Relaxation Behavior of Polymers at Low Temperatures

Abstract

The relaxation behavior of polymers at low temperatures, as studied by dynamic mechanical, dielectric, and nuclear magnetic resonance methods, is reviewed. Modulus values at 4 and 77°K are tabulated for a variety of both crystalline and amorphous polymers and are compared to room temperature values as well as to theoretical values deduced from intermolecular force constants. Specific attention is given to polymers that show a δ-type relaxation. At low frequencies, this relaxation occurs in the temperature region below 100°K. The δ-processes are shown to arise from reorientational motion of side chain methyl groups, as in PMMA or PVA, side chain ethyl groups, as in PEMA or PVPr, and side chain phenyl or phenyl-substituted groups, as in PS or PmCIS. Linear polymers, such as PE and PTFE, that have no side chain groups exhibit no δ-type relaxation. Consideration is also given to the effect of various internal and external variables on the low-temperature relaxation behavior. These variables include chemical structure, hydrogen atom substitution, crystallinity, impurities, drawing, and past history. An attempt is made to correlate the dielectric and nuclear magnetic resonance data with the mechanical data. Log frequency versus reciprocal temperature plots are given for several polymers of different chemical structure, and activation energies are estimated.