility of molecular chains and a weaker resistance to deformation, in agreement with previous observations [START_REF] Ebnesajjad | History of Polytetrafluoroethylene and expanded PTFE membrane[END_REF][START_REF] Gac | Yield stress changes induced by water in polyamide 6: Characterization and modeling[END_REF]. For Ext-PTFE, the opposite trend is evidenced. The Young's modulus increases while the yield stress remains constant as the equilibrium uptake increases with the notable exception of n-hexane. The presence of solvent contributes to the decrease of the nanocrystallite size in Ext-PTFE samples during the tensile tests. An early paper by Askadskii [START_REF] Askadskii | The Influence of the degree of crystallinity on the elasticity modulus of polymers[END_REF] showed that this change in the crystalline structure of semi-crystalline polymers can be related to the Young's modulus increase.

Concluding remarks

We reported on the effect of swelling on the structural and mechanical properties of Ext-PTFE and SPS-PTFE. The above results show that the sorption kinetics behavior of toluene, n-hexane, tetrahydrofuran and chloroform are pseudo-Fickian. To rationalize the sorption kinetics data the BH equation was used. Overall, we found that SPS-PTFE is characterized by property gradients from the surface to the bulk and that the equilibrium uptake is larger than for Ext-PTFE. An attempt was made to correlate the crystallinity of the native PTEFs deduced from DSC, the equilibrium uptake for several liquids and the apparent diffusion coefficient, in order to discriminate between the two kinds of samples explored. The combined effects of free energy of the interface between the processed PTFE and liquid, the Hansen parameters solubility, the molar volume and liquid type on the sorption rate are also discussed. It is found that the lower crystallinity of SPS-PTFE, acting as a barrier to solvent diffusion, decreases its resistance to swelling. XRD investigation exhibits a slight decrease in crystallite size in swollen SPS-PTFE. Tensile experiments reveal that the native SPS-PTFE has the best mechanical properties, and highlight a significant decrease of Young's modulus and yield stress after swelling due to a plasticization effect. For Ext-PTFE samples, the opposite trend is observed after swelling due to its higher crystallinity.

Fig. 1 :

 1 Fig.1: Principle of the SPS apparatus.

Fig. 2 :

 2 Fig. 2: Typical operating conditions for the SPS applied to PTFE powder.

Fig. 3 :

 3 Fig.3: DSC traces of the (a) first and (b) second heating-cooling cycle of Ext-PTFE and SPS-PTFE (Heating and cooling rate = 10°C/min). Scans are separated on the vertical scale for clarity of presentation. Schematics of the mixed amorphous crystalline structure of SPS-PTFE at: (c) the first and (d) second heating/cooling cycles. (e) Same for Ext-PTFE at both cycles. The schematics in (f) illustrates the chain folded crystal structure (FCC) and the extended chains crystal structure (ECC).

  the dispersive and polar components of surface free energy of the solid and liquid phases, sl γ is the IFE corresponding to the solid-liquid interface, and θ is the contact angle for the probe liquid on the solid phase.

Fig. 5 :

 5 Fig.5: Sorption kinetics of liquids for: (a)Ext-PTFE and (b)SPS-PTFE samples.

Figs. 6

 6 Figs.6 and 7. A linear part of the curve is observed only at the very beginning of the sorption experiments. It indicates that the diffusion is dominated by a rapid Fickian diffusion at the

Fig. 6 :

 6 Fig.6: A comparison for our sorption kinetics measurements to the BH model for Ext-PTFE samples (the red line shows the fit to the BH model).

Fig. 7 :

 7 Fig.7: Same as in Fig. 6 for SPS-PTFE samples.

Fig. 8 :

 8 Fig.8: Plot showing the effect of liquid molar volume on uptake at saturation for PTFE samples. (1): chloroform, (2): THF, (3): n-hexane, (4): toluene (solid lines are for guidance of the eye).

Fig. 9 :

 9 Fig.9: XRD patterns of (a) Ext-PTFE and (b) SPS-PTFE dry and swollen at equilibrium in several liquids. The red dashed line shows the (100) peak.

Figure 9 (

 9 Figure 9(b) presents XRD patterns of SPS-PTFE. Native SPS-PTFE exhibits the hexagonal structure as Ext-PTFE. Nevertheless, several significant differences between both types of samples can be observed: for SPS-PTFE, the (110) diffraction peak which is absent in the XRD pattern of Ext-PTFE has the highest intensity; the (200) and (220) peaks do not appear in the diffraction pattern; and the (210) peak has a strong intensity (Table8). These disparities

Fig. 10 :

 10 Fig.10: A plot reflecting the relationship between the apparent crystallite size of PTFE samples and the equilibrium uptake of several liquids.

Fig. 11 :

 11 Fig.11: FT-IR spectra of native PTFE and swollen Ext-PTFE and SPS-PTFE samples.

a

  key role in enhancing the mechanical properties since the DSC thermograms of these samples indicate the presence of ECC which are characterized by a high aspect ratio. The sintering process contributes also to the observed mechanical properties of SPS-PTFE by maintaining the compressive pressure at 25 MPa. This value is largely sufficient to provide an effective compaction of PTFE particles since the room temperature yield strength of PTFE is close to ~12.5 MPa and decreases by increasing temperature[START_REF] Gac | Yield stress changes induced by water in polyamide 6: Characterization and modeling[END_REF]. Additionally, other parameters such as the size of PTFE powder particles prior to processing, and distributionorientation-texture of crystalline phases, should be taken into account in order to fully understand the differences between the mechanical responses of SPS-PTFE and Ext-PTFE samples. Moreover, changing the load conditions, e.g DMA measurements by varying both temperature and frequency, eventually bring additional information.

