We present a general, convenient, and efficient synthetic concept for the coating of colloidal particles with a silica (SiO2) shell of well-defined and precisely controlled morphology and porosity. Monodisperse submicroscopic polystyrene (PS) particles were synthesized via two-stage emulsifier-free emulsion polymerization and subsequent swelling polymerization, enabling selective particle surface modification by the incorporation of ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers, which could be proven by zeta potential measurements as well as by determining the three-phase contact angle of the colloidal particles adsorbed at the air-water and n-decane-water interface. The functionalized particles could be directly coated with silica shells of variable thickness, porosity, and controlled surface roughness in a seeded sol-gel process from tetraethoxysilane (TEOS), leading to hybrid PS@silica particles with morphologies ranging from core-shell (CS) to raspberry-type architectures. The experimental results demonstrated that the silica coating could be precisely tailored by the type of surface functionalization, which strongly influences the surface properties of the colloidal particles and thus the morphology of the final silica shell. Furthermore, the PS cores could be easily removed by thermal treatment, yielding extremely uniform hollow silica particles, while maintaining their initial shell architecture. These particles are highly stable against irreversible aggregation and could be readily dried, purified, and redispersed in various solvents. Herein we show a first example of coating semiconducting CdSe/ZnS nanocrystals with smooth and spherical silica shells by applying the presented method that are expected to be suitable systems for applications as markers in biology and life science by using fluorescence microscopy methods, which are also briefly discussed.

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.

Paper-supported solvent-responsive elastomeric opal films based on hard-soft coreinterlayer- shell spheres featuring remarkably distinct iridescent reflection colors were investigated. By using extrusion and compression molding, elastomeric opal films could be obtained, which were incorporated into a porous paper sheet to build robust composites. Swelling of the opal paper composites caused by various solvents was accompanied with a tremendous photonic band gap shift of the reflection colors. The combination of the extraordinary optical properties of the elastomeric opal films used with the remarkable features of highly porous paper can be the basis for a whole family of polymer-based soft sensors featuring a fascinating optical, fast and reversible response. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)

An efficient and convenient thermal cross-linking protocol in elastomeric opal films leading to fully reversible and stretch-tunable optical materials is reported. In this study, functional monodisperse core-shell particles were arranged in a face-centered cubic (fcc) lattice structure by a melt flow process. A problem up to now was that un-cross-linked films could not be drawn fully reversibly and hence lost their optical and mechanical performance. After thermal cross-linking reaction, the obtained films can be drawn like rubbers and the color of their Bragg reflection changes because of controlled lattice deformation, which makes the cross-linked films mechanochromic sensors. Different techniques were developed for the cross-linking of the films a posteriori, after their preparation in the melt flow process. A photo-cross-linking approach was reported earlier. This study now deals with a very efficient thermo-cross-linking approach based on the chemistry of hydroxyl- and isocyanate-functionalities that form urethane bridges. The focus of the present work is the mechanism and efficiency of this cross-linking process for elastomeric opal films with excellent mechanical and optical properties.

A new strategy to build large-scale solvent-responsive elastomeric opal films that are candidates for a wide range of optical sensor applications is reported. Hard-soft core-interlayer-shell (CIS) beads were used to prepare paper-supported elastomeric opal films with remarkably distinct iridescent reflection colors. Extrusion and compression molding of the CIS beads directly on the top of a highly porous paper sheet followed by UV cross-linking led to polymer-paper composite films with a high tensile strength and an outstanding solvent resistivity. Due to the high porosity of the paper-sheets used, these composites could be easily swollen by various solvents. The swelling changed the crystalline lattice of the opals which provoked a tremendous photonic band gap shift and also enhanced the brilliance of these colors. After deswelling, the original opal structure was totally restored which means that the shift of the photonic band gap was completely reversible. This approach can become the basis for a whole family of polymer-based soft sensors with a fascinating optical response.

Core-interlayer-shell (CIS) beads featuring noncross-linked hard cores were used to prepare large and well-defined elastomeric opal films with remarkably distinct iridescent reflection colors. The matrix of the opal films was cross-linked by UV-irradiation after compression molding of the CIS beads mixed with a bifunctional monomer. Stress-induced deformation of the embedded PS cores lead to hexagonally arranged spheroid oblates with an aspect ratio of 2.5. Optical characterization shows that bead deformation provokes a tremendous photonic band gap shift of about 160 nm. Fully reversible shape transition from the spheroid oblates back to the spherical beads and hence full recovery of the original photonic band gap can be achieved.

A new strategy to achieve easily scalable triple stimuli-responsive elastomeric opal films for applications as stretch-tunable photonic band gap materials is reported. Novel monodisperse highly functional core-interlayer-shell beads are obtained by semicontinuous emulsion polymerization featuring a temperature-sensitive fluorescent rhodamine dye either locally restricted in the core or the shell of prepared beads. After extrusion and compression molding, homogeneous elastomeric opal films with fascinating stretch-tunable and temperature-dependent fluorescent properties can be obtained. Applying strains of only a few percent lead to significant blue shift of the reflected colors making these films excellent candidates for applications as deformation sensors. Higher strains up to 90% lead to a tremendous Bragg reflection color change caused by transition from the (111) to the (200) lattice plane. The well ordered opaline structure with its stop band at the emission frequency of the incorporated fluorescent dye shows remarkable angle dependent fluorescence suppression. Herein described elastomeric opal films can be valuable in a wide range of applications such as rewritable 3D optical data storage, tunable laser action, and sensing materials.

