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

Abstract

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 7.5×10 4 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.PACS. 36.20.-r Macromolecules and polymer molecules -61.10.-i X-ray diffraction and scattering -82.70.Gg Gels and sols

Dynamic Light-Scattering Study of Aggregating and Gelling Colloidal Disks

“…DLS experiments show a remarkably similar behavior of g 2 (t) for very different gel-forming systems such as silica (8,9), Laponite (10), polyurethane (11), polysaccharides (12), PMMA (13), and globular proteins (14). Usually, a fast decay is observed followed by a stretched exponential decay sometimes preceded by power law decay.…”

“…The moment when this occurs is highly system-dependent as is the fraction of trapped concentration fluctuations. DLS measurements of both cross-linking silica spheres (8) and flexible PMMA chains (13) showed that the relaxation of concentration fluctuations close to the gel point could be well described in terms of this model.…”

“…However, if qξ 1 interpretation in terms of internal modes of the individual fractal aggregates may be more appropriate (15). The slow decay has been attributed to self-diffusion of aggregates in a complex medium (8,13), to relaxation of the elastic modulus [9], or in terms of the mode-coupling theory developed for relaxation in glasses (10). In most interpretations the terminal relaxation time is expected to diverge at the gel point which may be defined as the moment when a macroscopic system-spanning cluster is formed.…”

Association of Telechelic Ionomers in Apolar Solvents

“…An interesting parallel with chemical gels can be drawn. In the latter case, slow modes due to structural inhomogeneities have a q-dependence of their relaxation times which is described by high exponents (s s V qd with d = 3-4) [57] but in this case the diffusion of inhomogeneities on large length scales is inhibited by the presence of permanent crosslinks. Both for ionomers and covalent gels, the slow mode is due to relaxation of the structural inhomogeneities which take place by different mechanisms.…”

Light Scattering Study of Turbid Heat-Set Globular Protein Gels Using Cross-Correlation Dynamic Light Scattering

“…In all cases the correlation functions were characterized by a narrow relaxation time distribution. We calculated the apparent diffusion coefficient from the relaxation rate as D a = /q 2 and found that data at different concentrations superimpose if D a /D c is plotted as a function of q · ξ , see The correlation functions of the protein gels are typical for a wide range of gelling systems (21)(22)(23)(24)(25)(26)(27). Before the gel point the concentration fluctuations relax via cooperative diffusion and internal motion.…”