Role of Local Polymer Conformations on the Diverging Glass Transition Temperatures and Dynamic Fragilities of Isotactic-, Syndiotactic-, and Atactic-Poly(methyl methacrylate)s

Abstract

The profound stereosequence dependence of the glass transition temperature (Tg) of poly(methyl methacrylate)s (PMMAs) offers the possibility to evaluate any conformational contributions to their glass formation processes. Close to Tg for a given polymer liquid, the viscosity and relaxation time increase dramatically, leading to the temperature dependence of its dynamics to deviate from Arrhenius behavior. The degree of that deviation is called dynamic fragility (m). The broad reported variations for Tg of over 70 K and for m of 90 necessitated new measurements for each sample using the same protocols to keep comparisons reliable. The limiting values of fictive temperature as a function of cooling rates were measured using differential scanning calorimetry (DSC) and were used to calculate m values of two stereoregular PMMAs, isotactic (i) and syndiotactic (s), as well as three atactic (a) PMMAs with different molecular weights. For a-PMMAs, m exhibited a positive dependence on molecular weight, and the m values for s- and i-PMMAs were consistent with the previous studies to be high and low, respectively. Heat capacity changes at Tg correlate negatively with m, probably due to the breakdown of their thermorheological simplicity. Conformational analyses were conducted at the dimer level to elucidate the potential role of local polymer conformations. Rotational isomeric state Monte Carlo simulations and conformational analyses provide insights into the molecular origin of the Tg and m differences of i-PMMA and s-PMMA. On the other hand, we demonstrated that the characteristic ratio is not a suitable parameter for characterizing PMMA flexibility and is irrelevant to m. A future focus on the local conformational geometry of glass-forming polymers is proposed.