Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n‐butyl methacrylate) networks

The linear viscoelastic and stress‐strain behavior of poly( n ‐butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c 0–1 × 10 −4 mole/cm 3 was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks ( c < 0.1 × 10 −4 mole/cm 3 ) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C 2 constant of the Mooney‐Rivlin equation.