Studies on Phase Separation in a-PMMA/PEG Gels

Stereo-irregular atactic poly(methyl methacrylate) ($a$-PMMA) is known incapable of forming gels in common solvents, irrespective of the solvent quality. However, we recently found a rigid opaque thermal-reversible $a$-PMMA gel in the solvent of the polyethyl glycol oligomer (PEG) (the PEG molecule mass differ from 400 to 4000 were used). FT-IR, dynamic mechanical temperature analysis and Solid state NMR measurements were used to study the gel properties and gelation mechanism. The \textit{in situ }IR studies in $a$-PMMA/PEG gel suggested that some $a$-PMMA segments were in the aggregated state in solution, which became a node in the solution. With decreasing temperature, the fraction of aggregated $a$-PMMA in solution increases, resulting in the formation of physical network finally. Spin diffusion was used to determine the size (\textit{$\xi $}) of domains in the gels. We found that, $a$-PMMA/PEG4000 was miscible (\textit{$\xi $ }$\sim $ 9nm), while $a$-PMMA/PEG1000 was micro phase separated (\textit{$\xi $ }$\sim $ 57nm) and $a$-PMMA/PEG400 was macro phase separated (\textit{$\xi $} $>$ 300nm). The $a$-PMMA self-aggregation was attributed to the depletion interaction that becomes important in the case of middle-sized solvents.