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The observed brittle fracture behavior of amorphous polylactides seems to be contradicted by the low value of C(infinity) = 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(infinity) was estimated in a number of ways, resulting in much higher values of C(infinity) = 11.7 and C(infinity) = 9.1 for poly(L-lactide) and L- and D-lactide copolymers, respectively. These high values of C(infinity) 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 lambda = 7. These materials with strengths up to 460 MPa could, nevertheless, be oriented much more effectively than linear, amorphous L/D lactide copolymers. |
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