Abstract

We have undertaken a systematic exploration of proton dispersion profiles of several families of synthetic polymers. The NMRD profiles of solid samples had been measured at 25 oC over relaxation fields ranging from 5 kHz to 20 MHz (measured by 1H Larmor frequency). So far, we have explored several classes of elastomers, styrenes, nylons and a polycarbonate.

The survey, though still in a preliminary and rather phenomenological phase, allows us already to draw several experimental conclusions of considerable interest in view of the expected rapid growth of NMRD studies of molecular dynamics of bulk polymers.

In many individual profiles, the R1 values range over more than three orders of magnitude. In virtually all cases, the variation continues down to the lowest measured frequencies, following curves which deviate sharply from the simple BPP model and which rarely exhibit any plateau, be it at high or at low frequencies (only in a few cases there seems to be a plateau between 5 and 10 kHz). This implies a wide spectrum of correlation times and dynamic models much more complex than what might appear from relaxation studies carried out at high (and fixed) frequencies. Though there are marked differences between individual profiles, the review as a whole exhibits a surprising internal coherence, making it possible to group the polymers into families according to the shapes of the profiles (it is comforting that such classification correlates well with the known chemical composition of the samples).

It is beyond any doubt, however, that the internal coherence of the review would be completely lost if it were not possible to measure reliable R1 values in the range from 100 to almost 10000 1/s. Indeed, while most of the studied polymers exhibit quite low R1 values at 20 MHz (in some cases close to 1, often around 10 and always below 100), the situation changes dramatically at fields around 1MHz and lower. Without the R1 values lying in the two decades above 100 1/s, FFC relaxometry of bulk polymers would be seriously handicapped.