Stereocomplex (SC) crystallites, formed between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA), show a melting point 50 °C higher than that of PLLA or PDLA homocrystallites, which makes it possible for SC crystallites to be reserved in the melt of PLLA in asymmetric PLLA/PDLA blends and to act as a rheological modifier and a nucleation agent for PLLA. Herein, by a rheological approach, a transition from the liquid-like to solid-like viscoelastic behavior was observed for the SC crystallites reserved melt, and a frequency-independent loss tangent at low frequencies appeared at a PDLA concentration of 2.0 wt %, revealing the formation of SC crystallite network. By a delicately designed dissolution experiment, the structure of the formed network was explored. The results indicate that the network are not formed by SC crystallites connected directly with each other or by bridging molecules, but by the interparticle polymer chains which are significantly restrained by the cross-linking effect of SC crystallites. Nonisothermal and isothermal crystallization show that the reserved SC crystallites can accelerate remarkably the crystallization rate of PLLA due to heterogeneous nucleation effect. Besides, a special PDLA concentration dependence, e.g., the overall crystallization rate is almost independent of PDLA content for the blends with PDLA content higher than PDLA percolation concentration (2.0 wt %), was also observed. The increase of nuclei density for the blends containing PDLA from 2 to 5 wt % was estimated from POM observations. The result of an enhanced nucleation but an unchanged overall crystallization rate reveals the confining effect of the SC crystallite network on PLLA crystallization. This confining effect can be ascribed to the restrained diffusion ability of PLLA chains owing to the SC crystallite network.

Melt processing of polymers is preferable in industry. Unfortunately, for high molecular weight (M w) poly­(l-lactide)/poly­(d-lactide) (PLLA/PDLA) blends, exclusive formation of stereocomplex (sc) crystallites from the melt has never been achieved. We proposed a new and simple approach to enhance the melt crystallization of sc crystallites from high M w PLLA/PDLA by using poly­(ethylene glycol) (PEG). The crystallizability of the PLLA/PDLA blend is greatly enhanced by PEG during crystallization from the melt. The resulted crystalline structures depend on the M w and content of PEG, and the crystallinity of sc crystallites increases with increasing content or decreasing M w of PEG. More importantly, exclusive formation of sc crystallites is achieved in the blends with 10% PEG having M w values of 1000 or 2000 g mol–1. Polarized optical microscopy (POM) observation shows that the spherulitic growth rates of sc crystallites are accelerated. The results demonstrate that segmental mobility of polylactides (PLA) chains plays a dominant role on the formation of sc crystallites from melt and provide a simple way to prepare sc crystallites from high M w PLA by melt blending and enlarge the applications of PLA.

Polymers experience degradation during storage and service. One of the main degradation mechanisms of plasticised-polymer products is the loss of plasticiser, which leads to poorer mechanical properties and eventual contamination of the surrounding environment. This paper addresses the kinetics and predictions of plasticiser migration from polymers to a surrounding gas phase, an important issue for plastic and rubber products exposed to high service temperature conditions and during accelerated ageing and testing. The features and factors influencing the two migration-rate-limiting modes (plasticiser evaporation and diffusion), as well as migration issues related to bio-based plasticisers and plasticiser-biopolymer systems, are discussed.

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