Thiol-Ene Elastomers Derived from Biobased Phenolic Acids with Varying Functionality

The synthesis and physical properties of thiol ene elastomers derived from plant-based phenolic acids were explored. Phenolic acids of varying functionality (ranging from 2 to 4 hydroxyl and carboxyl groups per molecule) and relative placement of functional groups (ortho, meta, para) were allylated and subsequently reacted with a multifunctional thiol using a photoinitiator. The thermal and mechanical behaviors of the resulting elastomers were characterized. The networks derived from difunctional allylated phenolic acids exhibited narrow glass transitions (indicating a high degree of network homogeneity) and glass transition temperatures (T-g) which correlated with their cross-link density. The para placement of allyl groups on the allylated phenolic acid produced a network with the highest cross-link density, T-g, modulus, tensile strength, and elongation at break (followed by ortho and then meta). As the functionality of the allylated monomer increased (to 3-4 allyl groups per molecule), the cross-link density remained high yet the T-g decreased, attributed to a lower concentration of benzene rings throughout the network structure (as all networks were prepared at the stoichiometric ratio of allyl and thiol functional groups). The networks derived from the higher functionality allylated phenolic acids also exhibited lower elongation at break and associated tensile strength and tensile toughness, likely due to increased heterogeneity of the networks (indicated by higher glass transition widths compared to the networks derived from difunctional allylated phenolic acids). All networks exhibited behavior consistent with an ideal elastomer (affine network) at low to moderate strains, albeit with lower moduli than predicted from the monomer chemical structure. At the high end of the strain ranges achieved, some of the networks exhibited strain hardening behavior. This work develops fundamental relationships between the molecular structure of the phenolic acids, including number and placement of functional groups, and the physical properties of the resulting networks.