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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

ABSTRACT:The involvement of microorganisms in the initial stage of maturation of natural rubber coagula was assessed with five latex treatments that varied in the initial quantity of microorganisms; the treatments ranged from latex added with an antimicrobial agent (3.4 Â 10 4 CFU/mL) to strongly inoculated latex (2.4 Â 10 7 CFU/ mL). After 0-6 days of maturation, the obtained rubber was characterized with respect to its physical and structural properties. The Wallace plasticity (P 0 ) and plasticity retention index (PRI) remained constant during maturation with the antibiotic-added treatment. PRI decreased with the maturation time, and the rate was proportional to the initial microorganism concentration. P 0 of all inoculated rubber increased for the first 2 days of maturation and decreased after 6 days of maturation. With respect to structural parameters, a higher initial microorganism content corresponded to a higher gel content and a lower weight-average molar mass after maturation, drying, and storage. The inoculated rubber showed a stable value for the number-average molar mass (M n ), in contrast to the noninoculated samples, for which an increase in M n during maturation was observed. The quantity of microorganisms significantly affected the physical properties and structure of the processed dry rubber. The mechanisms occurring during the initial stage of maturation are complex, and microorganisms are involved not only in the increase in sensitivity to thermooxidation but also in the crosslinking phenomenon between isoprene chains.