Maximum tensile properties of oriented polyethylene, achieved by uniaxial drawing of solution-grown crystal mats: Effects of molecular weight and molecular weight distribution

The effects of molecular weight (MW) and MW distribution on the maximum tensile properties of polyethylene (PE), achieved by the uniaxial drawing of solution-grown crystal (SGC) mats, were studied. The linear-PE samples used had wide ranges of weight-average (M-w = 1.5-65 x 10(5)) and number-average MWs (M-n = 2.0-100 x 10(4)), and MW distribution (M-w/M-n 2.3-14). The SGC mats of these samples were drawn by a two-stage draw technique, which consists of a first-stage solid-state coextrusion followed by a second-stage tensile drawing, under controlled conditions. The optimum temperature for the second-stage draw and the resulting maximum-achieved total draw ratio (DRt) increased with the MW. For a given PE, both the tensile modulus and strength increased steadily with the DRt and reached constant values that are characteristic for the sample MW The tensile modulus at a given DRt was not significantly affected by the MW in the lower DRt range (DRt < 50). However, both the maximum achieved tensile modulus (80-225 GPa) and strength (1.0-5.6 GPa), as well as those at higher DR(t)s > 50, were significantly higher for a higher MW Although the maximum modulus reached 225 +/- 5 for M-n >= 4 x 10(5), the maximum strength continued to increase with M-n even for M-n > 4 x 10(5), showing that strength is more strongly dependent on the M-n, even at higher Mo. Furthermore, it was found that each of the maximum tensile modulus and strength achieved could be expressed by a unique function of the Mn, independently of the wide variations of the sample MW and MW distribution. These results provide an experimental evidence that the M-n has a crucial effect on the tensile properties of extremely drawn and chain-extended PE fibers, because the structural continuity along the fiber axis increases with the chain length, and hence with the M-n. (c) 2005 Wiley Periodicals, Inc.