Hydration, phase separation and nonlinear rheology of temperature-sensitive water-solublepolymers

Abstract

The collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating and the phase diagrams of aqueousPNIPAM solutions with a very flat lower critical solution temperature (LCST) phaseseparation line are theoretically studied on the basis of cooperative dehydration (simultaneousdissociation of bound water molecules in a group of correlated sequence), and comparedwith the experimental observation of temperature-induced coil–globule transition by lightscattering methods. The transition becomes sharper with the cooperativity parameterσ of hydration. The reentrant coil–globule–coil transition and cononsolvency in a mixedsolvent of water and methanol are also studied from the viewpoint of competitive hydrogenbonds between polymer–water and polymer–methanol. The downward shift of thecloud-point curves (LCST cononsolvency) with the mol fraction of methanol due to thecompetition is calculated and compared with the experimental data. Aqueous solutions ofhydrophobically modified PNIPAM carrying short alkyl chains at both chain ends(telechelic PNIPAM) are theoretically and experimentally studied. The LCST of thesesolutions is found to shift downward along the sol–gel transition curve as a result ofend-chain association (association-induced phase separation), and separate fromthe coil–globule transition line. Associated structures in the solution, such asflower micelles, mesoglobules, and higher fractal assembly, are studied by ultrasmall-angle neutron scattering with theoretical modeling of the scattering function.Dynamic-mechanical modulus, nonlinear stationary viscosity, and stress build-up instart-up shear flows of the associated networks are studied on the basis of the affine andnon-affine transient network theory. The molecular conditions for thickening, strainhardening, and stress overshoot are found in terms of the nonlinear amplitudeA of the chain tension and the tension–dissociation coupling constantg.