Ilke Anac

The swelling behaviour of thermoresponsive hydrogel/silica nanoparticle composites

A new responsive nanocomposite material consisting of a poly-(N-isopropylacrylamide) hydrogel and “super-crosslinking” silica nanoparticles was prepared by mixing both components in solution, spincoating a thin film, and photocrosslinking by UV irradiation. Detailed analysis of the thermal response of these water-swollen films by means of surface plasmon resonance and optical waveguide spectroscopy revealed that the composite is very stable and has excellent responsive properties; it is of high optical homogeneity; admixture of the nanoparticles (up to 50%-wt) does not affect the critical volume collapse temperature; and swell-collapse cycles are highly reproducible and display only limited hysteresis. Thus, the composite is promising as a scaffold for further functionalisation and incorporation in sensors or actuators.

Analysis of optical gradient profiles during temperature- and salt-dependent swelling of thin responsive hydrogel films

Surface-attached, cross-linked hydrogel films based on thermoresponsive N-isopropylacrylamide with a dry thickness >1 microm were studied with surface plasmon resonance/optical waveguide mode spectroscopy (SPR/OWS) to monitor temperature-dependent and salt-induced changes of their swelling state. In combination with the reversed Wentzel-Kramers-Brillouin and Bruggeman effective medium approximation and by modeling the hydrogel film as a composite of sublayers with individual complex refractive indices, refractive index/volume fraction gradient profiles perpendicular to the surface are accessible simultaneously with information about local inhomogeneities. Specifically, the imaginary refractive index kappa of each sublayer can be interpreted as a measure for static and dynamic inhomogeneities, which were found to be highest at the volume transition collapse temperature in the layer center. These results indicate that the hydrogel collapse originates rather from the film center than from its boundaries. Upon addition of NaCl to a swollen hydrogel below its transition temperature, comparable optical loss characteristics as for the thermal gel collapse are observed with respect to inhomogeneities. Interestingly, in contrast to the thermally induced layer shrinkage and collapse, swelling increases at intermediate salt concentrations.

Chemical modification of chromium oxide surfaces using organosilanes

Alkylsiloxane layers that were prepared by reaction of alkyltrichlorosilanes, alkyltriethoxysilanes, and mono-, di- and trihydridoalkylsilanes with chromium oxide surfaces were assessed in terms of their hydrophobicity. Two general conditions were studied, in the vapor phase at elevated temperatures (70 degrees C) and in solution at 70 degrees C. The surfaces were characterized by contact angle analysis and X-ray photoelectron spectroscopy (XPS). The kinetics of the solution phase reactions of n-octadecylsilane and n-octadecyltrichlorosilane were also studied. Hydrophobic monolayers were achieved in all cases, but subtle differences in structure were found.

Poly(trifluoroethylene) adsorption and heterogeneous photochlorination reactions.

Heterogeneous (gas-solid) photochlorination reactions of poly(trifluoroethylene) (PF 3E) films were studied as a function of reaction time and light intensity. The rate of chlorination was found to be faster in high-intensity light when compared to the reaction in ambient light. PF 3E irreversibly adsorbed to oxidized silicon and covalently attached amine monolayers supported on silicon, producing hydrophobic thin films in the thickness range of 8-40 A. Adsorption conditions such as polymer concentration and solvent composition were investigated. Radical grafting of maleic anhydride to the polymer backbone resulted in increased adsorption on oxidized silicon.

Superhydrophobic polysiloxane filament growth on non-activated polymer coatings

Superhydrophobic polysiloxane nanofilaments were successfully grown on polymer coatings prepared by conventional emulsion polymerization without the application of any activation step for the first time using methyltrichlorosilane as the silane precursor. Both gas phase reactions under a controlled humidity environment and liquid phase reactions in standard petroleum ether which is open to air were applied during filament growth. Wettability, morphology and transparency of the obtained polysiloxane nanofilaments were characterized by contact angle measurements, scanning electron microscopy and UV-vis transmittance tests respectively. Advancing contact angle values between 166–172° and 157–169° were obtained via the gas phase reactions and the liquid phase reactions, respectively. It was also shown that polysiloxane nanofilaments can be grown on polystyrene coatings prepared by solution polymerization without any pre-activation step via the liquid phase filament growth reaction. It was determined that the presence of hydroxyl groups on a polymer coating is not required for the growth of polysiloxane nanofilaments, and only the formation of a water film having an adequate thickness on the polymeric coating surface is sufficient.