Influence of melt processing conditions on poly(lactic acid) degradation: Molar mass distribution and crystallization

Abstract

The degradation of poly(lactic acid) (PLA) during thermal-mechanical processing was studied and the influence of processing conditions on degradation rate was determined by size exclusion chromatography coupled with multi-angle light scattering (SEC-MALS). A two-parameter model accounting for both chain scission and recombination processes was used to describe the experimentally observed molar mass distribution. The degradation and recombination rate constants were determined for undried and dried PLA. It was highlighted that the effect of processing temperature (in the 170-210 °C range), processing time (until 30 min) and shear rate (rotor speed varying from 0 to 150 rpm) on molar mass reduction can be relatively well simulated insofar as self-heating related to the mechanical energy conversion into heat was taken into account. The influence of melt processing on the thermal behaviour of PLA was also investigated using temperature modulated differential scanning calorimetry (TMDSC). It was evidenced that the molar mass reduction affects the crystallizability of PLA. Cold crystallization temperature progressively decreases with decreasing molar mass and the metastable a' phase is formed in place of the stable ? phase. The a' phase can be partially converted into ? form during melting giving rise to a double-melting peak. The two peaks can be separated using reversing and non-reversing signals confirming that recrystallization of the a' form occurs. (Résumé d'auteur

Effects of Aging and Different Mechanical Recycling Processes on the Structure and Properties of Poly(lactic acid)-clay Nanocomposites

“…The structure of PLA nanocomposites can be significantly altered by the degradation that takes place during the service life and the recycling process, which includes a melt reprocessing at high temperature and shear stress. On one hand, it is known that PLA and PLA-based materials are susceptible to thermal, photochemical and hydrolytic degradation processes that lead to a decrease of the average molecular mass, which can negatively affect the thermal, optical and mechanical properties of the material [28]. On the other hand, the dispersion of the nanoparticles in the polymer can be also altered during the recycling process due to the shear stress, resulting in a different morphology of the material, which also leads to changes in the properties.…”

Characterisation of natural fibre reinforced PLA foams prepared by supercritical CO2 assisted extrusion

“…The melting of the crystalline phase occurs around 150°C. Typically two endothermic peaks are visible, at 149°C corresponding to melting of the less ordered α' crystals, and at 156°C corresponding to melting of the thermodynamically more stable α crystals [22]. In contrast, only a slight cold-crystallization exotherm is observable in the DSC curve of the PLA foam of 96.4% porosity, indicating notice- able inherent crystalline phase in this sample.…”

Functionalization of poly(lactide) with N‐phenyl maleimide using N‐acetoxy‐phthalimide during reactive extrusion

“…The system was calibrated with 12 polystyrene standards (EasiVial GPC/SEC calibration standards, Agilent Technologies, Santa Clara, CA) from 164 to 364,000 g mol −1 . A differential RI increment (dn/dc) of 0.0558 for PLA in THF was used …”