V Gaucher

J F Tahon

S Van Berkel

A Arias

C Rogeret

R Nourichard

S De

G Stoclet

S Xu

S De Vos

Influence of the molecular weight on mechanical behavior and associated strain-induced structural evolution of Poly(ethylene 2,5-furandicarboxylate) upon biaxial stretching

Keywords: biaxial stretching, Poly(ethylene 2, 5-furandicarboxylate), macromolecular orientation, crystallization, barrier properties

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

Introduction:

Poly(ethylene 2,5-furandicarboxylate) (PEF) knows an increasing interest due to, on the one hand its bio-based nature and, on the other hand, its interesting properties that could allow it to be used in a large field of applications such as packaging 1 . Due to their similar chemical nature, PEF is often compared to poly(ethylene terephthalate) (PET). Several studies have been carried out regarding the characterization of the PEF physical properties. These works have shown that PEF exhibits similar thermal properties as PET, i.e., a glass transition temperature (Tg) around 80°C, a melting temperature (Tm) in the 220-250°C temperature range and slow thermal crystallization kinetics 2,3,4,5,6,7 . The same can be said about the mechanical properties 8 , which are somewhat equivalent between the two polymers. However, as a main feature, PEF exhibits significantly better water and gas barrier properties in comparison with PET, which is explained by the reduced chain mobility of PEF as compared to PET. 9,10,11 Several studies have been published regarding the mechanical behavior of PEF upon uniaxial stretching in the rubbery state 12,13,14,15,16,1718 . They underlined the fact that the microstructure induced upon stretching is strongly dependent on the stretching conditions and that the strainhardening of the material, observed for some stretching conditions, is linked to the formation of an ordered structure (crystalline or mesomorphic) upon stretching 11,12 . These studies also showed that the content of ordered structure induced by uniaxial stretching could reach 35% 16 and that the crystalline structure involved is different from the one induced by thermal crystallization 12 . Regarding the relationships between mechanical behavior and structural evolution, Mao et al. have shown that local ordering of the chains occurs before the occurrence of strain-induced crystallization (SIC). They also pointed out that the onset of strain-hardening occurs in the meantime that strain-induced crystallization 13 suggesting a link between these two phenomena. On the other hand, in a recent study, Forestier et al. clearly demonstrate that PEF crystals are formed before the occurrence of structural hardening and that the PEF strain-induced crystallization process exhibits a "simple" twosteps path with no intermediate phases stable with no formation of intermediate order phase 16 .

Beyond uniaxial stretching, biaxial stretching is also of prime interest as it allows to simulate industrial processes such as bottle stretch-blow molding, biaxial film drawing or thermoforming. Regarding biaxial stretching of PEF, only an explorative study from van Berkel et al. has, to date, addressed this topic. In their work, van Berkel and his coworkers compared the mechanical behavior and the structural evolution in the rubbery state of PEF with that of PET [START_REF] Van Berkel | Biaxial Orientation of Poly(ethylene 2,5-furandicarboxylate)[END_REF] . They observed that strainhardening and strain-induced crystallization appear at higher stretching ratios for PEF than for PET. This retarded occurrence of strain-hardening in the case of PEF is ascribed to its higher mass between entanglements, as compared to PET. Besides they assume that in order to achieve the same amount of entanglement per chain, and consequently a comparable network of entanglements, a higher molecular weight for PEF is needed to meet the behavior of PET. In addition they also observe that, in like for uniaxial stretching 12 , the content of the crystalline phase induced upon stretching in the case of PEF is low (around 10% maximum). Finally, they observed that biaxial orientation has a beneficial impact on both mechanical and gas barrier properties of PEF, although the impact on barrier properties is significantly lower than in the case of PET. In particular they noted an increase of tensile modulus and tensile strength for the biaxially oriented films as well as some decrease of both O2 and CO2 permeability, even if limited.

Basically, this work of van Berkel et al. is the first one giving information about i) the mechanical behavior of PEF upon biaxial stretching and ii) the influence of biaxial orientation on the structure of PEF. However, in their study, only PEF samples with high biaxial orientation degrees, comparable to the ones encountered in industrial conditions, have been characterized. Consequently the whole process of strain-induced structural evolution during biaxial stretching remains unaddressed. In particular the origin of the strain-hardening upon biaxial stretching in the case of PEF remains unclear. Several studies dealing with the link between strain-hardening and SIC have been carried out in the case of other polymers [START_REF] Tassin | Biaxial stretching of pet films: A molecular description[END_REF][START_REF] Adams | Biaxial hot drawing of poly(ethylene terephthalate): Measurements and modelling of strain-stiffening[END_REF][START_REF] Roland | Thermal marking of amorphous poly(ethylene terephthalate)[END_REF][START_REF] Ray | Effect of strain rate and temperature on stress-strain properties of EVA-LDPE blends and the mechanism of strain hardening Die[END_REF][START_REF] Adams | Biaxial hot drawing of poly(ethylene terephthalate): Measurements and modelling of strain-stiffening[END_REF][START_REF] Roland | Thermal marking of amorphous poly(ethylene terephthalate)[END_REF][START_REF] Ray | Effect of strain rate and temperature on stress-strain properties of EVA-LDPE blends and the mechanism of strain hardening Die[END_REF][START_REF] Chandran | Biaxial orientation of poly (ethylene terephthalate). part II: The strain-hardening parameter[END_REF][START_REF] Ouchiar | Effect of biaxial stretching on thermomechanical properties of polylactide based nanocomposites[END_REF] . For example, Chandran et al. carefully analyzed the influence of the stretching conditions on both strain-hardening and strain-induced crystallization [START_REF] Chandran | Biaxial orientation of poly (ethylene terephthalate). part II: The strain-hardening parameter[END_REF] of PET and directly linked both phenomena. Similar studies carried out with PLA, an aliphatic crystallizable bio-based polyester, lead to the same conclusions [START_REF] Ouchiar | Effect of biaxial stretching on thermomechanical properties of polylactide based nanocomposites[END_REF][START_REF] Delpouve | Water barrier properties in biaxially drawn poly(lactic acid) films[END_REF] . In other words these studies indicate that strain-hardening originates from SIC. Nevertheless, on the other hand, studies carried out on atactic polystyrene (i.e. which is not able to crystallize) demonstrate that strain-hardening results from the extension of the entanglement network [START_REF] Chau | Cross deformation and stress relaxation of biaxially oriented polystyrene films[END_REF][START_REF] Zhang | Biaxial orientation behavior of polystyrene: Orientation and properties[END_REF] . Consequently, even if PEF seems to have a behavior close to the one of PET, determining the origin of PEF's strain-hardening, and the respective contribution of amorphous orientation and SIC on the mechanical behavior, could be of prime interest. Another unaddressed point regarding the PEF mechanical behavior and associated structural evolution is the effect of molecular weight. As suggested in [16], the later occurrence of strainhardening in the case of PEF as compared to PET is due to its higher Me. As suggested by van Berkel et al., this should be balanced by the achievement of a comparable network of entanglements (i.e. the achievement of the same amount of entanglements per chain) by increasing the molecular weight as already shown in the case of PET [START_REF] Bonnebat | Biaxially oriented poly(ethylene terephthalate) bottles: Effects of resin molecular weight on parison stretching behavior[END_REF] .

As a consequence, this study is devoted to determine the structure -mechanical behavior relationships of PEF upon biaxial stretching above Tg in order to determine the origin of the strainhardening. In addition, the influence of molecular weight on mechanical behavior of PEF and on the associated strain-induced structural evolution will be studied in order to determine if this parameter allows to tune the mechanical behavior of PEF towards the one of PET.

Experimental:

Materials and elaboration

Three different neat Poly(ethylene 2,5-furandicarboxylate) polymer samples with different molecular weights, supplied by Corbion, have been used. The polyesters were made by classical esterification-polycondensation in the presence of a Sb2O3 catalyst and SSP was applied when needed to increase Mw. The average number of entanglements per chain has been calculated as Mn/Me with Me the average molecular weight between entanglements, taken equal to 3500 g/mol from [15].

For sake of clarity the PEF exhibiting the lowest molecular weight will be denoted LMW-PEF, the one having the intermediate molecular weight will be denoted MMW-PEF and finally the material having the highest molecular weight will be denoted HMW-PEF.

Resins were dried before extrusion below a moisture content of 50 ppm, as determined Karl Fischer. Extruded cast films of the three resins have been elaborated using as single screw extruder. The melt temperature was maintained at 255 °C for the LMW and MMW samples, for the HMW sample a melt temperature of 280 °C was applied. The obtained cast was cooled using a 30 °C cooling drum. The average obtained sample width was 14 cm, and the thickness was around 400 microns.

Biaxial stretching

A Brückner Karo IV laboratory biaxial stretcher was used for the biaxial drawing of the specimens. Experiments were carried out on 10x10 cm² squares cut from the extruded films (thickness from 300 to 500µm). Simultaneous biaxial stretching was carried out at drawing temperatures (Td) of 100 °C using an initial stretching speed ( = ) of 200%.s -1 . Viscoelastic properties measurements have been carried out in order to ensure that, for these stretching conditions, the material is in its rubbery state (see Fig SI .1) Finally elongation ratios λMD x λTD up to 4.5x4.5 were tested. The mechanical behavior is represented by:

-The elongation ratio : λxx = with the initial length of the sample -The true stress calculated as: = with Fxx the applied force in the xx direction and S the cross-section calculated assuming the hypothesis of constant volume deformation.