Fig. 12 :

 12 Fig.12: A plot of the stress-strain relationships of (a) unswollen PTFEs (b) Ext-PTFE and (c) SPS-PTFE samples after swelling in different liquids: toluene, n-hexane, THF and chloroform. Room temperature.

  13(b)), we obtain the intercept on the y-axis which

.

  Only the positive solutions with smallest average deviation are selected.

Fig. 13 :

 13 Fig.13: Linear regression analysis of contact angle measurement for the different solvents and PTFE samples explored.

Table 1 :

 1 Molar mass, density and molar volume of solvents used for PTFE swelling[START_REF] Van Ness | Thermodynamics in[END_REF].

	Solvent	Structure	Molar mass (g/mol)	Density (g/cm 3 )	Molar volume (cm 3 /mol)
	Toluene		92.14	0.867	106.29
	N-Hexane		86.18	0.655	131.6
	Tetrahydrofuran		72.11	0.888	81.57
	Chloroform		119.38	1.489	80.67

Table 2 :

 2 Melting peak temperature Tm peak , Heat of fusion ∆Hf , Crystallization temperature Tc peak , crystallization enthalpy ∆Hc, Degree of crystallinity Xc deduced from DSC Scans

	Material Designation	Cycle number	Tm peak (°C)	∆Hf (J/g)	Xc (%)	Tc peak (°C)	∆Hc (J/g)
	SPS-PTFE	1 st	330 348	10.4 9.3	24.0	319 349	-19.8 -0.23
		2 nd	330	20.7	25.2	315	-22.2
	Ext-PTFE	1 st 2 nd	333 330	45 31	54.9 37.8	310 310	-0.23 -34.2

  Table 3 shows static contact angle of Ext-PTFE and SPS-PTFE samples ranging in nature from polar aprotic to nonpolar protic: a) water, b) toluene, c) n-hexane, d) THF, e) chloroform, and from the most polar to the less polar: water, glycerol, ethanol, diiodomethane.

Table 3 :

 3 Dispersion γ d and polar γ P surface tension of liquids (mJ/m²) and their contact angles

				(C.A) with PTFEs
		(a) Water	Glycerol Ethanol	Di-iodo-methane	(b) Toluene	(c) n-hexane	(d) THF	(e) Chloroform
	γ d (mJ/m²)	21.6	34	18.8	49.5	28.5	18.4	24.6	25.9
	γ 51.0	30	2.6	1.3	0	0	0	1.6
	γl(mJ/m²)	72.6	64	21.4	50.8	28.5	18.4	24.6	27.5
	C.A. Ext-PTFE	107±3	107±5	31±2	88±3	40±1	∼0±0	41±2	39±2
	C.A. SPS-PTFE	116±3	104±3	31±3	84±4	38±2	∼0±0	38±1	36±4

P (mJ/m²)

Table 4 :

 4 FSE and surface tension components of Ext-PTFE and SPS-PTFEFigure4shows the IFE of Ext-PTFE and SPS-PTFE. It decreases progressively from water, toluene, THF, n-hexane and Chloroform. Furthermore, IFE of each liquid in contact with Ext-PTFE is slightly lower than SPS-PTFE counterpart; this will approve spreading, wetting and sorption of liquid in Ext-PTFE.

		Ext-PTFE	SPS-PTFE
	γ p (mJ/m²)	0.45±2 .6	1.77±3.2
	γ d (mJ/m²)	18.49±0.63	18.49±0.33
	γs(mJ/m²)	19±3	20±4

Table 5 :

 5 Hansen solubility parameters (HSP) of liquids and equilibrium uptake for SPS-PTFE and Ext-PTFE samples.

	Material	Total HSP (cal/cm 3 ) 1/2 δT	HSP (cal/cm 3 ) 1/2 δD δP δH	Ra(cal/cm 3 )	RED	Qe (%) Ext-PTFE SPS-PTFE
	Water	14.8	7.4	10 8.1	20	5.12	-	-
	Toluene	18.2	18	1.4	2	3.88	0.99	0.17	0.91
	n-hexane	14.9	14.9	0	0	4.64	1.19	0.20	0.95
	THF	19.5	16.8 5.7	8	6.15	1.58	0.25	1.21
	Chloroform	18.9	17.8 3.1 5.7	4.15	1.06	0.46	2.03
	PTFE	16.7	16.2 1.8 3.4	R0=3.9	-	-	-

Table 6 :

 6 Calculated diffusion parameters from fit parameters to sorption kinetics curves using the BH empirical power law and Eq.(4).

	liquid	Di(m²/s)	KF=(π²Di/l²)×10 5 (s)	k	n
			Ext-PTFE
	toluene	1.45 10 -11	3.63	0.22±0.02	0.22±0.01
	n-hexane	3.72 10 -12	0.93	0.13±0.01	0.29±0.01
	THF	1.12 10 -11	2.78	0.24±0.01	0.20±0.01
	chloroform	1.07 10 -11	2.68	0.10±0.01	0.33±0.01
			SPS-PTFE
	toluene	2.65 10 -12	2.65	0.21±0.01	0.23±0.01
	n-hexane	5.15 10 -13	0.52	0.08±0.07	0.38±0.01
	THF	1.13 10 -12	1.13	0.13±0.01	0.30±0.01
	chloroform	2.32 10 -12	2.32	0.17±0.01	0.26±0.01

Table 7 :

 7 Calculated diffusion parameters from fit parameters to sorption kinetics curves using BH model in both SPS-PTFE and Ext-PTFE samples.

