Molecular dynamics in bulk and plasticized poly(vinyl acetate): a spin-probe study

Abstract

Poly(vinyl acetate) (PVA) in bulk and plasticized with di- n-butyl phthalate (DBP) over a wide composition range was spin-probed with the nitroxide radicals, 2,2-di- n-butyl-5,5-dimethyl-3-oxazolidinyl oxyl (DBOZ) and 2,2-di- n-nonyl-5,5-dimethyl-3-oxazolidinyl oxyl (DNOZ). The correlation times, τ c, for rotational diffusion of the probes over the temperature range 130–370 K were calculated from their electron spin resonance (e.s.r.) spectra by standard methods. Arrhenius plots of τ cfor all samples except pure DBP showed three distinct regimes: a high-temperature region of relatively high activation energy ( E a) in which E a −1 decreased linearly with increasing PVA content, a low temperature region of very low E a in the range 0–3 kJ mol −1, and an intermediate region of E a ca. 7 kJ mol −1 independent of composition. It was concluded that in the high-temperature region the probe responds to the glass-to-rubber relaxation of PVA and in the intermediate region to the γ or merged β, γ relaxation. In the low-temperature region it seems likely that a proportion of the probes can rotate freely in ‘holes’ or voids in the polymer, the remainder being immobilized within the glassy matrix. The values of E aa for rotation of DBOZ and DNOZ in pure bulk PVA were 69 and 56 kJ mol −1, respectively. It is tentatively suggested that this reversal of the order expected from the relative molecular volume of the two probes, is due to local plasticization by the long alkyl chains on DNOZ.