Superior thermal stability and fast crystallization behavior of a novel, biodegradable α-methylated bacterial polyester

Given their ubiquity in modern society, the development of biodegradable and renewably sourced plastics is essential for the creation of an environmentally sustainable society. One of the drawbacks for currently available biodegradable plastics such as poly(l-lactic acid) (PLLA) and polyhydroxyalkanoates (PHAs) is that it is difficult to simultaneously achieve mechanical flexibility and certain crystallization behavior in these materials, which limits their use as replacements for established petroleum-based plastics such as isotactic polypropylene (iPP). Here, we report the synthesis and characterization of a new biodegradable plastic, poly(3-hydroxy-2-methylbutyrate) [P(3H2MB)], which is a member of the bacterial PHA family whose members include an α-methylated monomer unit. Biosynthesis of P(3H2MB) was achieved using recombinant Escherichiacoli expressing an engineered pathway. Biosynthesized P(3H2MB) exhibited the highest melting temperature (197 °C) among the biosynthesized PHAs and improved thermal resistance. It also exhibited improved crystallization behavior and mechanical flexibility nearly equal to those of iPP. The primary nucleation rate of P(3H2MB) was faster than that of P(3HB), and the spherulite morphology of P(3H2MB) was much finer than that of P(3HB). This crystal morphology may result in more rapid crystallization behavior, increased transparency, and enhanced mechanical properties. The superior physical properties of P(3H2MB) have the potential to open new avenues for the production of high-performance biodegradable plastics for replacing petroleum-based bulk commodity plastics.

Multiple β Forms of Saturated Monoacid Triacylglycerol Crystals

We have investigated the polymorphism of triacylglycerol (TAG) crystals as they affect the qualities such as shelf life, mouth feel, and texture of chocolate and other products. Saturated monoacid TAGs, like trilaurin, are considered as models for TAG crystallization; however, there is still debate about the number of their polymorphs that exist. In this study, we characterized a set of novel polymorphs, β forms of saturated monoacid TAGs, which were obtained via different pathways depending on the crystallization history, by polarized light microscopy, X-ray diffraction, and differential scanning calorimetry. Saturated monoacid TAGs were crystallized as the unstable polymorphs, the α or β’ forms first, and then they were transformed into β forms by solid–solid transformations. The β form that had transformed from β’ changed its morphology by a polymorphic transformation, while the β form made from the α form kept its spherulite morphology. The β forms obtained showed different melting points. Additional heat treatment promoted further polymorphic transformation. Four novel β forms were found for each of the saturated monoacid TAGs, trilaurin, trimyristin, tripalmitin, and tristearin. They showed similar polymorphism with the same subcell packing.

Foaming behavior of poly(lactic acid) with different D-isomer content based on supercritical CO2-induced crystallization

Using supercritical carbon dioxide (sc-CO2) as a physical foaming agent, the effect of sc-CO2 on the formation of crystalline domains and subsequently on the foaming behaviors of the two grades of PLA with different D-isomer content were investigated in a wide foaming temperature range. The PLA’s final crystallinity is significantly increased with decreasing annealing temperature and by reducing the D-isomer content. Cellular structure results show that not only the crystallinity but also the crystalline morphology play an important role in cellular structure. A novel spherulite morphology including ringless and ring-banded morphology in the same spherulite was formed at lower foaming temperature, as a result, some entities were nonuniformly distributed in the PLA foams. Uniform and closed cellular structure were obtained when only the ring-banded spherulites were formed. An opened and interconnected cellular structure is tended to be formed because of the synergistic effect of high temperature and plasticization of CO2. Based on the crystallinity and morphology, a suitable foaming window as a function of temperature is proposed. It is found that PLA with 4.1% D-isomer content had much broader suitable foaming window range to produce homogeneous cellular structure.

Miscibility Study of Poly(Butylene Succinate) and Pine-Gum Blends

Miscibility between poly (butylene succinate) [PBS], a semi-crystalline polymer with a natural gum extracted from the pine tree (Pinus Caribaea – Hondurensis), was investigated in solution cast blends using Differential Scanning Calorimetry [DSC] and Fourier Transform Infrared Spectroscopy [FTIR]. The spherulite morphology of PBS in the blends was observed with polarized optical microscopy [POM]. Depression in the equilibrium melting temperature of PBS in the blends was determined using the Hoffman-Weeks plot method. The depression in the crystallization temperature of the blends with increasing pine gum ratio and the emergence of extinction rings in the spherulites of the blends confirmed the blends to be miscible at the molecular level. Infrared spectroscopy indicated that interactions occurred between the hydroxyl groups of the pine-gum and the carbonyl group of PBS.

Interfacial Effects on the Spherulitic Morphology of Isotactic Polystyrene Thin Films on Liquid Substrates

The influence of interfaces on the morphology of flat spherulites of isotactic polystyrene (iPS) grown in thin films on liquid substrates was investigated. Amorphous iPS thin films spin-cast from a solution were annealed for cold crystallization on glycerol and silicone oil (nonsolvents for iPS). The number density of grown spherulites was revealed to be higher on the glycerol substrate than on the silicone oil substrate. This implies that the primary nucleation rate of crystallization is greater at the iPS/glycerol interface than at the iPS/silicone oil interface. The results may be consistent with the previous findings that concern the molecular interaction between atactic polystyrene and nonsolvents at the interface. In some cases, holes were formed in the thin films during the cold crystallization due to dewetting, which also significantly affect the spherulite morphology via, for example, transcrystallization.