The effect of trans-stilbene unit in the compensation of birefringence of poly(methyl methacrylate) in the random copolymerization method and anisotropic molecule dopant method

We investigated the effects of trans-stilbene unit in compensating birefringence in the random copolymerization method and the anisotropic molecule dopant method. In the random copolymerization method, trans-stilbene methacrylate (TSMA) containing the trans-stilbene unit in the side chain was polymerized with methyl methacrylate (MMA) in solution polymerization to compensate the photoelastic birefringence and the orientational birefringence of poly(methyl methacrylate) (PMMA). In the anisotropic molecule dopant method, trans-stilbene was added to PMMA. In the both methods, the photoelastic birefringence and the orientational birefringence shifted from the negative side to the positive side with an increase in the concentration of trans-stilbene unit. 0.8 mol% of TSMA almost eliminated the photoelastic birefringence. Also, we demonstrated that poly(MMA/TSMA) exhibited no orientational birefringence with 1.9 mol% of TSMA. 2.0 mol% of trans-stilbene almost eliminated the photoelastic birefringence of PMMA. Similarly, we demonstrated compensating orientational birefringence with 2.0 mol% of trans-stilbene. Based on the results, the effects of trans-stilbene unit in compensation of orientational birefringence are almost the same in the two methods. However, in compensation of photoelastic birefringence, the trans-stilbene unit had 2.5 times higher effect in the random copolymerization method than that in the anisotropic molecule dopant method. Photoelastic birefringence is caused in elastic deformation below T_g, in which the side chains are mainly orientated while the polymer main chains are scarcely orientated. Therefore, we concluded that addition of trans-stilbene unit to the side chain enhanced the effect for compensating photoelastic birefringence.