Dual temperature and pH responsiveness of poly(2-(N,N-dimethylamino)ethyl methacrylate-co-n-butyl acrylate) colloidal dispersions and their films
2-(N,N-Dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) monomers were copolymerized to form p(DMAEMA/nBA) colloidal particles that exhibit pH- and temperature-responsive behavior. Aqueous p(DMAEMA/nBA) colloidal dispersions exhibit second-order lower critical solution temperature (II-LCST) transition, referred to as stimuli-responsive temperature (T-SR) for solid films, in the 27-37 degrees C range manifested by the particle size changes from 148 to 80 nm. Upon pH changes from 2 to 10, the particle size also changed from 205 to 139 nm. When p(DMAEMA/nBA) colloidal particles were allowed to coalesce which was facilitated by the presence of nBA component, the films retained their dual responsiveness and shrink in all the directions at T> T-SR while expanding upon pH changes from basic to acidic. Thermodynamic molecular simulations combined with spectroscopic analysis showed that the temperature-responsive behavior results from the collapse of the (dimethylamino)ethyl groups and twisting/buckling of the copolymer backbone, while the pH responsiveness is attributed to protonation/deprotonation of amine functional groups. Molecular simulations also showed that the total volume (V) above T-SR decreases by 21.6%, whereas a 13.26% increase occurs at the protonation state, which is in good agreement with the experimental data.