Abstract
The ability of PLLA, either amorphous or semicrystalline, to plastic deformation to large strain was investigated in a wide temperature range (Td = 70–140 °C). Active deformation mechanisms have been identified and compared for two different deformation modes—uniaxial drawing and plane-strain compression. The initially amorphous PLLA was capable of significant deformation in both tension and plane-strain compression. In contrast, the samples of crystallized PLLA were found brittle in tensile, whereas they proved to be ductile and capable of high-strain deformation when deformed in plane-strain compression. The main deformation mechanism identified in amorphous PLLA was the orientation of chains due to plastic flow, followed by strain-induced crystallization occurring at the true strain above e = 0.5. The oriented chains in amorphous phase were then transformed into oriented mesophase and/or oriented crystals. An upper temperature limit for mesophase formation was found below Td = 90 °C. The amount of mesophase formed in this process did not exceed 5 wt.%. An additional mesophase fraction was generated at high strains from crystals damaged by severe deformation. After the formation of the crystalline phase, further deformation followed the mechanisms characteristic for the semicrystalline polymer. Interlamellar slip supported by crystallographic chain slip has been identified as the major deformation mechanism in semicrystalline PLLA. It was found that the contribution of crystallographic slip increased notably with the increase in the deformation temperature. The most probable active crystallographic slip systems were (010)[001], (100)[001] or (110)[001] slip systems operating along the chain direction. At high temperatures (Td = 115–140 °C), the α→β crystal transformation was additionally observed, leading to the formation of a small fraction of β crystals.
Highlights
Poly(L-lactic acid) (PLLA), called polylactide, has received a lot of attention in recent years due to growing concerns about environmental issues
The A and CC samples used for further experiments
The results presented in this paper allowed us to identify the active deformation mechanisms and compare them for two different modes of deformation: uniaxial drawing and planestrain compression
Summary
Poly(L-lactic acid) (PLLA), called polylactide, has received a lot of attention in recent years due to growing concerns about environmental issues. Due to low crystallization rate, an amorphous PLLA is obtained under usual processing conditions It is brittle and exhibits reduced strength and dimensional stability at room temperature, often requiring modification before application. One of the possible routes of modification is the use of plastic deformation, which may result in modification of the structure and morphology of the polymer, its crystallinity, and the orientation of the crystalline and amorphous phases. This provides an opportunity to significantly improve the physical properties (including mechanical, barrier, and optical) through appropriate mechanical and/or thermal treatment. Compression, rolling, solid-state extrusion, or die-drawing can be applied to induce chain orientation in objects with large cross-sections, and to obtain materials with improved stiffness and strength, suitable for the production of technical parts or other items with larger dimensions, such as, e.g., biocompatible surgical implants
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