João Cabral
Imperial College London, UK

Patterning of soft matter provides exceptional routes for the generation of micro/nanostructured and functional surfaces. We describe the propagation of planar wavefronts of network formation emanating from an illuminated surface during photopolymerisation and report a 3D patterning approach based on coupling planar growth with precisely controlled, yet spontaneous, interfacial instabilities. Photopolymerisation is a complex spatio-temporal process that may lead to well-defined fronts, both stable and unstable. We investigate this light-driven frontal photopolymerisation (FPP) process with a combination of experiments, analytical and numerical modelling. Frontal growth of a series of multifunctional polymers is quantified and described with a ‘minimal’ coarse-grained model, in terms an order parameter Φ(x,t) characterising the extent of monomer-to-polymer conversion. The non-trivial aspects of FPP derive from the coupling of optical attenuation coefficient μ(x,t) and the growing non-uniform network h(x,t). The effect of temperature and nanoparticle fillers (silica, titania, and carbon nanotubes) is elucidated and agrees remarkably with model predictions. Using these results, FPP is demonstrated as a photolithographic3D fabrication process.

References:
[1] A. Vitale, M.G. Hennessy, O.K. Matar, and J. T. Cabral, “A unified approach for patterning via frontal photopolymerization”, Adv. Mater. 27, 6118–6124 (2015)
[2] M.G. Hennessy, A. Vitale, J. T. Cabral and O.K. Matar, "Role of heat generation during frontal photopolymerization" Phys. Rev. E 92, 022403 (2015)
[3] M.G. Hennessy, A. Vitale, O.K. Matar, and J. T. Cabral, "Controlling frontal photopolymerization with optical attenuation and mass diffusion" Phys. Rev. E 91, 062402 (2015)
[4] A. Vitale, M.G. Hennessy, O.K. Matar, and J. T. Cabral, "Interfacial profile and propagation of frontal photopolymerization waves" Macromolecules 48, 198-205 (2015)
[5] M. Mania, O. K. Matar, and J. T. Cabral, "Frontal vitrification of PDMS using air plasma and consequences for surface wrinkling", Soft Matter 11, 3067-3075 (2015)