Characterization of polymer networks using the dipolar correlation effect on the stimulated echo and field-cycling nuclear-magnetic resonance relaxometry

Chain dynamics In a series of styrene-butadiene rubbers (SBR) was studied with the aid of the dipolar correlation effect (DCE) and field-cycling NMR relaxometry (FCR). The typical lime scales of the two techniques are t>10(-4) s and t<10(-3) s, respectively, and therefore complementary. The crosslink density of he polymer networks was varied in a wide range. In order to prevent sinusoidal undulations of the stimulated-echo attenuation curves due to spin exchange between groups with different chemical-shift offsets, the DCE of the samples was examined using a modified radio frequency pulse sequence with additional pi pulses inserted in the free-evolution intervals. Residual dipolar couplings can thus be probed in samples where chemical-shift and dipolar interactions are of the same order. The dipolar correlations probed with the DCE in SBR networks turned out to exist on a time scale exceeding 300 ms. The short-time fluctuations (probed by FCR) and the long-time dynamics (probed by DCE) can be approached by power-law dipolar correlation functions with exponents -0.78+/-0.02 and - 1.5+/-0.1, respectively. he crossover time is in the order of 1 ms. In contrast to FCR, the DCE data strongly depend on the crosslink density but not on the temperature in a range from 30 to 80 degrees C. On this basis determinations of the crosslink density may be possible as an alternative to the usual mechanical torsion modulus measurements. (C) 1998 American Institute of Physics.