Azo-containing polymer brushes: photoalignment and application as command surfaces

Brush polymer surfaces, with respect to traditional films, are resistant to solvents and to high temperatures due to the covalent bonds between chain and substrate, which stabi!izes the film morphology [ The photo response of films of azopolymers has extensively been characterised [ either as cast films [ or Langmuir-Blodgett films [ However the peculiar characteristics of polymeric brushes, with the possibility of a fine control on the struct-ure, make them very attractive for application as command surfaces. A brush polymer containing photosensitive mesogenic side chains has been directly synthesised on glass and on silicon substrates by a grafting method involving polymerisation of the methacrylic azobenzenic monomer (4-methacryloyloxy- hexyloxy-4’-ethoxyazobenzene) under Atomic Transfer Radical Polymerisation (ATRP) conditions. Under these conditions the polymeric brush grows linearly with the reaction time; the polymerisation therefore could be considered as living, and these molecular brushes could be emp!oyed for making new biock copolymeric brushes. Different chain lengths have been obtained by fine tuning of the time of polymerisation. Brush growth was monitored by a combination of UV/vis absorbance and by nuil ellipsometry (see figure 2); the resulting structure was characterised by Atomic Force Microscopy, optical dichroism and confocai depolarised micro-Rarnan spectroscopy to measure the degree of molecular order. Both photomechanical effects (photoinduced expansion and shrinking) and photoaligment in the brush have been investigated as a function of temperature, illuminatìon and brush length. The resuits have been compared with those obtained by a parallei study carried out on spin—coated films of polymers composed by the same monomer and with high molecular weight. We then employed such brushes as photo controllable command surfaces in twistedlplanar switchabie iiquid crystal cells. An exampie of such celi is shown in figure 3; the celi was fihled with cyanobiphenyi 5CB, the command surface was provided by a polymeric brush (length i2nm) grown on glass while the reference surface was provided by rubbed polyimmide. The celi was prepared as twisted by macroscopic polarised illumination, while selected spots were reverted to pl Figure 3 Polarising opticai microscopy images ofLC celi (1000x800 im) These cells have been characterised by depolarised confocal micro Raman spectroscopy. The order parameter at the command surface has been measured as a function irradiation and brush length. The results are compared with those obtained on similar cells [ References [ T. Kowaiewski, R.D. McCuilough, and K. Matyjaszewski, Eur. Phys. i E 10, 5—16 (2003) [ A. Natansohn and P. Rochon, Chem. Rev. 102, 4139-4176 (2002).. [ P. Camorani, M.P. Fontana Phys. Rev. E 73, 011703 (2006) [ L. Cristofolini, S. Ari, M. P. Fontana, Phys. Rev. Lelters 85 4912 (2000). [ P.Camorani, M.P. Fontana, Mol CrystLiq Cryst, 465, 143 (2007)