Kerr effect of molecular liquids

The reorientational motions of molecules in the supercooled liquid state and in liquid-crystal-forming materials just above the clearing point may occur on a time-scale which is far longer than that encountered for normal low viscosity liquids. Such slow motions are conveniently studied using dielectric and dynamic Kerr-effect techniques in the ranges 10 to 10 Hz and 10 to lO s respectively. The present account describes the behaviour of representative systems, indicates the molecular factors which are involved and, where possible, gives a discussion of mechanisms for reorientation in specific liquid systems. 

We note that measurements in the range 10 to 10 Hz may be made quickly and with high precision using either a General Radio 1620-A or 1621 capacitance measuring assembly or a Hewlett Packard 4274-.A-multifrequency LCR meter. 

3 process. (i) carries with it only a small part of Ae 

The frequency-temperature locations for a, 3 and (a3) processes in all amorphous solid polymers are shown schematically in figure 1. For T Tg only the 3 process is observed. For 

A simple phenomenological theory has been given (4-7) which appears to rationalize all the experimental observations for the a, B and (aB) dielectric relaxations in amorphous solid polymers. It is assumed that a reference dipolar group in the polymer system may partially reorientate via motions in a temporary local environment (B process) but at sufficiently long times may completely reorientate by cooperative rearrangements of that local environment (a process). When the time scales of the two processes are sufficiently different, i.e. for T Tg or for T just above Tg, the dipole moment vector correlation function may be written as (4-6) 

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LOW FREQUENCY DIELECTRIC SPECTROSCOPY AND DYNAMIC KERR-EFFECT OF MOLECULAR LIQUIDS. 

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