N Dolmaire

F Méchin

E Espuche

J P Pascault

Modification of a Hydrophilic Linear Polyurethane by Crosslinking with a Polydimethylsiloxane. Influence of the Crosslink Density and of the Hydrophobic/Hydrophilic Balance on the Water Transport Properties

Keywords: reactive extrusion, thermomechanical properties, water sorption

A hydrophilic thermoplastic polyurethane (TPU) was modified by reactive extrusion to obtain in a first step a grafted and soluble material and to finally form by a hydrolysis condensation process a weakly crosslinked network. Different isocyanates were used as grafting agents and a α,ωdihydroxypoly(dimethylsiloxane) (PDMS) was used to modify the hydrophilic/hydrophobic balance of the material and the chain length between the crosslinks. The influence of the isocyanate functionality and of the PDMS content were studied on the network formation and on the thermomechanical and water sorption properties. The networks properties were also compared with those of a TPU/PDMS blend.

INTRODUCTION

Linear segmented thermoplastic polyurethanes (TPU) have been developed for many applications, such as biocompatible materials, membrane separation process, and breathable materials. The common way used to adapt the material to its use consists often in varying the nature of the soft and hard segments and the TPU composition [1,2]. Because of the hydrophilic character of polyoxyethylene (POE), TPU based on oxyethylene units as soft segments could represent promising candidates for breathable applications. Indeed, these applications require high water permeability coefficients and high water diffusion rates. A previous study [3] performed on a commercial POE-based TPU has shown that the water diffusion was fast in this material at low relative humidity. Nevertheless, it decreased at high relative humidity due to an important water uptake and a water aggregation phenomenon. A chemical modification consisting in crosslinking the material by hydrophobic and soft chains to decrease the swelling effect while keeping a high water diffusion rate was thus envisaged. Polydimethylsiloxane (PDMS) was chosen as modifier due to its hydrophobic character, its high mobility, and its high water diffusion rate. A two-step process was chosen for the modification to keep a thermoplastic, soluble material in a first step. Indeed, different routes allowing the incorporation of PDMS species in TPU have been reported in the literature [START_REF] Lake | Proceedings of the National Technical Conference of the Society of Plastic Engineers[END_REF][START_REF] Hill | [END_REF][6][7][8][9][10][11][12][13][14][15][16][17][18]. Some studies have focused on TPU/PDMS blends [START_REF] Lake | Proceedings of the National Technical Conference of the Society of Plastic Engineers[END_REF][START_REF] Hill | [END_REF][6][7][8][9]. In these systems, a poor miscibility is generally observed between the PDMS component and the TPU, leading to low interactions at the interface and as a consequence to a weakening of the mechanical properties for PDMS amounts higher than 2% by weight. Furthermore, a PDMS migration toward the surface has even been observed in some cases [9].

The other routes used to introduce PDMS in a polyurethane are based on chemical methods. They can be divided into two categories. The first one [10][11][12][13][14][15] essentially concerns the preparation of TPU copolymers with PDMS soft segments or the preparation of polyurethane grafted with PDMS. The second one [16][17][18] concerns polyurethane-PDMS networks and is based on the hydrosilylation reaction; therefore, it requires the use of two types of siloxane, one with a hydride function and the other with a vinyl function. These chemical methods allow to incorporate higher contents of PDMS compared to melt blending but they still present some disadvantages. Indeed, the water aggregation phenomenon cannot be highly limited in the first case and a network is immediately obtained in the second case.

More recently, a modification process allowing to blend any polymer bearing labile H atoms in TPUs in a first step and then to crosslink the modified material by hydrolysis/condensation in a second step has been proposed by Dassin [19] and Lambour [20] on hydrophobic TPUs. The aim of our study is to apply this method to a hydroxy-terminated PDMS and a hydrophilic TPU, and to study the consequences of these modifications on the final properties, particularly on the water transport properties.

EXPERIMENTAL Initial Materials and Reagents

The main characteristics of the different materials used in this study are presented in Figure 1. The reference linear polyurethane, TPU, was provided by Noveon (tradename: Estane® 58245). A previous study allowed to determine the chemical structure of this raw TPU material [3]. The soft segments were based on poly(oxyethylene) (POE), whereas the hard segments were composed of 4,4'diphenylmethane diisocyanate (MDI) extended with 1,4-butanediol (BDO). The TPU composition could be expressed as POE/MDI/BDO (1/3.34/2.34) and the average molar mass of the starting POE was 1500 g/mol. The urethane equivalent weight was equal to 356 g/mol.

