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 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.
通过进行双因素方差分析(ANOVA),研究了热老化对低分子量聚
碳酸酯影响的重要性。虽然这项工作的主要动机是在 LMW 2608(第一部分)的应力松弛结果中观察到较大的实验散差,但也研究了热老化对动态测试产生的二次转变(γ 和 β1)特性的影响。这两种测试都是使用动态机械分析仪进行的。统计分析验证了之前的观点,即二次转变对热老化不敏感。在准静态应力松弛试验中,使用方差分析评估了热老化时间和试验温度增加时的曲线拟合 KWW 参数(τ、Eo′ β′)。此外,还采用了另外两种统计技术来测试重复性--每种老化时间/测试温度组合的功率和达到 90% 重复性所需的观察次数。总之,τ 和 β′ 都能描述体积恢复的自回复性质,但 β′ 的重复性要高得多。不过,未松弛模量 Eo 被认为是热处理是否导致内在结构变化的不可靠指标。总之,研究表明,在所测试的置信
水平下,应力松弛测试结果的可重复性通常很差。