	Sample processing method	liquid	αF	D×10 12 (m²/s)	(s -1 ) KF= D/l²×10 5	(s -1 ) KR×10 5	De=KF/KR
		toluene	0.58	3.89	0.097	0.133	0.7
	Ext-PTFE	n-hexane THF	0.52 0.72	4.17 5.11	0.103 0.128	0.061 0.055	1.7 2.3
		chloroform	0.25	11.1	0.279	0.094	3.0
		toluene	0.35	9.70	0.972	0.108	9.0
	SPS-PTFE	n-hexane THF	0.16 0.29	5.00 3.33	0.500 0.333	0.089 0.094	5.6 3.5
		chloroform	0.34	4.45	0.446	0.110

Table 8 :

 8 XRD pattern attributions of Ext-PTFE and SPS-PTFE samples (S=sharp, W=weak, VW=very weak, and M=moderate).

Table 9 :

 9 Assignment of IR absorbance bands of swollen and native PTFE samples (W=weak,

	VW=very weak, M=moderate, S=sharp).
	IR band		Assignment
		Native PTFEs	Swollen SPS-PTFE
	620-640 W		CF deformation
	718,765 VW		CF2 scissoring
	720-780 M	-	C-Cl stretching (SPS-PTEF swollen
			by choloroform)
	1150, 1204-1210 S		CF2 symmetric stretching
	1436 W	C-F2 asymmetric stretching
	1456-1462, 1621 W	-	CH2 , CH3 deformation
	1580-1800 W	-	CH deformation
	2918-2930 W, 2845 W	-	CH2 , CH3 stretching

Table 10 :

 10 Young modulus and yield stress at elastic strain of 0.2% for Ext-PTFE and SPS-PTFE(dry and equilibrium swollen) samples.

		Ext-PTFE		SPS-PTFE
		E (MPa)	σe (MPa)	E (MPa)	σe (MPa)
	dry	270	0.6	480	1
	toluene	480	0.5	290	0.7
	n-Hexane	274	0.5	184	0.4
	THF	463	0.8	228	0.5
	chloroform	412	0.8	200	0.5

Acknowledgements

The authors thank the Center of Analysis of Cadi Ayyad University for FTIR and XRD characterizations. We gratefully acknowledge the assistance of G. Bonnefont for SPS experiments.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Appendix A

The dispersive and polar components of both Ext-PTEF and SPS-PTFE samples are calculated by applying Owen-Wendt's model and using contact angle measurements. We proceed by linear regression which permits to evaluate correctly the errors involved. The points on the x-and y-axis are calculated for each solvent using


Federico Lizzi

Christelle Goutaudier

Nina Attik

Philip Jackson

Ian Campbell

Ilham Mokbel

Brigitte Grosgogeat

Cyril Villat

Ion release characterization in phase separated borosilicate glass powders

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In 1969, Hench developed the first bioglass containing Na2O; CaO; SiO2 and P2O5. Since then, the initial Hench Bioglass ® has been widely studied and modified to improve its biological, antibacterial and mechanical properties. Moreover, bioglasses may exhibit phase separation that could have an impact on their biological properties. The particular structure and the properties deriving therefrom depend on their thermal history as well as their preparation technique and their composition. [START_REF] Jones | Review of bioactive glass: From Hench to hybrids[END_REF][START_REF] Li | Antibacterial 45S5 Bioglass®-based scaffolds reinforced with genipin cross-linked gelatin for bone tissue engineering[END_REF][START_REF] Lizzi | Mechanical characteristic and biological behaviour of implanted and restorative bioglasses used in medicine and dentistry: A systematic review[END_REF][START_REF] Fiume | Bioactive Glasses: From Parent 45S5 Composition to Scaffold-Assisted Tissue-Healing Therapies[END_REF][START_REF] Lefebvre | Sintering behaviour of 45S5 bioactive glass[END_REF][START_REF] Golovchak | Influence of phase separation on the devitrification of 45S5 bioglas[END_REF][START_REF] Durgalakshmi | Phase separation induced shell thickness variations in electrospun hollow Bioglass 45S5 fiber mats for drug delivery applications[END_REF][START_REF] Liu | Nanophase separation and structural ordering in silica-rich mixed network former glasses[END_REF] Among the different bioglasses investigated during these past five decades, borosilicate glasses have shown interesting bioactive properties in bone repair and in drug delivery systems [START_REF] Fiume | Bioactive Glasses: From Parent 45S5 Composition to Scaffold-Assisted Tissue-Healing Therapies[END_REF]. These glasses are also well known to exhibit phase separation. The earliest practical application of phase separation has been developed by Hood and Nordberg [START_REF] James | Volume Nucleation in Silicate Glasses[END_REF] in the Vycor process (SiO2, B2O3, Na2O glass system) for the production of high silica ware via a porous glass approach.

The amorphous phase separation (APS) causes an initially homogeneous single phase glass to separate into two or more phases of different compositions. The Gibbs free energy of the system with two or more distinct phases has to be lower than that of the system with one single homogeneous phase. The degree of interconnectivity of the two glass phases depends on the nature of the phase separation mechanism. This process can occur by a nucleation and growth process (which gives isolated spherical particles) or by spinodal decomposition where an interconnected structure is obtained [START_REF] Uhlmann | Microstructure of glasses does it really matter[END_REF].

