DYNAMIC HETEROGENEITY NEAR THE GLASS-TRANSITION

The basic principles of dynamic light scattering from viscoelastic flu ids are explained and illustrated with specific results obtained for b oth simple fluids and polymers. The origin and spectral form of the dy namic central peak due to slowly relaxing longitudinal density fluctua tions are discussed. The importance of measuring both the power spectr um and the time correlation function of the scattering is established. The contribution of optical anisotropy fluctuations associated with m olecular reorientation to the polarized spectrum is demonstrated for p olystyrene. The distribution of relaxation strengths for longitudinal density fluctuations was determined for poly(methyl methacrylate) and poly(n-butyl methacrylate) as a function of temperature near the glass transition. Dynamic heterogeneity associated with the chemical struct ure was demonstrated. A clearly resolved bimodal distribution was obta ined for the butyl sample. The butyl side chain served to isolate the ester side chain isomerization from the slower dynamics of the main ch ain motion.