Azobenzene modified poly(aryl ether ketone amide)s. 2. Photo- and thermo-responsive behaviour in dilute solution
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文摘
A number of azobenzene modified poly(aryl ether ketone amide)s with differing backbone geometries were evaluated for their photo- and thermo-regulated behaviour in dilute solution. Photoinduced trans → cis isomerization reactions were carried out by irradiating the polymer samples with ultraviolet light at wavelengths between 370 and 400 nm. Photostationary state compositions achieved under these conditions typically consisted of about 70 % of the higher energy cis isomer distributed along the polymer main chain. Reverse cis → trans isomerization of the backbone azobenzene moieties was triggered by either photochemical or thermal means and was monitored by optical absorbance and 1H n.m.r. spectroscopies. Thermally induced cis → trans return in each of the polymers obeyed the first-order rate law. Activation energies calculated for the ‘dark’ isomerization reaction fell near 21 kcal mol−1 for each of the polymer samples evaluated. These values were not dependent on the overall structure or molecular weight of the polymer backbone and were nearly identical to those determined for several lower molecular weight model compounds. Calculated half-lives for the isomerization of cis-azobenzene linkages buried in the polymer backbone ranged from 1 day near room temperature to about 1 h at the 60°C isotherm. Data gleaned from SEC experiments suggested that polymers endowed with conformationally restricted geometries underwent a two-fold reduction in hydrodynamic radius in response to ultraviolet light exposure. Photo-contractions in more flexible polymer samples appeared to be less dramatic, consistent with molecular modelling and dilute solution viscosity measurements.
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