Glass transition behavior of poly(methyl methacrylate) in compressed carbon dioxide revisited – New perspectives

Abstract

Glass transition behavior of poly(methyl methacrylate) (PMMA) in compressed carbon dioxide has been investigated with a relatively new technique, High-Pressure Torsional Braid Analysis (HP-TBA), using two polymer samples of widely differing molecular weights, with Mw = 89,700 and 457,000 g/mol. For each sample, the glass transition temperature was determined along constant-temperature and constant-pressure pathways at 35, 70, and 90 °C and 30, 50, 75, 100, and 200 bar, respectively. The results show that the observed Tg behavior depends not only on the molecular weight but also on the rate of pressurization. Based on the present observations new interpretations are provided for the differences in the data reported in the literature along with new insights regarding the presence or absence of the retrograde Tg behavior. This study suggest that high molecular weight PMMA samples may not display classic retrograde Tg behavior.