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.

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The plastic behavior of polyamide‐6 (or nylon‐6) films under uniaxial and biaxial tensile drawing is studied in relation to structural features. Quenched films in the mesomorphic β form are more ductile than films in the predominant stable α form. Films in the major γ crystalline form are intermediate between films in the β and α forms. Under uniaxial drawing, a great part of the β phase undergoes strain‐induced phase change into the α form, involving strain hardening of the material. The β→α phase change is more pronounced above 120 °C because of additional thermal reorganization. The γ form that is thermally stable up to 200 °C also undergoes a strain‐induced phase change above 120 °C, but the reorganization is much less important than for the β phase. Biaxial drawing can only be achieved below 120 °C with quenched films. This is likely due to the high mechanical anisotropy of the H‐bonded sheetlike structure of both the α and γ phases that is suspected to develop catastrophic splitting under a normal stress component, notably above 120 °C because of the collapse of the paraffin‐like van der Waals interactions between the sheets. The disordered distribution of the H bonds in the mesomorphic β structure of the quenched films is more appropriate for biaxial drawing. The strain‐induced β→α phase reorganization, which might have been a serious prejudice, proved to be much reduced during biaxial drawing below 120 °C. The mechanism of the strain‐induced phase change is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1224–1236, 2001