Synthesis and Physical Properties of Thiol–Ene Networks Utilizing Plant-Derived Phenolic Acids

Elastomeric polymer films synthesized through thiol–ene chemistry, suitable in applications as coatings and adhesives due to their ease of preparation and superior physical properties, are traditionally derived from petroleum sources. Of recent interest is the exploration of sustainable alternatives for the precursors to these materials. Here, we report the synthesis of thiol–ene networks through the photoinitiated reaction between allylated plant-based phenolic acids (salicylic acid and 4-hydroxybenzoic acid) and a multifunctional thiol, followed by isothermal annealing. Plant-sourced phenolic acids offer many advantages as biorenewable monomers: their rigid aromatic rings are expected to provide mechanical strength to the resulting polymers and the presence of multiple hydroxyl and carboxyl groups leads to ease of functionalization. Both phenolic acids produced networks with high degrees of homogeneity and few defects, as evidenced by narrow glass transitions and consistency of their tensile behavior with the ideal elastomer model at low-to-moderate strains. The 4-hydroxybenzoic acid based network, which had a higher cross-link density, exhibited a higher glass transition temperature, modulus, tensile strength, and elongation at break as compared to the salicylic acid based network. This work develops fundamental relationships between the molecular structure of the phenolic acids and the physical properties of the resulting networks

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 (<i>ortho</i>, <i>meta</i>, <i>para</i>) 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 (<i>T</i>_g) which correlated with their cross-link density. The <i>para</i> placement of allyl groups on the allylated phenolic acid produced a network with the highest cross-link density, <i>T</i>_g, modulus, tensile strength, and elongation at break (followed by <i>ortho</i> and then <i>meta</i>). As the functionality of the allylated monomer increased (to 3–4 allyl groups per molecule), the cross-link density remained high yet the <i>T</i>_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

Interfacial-Strain-Induced Structural and Polarization Evolutions in Epitaxial Multiferroic BiFeO₃ (001) Thin Films

Varying the film thickness is a precise route to tune the interfacial strain to manipulate the properties of the multiferroic materials. Here, to explore the effects of the interfacial strain on the properties of the multiferroic BiFeO₃ films, we investigated thickness-dependent structural and polarization evolutions of the BiFeO₃ films. The epitaxial growth with an atomic stacking sequence of BiO/TiO₂ at the interface was confirmed by scanning transmission electron microscopy. Combining X-ray diffraction experiments and first-principles calculations, a thickness-dependent structural evolution was observed from a fully strained tetragonality to a partially relaxed one without any structural phase transition or rotated twins. The tetragonality (c/a) of the BiFeO₃ films increases as the film thickness decreases, while the polarization is in contrast with this trend, and the size effect including the depolarization field plays a crucial role in this contradiction in thinner films. These findings offer an alternative strategy to manipulate structural and polarization properties by tuning the interfacial strain in epitaxial multiferroic thin films