Biodegradable polyhydroxybutyrate (PHB) films were fabricated using acetic acid as an alternative to common solvents such as chloroform. The PHB films were prepared using a solvent casting process at temperatures ranging from 80 °C to 160 °C. The crystallinity, mechanical properties and surface morphology of the films cast at different temperatures were characterized and compared to PHB films cast using chloroform as a solvent. Results revealed that the properties of the PHB film varied considerably with solvent casting temperature. In general, samples processed with acetic acid at low temperatures had comparable mechanical properties to PHB cast using chloroform. This acetic acid based method is environmentally friendly, cost efficient and allows more flexible processing conditions and broader ranges of polymer properties than traditional methods.

An enzyme activated time-temperature indicator (TTI) which produces a direct colour change concomitant to variations in integrated time and temperature conditions is described. This direct colour change is realised by degrading a dye-loaded polyhydroxybutyrate (PHB) film by a depolymerase enzyme. The degradation of the PHB film by the enzyme causes the release of the dye in solution, which in turn undergoes an optical transition from clear to coloured with elapsing time. Macroscopic and microscopic optical observations confirms the uniform distribution of the dye in the PHB film. The dye release kinetics, mediated by the enzymatic reaction, are tested at different temperatures ranging from 4 to 37 °C, and are used to determine the suitability of a dye-loaded PHB as a time-temperature indicator for fresh food products based on kinetic parameters previously reported. The kinetic analysis shows that the activation energy of the dye release process is 74 kJ mol , and that, at 37 °C, the dye would be totally released within 6 h. However, when incubated at 4 °C, the TTI requires in the range of 168 h (7 days) to release all the dye. These kinetics values highlight the potential of the TTI for monitoring fresh food products that have optimum shelf life around 4 °C.

The effect of dimensional constraint, imparted by a variation in film thickness, on the enzymatic degradation of polyhydroxybutyrate (PHB) is reported. The characterization of the crystalline structure and the surface topography of solvent-cast PHB thin films revealed strong correlations between film thickness and both crystallinity and crystal anisotropy, with the polymer film becoming more amorphous with decreasing thickness. The enzymatic degradation of the PHB films was characterized using a high precision diffraction metrology, which enabled the visualization of small variations in the degradation behavior. The results show that the degradation rate increases with decreasing thickness due to the corresponding decrease in crystallinity. However, in a nanoscopic ultra-thin PHB specimen, produced by μ-transfer molding, enzymatic degradation was impeded. The enzymatic degradation rate of the PHB films therefore was found to exhibit a discontinuous trend with respect to film thickness: initially increasing as film thickness was reduced, and then decreasing dramatically once the thickness was reduced to tens of nanometers. In this regime, enzymatic degradation was hindered by the absence of crystalline regions in the films. These results show that a nano-dimensional constraint on PHB films can result in specimens with a tunable response to extracellular enzymes.

Due to their unique set of properties, polymer composites reinforced with graphenic nanoparticles are materials of interest for applications such as actuators, sensors, and degradable electronic components. To implement polymer nanocomposites in such applications, it is necessary to understand how both processing conditions and aging affect their properties. This is especially important when the matrix is composed of a semicrystalline polymer susceptible to transformations due to aging. In this study, we investigate the physical properties of a biodegradable polymer nanocomposite, comprising a polyhydroxybutyrate (PHB) matrix loaded with graphene nanoplatelets (GNP) as a conductive filler. PHB/GNP nanocomposite films were prepared at different solvent casting temperatures ranging from 80 to 140 C. Results show that electrical resistivity decreased-from 42.3 Ω cm for 80 C to 3.01 Ω cm for 110 C and 1.5 Ω cm for 140 C-with increasing solvent casting temperature. Moreover, for nanocomposite films containing less than 10 wt% of GNP and processed at 80 C, we observed significant decrease in resistivity (>50%) over time when the sample was aged at room temperature. We postulate that this decrease in resistivity arises from the cold-crystallization of PHB, as observed by X-ray diffraction analysis, and the densification of the polymer matrix, which is a direct consequence of an increase in crystallinity of nearly 20% over 168 h of aging. These results show that understanding the aging behavior of nanocomposites made from semicrystalline polymers such as PHB is crucial when designing conductive polymer composites and active devices.