Instability-induced pattern formation of photoactivated functional polymers

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Abstract
Since the pioneering work of Turing on the formation principles of animal coat patterns [Turing AM (1952) Phil Trans R Soc Lond B 237(641):37–72], such as the stripes of a tiger, great effort has been made to understand and explain various phenomena of self-assembly and pattern formation. Prominent examples are the spontaneous demixing in emulsions, such as mixtures of water and oil [Herzig EM, et al. (2007) Nat Mater 6:966–971]; the distribution of matter in the universe [Kibble TWB (1976) J Phys A: Math Gen 9(8):1387]; surface reconstruction in ionic crystals [Clark KW, et al. (2012) Nanotechnol 23(18):185306]; and the pattern formation caused by phase transitions in metal alloys, polymer mixtures and binary Bose–Einstein condensates [Sabbatini J, et al. (2011) Phys Rev Lett 107:230402]. Photoactivated pattern formation in functional polymers has attracted major interest due to its potential applications in molecular electronics and photoresponsive systems. Here we demonstrate that photoactivated pattern formation on azobenzene-containing polymer films can be entirely explained by the physical concept of phase separation. Using experiments and simulations, we show that phase separation is caused by an instability created by the photoactivated transitions between two immiscible states of the polymer. In addition, we have shown in accordance with theory, that polarized light has a striking effect on pattern formation indicated by enhanced phase separation. Significance When azobenzene-containing polymer films are exposed to UV or visible light complex Turing patterns form on the polymer9s surface. But despite the large number of applications reported, a physical explanation for the pattern formation in this important class of materials and its dependence on both the lights intensity and polarization is still lacking. In this study, we present a general explanation for the pattern formation on these photoactivated azopolymer films. We believe that our findings are an innovative contribution to the field of pattern formation of functional polymers and photoactivated thin film engineering, which have the potential to boost the development of photoresponsive systems, such as molecular electronic devices.

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