A re‐examination of the sub‐Tm exotherm in polyamide 6: The roles of thermal history,water and clay

The sub‐T_m exotherms in polyamide 6 (PA6) have been carefully re‐examined by differential scanning calorimetry and X‐ray diffraction, considering the effects of processing and thermal history, addition of water and clay. The results obtained cast doubt on Khanna's proposal that sub‐T_m exotherm in PA6 comes from the release of strain energy absorbed during processing, and suggested that the origin of sub‐T_m exotherm is the γ?α transformation at the premelting temperature, namely, the less thermodynamically stable γ‐form (γ^ns) transforming into the more thermodynamically stable α‐form (α^s). The presence of water or clay in PA6 samples facilitated the formation of γ^ns at corresponding cooling rates, and enhanced the development of sub‐T_m exotherms. During the heating scan of PA6/clay composites, the initial γ^ns can be transformed into more stable (γ^s)^t and α^s at the same time, which can be thought as the origin of their sub‐T_m events. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2385–2393, 2009