Laser light scattering by polymer gels

Abstract

The frequency-integrated, absolute Rayleigh–Debye scattering measured as a function of scattering angle down to 1°, contains information about the supramolecular structural order exhibited by gels. For covalently crosslinked, swollen polymer networks this provides a measure for the degree of spatial non-randomness in crosslinking. The non-randomness index for a series of poly(2-hydroxyethyl methacrylate)(PHEMA) networks is found to vary systematically by a factor of over a hundred depending on the history of the network formation. Intensity-fluctuation spectroscopy at any given scattering angle provides a probe for the local viscoelasticity of a gel without applying an external driving force. Swollen PHEMA networks are found to exhibit a spectrum of long (10–3 s) relaxation times. In dilute aqueous agarose systems both above and below the sol→gel transition a similar spectrum of relaxation times is found. During the transition, the autocorrelation function exhibits oscillatory behaviour for several hours. This is attributed to the mass flow taking place during the microphase separation. Upon inducing the sol→gel transition in agarose solutions by quenching to various temperatures below the phase boundary, the frequency-integrated Rayleigh–Debye scattering is found to vary systematically a thousand fold. A Bragg maximum appears, caused by the regularly spaced spontaneous concentration fluctuations which occur in a nucleation-free, spinodal phase decomposition.