Poisoning by Purity: What Stops Stereocomplex Crystallization in Polylactide Racemate?

Abstract

Formation of stereocomplex crystals (SC) is an effective way to improve the heat resistance and mechanical performance of poly(lactic acid) products. However, at all but the slowest cooling rates, SC crystallization of a high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemate stops at a high temperature or does not even start, leaving the remaining melt to crystallize into homochiral crystals (HC) or an SC–HC mixture on continuous cooling. To understand this intriguing phenomenon, we revisit the SC crystallization of both high- and low-molecular-weight PLLA/PDLA racemates. Based on differential scanning calorimetry (DSC), supplemented by optical microscopy and X-ray scattering, we concluded that what stops the growth of SC is the accumulation of the nearly pure enantiomer, either PDLA or PLLA, that is rejected from the SC ahead of its growth front. The excess enantiomer is a result of random compositional fluctuation present in the melt even if the average composition is 1:1. The situation is more favorable if the initial polymer is not fully molten or is brought up to just above the melting point where SC seeds remain, as proven by DSC and X-ray scattering. Moreover, we find that not only is SC growth poisoned by the locally pure enantiomer but also that at lower temperatures, the HC growth can be poisoned by the blend. This explains why SC growth, arrested at high temperatures, can resume at lower temperatures, along with the growth of HC. Furthermore, while some previous works attributed the incomplete SC crystallization to a problem of primary nucleation, we find that adding a specific SC-promoting nucleating agent does not help alleviate the problem of cessation of SC crystallization. This reinforces the conclusion that the main problem is in growth rather than in nucleation.

Scalable Continuous Manufacturing Process of Stereocomplex PLA by Twin-Screw Extrusion

“…Another more recent study further described this phenomenon as “poisoning by purity”, in which the growth of the pure PLA homopolymer is rejected from the SC ahead of its growth front. They contributed this phenomenon to random compositional fluctuations present in the melt [ 67 ].…”

Local segmental miscibility dominating stereocomplex crystallization in polymer blends

“…In experiments, some groups proposed that the local segmental miscibility or its fluctuation plays an important role in the SC formation. 39 For instance, Yang et al utilized multi-walled carbon nanotube (CNT) grafted PLLA- block -PDLA copolymers in PLLA/PDLA blends to prevent partial phase separation and enhance molecular miscibility, thereby promoting the SC formation. 40,41 Our group had previously demonstrated, through molecular simulations, that the SC content in multiblock copolymers is influenced by the block number due to the local segmental miscibility of different blocks.…”

Enantiomers Self-Sort into Separate Counter-Twisted Ribbons of the Fddd Liquid Crystal─Antiferrochirality and Parachirality

“…48 Recently its slow growth was explained by an effect named "poisoning by purity", whereby local nonracemic concentration fluctuations cause the crystal growth surface to be blocked by the rejected surplus enantiomer. 49 It is conceivable that the relatively slow formation of the Fddd phase shown in Figure 9a,b is due to the same diffusion-controlled phenomenon. While the growth of Fddd is suppressed at a cooling rate <30 K/s (1800 K/min), PLA stereocomplex stops growing already at 5 K/min.…”