Surface dilatational rheological study of spread polymethacrylate monolayers modified by E/Z photoisomerisation

Photoinduced changes of the surface behavior of spread monolayers of three 4′ substituted phenyl-azo-phenoxy-alkyl-methacrylate polymers were characterized by using π/A isotherms, surface dilatational elasticities, and viscosities. The behavior of two homopolymers is compared, which differ by variation of the p-substitution from p OCH3 (polymer II) to OCF3 (polymer I) and a copolymer with OCF3 and 50% of the hydroxyethylmethacrylate (HEMA) moiety (polymer III). Based on the photokinetics in solution and spin-coated films the properties of the monolayers in two steady states are characterized, which differ in the content of the Z isomeric form. It is much higher upon ultra violet (UV) irradiation at 360 nm as in the case of the steady state established by exposure at 450 nm. On the molecular level the thermal Z/E isomerisation of the polymer I is significantly slower compared with that of the polymer II. In contrast to the copolymer III UV irradiation causes brittle monolayers of polymers I and II, which are not able to absorb higher surface pressure than about 10 mN m−1. The surface dilatational rheological behavior of the polymers was compared only for E isomer dominated monolayers, as the Z rich layers of polymer I and II could not be deformed without mechanical destruction. In contrast polymer III could bear deformation because of the monolayer softening effect of the HEMA group. The surface rheological behavior of all polymers is visco-elastic, with the order of the values of both, dilatational elasticities and viscosities, polymer II>I>III, and whereas polymer III could be compressed to higher surface coverage.