Liquid Crystalline Side Group Research Articles

Liquid crystalline side-group block copolymers with triblock structure: Investigations on the influence of the block arrangement on the morphology and the LC-phase behavior

Liquid crystalline triblock copolymers with LC inner block and amorphous outer blocks have been synthesized by living anionic polymerization and investigated using DSC, TEM, and small-angle x-ray diffraction. All samples of poly[styrene-block-2-(3-cholesteryloxycarbonyloxy) ethyl methacrylate-block-styrene] (PS-b-PChEMA-b-PS) show liquid crystalline behavior and phase separation between the blocks. Compared to triblock copolymers with PS inner block (PChEMA-b-PS-b-PChEMA) and diblock copolymers (PS-b-PChEMA) the LC block copolymers with PS outer blocks have the same properties. The LC behavior and the morphology do not depend on the block arrangement ; they are only influenced by the volume fractions of the blocks. Those samples in which the liquid crystalline subphase is not continuous (spheres) only a nematic phase was found, whereas in all samples with a continuous liquid crystalline subphase, the smectic A phase of the homopolymer was observed.

Phase behaviour of copolymers containing cholesterol and 4-methoxybiphenyl as mesogenic groups

Abstract Liquid crystalline side group copolymers containing cholesterol and methoxybiphenyl were synthesized and investigated by size exclusion chromatography, 1H NMR, polarization microscopy, X-ray diffraction, differential scanning calorimetry and additionally by measuring the birefringence and the optical reflection of the cholesteric phase. This cholesteric phase could be obtained by copolymerization of two monomers whose homopolymers exhibit only smectic phases. The cholesteric phase exists over a broad range of copolymer composition. For the cholesterol containing homopolymer an SA2 bilayer phase was observed, whereas the methoxybiphenyl homopolymer shows a highly ordered E phase below an SA1 monolayer phase. The layer periods of the SA phases of the copolymers depend on the composition. By measuring the birefringence the order parameter in the cholesteric phase could be determined.

Characterization of the motion of spin probes and spin labels in amorphous polymers with two-dimensional field-step ELDOR

The motion of nitroxide spin probes and spin labels in amorphous polymers is studied below the glass transition temperature with a two-dimensional pulsed electron double-resonance experiment. Polystyrene and a liquid crystalline side group polymer are studied using both spin probes and spin labels covalently bound to specific sites along the polymer chain. Two methyl acrylic polymers differing only in their side group structure and polyvinylacetate are compared and large differences in the molecular dynamics deduced from both the nuclear and the electron spin relaxation rates are observed as the glass transition is approached. The results demonstrate the complexity of small amplitude motion in simple polymers below the glass transition temperature and show that it is very sensitive to the packing in the polymer. © 1996 John Wiley & Sons, Inc.

Thermal and ferroelectric properties of two series of coloured chiral liquid crystalline side group copolymers

AbstractTwo series of liquid crystalline side group copolymethacrylates were synthesized and their thermal and ferroelectric properties determined. All copolymers contain the same azobenzene mesogen spaced from the backbone by a heptamethylene sequence but different chiral co‐compounds were used. The samples show either a monolayer or a bilayer smectic C* phase but only four polymers with a low content of the azo‐dye could be switched ferroelectrically. The formation of the smectic C* phase of some samples was found to be hindered. In order to study this phenomenon as a function of the molecular weight one copolymer was fractionated.

Ferroelectric liquid crystalline side group polymers derived from mesogenic vinyl ether monomers

Abstract Two classes of ferroelectric polymers, polysiloxanes and polyvinyl ethers, have been synthesized, starting from the same mesogenic vinyl ether as precursor. The polysiloxane materials obtained by polymer analogous reaction of vinyl ethers with H-siloxanes exhibit larger ranges of ferroelectric S*C phases than the corresponding polyvinyl ethers. The effect of changing the nature of the link of the terminal chiral end group on the phase behaviour and ferroelectric properties of the new side chain polymers is reported.

Liquid Crystalline Side Group Block Copolymers with n-Butyl Methacrylate as an Amorphous A-Block: Synthesis and Characterization

A second system of liquid crystalline (LC) side group block copolymers with n-butyl methacrylate as an amorphous A-block has been synthesized and characterized. The system poly(n-butyl methacrylate)/poly[2-((((3-cholesteryl)oxy)carbonyl)oxy)ethyl methacrylate] (PBMA/PChEMA) displays a similar phase diagram and macroscopic and microscopic interaction between the morphological structure and the LC behavior of LC side group block copolymers as the system polystyrene (PS)/PChEMA. A formation of a disordered (nematic) phase is also seen in those samples where the LC subphase is not continuous (spheres), whereas in all samples in which there is a continuous LC subphase, the smectic A phase of the homopolymer is formed. For volume fractions of PBMA (Φ PBMA ) 0.6-0.7, a cylindrical structure of the LC subphase was expected. The formation of the smectic A phase in the subphase like that displayed in the homopolymer PChEMA seems to be more favorable than the formation of an equilibrium morphology. The orientation of the LC subphase with respect to the orientation of the morphological elements is dependent on the morphological structure.

