A new in-situ technique for the measurement of photoviscosity effects in polymer solutions has been developed using a custom-modified, cone-and-plate rheometer. The new technique permits simultaneous irradiation of the polymer solution and continuous measurement of its viscosity in a temperature-controlled environment. An additional benefit of the technique is the greatly reduced sample volume compared to traditional capillary viscometers. However, adequate measures have to be taken to minimize the evaporation of the sample solution and to maintain the required temperature. Copolymers of methyl methacrylate and azobenzene monomers with a chromophore in the side chain have been synthesized and characterized using the new technique. This polymer exhibits a photoviscosity effect when exposed to UV irradiation (λ = 365 nm), with the reduced specific viscosity (ηsp/c) of the polymer being up to 77% lower than in the dark. However, the magnitude of the photoinduced change in ηsp/c was found to increase with the number of azobenzene units in the polymer chain, and the greatest effect was shown by a polymer with 42 mol % azobenzene loading.

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Photo-induced trans-cis isomerization of trans-4-methacryloyloxyazobenzene and its copolymer with methylmethacrylate, synthesized in this work, were investigated in a polar protic (CDCl 3 ) and a polar aprotic (DMSOd 6 ) solvent, using 1 H-NMR . The results were cross-correlated with trans-cis isomerization calculated from UV-visible spectra. The kinetics of the photoisomerization reaction of the monomer species under UV light irradiation is described by a simple first order exchange between the trans and cis forms of the monomer. The behavior was found to be similar for both solvents. The cis-to-trans reversion in the absence of irradiation is about 3% of the back reaction under irradiation. For the polymer in solution, the behavior was more complicated and is described by the sum of two equilibria, each of first order exchange kinetics of the trans form with two different types of cis isomer in the polymer; a first type of isomerization similar in behavior to that of the monomer and the second type much faster in cis/trans exchange rate. This bikinetic behavior is in agreement with that of the azobenzene moiety in PMMA as a solid phase mixture. The relative proportion of the two rates comprising the bikinetic effect is solvent-dependent.