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

The sub-Tm 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-Tm exotherm in PA6 comes from the release of strain energy absorbed during processing, and suggested that the origin of sub-Tm exotherm is the γ-α transformation at the premelting temperature, namely, the less thermodynamically stable γ-form (γns) transforming into the more thermodynamically stable α-form (αs). The presence of water or clay in PA6 samples facilitated the formation of γns at corresponding cooling rates, and enhanced the development of sub-Tm exotherms. During the heating scan of PA6/clay composites, the initial γns can be transformed into more stable (γs)t and αs at the same time, which can be thought as the origin of their sub-Tm events.