Figure 1. Chemicals and polymers used in this study

O C N CH 2 N C O MDI N N N RNCO RNCO OCNR CH 2 CH 3 CH 3 CH 3 with R = O O O + higher oligomers tIPDI NH-C-O-CH 2 -CH 2 -O-C-NH CH 2 O m NH-C-O-CH 2 -CH 2 -CH 2 -CH 2 -O-C-NH CH 2 O O O x TPU CH 3 -O-Si-CH 2 -CH 2 -CH 2 -NH 2 O-CH 3 O-CH 3 γ-APS HO Si O Si OH CH 3 CH 3 CH 3 CH 3 n

PDMS

The reagents chosen for the TPU modification were used without further purification. MDI was purchased from Bayer, and isophorone diisocyanate trimer ( t IPDI) and 3-aminopropyl trimethoxysilane (γ-APS) from Sivento. The structure and the molar mass of the α,ω-dihydroxypoly(dimethylsiloxane) (PDMS) used in this study (Dow Corning, DC3-0133) was determined by 29 Si NMR. The chain ends of this polymer were composed exclusively of SiOH and the number average molar mass was determined as 29,000 g/mol.

Processing

The TPU modifications were processed by reactive extrusion using a ZSK 25 Werner twin screw extruder. The different components were introduced in the extruder according to the scheme presented in Figure 2. The processing parameters were processing temperature, 180 °C; L/D = 36; screw speed, 400 turns/min; and outcome, 15 kg/h.

Figure 2. Scheme of the twin screw extruder used to prepare the modified TPU

Characterization Methods

Size Exclusion Chromatography

Size exclusion chromatography (SEC) experiments were performed in THF (1 mL/min, 35 °C) using a Viscotek chromatograph equipped with a VE1121 pump, VE5111 injector, and VE3580 refractometor, and three styragel HR 5E columns (Waters) in series.

Thermomechanical Analysis

Thermomechanical experiments were performed from -150 to 250 °C at a heating rate of 2 °C/min on 1-mm thick samples with a RSAII rheometer (Rheometrics). The length and the width of the samples were equal to 50 and 5 mm, respectively. The applied tensile strain was fixed to 0.1% and the frequency was equal to 1 Hz.

Scanning Electron Microscopy

The morphological analysis were performed with a Philips XL20 scanning electron microscope. The samples were metallized before analysis.

Water Vapor Transport Properties

The water transport properties were determined by sorption analysis. The method used was detailed in a previous study [3]. The samples (40 mg) were introduced in a Setaram B92 microbalance and maintained at a constant temperature [(21 ± 1) °C]. The dry weight of the samples was determined after desorption under vacuum (2.10 -6 mbar). The water vapor uptake (Mt) was then followed as a function of time at different water activities (aw) and the equilibrium sorption (M) was determined. Two characteristics were deduced from these experimental data: the water diffusion coefficient (D) and the sorption isotherm. D was calculated according to the following equation [3]: = 0.04909 / with t1/2, the time at half sorption and e, the thickness of the film.

The sorption isotherm represents the water uptake per dry weight of polymer at the equilibrium (M) as a function of the activity (aw).

RESULTS AND DISCUSSION

Material Preparation

Modification Schemes

Different steps of TPU modification were considered to study the influence of the crosslink density and of the hydrophobic/hydrophilic balance on the water transport properties. Two isocyanates were used: a difunctional isocyanate, MDI, and a polyfunctional isocyanate, t IPDI. The latter is an amorphous polyisocyanate, with a Tg = 70 °C. In a recent publication [21], the so-called trimer of IPDI was shown to be formed by many oligomers, not only trimer but also pentamer, heptamer, or higher molar mass oligomers of IPDI, resulting from the polycondensation of isophorone diisocyanate. According to these authors, the number average functionality of t IPDI would be = 3.24 (NCO equivalent weight: 243 g/mol). The modification steps and three possible ways of crosslinking are presented in Figure 3 for MDI. The first step consisted in the grafting of MDI onto the TPU backbone chain by means of the urethane functions. An allophanate bond was formed (eq 1) and the residual and dangling isocyanate function could then react with the γ-APS amine function (eq 2).

