No abstract

Usually, the nature of surface-induced nucleation in polymer blends is not easily disclosed. A novel approach for studying surface-induced crystallization in blends of semicrystalline polymers is proposed here. It consists of detecting variations in the crystallization kinetics of the dispersed phase with changing the crystalline state of the matrix through self-nucleation. It can be used only when the dispersed phase has a lower melting temperature than the matrix phase. As a case study, the crystallization behavior of dispersed polyethylene droplets in a polypropylene matrix was investigated. An enhancement of crystallization kinetics of polyethylene was achieved when the lamellar thickness of polypropylene increased, and it was proved by the formation of a transcrystalline layer of polyethylene at the interface, as observed by scanning electron microscopy. Compared to the self-nucleated neat polyethylene, the efficiency of the nucleating effect of polypropylene toward polyethylene was estimated around 140%. This result together with a very low value for the interfacial free energy difference as obtained from isothermal crystallization measurements is evidence that such surface-induced nucleation occurs through epitaxial growth. Moreover, a mechanism of polyethylene nuclei formation through epitaxy, which was proposed in the literature, was proved to be valid for blends of the two polymers through small- and wide-angle X-ray scattering structural analysis. While epitaxy between polyethylene and polypropylene was previously shown only for ideal systems such as thin-layered films, it is hereby reported for common melt mixed blends of the two polyolefins.

Poly(3-hydroxybutyrate) (PHB) is naturally accumulated by bacteria but can also be synthesized chemically. Its processability is limited, as it tends to degrade at temperatures above its melting temperature; hence, investigation into crystallization kinetics and morphology of PHB materials of both natural and synthetic origins is of great need and interest to get a better understanding of structure–property relationship. Accordingly, this contribution reports a first study of the crystallization and morphology of synthetic PHB materials of different molecular weights. These synthetic PHBs are racemic mixtures (50/50 mol %) of R and S chain configurations and are compared with an enantiopure bacterial R -PHB. Nonisothermal and isothermal crystallization studies show that R and S chains of PHB can cocrystallize in the same unit cell as the R -PHB. Most significantly, the results show that the presence of S chains decreases the overall crystallization rate, which could enhance the processability and industrialization of PHB-based materials.

Stereo-defects present in stereo-regular polymers often diminish thermal and mechanical properties, and hence suppressing or eliminating them is a major aspirational goal for achieving polymers with optimal or enhanced properties. Here, we accomplish the opposite by introducing controlled stereo-defects to semicrystalline biodegradable poly(3-hydroxybutyrate) (P3HB), which offers an attractive biodegradable alternative to semicrystalline isotactic polypropylene but is brittle and opaque. We enhance the specific properties and mechanical performance of P3HB by drastically toughening it and also rendering it with the desired optical clarity while maintaining its biodegradability and crystallinity. This toughening strategy of stereo-microstructural engineering without changing the chemical compositions also departs from the conventional approach of toughening P3HB through copolymerization that increases chemical complexity, suppresses crystallization in the resulting copolymers, and is thus undesirable in the context of polymer recycling and performance. More specifically, syndio-rich P3HB (sr-P3HB), readily synthesized from the eight-membered mesodimethyl diolide, has a unique set of stereo-microstructures comprising enriched syndiotactic [rr] and no isotactic [mm] triads but abundant stereo-defects randomly distributed along the chain. This sr-P3HB material is characterized by high toughness (U T = 96 MJ/m 3 ) as a result of its high elongation at break (>400%) and tensile strength (34 MPa), crystallinity (T m = 114 °C), optical clarity (due to its submicron spherulites), and good barrier properties, while it still biodegrades in freshwater and soil.