HYDRATION AND PHASE SEPARATION OF TEMPERATURE-SENSITIVE WATER-SOLUBLE POLYMERS

Collapse of a poly（N-isopropylacrylamide） （PNIPAM） chain upon heating and phase diagrams of aqueous PNIPAM solutions with very fiat LCST phase separation line are theoretically studied on the basis of cooperative dehydration （simultaneous dissociation of bound water molecules in a group of correlated sequence）, and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes sharper with the cooperativity parameter σ of hydration. Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves （LCST cononsolvency） with the mole fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends （telechelic PNIPAM） are theoretically and experimentally studied. The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association （association-induced phase separation）, and separate from the coil-globule transition line. Associated structures in the solution, such as flower micelles, mesoglobules and higher fractal assembly, are studied by USANS with theoretical modeling of the scattering function.

New insights into the effects of molecular weight and end group on the temperature-induced phase transition of poly(N-isopropylacrylamide) in water

In an attempt to clarify issues related to the molecular weight dependence of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in water,we prepared a library of PNIPAM samples of well-controlled molecular weight (7000 to 45000 g/mol) bearing identical groups on each chain end.The polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) with a bifunctional chain tranfer agent and further end group modification.The effects of the end group chemical structure,hydroxyethyl (HE),propargyl (Pr),chloroethyl (CE),n-butyl (nBu),n-hexyl (nHe),and isobutylsulfanylthiosulfanyl (IBS) on the phase transition temperature of aqueous PNIPAM solutions were investigated by high-sensitivity differential scanning calorimetry (HS-DSC),yielding the enthalpy ΔH and the endotherm maximum temperature (T M),and turbidimetry,providing the cloud point (T CP) of each solution.The T CP and T M of the PNIPAM sample of lowest molar mass (M n 7,000 g/mol,0.5 g/L) ranged,respectively,from 38.8 to 22.5 °C and 42.2 to 26.0 °C,depending on the structure of the end-group,whereas H showed no strong end-group dependence.The phase transition of all polymers,except,-di(n-butyl-PNIPAM),exhibited a marked dependence on the polymer molar mass.