Two possibilities could then be envisaged from these [TPU/MDI r1/γ-APS r2] systems where r1 = [NCO]0/[urethane]0 and r2 = [NCO]0/[NH2]0. (The values in brackets for r1 and r2 calculation refer to molar concentrations). The first one consisted in the crosslinking of the system by a hydrolysis/condensation process (eq 3). The second one consisted in adding PDMS in the grafted TPU to (i) modify the hydrophilic/hydrophobic balance of the polymer and (ii) increase the length of the chains between the crosslinks (eq 4). These systems were named [TPU/MDI r1/γ-APS r2/PDMS x], where x represents the weight amount of PDMS introduced per 100 g TPU.

N C O O C NH O CH 2 NCO + CH 3 -O-Si-CH 2 -CH 2 -CH 2 -NH 2 O-CH 3 O-CH 3 N C O O C NH O CH 2 NH (CH 2 ) 3 Si(OCH 3 ) 3 equation 2 C NH O 2 + 2 H 2 O N C O O NH (CH 2 ) 3 Si(OCH 3 ) 2 2 OH + 2 CH 3 OH N C O O NH (CH 2 ) 3 Si H 3 CO OCH 3 O Si OCH 3 H 3 CO (CH 2 ) 3 NH N C O O + H 2 O equation 3 N C O O C NH O CH 2 NH (CH 2 ) 3 Si(OCH 3 ) 3 C NH O (b)

Figure 3. Modification steps, crosslinking reactions, and possible side reactions

(continued from the previous page)

2 + HO Si O Si O Si OH CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 n O C N O N H (CH 2 ) 3 Si O Si O Si O Si O Si (CH 2 ) 3 N H N C O O OCH 3 OCH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 OCH 3 OCH 3 n equation 4 + 2 CH 3 OH N C O O C NH O H 2 C NH (CH 2 ) 3 Si(OCH 3 ) 3 C NH O N C O O C NH O CH 2 NCO + N H C O O CH 2 NCO + equation 5 (CH 3 -O) 3 -Si-CH 2 -CH 2 -CH 2 -NH 2 (CH 3 -O) 3 -Si-CH 2 -CH 2 -CH 2 -NH-C-NH O N C O O C NH O CH 2 NCO + H 2 O 2 + + CO 2 N C O O C NH O CH 2 NCO N C O O C NH O CH 2 NH 2 O C N O C H N CH 2 O H N C H N O CH 2 H N C N C O O O equation 6
Finally, a direct crosslinking of the MDI-grafted TPU can be observed if this material is exposed to moisture via the hydrolysis of the side isocyanate group, leading to an amine and its subsequent reaction with another pendent NCO function (eq 6). The modification by γ-APS is thus also a way to avoid such premature gelation.

The same modification schemes could be proposed with t IPDI. The main difference consisted in that case in the higher functionality of this isocyanate.

Preparation of the Grafted TPU

As previously mentioned, the TPU modifications were processed by reactive extrusion. For MDI based systems, the r1 and r2 values were fixed to, respectively, 0.11 and 2. Indeed, a r1 value equal to 0.11 led ideally to a graft with one reactive isocyanate function per 17 urethane functions, thus to a final weakly crosslinked network. Concerning r2, it was shown by Dassin [19] on another TPU that for this r1 value, a value of r2 < 2 (NH2 excess) led to the scission of urethane functions, whereas a higher value of r2 led to a lack of amine functions compared with isocyanate functions and as a consequence to the formation of insoluble fractions. As for PDMS, x was varied in the range 0-8 g/100 g TPU.

It was at first verified that, for the chosen extrusion conditions, MDI was indeed grafted onto the TPU. TPU and MDI were introduced at the same time in the twin screw hopper and the resulting product was cooled rapidly under a cold press. It was then dissolved in THF with an excess of di-n-butylamine to endcap any residual NCO group and to avoid further reaction. The SEC chromatograms relative to [TPU/MDI 0.11] was compared with the chromatogram relative to the TPU that was extruded alone in the same conditions (e.g., with the same screw profile and the same temperature profile). Indeed, according to the literature, two types of thermal degradation could occur for polyurethanes based on POE as soft segments. The first one concerns the scission of the urethane bonds and this phenomenon was shown to be reversible for T < 200 °C and under a slow cooling from the melt state [25][26][START_REF] Eisenbach | Advances in Elastomers and Rubber Elasticity[END_REF][START_REF] Yang | [END_REF][29][30][31].

