DYNAMIC PROPERTIES OF POLYACRYLAMIDE GELS AND SOLUTIONS

Decay time distributions have been obtained by Laplace inversion of the autocorrelation functions in dynamic light scattering for polyacrylamide (PAA) solutions and gels prepared in an identical manner except for the addition of crosslinking agent. The measurements were made as a function of PAA concentration, crosslinking concentration and temperature. The purpose was to establish the similarities and differences between the gel and the analogous solution and also to examine the connection between these variables and the dynamic behaviour of the gel. While the solutions have a comparatively complicated dynamic structure, the gels are precisely single-exponential at PAA concentrations above about 2%. Over the same concentration range, the solutions have a slowly relaxing component of narrow distribution in addition to the network mode formed through interchain entanglements. The slow mode, which has a strong negative concentration dependence, finally disappears at CPAA ≈ 10%. The dynamic polymer-polymer correlation length (ξH) for the permanent gel, which is considerably larger than that for the corresponding solution, is weakly dependent on concentration (ξH ∼ C-0.22 for both gels and semidilute solutions) and is independent of the temperature. ξH increases with increasing concentration of bisacrylamide Cbis, reaching a plateau in the vicinity of Cbis = 5%. The static screening length takes the same value in solutions and gels and is dependent on concentration: ξs ∼ C-0.65. The two correlation lengths become approximately equal at high crosslinking density. The longitudinal elastic modulus (Mos), evaluated from static light-scattering data, shows the theoretically expected dependence on PAA concentration. © 1990.