Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 2. Effect of molecular mass on the crystal growth rate and semicrystalline morphology

Abstract

The influence of molecular parameters on the crystallization kinetics and α/α ′-crystal polymorphism of poly(lactic acid) (PLA) was analyzed. Previous studies indicated that the content of L-and D-lactic acid affects both the crystallization rate and the formation of α-and α′-crystals of PLA. This preliminary investigation has now been completed by analysis of the influence of the chain length on the growth rate of the α-and α′-polymorphs. A series of linear PLA grades with molecular masses ranging from 60 to almost 600 kDa, and containing up to 3 wt.% D-lactic acid was investigated. Independent of the copolymer composition, an increase of the molecular mass leads to a lower maximum crystallization rate but does not affect the α/α′-crystal polymorphism of PLA upon melt crystallization, at least for the analyzed range of molecular mass and D-lactic acid content. Lower molecular mass causes an initial growth of hedritic superstructures, which gradually transform into spherulites. The D-lactic acid content also affects the morphology, as hedrites are observed also at higher molecular mass. Highly irregular and fibrillar structures grow in PLA grades with low stereoregularity and high molecular mass.

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

“…Finally, it appears that the ratio of the maximum growth rates of α′‐ and α‐crystals changes as a function of the d ‐isomer content but not of molar mass. Otherwise, the observed decrease of the maximum crystal growth rate with increasing molar mass is in line with general predictions available in the literature but also specific experiments performed on PLLA …”

“…POM micrographs with a λ‐plate inserted diagonally between crossed polars of a stereoregular PLLA (100% l ‐isomer) with M w = 576 kDa, isothermally crystallized at 140 °C. (a) is reprinted from reference , by permission from Springer Nature, Copyright (2015).…”

“…The morphology of PLLA spherulites is affected by both the molar mass and the d ‐isomer content of the chains, but not by the α′‐/α polymorphism . A few examples are provided in Fig.…”

“…POM micrographs PLLAs of various molar masses and d ‐isomer contents during isothermal crystallization at 140 °C. Reprinted from reference , by permission from Springer Nature, Copyright (2015).…”

“…Spherulite growth rate of PLLA of different d ‐isomer content (a) and different molar mass (b) as a function of the crystallization temperature. Adapted from (a) reference , Copyright (2016), with permission from Elsevier and (b) reference , by permission from Springer Nature, Copyright (2015).…”