Considering the borosilicate glass composition, boron impacts on the release into aqueous media of other elements such as calcium and, hence, on the bone metabolism. Boron has been linked with preventing calcium loss and bone demineralization in post-menopausal women and boron supplementation has been shown to reduce the effects of vitamin D deficiency in chicks. It also clearly influences bone-regulating hormones involved in bone growth and bone turnover. In terms of other elements used in silicate glasses, calcium is useful since it is a primary component of hydroxyapatite in bone but also in dental tissues (enamel and dentin) and so, is crucial for the restoration of damaged hard tissue. Potassium is a useful desensitizing agent. Aluminium has been considered as an important element for providing Al 3+ ions that are useful for the secondary self-hardening process in dental cements featuring polyacids. Nonetheless, Aluminium is recognized as a neurotoxin and the cause of some neurological diseases, such Alzheimer's and Parkinson's disease [START_REF] Naghii | The role of boron in nutrition and metabolism[END_REF][START_REF] King | The Effect of In Ovo Boron Supplementation on Bone Mineralization of the Vitamin D-Deficient Chicken Embryo[END_REF][START_REF] Palacios | The Role of Nutrients in Bone Health, from A to Z[END_REF][START_REF] Matis | In Vivo Study of Two Carbamide Peroxide Gels with Different Desensitizing Agents[END_REF][START_REF] Nicholson | Role of Aluminum in Glass-ionomer Dental Cements and its Biological Effects[END_REF][START_REF] Drago | Role of metal ions in the abeta oligomerization in Alzheimer's disease and in other neurological disorders[END_REF].

Considering the properties described above, phase-separated glass systems composed of SiO2-K2O-B2O3-CaO-Al2O3 could be of interest in medicine and dentistry due to their ion release potential and post-leaching increased porosity. However, the behaviour of such materials should be investigated in acidic conditions to evaluate their possible uses blended with polyacid solutions or in the oral environment where pH changes due to food consumption are experienced. In the work reported here, different compositions have been investigated in which SiO2 and K2O were fixed (45% and 15% in weight respectively) while other elements (Ca, Al, B) varied in order to assess the effect on phase separation. In the perspective of biomedical and dental use i.e. in glass ionomer cements (GIC), this study investigates the acid degradability and leaching profile of the ions released from novel borosilicate fillers into a nitric acid solution at different time points. The different composition of borosilicate samples have been obtained with a factorial experimental design (FED) that allows the efficient organization of the experiments required in scientific research or industrial process optimisation. The hypothesis is that the phase separation would directly influence the ion release behaviour and subsequently the poly-acid cement setting. Structural properties of the glasses, physico-chemical and morphological features related to the phase separation and ion release are investigated in this study and correlated to the glass composition.

Material and methods

Borosilicate glass preparation

Using a Factorial Experimental Design (FED) approach, a series of glass compositions based on the quinary system SiO2-B2O3-K2O-Al2O3-CaO (Table1) were prepared from raw materials purchased by different manufacturers:

-Microsil (Sibelco Benelux, Antwerpen, Belgium) 99% pure SiO2, -H3BO3 (Optibor TP, Borax Inc, Rio Tinto Borates, Greenwood Village, CO, USA), -K2CO3 (Altair Chimica SpA, Saline di Volterra, Italy), -Al2(OH)6 (Industrial Mineral Services, Madeley, United Kingdom) and, -CaCO3 (Ben Bennett Jr Ltd, Rotherham, United Kingdom).

As mentioned above, SiO2 and K2O were fixed at, respectively, 45% and 15% by weight in the developed glasses. The phase-separated borosilicate (PSBS) glasses were synthesized using the melt-quenching technique. The batch was melted at 1200-1300 °C (the temperature selected for each composition being based on visual assessment during melting) in a quartz crucible for 4 hours in an electrically heated furnace. The melt was quenched by pouring onto a flat steel plate. The resulting glasses were then heat treated at 700°C for 20 hours to induce phase-separation. Two duplicate batches (PSBS 7 and PSBS 10; PSBS2 and PSBS 6) were made in order to compare the results obtained after melting to avoid possible experimental errors. Finally, a fraction of each sample was milled with a Tema mill (Tema Machinery Ltd, Woodford Halse, United Kingdom) to obtain a particle size of about 50µm.

Characterization of the borosilicate glasses 2.2.1. X-ray powders diffraction

Powder X-ray diffraction (XRD) data were collected with a Bruker diffractometer with graphite monochromator (Bruker Ltd, Coventry, United Kingdom) using CuKα radiation. Data were collected in the range of 2°<2θ<60° in 0.05° steps. This technique was used to determine if the glass frit was fully amorphous after the heat treatment.

Particle size distribution analysis

The particle size distribution was measured by the Malvern Mastersizer 3000 laser diffraction particle size analyser (Malvern Panalytical Ltd, Malvern, United Kingdom). The glass powders were evaluated in liquid dispersion with ethanol to avoid unwanted aqueous dissolution and to ensure a consistent Dv99 of less than 50 µm and a Dv50 of less than 6.9 µm. Three collections of powders measurements were made for waiting the stabilization of the dispersion.

Differential thermal analysis

Thermal behaviour was analysed using differential thermal analysis (DTA) on a TGA/DSC 2 apparatus (Mettler Toledo, Viroflay, France). The measurements were conducted with a heating rate of 20°C/min and using nitrogen as both a protecting and a purge gas.

Approximately 50 mg of glass powder was placed into an alumina crucible and gently pressed to ensure good heat transfer. The powders were scanned over the 30 to 1200°C range. The calibration of the DSC equipment in terms of temperature was done by using the theoretical values of four metal standards for which the precise values of melting temperatures are well known (In; Al; Au and Pd). The relative uncertainty does not exceed 0.3% for temperature.

