POISSON RATIOS AND UPPER-BOUNDS OF INTRINSIC BIREFRINGENCE FROM BRILLOUIN-SCATTERING OF ORIENTED POLYMERS

To overcome the difficulties in measuring high-frequency shear constan ts of polymeric materials by ultrasonic or Brillouin scattering techni ques, we extrapolate results from oriented materials to zero birefring ence. Shear constants c44 in the high-frequency limit (GHz) are determ ined for polyethylene, poly(ethylene terephthalate), poly(vinyl chlori de), poly (methyl methacrylate), and polycarbonate using Brillouin sca ttering. Accurate values of Poisson's ratio are derived. The extrapola tion to full orientation using an amended Moseley relation gives upper bounds for the ''intrinsic birefringence.'' Changes in the character of the orientation process are easily detected by monitoring the mode numbers, which are defined by analogy to Poisson's ratio. Extrapolatio n of these ratios to their upper bound 0.5 gives an independent check of the maximum intrinsic birefringence. (C) 1993 John Wiley & Sons, In c.