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 °C. The degree of crystallinity (Xc) 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 Tg 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 °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.