Development of Thermo-Responsive Materials and Their Potential Application of Osmotic Control

Abstract

Smart materials response to not only the physical signals such as temperature, light, and magnetic field but also chemical signals such as ionic concentration, glutathione concentration, and pH, etc. Among them temperature has some advantages. Temperature could be easily applied to biosystem by heat pad or near-infrared illumination. It can be also cooled or heated reversibly. Thermoresponsive materials which consist of two or more components change physical or chemical properties by mild temperature changes. Among them I have more interested in water-soluble thermoresponsive materials. The water is not only present in a large amount enough to covers approximately 71% of the earth surface but also forming the body fluids of all living organisms on the earth. Therefore, water-soluble thermoresponsive materials could be applicate for biomedical application as well as water purification. In practice, these materials have been extensively researched in various applications such as cell culture dishes, chromatography, temperature-triggered drug release, or targeted drug delivery. Thermoresponsive materials in aqueous solution could be classified into two and the phase transition behavior in water could be explained by simplified thermodynamic equation.

(The Gibbs free energy of mixing: ΔGm, The enthalpy of mixing: ΔHm, The entropy of mixing: ΔSm, Temperature

T) Materials that can mix in solvent below a certain temperature and abruptly phase separate from solution above a certain temperature are referred as the lower critical solution temperature (LCST). Representative LCST materials are poly(N-isopropylacrylamide) of which structure harmoniously consist of hydrophilic moiety and hydrophobic moiety. In aqueous solutions of these polymers, ΔHm is negative due to strong interaction between water and polymer molecules and ΔSm is also negative because the hydrophobic moieties(isopropyl) of poly(N-isopropylacrylamide) are captured in ordered form of water molecules which actually existed in disordered form. Therefore, at a given composition and pressure, LCST phase separation occurs when ΔGm becomes zero as the negative TΔSm becomes dominant over the negative ΔHm upon reaching a specific temperature. On the contrary to LCST pahse transition, materials which are miscible in solvent above a certain temperature and abruptly phase separate from solution below a certain temperature are referred as the upper critical solution temperature (UCST). Representative UCST materials are poly(3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate which has both positively charged ammonium moiety and negatively charged sulfonate in every repeating unit, which can strongly interact with each other by electrostatic interaction generating positive value of ΔHm . Therefore, UCST phase separation occurs when temperature reaches below a certain temperature to give rise to zero value of ΔGm as the positive ΔHm becomes dominant over the positive TΔSm upon reaching a specific temperature. In order to synthesize the thermoresponsive materials previously, monomer should be synthesized and then polymerized through multiple steps. I have been focusing on researches for synthesizing water-soluble thermoresponsive materials in a simple way comparing to conventional methods in this work. In order to achieve above goal I purchased the commercially available amine rich polymer, branched polyethylenimine (b-PEI) which can be easily modified through simple reaction such as acylation, ring opening reaction, or methylation for synthesizing LCST materials or UCST materials. LCST properties were introduced into b-PEI by simple acylation. The resulting N-acylated b-PEI have similar structure with poly(N-isopropylacrylamide). The phase transition temperature of N-acylated b-PEI in aqueous solution can be controlled by the hydrophobicity of acyl group, degree of acylation, concentration, and pH in the range of 10-90 °C UCST properties were introduce into b-PEI by ring-opening reaction resulting in N-sulfopropylated b-PEI which has similar structure with poly(3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate. The phase transition temperature of N-sulfopropylated b-PEI in aqueous solution can be controlled by degree of acylation, molecular weight, concentration, and pH in the range of 10-90 °C Another way to introduce the UCST properties into b-PEI is to simply mix with hydrogen halide or by methylation of b-PEI. Although halide salts of b-PEI exhibited the UCST properties in water, the pH of solution is too low to apply for practical application. However, methylated b-PEI (MPEI) derivatives which possess 35-38% quaternary ammoniums with hydrophobic counter anions such as iodide (I-)or tetrafluoroborate(BF4-) exhibited UCST in mild pH range because of strong electrostatic interaction and hydrophobic interaction. Methylated b-PEI with more hydrophilic anion such as Cl- and Br- did not show UCST properties because the hydrophobicity of anions is not enough to have strong interaction with MPEI. The dependence of salt effect and molecular weight on phase transition temperature is also investigated. The new application such as a draw solute for forward osmosis (FO) is possible thanks to easy introduction of thermoresponsiveness into amine-rich polymer by above methods. The FO method is to draw fresh water from feed solution to a draw solution (Higher concentration than feed solution) by spontaneous osmosis through a semipermeable membrane. Following the Vant hoff equation,

(π: osmotic pressure, c: molarity of concentration, R: gas constant, T: temperatrure, V: volume, M, molecular weight, w: solute weight) thermoresponsive materials with low molecular weight are needed for inducing high osmotic pressure. Therefore, nBu-TAEA with low molecular weight, 356g/mol, was synthesized, which exhibited 20-30 ℃ phase transition temperature at 2.2 M in water. After phase separation at 55 ℃, the effective osmotic pressure of nBu-TAEA solution became lower than 0.15 M equivalent to physiological saline. Therefore, after fresh water could be drawn from 0.60 M NaCl solution equivalent to seawater, to 2.2 M of nBu-TAEA solution below phase transition temperature, the obtained fresh water in nBu-TAEA solution was released into 0.15 M equivalent to physiological saline above phase transition temperature. The thermoresponsive materials as a draw solute for forward osmosis could be energy efficient method for obtaining fresh water from seawater.