Compensation for birefringence of oriented polymers by random copolymerization method

Birefringence is caused by orientation of polymer chains during injection molding or extrusion, and is a large obstacle to optical applications. Transparent zero-birefringence copolymers were successfully synthesized by randomly copolymerizing two monomers having positive and negative birefringence, respectively. Since the anisotropic polarization of the negative birefringent monomer units is compensated by the positive birefringent monomer units on the same polymer chain, the birefringence is compensated on the length scale of only several monomer units and becomes zero for any degree of orientation. Methyl methacrylate (MMA) was used as the negative birefringent monomer, and 2,2,2-trifluoroethyl methacrylate (3FMA) as the positive birefringent monomer. Poly(MMA-co-3FMA) copolymer synthesized with a composition of MMA/3FMA=45/55 (wt./wt.) showed no birefringence for any draw ratio when a sample film was uniaxially heat-drawn to 3.5 times its original length. The total scattering loss of the zero-birefringence copolymer was 65.9 dB/km, which is comparable to those of homopolymers.