Synthesis of a Zero-Birefringence Optical Polymer by the Birefringent Crystal Dopant Method

Birefringence induced by the orientation of polymer main chains during an injection-molding or extrusion processing restricts the application of optical polymers to optical devices that require maintaining the polarization state of incident light, We propose the "birefringent crystal dopant method" to compensate the birefringence of polymers by homogeneous doping with an opposite birefringent needle-like crystal. Strontium carbonate (SrCO ₃) was selected for this method and synthesized, with particle lengths of 50-200nm and aspect ratios of 2-5. SrCO₃ was doped into poly(MMA/BzMA= 78/22(wt./wt.)) film. The film was uniaxially drawn at 130°C and 4mm/min. For the first time, the positive birefringence of the drawn copolymer film at a wavelength of 633nm was compensated by doping with 0.3wt.% of SrCO₃ without losing transparency and thermostability.