Light-weight, power-free mechanochromic sensors that can change and record the reflective color depending on the magnitude and rate of the applied force are fabricated from inverse opals by infiltrating the colloidal crystals of silica particles with uncrosslinked SU-8, followed by removal of the colloidal templates. The mechanical sensing range of the materials is high, 17.6–20.4 MPa. Due to elastoplastic deformation of the SU-8 films, the deformed structures and thus colors can be locked after the removal of the load, therefore establishing a quantitative relationship between the mechanical force and optical responses. In comparison, mechanochromic photonic gels reported in the literature typically detect force in the range of 10–100 kPa; once the load is removed, the structure and color return back to the original ones. The mechanochromic sensors are highly sensitive: the ratio of shift in the stopband wavelength to the change in applied strain is up to 5.7 nm per percent, the highest among literature. Comparison of finite element simulations with experiments confirms the elastoplastic deformation of the films and highlights that reconfiguration of pore shape under compression plays a key role in the mechanochromic response.

Two-dimensional nanowaves with long-range order are fabricated by exploiting swelling-induced buckling of one-dimensional (1D) nanowalls with nanofibers formed in-between during holographic lithography of the negative-tone photoresist SU-8. The 1D film goes through a constrained swelling in the development stage, and becomes buckled above the critical threshold. The degree of lateral undulation can be controlled by tuning the pattern aspect ratio (height/width) and exposure dosage. At a high aspect ratio (e.g., 6) and a high exposure dosage, nanofibers (30–50 nm in diameter) are formed between the nanowalls as a result of overlapping of low crosslinking density regions. By comparing experimental results with finite-element analysis, the buckling mechanism is investigated, which confirms that the nanofibers prevent the deformed nanowalls from recovery to their original state, thus, leading to long-range ordered two-dimensional (2D) wavy structures. The film with nanowaves show weaker reflecting color under an ambient light and lower transmittance compared to the straight nanowalls. Using double exposure through a photomask, patterns consisting of both nanowaves and nanowalls for optical display are created.

High-aspect-ratio shape-memory polymer (SMP) pillar arrays are investigated as a new type of dry adhesive based on buckling and interlocking mechanism. When two identical SMP pillar arrays are engaged at 80 °C, above the glass transition temperature at a preload larger than the critical buckling threshold, the pillars are deformed and become interweaved and/or indented with each other. After cooling to room temperature, strong pull-off forces are observed in the normal and shear directions, both of which are much larger than those from pillar-to-flat surface and flat-to-flat surface contact. From finite element anaylsis (FEA) and comparison of measured and calculated adhesion values using different contact mechanics models, it is shown that interweaved pillars are the main source that contributes to the pillar-to-pillar adhesion and the indented pillars set the lower limit, whereas the probability of interdigitation is very low. Further, it is found that interweaved pillars are primarily responsible for the decreased adhesion strength and increased anisotropy when the pillar spacing became larger. Finally, it is shown that the bonded pillars can be easily separated after reheating to 80 °C due to significant drop of modulus of SMPs.

A dual-functional copolymer, poly(4-styrenesulfonyl azide-co-t-butyl-methacrylate), with built-in photoacid labile and photocrosslinkable components was designed and synthesized by radical copolymerization. The mixture of copolymer and photoacid generators was spin coated on aminosilane treated Si wafers and polycarbonate (PC). When exposed to 365 nm UV light, photoacids were generated, which decomposed the acid labile groups, t-butyl-ester, to carboxylic acid in the exposed region, leading to drastic change of wettability from hydrophobic to hydrophilic after developing the film in an aqueous base solution. The patterned polymer film could be subsequently photoimmobilized on the substrate under 254 nm deep UV exposure through CH insertion via exited azide groups. ¹H-NMR and Fourier transform infrared spectra confirmed the synthesis of the copolymer, and the photodecomposition and photografting reactions occurred orthogonally at 365 and 254 nm, respectively, without interfering each other. On the patterned surfaces, including a hexagonal dot array and a gradient line array, we demonstrated selective wetting in the 365 nm exposed regions. On the gradient line array, we showed an interesting ratchet wetting pattern. Finally, we showed that the copolymer could be used to modify the wettability of PC while maintaining its high optical quality. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

The fabrication of three-dimensional (3D) diamond photonic crystals with controllable nanoroughness (≤120 nm) on the surface from epoxy-functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) is reported. The nanoroughness is generated on the 3D network due to microphase separation of the polymer chain segments in a nonsolvent during the rinsing step in holographic lithography process. The degree of roughness can be tuned by the crosslinking density of the polymer network, which is dependent on the loading of photoacid generators, the exposure dosage, and the choice of developer and rinsing solvent. Because the nanoroughness size is small, it does not affect the photonic band gap position of the photonic crystal in the infrared region. The combination of periodic microstructure and nanoroughness, however, offers new opportunities to realize superhydrophobicity and enhanced dye adsorption in addition to the photon management in the 3D photonic crystal.

