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

The influence of molecular parameters on the crystallization kinetics and alpha/alpha '-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 alpha- and alpha'- 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 alpha- and alpha'-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 alpha/alpha'-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.