Synthesis and Poling of A Crosslinked Second Order Nonlinear Optical Polymer with Liquid Crystalline Side Groups

Synthesis and Poling of A Crosslinked Second Order Nonlinear Optical Polymer with Liquid Crystalline Side Groups

Main Chain Order and Dynamics in a Liquid Crystalline Side-Group Polymer

Abstract The orientational distribution function for the C-CD3 bond on the perdeuterated polymethacrylate chain of a liquid-crystalline side group polymer was determined in the frozen SA phase by solid-state 2H NMR. The results are compared with the ones of two similar systems recently reported. The slow chain dynamics above the glass transition was studied via 2D 2H NMR, which indicates a diffusive motion constrained by the partial orientational order of the mesophase.

Effect of Mesophase Order and Molecular Weight on the Dynamics of Nematic and Smectic Side-Group Liquid-Crystalline Polymers

The dynamic moduli of side-group liquid-crystalline polymers (SG-LCPs) having methacry- late backbone, hexamethylene spacer, and phenyl benzoate mesogens are characterized as a function of molecular weight in the isotropic, nematic, and smectic phases. Molar mass was varied from 3 x lo6 to 3 x lo6 g/mol, corresponding to the range where the onset of entanglement is observed in the isotropic phase (approximately l-lOMe,,so). Nematic order produces a profound change in the dynamics of the entangled SG-LPCs relative to the isotropic phase; however, this effect is absent in the unentangled SG-LCPs. Oscillatory shear with large amplitude (yo z 40%) is effective in inducing macroscopic alignment in the nematic phase for all the SG-LCPs studied. Smectic order increases the elastic character of the fluid, but its incremental effect in a system that is entangled is relatively small. Large-amplitude shearing can be used to alter the microstructure in the smectic liquid: shearing the smectic phase produces a decrease in modulus, whereas shearing in the nematic phase followed by cooling into the smectic phase produces an increase in modulus.

Dielectric spectroscopy of an aligned liquid crystalline side chain polymer using high pressures

Abstract The complex permittivity of a liquid crystalline side group polymer has been measured at pressure from ambient up to 300 MPa. The samples were initially not aligned, then planar and finally homeotropically aligned by the field of a 7.2 Tesla magnet. Temperatures ranged up to 130°C, and measurement frequencies from 5 Hz to 5 MHz. The equilibrium permittivity es has been interpreted in terms of an effective dipole moment μ. The frequency of maximum loss has been analysed in terms of enthalpy of activation and volume of activation for α and β relaxation processes.

Redox active LC ionomers: LC ionomers as materials for compatible blends with amorphous ionomers

AbstractRedoxactive liquid crystalline (LC) side group polymers were prepared by copolymerization of liquid crystalline acrylates and ferrocene‐containing acrylates. The corresponding liquid crystalline ionomers were synthesized by a reversible redox reaction. The incorporation of tetra‐alkylated ferrocene derivatives leads to LC polymers with reduced redox potential. These polymers can be oxidized under mild conditions to the corresponding ionomers, which are much more stable in solution than LC ionomers containing monoalkylated ferrocenium ions. Dynamic mechanical measurements and small angle X‐ray scattering (SAXS) perfomed on LC ionomers prove the existence of ionic superstructures (clusters) which are typical for ionomers and a gelation of the LC ionomers.In biphasic polymer blends consisting of liquid crystalline and amorphous ionomers (partially sulfonated polystyrene) we investigated the influence of the concentration of ionic groups in the LC component on the size of phase separated regions. It was found that blends without ionic interactions are macroscopically phase separated. In blends consisting of two ionic components, microphase separated regions of approximately 100 nm were found. The size of phase separated regions decreases continuously with increasing concentration of ionic groups in the LC component and can therefore be adjusted by a reversible redox reaction.The incorporation of ionic groups in LC polymers leads on a macroscopic scale to a combination of the liquid crystalline properties of the LC component with the elastic behavior of the amorphous ionomer. Therefore microphase separated ionomer blends show homogeneous liquid crystalline textures and behave mechanically like reversible crosslinked ionomers.

The influence of lateral substituents in the mesogens of Liquid Crystalline Side Group Copolymethacrylates on the phase behaviour

The phase behaviour of lateral substituted side group copolymethacrylates was investigated. With structural changes it is possible to influence the phase behaviour of the polymers. The lateral substituents have a stronger influence in side group polymers then in low molecular systems because of the coupling of the mesogens via the main chain. According to simulations, the possibility of rotational motions of the side groups around their long axis is hampered by a lateral substitution. With the described structural changes in the mesogen we have synthesized side group copolymethacrylates on the border between the liquid crystalline and the amorphous state. In some further papers we will discuss the consequences of a lateral substitution in the mesogen in the described materials for the dielectrical and photochemical behaviour.