Surface wrinkles are commonly observed in polymer films driven by mechanical instability when the stress exceeds a critical value. Recently, wrinkling instability has been utilized as a versatile patterning platform to create unique surface patterns for a wide range of applications that are related to surface topography and its dynamic tuning. In this review, we discuss three mechanisms to create large-area surface wrinkles via thermal stress, osmotic pressure and mechanical stress applied on bilayer and gradient polymer films. We briefly compare the governing physics in each system, and how to control the wrinkle pattern order, characteristic wavelength and amplitude, orientation and interactions under different geometrical confinements. We then present various technological applications that harness wrinkling effects, including optical components, responsive microfluidic channels, thin-film metrology, tunable wetting and directed assembly of liquid crystal molecules, flexible electronics and particle sorting.

Composite hydrogel films are prepared by photocrosslinking of partially polymerized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) prepolymer solution dispersed with aragonite precipitated calcium carbonate (PCC) particles. The effects of PCC aspect ratio (AR, 4.2–6.6) and volume fraction (ϕ_PCC) are studied with respect to the prepolymer viscosity, the maximum packing fraction (ϕ_m), and optical properties. ϕ_m obtained from Kreigher–Dougherty fitting is found to decrease with increasing AR of PCC particles, from ϕ_m = 0.33 (AR = 4.2) to ϕ_m = 0.21 (AR = 6.6). Scanning electron microscopy images revealed that the particles are uniformly dispersed and randomly orientated in the polymer network when ϕ_PCC < ϕ_m. Optically transparent neat hydrogel films become opaque with increasing particle concentration and AR because of random light scattering from the PCC particles. A whiteness up to 80 is obtained from 100 μm thick composite hydrogel film in the wet state with PCC AR = 6.6 and ϕ_PCC = 0.19 (near ϕ_m). POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

We report the facile synthesis of vesicular nanoparticles via self-assembly of random copolymers in selective organic solvents. The polymers were synthesized via photopolymerization in bulk from acryloyl chloride (AC) containing a small amount of hydrolyzed acrylic acid (AA) at ambient condition. Fourier transform infrared spectroscopy of the photopolymerized product revealed two main chemical components: poly(acryloyl chloride) (PAC) and acid anhydride. The later peak intensified when increasing the initial AA composition in the monomer solution, suggesting that the hydrolyzed AC contributed to the formation of crosslinked anhydride, leading to amphiphilicity of the polymer with solvophobic anhydride and solvophilic PAC chains. At an optimal UV dosage (2000 mJ cm⁻²), vesicular nanoparticles (∼90 nm in diameter) were obtained from polymers assembled in acetone. The particle size and morphology were confirmed by scanning electron microscopy, transmission electron microscopy, and dynamic and static light-scattering measurements. Further, we found that the random copolymers self-organized into vesicles in merely good solvents of PAC chains, including acetonitrile, tetrahydrofuran, and 1,4-dioxane, but became aggregated and precipitated out in poor solvents of PAC, such as isopropanol, ethanol, toluene, xylene, and hexane. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Mechanical instabilities in soft materials, specifically wrinkling, have led to the formation of unique surface patterns for a wide range of applications that are related to surface topography and its dynamic tuning. In this progress report, two distinct approaches for wrinkle formation, including mechanical stretching/releasing of oxide/PDMS bilayers and swelling of hydrogel films confined on a rigid substrate with a depth-wise modulus gradient, are discussed. The wrinkling mechanisms and transitions between different wrinkle patterns are studied. Strategies to control the wrinkle pattern order and characteristic wavelength are suggested, and some efforts in harnessing topographic tunability in elastomeric PDMS bilayer wrinkled films for various applications, including tunable adhesion, wetting, microfluidics, and microlens arrays, are highlighted. The report concludes with perspectives on the future directions in manipulation of pattern formation for complex structures, and potential new technological applications.

Hydrogels with controlled surface patterns are useful for a range of applications, including in microdevices, sensors, coatings, and adhesives. In this work, a simple and robust method to generate a wide range of osmotically driven surface patterns, including random, lamellar, peanut, and hexagonal structures is developed. This method does not require the use of organic solvents for swelling, pre-patterning of the film surface, or coating of a second layer on the gel. The patterns are fabricated by exposing a photocurable formulation to light while open to air and then swelling, using oxygen inhibition of the radical polymerization at the surface to create a gradient of crosslinking with depth, which was confirmed by measuring the double bond conversion at the surface, surface mechanics, and molecule diffusion into the network. The modulus gradient, and hence osmotic pressure, is controlled by the crosslinker concentration, and the characteristic size of the patterns is determined by the initial film thickness. The patterns are stable in both swollen and dry states, creating a versatile approach that is useful for diverse polymers to create complex patterns with long-range order.