Hydrophobic Interaction, LCST effect

As is being discussed, polymers used to prepare micelles exhibit a LCST that can be deLned as the temperature at which the polymer phase separates (Heskinsand Guillet, 1968). Below the LCST, the polymer/micelle is soluble, but it precipitates at temperatures above the LCST. The diameter of these micelles rapidly rises at temperatures above the LCST, due to hydrophobic interactions that result in the aggregation of the micelles (Kohori et al., 1998). This effect of temperature on size was shown to be reversible, since the micellar architecture was maintained after lowering the temperature below the LCST (Chung et al., 1999). 

As mentioned above, however, a decrease in the LCST was observed when urea was added to an aqueous PNIPA solution. We thus tried to determine the LCSTs of both the PNIPA solution and the PNIPA-PAAc mixture in the presence of 4 M urea (Table 2). It is generally believed that urea breaks up the hydrogen bonds between solute molecules and also disrupts the cluster structure of water molecules ( structure breaking effect ). The latter brings about a weakening of the hydrophobic interaction between solute molecules (e.g., see Ref. 76). In the case of an aqueous PNIPA system, however, the addition of urea shifted the LCST to a low-temperature range. Therefore we cannot simply state that hydrophobic interaction between NIPA residues is weakened by the addition of urea. 

Reverse thermogelling polymers used to act as an effective injectable thermogel usually possess block architectures and a balanced structure of hydrophobicity and hydrophilicity. As temperature increases, the association of the polymers occurs due to increased hydrophobic interactions to show a temperature-sensitive sol-to-gel transition at a critical temperature, namely, lower critical solution temperature (LCST). Typical reverse thermogelling polymers include poly(N-substimted acrylamide)-based block copolymers [7-11], poly(vinyl ether)-based block copolymers, poly(ethylene oxide) (PEO)/poly(propylene oxide) (PPO)-based block copolymers [12-17] and PEG/polyester block copolymers [18-23], The representative structures of each class are shown in Fig. 1. In most cases, PEG was used as a hydrophilic block. All the themogelling hydrogels formed from the amphiphilic block copolymers mentioned above exhibit a sol-gel phase-transition in the physiological conditions in a tunable manner and have been intensively studied in recent years. 

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