Temperature-responsive polymer irradiation

In recent years, many kinds of temperature-responsive PNIPAAm and its copolymer hydrogels with other acrylic monomers have been synthesized [142]. Besides being used for hydrogels, NIPAAm monomer can be grafted on to polymer substrates by electron beam, irradiation or UV-initiated graft polymerization to achieve special modification of polymer surfaces. Thus NIPAAm has been grafted on porous polymer films such as LDPE, PP, or polyamide films in order to prepare novel films for pervaporation of liquid mixtures or separation membranes [150,151]. 

The UV irradiation technique has also been successfully employed in the preparation of novel macroporous cryogels based on hydrophobically modified high molar mass polyglycidol (Fig. 10) [19]. It is known that by varying the degree of modification of polymer, it is possible to obtain temperature-responsive materials with tunable properties [20]. For all copolymer compositions studied... 

Bilayer lipid structures are known to transform between a variety of morphologies. These transformations are controlled by the geometry or the chemistry of the molecular structures and by the presence of external stimuli. Giant vesicles, in the form of spherical lipid bilayers, are used as simplified models of biological cells. Their in vitro study using optical microscopy allows their responses to be observed when exposed to stresses such as temperature changes, UV irradiation, electric or osmotic shocks, or the addition of an external agent, such as amphiphilic polymers. 

Random and block copolymers of AzoVE and various vinyl ethers (exhibiting LCST-type phase separation) were prepared ([AzoVE]o = 0.5-5 mol%). For random copolymers (70) containing both thermally responsive and azobenzene units, phase separation occurred at a higher temperature imder UV irradiation compared with that imder visible light. This shift permitted solubility control of the polymer by irradiation with UV or visible light at a constant temperature [228]. 

In the phase separation process, however, it needed some induction period for the polymer to start the phase separation. Almost complete isomerization of the azobenzene pendant groups from the cis to the trans form is required to decrease the phase separation temperature below 19.5 °C. The phase separation process exhibited a non-linear response to the irradiation time or the number of photons. When the number of absorbed photons reached a critical value, the system underwent the phase separation and the polymer chain was shrunk. The photo-stimulated phase separation/dissolution cycle was not observed below 19.4 and above 26.0 °C. 

Poly(vinyl methyl ether), PVME, is a thermo-sensitive polymer. The aqueous solution has a Lower Critical Solution Temperature (LCST) of 37 °C. Therefore, PVME is soluble in water below its LCST, but insoluble above its LCST. When an aqueous solution of PVME is irradiated with y-rays the solution becomes PVME hydrogel [18, 19]. The gel shows thermo-sensitivity similar to the solution, and swells below 37 °C and shrinks above this temperature. It is important to form a fine porous gel structure to obtain quick response gels. There are two methods for the purpose. One is a method using micro-phase separation by heating. The other is a method using micro-phase separation by blending of polymer solutions. 

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