A New Type of Disorder Affecting the Dielectric Properties of Long-Chain Esters

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Abstract
The dielectric properties of a series of long-chain esters have been investigated in the solid state. Like other long-chain molecules containing 20 or more carbon atoms, most of the ester molecules showed evidence of hindered molecular rotation about their long axes in the crystal lattice. It was also found that higher dielectric constants resulted when the samples were rapidly frozen than when they were allowed to solidify gradually, the effect being more pronounced as the chain length was increased. In order to explain this behavior, it has been assumed that the esters may crystallize in an ordered lattice in which the dipoles are in a plane, or in a longitudinally disordered lattice in which some of the chains are reversed. In the latter case, the dipoles may or may not form two planes depending upon the position of the polar group in the chain. It has also been assumed that the degree of longitudinal disorder increases with increase in freezing rate. Once frozen in, however, it remains fixed until the sample is remelted. The potential barrier hindering rotation has been assumed to be much higher in the ordered state than in the disordered state because separation of the dipoles should lower steric and dipole interaction in the latter. Since the rotational transition temperature is proportional to a potential energy difference V0 related to the barrier against rotation, the transition temperature should increase with increase in longitudinal order. In a completely ordered ester or a nearly symmetrical disordered ester, the barrier may be sufficiently high to raise the transition point to a temperature near or above that of fusion. The experiments agree with these assumptions.

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