Rheology, mechanical properties, and barrier properties of poly(lactic acid)

Abstract

Knowledge of the fundamental parameters of the poly(lactic acid) (PLA) molecular chain and resulting macroscopic properties is important for successful application of this polymer in different domains. Rheological data show that PLA has the typical properties of a linear and semi-stiff polymer chain. The stereochemical composition of the atactic polymer chain does not impact the rheological, mechanical, and barrier properties of PLA. Most commercial PLA grades contain a large majority of l-lactic acid units, in which case the polymer is named PLLA. PLLA at room temperature is a brittle glassy polymer and its main fracture mechanism is crazing. Above the glass transition, semicrystalline PLLA shows extensive cavitation. Uniaxial deformation above, but near, the glass transition temperature leads to the formation of a mesophase, responsible for strain hardening. At higher temperatures, strain hardening is caused by induced crystallization. The PLLA oxygen barrier properties are comparable to those of polystyrene (PS). The water vapor barrier properties are higher than those of PS because of the higher polarity of the polymer chain. An increase in barrier properties can be obtained by specific crystallization techniques, multilayer strategies, or the addition of (nano)fillers.