Rod-like attapulgite/polyimide nanocomposites with simultaneously improved strength, toughness, thermal stability and related mechanisms

Abstract

Rod-like silicates (attapulgite, ATT) with covalently grafted polymer chains chemically identical to the matrix were incorporated into polyimide films, which resulted in prominent improvements in strength and toughness and elevation as high as 51 °C of the thermal decomposition temperature. The dispersion uniformity of ATT nanorods in the polyimide was confirmed by SEM observations and UV-vis spectra. High grafting efficiency of the polymer on the particle surface significantly confines the segmental motion of the polyimide, causing a 33 °C increase in the glass transition temperature and remarkable broadening in the primary transition of nanocomposite films. The decreases in strength and toughness of the samples with high particle contents indicate that there are other factors impacting the mechanical properties of nanocomposites, besides percolated particle networks and attractive interfacial interactions. The plastic deformation observed on the fracture surface of nanocomposites suggests that the introduction of free volume, due to the exclusive volume effect of particles and polymer chains, might play a crucial role especially for those samples with high particle contents. Some fundamental facts based on the free volume effect are proposed to elucidate the possible reasons why the strength and toughness of nanocomposites decrease at high particle contents, which implies that there probably exists a limit upon simultaneously reinforcing and toughening a polymer matrix by means of adding nanoparticles. Similarly, such a limit probably impairs the improvement in thermal stability of nanocomposites at high particle contents, although the segmental confinement effect and porous feature of rod-like silicates have led to pronounced improvements in the thermal stability of the polyimide.