title: Instability-induced pattern formation of photoactivated functional polymers.
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.
authors.affiliation: School of Engineering and Applied Science, Harvard University, Cambridge, MA 02138; Laboratory for Nanometallurgy and hgalinski@seas.harvard.edu antonio.ambrosio@spin.cnr.it.
authors.forename: Henning
authors.identifier: http://orcid.org/0000-0003-4148-7436
authors.initials: H
authors.lastname: Galinski
authors.affiliation: School of Engineering and Applied Science, Harvard University, Cambridge, MA 02138; CNR-Institute for Superconductors, Oxides and Other Innovative Materials and Devices (SPIN) and Dipartimento di Fisica, Università degli Studi di Napoli Federico II, 80126 Naples, Italy; and hgalinski@seas.harvard.edu antonio.ambrosio@spin.cnr.it.
authors.forename: Antonio
authors.initials: A
authors.lastname: Ambrosio
authors.affiliation: CNR-Institute for Superconductors, Oxides and Other Innovative Materials and Devices (SPIN) and Dipartimento di Fisica, Università degli Studi di Napoli Federico II, 80126 Naples, Italy; and.
authors.forename: Pasqualino
authors.initials: P
authors.lastname: Maddalena
authors.affiliation: Nonmetallic Inorganic Materials, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Iwan
authors.initials: I
authors.lastname: Schenker
authors.affiliation: Laboratory for Nanometallurgy and.
authors.forename: Ralph
authors.initials: R
authors.lastname: Spolenak
authors.affiliation: School of Engineering and Applied Science, Harvard University, Cambridge, MA 02138;
authors.forename: Federico
authors.initials: F
authors.lastname: Capasso
mesh_terms: D000465:Algorithms; D001391:Azo Compounds / Q000737:chemistry*; D007536:Isomerism; D008027:Light; D008956:Models, Chemical; D008968:Molecular Conformation / Q000528:radiation effects*; D055668:Photochemical Processes / Q000528:radiation effects; D011108:Polymers / Q000737:chemistry*
chemical_list: D001391:Azo Compounds; D011108:Polymers; C009850:azobenzene
keywords: azobenzene; pattern formation; phase separation; photoactivated functional polymers; spinodal decomposition