Prior to biaxial stretching the samples were heated during 30 s at Td in order to reach thermal equilibrium. The three principal axes of the films are defined as the machine direction (MD) which correspond to the extrusion direction, the transverse direction (TD) which corresponds to the direction perpendicular to the extrusion direction in the film plane and the normal direction (ND) which is the direction perpendicular to the film place. Regarding the mechanical behavior during biaxial stretching, for sake of clarity, only the stress recorded along the machine direction (MD) will be reported in the mechanical behavior plots. However, it was ensured for each test that the same behavior was recorded along the machine and the transversal direction (MD and TD). Finally, in order to ensure that stretching is homogeneous, an ink grid composed of 1x1cm² squares has been printed onto the film and analyzed after the film stretching.

Thermal properties and Crystallization kinetics analyses

Thermal properties of the materials were determined by means of Differential Scanning Calorimetry (DSC) analyses. A DSC Q20 (TA Instruments) apparatus was used. Temperature and heat flow were calibrated with a high purity indium sample using standard procedures. Experiments were carried out under nitrogen flow on about 10mg samples placed in aluminum crucibles. Isothermal crystallization kinetics from the melt at 165°C of the PEF films were determined using the following thermal cycle:

-Holding the sample at 280°C during 2min.

-Cooling down to 165°C at 50K/min.

-Isothermal during 120min.

-Cooling down to 25°C at 50K/min.

-Isothermal during 1min at 25°C -Heating up to 280°C at 10K/min Viscoelastic properties measurement Dynamic Mechanical Analyses were carried out on a RSA III apparatus from TA Instruments operated in tensile mode. Experiments were performed at the frequency of 1Hz, between 25°C to 180°C at a heating rate of 5°C.min -1 . Rectangular shaped samples with length and width of 20 x 6 mm² respectively were cut from PEF films.

Macromolecular orientation: Trichroic Infrared Analyses

The Fourier Transform Infrared (FTIR) spectra were collected in transmission mode on a Bruker VERTEX 70v spectrometer equipped with a DTGS detector and a rotating polarizer. Every FTIR spectrum consisting of 64 scans was recorded over the wavenumber range 550-3500 cm -1 with a resolution of 2 cm -1 . In our study trichroic infrared measurements were adopted to assess the orientation functions for biaxially stretched samples with a 3D texturing nature. For spectra along stretch axis, and were measured with the polarization parallel to the M and T directions respectively where reduced absorbance were recorded.

In order to obtain the spectrum SN through the film thickness, trichroic analysis was carried out by tilting the films about the T axis at 45° with respect to the IR beam, and the spectrum SMN was measured with the polarization in the MN plane. Then, the SN spectrum corresponding to a polarization through the thickness was computed from the following relation

= 2 ! "#1 % 1 2 ! & % 2 ! % 1" &
Where n is the effective refractive index of the polymer in the direction of the beam propagation.

Due to the lack of data regarding the refractive index of PEF, the value of PET (i.e. n = 1.58) has been used [START_REF] Wypych | Handbook of Polymers -2nd Edition[END_REF] .

Then, the S0 spectrum for biaxially drawn samples was calculated from the equation

= 1 3 ( ( "
Based on the calculated S0 spectra, a quantification of the macromolecular orientation degree has been performed. The orientation of a molecular chain axis i is usually described by three Herman's orientation functions fij where j = M, T or N holds for the directions defined above. The three orientation functions must obey the relationship

) ( ) ( ) = 0
For a transition moment of an infrared vibration along the molecular axis i, the Herman's orientation function can be calculated by means of the following equation

) + = 1 2 , + , % 1"
Where Aj and A0 are the band intensities in the Sj and S0 spectra respectively and j = M, T or N For sake of accuracy, the vibration bands used in the calculations should be distinct bands with absorbance below the saturation. In addition, these bands should be sensitive to orientational change, i.e., showing trichroic behavior when 3D texturing is present. In our study, the eligible band showing trichroic behavior is the band at 619 cm -1 , assigned to amorphous phase in PEF as once reported in [25].

Structural characterization

WAXS and SAXS measurements were performed on a Xeuss 2.0 apparatus (Xenocs) equipped with a micro source using a Cu Kα radiation (λ= 1.54 Å) and point collimation (beam size: 300×300μm²). The sample to detector distance, around 12 cm for WAXS and 2.5m for SAXS, is calibrated using silver behenate as standard. Through view 2D diffraction patterns are recorded on a Pilatus 200k detector (Dectris). Integrated intensity profiles are computed from the 2D patterns using the Foxtrot® software.

SAXS experiments were carried out to determine the nature of the plastic deformation mechanism(s) involved during cold drawing. To achieve this goal a homemade stretching device adaptable on the apparatus was used in order to analyze the sample during stretching. Besides, these tensile tests were carried out at room temperature (i.e. 20°C) using an initial stretching speed = 10 -3 s -1 . For the determination of the nature of the plastic deformation mechanism(s) involved, stretching is stopped just before sample's break and the acquisition is done on constrained sample.

Oxygen Transmission Rate measurement

Oxygen transmission rate (OTR) has been analyzed according to the ASTM D3985-17 norm and F1927-14. The film sample was analyzed in duplicate with the Mocon Oxtran 2/22L equipment. When the amount of sample area allowed it, the maximum sample area of 50cm² was used for the OTR measurement. A smaller film area of 5.64cm² was used for other foil samples. The OTR analysis was directly measured with 100% oxygen concentration as permeant gas. Analyses were performed at 23°C and 80% relative humidity. Enough conditioning time was applied to allow the sample to adjust to the relative humidity condition. Test was automatically stopped when the difference between the last 4 cycles of the measurement series showed less than 1% relative difference.

Mechanical properties of biaxially drawn samples

Mechanical properties of biaxially drawn samples were determined via tensile tests upon uniaxial stretching. The tests were performed on an Instron 4466 apparatus at room temperature (i.e. 20°C) using an initial stretching speed = 10 -3 s -1 . For each set of material, at least three dumbbell specimens having 22 mm and 6 mm in gauge length and width respectively were tested.

Results:

Initial materials characterization Thermal behavior of the three PEF samples has been investigated by means of DSC. Analysis showed that all the cast films are amorphous with a glass transition temperature (Tg) around 84°C ±1°C revealing no significant effect of molecular weight. To complete the potential influence of molecular weight was also investigated regarding its effect on crystallization kinetics and viscoelastic properties. Figure 1 depicts the isothermal DSC thermograms recorded at 165°C after cooling of the samples from the molten state.

Figure 1: Thermograms recorded during isothermal crystallization from the molten state at Tc = 165°C of LMW-, MMW-and HMW-PEF (endo up).

As can be seen, the crystallization kinetics are roughly similar for both LMW-PEF and MMW-PEF samples, while the one of HMW-PEF is significantly slower. While the crystallization halftime is around 20 and 30 minutes for LMW-and MMW-PEF, respectively, it is around 65 minutes for HMW-PEF. This retarded crystallization for HMW-PEF can be attributed to its higher molecular weight, which involves a decrease of the molecular mobility and thus slower macromolecular rearrangements required for crystal formation under these steady conditions. Even if the molecular weight influences the crystallization kinetics, no differences were observed regarding the crystallinity degree achieved, around 34% ±2% (standard melting enthalpy taken equal to 140 J/g [31]), as well as the melting temperature, around 205°C (DSC thermograms are depicted in Fig SI .2).

Finally the viscoelastic properties of the different materials have also been compared. The evolution of the storage modulus E' for the three different materials is shown in Figure 2. In the glassy state, the molecular weight has no significant influence on E' as all the materials exhibit a storage modulus around 3GPa. A sharp decrease of E', characteristic of the α relaxation, is also observed around 75°C for the three materials. On the other hand, an influence of the molecular weight can be observed in the rubbery state where a short rubbery plateau is observed for both LMW-and MMW-PEF while it spans a larger temperature range for HMW-PEF. Finally, the terminal transition characterized by the flowing of the sample involving a sharp drop of E' at the end of the rubbery plateau happens at much higher temperature for HMW-PEF due to its much higher molecular weight. On the other hand, LMW-and MMW-PEF have a higher ability towards disentanglement. Indeed for these two materials, the molecular weight is only 3 times higher than the critical mass (Mc = 2Me, with Me ≈ 3500 g/mol 2,32 ) to form entanglements while it is 7 times higher for HMW-PEF.

In summary, the characterization of the initial sample materials revealed that LMW-and MMW-PEF exhibit similar thermal and viscoelastic properties, while HMW-PEF differs strongly, as a direct result of its significantly higher molecular weight. Particularly this higher molecular weight leads to a higher number of entanglements per chain and consequently a slower disentanglement rate which explains the larger rubbery plateau. The mechanical behavior upon simultaneous biaxial stretching of LMW-, MMW-and HMW-PEFs has been studied on a wide range of stretching conditions. For sake of clarity, the purpose of the paper being to determine the relationships between the structure and the mechanical behavior of PEF, only the mechanical behavior for some sets of conditions will be presented and discussed here. Figure 3 depicts the mechanical behavior of PEF biaxially stretched according to the stretching conditions for which i) a biaxial stretching ratio of 4.5x4.5 could be achieved whatever the molecular weight and ii) the strain-hardening slope is the higher. The mechanical behavior observed is typical of an initially amorphous polymer stretched in the rubbery state such as PET and similar to the one reported by van Berkel et al in the case of PEF 16 . For all PEF samples, whatever their molecular weight, an elastic region at low strains is observed followed by a deformation zone where stress slowly increases finally followed by a strain-hardening stage where stress rapidly increases with strain. Contrary to the results reported for compression Elongation ratio (λ) Regarding the effect of the molecular weight, it appears that the increase of the molecular weight involves an increase of the applied stresses. This could be ascribed to the increase of the number of entanglement per chain with the increase of molecular weight which leads to a more robust entanglement network. However, the effect of molecular weight on the onset of strain-hardening appears limited even if a slightly earlier occurrence of strain hardening is observed with the increase of the molecular weight. Nevertheless, even if the effect on the onset of strain-hardening is limited, the increase of molecular weight involves an increase of the strain-hardening slope. Indeed the slope in that region, taken for 4.2 ≤ λ ≤4.5, increases from 0.9 MPa/% to 1.39 MPa/% for MMW-PEF and to 2.14 MPa/% for HMW-PEF. This more pronounced strain-hardening with the increase of the molecular weight has already been reported in the case of PET 33 and PEN [START_REF] Martins | Large deformation mechano-optical and dynamical phase behavior in uniaxially stretched poly(ethylene naphthalate)[END_REF] , but comparisons are difficult as units are not the same or as viscosity values are given rather than molecular weight data.

