ON THE VISCOELASTIC AND PLASTIC BEHAVIOR OF METHYLMETHACRYLATE-CO-N-METHYLGLUTARIMIDE COPOLYMERS

Both viscoelastic and plastic properties were investigated on random c opolymers of methylmethacrylate (MMA) and N-methyl-glutarimide (GIM) i n the range 0-76 mol%. All the measurements were performed on samples quenched from the melt in order to break free from physical aging effe cts. Dynamic mechanical experiments were performed at very low deforma tion and temperatures ranging from - 150 degrees C up to the glass tra nsition temperature (T-g) region. Increase in GIM amount improves the thermomechanical stability of the copolymers, as revealed by the incre ase of both T-g and alpha relaxation temperature. In the beta relaxati on region, the E '' loss peak first decreases in amplitude with increa sing GIM content and then broadens further and finally spreads out til l the onset of the alpha peak at the largest GIM amounts. A quantitati ve analysis of the beta relaxation phenomena was performed by consider ing the loss compliance J '' instead of the loss modulus E ''. It turn s out that in the low temperature range (-80 degrees C-0 degrees C) th e mechanical damping associated with the MMA motions is stronger for M MA-GIM than for MMA-MMA linkages; in addition, the mechanical damping associated with the motions of the GIM units is very low. By contrast, in the high temperature range (30 degrees C to about 100 degrees C), the mechanical damping associated with the MMA motions drops with incr easing GIM amount, whereas a significant damping coming from the GIM u nits is observed. These results suggest that the beta relaxation would mainly consist of MMA isolated motions at low temperature and of coop erative motions at higher temperature, involving the MMA units at GIM amounts lower or equal to 58 mol% and the GIM units at higher GIM cont ent. The stress-strain curves were determined at low strain rate (2 X 10(-3) s(-1)) and temperatures ranging from -120 degrees C to T-g. Ana lysis of the plastic deformation region shows that the yield stress de creases with increasing GIM amount at low temperatures. The opposite t rend shows up on the high temperature side of the beta relaxation, whe re strain softening peaks at intermediate GIM amounts. As a plausible explanation, the cooperative beta motions, whenever they exist, are su spected to be responsible for the decrease of both yield stress and st rain softening. These conclusions agree well with those of a previous study on methylmethacrylate-co-maleimide copolymers. They are also con sistent with our earlier identification of the microdeformation mechan isms involved in the stretching of methylmethacrylate-co-N-methylgluta rimide thin films. (C) 1998 Elsevier Science Ltd. All rights reserved.