Structure and dynamics in polystyrene/single-walled carbon nanotube nanocomposites via neutron scattering techniques

Abstract

Small angle and quasielastic neutron scattering (SANS and QENS) were used to investigate the cause of the minimum in the diffusion coefficient for polystyrene (PS)/single walled carbon nanotube (SWCNT) nancomposites (M. Mu, N. Clarke, R. J. Composto and K. I. Winey, Macromolecules, 2009, 42, 7091–7097). Radius of gyration (Rg) values for PS/SWCNT nanocomposites were obtained by fitting SANS data with the Debye equation, and were found to increase by ~450% (110 kg mol–1) and ~500% (230 kg mol–1), indicating agglomeration or incomplete contrast-matching of the matrix and the nanotubes.
Elastic scans recorded via QENS on ring and chain-labelled samples indicated that the PS rings were more mobile in both the polymer and the nanocomposite, and that adding SWCNTs increased the mobility of the chain at SWCNT concentrations above 1 wt%, especially around the glass transition temperature (Tg). Slower motions of the ring also increased, but only at 4 wt% loading. The stiffness values for the chain and ring were isolated for the first time, indicating reduced chain stiffness on addition of increasing levels of SWCNTs.
QENS peaks were Fourier transformed and the decay curves fitted with the KWW function. Only the data recorded at 177 °C returned relaxation times that could be resolved, suggesting that the motions at lower temperatures are slower than could be detected.
Tg values were extracted calorimetrically and from neutron data. The calorimetric Tg had a minimum at ~1 wt%. The neutron Tg was recorded from data on two spectrometers, IRIS (2-200 ps) and HFBS (100 ps–10 ns); the ring data recorded on IRIS increased relative to the bulk on loading, while the chain data recorded on HFBS decreased, indicating that the chain and rings are affected by SWCNTs on different timescales.
The neutron static structure factor was affected at loading levels of 0.1–3 wt%, and the effect was more pronounced for the chain than the ring.
This work clearly indicates that adding nanoparticles influences the local structure and fast local dynamics of PS/SWCNTs, and while it does not identify the origin of the minimum in the diffusion coefficient, it does narrow the time window where the origin must lie.