Self-Assembling of Thermally Responsive Block and Graft Copolymers in Aqueous Solutions

Various thermally responsive block and graft copolymers based on poly(Nisopropylacrylamide) (PNIPA) have been synthesised. Poly(ethyleneoxides) (PEO) with different chain lengths were used as hydrophilic blocks and grafts. The graft copolymers were synthesised in two steps. Firstly, two functional copolymers were prepared by free radical polymerisation of N-isopropylacrylamide (NIPA) with either glycidylmethacrylate (GMA) or N-acryloylsuccinimide (NASI). The reactivity ratios of NIPA and GMA were determined by two different methods. Secondly, an amino terminated PEO was allowed to react with the functional copolymers either in an organic solvent or in water, resulting in various amounts of side chains on the PNIPA backbone. Block copolymers were prepared by free radical polymerisation of NIPA using two PEO substituted macroazoinitiators. The copolymers exhibit reversible thermosensitivity in aqueous solutions typical to PNIPA, being completely water soluble at low temperatures but phase separate at the lower critical solution temperature (LCST) upon heating. In addition, a block copolymer of NIPA with a zwitterionic sulfobetaine, N,N-dimethyl-N-3-methacrylamidopropyl ammoniopropane sulfonate (SPP) was studied, which showed double thermosensitive properties precipitating upon cooling at the upper critical solution temperature (UCST) owing to the PSPP block, and upon heating at the LCST owing to the PNIPA block. All the graft and block copolymers, PNIPA-g-PEO, PNIPA-b-PEO and PNIPA-b-PSPP, formed spherical core-shell structures in dilute aqueous solutions at their critical temperatures. The factors determining the coil-to-globule transition of the thermosensitive part include hydrophobic interactions, i.e. intraand interchain interactions, and the solubilising effect of the hydrophilic shell on the shrinking backbone. Thermal properties of aqueous polymer solutions were studied by the combination of static and dynamic light scattering, fluorescence, EPR and UV spectroscopies, and also by viscosimetry. The LCST of the PNIPA-PEO block and graft copolymers critically depended on the amount of PEO and on polymer concentration. Above the critical temperature, the size, size distribution and structure of the nanoscopic aggregates sterically stabilised by the PEO shell can be adjusted by various factors such as the amount of PEO, polymer concentration and the heating rate of the sample. Two copolymers grafted in water either far below or close to the LCST of the backbone turned out to be able to partly remember the original conformation of the parent backbone copolymers in which they had been grafted. The resulting copolymer solutions and the localisation of the fluorescent and spin probes inside copolymers were dependent on both the chemical nature of the probes and the method of the sample preparation. Thermal properties of aqueous PNIPA-b-PSPP solutions can be influenced by the addition of sodium chloride. Salt enhances the water solubility of polysulfobetaine but on the other hand the solvent becomes poorer for PNIPA in the course of addition of salt. At room temperature, gradual addition of salt turns the mutual interactions from zwitterionic attractions between PSPP blocks to hydrophobic attractions between PNIPA blocks. At a certain concentration of salt the thermosensitivity of the PSPP block disappears. In pure water below the UCST, the block copolymers PNIPA-b-PSPP form loosely packed aggregates sterically stabilised by PNIPA shell whose size depends not only on the salt