The second mechanism is the oxidation of the POE segments [32][33][34]. It was thus very important to compare systems obtained with the same thermomechanical history, even if we verified by studying the neat TPU and a TPU/phenolic antioxidant system that the POE oxidation did not take place under our processing conditions.

Compared to TPU, the chromatogram relative to [TPU/MDI 0.11] was shifted to lower retention volumes and its width was more important (Fig. 4). Furthermore, no peak relative to di-nbutylamine/MDI/di-n-butylamine (elution volume: 30.9 mL) was observed. The peak noticed at an elution volume equal to about 32 mL was related to an excess of di-n-butylamine. It could then be concluded that all the MDI molecules that had been introduced in the system were grafted onto the TPU backbone chains via allophanate branching points. This result was recently confirmed by the study of the reaction of a model diurethane with excess MDI, for which the formation of allophanate as the only product was evidenced. No side product such as isocyanurates were detected in the bulk reactive system [35]. From an experimental point of view, the extrusion of all the systems based on t IPDI as isocyanate required the introduction of 1 g PDMS per 100 g TPU in the hopper as lubricant. Furthermore, to avoid crosslinking reactions in the twin screw extruder, two extrusion steps were performed for the [TPU/ t IPDI r1/γ-APS r2/PDMS x] systems. TPU, t IPDI, γ-APS, and 1 g PDMS per 100 g TPU were introduced in the twin screw extruder for the first extrusion step. The obtained material was then extruded with the remainder of PDMS in a second step. The x value reported in the formulation name represents the total amount of PDMS per 100 g TPU, thus it includes the PDMS added in the formulation as lubricant.

With these controlled extrusion conditions, it was possible to obtain modified but still non-crosslinked TPU. The modified TPU pellets were kept in hermetically sealed bags to avoid crosslinking reactions before the films process.

Finally a [TPU/PDMS 2.4] blend was also prepared to study a totally ungrafted and uncrosslinked material as reference.

Film Process and Crosslinking Step

Two types of samples were prepared. For thermomechanical analysis, plates with a thickness equal to 1 mm were obtained by compression molding of the modified TPU pellets at 165 °C for 5 min (1 min at 1 tonne and 4 min at 4 tonnes). The plates were then cooled under a cold press.

Water transport analysis was performed on thinner films (thickness e = 40 µm). The method used to process these films was already described [3]. It consisted in the coextrusion and blowing of a threelayer film (PE/PE/TPU or modified TPU). The TPU or modified TPU film was then removed from polyethylene PE layers. All the samples were kept in hermetically sealed bags before the crosslinking step. This last step consisted of a hydrolysis/condensation mechanism (eqs 3 and 4). For this purpose, samples were immersed in distilled water at various temperatures. The crosslinking kinetics was followed by the evolution of the insoluble fraction as a function of time for the different systems and for different conditions. Two methods were used to determine this parameter.

The first one consisted in a gravimetric method. The initial weight of the sample (m1) was determined.

The sample was then immersed in THF during a week (this time was shown to be sufficient for the total dissolution of the soluble fraction of the samples). The weight of the gel (m2) was measured after filtration and extraction of THF under vacuum. The insoluble fraction was determined by the ratio of the weight of the gel (m2) to the weight of the initial sample (m1).

The second method consisted in the analysis of the soluble part of the sample that corresponds to the uncrosslinked polymer by SEC. Using always the same mass of sample suspended in the same volume of solvent, the area of the peak was proportional to the amount of dissolved polymer, and the insoluble fraction could then be deduced.

Figure 5. Evolution of the insoluble fraction as a function of immersion time in water at 70 °C for [TPU/MDI 0.11/γ-APS 2] ( ) by gravimetric method for a sample thickness equal to 1 mm, ( ) by SEC for a sample thickness equal to 1 mm, ( ) by gravimetric method for a sample thickness equal to 40 µm

The results obtained with these two methods were compared for [TPU/MDI 0.11/γ-APS 2] plates immersed in distilled water at 70 °C. Figure 5 shows that both methods gave the same results but that a higher dispersion was observed using SEC. As a consequence, the gravimetric method was chosen for further studies.

