C. Boeffel

An infrared spectroscopic study of photo-induced reorientation in dye containing liquid-crystalline polymers

Abstract Infrared spectroscopy is used to study simultaneously the orientational behaviour of different segments of dye containing liquid-crystalline side group copolymers in sandwich type films of about 2 μm thickness. Under continuous irradiation with polarized light above and below T g of the polymers both azobenzene and phenyl benzoate side groups reorient preferentially normal to the film plane leading to a strongly biaxial orientation distribution. The analysis of the kinetics reveals that the reorientation is essentially a mono-exponential process with an additional faster process only found for the azobenzene dye and assigned to the initial trans to cis isomerization step. Investigation of an isotropic copolymer system containing azobenzene in the side groups shows that an anisotropy can be induced through irradiation with polarized light that is strongly dependent on temperature.

Broadband dielectric spectroscopy on side group liquid crystal polymers

Abstract Dielectric studies on three liquid-crystalline side group polymers were performed in the temperature range from 100 K to 450 K and in the frequency range from 10−1 Hz to 107 Hz. All samples exhibit an α relaxation with a WLF dependence of the activation energy as well as a β relaxation, which are comparable in the glassy state. Their mean relaxation times, their relaxation time distribution, in its asymmetry and half-width, and their activation energies are nearly unaffected by the spacer length, terminal group and/or main chain. These findings are supported by N.M.R. measurements. A third relaxation process occurring close to the clearing point is also observed and assigned to the cooperative rotation of the side group around the main chain.

1H spin diffusion coefficients of highly mobile polymers

Abstract Advanced solid-state n.m.r. techniques using 1H spin diffusion are useful tools for the non-destructive investigation of polymer morphologies and domain sizes. Calibration measurements of the spin diffusion coefficients are reported in this communication, combining 1H spin diffusion with 13C detection and electron microscopic measurements on a diblock copolymer of known morphology. The spin diffusion coefficients thus determined can now be used for quantitative determination of domain sizes in systems containing both rigid and mobile components.

Molecular dynamics at the glass transition: One dimensional and two dimensional nuclear magnetic resonance studies of a glass‐forming discotic liquid crystal

The molecular dynamics of the aromatic core as well as the hydrocarbon side chains have been studied in a new kind of glass‐forming discotic liquid crystal, based on a triphenylene core. It serves as a model compound in which only a limited number of degrees of freedom is activated at the glass transition. Slow motion with non‐Arrhenius temperature dependence (α process) is associated with the axial motion of the discs around the column axis. The geometry of this rotation is found to be neither a simple threefold jump as suggested by the pseudo threefold symmetry of the substituted triphenylene core nor does it correspond to small‐step rotational diffusion, established as a prominent feature of the rotational motions of polymers at the glass transition. Instead ill‐defined angular displacements with pseudo threefold symmetry are detected by two‐dimensional exchange NMR. A simple model combining small‐step diffusion and rotational jumps is presented that quantitatively accounts for the angular displacement...

2H NMR studies of phase behaviour and molecular motions of doped discotic liquid-crystalline systems

Abstract A discotic triphenylene monomer as well as a dimer and a main chain polymer, all substituted with heptyloxy side groups, were doped with an electron acceptor (2,4,7-trinitrofluorenone (TNF)) to give charge transfer complexes. These doped systems were aligned in a magnetic field, thus proving their liquid crystallinity. 2H NMR measurements show that the electron acceptor molecules are incorporated into the columns built of triphenylene cores. In the charge transfer complex with the triphenylene monomer almost all the electron acceptor molecules stack in the columns even close to the clearing temperature T 1, while for the dimer and especially for the polymer a significant fraction of the TNF molecules exhibits isotropic motion, which is attributed to their location in the region between the columns, already way below T 1. This isotropically distributed part increases on approaching T 1. Fast rotation of the discs around their column axes takes place in the monomer and is quenched in the dimer and ...

