Spatial Inhomogeneities of Polystrene Gels Prepared from Semidilute Solutions

The spatial inhomogeneity and the chain dynamics of polystyrene gels prepared by random cross-linking of functionalized polystyrene in semidilute toluene solution were investigated by static and dynamic light scattering. From polymer concentrations as low as 0.02 g/mL, continuous macroscopic gels were obtained, while at 0.01 g/mL, only microgels formed. In the concentration range 0.02−0.05 g/mL, the static correlation length, deduced from the angular dependence of the excess scattering intensity by the Debye−Bueche method, increases from ∼10 to 50 nm with rising concentration, while concurrently the root-mean-square concentration fluctuation decreases markedly. The spatial concentration fluctuation approaches the mean concentration at the gelation threshold, whereas at 0.05 g/mL, it amounts to only 10% of the mean. A quantitative comparison with some literature data shows that random cross-linking leads to less heterogeneity than cross-linking copolymerization, when gel formation takes place at the same concentration. The dynamically fluctuating (liquidlike) scattering component, obtained from dynamic light scattering, equals that of a corresponding polymer solution only at concentrations well above the gelation threshold. At the lowest polymer concentration where a continuous gel was obtained, it is appreciably higher than that of a solution. Presumably, this difference is caused by the sol fraction consisting of highly branched structures or cross-linked clusters, which contribute substantially to the scattering just above the gelation threshold.