The influence of the hydrolysis/condensation temperature and of the film thickness were studied on [TPU/MDI 0.11/γ-APS 2] systems. The kinetics became faster as the hydrolysis/condensation temperature increased (Fig. 6). The maximum insoluble fraction was obtained after 50 h at 70 °C, whereas 150 h were required at 40 °C to achieve a constant value of insoluble fraction. More generally, the kinetics relative to our system was fast compared with the kinetics reported by Dassin [19] and this could be explained by the hydrophilic character of our initial TPU. Indeed, the only difference between Dassin's formulation and our system was the nature of the TPU. This highly hydrophilic character could also explain that the initial insoluble fraction of films was quite important in our systems (comprised between 10 and 25%) despite the precautions used to store the films, and that an increase of the sample thickness from 40 µm to 1 mm did not lead to significant variations of the kinetics (Fig. 5). At last, we could observe that whatever the hydrolysis/condensation conditions, the [TPU/MDI 0.11/ γ-APS 2] system was never totally crosslinked, whereas a totally crosslinked [TPU/MDI 0.11] could be obtained (crosslinking mechanism in this case: eq 6). The same trend was observed by Dassin [19] and it could be partially explained by the scission of some allophanate bonds by γ-APS species as discussed in eq 5. As the maximum insoluble fraction was a little higher when the hydrolysis/condensation was performed at 40 °C (80%), this temperature was chosen for the hydrolysis/condensation of all the systems.

Figure 6. Influence of the hydrolysis/condensation temperature on the evolution of the insoluble fraction as a function of time for [TPU/MDI 0.11/γ-APS 2]: ( ) T = 70 °C, ( ) T = 40 °C

The addition of PDMS in TPU/MDI/γ-APS systems did not lead to significant modifications neither of the kinetics nor of the maximal insoluble fraction (Fig. 7). The same conclusion could be reached for the systems based on t IPDI (Fig. 8). Nevertheless, the insoluble fraction relative to these systems was very low (comprised between 30 and 40%). This last result could be explained in part by the lower r1 value used for TPU/ t IPDI/γ-APS and TPU/ t IPDI/γ-APS/PDMS. Indeed, it was shown by Dassin [19] that a decrease in r1 from 1.1 to 0.6 in TPU/MDI/γ-APS systems led to a decrease in the final insoluble fraction from 80% to 60%. The final goal of the modifications we performed on TPU was to crosslink this material by PDMS chains.

To evaluate the content of PDMS linked to the networks, the 40-µm thick films were placed during 330 h in distilled water at 40 °C to achieve the maximum insoluble fraction. An extraction by Soxhlet in heptane was then performed. Heptane was chosen since it is a solvent of PDMS, whereas it cannot dissolve the TPU. After the extraction step, the films were analyzed by elementary microanalysis. The [TPU/PDMS 2.4] blend was also studied for comparison. The results are presented in Table 1. a The precision on the Si amount is ± 0.2%. b The precision on the PDMS amount is ± 0.5%.

All the PDMS could be extracted from the [TPU/PDMS 2.4] blend and the [TPU/ t IPDI 0.06/PDMS 1.4] system, whereas no great modification of the Si content was observed before and after the extraction process for the networks containing the alkoxysilane and the PDMS.

These observations first validated the method used to quantify the amount of linked PDMS and then showed that the silanol sites brought by γ-APS were necessary to link the PDMS species to the network. Thus in all networks containing γ-APS and PDMS, all the PDMS species introduced were linked to the network.

Morphologies

The morphology of the uncrosslinked and crosslinked samples were analyzed by SEM [Fig. 9(a-d)]. PDMS rich domains were clearly observed on the uncrosslinked samples [Fig. 9(a,c)]. The mean size of these domains only depended on the initial amount of PDMS but not on the method used to process the films, contrary to their shape. Indeed, spherical rich PDMS domains were observed on the plates, whereas oriented domains were observed on the films processed by coextrusion and blowing. After crosslinking, whatever be the samples, the PDMS-rich domains could no longer be distinguished in the micrographs, showing that the interfacial adhesion was greatly improved by the crosslinking reaction [Fig. 9(b,d)].

Thermomechanical Properties

The evolution of E' was reported in Figure 10 for the TPU and the crosslinked [TPU/MDI 0.11/γ-APS 2] system. No differences were observed between the two curves in the range -150 to 110 °C. The position of the main relaxation did not vary going from neat TPU to the crosslinked [TPU/MDI 0.11/ γ-APS 2], showing that in a rough approach, the soft segment mobility and the phase separation between the soft and the hard segments were not affected by the TPU modification.

