Poly(lactic acid): Influence of chain structure and processing on materials properties

Poly(lactic acid) (PLA) is a biodegradable polyester produced from renewable resources. Similar to other biodegradable polyesters, PLA displays crystal polymorphism. Only two crystal modifications grow upon usual processing conditions: ? crystals at high crystallization temperature and ?' crystals at low temperatures. PLA films containing different ratios of ?'?and ? crystals were prepared by annealing quenched PLA at different temperatures. It was found that the polymorphic structure of PLA significantly affects its mechanical, thermal and barrier properties. The ? crystal modification provides a better barrier to water vapor and a higher Young's modulus, compared to films containing ?' form, but a lower elongation at break. The varied barrier and mechanical properties were correlated to the different packing of PLA chains in the two analyzed polymorphs, as well as to the morphology of PLA spherulitic superstructures. Quantitative analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure of PLA composed of a crystal phase and two amorphous fractions with different mobility, for all the analyzed thermal histories. A higher coupling of the amorphous chain segments with the crystal phase was found in the presence of ? crystals, probably due to the slightly larger lattice dimensions and the looser arrangements of PLA chains in the ?' structure. Moreover, the influence of chain structure of PLA on ? / ?' polymorphism was studied. The monomer, lactic acid, has two optically active forms called L-lactic acid and D-lactic acid. Commercial poly(lactic acid) grades are usually based on an L-rich mixture, as the majority of bacteria used in fermentation processes mainly produce L-lactic acid, and typically comprise a minimum of 1-2% D- units. Incorporation of D-lactic acid monomer leads to a decrease of the overall crystallization rate of PLA, as well as of the rate of spherulite growth (G) of both the crystal modifications. The relative crystallization rates of ? and ?' forms are highly affected by stereoregularity, especially in the PLA grades that have a high crystallization rate. A high D-lactic acid content results not only in an overall slower crystal growth, but also in a varied temperature range where each of the two crystal modifications prevail, with a shift to lower temperatures of both the maxima of the G vs. temperature plots, indicating that inclusion of D-lactic acid units in the PLA chain affects crystallization rate of both ? and ?' crystal modifications.