Tuning the upper critical solution temperature of acrylamide and acrylonitrile copolymers with ionic monomers

In this research, poly(acrylamide-co-acrylonitrile) (poly(AAm-co-AN)) copolymers were synthesized by free radical polymerization to evaluate the upper critical solution temperature (UCST) of their aqueous solutions. Because of the combined effects of hydrogen bonding and electrostatic interactions, by altering the copolymer composition, pH, and ionic strength of the solution, one may adjust the cloud point. The effect of AN to AAm ratio in the poly(AAm-co-AN) on UCST was investigated, and then the effect of introducing anionic and cationic monomers into the copolymer, as well as the effect of pH on the UCST of aqueous solutions were evaluated. For this purpose, acrylic acid (AA) was added as an anionic monomer and 2-(acryloyloxy)ethyltrimethylammonium chloride as a cationic monomer. Attenuated total reflectance-Fourier transform infrared spectroscopy proton nuclear magnetic resonance (¹H-NMR) and zeta potential analyses were used to verify the synthesis of copolymers. The turbidity of the aqueous solutions versus temperature was evaluated by ultraviolet-visible spectrometry (UV-Vis). The hydrodynamic radius of the polymeric chains was measured by dynamic light scattering. The results show that by increasing the AN content of the copolymer the UCST increases and by introducing ionic monomers in the copolymer composition the UCST was decreased. Also, pH has a great effect on UCST and this effect is different for anionic and cationic polymers. The results of this research give a good insight into the UCST behavior of ionic polymers in solution, which is especially important for polymers used in smart applications such as sensors and membranes.