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.

Influence of α′‐/α‐crystal polymorphism on properties of poly(l‐lactic acid)

“…The mechanical properties of PLLA have also been investigated by several research groups, because the poor mechanical properties of PLLA, mainly its intrinsic brittleness, have so far limited its commercial development . Similar to other semicrystalline polymers, the mechanical properties of PLLA are affected by crystal fraction and morphology, whose effects on Young's modulus and the brittleness of PLLA are detailed by Tsuji and Ikada .…”

“…Barrier properties of PLLA were investigated by several authors as they are of fundamental importance for applications such as food packaging or drug delivery. Unfortunately contradictory results appear in the literature; 123,[131][132][133] hence a clear understanding of the influence of structure and morphology on permeability to gas and vapors of PLLA has not been achieved yet. The permeability (P) to gas and vapors is determined as the product of diffusivity (D) and solubility (S): P = DS.…”

“…123 The mechanical properties of PLLA have also been investigated by several research groups, because the poor mechanical properties of PLLA, mainly its intrinsic brittleness, have so far limited its commercial development. 132 Similar to other semicrystalline polymers, the mechanical properties of PLLA are affected by crystal fraction and morphology, whose effects on Young's modulus and the brittleness of PLLA are detailed by Tsuji and Ikada. 153 A systematic investigation of the influence of ′ -/ polymorphism on Young's modulus (E) and elongation at break ( r ) is described by Cocca et al 123 PLLA samples containing various ratios of ′to -crystals were prepared by isothermal cold crystallization in a wide temperature range, and the relative content of ′ -and -crystals was quantified by X-ray diffraction.…”

Crystallization of poly( l ‐lactic acid) in solution with the mosquito‐repellent N , N ‐diethyl‐3‐methylbenzamide

“…PLLA products may be obtained by classical melt‐processing technologies including injection molding or extrusion . Depending on the melt‐processing conditions, PLLA may partially crystallize to form a semicrystalline structure, which literally affects all application‐relevant properties including the stiffness, barrier characteristics, or the degradation/bioresorption kinetics . Though at favorable melt‐crystallization conditions a large crystal fraction up to 60‐70% may be formed, PLLA can be classified as a rather slowly crystallizing polymer as the minimum crystallization half‐time is of the order of magnitude of few minutes .…”

Effect of the Addition of Natural Rice Bran Oil on the Thermal, Mechanical, Morphological and Viscoelastic Properties of Poly(Lactic Acid)

“…Actually also in the terminal zone at low frequency, which defines the zero-shear viscosity η o , a small decrease in viscosity is observed for pure PLA, probably due to an original strongly entangled structure not completely destroyed before the beginning of the rheological test. The η o value for PLA at 175 • C is about 5.8 10 6 mPa s. This value is in the range typical of commercial PLA (M w = 140-160 kg/mol), i.e., 10 5 -10 7 mPa s [65]. volume and the polymer chain mobility.…”