Synergistic interactions of plasticizers and nanoclays in hydrophilic starch based bionanocomposites

Abstract

Depletion of non-renewable resources and exorbitant levels of carbon dioxide emissions have questioned the further usage of traditional plastics. The imbalance in global sustainability has necessitated the development and use of biodegradable polymers. This research work set sights on understanding the synergistic interactions of plasticizer, starch and Na[superscript]+-montmorillonite (MMT). Having a clear view of the molecular behavior within this ternary system ameliorates the performance of the end product and fabrication of the tailored-property biodegradable polymers. Conversely, the explicit information of the synergistic Interactions of Plasticizers and MMT in hydrophilic starch is still vague. Hence a great degree of work and effort has been put forward through this work to understand the fundamental principles of competitive interaction.The study emphasizes on modelling the kinetics of various components in the system such as crystallisation of the long chain starch, water diffusion from the polymer matrix and the interaction between the MMT molecules and the water molecules. It was observed that the interactions between MMT and plasticiser were enhanced upon increasing the MMT loading. The high hydrophilicity of plasticizer ensured a stronger interaction of plasticizer/MMT which overtook the MMT/MMT interaction causing a dimensional increase in basal spacing in the high plasticiser loading samples.GAB model based on the multi-layer kinetics has been applied in the current work to understand the isotherm behaviour. Small Angle X-ray Scattering (SAXS) was employed to determine the diffraction pattern and intensity of the samples at the micro/nano structural level and thereby fathom the orientation of the crystalline domains. Differential Scanning Calorimetry was carried out to indicate the changes in T[subscript]g/T[subscript]m values of various samples and the corresponding degree of plasticization. The obtained results were then interpreted with Avrami equation to reach solid conclusions.The crystallization ability and intermolecular hydrogen bond strength of the applied plasticizer were the key parameters that affected the crystallisation process. The crystallisation mechanism in xylitol-plasticized samples differed from that of glycerol/sorbitol-plasticized samples. It was also determined that the molecular size of the plasticizer was another significant element that influenced the crystalline domain formation. The interaction process was defined into three stages based on the concentration of plasticizer used. The plasticizer concentration for the formation of a loosen-soft polymeric network and tighten-firm polymeric network was categorised. Correspondingly, the ‘threshold’ value for glycerol, xylitol and sorbitol-plasticized systems was characterised to be 5%, 10% and 5%, respectively. Morphological observations such as Wide Angle X-ray Diffraction (WAXD) and Transmission electron microscopy (TEM) has been utilised to examine the morphology formed in all nanocomposite samples.A relatively unsaturated loosen-soft polymeric network was formed and the starch/plasticizer interaction and starch/MMT interaction transpired without interfering each other within the threshold limits. Nonetheless, beyond the threshold, a relatively saturated tighten-firm polymeric network was observed as confirmed by x-ray scattering results, the molecular dynamic modelling results and Positron Annihilation Lifetime Spectroscopy (PALS) measurements from representative samples. Plasticizer/plasticizer interactions significantly altered the MMT exfoliation process and crystallization behaviour of the corresponding sample.The complex interactions existing in the polymeric system were found to be dependent on several main factors including type of the plasticizer and the relative ratio of plasticizer and MMT. These factors have been kept in mind whilst designing the modes of experiment and understanding the related outcomes. The incidence of excess moisture successfully modified the interactions amongst starch/ MMT/plasticizer. Water molecules behaved like a typical plasticizer and occupied most of the small voids between the starch polymers throughout the polymeric network.