DEUTERIUM-NMR STUDY OF MICROSCOPIC EFFECTS IN COLLAPSING POLYACRYLAMIDE GELS

Deuterium-NMR spectra and relaxation measurements for selectively deuterated acrylamide units in polyacrylamide (PA) gels exhibiting collapse transitions are presented. The main aim of the present study is to investigate the microscopic dynamics of the segmental units of the polymer network while the entire gel is approaching the collapse transition accompanied by the critical long-wavelength diffusivelike excitations. The D nucleus, with the electric quadrupole interaction, is directly sensitive to the chain dynamics because of the random modulations of the electric-field-gradient components in the frame of reference of the external field, induced by the motion of the monomer axes. From the analysis of the spectra and of the relaxation rates, it is shown how, while the macroscopic motions exhibit critical slowing down, the dynamics of the segmental units is described by practically isotropic local modes, weakly coupled to the density fluctuations of the PA network. Quantitative estimates of the effective correlation times are given.