F. Sahlén

Dielectric relaxation of liquid crystalline side-chain poly(vinyl ether)s

Dielectric permittivity and loss have been measured over the frequency range 10−2 Hz–10 kHz between 100 K and 350 K for samples of two mesomorphic side-chain poly(vinyl ether)s, poly(4-(11-(vinyloxy)undecyloxy)-4-ethoxyphenylbenzoate) (P-EtO) and poly(4-(11-(vinyloxy)undecyloxy)-4-cyanophenylbenzoate) (P-CN) of different degree of mesogen group orientation. X-ray scattering showed that P-EtO was in a semicrystalline state below 345 K, whereas P-CN displayed a titled smectic structure at temperatures lower than 315 K. Four relaxation transitions were found in P-EtO: α, the glass-rubber transition occurring at 290–300 K, and three subglass processes referred to as β, γ and δ. P-CN exhibited only three dielectric processes, α, β and γ. The low temperature process, δ, was absent in P-CN and could be assigned to torsion about the pendent phenyl-carbon-ether-oxygen bond. Subglass processes β and γ exhibited an Arrhenius temperature dependence with activation energies of 60–105 and 32 ± 2 kJ mol−1, respectively. The activation energy of the γ process was insensitive to morphology and it was assigned to local motions in the spacer group. The activation energy of the β process varied considerably among the studied polymers with higher values for the highly ordered P-EtO than for P-CN. It is suggested that the β process leads to reorientation of the carboxylic group in the phenyl benzoate moiety.

A pyroelectric liquid crystal polymer (PLCP) for second-harmonic generation

Abstract In order for a material to show a non-vanishing second order non-linear susceptibility, the material has to have a polar axis. This is hard to achieve in a liquid crystal system because of the strong quadrupolar order along the director. Electrostatic poling of polymers and polymer liquid crystals at high temperature and subsequent quenching to lower temperature only gives a small value of the polar order parameter and, in addition, results in materials which are not in thermodynamic equilibrium and therefore not stable over long times. Starting with a ferroelectric liquid crystal (which has polar order perpendicular to the director) we have succeeded in making a material with true polar order. It is not ferroelectric, although the ferroelectric properties of the starting and intermediate materials are basic for the procedure and for the final product which can be used for frequency doubling, of importance, for instance, in a wide area of optic communication applications.

Structural assessment of liquid-crystalline side-chain poly(vinyl ether)s: dependence on terminal group, orientation and temperature

Abstract Oriented and non-oriented samples of two liquid-crystalline side-chain poly(vinyl ether)s, poly-[11-(vinyloxy)undecyloxy]-4-cyanophenylbenzoate (poly(1)) and poly4-[11-(vinyloxy)undecyloxy]-4-ethoxy-phenylbenzoate (poly(2)), have been studied by small-angle and wide-angle X-ray scattering techniques, polarized light microscopy and differential scanning calorimetry. The terminal group of the mesogenic unit played an important role in the organization of the side chains. With increasing temperature, poly(1) exhibited a smectic C, a transitional ordered smectic, a smectic A and finally an isotropic phase, whereas poly(2) showed a crystal B, a smectic A and an isotropic phase. Both polymers showed a glass transition at 310 K. The global alignment of the mesogenic groups did not affect the internal order of the smectic phases.

Dielectric relaxation of liquid crystalline polymers used in non-linear optical materials

Abstract Dielectric permittivity and loss have been measured over the frequency range 0.1–1000 Hz between 80 K and 300 K for side-chain and network liquid crystalline polyacrylates obtained by photopolymerization of smectic monomers. The polyacrylates were based on mixtures of chiral monofunctional monomers with a lateral nitro group attached to the outer phenyl group of the mesogen and an ester group in the mesogen and nonchiral bifunctional monomers, either with an ester group in the mesogen or with an ester group in the mesogen and a lateral nitro group, creating networks with different crosslink densities. Poled copolymers based on these monomers exhibit second-order nonlinear optical properties. Polymers based only on monomers with the chiral and nitro groups exhibited a pronounced γ process, associated with segmental motions in the methylene spacer groups and only vague signs of the high-temperature subglass process (β) associated with reorientation of the mesogen. The polymers with the bifunctional monomer without the nitro group exhibited clearly both γ and β processes. The dielectric data indicate that the ester groups in the bifunctional units undergo conventional reorientation according to the β mechanism and that the β process is suppressed in the pendant chains with the lateral nitro groups. The β process involves a coordinated torsion about two bonds in the mesogen and the swept-out volume involved in the motion becomes extensive when a lateral nitro group is present.

Smectic A organisation in copolymers of i-butyl vinyl ether and 11-[(4′-cyano-4-biphenyl)oxy]undecanyl vinyl ether as assessed by X-ray scattering

SummaryLiquid crystalline copoly(vinyl ether)s based on i-butyl vinyl ether and 11-[(4′-cyano-4-biphenyl)oxy]undecanyl vinyl ether of various compositions were synthesized. The copolymers exhibited smectic A mesomorphism with a layer thickness strongly increasing with increasing i-butyl vinyl ether content. The intermesogenic distance remained unchanged. A model of the layer structures is given based on structural assessments from small-angle X-ray scattering data.

