Peculiar α-β relaxations of Syndiotactic-Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) exhibits significantly tacticity-dependent properties, e.g. the glass transition temperature (Tg), the characteristic ratio (C infinity), the specific volume and the gas permeability coefficient. Syndiotactic (s) PMMA is the most peculiar, exhibiting complex alpha-beta relaxations. We first make brief reviews about key experimental facts of s-PMMA relaxations: 1) unusual alpha-beta split; 2) tacticity dependence of Tg; 3) discrepancy between chain stiffness and Tg. Based on the chain conformation analysis of Vacatello and Flory in 1986, and combined with the experimental results from dielectric spectroscopy (DS), dynamic-mechanical analysis (DMA), nuclear magnetic resonance (NMR), we have demonstrated that the molecular origin of the peculiar alpha-beta relaxations resulted from the all-trans extended conformations which are predominant in syndiotactic PMMA. This local extended (and curved) structure, as a bigger jumping unit, induces the formation of domains with slow dynamics. The percolation of slow domains causes the freezing of the large-amplitude motion of main chains, and then a novel alpha transformation occurs at a higher temperature eventually. The high-specific volume state below Tg provides a space for the constraint rotation of ester groups, showing a strong beta relaxation even far from the Tg. Therefore, many related experimental phenomena, e.g. the tacticity-dependence of properties, the effect of alpha-methyl, the effect of side-chain n-alkyl, the effect of molecular weight, and otherwise are easily resolved. Since adjacent bond angles are unequal in all-trans extended conformations, the backbone is curved, hence the sPMMA chain tends to form quasi-cyclic structure, leading to the reversely correlation between the overall chain size (C infinity) and the local stiffness (Tg). Lastly, the impact of these findings on the glass theory is presented in conclusions.