STUDIES ON UV AND THERMALLY RADICAL-INDUCED CROSS-LINKED POLYMER NETWORKS AS CHARGE-TRANSPORT LAYERS IN ELECTROPHOTOGRAPHIC COATING APPLICATIONS

A number of both UV and thermally radically induced cross-linked epoxy acrylates and vinyl polymers/copolymers (cross-linked with divinyl be nzene) have been prepared and predoped with different charge-transport media (CTM) and examined as alternative potential durable wear resist ant charge-transport layers (CTL) for electrophotographic applications . The CTMs studied were tri-p-tolylamine (TpTA), 2,5-bis(4-diethylamin ophenyl-1,3,4-oxadiazole) (OXD), p-diethylaminobenzaldehyde-diphenylhy drazone (DEH) and p-diethylaminophenyl benzaldehyde methylphenyl hydra zone (T191). The polymer binders were based on epoxy acrylates, epoxy resins (amine cured) and divinyl benzene cross-linked styrene/styrene- co-methyl methacrylate, styrene-allyl methacrylate and poly(vinyl cinn amate) and compared with the properties of a conventional polycarbonat e CTL. The structure of the cross-linked polymer, nature of the CTM an d its concentration controlled the extent of cross-linking and mobilit y of charge within the coating. UV cured systems were found to fail as CTLs due to the presence of high levels of hydroxy groups on the poly mer backbone, residual initiators and photosensitivity and photolysis products of the CTM's trapping and inhibiting charge migration. Polyme rs containing styrene units, on the other hand, display good charge tr ansport properties compared with polycarbonate due to strong interacti ons of the pendant phenyl groups stabilising the cation radicals produ ced on illumination. However, these coatings, although hard were found to be brittle in nature. Copolymerisation with methyl or allyl methac rylate reduced the brittleness but also the mobility of charge due to spacing of the phenyl rings along the polymer network. Thermal cross-l inking of a polymer that undergoes a 2 + 2 cycloaddition, poly(allylci nnamate), gave a hard tough coating that exhibited promise as an effec tive CTL when compared to that of polycarbonate. (C) 1998 Elsevier Sci ence S.A. All rights reserved.