Selective Doping of Photochromic Dye into Nanostructures of Diblock Copolymer Films by Vaporization in a Vacuum

Poly(phenyl methacrylate) (PPhMA), poly(methyl methacrylate) (PMMA), poly(tert-butyl methacrylate), polystyrene (PS), poly(4-methylstyrene), poly(p-tert-butylstyrene), and polycarbonate (PC) were doped with the photochromic dye cis-1,2dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) using a simple vacuum process termed the "vapor transportation method." The CMTE-doped polymer molds and powder were examined by optical microscopy, SEM/energy-dispersive Xray (EDX) elemental analysis, Fourier transform infrared absorption (FT-IR), and transmission electron microscopy (TEM). CMTE-doped PS, PC, and PMMA molds showed photochromism. Optical microscopy and sulfur (S) elemental analysis confirmed formation of CMTE-doped regions, with rapid CMTE concentration decreases at the CMTE-doped region/polymer substrate interface. CMTE doping rates into PS, PC, and PMMA were estimated by measuring the depths of the doped regions. CMTE showed more efficient penetration into PS than into PC or PMMA. FT-IR measurements showed saturated concentrations of CMTE in the CMTE-doped polymer powder. PS and PPhMA, with phenyl groups in their side chains, showed the highest concentrations of CMTE among PS derivatives and PMMA derivatives, respectively. Our results indicated that the affinity between CMTE and aromatic groups in these polymers, based on "(pi)-(pi) interactions," enhances CMTE doping. Ultrathin sectioned TEM images of CMTE-doped diblock copolymers, "PS-rich PS-block-PMMA (PS-bPMMA)," and CMTE-doped "PMMA-rich PS-b-PMMA" films confirmed that CMTE was dispersed selectively into PS nanodomains in "PMMA-rich PS-b-PMMA" and the PS matrix in "PS-rich PS-b-PMMA." We demonstrated selective doping of CMTE into the PS regions of PS-b-PMMA.