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RESEARCH PRODUCT

Investigation of the glass transition and melting of polymers by nuclear magnetic resonance

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description

Investigations of the glass transition and melting of partially crystalline polymers by nuclear magnetic resonance provide valuable information on the nature of noncrystalline regions. The broad line NMR signal was separated into a narrow component and a broad component by using a new method which does not imply the ambiguity usually present. From the intensity of the narrow component the mobile fraction was determined. This fraction was extrapolated to “infinite” temperature. By subtracting the mobile fraction from the noncrystalline fraction, the noncrystalline rigid fraction was obtained. Comparison of the measured second moments of the two components of the NMR line with calculated second moments of chains with fixed ends shows the following: The mobile fraction is formed by chains with either one end fixed or both ends fixed in a comparatively small end-to-end distance (for example, loops with adjacent reentry); the noncrystalline rigid fraction is formed by chains whose ends are fixed in a comparatively large or medium end-to-end distances (for example, tie molecules connecting different crystals). More detailed information on the chains forming the mobile fraction as well as on the structure in the melt was obtained by high resolution NMR. It was shown that near range order in the melt, for example, the formation of bundles of parallel chains, causes deviations from the lorentzian shape of the NMR line. During primary crystallization the chains within the bundles of parallel chains crystallize. During secondary crystallization, extremely mobile chains outside these bundles are incorporated into the crystals.