Phase-structure and mechanical properties of isothermally melt-and cold-crystallized poly (L-lactide)

The effects of crystallinity differences induced by isothermal melt- and cold-crystallizations on thermal, mechanical and morphological behavior of poly (L-lactide) (PLLA) have been investigated. PLLA samples were crystallized from the melt and annealed from the glassy state at 80, 100 and 120 degrees C. The degree of crystallinity (X-c) and rigid amorphous phase (RAP) of PLLA was found to increase by crystallizing the samples at higher temperatures. Dynamic mechanical analysis (DMA) results suggest the presence of a rubber-like structure composed by both amorphous and crystalline phases for crystallized specimens. When samples are cold-crystallized, the structural integrity about T-g can be better kept, prompting to a smaller E' reduction after glass transition. Improvements in Young's modulus from 1027 MPa for quenched PLLA to 1401 MPa for the sample melt crystallized at 120 degrees C together with ductility reduction are obtained as the crystallization temperature increases. The tensile stress-strain curves at a range of temperatures, comprising below and above glass transition, have provided a mean for computing the mechanical properties ready for being used in linear elastic, visco-elastic and hyperelastic computing models. Polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) analysis revealed completely different morphologies for melt-crystallized and cold-crystallized samples. When PLLA was crystallized from the melt surface roughness increases up to 566 nm, while the increase in spherulite diameter is accompanied by a monotonous decrease of the nucleation density. However, when PLLA was cold-crystallized the obtained semicrystalline structure is independent of the crystallization temperature because nucleation occurred upon quenching. (c) 2012 Elsevier Ltd. All rights reserved.