Investigation of free volume characteristics of the interfacial layer in poly(methyl methacrylate)–alumina nanocomposite and its role in thermal behaviour

Abstract

The free volume characteristics (free volume size, density and size distribution) of the interfacial layer (IL) in PMMA–alumina (0.3 wt%) nanocomposites were investigated using the well-established positron annihilation lifetime spectroscopy (PALS) technique. Differential scanning calorimetry (DSC) measurements were carried out to investigate the glass transition process of the PMMA matrix in these nanocomposites. PALS results show significant variations in the free volume structure of the nanocomposite, contrary to the assumptions made in earlier reports, i.e., free volume in PMMA remains unaffected on alumina loading. The study shows that as a result of strong interfacial interaction between PMMA and alumina, a denser region is created in the vicinity of alumina particles (interfacial layer), having smaller-sized nanoholes with narrower size distribution compared to bulk PMMA. This observation is consistent with the studies wherein an interfacial layer with constrained segmental motion was inferred from spectroscopic and thermal analysis techniques. The glass transition temperature, T_g, determined from DSC is observed to vary with alumina loading qualitatively similar to the variation in the fractional free volume of PMMA. The study indicates that the critical free volume for PMMA to undergo glass transition process depends on the interfacial layer fraction and hence varies as a function of alumina loading.