1,720,971 research outputs found
Q-sensitive quartz crystal microbalance as a fast probe for viscoelasticity in polymeric film
No full textOptical control of structural morphology in azobenzene containing polymeric liquid crystals.
No full textAzo-containing polymer brushes: photoalignment and application as command surfaces
No full textBrush 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
SNOM Imaging of Photoinduced Microstructures in Azo-Polyacrylates
No full textPolymer liquid crystals with a photosensitive azobenzene side chain (azo-scPLC) are interesting materials because of their potential in optical switching and high density optical data storage applications. By illumination with blue linearly polarized light, the azobenzene chromophores undergo repeated trans-cis-trans isomerization cycles, resulting in reorientation of the side chains perpendicular to the electric field direction.
We investigate the structural effects due to molecular reorientation in azo-scPLC on the scale of 10-100 nm by using a scanning near-field optical microscope (SNOM). We have selected a SNOM contrast mechanism suitable for domain discrimination by modulating the polarization at the input of the SNOM fiber probe and measuring the sample response with a lock-in technique, since the SNOM optical signal is polarization dependent. We image topographic and optical spontaneous structures of a Langmuir-Schaeffer multilayer of azo-scPLC and photoinduced micron-size patterns previously created by far-field laser illumination.
We also present preliminary results in nanowriting, obtained by a pump and probe technique performed through the SNOM probe
Optical bit stability and relaxation processes in a liquid crystal polymer with a photosensitive azo dye molecule as side group
No full text
Comparative Study of Photomechanical Effects in Linear and Star Azo-Polymers
No full textWe investigated the influence of the polymer macromolecular geometry and chain
length in azobenzene containing polymeric liquid crystals (azo-PLC). In particular
we studied the photomechanical effects (photoinduced expansion and shrinking)
on monodispersed azo-PLCs (polydispersity index 1.08–1.17) prepared from
the same monomer and different molecular weights, both in linear and 3-arm star
conformation.
The characterization has been carried out in thin films by high resolution null
ellipsometry for a simultaneous and independent determination of both mechanical
and optical properties (e.g., thickness and refractive index). The measurement
system has been set up in a pump probe configuration for a real time characterization
of the photoinduced dynamics. The effect of coupling introduced by the
polymer star configuration is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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