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

Abstract

AbstractThe 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 β1) 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, Eo′ 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, Eo, 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.