Jean-François Tassin

Development of molecular orientation in sequentially drawn PET films

A study has been made of the transverse stretching of poly(ethylene terephthalate) film previously drawn longitudinally at constant force and width. The crystalline phase has been characterized using wide-angle X-ray diffraction: the orientation of the chain axis and the normal to the phenylene ring have been determined, as well as the crystallite size. Infra-red spectroscopy and refractive index measurements have been used to characterize the orientation of the material averaged over the crystalline and amorphous phases. The results showed that the crystals, which were preferentially aligned with their chain axes along the first stretching (longitudinal) direction, break down during the second drawing and that recrystallization takes place to produce crystallites with their c-axes aligned along the transverse stretching direction. The observed increase in the tendency for the normal to the phenylene ring to be normal to the film plane correlates with the increase in crystallite size along the chain axis for crystallites oriented with their chain axes in the transverse direction.

Gelation studies of a cellulose-based biohydrogel: The influence of pH, temperature and sterilization

The present paper investigates the rheological properties of silated hydroxypropylmethylcellulose (Si-HPMC) biohydrogel used for biomaterials and tissue engineering applications. The general property of this modified cellulose ether is the occurrence of self-hardening due to silanol condensation subsequent to a decrease in pH (from 12.4 to nearly 7.4). The behavior of unsterilized and sterilized Si-HPMC solutions in diluted and concentrated domains is first described and compared. In addition, the influence of physiological parameters such as pH and temperature on the rate of the gelation process is studied. In dilute solution, the intrinsic viscosity ([eta]) of different pre-steam sterilization Si-HPMC solutions indicates that macromolecular chains occupy a larger hydrodynamic volume than the post-steam sterilization Si-HPMC solutions. Although the unsterilized Si-HPMC solutions demonstrate no detectable influence of pH upon the rheological behavior, a decrease in the limiting viscosities (eta(0)) of solutions with increasing pH is observed following steam sterilization. This effect can be explained by the formation of intra- and intermolecular associations during the sterilization stage originating from the temperature-induced phase separation. The formation of Si-HPMC hydrogels from injectable aqueous solution is studied after neutralization by different acid buffers leading to various final pHs. Gelation time (t(gel)) decreases when pH increases (t(gel) varies from 872 to 11s at pH 7.4 and 11.8, respectively). The same effect is observed by increasing the temperature from 20 to 45 degrees C. This is a consequence of the synergistic effect of the increased reaction rate and acid buffer diffusion. pH and temperature are important parameters in the gelation process and their influence is a key factor in controlling gelation time. By adapting the gel parameters one could propose hydrogels with cross-linking properties adapted to clinical applications by controlling the amount of pH of neutralization and temperature.

Uniaxial planar deformation of PET films: 2. Characterization of the orientation from measurements of refractive indices

Abstract Refractive indices along the three principal directions of uniaxial planar deformed poly(ethylene terephthalate) (PET) films have been used to calculate the second moments of the orientation distribution of the phenyl rings normal with respect to the three principal directions of the sample. Under our stretching conditions, the phenyl rings show a strong tendency to orient their plane parallel to the plane of the films. The orientation of the chain axis can also be evaluated from the same data. A good correlation is observed between this indirect measurement and direct, but more tedious, techniques such as infra-red dichroism.

Study of the molecular structure of PET films obtained by an inverse stretching process Part 2: crystalline reorganization during longitudinal drawing

The transverse stretching, under a constant drawing force, of a one-way drawn poly(ethylene terephtalate) film has been studied. This type of stretching leads to an equilibrium deformation under load which increases mainly with the applied stress and to a lesser degree with temperature. The crystalline phase of such samples has been characterized using wide-angle X-ray diffraction. The orientation of the chain axis and the normal to the phenylene ring have been analysed as well as the crystalline size, as a function of the draw ratio and the stretching temperature. The crystallinity is constant during stretching. The orientation distribution of the chain axes as well as the length of the crystals shows that the crystals which were preferentially oriented with their c-axes along the first stretching direction rotate rapidly towards the actual draw direction and grow up in length as the stretching proceeds. At the same time, the length of the crystals aligned along the other direction of the film plane decreases rapidly, indicating a break-down of some of these crystals. The orientation in the crystalline phase is uniquely controlled by the draw ratio. No relaxation with temperature is observed. This stretching method is compared with the constant speed drawing of one-way drawn films, where no rotation of large crystallites can be detected.

Molecular structure and gross melt fracture triggering

We report in this paper rheological and rheo-optical experiments on a linear and a star-branched statistical copolymer of styrene butadiene rubber (SBR). The aim of this work is to study the influence of molecular structure on gross melt fracture defect. First, the rheological behavior in shear and in elongation of both materials are investigated. Then, the experimental results obtained by various rheological techniques enable us to compare the level of apparent shear rates and elongational stresses applied to each polymer at the onset of flow instabilities. The critical apparent shear rates and elongational stresses are found to be higher for the star-branched SBR copolymer. Moreover, the shear behavior of solutions of SBR copolymers in toluene is investigated using classical and optical rheometry. These results allow us to compute their stress optical coefficients. Birefringence experiments performed with a specific two-dimensional slit die corroborate axisymmetric observations. The level of the first normal stress difference along the flow axis is higher for the star-branched SBR copolymer than for the linear one before gross melt fracture occurrence. Thus, all experiments illustrate the dependence of gross melt fracture defect on molecular structure.

