The glass transition temperature of PMMA: A molecular dynamics study and comparison of various determination methods

Abstract

The glass transition temperature (Tg) governs the mechanical and physical performances of polymeric materials and thus their ultimate applications. Although an extensive body of research has focused on the study and determination of the Tg, thermal events at and around the Tg at the molecular level have not yet been fully understood. It is widely believed that, at and around the Tg, the intermolecular interactions and the structure of polymer change resulting in dramatic variations of the bulk properties of polymers. Therefore, the Tg could be determined by tracing the changes observed in macroscopic (bulk) and microscopic properties as a polymer system cools down. In this study, we attempted to estimate the Tg of isotactic polymethylmethacrylate (is-PMMA) employing the molecular dynamics simulations based on the united atom model. To achieve this, the polymer properties including the thermal conductivity, volume, thermal expansion and Young’s module were examined. Moreover, microscopic properties such as the radial distribution function (RDF) and motions of the polymer chains by the mean squared displacement (MSD) function and the non-bonded energy were assessed. It was shown that a unique break appears on the property-temperature curves around 440K irrespective of the MD simulation method. The Tg values obtained in this work were quite consistent with the experimental results reported in the literature. The study also indicated that the Tg increases with increasing the cooling rate and molecular weight of the polymer.