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.

No abstract

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.

Hyaluronic acid ͑HA͒ is an anionic biopolymer that is almost ubiquitous in biological tissues. An attempt is made to determine the dominant features that account for both its abundance and its multifunctional role, and which set it apart from other types of biopolymers. A combination of osmotic and scattering techniques is employed to quantify its dynamic and static properties in near-physiological solution conditions, where it is exposed both to mono-and divalent counterions. An equation of state is derived for the osmotic pressure ⌸ in the semidilute concentration region, in terms of two variables, the polymer concentration c and the ionic strength J of the added salt, according to which ⌸ = 1.4ϫ 10 3 c 9/4 / J 3/4 kPa, where c and J are expressed in mole. Over the physiological ion concentration range, the effect of the sodium chloride and calcium chloride on the osmotic properties of HA solutions is fully accounted for by their contributions to the ionic strength. The absence of precipitation, even at high CaCl 2 concentrations, distinguishes this molecule from other biopolymers such as DNA. Dynamic light scattering measurements reveal that the collective diffusion coefficient in HA solutions exceeds that in aqueous solutions of typical neutral polymers by a factor of approximately 5. This property ensures rapid adjustment to, and recovery from, stress applied to HA-containing tissue. Small angle x-ray scattering measurements confirm the absence of appreciable structural reorganization over the observed length scale range 10-1000 Å, as a result of calcium-sodium ion exchange. The scattered intensity in the transfer momentum range q Ͼ 0.03 Å −1 varies as 1 / q, indicating that the HA chain segments in semidilute solutions are linear over an extended concentration range. The osmotic compression modulus c ‫ץ‬ ⌸ / ‫ץ‬c, a high value of which is a prerequisite in structural biopolymers, is several times greater than in typical neutral polymer solutions.