Two switchable, mesoscopically periodic materials were created by combining crystalline colloidal array (CCA) self-assembly with the temperature-induced volume phase transition of poly(N-isopropylacrylamide) (PNIPAM). Body-centered-cubic CCAs of hydrated, swollen PNIPAM particles Bragg-diffract infrared, visible, and ultraviolet light weakly, whereas arrays of compact shrunken particles diffract efficiently. A tunable diffracting array was also created by embedding a CCA of polystyrene spheres within a PNIPAM hydrogel that swells and contracts with temperature; thus the array lattice constant varies with temperature, and the diffracted wavelength was thermally tunable across the entire visible spectrum. These materials may find applications in many areas of optics and materials science.

Highly charged fluorinated monodisperse spherical particles of diameters between 50 and 250 nm were synthesized from 1H,1H-heptafluorobutyl methacrylate by emulsion polymerization. These particles have a low refractive index of 1.386. High particle surface charge densities were obtained by minimizing the polymer molecular weight. These colloids formed well-ordered crystalline colloidal arrays (CCAs), after dialysis and ion exchange, which Bragg diffract light at wavelengths from the near-IR down to 270 nm in the UV. The diffraction bandwidths in water are very narrow (<10 nm) due to the closeness of refractive index of the colloidal particles to that of water. These CCAs are excellent materials for narrow band notch optical filters. In addition, these fluorinated CCAs can be easily refractive index matched to a predominately aqueous medium. We have covalently attached dyes to the colloidal particles to prepare absorbing CCAs. We photopolymerized these dyed CCAs within a polyacrylamide matrix to form polymerized crystalline colloidal array (PCCA). These semisolid PCCAs can withstand vibrations, ionic impurity addition, and thermal shocks while maintaining the CCA ordering. The medium within the PCCA can easily be exchanged to exactly refractive index match the CCA. These refractive index matched dyed PCCAs may have applications in optical limiting, computing, and nanosecond fast optical switching devices as discussed in the accompanying paper.

We have developed a method to synthesize highly charged, monodisperse ca. 100 nm poly-(methyl methacrylate) (PMMA) colloidal particles containing covalently linked absorbing dyes. We acylate these dyes with methacryloyl chloride and add these functionalized dyes during an oil-in-water emulsion polymerization of the PMMA colloids. The high charge was obtained by copolymerizing with the ionic sulfonated surfactant After purification these monodisperse colloids spontaneously self-assemble into crystalline colloidal arrays (CCA). We utilize this colloid to fabricate an ordered array of absorbing colloidal spheres locked into a hydrogel matrix. We exchange the water in the medium to approximately match the real part of the array refractive index. This forms a novel submicron periodic system where a body-centered cubic array of absorbers interact with electromagnetic radiation with little scattering or diffraction of the incident light.

The stress-whitened damage zone that formed ahead of a semicircular notch during slow tensile loading has been measured from optical micrographs of translucent blends of poly(vinyl chloride) (PVC) with experimental chlorinated polyethylene (CPE) resins. When the zone was small, the plane strain condition applied and from the elastic stress distribution a constant mean stress condition was found at the boundary of the crescent-shaped zone. The critical mean stress did not depend on the chlorine content or the chlorine distribution of the experimental CPE resin used in the blend. While the critical mean stress decreased as the amount of CPE in the blend was increased, the critical volume strain, calculated from the bulk modulus, was independent of composition and was thought to be the controlling parameter for stress-whitening. When the zone was larger, the shape was qualitatively described by concepts of stress redistribution in the presence of a plastic zone ahead of the notch. Macroscopic flow and necking were only detected near the maximum in the stress-displacement curve.

The development of optical technologies requires the fabrication of reliable optical switching and limiting devices. Optical switches modulate the transmission or reflection of incident light, while optical limiters serve to limit transmission to prevent the transmitted light intensity from exceeding a defined level. A major application of optical limiters is to protect delicate sensors.