This project investigates the moisture barrier properties of bio-based polymers and ways of improving them. The first section addresses the effect of crystallinity on the water permeability of poly(lactic acid) (PLA). The second section investigates PLA/talc composites and PLA/ montmorillonite nanocomposites. The third section is focused on a new polymer, polybutylene succinate (PBS), and its nanocomposites with montmorillonite.
In the first section, the water vapour transmission rates (WVTR) through samples of polylactic acid of different crystallinities have been measured. Three different grades of commercial PLA were used with different ratios of L-lactide and D-lactide to give a range of crystallinities from 0 to 50%. Sheets of PLA were prepared by melt compounding followed by compression moulding and annealing at different temperatures and for different times to give the range of crystallinities required. Crystallinity was measured by differential scanning calorimetry (DSC) and the morphology of the samples was observed under crossed polars in a transmitted light microscope. Water vapour transmission rates through the films were measured at 38°C and at a relative humidity of 90%. It was found that the measured values of WVTR decreased linearly with increasing crystallinity of the PLA from 0 to 50%. The results are discussed in terms of the effect of crystallinity on solubility and shown to fit the tortuous path model. The model was also successfully used to explain published data on water permeability of polyethylene terephthalate.
In the second section, a series of PLA/talc composites and PLA/ montmorillonite nanocomposites were prepared by melt compounding followed by compression moulding. The morphologies of the composites were investigated using transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD) and it was found that the fillers were well dispersed in the polymer matrix. The average aspect ratio of the compounded talc was found to be 8, and that of the nanoclay was found to be 50. Water vapour transmission rates (WVTR) through the films were measured at 38°C and at a relative humidity of 90%. It was found that the measured values of WVTR decreased with increasing filler content and the results gave good agreement with predictions from the Nielsen tortuous path model.
In the third section, PBS/ montmorillonite nanocomposites were prepared by melt compounding followed by compression moulding. The melting and crystallisation behaviour of the pure PBS samples were investigated using differential scanning calorimetry (DSC) and cross polarised optical microscopy. A slight decrease of the degree of crystallinity was found in PBS containing 5% nanoclay. The morphology of the composites was investigated using transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD) and it was confirmed that that composite structures were intercalated. Water vapour transmission rates (WVTR) through the PBS sheets were measured using a MOCON Permatran-W®398. The measured values of WVTR decreased with increasing nanoclay content. However, the experimental values were all higher than the values predicted by the Nielsen tortuosity model. This result shows that in the case of PBS, which is a highly crystalline polymer, the nanoclay is not as well dispersed and is not as effective in reducing water vapour permeability as in the case of PLA.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.