(576h) Ultrasonic Synthesis of Temperature Responsive Copolymer and Its Characterization

Poly(N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer with a lower critical solution temperature (LCST). Poly (2-hydroxyethyl methacrylate) (PHEMA) has a high mechanical strength, so that a copolymer of Poly(NIPAM-co-HEMA) is expected to be applied as advanced functional polymeric materials in various fields, such as sensors, actuators, drug delivery system, and so on. The properties of the copolymer are dependent on the molecular weight distribution and copolymer composition.

Ultrasonic irradiation to liquid results in cavitation phenomena, i.e., formation and collapse of micro scale bubbles. Collapsing bubbles cause local high temperature and high pressure field, and extremely high shear flow in liquid. When ultrasound is irradiated to a solution containing monomer, polymerization proceeds by generating radicals owing to local high temperature, and the synthesized polymer decomposes due to high shear stress, simultaneously. Thus, the molecular weight and polydispersity of polymer is expected to be controlled by changing the condition of ultrasonic irradiation. Ultrasonic irradiation to monomer solution was applicable to produce various vinyl polymers.

In this research poly (N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) was synthesized using ultrasonic irradiation in a mixed solvent of water and ethanol without chemical initiator. The effects of ultrasonic power intensity, and monomer molar ratio on time course of conversion to polymer, number average molecular weight, and polydispersity were investigated in order to synthesize the polymer with low molecular weight distribution (i.e., low polydispersity). The responsive temperature of synthesized polymer was also evaluated.

Higher ultrasonic power intensity resulted in higher conversion to polymer and lower molecular weight. This is because the higher amount of radical is generated at higher ultrasonic power intensity. The polydispersity was less than 1.5 for all the case. Thus, the molecular weight can be controlled by ultrasonic power intensity while maintaining low polydispersity. Higher NIPAAM ratio resulted in higher polymerization rate, faster polymer degradation rate, and lower polydispersity. PNIPAAM radical has lower reaction activity with ethanol than PHEMA radical, so that PNIPAAM polymerizes faster than PHEMA.

The transmittance of aqueous solution of copolymer decreased abruptly at a lower critical solution temperature (LCST) which depends on the monomer molar ratio. The lower the molar ratio of NIPAM, the lower the LCST because HEMA is more hydrophobic than NIPAM. The higher polymerization rate resulted in the higher LCST. Those results suggested that the responsive temperature of the copolymer can be controlled by changing the molar ratio of NIPAM and the ultrasound intensity.