The origin of the effect of molecular weight on the strain-hardening slope will be discussed later in the manuscript.

In order to determine the structural origin of strain-hardening in PEF, and also in order to determine the origin of the molecular weight effect, a particular attention has been paid to the characterization of the biaxially stretched samples.

Indeed, as previously mentioned, strain hardening can have two origins:

-strain-induced crystallization as demonstrated in the cases of PET or PLA -high extension and solicitation of the entanglement network as for PS, i.e. macromolecular orientation

Structural characterization

The strain-induced structural characterization has been assessed by means of WAXS. Figure 4 depicts the integrated intensity profiles calculated from 2D (face-on) WAXS patterns for the three PEF resins biaxially stretched at different elongation ratios.

Figure 4: WAXS Integrated intensity profiles as a function of the elongation ratio for (a) LMW-PEF, (b) MMW-PEF and (c) HMW-PEF

It first appears that all the films are initially amorphous whatever the molecular weight which indicates the quenching speed of the cast film during production was sufficient to prevent crystallization. Regarding LMW-PEF, it appears that for these biaxial stretching conditions, the film remains fully amorphous whatever the elongation ratio, even in the strain-hardening region. In the case of MMW-PEF, the appearance of diffraction peaks at 2θ = 16.5° and 2θ = 20.9° is noted for stretch ratios λxλ ≥ 4x4, attesting of the occurrence of a strain-induced crystallization. The intensity of these peaks slightly increases between stretch ratios of 4x4 and 4.5x4.5 even if it remains low, indicating only modest levels of crystallinity. Finally for HMW-PEF, the appearance of diffraction peaks is noted for a stretch ratio of λxλ = 3.5x3.5. The intensity of these peaks drastically increased in the stretched film with areal stretch ratio 4x4 and then remains roughly constant at 4.5x4.5. Consequently, results indicate that the increase of molecular weight allows for, and even favors, the strain-induced crystallization process of these cast PEF films.

It can be noted that for MMW-and HMW-PEF the onset of strain-induced crystallization roughly corresponds to the onset of the strain-hardening. However, even if a strain-hardening stage is observed for LMW-PEF, no strain-induced crystals were detected. To go further, the crystalline structure induced by biaxial stretching has been analyzed by means of 2D-WAXS and 2D-SAXS. Regarding SAXS (data not shown here) no scattering characteristic of the presence of stacked crystals was observed whatever the sample.

As an illustration, Figure 5 Regarding the WAXS pattern recorded along ND, isotropic diffraction rings are observed. On the other hand diffraction arcs are observed on the pattern recorded along MD. This clearly shows that the crystalline structure induced upon biaxial stretching is orthotropic, i.e., PEF crystals lie parallel to the film plane. This particular crystalline orientation, often encountered in the case of biaxially oriented polymer, should probably play a role in the improvement of barrier properties previously reported 16 , by restricting segmental motion of amorphous chain parts arrested by microcrystals.

The type of crystalline structure involved by biaxial stretching has been also assessed. Figure 6 depicts the comparison between thermally crystallized PEF and PEF having crystallized upon biaxial stretching. The comparison seems to show a satisfactory agreement between the position of the diffraction peaks of the thermally crystallized and "biaxially crystallized samples" indicating that it is the "standard" α or α' crystalline form which is induced upon biaxial stretching. For uniaxial stretching, on the contrary it has been reported that the crystalline structure differs from the α or α' structure 12 . Indeed the two characteristic peaks observed at 2θ = 3.2° and 2 θ = 6.4° in uniaxially stretched PEF are not seen in this biaxially oriented sample. This shows that the nature of the molecular orientation process, i.e., uniaxial or biaxial, indeed plays a key role on the nature of the crystalline phase induced.

In order to complete this study, the evolution of the amorphous microstructure upon biaxial stretching has also been assessed using FTIR measurements. In order to determine the contribution of the macromolecular orientation of the amorphous chains to the mechanical behavior, the Herman's orientation parameter of the amorphous macromolecules has been determined for the three biaxially stretched materials up to different elongation ratios. Figure 7 depicts the Wilchinksi diagrams of the orientation functions of the PEF with different molecular weights and the evolution of the orientation parameter as a function of the elongation ratio.

) MMW-PEF, (c) HMW-PEF and (d) evolution of the orientation parameter fcN as a function of the elongation ratio (The line represents the critical value of the orientation parameter above which SIC is observed).

Figure 7 (a)-(c) reveal that the evolution of the orientation parameter is similar irrespective of molecular weight. The vibration band characteristic for the amorphous phase of PEF at 619 cm -1 tends to orient towards the normal direction (ND) to the film plane with the increase of the elongation ratio. Since the vibration band considered at 619 cm -1 is perpendicular to the chain axis [START_REF] Araujo | Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains[END_REF] , the evolution tendency towards ND indicates in fact that the macromolecules tend to orient in the MT plane during biaxial stretching. In addition the fact that all points fall upon the median of MT plane suggests an equiaxial orientation parameter along MD and TD, however combing the former 2D-WAXS results the overall texture should be "isotropic" in the plane of film.

A quantitative analysis of the evolution of the orientation parameter as a function of strain (Figure 7.d) indicates that the molecular weight has no significant effect until an elongation ratio of 3x3. Up to this areal stretch ratio the orientation parameter monotonously increases with strain. This can be ascribed to the fact that, in this deformation range, the macromolecular network is slightly constrained and plastic deformation rather occurs by chains unfolding and reorganization. Consequently, at this point the fact that LMW-PEF, which exhibits less entanglements per chain, than HMW-PEF does not infer on the mechanical behavior observed. Above this 3x3 elongation ratio indeed significant differences are observed between the PEF samples of different Mw. Especially the orientation parameter strongly increases with strain for HMW-PEF, while it increases slowly with strain for LMW-PEF. This trend is in line with the mechanical behavior of the materials where an increase of the strain-hardening slope was observed with the increase of the molecular weight. This slower orientation for LMW-PEF than for HMW-PEF can be ascribed to the fact that, due to the lower Biaxial elongation ratio (λ) number of entanglements per chain, the macromolecules have the possibility to slip and to disentangle, which disadvantages macromolecular orientation. In other words, the increase of the molecular weights involves a decrease of the disentanglement rate upon stretching which favors the macromolecular orientation. A comparison with WAXS data also showed that the onset of straininduced crystallization is observed at the same orientation parameter for both MMW-and HMW-PEF (fcN ≈ 0.18). This result is of prime interest as it tends to prove the existence of a critical amorphous orientation for the beginning of crystallization, as for PET [START_REF] Lebourvellec | Amorphous orientation and induced crystallization in uniaxially stretched poly(ethylene terephthalate glycol)[END_REF] . This conclusion seems to be confirmed by the behavior of LMW-PEF for which no crystallization was observed and which also exhibit an orientation parameter fcN ≤ 0.18 over the entire studied areal stretch ratio range.

Finally, a remaining question is the origin of the strain-hardening stage. At this point one can state that both amorphous orientation and presence of crystals are at the origin of the phenomenon. However, later results also show that the presence of crystals involves -and perhaps even facilitates -an increase of the amorphous orientation parameter. Therefore, one can wonder if the slope of the strain-hardening is determined, by the high rigidity of crystals or rather by the fact that the amorphous chains are highly oriented and arrested in a constantly increasing network of crystals. In an attempt to answer this question, the evolution of the slope in the strain-hardening region has been plotted as a function of fcN, as depicted in Figure 8. A reasonably good correlation is observed between fcN and the slope of the strain-hardening except for high fcN values (i.e., for high elongation ratios). This suggests that, in the case of PEF, strainhardening mainly originates from the extension of the macromolecular network. Particularly, the increase of the amorphous phase orientation leads to an increase of the strain-hardening slope. This conclusion seems in opposition with the observations of PET where SIC is believed to be the predominant phenomenon at the origin of strain-hardening. However one have to keep in mind that in PEF the crystallinity achieved upon biaxial stretching is very low and that's why crystals play a modest role regarding the material stiffening. However, even if low crystal contents are induced, it can be seen that they play a role on macromolecular orientation as shown by the deviation from the two last point representing the higher orientation values. Particularly it can be assumed that, the strain-induced crystals plays the role of crosslinking/entanglement points which favor/force the amorphous macromolecular orientation.

