AZOBENZENE MODIFIED POLY(ARYL ETHER KETONE AMIDE)S - 2 - PHOTO-RESPONSIVE AND THERMORESPONSIVE BEHAVIOR IN DILUTE-SOLUTION

A number of azobenzene modified poly(aryl ether ketone amide)s with di ffering backbone geometries were evaluated for their photo-and thermo- regulated behaviour in dilute solution. Photoinduced trans --> cis iso merization reactions were carried out by irradiating the polymer sampl es with ultraviolet light at wavelengths between 370 and 400 nm. Photo stationary state compositions achieved under these conditions typicall y consisted of about 70% of the higher energy cia isomer distributed a long the polymer main chain. Reverse cis --> trans isomerization of th e backbone azobenzene moieties was triggered by either photochemical o r thermal means and was monitored by optical absorbance and H-1 n.m.r. spectroscopies. Thermally induced cis --> trans return in each of the polymers obeyed the first-order rate law. Activation energies calcula ted for the 'dark' isomerization reaction fell near 21 kcal mol(-1) fo r each of the polymer samples evaluated. These values were not depende nt on the overall structure or molecular weight of the polymer backbon e and were nearly identical to those determined for several lower mole cular weight model compounds. Calculated half-lives for the isomerizat ion of cis-azobenzene linkages buried in the polymer backbone ranged f rom 1 day near room temperature to about 1 h at the 60 degrees C isoth erm. Data gleaned from SEC experiments suggested that polymers endowed with conformationally restricted geometries underwent a two-fold redu ction in hydrodynamic radius in response to ultraviolet light exposure . Photo-contractions in more flexible polymer samples appeared to be l ess dramatic, consistent with molecular modelling and dilute solution viscosity measurements. (C) 1998 Elsevier Science Ltd. All rights rese rved.