The stability mechanisms of an injectable calcium phosphate ceramic suspension

Calcium phosphate ceramics are widely used as bone substitutes in dentistry and orthopedic applications. For minimally invasive surgery an injectable calcium phosphate ceramic suspension (ICPCS) was developed. It consists in a biopolymer (hydroxypropylmethylcellulose: HPMC) as matrix and bioactive calcium phosphate ceramics (biphasic calcium phosphate: BCP) as fillers. The stability of the suspension is essential to this generation of “ready to use” injectable biomaterial. But, during storage, the particles settle down. The engineering sciences have long been interested in models describing the settling (or sedimentation) of particles in viscous fluids. Our work is dedicated to the comprehension of the effect of the formulation on the stability of calcium phosphate suspension before and after steam sterilization. The rheological characterization revealed the macromolecular behavior of the suspending medium. The investigations of settling kinetics showed the influence of the BCP particle size and the HPMC concentration on the settling velocity and sediment compactness before and after sterilization. To decrease the sedimentation process, the granule size has to be smaller and the polymer concentration has to increase. A much lower sedimentation velocity, as compared to Stokes law, is observed and interpreted in terms of interactions between the polymer network in solution and the particles. This experimentation highlights the granules spacer property of hydrophilic macromolecules that is a key issue for interconnection control, one of the better ways to improve osteoconduction and bioactivity.

Association of telechelic ionomers in apolar solvents

The associative properties of well-defined halatotelechelic ionomers in an apolar solvent were studied by means of dynamic mechanical spectroscopy, small-angle X-ray scattering, as well as static and dynamic light scattering. The effect of end-functionalisation was investigated by comparing results on polystyrene (PS) and polyisoprene (PI) precursors using PS and PI of the same molar mass modified at one or both ends with sulfonate groups. The molar-mass dependence was investigated by comparing difunctionalised samples with different molar masses and the influence of the backbone by comparing polystyrene and polyisoprene. Experimental data suggest a relatively simple model for the association of this material. At low concentrations, the polymer chains organise themselves into well-defined reversed micelles, so-called flowers. Above a certain concentration C+, the flowers associate into larger aggregates according to an open association mechanism. At a critical concentration Cgel, the aggregates fill the whole space and a transient network is formed most likely by a percolation process. A semi-quantitative description of the associating process can be obtained by considering an open association mechanism properly combined with inter-particle interactions.

Amorphous phase orientation in biaxially drawn poly(ethylene terephthalate) films

An experimental study of the development of molecular orientation in the amorphous phase during the transverse drawing of one-way drawn poly(ethylene terephthalate) films has been carried out. Both the behaviours of phenyl ring normals or chain axes and of fluorescent rigid probes have been considered. The influence of the size of the crystallites in the one-way drawn films has been studied. It is first shown that the phenyl ring normals in the amorphous phase tend to become more perpendicular to the draw direction, as expected. However, there is no marked tendency for the rings to lie in the plane of the film, on the contrary with the crystalline phase. The chain axes, as well as the fluorescent probes, tend to align along the second draw direction. In films possessing initially long crystals, this reorientation appears less efficient at equivalent draw ratios. A memory of the state of the amorphous orientation in the one-way drawn films is observed in the sense that more relaxed one-way drawn films will lead to broader distributions in the final biaxially stretched films.

Molecular description of constant-load stretching of amorphous poly(ethylene terephthalate) films

Abstract This paper is devoted to a molecular description of the deformation of amorphous isotropic poly(ethylene terephthalate) (PET) films at temperatures slightly above the glass transition temperature under constant load. The deformation is qualitatively described by chain relaxation phenomena occurring before stress-induced crystallization, which are followed by the equilibration of a rubber-like network. The junction points include both trapped entanglements and crystalline units. The structure of this network is characterized by the number of segments between crosslinks. This parameter is calculated by comparing the predictions of the rubber elasticity theory (without Gaussian approximation) with the experimentally observed draw ratios under given conditions of temperatures and loads. It is shown that light loads induce soft networks leading to high draw ratios. The predictions of the molecular orientation derived from this treatment are in good agreement with birefringence data on a large variety of samples.

The influence of initial morphology of one-way drawn poly(ethylene terephthalate) films on their transverse drawing

Abstract A study of the development of molecular orientation and crystalline structure during the transverse drawing of one-way drawn poly(ethylene terephthalate) films has been carried out. The influence of the size of the crystallites in the one-way drawn film and the influence of the transverse drawing temperature have been studied. It is shown that the deformation mechanisms appear to depend on the size of the crystals initially present. Films containing small crystals lead to a higher orientation and larger crystalline structures oriented along the transverse direction. The influence of the second drawing temperature can be understood as a result of the importance of molecular relaxation and crystallization, both of which are enhanced by elevated temperatures.