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Parsons, Nathan
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- 2018An investigation into the crystalline morphology transitions in poly-L-lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regimecitations
- 2018An investigation into the crystalline morphology transitions in poly-L- lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regimecitations
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article
An investigation into the crystalline morphology transitions in poly-L-lactic acid (PLLA) under uniaxial deformation in the quasi-solid-state regime
Abstract
The mechanical behaviour, crystalline and macromorphology structure development during uniaxial deformation and annealing of poly-L-lactic acid (PLLA), with varying strain rate and draw temperatures (<i>T</i><sub>d</sub>) above <i>T</i><sub>g</sub>, have been investigated using small- and wide-angle X-ray scattering (SAXS/WAXS), microscopy, thermal and mechanical techniques. The mechanical behaviour of PLLA, was strongly dependent on <i>T</i><sub>d</sub> where embrittlement and eventual failure were observed as <i>T</i><sub>d</sub> was increased, during uniaxial drawing of the amorphous polymer. This was mirrored in the bulk surface morphology where crazing, microvoiding and cavitation occurred with increasing <i>T</i><sub>d</sub>. SAXS/WAXS data showed that strain-induced crystallization occurs on drawing, but crystallite orientation decreased with increasing <i>T</i><sub>d</sub>, due to chain relaxation at temperatures ≥30 <sup>o</sup>C above <i>T</i><sub>g</sub>. However, no long-range oriented lamellar macromorphology was observed post-draw directly and only developed in the samples that were step annealed at temperatures above <i>T</i><sub>d</sub>.Also, the disordered α’ crystal form was observed post-draw at <i>T</i><sub>d</sub> between 60 – 80 <sup>o</sup>C, whereas <i>T</i><sub>d</sub> ≥ 90 <sup>o</sup>C, resulted in the ordered α crystal form directly.However, on annealing at temperatures of ≥110 <sup>o</sup>C, the α’- α crystal transition ensued and in all samples, an oriented lamellar macromorphology developed. Therefore, <i>T</i><sub>d</sub> and post-draw annealing, have a significant influence on the mechanical properties, crystallinity and crystalline phase transformation in PLLA, which in-turn, affects the polymers medical and industrial applications.