Concentration dependence of the static properties of randomly branched poly(methyl methacrylate) studied by static-light scattering

We study the static properties of solutions of randomly branched poly(methyl methacrylate) (PMMA) obtained near the gelation threshold by measuring their structure factor using static-light scattering experiments. The picture of the initial (not yet diluted) system is consistent with a hierarchical interpenetration of the aggregates, as predicted by the percolation theory. The dilution creates in the system a characteristic length dependent on concentration, beyond which excluded-volume interactions are screened. This length is proportional to the apparent radius of gyration Rga. The concentration dependences of Rga and the apparent molar mass Ma associated with Rga are determined by a Guinier analysis of the light scattering data. The self-similarity of the size distribution of the aggregates is clearly shown by the master curve representing the static structure factor of the system $S_z(q)$ as a function of $q\cdot R_{\rm ga}$.

Characterization of the structure and the size distribution of branched polymers formed by co-polymerization of MMA and EGDMA

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl methacrylate (EGDMA) in solution leads to the formation of polydisperse branched PMMA which grows in size until the system gels. The structure and the size distribution of the PMMA aggregates were characterized at infinite dilution using static and dynamic light scattering and size exclusion chromatography (SEC). The reaction extent was measured using SEC and Raman spectroscopy. The results show that the structure and size distribution of PMMA aggregates formed close to the gel point are compatible with those of percolating clusters. The structure factor of semi-dilute solutions of PMMA aggregates is the same as that of dilute solutions at distance scales much smaller than the correlation length of the concentration fluctuations (ξ). However, the cut-off function of the pair correlation function at ξ for semi-dilute solutions is more gradual than the cut-off function at Rgz for dilute solutions

Dynamic and static light scattering study of the formation of cross-linked PMMA gels

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl metacrylate (EGDMA) was investigated in solution at different molar ratios R = [EGDMA]/[MMA] between 0 and 0.05. Initially mainly linear PMMA was formed with weight average molar mass 7.5x104 g/mol independent of R. At larger reaction extents branched polymers were formed and the systems gelled. The scattering intensity rose initially with the reaction extent, but reached a plateau value at larger reaction extents. The plateau value increased strongly with R. Dynamic light scattering showed the appearance of a slow relaxation not observed in linear PMMA solutions. The data can be interpreted by assuming that the excess scattering originates from the branching points and relaxes through self diffusion of the branched particles. The results agree with predictions of the percolation model for gelation and Rouse dynamics. Viscosity measurements corroborate this interpretation. Measurements on a progressively diluted sample quenched close to the gel point again showed quantitative agreement with the percolation model for gelation

Characterization of Randomly Branched Polymers Formed by End-Linking Linear Polystyrene Using Controlled Free Radical Polymerization

Randomly branched polystyrene (PS) was synthesized by end-linking monodisperse linear PS chains with divinylbenzene using controlled free radical polymerization. Samples obtained at different reaction extents were characterized using size exclusion chromatography and static and dynamic light scattering. The results are compared with those obtained on randomly branched poly(methyl methacrylate) (PMMA). The overal structure of branched PS was found to be the same as that of branched PMMA despite the very different local structure. The results strongly support the idea that the formation of polymeric gels is a universal process that may be described by the percolation model