Thermo-responsive polymers lower critical solution temperature

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. 

Poly(7V-isopropylacrylamide) (PNlPAAm) is a well-known thermo-responsive polymer and exhibits a lower critical solution temperature (LCST) of 32°C in water. It assnmes a random coil structure (hydrophilic state) below the LCST and a collapsed globnlar stractnre (hydrophobic state) above. Because of this sharp reversible transition, this polymer finds a vast array of applications,... 

LIP Li, P.-F., Wang, W., Xie, R., Yang, M., Ju, X.-J., and Chu, L.-Y., Lower critical solution temperatures of thermo-responsive poly(A -isopropylaciylamide) copolymers with racemate or single enantiomer groups, Polym. Int, 58, 202, 2009. 

Thermo-responsive polymers have been used to release encapsulated drugs in response to temperature changes. These polymers typically undergo a rapid and reversible hydration-dehydration at lower critical solution temperature (LCST, from soluble to insoluble) or upper critical solution temperature (UCST, from insoluble to soluble) upon heating. [71] For example, a thermo-responsive polymer, poly (2-isopropyl- 2-oxazoline) (PiPrOx), was conjugated to pH-responsive poly(benzyl ether)dendrons, which exhibited sharp thermal transitions as well as pH-dependent behaviors (Fig. 4a).[105]... 

One of the more recently developed material classes is that of thermo-responsive polymers. This accounts mainly for those polymers showing a so-called lower critical solution temperature (LOST) or - to a much less extent - an upper critical solution temperature (UCST), meaning polymers that change their solution status significantly upon temperature changes. In general, this behaviour is the result of a delicate balance of hydrophilic and hydrophobic groups in the polymer. The major... 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

Duan et alP also reported the synthesis of poly(A -isopropylacrylamide)-silica composite microspheres by using inverse Pickering suspension polymerization with various sizes of silica particles as stabilizers. Figure 1.14 shows examples of such microgels stabilized by silica particles with mean diameters of 53, 301, 500 and 962 nm. To generate these nanocomposite structures, droplets of an aqueous solution of 7V-isopropylacrylamide were first dispersed in toluene and then stabilized by silica particles. The monomer was subsequently polymerized to obtain polymer silica composite microspheres. It was also observed that the thermo-responsive behavior of the polymer was not affected in the presence of silica, as a lower critical solution temperature of 32 °C for the poly(A -isopropylacrylamide) was also observed in the polymer-silica microspheres. 

Figure 1.1 Curves showing phase transition phenomenon, (a) Lower critical solution temperatnre (LCST) and (h) npper critical solution temperature (UCST) phase transition behaviors of thermo-responsive polymers in solution.

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