The glass transition temperature was measured at the inflection point of the sigmoid characterizing it.

Static dissolution experiments

Static dissolution experiments were performed for all the PSBS samples. The glass powders (0.4 g each) were mixed with an HNO3 0.003M aqueous solution (40.0 mL) in a flacon tube.

The mixtures were stirred in a 37°C thermostatically controlled reactor. The pH of the mixture was recorded during the entire dissolution process. The absolute uncertainty of the pH values is u(pH) = 0.02 after calibration of the meter with pH 4; 7 and 10 buffers. At different time points, the solutions were centrifuged and collected to quantify the ions released.

Inductively coupled plasma atomic emission spectroscopy

The amount of ions released in the collected solutions was measured by inductively coupled plasma atomic emission spectroscopy (Vista-MPX CCD Simultaneous ICP-OES, Varian Inc., Palo Alto, USA). The elements detected and the corresponding absorption wavelengths are reported in table 2. Calibration curves were obtained by preparing multi-elements standard solutions containing B, Ca, K and Si (dilutions were made in HNO3 solutions). Aluminium was not detected with the ICP measurements for all the glasses because it is highly insoluble in aqueous solution from pH=2 to 10. Due to its amphoteric nature, Al2O3 can react with both bases and acid. According to the pH of the medium, it precipitates as aluminium hydroxide or alkaline aluminate as it is released in the solution. Three replicates were collected for each measurement. The concentration values obtained from different emission lines for each element were averaged and presented as percentage of dissolved element ([element]%), calculated as the ratio between the measured concentration and the maximum concentration that could be obtained if total dissolution of the glass occurred. The main source of uncertainty is due to the scattering of the experimental points emerging from analysis procedure. The relative standard uncertainty ur([element]%) is estimated as twice the value of the variation coefficient of the 3 replicates.

Consequently, the mean values of ur are as follow: ur([element]%) = 3% for Boron, Calcium and Silicon, and ur([element]%) = 5% for Potassium. These values are in good agreement with those commonly accepted for ICP technique.

Scanning electron microscopy (SEM)

Glass bulk specimens were mounted in a methyl methacrylate resin (Resine mecaprex KM-U, Prési, Brié-et-Angonnes, France) then polished by a NWF+ (Rough-finishing cloths, Prési, Brié-et-Angonnes, France) with a 3µm diamond suspension, in order to obtain a flat surface.

The morphology of the samples was examined using a scanning electron microscope incorporating EDX analysis (FEI Quanta FEG 250, Thermo Fisher Scientific, Villebon-sur-Yvette, France) before and after two acid treatments. The leaching of bulk glasses was performed using (i) ambient conditions comprising a 1 M HNO3 etch at 37°C for 48 hours and (ii) "hard" conditions (as (i) but at 90°C). Then, the glasses were rinsed with deionized water before analysis. The hard condition was carried out to speed up the dissolution process to more completely etch out the most soluble phase (postulated to be the boron-rich one) from the samples.

X-ray fluorescent analysis (XRF)

XRF analysis was used to analyse the oxide type and content in the glass powder obtained after the dissolution experiments featuring the hard condition. The material was ground to a powder and ignited to attain a constant weight; the ignited material was mixed with a flux and a glass melt formed. The melt was casted into a bead and run on the Panalytical Axios WD XRF spectrometer (Malvern Panalytical Ltd, Malvern, United Kingdom). A bulk chemical analysis has then obtained.

Results

3.1.Glasses characterization

All the PSBS glasses were amorphous according to the XRD results before and after the heattreatment. However, whilst the as-quenched samples appeared consistently transparent and colourless, the thermal treatment at 700°C for 20 hours induced some of the samples to turn opalescent. The intensity of this phenomenon varies from a slight milky effect until completely opaque. The diagram in figure 1 shows that the composition influences the opacity of the samples and suggests lines (isopleths) associated with intense, mild or no opacification.

In the absence of crystalline phases, the opacification is most likely linked to a phase separation leading to two glass phases with different refractive index (RI).

With the SiO2 and K2O content being fixed, Janecke's coordinates ( ) were employed to enable glass compositions to be plotted on a triaxial diagram. This allows better visualisation of the impact of varying Al2O3, CaO and B2O3 content (fig. 1):

= ∑
where mi represents the mass of each component Al2O3, CaO and B2O3

Powder X-ray diffraction (XRD) analysis was performed for every sample after the heat treatment. All the XRD spectra patterns show a characteristic amorphous shape and do not demonstrate the formation of a secondary amorphous phase. Figure 2 shows some typical XRD results of glasses without Al2O3 (Figure 2a) and without CaO (figure 2b).

For samples without Al2O3, the superposition of the resulting patterns shows a 2θ angle shift of the typical amorphous halo. It has been found to be related to the level of CaO addition in the sample compositions (Table 3).

DTA analyses were carried out on four carefully chosen samples to evaluate the influence of composition on the glass structure. Thermograms for PSBS 1, 3, 4 and SBK are showing a principal glass transition Tg, corresponding to the major amorphous phase (Figure 3). Other less pronounced shifts in the DTA curves are evidence of the presence of a second phase, probably corresponding to the other phase formed after the heat treatment. The Tg is influenced by the introduction of alkaline earth oxides and aluminium addition (Table 4), considering that the w% potassium is the same for all samples. Ca 2+ causes an increase in the Tg temperature in respect to the three component glass PSBS SBK. However, the order of Tg increase does not correlate to calcium loading suggesting that aluminium also plays a significant role in creating increased glass network connectivity and so a higher Tg. The presence of phase separation serves to further complicate the situation.

