Polyhydroxybutyrate (PHB) is a sustainable polymer that is a promising candidate for replacing petroleum-based plastics in food packaging. Fillers are used to improve the mechanical properties of PHB composites, simultaneously changing the crystallinity of the polymer matrix. However, it is not well understood how fillers affect crystallisation and microstructure, and thus the resulting mechanical properties of the composite. This review summarises simulation work on polymer nucleation and crystallisation and how nucleation is influenced by different types of polymer-filler interfaces. Experimental studies of PHB composites with a wide variety of fillers are reviewed to find trends between the filler type, crystallinity and mechanical properties. It is clear that fillers act as nucleants that increase the number of spherulites while reducing spherulite size. This behaviour is apparent for almost all fillers regardless of filler chemistry or topology. However, the data obtained from the literature do not seem to produce strong conclusions about the effect of the degree of crystallinity on the tensile properties of PHB-filler composites, although there are some weak trends that indicate the importance of microstructure. In order to enable prediction and control of PHB composite properties, further systematic studies are required to elucidate the effect of specific filler types and the connection between crystallinity, microstructure and mechanical properties.

The polymer glass transition is an important property in a wide variety of applications. The glass transition temperature of a polymer composite or confined thin film can be significantly different to the pure polymer. Molecular dynamics simulations are useful for providing molecular level insight and prediction, particularly at interfaces, that are not easily observable experimentally. However, there are significant methodological uncertainties in calculating the polymer glass transition temperature using molecular dynamics simulations. In this work we investigate how the cooling method, fitting range and statistical variation affects the calculated glass transition temperature of polyethylene. We found that it is necessary to perform multiple independent simulations to obtain statistically significant results, and that appropriate fitting ranges must be chosen. The methodological findings were used to investigate the difference in glass transition temperature between pure polyethylene and a polyethylene film confined between graphene surfaces. It was found that the glass transition temperature of a 9 nm thick confined film was higher than bulk polyethylene by approximately 15 K.

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