Chain entanglement, mechanical properties and drawability of poly(lactide)

Abstract

The observed brittle fracture behavior of amorphous polylactides seems to be contradicted by the low value ofC ∞ =2 determined for poly(L-lactide) by Flory and coworkers. Such very flexible polymer chains deform by shear yielding, and fracture in a ductile manner. In this study,C ∞ was estimated in a number of ways, resulting in much higher values ofC ∞ =11.7 andC ∞=9.1 for poly(L-lactide) and L- and D-lactide copolymers, respectively. These high values ofC ∞ and the low entanglement density account for the brittle fracture behavior of amorphous poly(lactide), as well as for the maximum attainable draw ratios of poly(L-lactide) networks and melt spun fibers. Bulk polymerized poly(L-lactide) networks, where crystallization during polymerization impedes severe entangling, could be hot-drawn most effectively to draw ratios of 8–16, resulting in very strong materials with tensile strengths of 550–805 MPa. By comparison, amorphous, non-crystallizable L/D lactide networks, which do not crystallize during polymerization, could be drawn less, to λ=7. These materials with strengths up to 460 MPa could, nevertheless, be oriented much more effectively than linear, amorphous L/D lactide copolymers.