An efficient and stable star-shaped plasticizer for starch: cyclic phosphazene with hydrogen bonding aminoethoxy ethanol side chains

Abstract

Starch is a sustainable polysaccharide with many existing and foreseen applications. Even if a plethora of routes have been described to plasticize it, e.g. with glycerol, sorbitol, and polyols, there remain major challenges related to retrodegradation, stability, and plasticizer leaching. Here we describe plasticization of starch using star-shaped molecules to combine the seemingly conflicting requirements: efficient plasticization typically assigned to low molecular weight hydrogen bonding plasticizers and reduced leaching typical for high molecular weight molecules. Efficient plasticization is allowed by the short, flexible, and hydrogen bonding dangling side chains, which are connected to the core of the plasticizer, leaving crystalline starch domains to allow self-reinforcement. The star-shaped plasticizer, a cyclic phosphazene having six covalently bound aminoethoxy ethanol side groups, was synthesized via nucleophilic substitution, and a series of films using rice starch and different weight fractions of the plasticizer were prepared by drop casting. Incorporating ≥ca. 10 wt% of the plasticizer leads to transparent plasticized films. FTIR indicates that the samples having ≤ca. 60 wt% of the plasticizer involve both plasticized amorphous domains and reinforcing crystalline domains, suggesting self-reinforced nanocomposite structures. The composition with 20 wt% of the plasticizer shows a high tensile modulus of 1.12 GPa and a yield strength of 20.9 MPa, still showing a large strain of 8.8%. No retrodegradation is observed after two months and no clear tackiness is observed even during aging of one year, suggesting stability and suppressed plasticizer leaching. We suggest that the star-shaped hydrogen bonding plasticizers show promising potential to optimize starch-based materials for advanced applications.