Abstract
Poly(L-lactic acid)（PLLA）is biodegradable and biocompatible polymer extensively used in bone fixation, drug delivery, and tissue engineering. The purpose of this study attempted to find the mechanical properties by PLLA thin film stretched. It was revealed that there is distinctive nano-mechanical behavior. First, crystallites disperse in PLLA thin film which was observed by atomic force microscope（AFM）and nucleation of crazes is always in the amorphous region between crystallites. Crystallinity could control craze widening, and it depends on PLLA films preparation. In low crystallinity, as strain increases, crystallites deform slightly and some of them were probed in crazes. As crazes growth most crystallites arrangement in crazes is parallel to stretching direction to form chain structure. This chain structure seems due to lamella cleavage arises from stress induced local melting. Violently, high crystallinity limits crazes width almost under 2μm which is smaller than in low crystallite density. The growth of craze sometimes is by small crazes merged together and PLLA film is very though showing no micro fracture at large strains beyond 20％. After stretched, crystallites outside of crazes region deform slightly which was intra-lamellae slip, but the amorphous region is the major deformation region. The craze formation was amorphous dominated but lamellae limit craze extend.