Hydration, Phase Separation and Nonlinear Rheologyof Temperature-Sensitive Water-Soluble Polymers

1F. Tanaka1, T. Koga1 , I. Kaneda2 and F. M. Winnik3
Kyoto University, Japan, 2Rakuno Gakuen University, Japan3
The University of Montréal, Canada

η+(t) (Pa s)

The molecular origin of temperature(heat)-sensitivity of poly(N-isopropylacrylamide) (PNIPAM) in water istheoretically studied on the basis of cooperative (de)hydration. The observed sharp collapse of a chain on heating above
32℃ and very flat LCST line (horizontal up to 20 wt%) are shown to be caused by the hydrogen bonds between watermolecules and amide groups on the chain with cooperativity parameter smaller than σ = 0.3[1].
A PNIPAM chain exhibits peculiar conformational changes upon addition of a second water-miscible solvent such as
methanol. Although the second solvent is a good solvent for the polymer, the chain collapses in the certain€ of the second solvent. The tendency forcompositions of mixed solvent, followed by eventual reswelling under majority
phase separation is strongly enhanced by the presence of the second solvent. The LCST drop is the largest, from 31.5°C down to -7 °C, for the specific molar fraction xm = 0.35 of methanol. This enhanced phase separation in mixed goodsolvents is known as cononsolvency. We propose a new mechanism of cononsolvency by introducing the concept ofcompetitive hydrogen bonds: water and methanol strongly compete with each other to form hydrogen bonds with thepolymer chain [2], and calculated the mean square end-to-end distance of a chain and the cloud point curves.
The solution properties drastically change when PNIPAM chains are hydrophobically modified. In particular,telechelic PNIPAMs exhibit a variety of associated structures, from flower micelles to gel networks. The LCST ofthese solutions is found to decrease along the sol-gel transition curve as a result of end-chain association(association-induced phase separation) . The LCST splits from the collapse transition line. We relate the magnitude ofthe LCST decrease to a hydration cooperativity parameter. The LCST.
decreases substantially (ca 100 K) in the case of random hydration (forγ (s-1)
210.
7γ=7s-1PEO σ = 1.0 ), whereas only a small shift (ca 5 to 10 K) occurs in the case
1020of cooperative hydration (for PNIPAM smaller than σ = 0.3)[3]. These10results are compared with experimental observations of the cloud points
12010€ of tel-PEO and tel-PNIPAM solutions.
The nonaffine transient network theory is€developed to study the timedevelopment of the shear and normal stresses under start-up shear flows in-2-1
01
10101010t(s)networks formed by self-assembled telechelic PEO and PNIPAMchains[4]. The initial slope, strain hardening, and overshoot of the shear stressFig.1 Stress buildup in start-up shear
flow: comparison of the theory withare studied in detail in relation to the nonlinear tension-elongation curve of theexperimental data on telechelic PEO.
elastically active chains in the network. The critical value of the shear rate
for strain hardening (upward deviation of the stress from the reference curve defined by the linear moduli) is
calculated as a function of the amplitude A of the nonlinear term in the tension of a chain. It is approximately 6.3 (inthe time unit of the reciprocal thermal dissociation rate) for a nonlinear chain with A = 10 . The overshoot time when
the stress reaches a maximum, and the total deformation accumulated before the peak time are obtained in terms ofthe molecular parameters of the polymer chain. These theoretical results are compared with recent rheological€experiments of the solutions of telechelic hydrophobically modified poly(ethyleneoxide)s carrying short branched alkylchains (2-decyl-tetradecyl) at both ends.
[1] Okada, Y.; Tanaka, F.; Kujawa, P.; Winnik, F. M., J. Chem. Phys. 2006, 125 (24), 244902[1-11].
[2] Tanaka, F.; Koga, T.; Kojima, H.; Winnik, F. M., Macromolecules 2009, 42 (4), 1231-1330.

[3] Okada, Y.; Tanaka, F.; Kujawa, P.; Winnik, F. M., J. Chem. Phys. 2006, 125 (24), 244902[1-11].
[4] Tanaka, F.; Koga, T., Macromolecules 2006, 39 (17), 5913-5920.
[5] Koga, T.; Tanaka, F.; Kaneda, I.; Winnik, F. M., Langmuir 2009, 25 (15), 8626-8638.

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