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Colloidal particles were derivatized with end-grafted polymethylmethacrylate polymer brushes containing varying concentrations of spirobenzopyran photochromic molecules. The polymers were grown from initiator-functionalized silica particles by an atom-transfer radical polymerization (ATRP). These core-shell colloids formed stable suspensions in toluene with the spirobenzopyran in its closed, nonpolar form. However, UV-induced photoswitching of the photochrome to its open, polar merocyanine isomer caused rapid aggregation. The nature of this colloidal stability transition was examined with respect to the spirobenzopyran content in the polymeric brush and solvent polarity. Turbidimetry, wettability studies, UV-vis spectroscopy, suspension rheology, SEM, and visual inspection were utilized to characterize the system photoswitchability. It was found that the system exhibiting the greatest transition in toluene was the copolymer brush composed of 20% spirobenzopyran and 80% methyl methacrylate.

The force profiles between negatively charged silica and titania surfaces in solutions containing additives of either rigid spherical nanoparticles or polyelectrolyte chains, also negatively charged, were measured using an atomic force microscope (AFM). The effects of various solution conditions (e.g., additive concentration, solution ionic strength, pH) on the nature of the measured force profiles were investigated. The primary focus of the work, however, was the long-range oscillations in the force profile and the specific dependence of the wavelength of these oscillations on both solution chemistry and the bulk additive concentration. In the case when spherical nanoparticles (Ludox silica) were used as depletants, the characteristic spacing between macromolecules in the gap region followed the space-filling behavior expected of the bulk suspension (i.e., the wavelength of the oscillations in the force profile scaled with the bulk nanoparticle concentration, c, as c -1/3 ). In addition, the actual magnitude of this spacing was approximately equal to n -1/3 , where n is the bulk number density. For a system in which the additive was potassium polyacrylate, measurements of the force profile was made in both the dilute and semidilute regimes. Good agreement was found between the experimental and theoretical chain-chain spacing in both solution regimes, indicating that the spacing between the polyelectrolyte coils in the gap region is controlled by the bulk behavior. Specifically, the chains were space-filling in the dilute regime (i.e., the wavelength of the oscillations scaled as c -1/3 ) and formed a mesh in the semidilute regime, with the wavelength of the oscillations scaling as c -1/2 . In addition, the concentration at which this change in scaling behavior occurred agreed with the expected overlap concentration.

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This work reports on the grafting of methyl methacrylate polymer brushes containing spirobenzopyran pendant groups from flat silica surfaces and colloidal particles utilizing atom transfer radical polymerization (ATRP). The reaction conditions were optimized with respect to the kind of surface bound initiator, the type of halide and ligand used in the catalytic complex, the presence/absence of untethered initiator, and solvent type. This enabled synthesis of coatings up to 80 ± 3 nm thick with controlled spirobenzopyran content. While polymerization kinetics indicate the presence of chain termination reactions, the “living” character of the process is confirmed by controlled formation of block copolymer brushes. UV/vis spectroscopy was used to characterize the UV-induced isomerization of spirobenzopyran to zwitterionic merocyanine and the thermal back-reaction. Spectral and kinetic analyses of this latter bleaching process points to the existence of free and associated merocyanines in the polymeric brush in both tetrahydrofuran and toluene. However, stabilization of merocyanine species by the polymer matrix is considerably greater in toluene with thermal back-reaction rates approaching those determined for solid dry films.