NMR deuteron dilution technique in senticrystallline polymers

Abstract

A NMR method is developed whereby the manner of chain folding in lamellar polymer crystals can be investigated. General expressions are given for the second and fourth moments of the 1H NMR spectrum of cocrystallized mixtures of partially deuterated and fully deuterated polymers including the influence of protons in amorphous regions, residual protons due to incomplete deuteration, and proton deuteron coupling. Models considered are chain folding through random reentry, adjacent reentry, and molecular clustering. Explicit calculations were performed for polyethylene cocrystals. Furthermore, a numerical fitting procedure is given for determining the second moment from the NMR solid echo taking into account the fourth-order “M 4ε correction” for relatively large pulse distances. Experiments in cocrystallized mixtures of half- and fully deuterated polyethylene yield constant second moments for pulse distances between 5 and 10 μsec. The accuracy suffices to differentiate between the different models of chain folding.