Controlling surface properties [of] polymer materials through photodirected thiol-ene wrinkle systems

Kloxin, Christopher J.Wagner, Norman J.Wrinkling/buckling on elastomers represents a cost-effective approach to creating surface topography, leading to a broad range of coating and templated assembly applications. Despite the versatility of wrinkling, several challenges hinder the development of wrinkled performance materials including a limited ability to confine and orient the wrinkles and the lack of commercial scalability due to the processing techniques that are currently utilized in wrinkle formation. ☐ Using thiol-ene ‘click’ chemistries, characterized by rapid kinetics and high selectivity under ambient conditions, we have developed rapidly-curing photo-wrinkle systems. To generate wrinkles, tetra-thiol and excess di-acrylate, embedded photoinitiator and photoabsorber, is reacted to form a thick acrylate-rich elastomer. Upon straining and irradiating the material with UV light, the photoinitiator triggers free radical polymerization of the pendant acrylates in the network, while the photoabsorber confines the light to a thin layer at the surface of the elastomer, thus creating the conditions necessary for wrinkling. Light affords spatiotemporal control over wrinkle formation, which enables facile wrinkle alignment and confinement, the formation of complex patterns with multiple distinct wrinkle wavelengths, and the formation of gradient wrinkles, all under ambient conditions. ☐ Leveraging oxygen inhibition of the free radical polymerization, we can also post-functionalize the wrinkled surfaces through sequential thiol-ene reactions of functional monomers. By first reacting the surface acrylates to excess tetra-thiol in solution, which converts the acrylate-rich surface into a thiol-rich surface, we develop a functionalization scheme that enables photopatterning of chemical moieties using a second photoinitiated thiol-ene reaction. As a demonstration, we employ these wrinkled substrates as cell culture platforms for the alignment of human mesenchymal stem cells (hMSCs) towards tissue engineering applications. Specifically, substrates functionalized with an RGDS-containing peptide showed drastic increases in hMSC density and spreading. Importantly, when cultured on wrinkled substrates, these hMSCs exhibit cell alignment along the troughs of the wrinkle structures, demonstrating the importance of both topography and chemistry in controlling surface properties. ☐ Alternatively, wrinkles can be formed via flowcoating and polymerizing thiol-ene monomer thin films, which can be transferred onto a softer thiol-ene elastomeric substrate. The flowcoating process enables formation of sub-micron wrinkle wavelengths by controlling film thickness and modulus mismatch between the film and the substrate layers. Through photopatterned UV light, wrinkle features can be spatially confined and aligned, and using a layer-by-layer process, wrinkle features can be discretely tuned across the surface. Importantly, due to the modular nature of thiol-ene ‘click’ chemistry, monomers can be exchanged to not only independently control the modulus of the film and the substrate, but may also facilitate future development of functional and stimuli-responsive polymer thin film systems. ☐ The versatility of the thiol-ene polymerization allows the design of more intricate polymer networks towards wrinkling applications. Through careful selection of the monomers used and precise stoichiometric control, the ‘click’ nature of the thiol-ene reaction ensures high tunability of the of the elastomer, ultimately enabling tailored control over modulus, chemistry and topography for targeted material applications.University of Delaware, Department of Chemical and Biomolecular EngineeringPh.D