Properties of BO films

The influence of molecular weight and biaxial stretching on some end-to-use properties of PEF have been determined. The oxygen permeability as a function of the biaxial elongation ratio are displayed in Regarding the effect of the biaxial stretching ratio no significant effect is observed for draw ratios up to 3x3 while the OTR value decreases for higher elongation ratios. This stretching range where a decrease of the OTR value is measured corresponds to the range where strain-hardening is observed on the mechanical curves. To go further, the OTR values have been plotted as a function of the macromolecular orientation degree ( Regarding stress at break, the trend is similar to what was observed for OTR measurements: there is no significant effect up to λxλ = 3x3, while an increase of the stress at break is observed for higher values. In addition this increase is fairly higher for HMW-PEF, probably due to the presence of a significant amount of crystal. On the other hand, an interesting feature is observed regarding the effect of biaxial stretching on the stretchability of PEF. Indeed, while the as-cast films are brittle, whatever the molecular weight, a ductile behavior is observed for elongation ratios above 2x2. Particularly, a strain at break as high as 170% can be achieved for LMW-and MMW-PEF when they are biaxially stretched at λxλ = 2x2. This brittle to ductile transition induced by biaxial stretching is of prime interest as it drastically improves the toughness of the PEF films. Finally it appears that, for λxλ ≥ 2x2, the strain at break decreases with the increase of the biaxial draw ratio. This is explained by the fact that the macromolecular orientation induced by biaxial stretching limits further elongation of the macromolecules during the uniaxial stretching test.

Origin of the brittle to ductile transition

Recently Xu et al. report a similar brittle to ductile transition induced by biaxial stretching in the case of Polylactide (PLA) [START_REF] Xu | Brittle-to-ductile transition of PLA: the influence of macromolecular orientation eXPRESS[END_REF] . In their study, authors show that this behavior transition is linked with a transition of the elementary plastic deformation mechanisms involved. Particularly they observed that the brittle behavior arises from the occurrence of crazing during stretching in the glassy state while shear banding is involved when the material is ductile. In addition they put in evidence that this brittle to ductile transition occurs when a critical molecular orientation parameter is reached, i.e. fcM=fcN≈0.07 for PLA. In order to try to understand the origin of this brittle to ductile transition in the case of PEF, BO samples have been analyzed by means of SAXS (Figure 11). presence of crazes. In addition, optical microscopy analyses revealed the presence of shear bands at the surface sample, explaining the ductile behavior observed (Figure 12). In order to assess the origin of this transition of plastic deformation mechanisms, MMW-PEF films were biaxially drawn at intermediates draw ratios between 1x1 and 2x2. The corresponding SAXS patterns (b-d) show a decrease of the scattered intensity with the increase of the draw ratio which is explained by a decrease of the crazes density. Consequently macromolecular orientation has an inhibiting or at least a restricting effect on the formation of crazes at the benefit of shear banding. Further investigations are under progress in order to determine how macromolecular orientation infers on the crazing mechanism.

Conclusion

The influence of the molecular weight on the mechanical behavior and associated structural evolution of PEF has been investigated. The influence of molecular weight on the mechanical behavior is limited as almost the same behavior is observed whatever the molecular weight. Particularly a strain-hardening stage is observed under appropriate biaxial stretching conditions, only the slope of strain hardening increases when increasing molecular weight. However, the molecular weight strongly influences the strain-induced structural evolution. Indeed while the low molecular weight PEF remains amorphous whatever the biaxial draw ratio, a strain-induced crystallization process is observed for higher molecular weights. This explains the higher slope of strain-hardening observed. To go further, the orientation degree of the macromolecules upon BO has been measured. Results indicate that strain-induced crystallization is driven by the orientation degree of the macromolecules. Particularly, a critical value of 0.18 has to be reach in order to initiate SIC. Even if SIC occurs, it was also shown that the mechanical behavior is mainly governed by the macromolecular orientation of the amorphous chains, due to the limited content of crystal induced. This faster macromolecular orientation kinetics for high molecular weight PEF is explained by the fact that long chains support more entanglements than small one, leading to a more constrained macromolecular network. Particularly, the disentanglement of the macromolecules is disfavored when the molecular weight increases. The effect of biaxial stretching on the resulting properties has also been analyzed. On the one hand it was shown that strain-induced crystallization leads to better oxygen permeation properties. On the other hand it has been evidenced that, whatever the molecular weight, biaxial stretching at sufficient draw ratios (i.e. when a critical macromolecular orientation degree is attained) leads to ductile films. This brittle to ductile transition has been explained by a transition in terms of elementary plastic 500µm deformation mechanisms from crazing to shear banding. Particularly, the increase of the orientation degree, limits or avoids crazing. Still regarding end-use properties, it has been shown that biaxial stretching has a positive effect on oxygen permeability. This improvement of barrier properties has been linked, on the one hand to the macromolecular orientation and, on the other hand, to the formation of crystals during stretching which strongly infers on this parameter. Another result of this work is that while the increase of the molecular weight has a positive effect on SIC, the opposite is observed for thermally induced crystallization were the increase of the molecular leads to a decrease of the crystallization speed. Finally, from a practical point of view, this work shows that a critical molecular weight has to be used, not for observing strain-hardening, but rather to allow strain-induced crystallization in order to improve the end-to use properties of the material.

Figure 2 :

 2 Figure 2: Evolution of the storage modulus E' as a function of the temperature for LMW-, MMW-and HMW-PEFs (heating rate 5°C/min, f = 1Hz)

  behavior of PEF upon biaxial stretching and associated strain-induced structural evolution

Figure 3 :

 3 Figure 3: Influence of the molecular weight of PEF on the mechanical behavior during simultaneous biaxial stretching(Td = 100°C, -= 2. /0 , for sake of clarity only the stress recorded along MD is depicted, however it was ensure that 1 = 1 )

Figure 5 :

 5 Figure 5: (a) face-on and (b) edge-on WAXS patterns of HMW-PEF biaxially stretched up to λxλ = 4.5x4.5.

Figure 6 :

 6 Figure 6: integrated intensity profiles of thermally crystallized PEF and biaxially oriented HMW-PEF (sample biaxially stretched up to λxλ = 4.5x4.5 at 100°C and = 200%/., the indexation of the diffraction peaks has been made on the basis of the results from [17] ).

Figure 7 :

 7 Figure 7: Wilchinski diagrams of the orientation function for biaxially stretched (a) LMW-PEF, (b) MMW-PEF, (c) HMW-PEF and (d) evolution of the orientation parameter fcN as a function of the elongation ratio (The line represents the critical value of the orientation parameter above which SIC is observed).

Figure 8 :

 8 Figure 8 : Evolution of the strain-hardening slope as a function of the orientation parameter fcN for LMW, MMW-and HMW-PEF (the slope of the strain hardening was determined in the 3x3 ≤ λxλ ≤4.5x4.5 region)

Figure 9 Figure 9 :

 99 Figure 9.a and

Figure 9 Figure 9 Figure 9 Figure 10 :

 99910 Figure 9.b). It appears that there's a good correlation between OTR value and macromolecular orientation. Considering that very low crystallinity is achieved in that case, it can be assumed that the improvement of barrier properties observed originates from macromolecular orientation.Regarding the effect of molecular weight,

Figure 11 :

 11 Figure 11 : (a-e) SAXS patterns of as-cast and BO MMW-PEF samples taken just before breaking during the uniaxial tensile test at room temperature (stretching axis is horizontal)The SAXS pattern recorded for the cast-film (a) uniaxially drawn at room temperature clearly indicates that crazing is involved which explains the brittle behavior of the PEF cast-film. In opposition, for a MMW-PEF film biaxially drawn up to λxλ = 2x2 (e), there's no more evidence of the

Figure 12 .

 12 Figure 12. Optical observation of a MMW-PEF film pre-oriented at 2x2 after rupture upon uniaxial deformation.

Table 1 :

 1 Their respective molecular weight values are listed in Table 1. Molecular characteristics of the PEF resins used.

	Abbreviation	Mn (kg/mol)	Mw (kg/mol)	Mw/Mn	Average number of entanglements per chain
	LMW-PEF	23	39	1.7	≈ 6.5
	MMW-PEF	32	54	1.74	≈ 9
	HMW-PEF	51	89	1.76	≈ 14.5

  Mn molded PEF plaques in[16], even if stretching conditions are comparable, a marked strain-hardening is observed in the case of each PEF for λ ≥ 4, comparable to the one reported in PET.

		250
			LMW_100°C_200%/s
			MMW_100°C_200%/s
			HMW_100°C_200%/s
		200
	True stress (MPa)	100 150
		50
		0
		1.0	1.5	2.0	2.5	3.0	3.5	4.0	4.5

Acknowledgement

This manuscript is a tribute to the 50 year anniversary of the French Polymer Group (Groupe Français des Polymères -GFP). The authors are indebted University of Lille and Région Haut-de-France for funding the PhD thesis. Financial support from Région Nord Pas-de-Calais and European FEDER for both biaxial stretching and SAXS laboratory equipment is also gratefully acknowledged.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.


Kevin Maëllys

Girault Guillaume

Caspar Yvan

Cherfa Moulay Ali

Mendy Christiane

Maurin Max

Ponsart Claire

Madani Nora
email: nora.madani@anses.fr

High-resolution melting PCR assay as a powerful tool for the epidemiological surveillance of tularemia in Western Europe

Keywords: Francisella tularensis, High-resolution melting PCR, surveillance, tularemia

Canonical single nucleotide polymorphisms are used to classify

Francisella tularensis.

•

Discrimination assays were designed for 113 markers identified in Western Europe.

•

It enables fast discrimination of Francisella tularensis isolates in Western

Europe for epidemiological surveillance and outbreak investigation.

