Modulating Barrier Properties of Stereocomplex Polylactide: The Polymorphism Mechanism and Its Relationship with Rigid Amorphous Fraction

Abstract

The barrier properties of semicrystalline polymers are crucial for their performance and their use as packaging materials. This work uncovers the mechanism of polymorphism modification (α, α′ and stereocomplex-crystals) and its combined effect on the oxygen and water vapor barrier properties of semicrystalline stereocomplex polylactide (SCPLA). A polymorphic selective filler-type nucleator was employed to eliminate the temperature effect on the development of polymorphism and rigid amorphous fraction (RAF), allowing correlations of barrier properties with different crystal forms and RAF combinations under the same amorphous composition (SCPLA). The oxygen and water vapor barrier performances strongly correlated with crystallinity and crystal form but were not monotonically related to the RAF quantity. The study proposes that the chain conformation of intermediate phases between the crystalline and amorphous phases differs with the associated crystal forms, thereby leading to different RAF “qualities” and contributing to different gas diffusion and solubility coefficients of the amorphous regions. RAF’s per unit excess free volume may be varied with crystal forms, for instance: α′ ≫ SC > α. Therefore, SCPLA with α′ crystals exhibited high oxygen and water vapor permeabilities. Those with high SC and α crystals showed similar barrier behaviors governed by Henry’s law dissolution and followed a linear “two-phase” relationship with total crystallinity.

Unlocking the secrets of high-water barrier stereocomplex polylactide blend extrusion films

“…Chen et al reported similar findings on SC-PLA and the distribution between the formation of α-crystals and SC-crystals; they reported that at 140 °C, the α-crystals formed in a much higher abundance than SC-crystals. 41 This study was conducted at 160 °C but yielded similar findings, which means the overall X C improves the barrier more than the individual X HC or X SC formed. Additional studies could be conducted to decouple the role of the α-crystals and the SC-crystals.…”

“…The peaks at 2 θ = 12.0, 20.8, and 24.1 correspond to the SC-crystal. 41,47 As shown in Fig. 5, the α-crystal dominates in all the blends except the 50/50 PLLA/PDLA blend.…”

“…2d and f show distinct peaks corresponding to α-crystals (2 Θ at 14.9°, 16.8°, and 19.2° associated with the 010, 110/220, and 203 crystal planes) and SC-crystals (2 Θ at 12.0°, 20.8°, and 24.1° associated with the 110, 300/030 and 200 crystal planes) not present in the non-annealed samples. 40,41 Brochu et al 37 reported the nucleating effect of PDLA on PLLA; they looked at PLLA/PDLA blends produced by solvent casting and showed that racemic crystallites formed with as little as 10% PDLA. This finding is consistent with the findings of Tsuji and Ikada, 19 indicating that a 50/50 PLLA/PDLA blend produced via solvent casting had complete crystallization at 5 min, while PLLA and PDLA did not show appreciable crystallization until 10 min.…”