Polylactide-based dynamic networks with varying structures and physical properties suitable for potential application as protective polymer coatings

Abstract

Polylactide (PLA)-based polymer networks containing groups with easily thermally dissociating covalent bonds were synthesized by coupling together oligomeric PLAs bearing hydroxyl groups with low-molecular-weight functional diols (introducing reversible bonds) using di- or tri-isocyanates. The formation of networks, characterized by high gel content (97-98%) and a comparable density of reversible bonds and urethane linkages, was confirmed by FTIR analysis.The polymer networks incorporating thermally reversible disulfide or tetraphenylethane (TPE) groups, which impart their dynamic character, were composed of different building blocks, resulting in varying physical properties. DSC and TGA analyses revealed that the networks showed no significant differences in thermal properties, with glass transition temperatures (T_g) ranging from 43 to 70 °C and maximum decomposition temperatures (T_{max. decomp.}) between 246 and 277 °C.Solubility tests in CDCl₃ and DMSO indicated high resistance to these solvents, while the networks degraded in alcohols. Alkaline and enzymatic hydrolysis studies revealed the lowest resistance for the most hydrophilic network, containing disulfide groups. Mechanical properties were evaluated by tensile testing, while the elastic modulus and hardness were assessed via nanoindentation method. Networks containing reversible TPE groups exhibited the highest hardness and environmental resistance. These systems were further tested for adhesion to steel and ceramic substrates, demonstrating potential as coating materials. It was also shown that minor surface damage, such as scratches, could be readily repaired at elevated temperatures due to the presence of reversible TPE groups in the network structure.In conclusion, polymer films derived from PLA-based networks containing reversible TPE groups demonstrated high mechanical strength, enhanced resistance to solvents and degradative environments, and the ability to undergo thermal repair. These features highlight their suitability for protective coatings under more demanding conditions.