The molecular weight dependence of the lower critical solution temperature of poly(N,N-diethylacrylamide) was studied with 11 samples of the polymer with a number-average molecular weight (M n ) ranging from 9.6 × 10 3 to 1.3 × 10 6 g mol -1 and relatively narrow polydispersity indices from 1.19 and 2.60. These samples were obtained by fractional precipitation of the polymer. LCST values of polymers were measured by turbidimetry and microcalorimetry. An inverse dependence of LCST on the molar mass was found and the LCST of the samples remained more or less a constant above a critical molecular weight of ca. 2 × 10 5 g mol -1 . The enthalpy and the entropy changes as well as the LCST of the polymer depend strongly on the molar mass of the polymer, especially in low molecular weight range.

Poly(N,N-diethylacrylamide) (PDEA) possesses a lower critical solution temperature (LCST) in aqueous media. The solution properties of PDEA at various temperatures have been characterized with techniques such as rheology and dynamic light scattering. There is a decrease in the coil size before the phase transition due to a coil-to-globule transition. At the LCST, rheological and dynamic light scattering studies have also confirmed an aggregation phenomenon. This aggregation modifies the rheological properties of the polymer solutions. High frequencies hinder the phasetransition process and reduce the LCST of the polymers.

To test the effect of the matrix polymer on diffusion, we have measured the self-diffusion coefficients of water and poly(ethylene glycol) of a molecular weight of 600 (PEG-600) in aqueous systems of selected polymers using the pulsed-gradient spin-echo NMR technique. The polymers used in this study include poly(vinyl alcohol) (PVA), hydroxypropyl methyl cellulose (HPMC), poly(N,N-diethylacrylamide) (PNNDEA), and poly(N-isopropylacrylamide) (PNIPA). The polymer concentrations varied from 0 to 0.38 g/mL. The effect of the polymer network on the self-diffusion coefficients of the solvent (water) and a solute (PEG-600) was investigated by analyzing the diffusion data with the use of the free volume model of Yasuda et al. [Yasuda, H.; Lamaze, C. E.; Ikenberry, L. D. Makromol. Chem. 1968, 118, 19], the diffusion model proposed by Phillies [Phillies, G. D. J. Macromolecules 1986, 19, 2367], and the model of Petit et al. [Petit, J.-M.; Roux, B.; Zhu, X. X.; Macdonald, P. M. Macromolecules 1996, 29, 6031]. The results obtained with PVA, HPMC, PNNDEA, and PNIPA are used to evaluate the applicability of these models in polymer−water−solute ternary systems. The physical significance of the parameters used in the models is discussed.

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