Ultrasound stimuli on viscometric change of aqueous copolymers having acrylic acid and N-isopropyl acrylamide for thermo-sensitive segments

Ultrasound (US) was used to change the shear viscosity of an aqueous solution of copolymers having acrylic acid (AA) and N-isopropylacrylamide (NIPAM) segments. The US effect on the shear viscosity of the copolymers containing 10, 50 and 90 mol% of the NIPAM group having thermo-responsible property was examined when the US was exposed to the aqueous solution at different temperatures. The shear viscosity of the solution had a significant change at about 30–35 °C when the viscosity was measured in the range of 0–60 °C. While, the viscosity decreased with the increase of the temperature, the US operated at 28, 45 and 100 kHz also induced significant reduction of the shear viscosity. Evidence of the US effect on the shear viscosity reduction was observed by measurement of FT-IR spectra of the copolymer solution when the US was exposed. It was noted that considerable change of the spectra at the 28 kHz US was observed relative to that of the 45 and 100 kHz US. The tendency of the change in the shear viscosity and IR spectra was almost similar. Furthermore, the shear viscosity and IR spectra changed gradually to recover its original value as the US was stopped. This was due to the breaking and reformation of the hydrogen bonding between NIPAM and AA segments when the US was exposed and stopped, respectively.

Research highlights
► Ultrasound changes viscosity of copolymers of acrylic acid and N-isopropylacrylamide.
► Hydrogen bonding of copolymers in aqueous solution is broken by ultrasound exposure.
► Hydrogen bond networks broken in the copolymers can be regenerated without ultrasound.