Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy

Abstract

A method for measuring the size and size probability distribution of free volume regions in polymeric materials using ultrafast infrared (IR) polarization-selective pump–probe experiments is presented. Measurements of the ultrafast dynamics of a vibrational probe (the CN stretch of phenyl selenocyanate) in poly(methyl methacrylate) show that the probe dynamics are highly confined. The degree of confinement was found to be both time-dependent and dependent on the vibrational frequency of the probe molecule. The experiments demonstrate that different vibrational frequencies correspond to distinct subensembles of probe molecules that have different dynamic properties determined by their local structural environments. By combining the degree of dynamical confinement with the molecular size of the probe molecule, the free volume element size probability distribution was determined and found to be in good agreement with the best established experimental measure of free volume. The relative probability of a free volume element size is determined by the amplitude of the nitrile absorption spectrum at the frequency of the measurement. The inhomogeneous broadening of the spectrum was linked to the vibrational Stark effect, which permits site selectivity. The observed dynamics at each frequency were then associated with a different size free volume element and distinct local electric field. The multiple timescales observed in the pump–probe experiments were connected to local structural fluctuations of the free volume elements.

ROAMing in mutable voids: Polymer free volumes from wobbling vibrational probes

“…Thus, careful analysis of the anisotropy of the restricted reorientation of the vibrating probe molecule provides a size distribution of the FVEs in the polymer or glass sample (9). Hoffman et al (9) carefully compared the FVE distributions from their ROAM analysis with existing measures and found that one of their two models quantitatively agrees with FVEs measured by PALS.…”

“…The ROAM described by Hoffman et al (9) combines several standard methods of infrared vibrational spectroscopy (14-16), together with a sophisticated analysis to provide information about free volumes in condensed matter. The experimental methods are based on ultrafast vibrational spectroscopy with a femtosecond laser source, thus providing coherent broadband probing with a bandwidth exceeding 100 cm −1 .…”

“…A vibrational probe molecule with an intense infrared absorption such as a nitrile stretch is used to provide high sensitivity. Hoffman et al (9) used phenyl selenocyanate (PhSeCN), although they point out that different probe molecules may be used. The reorientation time correlation function for the probe molecule is measured.…”

“…In PNAS, Hoffman et al ( 9) describe a powerful method for characterizing the size distribution of polymer FVEs, with the added twist of also being able to characterize the temporal evolution of the FVEs, which is lacking in the previously established methods. Hoffman et al (9) name their technique the restricted orientation anisotropy method (ROAM).…”

“…Hoffman et al ( 9) demonstrate the ROAM method by characterizing samples of the glass-forming polymer poly(methyl methacrylate) (PMMA), which is popularly known as Plexiglas, Lucite, or Perspex. One reason Hoffman, et al (9) selected PMMA as a sample is that the FVEs in this material have been widely characterized by several other methods. The essential feature of ROAM is that when a vibrating molecule is confined to a polymer or glassy material, its reorientation is constrained to wobble in a restricted volume rather than undergo completely random diffusive angular jumps, as would occur in a low-viscosity liquid.…”