Surface Properties of Thermoplastic Starch Materials Reinforced with Natural Fillers

Abstract

The self-association force of water on the surface of a composite polymeric material is a physicochemical process dominated by cohesive forces and van der Waals-type interactions existing below the material surface. Perturbations in the chemical potential of water, brought about by the interaction between it and a polymeric surface, induce compensatory structural changes. Thus, the structure of water on the surface of a composite polymeric material reveals the hydrogen bond interactions taking place beneath it, which are key to understanding the properties of thermoplastic starch (TPS) materials. In the literature, there is a broad consensus based on empirical results that a contact angle (θ) greater than 65° defines a hydrophobic surface. These findings suggest that there are at least two different types of water structures that exist as a response to interactions occurring within the composite polymers. One of these is formed when there is a low density of “Lewis sites”, and the other when there is a high density of “Lewis sites” on the surface of the thermoplastic materials. This second scenario produces the collapse of the water structure, i.e., the collapse of the hydrogen-bonded network. In spite of the physicochemical response of water to the intra- and intermolecular interactions that occur on composite materials, these have not been studied as a means to modify the surface behavior of TPS materials. This could be achieved by incorporating natural fillers that have a plasticizer or crosslinking effect on their structure. In this chapter, we analyze the surface properties of starch-based composite materials as an indirect measure of the interactions that occur within them, mainly as regards plasticizing effects and crosslinking reactions.