A decrease in the modulus was observed for both systems at around 110 °C. This phenomenon was important for the neat TPU, corresponding to the mixing of the hard and soft phases and to the melting of the hard domains followed by the TPU flow. For the crosslinked [TPU/MDI 0.11/γ-APS 2] film, only a small decrease in E' was observed in this temperature range, once again corresponding to the melting of the hard segments. Indeed, for this last system, the chemical crosslink points prevented the TPU flow. The introduction of PDMS in the crosslinked systems did not lead to significant E' variations and the main relaxation of PDMS at around -120 °C was not observed (Fig. 11). In fact, supposing that the network structure was nearly the ideal one with no intramolecular SiOH reactions, we calculated the ratio of SiOH brought by the PDMS to the Si(OR)3 brought by the alkoxysilane for the highest value of x used, for example, x = 7.5; in this case, 1 silanol was theoretically brought per 26 Si(OR)3. The crosslink density was thus only very slightly affected by the introduction of PDMS, explaining the invariance of the viscoelastic properties. Compared to MDI based network, the films based on tIPDI as grafting agent led to a lower thermal stability (Fig. 11). This could be related to the lower insoluble fraction of these networks (35 wt % instead of 80 wt %).

Water Transport Properties

As the water molecules participate in the hydrolysis/condensation phenomenon, the water transport properties could only be studied on unreactive systems, such as TPU/PDMS blends or on systems whose crosslink density was already optimum. The water transport mechanism of the neat TPU film was detailed in a previous study [3]. It was shown that the TPU sorption isotherm was of BET type III: a Henry mechanism governed the transport properties in the activity range from 0 to 0.6 as underlined by a constant diffusion coefficient in this activity range and a linear increase of the water uptake as a function of the activity (Fig. 12) and a clustering phenomenon was at the origin of the high increase in the water uptake and of the decrease in the apparent water diffusion coefficient in the range of activity from 0.6 to 1. The water sorption capacity was shown to be directly proportional to the oxyethylene content in the material. Compared to neat TPU, the crosslinked [TPU/MDI 0.11/γ-APS 2] exhibited a lower water diffusion rate (Fig. 12) and a lower water solubility at high activity (in the range 0.6-1) (Fig. 13). Nevertheless, the sorption mechanisms remained unchanged, with a predominant Henry sorption at low activity and a clustering phenomenon at high activity. PDMS was introduced in the grafted TPU to form hydrophobic segments with a high mobility between the crosslinks. To study the influence of crosslinking in such systems, we compared the water transport properties of a [TPU/PDMS 2.4] blend and of the crosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 2.1] system. Indeed and as previously mentioned, it was not possible to study the uncrosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 2.1] system due to an evolution of the structure and properties in wet conditions. The results are presented in Figures 121314. Introducing this little amount of PDMS by melt blending with TPU led to an important decrease in the water uptake at high activity (about 30%) (Fig. 14). The SEM analysis performed on uncrosslinked systems containing PDMS species clearly evidenced a dispersed PDMS rich phase. It was thus possible to consider these systems as composed of a hydrophilic continuous phase (e.g., the TPU phase) and a dispersed phase with a very low water sorption capacity (i.e. the PDMS phase). A theoretical water uptake could then be calculated at each activity, taking only into account the TPU phase. This theoretical water uptake was much higher than the experimental one and this difference could only be explained by the migration of a part of the PDMS to the sample surface, leading to the formation of a hydrophobic layer at the film surface. Concerning the water diffusion coefficient, no great modification of this transport parameter was noticed in the presence of PDMS (Fig. 12). This result was not surprising, since PDMS is known to have a high water diffusion coefficient.

