(703e) Dynamic Relaxation Properties of Polymer Nanocomposites

The introduction of nanoscale filler particles into a polymer matrix results in the creation of vast amounts of polymer-particle surface area and the potential for physical confinement of the polymer chains at the nanoscale. Polymer-particle interactions, as well as confinement effects, can substantially alter the local and segmental mobility of the matrix polymer chains, as reflected in the dynamic relaxation properties of the nanocomposite. In this study, we explore the sub-glass and glass-rubber relaxation characteristics of a series of model glassy polymer nanocomposites based on poly(etherimide) [PEI] and poly(methyl methacrylate) [PMMA] in combination with silica (SiO₂) nanoparticles.

Evaluation of the thermomechanical properties of polymer nanocomposites, and their correlation with filler loading, dispersion and the quality of the polymer-particle interface is an area of intense interest. In filled polymers, the inclusion of inorganic particles can lead to an increase in measured glass transition temperature (Tg) owing to interactions and potential adsorption phenomena that limit segmental mobility. Conversely, in systems that exhibit poor wetting characteristics, decreases in Tg have been reported. In the current work, we examine a series of PEI and PMMA nanocomposites formulated with varying levels of native and modified SiO₂ nanoparticles. Dynamic mechanical analysis and broadband dielectric spectroscopy are used to elucidate polymer chain dynamics across the glass-rubber relaxation in relation to nanocomposite morphology, and the nature of the polymer-particle interaction.

The inclusion of native SiO₂ nanoparticles (~10 nm diameter; up to 30 wt% loading) in the PEI matrix had only a very modest influence on the glass transition properties of the resulting materials; a ?bulk' Tg of ~225°C (1 Hz) was observed for all samples, essentially independent of polymer loading. The introduction of various surface-modified (i.e. hydrophobic) fumed silicas led to enhanced particle dispersion, and also to the emergence of a dual-Tg response that encompassed a primary glass transition at 225°C, as well as a second Tg event offset upwards by 60°C to 100°C. This high-temperature transition appeared to correspond to a distinct glass-rubber relaxation originating from regions of reduced mobility in the vicinity of the polymer-particle interface. The relative intensities of the two glass transitions, as well as the position and breadth of the high-temperature relaxation, correlated with the degree of particle loading and the character of the nanoparticle surface. Complementary dielectric spectroscopy measurements were undertaken to further establish the morphological basis for the observed dual-Tg response, and to probe the influence of the nanoparticles on the dynamic relaxation characteristics of the composites across the sub-glass region. Parallel studies on PMMA-SiO₂ composites indicated similar response properties, as related to the underlying polymer-particle compatibility.