1H NMR study of temperature collapse of linear and crosslinked poly(N,N-diethylacrylamide) in D2O

Changes in the dynamic structure of poly(N,N-diethylacrylamide)(PDEAAm) gels in D2O during a temperature-induced phase transition (collapse), were investigated by a combination of high-resolution, broad-line and multiple-pulse MW4 1H NMR methods. Both physical gels and solution of linear PDEAAm, and gels of chemically crosslinked PDEAAm were studied. Very similar behavior was found in both cases, indicating the formation of compact globular structures in the collapsed state. However, a lower mobility of chemically crosslinked PDEAAm results in up to 3–4°C shift of the transition region towards lower temperatures, in comparison with the linear polymer. In all cases, the transition as observed by NMR appeared as continuous, even for PDEAAm containing 1–6 mol% of sodium methacrylate units; this is probably due to the fact that high-resolution NMR spectra are sensitive only to the fast first stage of the deswelling process. The results of MW4 multiple-pulse experiments (T2eff relaxation measurements) show that the correlation time of the motion of collapsed segments is 2 μs.