Relaxation in poly(alkyl methacrylate)s: Change of intermolecular coupling with molecular structure, tacticity, molecular weight, copolymerization, crosslinking, and nanoconfinement

The voluminous amount of data in the literature on the structural α- and the Johari-Goldstein β-relaxations of the poly(n-alkyl methacrylate)s allows a systematic study of the interrelation between the two important relaxation processes. The data bring out the systematic changes in the interrelation between the structural α- and the Johari-Goldstein β-relaxations with changes in molecular structure, molecular weight, tacticity and size (by nanoconfinement), and modifications by copolymerization, and crosslinking. The results can all be interpreted as primarily due to changes in intermolecular coupling, which have significant effects on the many-molecule dynamics constituting the structural α-relaxation, but not on the precursory Johari-Goldstein β-relaxation. Theoretically, the Coupling Model predicts a relation of intermolecular coupling (or degree of cooperativity of the α-relaxation) to the ratio of the α- and the β-relaxation times, and a correlation of intermolecular coupling to the steepness or “fragility” index. The predicted relation and correlation are compared with experimental data of the poly(alkyl methacrylate)s.