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∞), the specific volume and the gas permeability coefficient. Syndiotactic (s) PMMA is the most peculiar, exhibiting complex α-β relaxations. We first make brief reviews about key experimental facts of s-PMMA relaxations: 1) unusual α-β 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 α-β 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 α 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 β relaxation even far from the Tg. Therefore, many related experimental phenomena, e.g. the tacticity-dependence of properties, the effect of α-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 s-PMMA chain tends to form quasi-cyclic structure, leading to the reversely correlation between the overall chain size (C∞) and the local stiffness (Tg). Lastly, the impact of these findings on the glass theory is presented in conclusions. [Display omitted] •Molecular origin of the peculiar α-β relaxations results from the all-trans extended conformationsin s-PMMA.•The local extended -structure, as a bigger jumping unit, induces the formation of domains with slow dynamics.•The local stiffness instead of the overall chain size determines the occurrence of the glass transition.•Molecular image of s-PMMA relaxations provides insights to understand the precise nature of the glass transition.