Importance of Low‐Temperature Melt‐Mixing on the Construction of Stereocomplex Crystallites with Superior Nucleation Efficiency in Asymmetric Poly(l‐lactide)/Poly(d‐lactide) Blends

Abstract

The nucleation efficiency (NE) of stereocomplex crystallites (SCs) formed in asymmetric poly(L‐lactide)/poly(D‐lactide) (PLLA/PDLA) blends is generally unsatisfactory because the competition between stereocomplexation and chain mixing involved in the melt‐mixing process can cause low formation efficiency and even severe aggregation of SCs. Herein, it is attempted to achieve high‐efficient formation of finely dispersed SCs particles by designing a unique melt‐mixing procedure, where the mixing of PLLA with 0.75 wt% PDLA is first performed at elevated temperatures (far above the melting temperature of SCs) to allow the homogeneous mixing of PLLA/PDLA chains and then at a low temperature (slightly above that of homocrystallites) to permit the full stereocomplexation of the premixed chains. It is found that the SCs formed in the blends exhibit unexpectedly low NEs (e.g., 54.5%), much inferior to that (73.6%) in the counterpart without undergoing premixing. This is because the introduction of premixing leads to a remarkable deterioration in the amount of SCs particles formed, despite decreased particle size, highlighting that the direct mixing at low temperatures of 170–180 °C (about 20–30 °C lower than that used in common melt‐processing of PLA) is more effective for the construction of SCs with superior NE. The mechanisms for these striking findings are discussed.

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

“…On the basis of elaborate exploration, many strategies have been reported to promote the formation of SC. These include the synthesis of stereo-block PLLA- b -PDLA, − melt blending at a temperature between the melting points of SC and HC, , adding nucleating agents, , flow-induced crystallization, ,, high temperature annealing, , etc. In spite of all this effort, crystallization of pure SC still remains an unsolved challenge, especially in a high-molecular-weight (HMW) PLLA/PDLA racemate. − …”

Toward Heat-Resistant and Transparent Poly(l-lactide) by Tailoring Crystallization with an Aliphatic Amide as a Nucleating Agent

“…Unfortunately, despite the high nucleation efficiency, the agglomeration of these particles in the PLLA matrix and the refractive index mismatch between them result in a loss of transparency . Interestingly, stereocomplexed polylactide particles obtained by stereocomplexation between PLLA and its enantiomer poly­( d -lactide) (PDLA) have been evidenced as a highly efficient bio-NA for PLLA crystallization, but their effectiveness for reducing crystal size is rather limited owing to the self-networking within the matrix. − In this context, much attention has been given to the organic compounds (e.g., 1,3,5-benzene-tricarboxylamide derivatives , and oxalamide compounds , ) that can dissolve in PLLA melt to ensure good dispersion at the molecular level and subsequently self-organize into regular supramolecular structures upon cooling to nucleate PLLA crystallization on their surface. Such supramolecular NAs are highly desirable for the effective control of the crystalline superstructure of PLLA. , For example, Ma et al reported that some oxalamide compounds are capable of organizing into fibrillar structures to induce the rapid growth of PLLA lamellae, forming shish-kebab-like structures.…”