Sampling Free-Volume in Polymer Glasses by Vibrational Dynamics

We propose a spectroscopic method to measure free volume in polymer glasses. It is based on the analysis of well-behaved infrared (IR) bandshapes generated by molecular probes residing in the substrate. Fourier analysis and least-squares curve resolution of these profiles provide information on the dynamics of the probe during the relaxation process. This information can be correlated with the space available to the molecular reporter, which, in turn, is connected to the free-volume. The proposed approach was applied to three polymer glasses differing by free-volume content and interactive character, namely poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) and a commercial poly(ether imide) (PEI). Carbon dioxide was the selected probe owing to its suitable vibrational behavior. The two-step model of Gordon was found to describe accurately the dynamics of the relaxation process. An initial period of unhindered rotation during which the probe behaves as the isolated gaseous molecule was identified in PTMSP. This period was clearly detected also in an aged PTMSP sample and in PPO, while for PEI it was less obvious. The Gordon model was less accurate in predicting the dynamic behavior of CO2 in PEI, likely because of the occurrence of probe-to-substrate interactions. It is shown that the vibrational dynamics parameters obtained by band shape analysis are sensitive and linearly related to the fractional free volume.