Abstract
At room temperature, poly(N-isopropylacrylamide) (PNIPAM) is soluble in water and methanol, but it is not soluble in certain water/methanol mixtures. This phenomenon, known as cononsolvency, has been explored in great detail experimentally and theoretically in an attempt to understand the complex interactions occurring in the ternary PNIPAM/water/co-nonsolvent system. Yet little is known about the effects of the polymer structure on cononsolvency. To address this point, we investigated the temperature-dependent solution properties in water, methanol, and mixtures of the two solvents of poly(2-cyclopropyl-2-oxazoline) (PcyPOx) and two structural isomers of PNIPAM (Mn ∼ 11 kg/mol): poly(2-isopropyl-2-oxazoline) (PiPOx) and poly(2-n-propyl-2-oxazoline) (PnPOx). The phase diagram of the ternary water/methanol/poly(2-propyl-2-oxazolines) (PPOx) systems, constructed based on cloud point (TCP) measurements, revealed that PnPOx exhibits cononsolvency in water/methanol mixtures. In contrast, methanol acts as a cosolvent for PiPOx and PcyPOx in water. The enthalpy, ΔH, and temperature, Tmax, of the coil-to-globule transition of the three polymers in various water/methanol mixtures were measured by high-sensitivity differential scanning calorimetry. Tmax follows the same trends as TCP, confirming the cononsolvency of PnPOx and the cosolvency of PiPOx and PcyPOx. ΔH decreases linearly as a function of the methanol content for all PPOx systems. Ancillary high-resolution 1H NMR spectroscopy studies of PPOx solutions in D2O and methanol-d4, coupled with DOSY and NOESY experiments revealed that the n-propyl group of PnPOx rotates freely in D2O, whereas the rotation of the isopropyl and cyclopropyl groups of PiPOx and PcyPOx, respectively, is limited due to steric restriction. This factor appears to play an important role in the case of the PPOxs/water/methanol ternary system.
Highlights
At room temperature, poly(N-isopropylacrylamide) (PNIPAM) dissolves readily in water as well as in alcohols
Okada and Tanaka extended to PNIPAM, the concept of cooperative hydration originally developed to explain the solubility of poly(ethylene glycol) in water.[14]. Their model speculates that the formation of an amide−water hydrogen bond on one repeat unit of a dehydrated PNIPAM chain facilitates the hydration of the adjacent repeat unit, and the extension of hydration to longer sequences
Tanaka et al used the concept of hydration cooperativity in their model of the PNIPAM cononsolvency in water/methanol mixtures with the added assumption that methanol molecules interfere with the cooperative hydration of the amide units on the polymer
Summary
Poly(N-isopropylacrylamide) (PNIPAM) dissolves readily in water as well as in alcohols. The PPOxs were prepared following standard procedures,[19] as described in detail in the Supporting Information document Their chemical structure is shown in Scheme 1. Like PNIPAM, PVME and PDEAM are water-soluble polymers that exhibit a temperature-dependent coil-globule transition in water. The addition of methanol to an aqueous solution of PVME or PDEAM does not lead to cononsolvency It results in the opposite effect: the temperature range in which the polymer is soluble increases. We built the phase diagrams of the three PPOxs in water/methanol mixtures via turbidimetry measurements and determined the enthalpy and phase transition temperature of the three polymers in mixed water−methanol solutions via microcalorimetry.
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