Highly functional green materials platform: Starch-ionic liquid-carbon nanotube polymer melt nanocomposites

Abstract

Starch is a promising biodegradable polymer material which has attracted attention recently. A renewable and biodegradable resource can potentially address the shortage of conventional petroleum-based polymers and help solve the problem of environmental pollution from the use of these polymers. To use starch in polymer processing, gelatinisation, which is the process to disrupt the native starch structure, must happen to turn the starch in to an amorphous thermoplastic starch (TPS). However, water, the conventional gelatinisation agent and plasticiser, is extremely volatile and will evaporate during thermal processing. This leads to high melt viscosities, processing difficulties and hence poor product quality. In addition the mechanical properties of the resulting TPS products are not as high as that of the currently used petroleum-based polymers that they seek to replace.Therefore, potential solutions for TPS processing and application development is to use other plasticisers to aid processing and/or to reinforce TPS with nano-particles. One of the key tasks of this project was to understand the plasticizing effects of ionic liquids (ILs) in TPS processing and interpreting these in terms of the molecular interactions between the components. In addition the role of adding nano-particles, in the form of carbon nanotubes, in an attempt to strengthen TPS was investigated. The ultimate aim was to provide design guidelines to produce high value added biodegradable nanocomposites.Three types of ionic liquids (ILs) were considered: 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]), 1-allyl-3-methylimidazolium chloride ([Amim]Cl) and 1-butyl-3- methylimidazolium chloride ([Bmim]Cl). They were processed in batches of 150g by thermal mixing. Their behaviours were compared to glycerol as plasticisers of TPS, and their effects on TPS processing, water uptake during conditioning, mechanical properties, thermal stabilities, crystallisation behaviour and molecular interaction were studied. Both spectroscopic and X-ray analysis techniques were used to investigate the interactions that occurred, and these were related to the material characteristics. Then, multi-walled carbon nanotubes (MWCNTs) were introduced to the TPS matrix to investigate their behaviour as a strengthening agent and also to potentially create a functional material with useful electrical properties.The results demonstrated that the ILs were effective plasticisers for TPS. However, whilst their use improved the flexibility of TPS, mechanical strength and thermal stability were decreased. The molecular interactions between TPS and ILs were found to differ between TPS and glycerol. This led to differences in the crystalline structures formed, 2% to 4% more water uptake during conditioning and hence up to a 90% decrease in Young’s modulus. In particular the effect of water uptake during conditioning was identified as the key contributor to the ultimate mechanical properties. The IL plasticised TPS were also found to be less thermally stable.In order to improve these poorer mechanical properties, the introduction of MWCNTs was investigated. Whilst it was possible to introduce MWCNTs to the TPS matrix, the loading achieved was not sufficient to make a significant effect on the material and electrical properties. Experiment results have shown that the CNTs did however have an effect on the crystallisation behaviour, resulting in a reduction of the differences observed between different ILs. Overall, this fundamental study on starch/ILs/carbon nanotubes composites has provided a better understanding of the interaction mechanisms and their impacts on material properties. This knowledge can be a guide and applied to further optimizing studies of starch/ILs/carbon nanotubes composites and eventually industrial applications in the future.