Structure, Dynamics, and Mechanical Properties of Polyimide-Grafted Silica Nanocomposites

A continuing challenge in polymer nanocomposites (PNCs) is to control nanoparticle (NP) dispersion and understand its role in property enhancements. Previous studies have been focused on the flexible polymer chain systems. In this study, we report the structure, glass transition behavior, segmental dynamics, and mechanical properties of polyimide (PI) nanocomposites that consisted of either bare or PI-grafted silica NPs to discriminate the role of grafted rigid chains in polymer–NP interactions and dynamic response of such hybrids. Silica NPs are well dispersed in the poly(amic acid) (the precursor of PI) nanocomposites. After thermal imidization, the aggregate structure and self-assembled small clusters of NPs are observed in ungrafted and grafted silica NP-filled PI composites, respectively. The glass transition temperature (Tg) shifts to high temperature by the addition of silica NPs resulting from strong polymer–NP interactions and steric hindrance, and Tg deviation is more visible with increasing the molecular weight of grafted PI chains. The α-relaxation dynamics are suppressed in PI nanocomposites, but there is no interfacial layer relaxation detected because of the rigid chain characteristic of PI. The accelerated Maxwell–Wagner–Sillars polarization process is noted in the presence of PI-grafted silica NPs with high molecular weight. PI-grafted silica NPs are effective in improving polymer–NP interfacial adhesion, resulting in superior mechanical properties to those of the bare silica NP-filled composites. Moreover, high molecular weight grafted PI chains are beneficial in the mechanical enhancement of the resulting PI nanocomposites. These findings provide new insight into the fundamental understanding of chain packing and relaxation dynamics of PNCs with rigid polymer chains and therefore provide guidance in designing such materials with desired macroscopic properties.