Thermo-shrinking polymers

One of the very useful applications involving thermo-shrinking polymers is a polymeric tool (a gel hand) made of three layers of PNIPAAm, PAAm, and an inert spacer (Figure 5.73). This gel hand can be used like a tweezer to pick up a target compound in aqueous solution by simply raising the temperature above LCST and to release the compound below LCST. 

FIGURE 5.73 A schematic illustration showing how thermo-shrinking polymer PNIPAAm can be used as a gel hand to isolate a target compound. 

Smart colloids are colloidal dispersions for which certain properties, such as size and structure, can be altered by changing an external influence, such as temperature, eg, cross-linked polymer gels of poly(iV-isoproplyaciylamide). Such polymer systems can swell or shrink in response to temperature changes, and are also termed thermo-shrinking polymers. (see References 122-124). 

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. 

Both PNIPAAm and PVME exhibit unique thermo-shrinking properties. Thus, as an aqueous solution is heated beyond a certain point, the polymer shrinks and a phase separation occurs. This temperature is commonly referred to as the lower critical solution temperature (LCST). For PNIPAAm, it lies between ca. 30 and 35°C, the exact temperature being a function of the detailed microstructure of the macromolecule. Below LCST, the polymer is soluble in the aqueous phase, as the chains are extended and surrounded by water molecules. Above the LCST, the polymer becomes insoluble and phase separation occurs. Because of the abrupt nature of these transitions and their reversibility (which allows repeated thermal switching) these polymers have stirred up particular interest in the field of science and engineering since their first appearance in the open literature in 1956. 

Y. Kaneko, R. Yoshida, K. Sakai, Y. Sakurai and T. Okano, Temperature-responsive shrinking kinetics of poly(Y-isopropylacrylamide) copolymer gels with hydrophilic and hydrophobic comonomers, J. Membr. Sci., 1995, 101, 13 Y.H. Lim, D. Kim and D.S. Lee, Drug releasing characteristics of thermo- and pH-sensitive interpenetrating polymer networks based on poly(W-isopropylacrylamide), J. Appl. Polym. Sci., 1997,... 

---