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.

Light and small angle neutron scattering results are presented for a series of poly(dimethyl siloxane) (PDMS) gels, cured in the dry state by endlinking with a trifunctional crosslinking agent, ethyl triacetoxy silane (ETAS). For gels swollen in octane, the scattered light and the smallest angle neutron response are dominated by scattering from aggregates of polymerized ETAS, the mean radius of which was found in this system to be ∼800 Å, and which have a smooth surface in the length scale between 500 and 100 Å. The internal surface area of this precipitate was roughly 200 cm2 per cm3 of the swollen gel. Only about one third of the free ETAS participates in this condensed phase. The rest is disseminated as oligomers or monomers throughout the gel. The gel itself appears as a solution-like structure with in addition permanent waves in the polymer concentration distribution. In this system, the permanent waves could be modeled by a Gaussian function of characteristic width ΔR=80 Å and relative amplitude 〈Δc2〉1/2/〈c〉≊0.1. The polymer–polymer correlation length ξ describing the shorter range fluctuations is substantially modified in comparison with an unperturbed polymer solution at the same overall concentration. For the uncrosslinked solution at volume fraction φ=0.16 it is found that ξ=10.3 Å, while for the crosslinked gel swollen to the same degree, ξ=18 Å.

Combined measurements are described involving elastic and quasi-elastic neutron scattering, quasi-elastic light scattering, nuclear magnetic resonance, and swelling pressure on an end-linked poly-(dimethylsiloxane) (PDMS) gel swollen to equilibrium in a good solvent (toluene) and the equivalent solution. The factors affecting the collective diffusion coefficient are considered. The swelling pressure measurements show that the osmotic modulus is appreciably depressed in the gel. The neutron spin-echo measurements reveal no difference in the dynamic response at intermediate and high values of the scattering vector Q. Elastic neutron scattering at small Q detects nonuniformities in the network structure, which are absent from the solution. These nonuniformities play a major role in the dynamic response of the system at lower Q and appear to be the cause of the observed reduction in osmotic pressure. The NMR measurements show a small increase of the solvent mobility in the gel, which is consistent with the appearance of structural nonuniformities in the system.

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Structural properties of rabbit skeletal myosin head (S1) and the influence of the DTNB light chain (LC2) on the size and shape of myosin heads in solution were investigated by small angle x-ray scattering. The LC2 deficient myosin head, S1 (-LC2), and the S1 containing LC2 light chain, S1 (+LC2) were studied in parallel. The respective values of the radius of gyration were found to be (40.2 +/- 0.5) A and (46.7 +/- 1) A, while the maximum dimension was (190 +/- 15) A for both species. The large difference between the two Rg values suggest that LC2 is located close to one extremity of the myosin head, in agreement with most electron microscopy observations. All models derived from the x-ray scattering pattern of the native myosin head share a common overall morphology, showing two main regions, an asymmetric globular portion which tapers smoothly into a thinner domain of roughly equivalent length making an angle of approximately 60 degrees, with a contour length of approximately 210 A.

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