Small conformationally mobile molecules as probes for molecular mobility in glassy polymers

Abstract

A new method for studying molecular mobility in glassy polymers—the method of conformationally inhomogeneous probes—is described. A small quantity of a low-molecular-weight compound (probe) having unidentical conformations with different vibrational spectra is introduced into a polymer. The thermodynamic and activation parameters responsible for the concentrations of the conformers and the rates of conformational transitions are connected with both the intramolecular interactions and the properties of the medium (e.g. polymer). Information about the kinetics and equilibria of the conformers could be obtained by investigating the vibrational spectra. This information may be the key to understanding the molecular mobility and free-volume distribution in polymers. The following compounds were used as probes: 1,2-bromofluoroethane, methyl dichlorophosphate and trans-1,2-dichlorocyclohexane. The polymers poly-styrene, poly(vinyl chloride), poly(methyl methacrylate), poly(vinyl acetate), poly(vinylbutyral), poly-propylene and polybutadiene were investigated. The temperature dependences of the absorption-band intensities of the probes were studied. Freezing of the conformational transitions in the probe molecules was found at temperatures T f characterizing specific polymer-probe systems. The T f are close to the temperatures of the relaxation transitions in the pure polymers. The possibilities of the method are discussed.