Observation of the PSBS3 at T above the Tg

Only one sample, the most calcium-rich sample PSBS 3, presents an exothermic peak at 787.8°C in the DTA curve. This heat production phenomenon demonstrated by a rapid change in the heat flow, appears when a glass arranges into a more ordered structure characteristic of crystals. This crystallisation temperature (Tc) converts the glass into a glass-ceramic.

The crystallization process has been amplified by a heat treatment at different time-points (5, 10 and 20 hours) and at a temperature (800°C) higher than the phase separation heat treatment. Pictures of the resulting sectioned samples were taken with the optical microscope and show the development of the crystallisation process from the external surface to the core of the bulk sample (Fig. 4).

The crystalline phases are revealed by XRD analysis (Fig. 5) and identified as:

-Potassium Boro-Silicate (KBSi2O6), -Calcium Borate (CaB2O4), -Tri-Calcium Silicate (Ca3SiO5).

3.2.Static dissolution experiments

In a first step, the PSBS powder samples have been compared by measuring the pH variation in HNO3 (0.003M aqueous solution). All glasses show the same characteristic pH curve typical of bioactive glasses when dispersed in aqueous media (Fig. 6). A rapid pH increase takes place during the first minutes, after which the pH increases only slightly. This phenomenon is due to the ionic exchange between the cationic network modifiers and H + in the HNO3 solution. After the fast pH rise, stabilization at between pH 8 and 9 occurs. The pH rise is considerably accelerated by the heat-treatment suggesting a low chemical durability phase with a higher concentration of potassium.

Hydrolysis of the silica network is known to occur at higher pH values (usually above pH=10), resulting in congruent dissolution of silicate glasses [START_REF] Hench | Physical chemistry of glass surface[END_REF]. Consequently, this phenomenon must be negligible in the conditions used.

The comparison of samples with and without Al2O3 reveals a different ion leaching behaviour during time (Fig. 7). PSBS 16 (opalescent glass, 15 w% of Al2O3) releases calcium at a higher kinetic rate (∆Ca/∆t = 2.98 Ca%/min). For PSBS8 (transparent glass, without Al2O3), the release of potassium and boron is congruent and happens at a faster kinetic rate than calcium (∆K/∆t = 2.74 K%/min; ∆B/∆t = 2.66 B%/min). Silicon release is very low for both the glass samples. This is to be expected as it is a network former and not prone to leaching. It is also likely that silicon is dominant in the more durable of the two phases. Boron and potassium are released much more readily than silicon. Fast potassium release is to be expected but boron, like silicon, might be expected to play a network former role in the generation of stable glasses. However, glasses with a high Boron content can have lower durability and in this case, the release can be similar to a network modifier ion such as K + . The formation of NBO (non-bridging oxygens) weakens the structure of glass and makes the species soluble and more accessible to water. The depolymerisation of the silica network by the creation of NBO generates larger cavities, favouring the invasion of nitric acid solution and the exit of the soluble elements. Two mechanisms of dissolution are hypothesized for PSBS8 and PSBS16; after 125 minutes in HNO3, a rapid increase in ion release is detected in PSBS8. The phenomenon is not detected in PSBS16 where after 125 minutes it appears that the ion release reaches a plateau. PSMS8 is postulated to have a weaker glass phase (featuring potassium and boron at high levels) that more readily dissolves. In addition, the faster release of calcium from the opalescent glass could bring about the formation of a precipitate layer on the glass surface that delays further release of other species resulting in a plateau. The release of aluminium ions is not reported because detection is at too low concentrations. This suggests Al ends up in the resistant phase and could encourage the progress of phase separation during the heat-treatment promoting the formation of opalescent glasses.

A comparison between two samples PSBS3 and PSBS5 (transparent and opalescent respectively, with the same amount of calcium in the compositions) has been made in figure 8. The ion concentration profiles suggest that the dissolution rate of Ca and B are enhanced by the heat treatment in all cases. Moreover, it is confirmed that different leaching rates exist between the opal and clear samples. In PSBS3 potassium ions are released with a higher kinetic rate than PSBS5 (∆K/∆t = 9.5E-3 vs. 5.3E-3), while for calcium ions the opposite tendency is observed (∆Ca/∆t = 8E-3 vs. 11.2E-3).

In figure 9 are shown the boron, calcium, potassium and silicon release trends for glasses belonging to specific isopleths for the two main glass series (transparent and opalescent)

identified in figure 1. The isopleths identify compositions with a constant amount of aluminium. The ion release profiles confirm that the opalescent glasses release calcium with a faster kinetic rate than the transparent ones (average comparison of the kinetic rate is 4 and 2.65 ∆Ca/∆t respectively). Potassium is released faster in the transparent samples (average rate: 3.18 ∆K/∆t) compared to the opalescent ones (average kinetic rate: 2.38 ∆K/∆t). The dissolution mechanism for both glass series is found to be dependent from the B2O3/CaO ratio. This value has been represented in the graphs using the Janecke's coordinates without taking in consideration the amount of Al2O3 because it is assumed to be constant along the isopleths. Higher amounts of boron in the composition induce a slower release of potassium, calcium and boron which is justified by a higher borosilicate character in the weaker phase of the glasses.

