Effect of dopant structure on refractive index and glass transition temperature of polymeric fiber-optic materials

Graded-index plastic optical fibers, composed of doped polymers, have advantages over conventional glass optical fibers, but need to be developed further for practical application. Here, a variety of aromatic sulfide dopants were synthesized, and their effects on the refractive indexes and glass transition temperatures (T_g) of poly(methyl methacrylate) and methyl 2-chloroacrylate/2,2,2-trichloroethyl methacrylate copolymers were studied. While polymers containing large dopants exhibited relatively high refractive indices, their T_g values were low, making these materials unsuitable for graded-index plastic optical fiber applications. Six dopants yielded polymers that exhibited higher T_g values than the conventionally used (diphenyl sulfide)-doped polymer. The dopant dibenzothiophene, in particular, yielded polymers with the highest refractive indexes and T_g values, and polymers containing (phenylthio)benzene dopants also performed well.