Dynamic light scattering from polymer gels: spring-rotor model

Dynamic light scattering behaviour in the form of periodic oscillating correlation functions has been found from measurements on both physically and chemically crosslinked hydrogels. The former were aqueous methyl cellulose at the thermal gelation temperature and the latter were poly(N,N‐dimethylacrylamide‐co‐methyl methacrylate‐co‐ethylene dimethacrylate) at swelling equilibrium in water. In order to explain the oscillating behaviour, a spring‐rotor model is proposed in which the molecular motions inside a gel are modelled as vibrations of springs having various frequencies. These frequencies are equivalent to the rotational frequencies when the free rotor theory was used to process the oscillating correlation functions by a modified CONTIN computer program. The validity of this model is supported by experimental data in three ways. (1) The model fits the experimental data almost perfectly. (2) The main peak positions of the obtained frequency distribution are not affected by the scattering angle. (3) For the chemically crosslinked hydrogels differing only in content of ethylene dimethacrylate, the mean vibrational frequency of the gel spring is higher the shorter the spring, i.e. the lower the average molecular mass between crosslinks. © 1998 SCI