For the crosslinked TPU/MDI/γ-APS/PDMS films, it was previously shown that the PDMS rich phase could no more be distinguished in the micrographs and that all PDMS species were linked to the network. Furthermore, only one Si(OR)3 function per 26 should theoretically react with PDMS for the formulation containing the highest content of PDMS (7.5 g PDMS/100 g TPU). Thus, the introduction of PDMS did not drastically modify the network structure compared with [TPU/MDI 0.11/γ-APS 2] and as a consequence the water transport properties. Indeed, no significant change of the water uptake was noticed between [TPU/MDI 0.11/γ-APS 2] and [TPU/MDI 0.11/γ-APS 2/PDMS x] (Fig. 13), and the introduction of PDMS only slightly increased the kinetics (Fig. 12). The water sorption properties of the networks based on MDI and t IPDI were compared (Fig. 15). The water uptake of [TPU/ t IPDI 0.06/γ-APS 1.4/PDMS 2.1] was higher than the water uptake of [TPU/MDI 0.11/γ-APS 2/PDMS 2.1], whereas the water diffusion coefficients of both systems were very similar (Fig. 12). The first trend could be explained by the low insoluble fraction of the [TPU/ t IPDI 0.06/γ-APS 1.4/PDMS 2.1] compared with [TPU/MDI 0.11/γ-APS 2/PDMS 2.1]. Nevertheless, it was then difficult to understand why no modification of the water diffusion was observed between these two systems. The global insoluble fraction (about 35 wt %) was not an enough representative of the [TPU/ t IPDI 0.06/γ-APS 1.4/PDMS 2.1] network to be considered as the governing factor for the water transport properties. The architecture of the systems had to be taken into account. Indeed, t IPDI has a higher functionality than MDI but the insoluble fraction of the crosslinked t IPDI-based systems was very low. We could thus presume that a small part of the polymer was highly crosslinked, leading to a low permeable phase, whereas an important part of the material remained soluble, composed of linear or grafted chains with a high mobility and a high swelling capacity. The final properties of this system resulted from the contribution of these two phases and thus became very complex.

In conclusion, for all the studied crosslinked systems, the POE content that governed the water sorption capacity of the initial TPU no longer played the major role. Indeed, the water sorption isotherms relative to the different networks represented as the water uptake per oxyethylene unit did not define a single curve (Fig. 16). It seemed, thus, that the insoluble fraction and the network architecture became the major parameters governing the water sorption capacity and diffusion in these networks.

CONCLUSIONS

This study has shown that it was possible to modify by reactive extrusion a hydrophilic TPU to obtain in a first step a grafted soluble and processable material and to finally form a weakly crosslinked network. The reaction schemes consisted in the grafting of a di-or polyisocyanate (respectively, MDI and t IPDI) onto the urethane bonds by an allophanate link. The dangling isocyanate functions could then react with an aminoalkoxysilane. Two possibilities were then envisaged: the formation of the network by the direct hydrolysis/condensation of the Si(OR)3 brought by the γ-APS and the addition of hydroxy-telechelic PDMS species, followed by hydrolysis/condensation, to modify the hydrophilic/hydrophobic balance and to introduce chains with high mobility between the crosslinks.

It was shown that the introduction of γ-APS as a third reactant in the TPU/isocyanate systems led to the scission of some allophanate bonds and to non-totally crosslinked networks. The introduction of a fourth component, PDMS, did not modify neither the hydrolysis/condensation kinetics nor the maximum of insoluble fraction. After the hydrolysis/condensation step, PDMS was linked to the network and the PDMS dispersed phase could no longer be distinguished by SEM, confirming the improved interfacial adhesion. The insoluble fraction of the networks based on MDI was higher than the insoluble fraction of the networks based on t IPDI (80% and 35%, respectively). This result could be explained in part by the lower urethane to isocyanate ratio used for the systems based on t IPDI.

Concerning the thermomechanical properties of the different networks, it was shown that the thermal stability increased with the increase in the global insoluble fraction: crosslinking prevented the TPU flow. Contrary to neat TPU, the POE content no longer governed the water sorption properties of the networks. The crosslink density and the architecture of the networks became the major factors. Indeed, a decrease in the sorption capacity was observed between the neat TPU and the TPU/MDI/γ-APS network due to crosslinking. On the other hand, no significant modification of the water uptake was observed between the TPU/MDI/γ-APS network and the TPU/MDI/γ-APS/PDMS network. Furthermore, even if a small increase of the diffusion rate was noticed between TPU/MDI/γ-APS and TPU/MDI/γ-APS/PDMS networks, this parameter remained low compared with TPU/PDMS blends. These results could be explained by the low content of PDMS introduced in the network and by the fact that the PDMS species were linked to the network. Indeed, the covalent bonds created between γ-APS and PDMS during the hydrolysis/condensation step prevented the PDMS migration to the surface and the formation of an impermeable layer. Furthermore, it improved the interactions between the hydrophilic and the hydrophobic part of the systems. As a consequence, the effects of the hydrophobic chains on the water transport properties were attenuated compared with TPU/PDMS blends.

