Cross-Linked Polymers and Swollen Gels

From the organic point of view, reversible contraction/dilation phenomena should be observed in photochromic networks below their glass transition temperatures, i.e. in the rubbery state where segment mobility is important. Considering that isomerization occurs in the rubbery state at a rate similar to that in solution, it should be expected that the use of isomerization reactions for photocontractile behavior should be optimal with easily deformable networks, i.e. swollen gels and rubbery materials. 

It is on the basis of Lovrien s ideas concerning the photoregulation of polymer conformation in solution by means of photochromic species that Van der Veen Prins reported a first photomechanochemical model transducer, consisting of water-swollen membranes of poly(2-hydroxyethyl methacrylate) cross-linked with ethylene glycol dimethacrylate (1.1 wt-%) in the presence of a sulfonated bis-azostilbene dye (chry-sophenine G), the ratio chrysophenine/2-hydroxyethyl methacrylate being 1/400. 

On irradiation the t-t-t-dye is isomerized to its c-t-c-isomer this conformational change causes a decrease of dye-polymer interactions resulting in a gel contraction of 1.2%, i.e. a change in volume of about 3.6%. In the dark the gel recovers its original dimensions the rate of recovery is a function of temperature and parallels the increase of optical density of chrysophenine at 400 nm. 

they reported the photoregulation of the degree of ionization and swelling of poly(methacrylic acid) membrane cross-linked with 1 mol-% ethylene glycol dimethacrylate onto which positively charged p-phenylazophenyltrimethylammonium ions (PTA) were absorbed (Eq. (9)). 

Photoisomerization of trans PTA to cis PTA causes a change of the degree of ionization (by half a pK unit) and thus of the degree of swelling. 

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