MOLECULAR WEIGHT(X10" =)Figure 7. Dependence of the percentage of the y spherulites in the crystalline regions on the molecular weight for PVF2 samples crystallized at 433 K for 24 h.longitudinal propagation is increased. On the other hand, the transformation is impeded by the interlamellar amorphous phase, which acts as a barrier to the transverse propagation. Both the percentage of the y spherulites in the crystalline regions and the content of the interlamellar amorphous phase increase with the head-to-head defect concentration and the molecular weight. As a result, the degree of the a -* y transformation appears to be a complicated function of both of these molecular parameters.

An investigation is described of a set of chemically cross-linked polyacrylamide hydrogeis prepared by copolymerization of acrylamide in presence of different amounts of bisacrylamide. These networks were studied by using osmotic and mechanical measurements, dynamic light scattering, and small-angle X-ray scattering (SAXS) techniques. The form of the concentration dependence of the swelling pressure was found to vary with the cross-linking density, water being a good solvent for the loosely cross-linked gels, while at higher bisacrylamide content the solvent power diminishes. The ratio of the longitudinal osmotic modulus obtained from mechanical and osmotic observations to that from dynamic light scattering w a~ found to increase with increasing cross-linking, in agreement with SAXS observations of the amplitude of the concentration fluctuations associated with the static heterogeneities in the sample. Furthermore, for all the gels studied, the values of the correlation lengths determined by three independent methods-SAXS, dynamic light scattering intensity, and collective diffusion coefficient measurements-were consistent with each other.

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Poly(dimethylsiloxane) (PDMS) networks swollen in toluene are investigated both by smallangle neutron scattering (SANS) and by swelling pressure measurements. The scattering signal from the gels, measured as a function of swelling, is resolved into two components, a solution-like part and a static part. The forward scattering intensity of the solution-like part contains all the thermodynamic information relevant to the gel. The absolute intensity is obtained by calibrating against that of a standard water sample. The Flory-Huggins theory of polymer solutions is adopted to derive the swelling pressure of the gel from the scattering measurements. In addition, the swelling pressure is determined independently at different degrees of dilution of the gels. A comparison is made between these direct macroscopic observations and the results deduced from the scattering intensity.

Carbon fibers, obtained by carbonizing poly(p-phenylene benzobisoxazole) (PBO) fibers at 900 °C, graphitize extensively upon heat treatment at higher temperatures (2700 °C). In this work, XRD, Raman spectroscopy, and HRTEM are used to monitor the structural and nanostructural transformations of the carbon material under heat-treatment at several temperatures in the interval 900–2800 °C. These different techniques provide complementary information, especially regarding the spatial resolution they achieve. They highlight a specific nonconventional mode of graphitization for this unexpectedly graphitizable precursor. The reliability in the determination of L a crystallite sizes from these three techniques is compared and discussed. The existence of four steps in the graphitization of PBO-derived carbon fibers is inferred.