2

Short communication

Francisella tularensis is the causative agent of tularemia, a zoonosis that can affect more than 350 animal species [START_REF] Mörner | The ecology of tularaemia[END_REF]. In Europe, the only subspecies known to cause tularemia is F. tularensis subsp. holarctica. As it is a highly clonal pathogen, classification of closely related subpopulations of F. tularensis at a strain level relies on highly resolutive markers [START_REF] Johansson | Genotyping of Francisella tularensis, the causative agent of tularemia[END_REF] and is required for outbreak investigation and long-term surveillance. Selection of polymorphic variable-number tandem repeats (VNTR) for multipleloci VNTR analysis (MLVA) allowed the differentiation of five clades for subspecies holarctica (B.I, B.II, B.III, B.IV and B.V) [START_REF] Johansson | Worldwide Genetic Relationships among Francisella tularensis Isolates Determined by Multiple-Locus Variable-Number Tandem Repeat Analysis[END_REF]. However, fast evolution and instability of tandem repeats can be responsible for the inherent homoplasy of some VNTRs [START_REF] Lankester | II.-On the use of the term homology in modern zoology, and the distinction between homogenetic and homoplastic agreements[END_REF]. Convergent evolution makes VNTRs unsuitable markers to infer phylogenies. Since the decline of the sequencing costs, identification of canonical single nucleotide polymorphisms (canSNP) has become a reference method for high resolution discrimination of bacterial strains [START_REF] Keim | Anthrax molecular epidemiology and forensics: using the appropriate marker for different evolutionary scales[END_REF][START_REF] Girault | Highresolution melting PCR analysis for rapid genotyping of Burkholderia mallei[END_REF]. Four main clades have been described for F. tularensis subsp. holarctica : B.4, B.6, B.12 and B.16, corresponding to clades B.II, B.IV, B.I and B.V respectively [START_REF] Vogler | Phylogeography of Francisella tularensis: Global Expansion of a Highly Fit Clone[END_REF]. Since, the number of newly identified canSNPs is constantly increasing over time and across studies [START_REF] Vogler | Phylogeography of Francisella tularensis: Global Expansion of a Highly Fit Clone[END_REF][START_REF] Chanturia | Phylogeography of Francisella tularensis subspecies holarctica from the country of Georgia[END_REF][START_REF] Gyuranecz | Phylogeography of Francisella tularensis subsp. holarctica , Europe[END_REF][START_REF] Karadenizli | Genomic analyses of Francisella tularensis strains confirm disease transmission from drinking water sources, Turkey, 2008, 2009 and 2012[END_REF][START_REF] Karlsson | The phylogeographic pattern of Francisella tularensis in Sweden indicates a Scandinavian origin of Eurosiberian tularaemia[END_REF][START_REF] Dwibedi | Long-range dispersal moved Francisella tularensis into Western Europe from the East[END_REF][START_REF] Myrtennäs | Introduction and persistence of tularemia in Bulgaria[END_REF][START_REF] Schulze | High and novel genetic diversity of Francisella tularensis in Germany and indication of environmental persistence[END_REF]Wittwer et al., 2018). Studies showed that isolates from Western Europe belonged to numerous subclades of clade B.11, a subclade of B.6, and that these subclades have been able to form spatio-temporal clusters within a country [START_REF] Allue | Tularaemia outbreak in Castilla y León, Spain, 2007: an update[END_REF][START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF].

High Resolution Melting-PCR (HRM-PCR) analysis is a technique that allows the discrimination of minor genetic variations on amplicons, such as SNPs, through their melting temperature (Tm) [START_REF] Tamburro | High Resolution Melting as a rapid, reliable, accurate and cost-effective emerging tool for genotyping pathogenic bacteria and enhancing molecular epidemiological surveillance: a comprehensive review of the literature[END_REF]. It enables rapid, reliable and low-cost surveillance of infectious diseases, as well as fast detection of a clonal outbreak. In this study, we developed a genotyping tool based on HRM-PCR to discriminate closely related strains belonging to clade B.11 and its subclades, with the highest possible resolution. A hierarchical HRM-PCR assay was developed using a total of 109 canSNPs retrieved from the CanSNPer database [START_REF] Lärkeryd | CanSNPer: a hierarchical genotype classifier of clonal pathogens[END_REF] and [START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF] (Figure 1). The updated version of the database is available on the following link:

https://github.com/adrlar/CanSNPer. CanSNPs were classified in "orders of discrimination" (D1-D9) to progressively differentiate the levels of resolution (Figure 1). The SNP-discriminative typing tool was designed to be used according to the purpose of the study (high level of resolution for outbreak investigation or low level of resolution for long-term surveillance).

Using Primer 3+ software [START_REF] Untergasser | Primer3-new capabilities and interfaces[END_REF] [START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF], should be typed in priority during HRM-PCR assays.

Such clades also present more subclades than other clades, which reflects the evolution of these clades and therefore, their persistence in Europe. Because of PCR inhibitors present in biological samples, HRM-PCR is more adapted to bacterial strains. HRM-PCR was tested on highly positive biological samples from a marmoset (Callithrix pygmaea, Ct ~20) and gave satisfying results (Figure 2).

Contact with hares is one of the most important risk factors for humans in Western Europe as several outbreaks have been associated with hunting or handling hares in these geographical areas [START_REF] Allue | Tularaemia outbreak in Castilla y León, Spain, 2007: an update[END_REF][START_REF] Mailles | Bilan de 10 années de surveillance de la tularémie chez l'homme en France[END_REF][START_REF] Otto | Hareto-Human Transmission of Francisella tularensis subsp. holarctica, Germany[END_REF][START_REF] Hestvik | Francisella tularensis in muscle from diseased hares -a risk factor for humans?[END_REF][START_REF] Koene | Phylogeographic Distribution of Human and Hare Francisella Tularensis Subsp. Holarctica Strains in the Netherlands and Its Pathology in European Brown Hares (Lepus Europaeus)[END_REF]. Thus, wildlife surveillance is of great importance not only for animal health, but also for human health, especially because human cases might be under-declared.

Indeed, as symptoms are not specific and often of mild severity, patients might be underdiagnosed or might not even request medical consultation. Moreover, most genetic clusters were identified with strains isolated from hares [START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF] and it has been previously demonstrated that the European Brown hare could be useful as sentinels of environmental risk [START_REF] Moinet | Spatiotemporal dynamics of tularemia in French wildlife: 2002-2013[END_REF]. It is fair to consider that these systems of surveillance (in humans and in wildlife), along with environment surveillance, should be complementary for a One-Health surveillance. As there seems to be no host specialization concerning F. tularensis in Western

Europe [START_REF] Dwibedi | Long-range dispersal moved Francisella tularensis into Western Europe from the East[END_REF][START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF] it is expected that strains isolated in vectors or in the environment would share identical genotypes as humans and other animals.

Therefore, our SNP-discriminative HRM-PCR assay would greatly help epidemiological surveillance of tularemia in Western Europe. First, this tool will enable highly resolutive discrimination of F. tularensis subsp. holarctica isolates to enhance knowledge about tularemia in Western Europe and to detect abnormal incidence of the disease due to a particular genotype. Secondly, this tool can be used for a biodefense purpose as it will allow to identify a clonal outbreak.

In conclusion, we developed a rapid, cost-effective and robust molecular typing tool for the discrimination of F. tularensis subsp. holarctica from bacterial strains or highly positive samples, with a high resolution in order to introduce a One-Health epidemiological surveillance of tularemia in Western Europe.

Wittwer, M., Altpeter, E., Pilo, P., Gygli, S. M., Beuret, C., Foucault, F., et al. (2018). Population Genomics of Francisella tularensis subsp. holarctica and its Implication on the Eco-Epidemiology of Tularemia in Switzerland. Front. Cell. Infect. Microbiol. 8. doi:10.3389/fcimb.2018.00089.

Figure legends

Figure 1. CanSNP nomenclature generated by CanSNPer [START_REF] Lärkeryd | CanSNPer: a hierarchical genotype classifier of clonal pathogens[END_REF]. Clades for which a HRM-PCR assay was designed are indicated with green circles; clades for which HRM-PCR primers need to be validated on samples are indicated with green circles filled with a black star; countries (Austria, Belgium, France, Germany, Italy, Netherlands, Spain, Switzerland) in which each clade (D5) has been identified are indicated with flags. Data have been retrieved from the NCBI database (https://www.ncbi.nlm.nih.gov/genome/genomes/511) as well as from Kevin et al [START_REF] Kevin | Phylogeography and Genetic Diversity of Francisella tularensis subsp. holarctica in France (1947-2018)[END_REF]. are listed in supplementary table I.

Figure 2 .

 2 Figure 2. Normalized melting curve obtained by HRM-PCR assay (canSNP B.304) on spleen samples from a marmoset (Callithrix pygmaea) diluted to the 1/2, 1/4, 1/8 and 1/16 (red). Positive control for B.304 was included (blue). Expected melting temperatures (Tm)

Funding

This work was supported by the French National Research Agency (ANR) and the Direction Générale de l'Armement (DGA) (No. ANR-15-ASTR-0021-01). M. Kevin is a PhD student co-supported by Université Paris-Est and DGA grants.

Supplementary table II.

List of strains used for in vitro validation of HRM-PCR primers.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Creating Products in the Absence of Markets: A Robust Design Approach

Keywords: New Product Development, uncertainty, high-technology venture

Purpose -The purpose of this study is to examine how firms deal with a situation of true uncertainty about their potential markets and technologies. Specifically, we ask how firms can create products when the corresponding market does not exist.

Design/methodology/approach -This paper is based on a longitudinal study of a high-tech firm, combined with analysis of existing theory in Product Design and Entrepreneurship.

Findings -Markets and products are usually a defining choice made early on by firms in their strategic process. Such a choice guides their development by providing a 'stable concept' to which decisions can be related to. When markets do not exist yet, however, this approach is not effective: Early choice of products and markets limits firms' flexibility by constraining their ability and willingness to adapt, while fundamental new technical and market information is likely to emerge during the project that will prove the initial assumptions wrong.