Scanning electron microscopy

SEM analyses of the PSBS 8 and PSBS 16 glass bulk samples were undertaken to find evidence for two amorphous phases and to confirm that phase separation was occurring. It was expected to find similarity with the sodium borosilicate system of the Vycor glasses. The etching process in normal conditions is found to be effective for revealing new details of the glass surface. A porous structure is detected for the opalescent sample PSBS 16 while a homogeneous surface without narrow veins is observed in the transparent glass PSBS 8 (see lower images in figure 10), proving that the surfaces are different and a distinct reaction with the nitric acid is going on. In order to increase the dissolution phenomenon, the glasses were immersed in nitric acid in hard conditions (temperature = 90°C). This hard acidic condition reveals more details for the interpretation of the glass structures:

-The opalescent bulk glass (PSBS 16) shows a high level of structure retention with the leach surface again showing porosity. EDX analysis identified pure silica (Fig. 11),

-The transparent bulk glass (PSBS 8) has dissolved significantly, leading to structure collapse and leaving, at the bottom of the reaction beaker, a white amorphous powder composed of almost pure silica as shown by XRD and XRF analyses (Fig. 12).

Discussion

The present study illustrated the properties of borosilicate glasses in relation to their thermal history and contrasts ion leaching behaviour before and after heat treatment of a range of compositions. The selected compositions simulated the SiO2 content of 45S5 glass [START_REF] Hench | The story of Bioglass ®[END_REF] and using K2O as alkali selected a compositional range based on reported development of phase separated structure [19]. In addition to compositional factors affecting the parent glass properties, thermal treatment was shown to consistently alter phase structures, depending on composition, and therefore properties as shown by relative pH changes vs time (figure 6) and relative leaching (figures 8 and 9) [START_REF] Kreidl | Phase separation in glasses[END_REF].

Two obvious glass types were apparent after heat treatment observed as transparent and opalescent products. Results indicated that opalescence must be the result of glass in glass phase separation, as crystallisation was typically not encountered with the selected heat treatment regime. Although difficult to see in figure 3, second Tg values were apparent even for transparent glasses. Only at high calcium levels and/or higher heat treatment temperatures was a crystallisation event seen (figure 3). Transparency in phase-separated glasses can either be a result of forming two phases with the same refractive index or the presence of one phase as nano-size entities [START_REF] Arstila | Influence of heat treatment on crystallization of bioactive glasses[END_REF].

Dissolution behaviour was considered typical of bioactive glasses in aqueous media [START_REF] Jones | Review of bioactive glass: From Hench to hybrids[END_REF][START_REF] Zhang | In situ pH within particle beds of bioactive glasses[END_REF]. However, the influence of heat treatment on individual cases must always be considered in connection with compositional factors. Heat treatment creating both opalescent and transparent phase-separated glasses leads to a change in ion dissolution behaviour (figure 8).

Thereby, a chemically weaker phase and a chemically stronger phase are created [START_REF] Rafferty | An investigation of amorphous phase separation, leachability and surface area of an ionomer glass system and a sodium-borosilicate glass system[END_REF] in both cases but much more work is needed to identify the composition and size / availability of the weaker phase in both cases.

Microscopy (figure 10) showed some evidence of phase separated structures with differences evident in between opalescent and transparent phase separated glasses. Opalescent glasses show clear evidence of smaller, discrete areas of weak glass becoming leached out.

Microscopic analysis for the transparent glass is less clear: A vein-like structure before leaching vanishes after leaching. The fact the transparent samples collapse to a powder after "hard" acid leaching suggests a weaker continuous glass layer with discrete, perhaps nanosize regions of a high silica durable phase that ultimately becomes powder (figure 13).

The final structures and compositions of the individual amorphous phases dictate the leaching of elements and the nature of any relict structure or material. It is doubtful whether SEM-EDX can provide compositional information on each phase. Techniques such as Helium Ion Beam Microscopy may be needed. Alternatively, more work could be performed on analysis on the remaining phase (assuming 100% leaching of the weak phase can be guaranteed) to provide, by difference, the composition of the leached phase. It would be interesting to compare the calculated leached phase to the ppm levels of ions leached from the phase versus time. Whilst it is clear that calcium and aluminium ions play a significant role in creating glass structure connectivity and so degree of leaching in acids, until the relative level of calcium, aluminium and boron in the weakest phase is known, it is difficult to connect structure to leaching trends [START_REF] Bunker | Molecular mechanisms for corrosion of silica and silicate glasses[END_REF]. Of particular interest in the field of GICs is the performance / behaviour of transparent phase-separated glasses that contain no aluminium. Although not reported here, very preliminary data suggests a powdered version of the transparent glass delivers an impressive set strength. In the absence of alumina, it is postulated that leaching of boron (as borates) creates cross-linking opportunities.

In summary, the present study suggests a free alumina borosilicate glass with an enhanced bioactivity and apatite forming ability. Despite the need for much more work, it is clear that the ability to influence the dissolution behaviour of these glasses indicates potential development for improved biomedical materials such as glass ionomer cement in restorative dentistry, in particular. Formulation of Al free varieties of dental cements has already been proposed in previous studies with different approaches [START_REF] Neve | Development of novel dental cements. I. Formulation of aluminoborate glasses[END_REF][START_REF] Kim | Development of a novel aluminum-free glass ionomer cement based on magnesium/strontium-silicate glasses[END_REF][START_REF] Gomes | Aluminum-free glass-ionomer bone cements with enhanced bioactivity and biodegradability[END_REF].

Figure 10

 10 Figure 10 represents the analysis of the glass surface with the back-scatter detector and shows two different structures for the opalescent and the transparent surfaces. The left-hand images (opalescent sample) present a number of sparse droplets ranging in size between 20 and 30 nm whilst the right-hand images suggest narrow veins surrounded by a homogeneous phase.

Figure 1 .

