Relaxation behavior of polymethacrylamide and polyacrylamide and of their copolymers with 2-hydroxyethyl methacrylate

Abstract

A free-oscillating torsional pendulum was used to investigate the effect of the concentration of the HO—CH₂—CH₂—COO— and NH₂ CO— side chains and α-methyl groups of the main chains on the mechanical relaxation behavior of polymethacrylate and polyacrylate polymers and copolymers in the glassy state. The intensity of the low-temperature (γ) dispersion is linearly proportional to the volume fraction of 2-hydroxyethyl methacrylate (HEMA) in both series of copolymers and is independent of the content of the α-methyl groups. The temperature of the γ-dispersion, which for PHEMA is about —133°C (1 Hz), is lowered by the incorporation both of methacrylamide (MAAm) and—although to a lesser extent—of acrylamide (AAm) units. The secondary (β₁) process of PHEMA (28°C; 1 Hz) is shifted toward higher temperature with increasing amount of MAAm, so that for the dry PMAAm it cannot be observed at temperatures up to 120°C. On the other hand, PAAm is characterized by the secondary relaxation at —25°C (1 Hz); however, the corresponding relaxation of the HEMA-AAm copolymers is somewhat indistinct. The effect of water leads to a decrease in temperature and an increase in the intensity by the β₁-relaxation of PAAm, while in the case of PMAAm a new dispersion appears around —120°C. The dependence of molecular mobility on the copolymer composition and water content is qualitatively interpreted in terms of steric hindrances and intermolecular interactions.