Dual pH- and Thermo-Responsive Self-Assembly of the P(NIPAM-co-AA) Hydrophilic Random Copolymer in Pure Aqueous Solution

The random poly(N-isopropylacrylamide-co-acrylic acid) copolymer(P(NIPAM-co-AA)) was synthesized and its stimuli-responsive self-assembly in pure aqueous solution was investigated. P(NIPAMco-AA) is a dual pH- and thermo-responsive hydrophilic random copolymer because it can self-assemble in pure aqueous solution in response to the stimuli of pH and temperature. The morphologies of P(NIPAM-coAA) assemblies were imaged by transmission electron microscopy, and the radius(Rh) and radius distribution were determined by dynamic and static light scattering. The lower critical solution temperature(LCST) of P(NIPAM-co-AA) and the zeta potential change of the aqueous solution of P(NIPAM-co-AA) with respect to pH were measured. The driving force for pH- and thermo-responsive self-assembly of P(NIPAMco- AA) was deduced from the protonation degrees of two different chain units based on LCST and zeta potential data. The response of the special microstructure of the assemblies to environmental stimulation was confirmed by combining the above results with Fourier- transform infrared spectroscopy, which can provide information about the enriched chain units on the surface of assemblies. The experimental results showed that the P(NIAPM-co-AA) random copolymer can be easily synthesized and is both pH- and thermoresponsive. The stimuli-responsive self-assembly behavior of P(NIAPM-co-AA) in pure aqueous solution can potentially be applied to drug delivery.