Synthesis of temperature responsive poly(N-isopropylacrylamide) using ultrasound irradiation

Ultrasound was employed to synthesize poly(N-isopropylacrylamide) [poly(NIPAM)] either as aqueous solutions or microgels in the absence of a chemical initiator. Poly(NIPAM) of different microstructures can be readily prepared via ultrasound irradiation by varying the reaction temperature. At a preparation temperature of 20 °C, poly(NIPAM) was formed in aqueous solutions, whereas, at a higher preparation temperature (beyond the lower critical solution temperature of approximately 32 °C), poly(NIPAM) microgels were formed. In addition, the high shear gradients generated by the acoustic cavitation process aid to control or vary the molecular weights of poly(NIPAM) formed in aqueous solutions. The swelling behavior of poly(NIPAM) at different concentrations of sodium dodecyl sulfate (SDS) was also studied. An increase in the transition temperature and hydrodynamic size of the particles was attributed to SDS binding to the polymer network through hydrophobic interactions. Light scattering data reflected the formation of larger microgels at low cross-linker concentrations. The encapsulation of rhodamine B within the microgels was achieved by sonicating the monomer containing the dye. The subsequent release of the dye was consistent with Fickian diffusion and the diffusion coefficient of the dye was estimated as 4.0 × 10^-12 and 3.6 × 10 ^-11 m² s^-1 at 20 and 40 °C, respectively. From the diffusion coefficients, the viscosities of the polymer samples at 20 and 40 °C were determined using the Stokes-Einstein equation to be 77 and 10 cP, respectively.