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

Spread monolayers of a photochromic trifluoromethoxy substituted metacrylate copolymer were characterised by π/A isotherms, Brewster angle microscopy (BAM) and surface dilatational rheological parameters for light induced trans–cis isomerisation at the air/water interface. Using appropriate irradiation Langmuir films of the two isomers can be characterised almost separately. The BAM images confirm a homogeneous structure for the two isomers, differing only in reflectivity. The surface dilatational behaviour and the π/A isotherms of the films of maximum trans and cis conformation differ strongly. The monolayer of maximum trans conformation exhibit increasing surface dilatational elasticity and viscosity when the monolayer is compressed. The resulting surface rheological behaviour is viscoelastic with a high dilatational 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 cis content starts to increase at lower surface coverage and exceeds the values of the trans conformer film over a wide range of surface coverage, but it levels off at 35 mN m−1. Starting from 0.45 nm2 per repeated unit the monolayer becomes highly compressible. The results are explained by an increase of the minimum average surface demand of the copolymer azobenzene side-groups for the cis isomer and a folding of the monolayer with increasing surface pressure.