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.

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.