Self-folding microscale origami patterns are demonstrated in polymer films with control over mountain/valley assignments and fold angles using trilayers of photo-crosslinkable copolymers with a temperature-sensitive hydrogel as the middle layer. The characteristic size scale of the folds W = 30 μm and figure of merit A/ W (2) ≈ 5000, demonstrated here represent substantial advances in the fabrication of self-folding origami.

A series of copolymers containing covalently attached benzophenone (BP) photo-cross-linkers were synthesized, and their UV-induced gelation was monitored as a function of the extent of BP conversion. For poly(methyl methacrylate) copolymers, the recombination yield between BP-and aliphatic-centered radicals was estimated and compared to that for dimerization of each species, directly confirming that the high gelation efficiencies observed for these copolymers arise due to the additional cross-linking pathways provided by covalently incorporated BP, as compared to doping with a small-molecule cross-linker. The placement of the hydrogen species most susceptible to abstraction by triplet benzophenone is found to greatly influence gelation efficiency, since radical generation on the polymer backbone typically increases the probability of dislinking events, while hydrogen abstraction pendent to the copolymer backbone tends to enhance cross-linking. Finally, the presence of atmospheric oxygen during photo-cross-linking was found to yield only modest changes in the gelation behavior of these copolymers.

Colorimetric temperature sensors are prepared from photo-crosslinkable polymers by sequentially spin-coating and crosslinking alternating layers of poly(N-isopropylacrylamide) and poly(para-methyl styrene). Layer thicknesses and copolymer chemistries are chosen to provide robust colorimetric temperature sensors that cover nearly the full visible spectrum.

722 wileyonlinelibrary.com of polymer fi lms, [ 2e,f , 2h , 6 ] this represents a substantial limitation, since many processing steps are therefore required to obtain high refl ectance effi ciencies.Signifi cant progress has been made toward increasing the refractive index of polymers, including by incorporation of pi-conjugated electronic systems, [ 7 ] sulfur, [ 8 ] halogen atoms, [ 9 ] or phosphorous. [ 10 ] These approaches have led to refractive indices as high as 1.7-1.8, but typically suffer from issues of limited polymer solubility, absorption of visible light, and challenging syntheses. As an alternative approach, polymer-inorganic hybrid materials incorporating high index nanoparticles such as TiO 2 , [ 11 ] ZrO 2 , [ 2h , 12 ] ZnS, [ 13 ] and ZnO [ 14 ] have yielded refractive indices as high as 1.8-2.0 and good optical clarity. (Incorporation of air voids to lower refractive index is another possibility to increase contrast, [ 15 ] although of less utility for hydrogel-based responsive materials due to the tendency for the voids to be fi lled by water.) While TiO 2 is an attractive choice for the inorganic component due to its high refractive index, its use in polymer composites is potentially complicated by its ability to photochemically degrade organic materials under UV irradiation, especially in aqueous environments. [ 16 ] On the other hand, ZrO 2 nanoparticles have reasonably high refractive index, are nearly transparent from the UV to the infrared, and are chemically inert, [ 17 ] making them well suited for incorporation in responsive polymer-based photonic multilayers. However, while embedding inorganic particles within responsive polymer matrices is a common route to obtain 3D photonic materials with robust and tunable structural color, [ 15a ] the study of polymer/inorganic hybrids for fabrication of 1D responsive photonic materials remains in its early stages. [ 2h , 18 ] In particular, the incorporation of high-index inorganic nanoparticles within solvent-swellable polymer multilayers remains largely unexplored.In the current report, we extend the approach recently described by our group for preparing thermochromic photonic multilayers from benzophenone-containing photocrosslinkable polymers, [ 2e ] by incorporating ZrO 2 nanoparticles within the polymer layers. We show that particles can be incorporated up to volume fractions of nearly 0.5 in both hydrophobic and hydrophilic polymer layers, simply by spin coating from mixed solutions and UV crosslinking. The presence of nanoparticles substantially increases the refractive indices of the nanocomposite layers, and therefore the effi ciency of light Photocrosslinkable Nanocomposite Multilayers for Responsive 1D Photonic CrystalsSeog-Jin Jeon , Maria C. Chiappelli , and Ryan C. Hayward * A straightforward method to increase the refractive index of photocrosslinkable polymers by incorporation of high index inorganic nanoparticles is demonstrated and shown to enhance the refl ection effi ciency of thermochromic 1D photonic multilaye...

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