ON THE PLASTIC AND VISCOELASTIC BEHAVIOR OF METHYLMETHACRYLATE-BASED RANDOM COPOLYMERS

Both plastic and viscoelastic properties were investigated on neat pol ymethylmethacrylate (PMMA) and methylmethacrylate-based random copolym ers including N-substituted maleimide units. A strong coupling was evi denced between the glass transition motions and the secondary transiti on motions for PMMA samples rapidly quenched from the molten state. Th is coupling was shown to diminish markedly in the case of aged PMMA an d to disappear almost completely in the case of MMA-maleimide copolyme rs having a maleimide content as small as 5 mol%. Thus, it strongly su ggests that a greater compacity of the glassy PMMA matrix and (or) the presence of stiff maleimide units in the copolymers are sufficient to hinder the early occurrence of large scale conformational motions in the secondary transition region. In addition, it was shown that the st rain softening that follows the yield point is also important for aged PMMA and maleimide-rich, copolymers, but tends to vanish in the other samples. Therefore, the probable origin, at least in this series of m aterials, of the strain softening would be the different nature of the molecular motions that occur in the secondary relaxation region and i n the glass transition region.