Mechanical relaxations in heat-aged polycarbonate. Part II: Statistical Analysis of low-molecular weight data

The significance of heat-aging effects on low-molecular-weight polycarbonate has been studied by performing a two-factor Analysis of Variance (ANOVA). Although this work was primarily motivated by the large experimental scatter observed in stress relaxation results for LMW 2608 (Part I), the effect of heat-aging on the characteristics of secondary transitions (γ and β 1 ) generated by dynamic testing was also investigated. Both types of tests were performed using a dynamic mechanical analyzer. The statistical analysis verified an earlier suggestion that both the secondary transitions were insensitive to heat-aging. In the quasi-static stress relaxation tests, the curve-fitted KWW parameters (τ, E o , β') were evaluated using ANOVA for increasing heat-aging time and test temperature. Two other statistical techniques were also applied to test repeatability-the power of each aging time/test temperature combination and the number of observations needed to achieve 90% repeatability. In conclusion, both τ and β' could describe the self-retarding nature of volume recovery although the repeatability of β' was substantially higher. However, the unrelaxed modulus, E o , was found to be an unreliable indicator of whether heat-treatment had caused changes in the intrinsic structure. Overall, the study showed that the repeatability of the stress relaxation test results is generally very poor for the confidence levels tested.