Microstructure and crystal modulus of polyamide 4 α-form in the direction parallel to the chain axis

Polyamide 4 (PA4) is a lactam-based aliphatic synthetic polymer with promising properties and admirable biodegradability. In this study, oriented PA4 fibers were fabricated by dry spinning followed by thermal drawing. Elastic modulus (E_l) of the crystalline regions (crystal modulus) of PA4 fibers in the direction parallel to the chain axis was evaluated from the X-ray diffraction peak shift owing to the tensile stress. The apparent E_l value of PA4 is 67 GPa for meridional 004 reflection and 119 GPa for 0010 reflection, respectively. The reason for the different E_l is that the decrease of crystallite size of PA4 by the applied stress brought extra shifts of 004 reflection peak to lower angle based on the Laue factor. In contrast, 0010 reflection peak received less effects from this effect, and the E_l value of 119 GPa obtained from 0010 reflection was considered to be a reliable value. The observed E_l value of PA4 is much lower than the calculated values assuming fully extended planar zigzag skeleton for PA4. Thus the low E_l value is attributed to the highly contraction (−1.9 %) from the fully extended conformation as observed for another polyamides.