We proposed the “birefringent crystal dopant method” in order to compensate the birefringence of polymers. In this method, the birefringence was compensated by homogeneous doping with a needlelike crystal which has the opposite birefringence to that of the polymers. Strontium carbonate (SrCO 3 ) was selected for this purpose because it has a large birefringence and a needlelike shape. SrCO 3 was synthesized, with a length of about 200 nm and a width of about 20 nm, and doped into a poly(MMA/BzMA=78/22(w/w)) film. The film was drawn at 130 °C and 4 mm/min. The orientational birefringence of the drawn polymer film at a wavelength of 633 nm was compensated by doping with 0.3 wt % SrCO 3 without losing its transparency and thermostability. The analysis of infrared dichroism of the films showed that the birefringence of the films was compensated at a low concentration of SrCO 3 because of its orientation-inhibition effect and its high orientation degree as compared with that of the polymer main chains.
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Ohkita et al. (2004) studied this question.