Carbohydrates as Hard Segments for Sustainable Elastomers: Carbohydrates Direct the Self-Assembly and Mechanical Properties of Fully Bio-Based Block Copolymers

A series of fully bio-based block copolymers (BCPs) consisting of maltooligosaccharides (maltose, maltotriose, maltotetraose, and maltohexaose; A block) and poly­(δ-decanolactone) (PDL; B block), with ABA-, A₂BA₂-, A₃BA₃-, A­(BA)₂-, and A₂(BA)₂-type architectures, were synthesized to demonstrate the potential of oligosaccharides as novel hard segments for bio-based elastomers. To understand the correlation between the BCP molecular structure and material properties, the BCPs were designed to have comparable molecular weights (ca. 12K) and total numbers of glucose units (12). Morphological analysis revealed the formation of body-centered-cubic sphere and hexagonally close-packed cylinder (HEX) morphologies depending on the branched architecture (interdomain distance 9.7–14.4 nm). While the PDL homopolymer is a viscous liquid due to its low T_g and amorphous nature, all BCPs exhibited elastomeric properties, confirming that the oligosaccharide blocks segregated to form the hard domains to cross-link the rubbery PDL chains. Tensile testing revealed that the mechanical properties of the BCPs were mainly determined by the microphase-separated structure and less affected by the length of each oligosaccharide chain. The HEX-forming A₂BA₂- and A₃BA₃-type BCPs exhibited Young’s moduli of ∼6 MPa, which is comparable to well-known styrene-based thermoplastic elastomers. Furthermore, a readily available polydisperse maltooligosaccharide was employed to synthesize an A₂BA₂-type BCP with a higher molecular weight PDL block (20K), which exhibited a Young’s modulus of ∼6 MPa and an elongation at break of ∼700%. These results demonstrate that oligosaccharides are a sustainable alternative to the petroleum-derived synthetic hard segments (e.g., polystyrene), thereby opening up a new avenue for fully bio-based soft material design.