Photodirected Control of Topography using Thiol-ene “Click!” Chemistry
Stephen J. Ma1, Dr. Norman J. Wagner1,2, Dr. Christopher J. Kloxin1,2,3
1) University of Delaware, Department of Chemical and Biomolecular Engineering
2) Center for Molecular and Engineering Thermodynamics
3) University of Delaware, Department of Materials Science and Engineering
Wrinkling/buckling on elastomers represent a cost-effective approach to creating surface topography, leading to a broad range of applications in antifouling coatings,1,2 optical coatings and tunable lenses3,4, substrates for directed cell growth5,6, and enhancements in solar cell efficiency.7,8 One of the ongoing challenges of wrinkling is the limited ability to confine and orient the wrinkles toward the development of performance materials.
Using an acrylate-rich thiol-ene elastomer, embedded with photoinitiator and photoabsorber, we have developed a rapidly curing wrinkling system.9 Upon irradiating the strained elastomers with UV light, the photoinitiator will induce radical polymerization of the excess acrylates while the photoabsorber confines the light to a thin skin layer at the surface, creating the necessary conditions for
wrinkle formation on the order of seconds. In conjunction with photomasked UV light, these wrinkles can be easily confined and oriented to generate complex patterns of multiple distinct wrinkle wavelengths and wrinkle gradients. With the addition of a photoorthogonal photoinitiator, we can also ‘lock' the shape of the elastomer through further bulk radical polymerization.
Beyond the ability to easily orient and align wrinkles through photopatterning, our wrinkle system is also highly versatile. The ‘click' nature of thiol-ene chemistry ensures high tunability of elastic modulus, network architecture and surface functionality of the elastomer, through the types of monomers used and careful stoichiometric control. The presence of excess acrylates also enables post-functionalization of various moieties at the surface of the films.
Additionally, recent advances in particle synthesis through thiol-ene chemistry,10,11,12 has opened up new avenues in wrinkle formation. By covalently attaching these particles atop strained thiol-ene elastomers, it is possible to form wrinkles with an additional level of surface roughness on the size-scale of the particles themselves, thereby enhancing the surface properties of the materials. More interestingly, these particles offer new potential platforms for generating surface topography on 3D objects, enabling, for the first time, the experimental study of wrinkle formation on spheres.
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