Influence of molecular tilt angle on the SHG response of pyroelectric liquid crystal polymers

Abstract Second harmonic generation in novel pyroelectric liquid crystal polymers (PLCP) made from a series binary mixtures, was studied using 1100 nm as the fundamental wavelength. The PLCPs were prepared by photo-polymerization of binary mixtures of two monomers which exhibit a smectic C ∗ phase, A2c (4″-( R )-(−)-2-[(10-acrylo-yloxy)decyl]oxy-3-nitrophenyl 4-{4′-[(11-acryloyloxy)-undecyloxy]phenyl}benzoate) and Alb (4″-(( R )-(+)-2-octyloxy)-3″-nitrophenyl 4-(4′-(11-acryloyloxy)undecyloxy)-phenyl)-benzoate). The highest d 16 and d 23 coefficients were found to be in the range 0.65–0.8 pm/V, and differed depending on the detailed preparation of the sample. All cases of polymers formed from the chiral smectic C∗ phase showed an SHG-signal with no external field present, indicating that polar order became fixed. The SHG-signal was found to increase with the tilt angle of the FLC molecules.

Thermal reversibility of ordered, photocrosslinked liquid crystalline poly(vinyl ether)s

Anisotropic networks were prepared by photoinitiated cationic bulk polymerization of aligned liquid crystalline mixtures of mono- and bifunctional vinyl ethers. The thermal reversibility of the macroscopic order of the networks was assessed by infra-red spectroscopy measuring the dichroic ratio of the oriented samples at different temperatures between 20°C and 200°C. The order parameter gradually decreased with increasing temperature and the relative reduction decreased with increasing crosslink density. For systems showing smectic mesophase transitions, the decrease in order parameter was concentrated in the transition temperature ranges. The almost complete recovery of alignment after cooling to 20°C was due to the network character of these polymers. A permanent loss in order parameter of about 15% after the first heating and cooling cycle was recorded. This was attributed to a thermally induced polymerization occurring during the first heating scan.

Preparation and Characterization of Crosslinked Liquid Crystalline Polymer Films as Nonlinear Optical Materials

Liquid crystalline monomers have been synthesized and characterized in purpose to make polymeric second-order nonlinear optical (NLO) materials. Crosslinked polymers have been made to thermally stabilize the orientation created in aligned samples. The orientation has been measured by means of x-ray scattering and infrared dichroism. Oriented, crosslinked poly(vinyl ether)s were made by cationic polymerization whereas crosslinked, nonlinear optical poly(acrylate)s were made by radical polymerization. Polymerization was initiated both thermally and photochemically. The NLO activity of the poly(acrylate)s was measured as the Pockels effect and the second harmonic generation signal, giving values of 0.15 pm/V and 0.40 pm/V, respectively.

Dynamics of liquid crystalline side-chain poly(vinyl ether)s

Abstract Segmental mobility between 100 and 350 K is reported for a series of mesomorphic side-chain poly(vinyl ether)s with 4–11 methylene carbon atoms in the spacer and phenyl benzoate and stilbenyloxy groups used as mesogens. A glass-rubber transition (α) appears in the vicinity of 300 K. Three subglass processes, referred to as β, 7, and δ obey the Arrhenius and Cole-Cole equations. The δ-process is present only in polymers with flexible and dielectrically active tail groups, e.g., in polymers with an ethoxy group, in which case the process can be assigned to torsion about the pendant phenyl-carbon-ether-oxygen bond. The activation energy (35 ± 5 kJ ·−1) of the β-process is insensitive to the morphology and is assigned to local motions within the spacer. The activation energy of the β-process varied considerably among the studied polymers (60 to 126 kJ·mol−1). The absence of the β-process in the stilbenyloxy polymer suggests that this process is due to reorientation of the carboxylic group in the phen...

The influence of chiral strength on the spontaneous polarization and the second-order nonlinear optical susceptibility in ferroelectric liquid crystals

Abstract The influence on the spontaneous polarization and the second-order nonlinear optical susceptibility of the mole fraction of the chiral component in binary mixtures of ferroelectric liquid crystals was studied theoretically. The model is based on the different molecular sizes which the molecules occupy in the mixture and on a model of the rotational distribution of the lateral dipole moment and the hyperpolarizability. The model is found to give a nonlinear relationship between the spontaneous polarization or the NLO-susceptibility and the mole fraction. A comparison is made with spontaneous polarization measurements performed on a series of systematically synthesized chiral and non-chiral liquid crystal structures, which where mixed in different proportions and combinations. The agreement with experiment is very good, and some model parameters are evaluated. The usefulness of the model is discussed.