We show an alternative approach where products and markets actually result from a generic process of products and markets exploration driven by the firm. We suggest that this approach forms a robust design in that it allows the firm to deal with the uncertainty by simultaneously developing its products and exploring markets, while preserving the flexibility to adapt to the changing environment.

Practical implications -

The practical implication of this paper is to suggest an alternative approach to deliberate planning in high-tech ventures. With this approach, rather than markets and products, strategy defines a market and technology exploration process.

Originality/value -The paper is original in three ways: 1) It links the product design and market exploration processes in high-tech firm development; 2) It is based on an in-depth longitudinal study; and 3) It results from an academic-practitioner collaborative work.

INTRODUCTION

The purpose of this study is to examine how firms deal with a situation of true, "Knightian" [START_REF] Knight | Risk, Uncertainty and Profit[END_REF] uncertainty that characterizes emerging or disruptive environments. Using an in-depth case study approach, we seek to understand how the firm evolves from an initial situation of uncertainty about its potential markets and products to a situation where an opportunity has been defined and the corresponding products are created. The choice of products and markets are both essential components of a firm's strategy [START_REF] Grant | Contemporary strategy analysis[END_REF]. It is thought that these provide stability that will guide the firm's development, and possibly define its identity. The early and appropriate definition of these strategic components, even before the firm's creation, is assumed to be an important success factor, as exemplified by the institutional importance given to business planning [START_REF] Honig | Entrepreneurship Education: Toward a Model of Contingency-Based Business Planning[END_REF]. Underpinning this assumption is the predominant use of causal rationality in management. Causal rationality assumes pre-determined and well-structured ends, and formulates the decision problem as one of discovering the best possible means to achieve those ends. Causal reasoning proceeds inward by breaking given goals into sub-goals and sub-sub-goals to specific individual tasks [START_REF] Sarasvathy | Effectuation in the management of Knightian uncertainty: Evidence from the Realnetworks case[END_REF]. Causal rationality, with its emphasis on maximizing expected return or minimizing expected costs and avoiding surprises through accurate prediction and comprehensive competitive analysis, has long served as the normative foundation for both research and pedagogy in business management. It implies the definition of a clear goal, and works with a logic of optimization; i.e., given a particular goal, what is the optimal way (in terms of resources) to reach it. High-tech firms, however, seek opportunities in disruptive contexts where neither their products nor the corresponding market generally exist at the time of the disruption. In this context, goals cannot be defined easily, markets cannot guide product development, and causal rationality does not work anymore. How can firms create products when the corresponding market does not exist? We organize the paper as follows. We first highlight and discuss the core concepts of entrepreneurship and product design relevant to our issue. Following this discussion, we describe our methodology and then report on our findings. We then discuss those findings and draw their managerial implications for high-tech venture management.

THEORETICAL BACKGROUND

To characterize the disruptive contexts in which high-tech firms operate, [START_REF] Knight | Risk, Uncertainty and Profit[END_REF] distinguished between risk and uncertainty using the concepts of probability. Risk consists of a future with a known distribution, or a distribution that can be estimated by studying events over time. For instance, in the case of a box with six red balls and six blue ones the probability of a particular color being drawn (an event) is perfectly known. On the contrary, "Knightian" uncertainty consists of a future whose distribution is not only unknown, but objectively unknowable. The key difference between Knightian uncertainty and risk is that Knightian uncertainty involves dealing with a future that has no discernible distribution whatsoever, not even in theory. In this case, therefore, neither the calculus of a priori probability nor techniques of statistical estimation can work [START_REF] Sarasvathy | Effectuation in the management of Knightian uncertainty: Evidence from the Realnetworks case[END_REF], because the involved objects do not exist yet. To reuse the box example, we do not know whether there is a box, how many balls there are, and the concepts of box and balls might even not be defined. For instance, key elements of the Internet were invented in the late sixties, and commercial application only became significant in the early nineties (Slater, 2002). Such applications were totally unanticipated by the original inventors of the technology: they designed the Internet for military use, ie to provide a secure communication network in case of nuclear war; it is now used for rather different applications such as book buying, television watching, music distribution and game playing. While firms in established industries (e.g. banks, insurance companies) face risk, those in emerging industries face Knightian uncertainty. They trade in a context where prediction is impossible [START_REF] Knight | Risk, Uncertainty and Profit[END_REF], goals are not predetermined [START_REF] March | The technology of foolishness[END_REF], and the environment does not independently select the outcomes [START_REF] Weick | The Social Psychology of Organizing[END_REF]. Early choice of products and markets, in such a context, limits firms' flexibility by constraining their ability and willingness to adapt [START_REF] Bhide | How Entrepreneurs Craft Strategies that Work[END_REF][START_REF] Fredrickson | Inertia and creeping rationality in strategic decision processes[END_REF]; as a result, firms are particularly sensitive to disruptions and turbulence and every discontinuity generates an erratic trajectory. [START_REF] Sarasvathy | Causation and effectuation: toward a theoretical shift from economic inevitability to entrepreneurial contingency[END_REF] studied how entrepreneurs deal with a situation of Knightian uncertainty. She showed that they inverted the principles of causal reasoning and that the inversions together constituted a comprehensive new logic that she calls "Effectuation". Effectuation is a sequence of non-predictive strategies in dynamic problem solving that is primarily means-driven, where goals emerge as a consequence of stakeholder commitments rather than vice versa. An alternative to causal (predictive) rationality, effectuation suggests that "Knightian" actors succeed by taking a progressive approach to the definition of their products and markets [START_REF] Wiltbank | What to do Next? The Case for Non-Predictive Strategy[END_REF]. What matters, therefore, is not which products and markets they choose ex-ante, but how, in the absence of current markets for future products, such products get created by the firm [START_REF] Shane | The promise of entrepreneurship as a field of research[END_REF] and how this process allows the firm to resolve the Knightian uncertainty. Traditional models for product development projects have emerged from observations in relatively mature, industrial environments, such as the automobile industry (e.g. [START_REF] Wheelwright | Revolutionizing Product Development[END_REF]. Their focus is on developing a structured process with clearly defined and sequential phases for the development project through which the future product is defined, transferred to manufacturing, and rolled out to the market [START_REF] Iansiti | Shooting the Rapids: Managing Product Development in Turbulent Environments[END_REF]. Those models imply a clear separation between concept development, specification definition, and implementation. Underlying them is the assumption that products are developed in order to serve markets that already exist and have been selected by the firm. As such they adopt a causal logic and tend to pose the problems in terms of optimization. They work well in environments in which technology, product features and competitive requirements are predictable, and no new information is expected that would significantly change the specifications. In Knightian uncertainty, however, markets do not exist yet, and fundamental new technical and market information is likely to emerge during the project. Uncertainty is such that even the product concept itself can be modified, or even changed completely. For instance, RealNetworks, the current leader in media streaming technology over the Internet, started out as a project to create a TV channel [START_REF] Sarasvathy | Effectuation in the management of Knightian uncertainty: Evidence from the Realnetworks case[END_REF]. In this context, the success factor shifts from the capability for optimized project execution to the capability to react to new information during the course of the project by changing the product specifications [START_REF] Brown | Product Development: Past Research, Present Findings, and Future Directions[END_REF][START_REF] Iansiti | Shooting the Rapids: Managing Product Development in Turbulent Environments[END_REF], possibly to a large extent. To model the exploratory nature of the design process in such environments, and the blurring of the distinction between the specification and implementation phases, [START_REF] Hatchuel | C-K Theory: Notions and applications of a unified design theory[END_REF] introduced the Concept-Knowledge (CK) design theory. CK is based on the articulation of knowledge with the exploration of new products concepts. A unit of knowledge is a proposition with a logical status (true or false). For instance, in designing a car with a platform strategy, a unit might be "The car must use Platform X". A concept is a notion or proposition without a logical status. It is an innovative proposition used as a basis for initiating a design project. A "flying boat", for instance, is neither true, nor false, nor uncertain. The design process starts when unknowns are created by proposing a new concept (going from Knowledge 'K' to Concept 'C'). This initial concept is then expanded using properties that have a logical status in K. Concepts are expanded through design spaces, which need existing knowledge, and also generate discoveries and investigation processes that in turn expand the initial concept [START_REF] Hatchuel | Building innovation capabilities: the development of designoriented organizations[END_REF]. The design process stops when this dual expansion of concepts and knowledge leads to a concept that can be proved either true or false with the resulting knowledge (flying boat has become a "hydropter"). CK theory is directly applicable in design situations where the work is not based on detailed specifications or a well-identified definition of the object to be designed. It helps the researcher describe the design work not as a deterministic process of optimization, but as a creative exploration, thus meeting one of the design requirements in Knightian uncertainty [START_REF] Hatchuel | C-K Theory: Notions and applications of a unified design theory[END_REF]. With these core concepts as a starting point, we report empirical findings that suggest how a firm may overcome Knightian uncertainty. Taken together, the findings show that WSoft1 succeeded in resolving Knightian uncertainty by defining its stable concept as a generic process of products and markets exploration.

