Structural and mechanical behavior of nylon‐6 films. II. Uniaxial and biaxial drawing

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.