A new apparatus for measuring the birefringence of a polymer solution under a continuous a.c. electric field has been developed. The birefringence signal is modulated by sinusoidal amplitude modulation (AM) and frequency modulation (FM) of the applied electric field in the frequency range from 0.1 Hz to 100 kHz. The birefringence response is detected with a digital lock-in system with a sinusoidal reference wave. Digital data processing ensures a high accuracy even at ultra-low frequencies. The data for nitrobenzene indicate that a birefringence as low as 10 -13 can be accurately measured for an optical path of 1 cm. Illustrative results for poly(γ-benzyl-L-glutamate) in m-cresol and DNA in aqueous NaCl are presented. The FM technique yields the frequency derivative of the AM curve and gives the relaxational frequency of these solutions clearly.
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Ookubo et al. (1980) studied this question.