Photo-induced reaction of dye in polymer media—ii. annealed polymer matrices

Abstract

Aberchrome 540 dye is amber in its ring-opened form (A) [(E)-α-(2,5-dimethyl-3-furylethylidene) (isopropylidene) succinic anhydride]. Irradiation with u.v. ( γ > 300 nm) causes the molecules to undergo conrotatory ring-closure and the dye is in its red form (B) [7,7a-dihydro-2,4,7,7,7a-pentamethyl-benzo [b]furan-5,6-dicarboxylic anhydride]. The red dye B may be bleached by white light to revert to A. This paper deals with the kinetics of the photobleaching of B with white light in various annealed polymer matrices viz. polystyrene, poly( p-t.butyl styrene), poly(methyl methacrylate), poly(n.butyl methacrylate) and poly(vinyl acetate). Thin films were prepared from dispersing the dye in polymer solution and solution-cast on quartz plates. The kinetics of the photobleaching of the annealed samples were followed with a spectrophotometer by monitoring the decreasing absorbance of the red dye at γ max of 493 nm with increasing exposure times. The experiments were conducted over a temperature range of 298 to 338 K. The kinetics of photobleaching of dye follow a simple first-order reaction, allowing for extraction of the pseudo first-order rate constants ( k) and the apparent thermal activation energies ( E a). The results are compared with those obtained previously for the same dye reaction but in unannealed polymer media. Discussion is made in terms of polymer free volume and the intermolecular interactions between the dye molecules and the dipolar polymer side-groups. The k values for the dye reaction remain unchanged among the non-dipolar matrices whether the media are annealed or not. In dipolar polymers, however, k and E a values obtained from each annealed polymer sample differ from those derived from the unannealed material.