Critical strains in tensile deformed polyamide 6 and 6/66 copolymer

Abstract

AbstractThe recovery properties of dry and water saturated polyamide 6 (PA6) and its copolymer PA6/66 (ratio 4:1 by mol) were studied at elevated temperatures above the glass‐transition temperature in uniaxial tensile tests. The data yield critical points along the true stress–strain curves at which the differential compliance and the recovery property change. These critical points include the onset of the plastic deformation (point A), the yield point (B), and the point where the elasticity of the samples reaches a plateau value (C). The strains at points A and B remain constant, whereas the strain at point C varies with temperature. The invariance of the critical strains at points A and B is assumed to be the result of the homogeneous strain distribution in the system and the general activation of the intralamellar block slip mechanism at low deformations. The strain at point C, being related to the properties of the entangled network, varies because the effective entanglement density of the network changes due to the change in the hydrogen bond number with temperature. With the Gaussian model of Haward and Thackray, we calculated the network moduli. From these data, we derived that the network stress remains constant at point C. At point C, the deformation mechanism starts to change from the block slip mechanism to a stress‐induced melting–recrystallization process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 87–96, 2005