New Insights into the Brill Transition in Polyamide 11 and Polyamide 6

The thermal stability of several crystal polymorphs in polyamide 11 (PA11) and polyamide 6 (PA6) has been investigated by means of in situ X-ray experiments. In the case of PA11, δ′ and α′ phases display a Brill transition far below the melting point. Both phases transform into a (pseudo)-hexagonal HT δ phase above 100 °C. The latter turns back into the most stable α′ phase upon cooling. PA6 exhibits similar thermally induced crystal transitions. In situ X-ray investigations refute the occurrence of a β → α transition upon heating, whereas the existence of a pseudohexagonal HT phase is suggested as in PA11. For both polymers, the present study underlines the major role of crystal perfection of the most stable α phase on the existence of a Brill transition. The combination of in situ structural information with thermal analysis allows to propose a thermodynamic scheme to describe the Brill transition in both polyamides.