Strain-recovery kinetics in rubber toughened polymethymethacrylate

The viscoelastic nature of polymers influences the material's behaviour not only during its deformation up to strains beyond yielding, but also during and after its unloading. In the present work the recovery of the strain after the removal of the applied stress was studied as a function of time and temperature on rubber toughened polymethylmethacrylates (RTPMMAs). Constant strain rate tests under uniaxial tensile loading were performed at room temperature on three RTPMMAs, differing in dispersed phase content. Identical samples were deformed up to the same strain level and then unloaded. The residual strain was measured as a function of time at different constant temperatures. By applying a time-temperature equivalence, a residual strain versus time master curve was obtained and the relevant shift factors were determined. Isothermal strain recovery on samples deformed up to the same strain was also measured at different temperatures, in the range between room temperature and the material's glass transition temperature. The residual strain after a given time was plotted as a function of temperature, obtaining an isochronous strain-recovery curve. Using the appropriate shift factors, an isothermal strain-recovery curve was then determined. The latter and the strain-recovery master curve are similar in the range of time over which recovery takes place, although an effect of the applied strain level on the curves' shape was observed. The effects of the applied strain level and of the rubber particles content on the strain-recovery kinetics were evaluated.