Time-resolved scattering spectra of flowing polymer-based colloidal opals are presented. Broadband spectra reflecting dynamic structural changes during a shear-ordering process reveal four distinct regimes of crystal growth and decay identified under different flow conditions.

Synthetic opals result from the crystallization of monodisperse silica or polymer beads of submicroscopic size. The beads self-organize to the face-centered cubic (fcc) lattice from which light is reflected wavelength selectively. At diameters of 0.15-0.3 mu m, colors are singled out of white light by diffraction from the I I I plane of the lattice, the reflected color depending on the spacing a I I,. With elastic opal films of core-shell (CS) beads, this spacing and, thereby, the color can be changed by deformation. This mechanochromic effect has so far been studied only on opals made of beads that were not chemically interconnected so the deformation was partly irreversible. In this study, opal films of polymeric core-shell beads were prepared by a melt-flow technique developed earlier in this institute. Afterward, the films were photo-cross-linked. They deformed indeed reversibly, however, with mechanical hysteresis effects. The strained fcc lattice causes a blue shift of the reflected color, which is indicative of a hardsphere deformation mechanism. The shift is strong enough to switch monochromatic light on and off by only a few percent strain.

Semiaromatic polyamides are high-performance thermoplastics with a huge potential but also a high price. In this study, the condensation kinetics of aliphatic diamines with aromatic diacids, diesters, and polyesters were investigated, to find ways of producing semiaromatic polyamides at lower costs. The condensation of hexamethylene diamine with phthalic acids and their dimethylesters are discussed in this Part I. The amidation of phthalic acids turned out to proceed like the amidation of adipic acid leading to PA66, only more slowly. The amidation of dimethylphthalates proved to be difficult. It is accompanied by a side reaction of amino alkylation which prevents the growth of long polyamide chains. This side reaction was investigated kinetically in detail, using model compounds that were unable to polymerize. The aim was to provide a basis for attempts to amidate polyesters, in particular, PET, which will be reported in Part II.

Polyphthalamides are distinguished by outstanding properties. Only their price keeps them from replacing PA66 in many applications, in particular; in automobiles. This article deals with a simple, economically attractive process of upcycling postconsumer polyethylene terephthalate (PET) waste to polyphthalamides. Basic studies on the amidation of acids and esters were described in Part I. The amidation of esters is accompanied by a side reaction of amine alkylation, which, in the case of polyesters, leads to branching and crosslinking. But the crosslinking can be avoided in mixtures with excessive diamine. Therefore, a heterogeneous amidation process was developed in which solid PET granules are simultaneously dissolved in and amidated by a liquid diamine. As long as the PET diamine system is heterogeneous, it reacts under amino excess conditions. Later, when all PET is dissolved, it comes back to stoichiometric conditions again, so long-chained polyamides can be obtained. In terms of molar mass and viscosity, this amidation of solid PET proceeds very similarly to the amidation of terephthalic acid (TPA), which is so far the common route towards polyphthalamides. PET waste could replace TPA.

In the present paper, the optical and structural properties of an opaline photonic crystal consisting of a closest packing of nanospheres have been studied via spectroscopic ellipsometry. This method allows for a non-destructive determination of the effective refractive index without any assumption on material properties. Further we are able to gather information about the structural constitution of the opal by introducing a multi-layer model. The layer distance and the maximum size of the nanospheres can be extracted.

In recent years, materials with a periodically modulated refractive index, with periods in the range of light wavelengths, have attracted much attention because of their unique optical properties caused by Bragg scattering of visible light. 3-d structures can be obtained by crystallizing spherical colloidal particles via sedimentation or drying of dispersions. Similar colloidal crystals are found in natural opals where monodisperse silica spheres of submicron diameters are arranged to a face-center-cubic (fcc) lattice. In this chapter, films are reviewed that were produced by a novel technique based on shear flow in the melts of specially designed core-shell latex spheres: as the melt of these latex spheres flows between the plates of a press, the spheres crystallize along the plates, layer by layer, and the cores assume the hexagonal order that corresponds to the (111) plane of the fee lattice. This process is fast and yields large-area films, thin or thick. In colloidal crystals with a sufficiently high refractive index contrast, the Bragg peaks widen to complete band gaps, so light cannot propagate in any direction in such a photonic crystal. To enhance refractive index contrast in our films, the colloidal crystalline structure was inverted. Two strategies will be described by which the cores are removed, leaving mesopores in crystalline order.

Blends were prepared from seven polymers in various combinations in the entire composition range. The Flory-Huggins interaction parameter (chi (12)) was used for the quantitative estimation of miscibility. The determination of chi (12) was attempted by several experimental techniques including the measurement of transparency, glass transition temperature, solvent diffusion and mechanical properties. The relatively simple methods used for the estimation of miscibility work surprisingly well. Solvent absorption can be determined easily for practically all blends, thus the method offers a quantitative measure of component interaction if the solvent is selected properly. After appropriate data reduction, the composition dependence of mechanical properties also supplies a quantitative estimate of compatibility. Although the approach presented in the paper reflects well the general correlation between miscibility and properties, it must be refined and improved in order to obtain a reliable estimate of blend performance.

Most blends are two-phase systems that exhibit coarse morphologies with weak interfaces between the phases. It is well known that these blends can be compatibilized by suitably adapted block copolymers is known. Less known is that random copolymers can compatibilize blends as well, forming interphases. In this study, block and random copolymers of styrene and methylmethacrylate were tested in direct comparison, in a blend of polystyrene and polyvinylchloride. The random copolymers are inferior, as regards the phase morphologies, but competitive, as far as the tensile strength is concerned.