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 stud ied with the aid of the dipolar correlation effect (DCE) and field-cyc ling NMR relaxometry (FCR). The typical lime scales of the two techniq ues are t>10(-4) s and t<10(-3) s, respectively, and therefore complem entary. The crosslink density of he polymer networks was varied in a w ide range. In order to prevent sinusoidal undulations of the stimulate d-echo attenuation curves due to spin exchange between groups with dif ferent chemical-shift offsets, the DCE of the samples was examined usi ng 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 interac tions 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 dyna mics (probed by DCE) can be approached by power-law dipolar correlatio n 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 DC E data strongly depend on the crosslink density but not on the tempera ture 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 usua l mechanical torsion modulus measurements. (C) 1998 American Institute of Physics.