Surface dilatational rheological study of the trans-cis isomerisation of copolymers with trifluoromethoxy substituted metacrylate monolayers

Spread monolayers of a photochromic trifluoromethoxy substituted metacrylat e copolymer were characterised by pi/A isotherms, Brewster angle microscopy (BAM) and surface dilatational rheological parameters for light induced tr ans-cis isomerisation at the air/water interface. Using appropriate irradia tion Langmuir films of the two isomers can be characterised almost separate ly. The BAM images confirm a homogeneous structure for the two isomers, dif fering only in reflectivity. The surface dilatational behaviour and the pi/ A isotherms of the films of maximum trans and cis conformation differ stron gly. The monolayer of maximum trans conformation exhibit increasing surface dilatational elasticity and viscosity when the monolayer is compressed. Th e resulting surface rheological behaviour is viscoelastic with a high dilat ational viscosity. The surface pressure increases up to 50 mN m(-1) before it starts to level off. The surface pressure of a monolayer with maximum ci s content starts to increase at lower surface coverage and exceeds the valu es of the trans conformer film over a wide range of surface coverage, but i t levels off at 35 mN m(-1). Starting from 0.45 nm(2) per repeated unit the monolayer becomes highly compressible. The results are explained by an inc rease of the minimum average surface demand of the copolymer azobenzene sid e-groups for the cis isomer and a folding of the monolayer with increasing surface pressure. (C) 2000 Elsevier Science B.V. All rights reserved.