Photochemical grafting of acrylated azo dyes onto polymeric surfaces. VII. Kinetics and mechanism of dye-sensitized photoinitiation of some diacrylates

Abstract

Photoinitiated grafting and graft polymerization of liquid mixtures, adsorbed onto polypropylene, polycaprolactam, and poly(ethylene terephthalate) films, containing one of four acryloxy-substituted aromatic diazenes and one of four diacrylate comonomers, in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as photoinitiator, were investigated kinetically at 30 ± 2°C. Irradiation was carried out polychromatically, with impinging photoenergy from 2.1 × 10−8 to 20.5 × 10−8 einstein s−1 cm−2; in some of the runs the ultraviolet radiation was filtered. The ratio R between the molar concentration of photoinitiator and the sum of concentrations of dye and diacrylate varied between 0.005 and 0.095; the ratio M between the molar concentration of dye and diacrylate varied between 0.005 and 0.046. The moles of dye and diacrylate n initially deposited per unit apparent polymeric surface S varied between 2 and 74 μmol cm−2. No selective effect was shown by the presence of dyes in the comonomer mixture. The surface density of grafted molecules at the end of the grafting process was not affected by the photoinitiator concentration (for 0.030 < R < 0.095), by diacrylate or dye concentrations, or by the kind of polymer substrate. This parameter, on the contrary, clearly depended on n/S, and linearly up to n/S ≅ 30 μmol cm−2. Quantum efficiencies for the two consecutive kinetic processes of grafting and graft polymerization (Φ1 and Φ2, respectively) were evaluated. The dependency of Φ1 on R, as well as of Φ2 on n/S, are critically discussed on the basis of the proposed mechanism involving grafting of an oligomeric chain (Φ1 values up to the order of 102) followed by a step-by-step graft polymerization (Φ2 limiting values of the order of unity). The sensitizing effect of acryloxy-substituted aromatic diazenes on both Φ1 and Φ2 is evidenced.