MATERIAL AND METHODS

Approach

The paper is based on a longitudinal case study of WSoft, a small European firm developing mobile phone software. We seek to explain how the firm was able to resolve the Knightian uncertainty about its products and markets. Case study research, whether using single or multiple cases, has not only been used effectively for descriptive and exploratory purposes, but also for explanatory purposes including frame-breaking causal inferences about complex and important phenomena [START_REF] Sarasvathy | Effectuation in the management of Knightian uncertainty: Evidence from the Realnetworks case[END_REF]. As opposed to statistical generalization, case study research allows analytical generalizations [START_REF] Yin | Case Study Research: Design and Methods[END_REF]. This study contributes to the integration of concepts and theories by using the extended case method, which aims to integrate and synthesize existing bodies of work. In contrast to the Grounded Theory approach [START_REF] Glaser | The Discovery of Grounded Theory: Strategies for Qualitative Research[END_REF], the primary focus of the extended case study is not to build new theory. Rather, its method is to integrate and extend existing theories through an iterative process of traveling back and forth between the data, pertinent literature and emerging theory [START_REF] Danneels | The Dynamics of Product Innovation and Firm Competences[END_REF]. Specifically, we seek to integrate the entrepreneurial theory of opportunity and the product development theory in the context of emerging markets. As with any idiographic research, we recognize that the entrepreneurial processes that WSoft employed can be unique and that such processes may be difficult to identify and measure. In such cases, it is the generative mechanisms or underlying processes that are generalizable and not the unique phenomena described [START_REF] Tsoukas | The Validity of Idiographic Research Explanations[END_REF]. The challenge here, therefore, is in identifying these processes, and the drivers of such processes, in the creation and early market development of WSoft.

Choice of firm

We chose this firm as a case study because, as a player in the market for mobile phone software, it exemplifies the phenomenon of interest. The mobile phone industry is characterized by rapid and unpredictable evolution: pioneers have exited the market (e.g. Alcatel, Siemens, Philips), merged (e.g. Ericsson), or are currently in difficulties (e.g. Motorola). Asian competitors, once poised to overtake them, have had various degrees of success: while Korea's Samsung has become the second largest manufacturer worldwide in just a few years, Taiwan's BenQ's attempt has failed and the company has since exited the market. The industry is also characterized by high profile industry-wide product failures, such as the WAP mobile Internet in 2001, and unexpected successes such as text messaging. Repeated disruptions threaten to undermine the position of actors who cannot react fast enough. For instance, Nokia's leading position was compromised when the company failed to anticipate the sudden popularity of clam-shell phones or of the camera feature in 2003. While the origin of the mobile phone industry can be traced back to the 1970s, it is still undergoing significant disruptions, and as such is typical of a situation of Knightian uncertainty. It is in this context that WSoft attempts to develop its products and markets, and establish itself as leader. While this leadership position is not yet reached, and may not be reached, the firm has successfully dealt with a situation of Knightian uncertainty: it is growing profitably and is recognized as a key player, having won designs with important clients in its field.

Data collection

Data was collected through a two-year full-time presence in the company. Our primary source of data was the cofounder and CEO of the firm, who is part of the research team as a reflective practitioner [START_REF] Schön | The Reflective Practitioner[END_REF]) and a co-author of this article. Other sources of data were: 1) interviews with other founders of the firm; 2) accounts of actions provided by the firm in the form of press releases; 3) analysis of internal reports as well as emails exchanged during the studied period; and 4) analysts' reports (both on firm and industry) whenever available.

To gather industry related background information, we relied on multiple sources such as news.com and telecoms.com, online industry trade magazines, as well as electronic databases of press articles such as Factiva. As typical in inductive research [START_REF] Eisenhardt | Building Theories from Case Study Research[END_REF], the data collected from these multiple sources served as the basis for our own detailed case study on the mobile software industry and WSoft's role in it. To establish the validity of the reconstruction process, we compared our case history with studies in close fields of the software industry, such as [START_REF] Sarasvathy | Effectuation in the management of Knightian uncertainty: Evidence from the Realnetworks case[END_REF] study of RealNetworks, Cusumano and Yoffie's (1998) study of Netscape, as well as [START_REF] Battelle | The Search -How Google and its Rivals Rewrote the Rules of Business and Transformed our Culture, Portfolio[END_REF] study of Google. We used the case histories (our own and others) to identify the product and market related actions taken by the firm, and to create tables to organize this data chronologically. These different sources enabled us to examine the data from multiple vantage points [START_REF] Glaser | The Discovery of Grounded Theory: Strategies for Qualitative Research[END_REF] and triangulate facts and inferences. For example, press releases provide detailed information on entrepreneurial actions, often including top managers' vision of the market and expected consequences of these actions. However, analysis of press releases was limited: they tend to emphasize the positive aspects of the various actions [START_REF] Rindova | Continuous "Morphing": Competing Through Dynamic Capabilities, Form and Function[END_REF], and it was not possible to supplement the analysis with media reports examining the same actions owing to the limited visibility of the firm at that time.

Analysis

Our unit of analysis consisted of the decision-events that occurred in the creation of the firm pertaining to technology, products and markets choices. We performed a process trace of the decision-events as they occurred, using the event listing method prescribed by [START_REF] Miles | Qualitative Data Analysis[END_REF]. We began by listing the decision-events in their chronological order and then examining the relations between product-related, technology-related and market-related decision-events. We then performed a trace of the domain-knowledge explored by the firm as such decision-events occurred. Such tracing allowed us to examine the role of learning in the development of the firm in relation to these decision-events. As typical in qualitative research, the validity of our insights was checked with senior executives of the firm as well as with other academic members of the research team.

RESULTS

Products and markets do not define the stable concept

Most companies start with an initial target market or product, and WSoft is no exception. Starting in 1998, WSoft's initial business idea was to provide IT services in the field of Web sites development. One year later, trying to get into the product business, the firm initiated a project called SH aimed at creating an inter-personal communication platform over the Internet. The firm tried, but failed, to raise money for SH, and the project was discontinued. In early 2000, the firm got a contract to develop mobile phones services (using the WAP standard) for a large mobile operator, entering the mobile market for the first time. Based on this experience, and switching back to the product business, it developed a WAP simulator that was sold, with moderate success, to a few wireless operators. In 2002, this simulator was extended and transformed into a browser, and ported to personal digital assistants (PDA). The idea was to use it to develop mobile access to corporate information systems. WSoft would work with IT services providers who would sell the product to their corporate clients. However, the market didn't take off in large part due to the lack of maturity of mobile communication at that time, and the firm exited the market. The browser was then adapted to fit onto a mobile phone, to target mobile phone manufacturers. Despite a certain interest, the product was sold only to a few clients. In 2003, however, upon a client's suggestion, it was further transformed into a user interface engine. Introduced in 2004, the new product has since been successful, and the firm is now growing profitably. During the period, the firm's products and markets changed radically several times, yet the firm remained the same in the sense that its key constituents didn't change. Its business model, defined as the development and licensing of software, didn't change. Its management and shareholding didn't change. Its employee turnover was nil, and the firm didn't experience any traumatic change or crisis. Neither markets nor products, therefore, can adequately define the "stable concept" of the firm, which must therefore be defined differently.

The evolution of products and markets was not random

Was there, indeed, a stable concept besides the structure itself? The firm's trajectory might have been purely chaotic, with markets and products being randomly tested out. Upon close examination, however, the changes in markets and products clearly did not appear as random; the evolution of the firm followed an iterative, path-dependent process contingent upon (1) relationships forged by the firm, (2) the firm's product development thrust, and (3) its environment. For instance, the WAP contract was obtained for reasons falling into these categories. Firstly, the management happened to know the client's project manager. Secondly, the firm could claim it did know the technology, which it had learned with the SH project which played the role of complementary asset [START_REF] Tripsas | Unraveling the process of creative destruction: complementary assets and incumbent survival in the typesetter industry[END_REF] (i.e. an asset whose development had not been considered a priority but whose importance suddenly rose upon a change in the firm's context) and such competences were scarce at the time, and larger, established service providers were too slow to react. Thirdly, the client was in a hurry. The importance of the firm's product development thrust is exemplified by how the market for the user interface engine came about. The firm had been trying to sell its mobile phone browser with limited success for some time when a client suggested that the technology could be adapted to create a user interface (UI) engine. User interface had become a major headache for manufacturers as phones were becoming more and more sophisticated, and such engine could well provide a solution to this growing issue. Within six months, WSoft was able to transform the browser to that effect and begin addressing this opportunity with the resulting new product. More generally, our analysis of the firm's chronology of decision-events shows that each product/service and market iteration was possible because it was in some way connected to the existing trajectory, thus allowing a trajectory of cumulative learning. A simplified, high-level version of the chronology of action-events is shown in Figure 1. The firm started out as a provider of development services. It first focused on Web applications, then moved into the related field of mobile Web applications. It then used its newly acquired knowledge to create a mobile Web browser using the Java technology. This product was adapted for several platforms ending on a mobile phone. Another iteration transformed it into a UI engine, and a final iteration consisted into rewriting it from the Java programming language into C.

Figure 1. Chronology of Decision-events

The fact that (1) neither products nor markets can adequately define the 'stable concept' of the firm; and (2) the evolution of the firm's products and markets was not random, but driven by an iterative path-dependent learning process, this suggests that the firm defined its stable concept as a generic process of product and market exploration. This begs the following question to be asked: how does product design theory relate to this proposition?

Market exploration and product development based on lineage management

Complex software such as that for mobile phones takes months, sometimes years to develop. How was the firm able to drive this development while not knowing in advance which markets it would target? We observed that the last product the firm developed from scratch was the WAP simulator introduced in 2000, seven years ago. All of the firm's subsequent products derived from it, as illustrated in Figure 2.

