Poly(ether imide) Alumina Nanocomposites: Interphase Properties Determined from Free Volume Element Radius Distributions

Ultrafast infrared (IR) spectroscopy was used to characterize the free volume element (FVE) radius probability distributions (RPDs) of poly(ether imide) (PEI) alumina nanocomposites. The nanocomposites (0–2 wt %) were prepared with 20 nm diameter spherical Al₂O₃ nanofillers and a small amount of phenyl selenocyanate (PhSeCN) as IR vibrational probes. Restricted orientation anisotropy method (ROAM), an ultrafast IR technique, was used to measure FVE radii. The results yield RPDs as a function of the nanoparticle concentration. The RPDs were decomposed into bulk PEI and interphase region contributions. The ROAM results demonstrate that the polymer chain packing in PEI nanocomposites is significantly altered from that of pure PEI. The average FVE radius increases with increasing nanofiller content. The RPDs indicate that subensembles with smaller radii are disproportionately affected by the presence of the Al₂O₃ nanofillers, causing the width of the distribution to narrow. The FVE RPDs for the interface regions reveal a distribution with an average radius ∼0.2 Å larger but significantly narrower than the pure PEI distribution. Finally, the interface volume fraction for each nanocomposite sample was determined from the differences in the RPD curves, and the effective interfacial layer thickness was found to be 19.2 ± 0.5 nm. The results demonstrated that FVE characteristics are strongly affected by the proximity to nanoparticles. The nature of the FVEs in the interfacial regions provides information about the microscopic origin of the polymer nanocomposite material’s properties.