(433b) Self-Assembly in Aqueous Solution and Nano-Morphology of a Dual-Responsive Hydrophilic Random Copolymer Poly(NIPAM-co-AA)

A dual-responsive hydrophilic random copolymer (HRC) poly(NIPAM-co-AA) is synthesized in this paper using AIBN as a free radical initiator in t-butanol at 70 °C. The solvent affinity of the hydrophilic chains and the hydrophobic chains in poly(NIPAM-co-AA) molecules could be reversed in aqueous solution depending on pH and/or temperature change, to produce poly(NIPAM-co-AA) nano-objects with various morphology including spheres, rectangles, and rods. In the present study, particular attention is paid to nano-morphology and nano-structure of these assemblies. Such nano-object exhibits thermo-responsive behavior: heating to 40 °C or 60 °C (above the lower critical solution temperature, LCST, of the copolymer) and keeping pH 4.0 causes self-assembly due to the onset of a hydrophilic-to-hydrophobic transition of NIPAM chains. TEM image of the self-assembly reveals a sphere that consists of NIPAM chains mainly inside and AA chains almost outside, and that is because the NIPAM chains are transformed from hydrophilic chains to hydrophobic chains and decreasing the hydrogen bonding interactions between them and water. And such nano-object also exhibits pH-responsive behavior: decreasing to pH 0.9 or increasing to pH 6.0 and keeping the temperature at 20 °C (below  LCST) causes self-assembly due to the onset of a hydrophilic-to-hydrophobic (and the latter hydrophobic-to-hydrophilic) transition of AA chains. TEM image of the self-assembly corresponding to pH 0.9 reveals a rectangle that consists of AA chains mainly inside and NIPAM chains almost outside. However, TEM image of the self-assembly corresponding to pH 6.0 reveals a rod that consists of NIPAM chains mainly inside and AA chains almost outside. Particular attention is such nano-object exhibits pH and thermo dual-responsive behavior: increasing to pH 6.0 and heating to 60 °C (above LCST) also causes self-assembly due to the onset of both the hydrophilic-to-hydrophobic transition of NIPAM chains and the hydrophobic-to-hydrophilic transition of AA chains. TEM is used to investigate the nano-morphology changes that occur during the sphere-to-rectangle-to-rod duo to the pH and/or temperature responsive behavior. In addition, AuD-TEM (in situ reduction and gold doping-TEM coupling technology) is used to investigate the nano-structure. The rectangle (pH 0.9, 20 °C) and rod (pH 6.0, 20 °C) exhibits a sponge nano-structure due to the hydrophilicity of a small quantity of NIPAM chains or AA chains inside.