Figure 2. Succession of products

While today's user interface engine for mobile phones (Product KR) and the initial simulator (WPW) have nothing in common, we created a CK [START_REF] Hatchuel | C-K Theory: Notions and applications of a unified design theory[END_REF] reconstitution of the design history which shows that the evolution followed an iterative, path-dependent process, from one product concept to another, always with a strong common technological base between the two. An extract of this reconstitution, describing the transition from WPM to KR (step 3 to 4), is shown in Figure 3. As Figure 3 shows, there is a constant exchange from C to K and K to C back and forth. Different types of knowledge are used (return from experience, market related and technical/design expertise) to every single logical component of the initial product, thus allowing a continuous progress from one product concept to another. We start from the concept: "A UI engine for mobile phones" (other approaches are possible to address the UI need), try a solution along one branch, backtrack, explore another branch, etc. until we have completely defined what "A UI engine for mobile phones" means in terms of knowledge: a functional layer using parameters and developed in the C language. In between, a series of intermediary products have been tried out, including the product WPM. KR, the end-product, is the result of this exploration. This sequence of products constitutes a lineage [START_REF] Hatchuel | Design-oriented Organizations: Towards a unified theory of design activities[END_REF], i.e. an evolving asset that provides both continuity and variation to the design process. Lineage management helped the firm drive the exploration in technology and customer needs on one side, and organize value-creating reuse of acquired knowledge on the other side. The trajectory of domain-knowledge explored by the firm, from its inception to the existing product-market, is depicted in Figure 4. Four main domains are distinguished: platform (for which WSoft creates products), customer, Field (the actual business problem solved by WSoft's product or service), and Technology (that needs to be mastered to address the business problem). Mapping the evolution of products with these domains shows the continuity of the process and the gradual stabilization of the domains, which stop changing. The first to be set is the platform, which is defined when WSoft focuses on embedded systems. Then comes the type of customer (mobile phones manufacturers), then the field (User Interface) and finally the technology, when the product is rewritten from the Java to the C language. When an opportunity was identified, instead of creating a product from scratch, which would have taken too long and de facto exclude the firm from this opportunity, the current product went through another iteration, leveraging hitherto accumulated knowledge. The technological solution embodied the learning and allowed the business concept to function.

DISCUSSION

The purpose of this study was to understand how high-tech firms deal with a situation of true, Knightian uncertainty about their potential products and markets. We studied a wireless software firm that was successful in its attempt. The study highlighted three factors that we believe contributed to this success. Firstly, the firm did not define its stable concept in terms of products and markets. Secondly, this did not mean that the evolution of products and markets was random, but rather path-dependent based on cumulative learning, the result of an organized exploration. Thirdly, the exploration of markets and the products development were based on lineage management, reflecting the strategic importance of product design in the firm's strategy.

Lineage-based strategy for startup development

Product innovation has long been shown to drive learning about customers and technologies [START_REF] Maidique | The New Product Learning Cycle[END_REF]. Underpinning the firm's concept is a product design strategy relying on a generic technical solution evolving as a lineage that provides both continuity and variation to the design process. The continuity reflects the cumulative learning of previous iterations, while the variation allows local explorations. It was not, however, just a case of technology-push development process as is so common in technology ventures. The CK reconstitution of design and market trajectory shows a simultaneous process of products and markets design, in effect a combination of both technology push and market pull [START_REF] Berthon | To Serve or to Create: Strategic Orientations Towards Customers and Innovation[END_REF]. The design process allowed the creation of products that supported market exploration, but market exploration fed the design process in return, until both converged to an opportunity. This ability to adapt to the evolution of the target market differs from similar concepts such as mass customization, a concept introduced by [START_REF] Davis | Future Perfect[END_REF]. Mass customization is an effort to resolve the paradox of combining the ability to adapt a product to each customer while maintaining economies of scale. Mass customization differs at least in two ways with lineage management. Firstly, mass customization is effective in a clearly defined market where the customization accounts for minor differences among customers within an existing segment, such differences being predictable. Put another way, the variation occurs within a finite space defined by a small number of parameters, and all possible variations are perfectly known in advance. Secondly, mass customization is static: instances of the product are customized, but the product itself doesn't change: this is a condition to reap economies of scale. Lineage, on the contrary, involves profound changes in the product itself to allow addressing new and possibly unexpected market needs. Lineage management also differs from the platform approach. [START_REF] Gawer | Platform Leadership[END_REF] define a platform as "an evolving system made of independent pieces that can each be innovated upon" [START_REF] Gawer | Platform Leadership[END_REF]. By organizing the ex-ante sharing of given architecture and/or common elements of product or process, the platform embodies the firm's existing knowledge; what is expected from a derivative project is not new knowledge, but economies of scope. A platform is static by definition: while its constituting elements might evolve. The platform approach is therefore not a strategy for exploration learning, but an efficient one for exploitation learning. By defining the space of concepts within which the market exploration can take place, the iterative, cumulative, and path-dependent lineage-based product design process actually limits the opportunities that the firm can pursue: what can be achieved in terms of new product from an existing technology base with one iteration is necessary limited. While this might seem to be an inconvenience, [START_REF] Moore | Crossing the chasm[END_REF] showed the benefit of focus in uncertain environments. It limits unnecessary variation and avoids a random walk of products and markets. It supports the effectual approach of development by providing a product basis upon which the firm can base its 'next' effectual iteration.

Robust design through second order competencies

As noted by [START_REF] Teece | Dynamic capabilities and strategic management[END_REF], deciding, under significant uncertainty about the future states of the world, which paths to commit to and when to change paths is the central strategic problem confronting the firm. While technological and customer competences can be thought of as first-order competences, the ability to identify, evaluate, and incorporate new technological and/or customer competences into the firm (i.e. a competence at explorative learning and design by exploring and designing new markets or new technology can be thought of as a second order competence [START_REF] Collis | Research Note: How valuable are organizational capabilities?[END_REF]). Second order competences enable a company to renew itself through building new first-order competences related to technologies and markets [START_REF] Danneels | The Dynamics of Product Innovation and Firm Competences[END_REF]. They provide flexibility and allow cumulative learning. The definition of the stable business concept in terms of second order competences allows to simultaneously explore markets to create value with the generic technical solution, and learn from the exploration in terms of markets and products. It forms the basis of a "robust design" [START_REF] Hargadon | When innovations meet institutions: Edison and the design of the electric light[END_REF] of the firm, defined as the ability to simultaneously make advances while preserving the flexibility to respond to new situations.

Surviving Knightian uncertainty

This 'robust' approach allows the firm to solve the problem of Knightian uncertainty because it combines exploration and exploitation [START_REF] March | Exploration and Exploitation in Organizational Learning[END_REF]. The general model of strategy process is that of an initial period of exploration, often before the firm's creation, which ends up when an opportunity is identified, after which the firm turns to exploitation based on previously capitalized knowledge on products and markets. Firms such as Netscape [START_REF] Cusumano | Competing on Internet Time: Lessons from Netscape and its Battle with Microsoft[END_REF] or Google [START_REF] Battelle | The Search -How Google and its Rivals Rewrote the Rules of Business and Transformed our Culture, Portfolio[END_REF] are typical of such approach. In our case, however, explorative learning is not limited to the initial phase, but constant in the development of the firm. This learning process is structured in terms of reuse and leveraging of existing knowledge and competencies into revenue generating products on the one hand, and prudential exploration of new markets opportunities and technology development on the other. Knightian contexts are characterized by complete uncertainty about products and markets. In those contexts, the role of exploration is crucial. Developing customer and technology competences, and then exploiting them, is not enough. Envisioned markets might not come about. New products might fail. New opportunities might emerge unexpectedly that could alter the productmarket pair of the firm. Exploration results in greater ability to adapt to changes, and thus supports future viability [START_REF] Danneels | The Dynamics of Product Innovation and Firm Competences[END_REF]. While exploitation provides vital short-term resources, exploration enhances the adaptation of the organization to a changing environment because it increases the variance of organizational activities [START_REF] Mcgrath | Exploratory learning, innovative capacity, and management oversight[END_REF]. This suggests that the firm should maintain equilibrium in terms of exploration and exploitation, despite likely institutional pressures from shareholders that call for a switch to exploitation. More generally, the study suggests that Knightian uncertainty is resolved by the definition of the stable business concept of the firm in terms of product and market exploration. The concept allows an iterative process of technology push and market pull, driven by a deliberate strategy of learning.

Managerial implications

The practical implication of this paper is to suggest that high-technology firm management might go beyond two traditional oppositions. Firstly, between deliberate and emergent strategies [START_REF] Mintzberg | Of strategies, deliberate and emergent[END_REF], and secondly, between exploitation and exploration [START_REF] March | Exploration and Exploitation in Organizational Learning[END_REF]. A deliberate strategy of learning and lineage management is combined with an emergent strategy of market development: markets cannot be anticipated, but they can be created by actively driving a learning and technology development strategy. The articulation between the deliberate and emergent paths is provided by the product design strategy based on lineages. Exploration does not stop at an early stage but is pursued and combined with the exploitation of current products and markets. Rather than markets and products, the overall firm's strategy consists in defining a market and technology exploration process to resolve Knightian uncertainty.

CONCLUSION

Firms in disruptive environment have to deal with a situation of true uncertainty about their potential markets and technologies. We asked how firms can create products when the corresponding market does not exist. This case study suggests that instead of defining the firm in terms of markets or products, a definition as a generic process of learning and organizational development constitutes a robust approach that allows the firm to resolve the uncertainty by simultaneously developing its products and exploring markets, while preserving the flexibility to respond to new situations. This paper results from a research initiated two years ago on the development mode of high-tech startups. Insight on the relation between the design process and the market exploration has already been gained. While our findings are largely consistent with the recent literature on Product Development and Entrepreneurship, they must be confronted to a larger sample of firms in order to mitigate the issue of contingency. The idea of a second order stable concept was not deliberately defined by WSoft but rather emerged empirically from an initial lack of market and technology success. We think entrepreneurial firms would benefit from having a normative framework from the outset to increase their effectual performance. Future research will aim at creating such framework.

Figure 3 .

 3 Figure 3. Reconstitution of Product Design history, from product concept WPM to product concept KR

Figure 4 .

 4 Figure 4. Domain-knowledge explored by the firm

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