The influence of the network architecture was underlined for the networks based on t IPDI as isocyanate. These networks presented the same water diffusion coefficient as the networks based on MDI despite a very low insoluble fraction (35%). Because of the high functionality of t IPDI, we presumed that this insoluble phase was highly crosslinked. The transport parameters resulted then from the contribution of this small but highly crosslinked part of the material and from the contribution of a still soluble phase that represented 65% of the total material. Thus, the global insoluble fraction could not totally explain the transport properties of networks with different architectures.

Figure 3 .

 3 Figure 3. Modification steps, crosslinking reactions, and possible side reactions

Figure 4 .

 4 Figure 4. Chromatograms relative to TPU and [TPU/MDI 0.11] obtained with the same thermomechanical history

Figure 7 .Figure 8 .

 78 Figure 7. Evolution of the insoluble fraction as a function of time at 40 °C for ( ) [TPU/MDI 0.11/γ-APS 2] and ( ) [TPU/MDI 0.11/γ-APS 2/PDMS 7.5]

Figure 9 .

 9 Figure 9. SEM micrographs of (a) uncrosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 7.5] plate; (b) crosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 7.5] plate; (c) uncrosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 7.5] film; and (d) crosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 7.5] film

Figure 10 .

 10 Figure 10. Evolution of E' as a function of time for (a) TPU, (b) [TPU/MDI 0.11/γ-APS 2] crosslinked at 15%, and (c) [TPU/MDI 0.11/γ-APS 2] crosslinked at 80%.

Figure 11 .

 11 Figure 11. Evolution of E' as a function of time for (a) TPU, (b) crosslinked [TPU/MDI 0.11/γ-APS 2/PDMS 7.5] (insoluble fraction: 80 wt %), and (c) crosslinked [TPU/ t IPDI 0.06/γ-APS 1.1/PDMS 7.2] (insoluble fraction: 35 wt %)

Figure 12 .

 12 Figure 12. Evolution of the water diffusion coefficient as a function of the activity for ( ) TPU, ( ) [TPU/PDMS 2.4], ( ) [TPU/MDI 0.11/γ-APS 2], (-) [TPU/MDI 0.11/γ-APS 2/PDMS 2.1], and ( ) [TPU/ t IPDI 0.06/γ-APS 2/PDMS 2.1]

Figure 13 .

 13 Figure 13. Water sorption isotherm relative to ( ) TPU, ( ) [TPU/MDI 0.11/γ-APS 2], and (-) [TPU/MDI 0.11/γ-APS 2/PDMS 2.1]

Figure 14 .Figure 15 .

 1415 Figure 14. Water sorption isotherm relative to ( ) TPU and ( ) [TPU/PDMS 2.4]

Figure 16 .

 16 Figure 16. Water uptake per oxyethylene unit for ( ) TPU, ( ) TPU/MDI-based systems, and ( ),TPU/ t IPDI based systems

Table 1 .

 1 Amount of PDMS Expressed in Weight Percent Contained in the Systems Before and After Extraction

	Samples	% Si before extraction a	% PDMS before extraction b	% Si after extraction a	% PDMS after extraction b
	TPU/PDMS 2.4	0.91	2.4	0.11	0.3
	TPU/MDI 0.11/γ-APS 2/PDMS 2	1.18	2.1	1.65	3.7
	TPU/MDI 0.11/γ-APS 2/PDMS 7.5	3.22	7.5	3.30	7.7
	TPU/ t IPDI 0.11/PDMS 1.4	0.53	1.4	0.05	0.2
	TPU/ t IPDI 0.11/γ-APS 2/PDMS 2.1	1.00	2.1	0.92	1.9
	TPU/ t IPDI 0.11/γ-APS 2/PDMS 7.1	3.11	7.2	3.42	8.1

Authors wish to express their thanks to Fernande Boisson (FR CNRS # 2151, Vernaison, France) for the NMR analysis, and to Mr Didier Lagneaux (Noveon, Frontonas, France) for his help with reactive extrusion and fruitful advice and discussions. Financial support from Région Rhône-Alpes is also gratefully acknowledged (join project: Nouveau matériau polymère pour membrane imper respirante. Partners: IMP, Mrs Méchin, Mrs Espuche, Mr Pascault; Noveon Inc: Mr Lagneaux; Alpex Protection, Mr Cros; IFTH, Mr Couvret).