Phase Separation in Poly(N,N-diethylacrylamide)/D2O Solutions and Physical Gels as Studied by1H NMR Spectroscopy

The structural-dynamic changes during temperature-induced phase separation in poly(N,N-diethylacrylamide) (PDEAAm)/D2O solutions and physical gels in a broad range of concentrations (c = 0.5–50 wt.-%) were studied by conventional high-resolution and MAS 1H NMR spectra. In the whole concentration range, irrespective of whether the studied system is a solution (c < 5 wt.-%) or physical gel (c > 5 wt.-%), the phase transition is manifested by line broadening (linewidth ≈ 3.6 kHz) of a major part of PDEAAm units, evidently due to the formation of compact globular-like structures. 1H MAS NMR spectra have shown that this broadening is not due to near-static dipolar interactions. The respective motion is effectively isotropic with correlation time ≈ 1 μs, probably corresponding to Brownian tumbling of the whole globular particles. The thermotropic phase transition, as revealed by NMR, is not discontinuous, but ≈ 6 K broad (302–308 K). Above the LCST transition, the fraction p* of PDEAAm segments in globular-like structures is between ≈ 0.94–1.0, independent of the polymer concentration. At temperatures below the LCST a certain preaggregation of PDEAAm is indicated by smaller line broadening (linewidths ≈ 100 Hz) in the 1H NMR spectra.