PHENYLENE MOTION IN POLYCARBONATE - INFLUENCE OF TENSILE-STRESS AND CHEMICAL MODIFICATION

Deuteron NMR was utilized to study phenylene group motions in glassy polycarbonate (PC) as a function of tensile stress and chemical modification. A special stretching device was constructed allowing the application of forces up to 1500 N during the measurement. While cold drawing of PC effects motional restrictions equivalent to a temperature shift of 10 K, a reversible strain near the yield point enhances the ring dynamics slightly. Methyl group substitution at the ortho positions gives rise to even more constraints, shifting the onset of fast ring flips by about 180 K.

Order in thin films of a liquid crystalline polymer

Ordering effects of a side group liquid crystalline polymer PMA64 have been observed by the x‐ray reflectivity technique. Thin films on float glass substrate are investigated as a function of temperature and film thickness. At 100 °C, a phase transition from a smectic‐A to a nematic phase is observed. The layers of the smectic‐A phase are oriented parallel to the surface giving rise to a Bragg peak which is coherently superimposed on Kiessig fringes. The Kiessig fringes are caused by interference of beams reflected at the polymer surface and the polymer–glass interface, while the Bragg peak originates from the electron density modulation perpendicular to the surface. As a function of film thickness, we observe a significant deviation of the integrated intensity of the Bragg peak as compared to the integrated intensity of a Bragg peak due to perfect periodic one‐dimensional density modulations. A model which includes the influence of the interfaces as well as distortions of the first and second kind is use...

Dynamics of flow‐induced alignment of side‐group liquid‐crystalline polymers

The dynamics of oscillatory shear‐induced alignment in side‐group liquid‐crystalline polymers (SGLCPs) are investigated by measuring transmittance and viscoelasticity during and after the alignment process. We compare the dynamic moduli and transmittance of shear‐aligned samples with those of magnetically aligned samples. Relative to an unaligned nematic, magnetic alignment does not appreciably alter the dynamic moduli. This suggests that, at small strains, distortion of the polymer backbone dominates the linear viscoelastic response so that the moduli are insensitive to macroscopic director alignment. With increasing strain, deviations from linear behavior are manifested by an increase in the dynamic modulus. This strain hardening may be due to the tendency of the mesogens to orient perpendicular to the backbone in the present SGLCP, since the mesogens attached to a given strand may be forced to adopt an intermediate orientation between that dictated by the backbone segment and that dictated by the local...

Convective director structures upon continuous rotation of nematic side group polymers

Abstract We present the first study of convective director structures in nematic side group polymers. A thin liquid crystal cell (10–500 μm) was continuously rotated about an axis perpendicular to the field of a 7T NMR magnet. The director behaviour was followed by deuteron NMR as well as by polarization microscopy. While by optical studies the development of periodic director structures can be directly monitored, the analysis of the NMR lineshape gives detailed information about the director distribution in these structures. The development of the structures depends sensitively on the rotation frequency and is discussed in terms of non-linear amplification of long wavelength director fluctuations due to the coupling between director rotation and viscous flow of the nematic.

Dynamics and structure of a flexible columnar liquid crystal based on tetrabenzocyclododecatetraene

Abstract The dynamical and conformational behaviour of a flexible tetrabenzocyclododecatetraene derivative exhibiting a columnar mesophase has been studied by a combination of deuteron solid state NMR spectroscopy and molecular dynamics (MD) simulations. As shown by two-dimensional (2D) exchange NMR, the mesophase is characterized by slow axial reorientations (∼10−3s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D spectra of the core methylene sites are sensitive to the molecular conformation and reorientation mechanism. Motional narrowing of one-dimensional (1D) spectra reveals additional fast librations due to the internal flexibility of the mesogenic moiety. The various reorientation pathways comprising interconversions and pseudo-rotations between different energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is ascribed to a complex axial motion involving ro...