 1 Figure 1. Triaxial diagram showing the relation between the visual aspect and the samples composition in the quinary system SiO2-B2O3-K2O-Al2O3-CaO (composition expressed in Janecke's coordinates, SiO2 and K2O being fixed at 45w% and 15w% respectively).

Figure 2 .

 2 Figure 2. Characteristic amorphous XRD patterns of glasses without Al2O3 (a) and without CaO (b).

Figure 3 .

 3 Figure 3. DTA analysis showing the thermal response of PSBS samples with an up scan analysis. The glass transition (Tg) and crystallization phenomenon (Tc above the heat-treatment temperature) are indicated for each curves (relative uncertainty on temperature ur(T) = 0.3%).

Figure 4 .

 4 Figure 4. PSBS 3 section showing the development of the crystalline phase after 5h (a), 10h (b) and 20h (c) of 800°C heat-treatment.

Figure 5 .

 5 Figure 5. XRD patterns of PSBS 3 heat-treated at 800°C during 5, 10 and 20h.

Figure 6 .Figure 7 .

 67 Figure 6. Characteristic pH variation in HNO3 acid for PSBS before (dark blue line) and after (light blue line) the heat treatment (u(pH) = 0.02)

Figure. 8 .

 8 Figure. 8. Comparison of ion release in a transparent PSBS3 (left column graphs) and an opalescent PSBS5 glass sample (right column graphs). The comparison has been made within a heat-treated sample (solid line) and a melt-quenched sample (dotted line) in terms of: the leaching of different ions: boron (a,b), calcium (c,d), potassium (e,f) and silicon (g,h). The y-axis indicates the percentage of the available element in the glass that has dissolved into the acid solution (relative uncertainty on percentage of dissolved element, ur([element]%) = 3% for Boron, Calcium, Silicon and 5% for Potassium).

Figure 9 .

 9 Figure 9. Comparison of ion kinetic release in opalescent (dotted line) and transparent (solid line) glasses. The kinetic release of the ions is calculated as the percentage of the single element dissolved per minute and is related to the B2O3/CaO ratio, expressed here as the B2O3 and CaO amount in the glasses considering the other elements constant (relative uncertainty on percentage of dissolved element, ur([element]%) = 3% for Boron, Calcium, Silicon and 5% for Potassium).

Figure 10 .

 10 Figure 10. SEM micrographs showing the glass microstructures before (a,b) and after (c,d) the dissolution test in ambient condition (HNO3 1M aqueous solution, T = 37°C). PSBS16 is presented on the left while PSBS 8 is showed on the right.

Figure 11 .

 11 Figure 11. Characterization of retained structures for PSBS 16 after hard condition dissolution experiment (HNO3 1M aqueous solution, T = 90°C ; a: surface analysis; b: EDX spectrum).

Figure 12 .

 12 Figure 12. Characterization of the residual glass powder for PSBS 8 after hard condition dissolution experiment (HNO3 1M aqueous solution, T = 90°C ; a: XRD pattern; b: chemical XRF). The XRF analysis shows the powder after the dissolution (orange column) compared with the initial composition of the glass (blue columns).

Figure 13 .

 13 Figure 13. Proposed glass PSBS8 microstructure after the heat treatment: A nucleated structure with a chemically weak continue phase surrounding a droplet phase.

Table 1 .

 1 Composition of the experimental borosilicate glasses (composition expressed in w%, SiO2 and K2O being fixed at 45w% and 15w% respectively).

	Sample	B2O3	Al2O3	CaO
	name
	PSBS 1	22.5	7.2	10.4
	PSBS 2	26.2	3.4	10.4
	PSBS 3	25.0	0.0	15.0
	PSBS 4	20.0	11.5	8.5
	PSBS 5	20.0	5.0	15.0
	PSBS 6	26.2	3.4	10.4
	PSBS 7	25.9	8.1	6.0
	PSBS 8	34.0	0.0	6.0
	PSBS 9	30.3	0.0	9.7
	PSBS 10	25.9	8.1	6.0
	PSBS 11	34.4	5.6	0.0
	PSBS 12	30.0	10.0	0.0
	PSBS 13	22.5	11.5	6.0
	PSBS 14	22.5	2.5	15.0
	PSBS 15	29.1	4.8	6.1
	PSBS 16	21.4	7.1	11.5
	PSBS SBK 40.0	0.0	0.0

Table 2 .

 2 Elements detected with ICP measurements and respective emission lines.

	Sample	PSBS	PSBS 8	PSBS 9	PSBS 3 PSBS 11 PSBS 12
		SBK
	2θ angle	24.55	25.80	26.64	27.83	26.80	26.06
	(degree)
	d-spacing (Å)	3.69	3.41	3.29	3.19	3.53	3.45
	CaO (w%)	0.0	6.0	9.7	15.0	0.0	0.0
	Al2O3 (w%)	0.0	0.0	0.0	0.0	5.6	10.0

Table 3 .

 3 Evolution of the diffraction parameters in relation with the calcium content (w%) in differentAl-free PSBS glass samples.

	Sample	PSBS	PSBS 1 PSBS 3 PSBS 4
		SBK
	Al2O3 (w%)	0	7.2	0	11.5
	CaO (w%)	0	10.4	15	8.5
	HT Tg (⁰C)	438.9	598.1	610.7	589.6
	Tc (⁰C)	n/o	n/o	787.8	n/o

Table 4 .

 4 Glass transition temperature of the heat-treated glasses (HT) compared with the Al2O3 and CaO content in the sample (relative uncertainty on temperature ur(T) = 0.3%).

Acknowledgements

The research leading to this research article has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement n•608197.

Conflicts of interest

The authors declare no conflicts of interests.