The evolution of structure and properties in poly(p-phenylene terephthalamide) ®bers Y. Rao, A.J. Waddon, R.J. Farris * Polymer Science and Engineering Department, University of Massachusetts Amherst, Amherst, MA 01003, USA Received 1 May 2000; received in revised form 1 December 2000; accepted 6 December 2000 Abstract The evolution of the mechanical properties and structure of poly(p-phenylene terephthalamide) (PPTA) ®bers with different post-treatment methods involving heat, tension, hydrostatic pressure, and different environments was systematically investigated. Wide-angle X-ray diffraction measurements reveal that the crystal structure of PPTA ®ber is not stable and changes upon post-treatment. The cooperative changes in the modulus and two structure parameters Ð the misorientation angle and the paracrystalline parameter upon treatment Ð indicate a direct structure±property correlation. After studying free-length annealing and heat-tensioning of ®bers, several structure para- meters Ð the c-dimension of lattice constants, the paracrystalline parameter, the intensity ratio between (110) and (200), and the orientation angle Ð were found to be affected greatly by the tension applied during heat stretching; while other structure parameters such as apparent crystal sizes, equatorial crystallinity and a, b dimensions of the lattice constant are insensitive to the applied tension but determined by the applied temperature and time. A sudden change in the crystal structure at 4008C suggests a a-relaxation in the crystalline region, which is supported by the DMTA and TMA measurements. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Poly(p-phenylene terephthalamide) Kevlar w ®bers; Structure and properties; Wide-angle X-ray diffraction 1. Introduction Poly(p-phenylene terephthalamide) (PPTA) ®ber is known to have very high modulus and strength and good thermal stability [1]. The property of the ®ber can be greatly enhanced through post-treatment. In order to achieve better performance, it is important to understand what structure change is caused by post-treatment and how this structure change relates to the property enhancement. In recent years several studies were made on the structure± property relation [2±5]. However, no systematic study of the structure change upon treatment has been reported. PPTA is a highly crystalline polymer [4,6±13]. A single- phase structure with crystalline imperfection describes its morphology well [9]. The crystal structure of PPTA shown by Northolt and Aartsen is a pseudo-orthorhombic [6,7]. Panar and coworkers suggested a paracrystalline structure of the crystal with a second order distortion g II of 2.5% along c-axis [9]. These imperfect crystals are packed to form pleated sheets and those pleats can be observed using polarized optical microscopy [5,8,14,15]. Wu et al. [14] studied the crystal structure change of PPTA ®ber under annealing temperatures ranged from 380 to 4808C at either no-tension or constant-tension modes. They proposed a residual-stress free c-dimension of a crystal. Then, an increase in c-dimension was observed for all treatments. The higher the tension, the bigger the increment is; and the higher the annealing temperature, the smaller the increment. They also discussed the change in the apparent crystal size along c-axis and the degree of orientation. Lee and coworkers did a similar study on the structure and property development of PPTA during heat treatment under tension [5]. They observed a correlation between the modulus and the orientation as well as the pleating. Other structure parameters such as crystal size and paracrystallinity were found to be irrelevant to the property. It was reported that the ®ber tenacity does not change much at low and intermediate temperatures for short time annealing but drops when the temperature exceeds 4508C and degradation occurs [8]. Several hours' exposure to high temperature deteriorates the strength but does not affect the modulus much [15]. The crystallites were found to grow laterally along the b-axis upon thermal annealing [16]. Different researchers have focused exclusively on certain structure parameters, narrow range of treatment temperature or speci®c method in their study of the structure change Polymer 42 (2001) 5925±5935 0032-3861/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00906-X www.elsevier.nl/locate/polymer * Corresponding author. Tel.: 11-413-577-3125; fax: 11-413-545-0082. E-mail address: rjfarris@polysci.umass.edu (R.J. Farris).