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

Poly(methyl methacrylate) (PMMA) exhibits significantly tacticity-dependent properties, e.g. the glass transition temperature (T g), 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 T g ; 3) discrepancy between chain stiffness and T g. 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 T g provides a space for the constraint rotation of ester groups, showing a strong β relaxation even far from the T g. 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 (T g). 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. [ABSTRACT FROM AUTHOR]