Broadband Dielectric Spectroscopy on Ferroelectric Liquid Crystalline Polymers

Abstract The collective and molecular dynamics of ferroelectric liquid crystalline side group polasiloxanes has been investigated be use of broadband dielectric spectroscopy (10−2Hz – 109Hz). In the lower frequency range (>106Hz) two collective relaxations, Goldstone - and soft - mode, could be observed. They are assigned to the fluctuations of the phase and the amplitude of the helical superstructue, respectively. In the high frequency regime (106Hz – 109Hz) one dielectric relaxation, the ᵬ - relaxation, is found. It is assigned to the hindered rotation (libration) of the mesogene around its long molecular axis. It shows an Arrhenius - like temperature dependence with no deviations at the phase transition S A /S* C . A dilution of the mesogenes causes a decrease of the collectivity but remains the local dynamics uninfluenced.

Surface- and interfacial tensions of liquid crystalline polymers

Surface tensions (γA) of two side group liquid-crystalline (lc) polymethacrylates and the interfacial tensions (γAB) between the lc-polymers (LCP) and branched polyethylen respectively polymethylmethacrylat were measured in the isotropic and nematic phase with the pendant-drop method. The surface tensions vs. temperature characteristics show anomalous behavior. The slope is initially negative but changes its sign near the isotropic-nematic transition. This behavior is in agreement with surface-tension measurements on low-molar-mass liquid crystals from other authors. The interfacial tension mainly depends on the density difference Δρ between the two phases, at the phase-transition so we observed a jump of the interface-tension values and a change of the slope. The pendant-drop apparatus contains a video image processing system, that allows to acquire the pendant-drop profile with a personal computer. The program used to analyse the experimental data is based on the “robust-shape method”. This algorithm allows to calculate both the surface-tensions and the interface-tensions respectively with a personal computer.

Two-dimensional electron paramagnetic resonance spectroscopy of nitroxides: Elucidation of restricted molecular motions in glassy solids

The combination of concepts of two-dimensional (2D) spectroscopy with the well-known field step electron–electron double resonance (ELDOR) method offers a practical route to recording 2D ELDOR spectra covering the full spectral range needed for electron paramagnetic resonance (EPR) of nitroxide spin labels in the solid state. The 2D ELDOR pattern provides information about molecular reorientation measured in real time, the anisotropies of electron phase, and electron spin-lattice relaxation as well as nuclear spin-lattice relaxation all of which are connected with the detailed geometry of the molecular reorientation. Thus, in 2D ELDOR the same electron spin probes the motional behavior over a wide range of correlation times from 10−4 to 10−12 s. An efficient algorithm for simulating 2D ELDOR spectra is derived, based on analytical solutions of the spin relaxation behavior for small-angle fluctuations and offers a means of quantitatively analyzing experimental data. As an example, the motion of nitroxide spin labels in a liquid-crystalline side-group polymer well below its glass transition is determined as a β-relaxation process with a mean angular amplitude of 5° and a distribution of correlation times with a mean correlation time of 0.9×10−10 s and a width of 2.5 decades.

The β‐relaxation in low‐molecular‐weight and in polymeric side group ferroelectric liquid crystals

AbstractDielectric spectroscopy in the microwave regime (106–1010 Hz) is employed to study the molecular dynamics of a set of ferroelectric liquid‐crystalline side group polymers with variable spacer length and different molecular weight, and on their low‐molecular‐weight counter‐part. One relaxation process is observed, the β‐relaxation, which is assigned to the hindered rotation of the mesogens around their long molecular axis. This librational process is nearly uninfluenced by the molecular weight and depends only slightly on the spacer length. At the smectic‐smectic phase transition SA/SC* the β‐relaxation does not decrease in its relaxation strength and shows no slowing down of its relaxation rate. This is in pronounced contrast to the common explanation of the origin of ferroelectricity in liquid crystals and requires a new interpretation.

Structure and order in partially oriented solids by three-dimensional magic angle spinning nuclear magnetic resonance spectroscopy

A new rotor-synchronized carbon-13 three-dimensional magic angle spinning NMR experiment for the extraction of orientational distribution functions from partially ordered solid materials is presented. Problems with overlap of two-dimensional spinning sideband patterns inherent in earlier methods are overcome by extension to three dimensions, allowing the degree of order for each resolvable carbon site to be measured. The new technique is demonstrated on a frozen smectic liquid-crystalline side group polymer which shows an order gradient from the aligned mesogen to the disordered polymer main chain.

Molecular weight dependence of the rotational diffusion constant and the rotational viscosity of liquid crystalline side-group polymers

Nematic, side-group polymethacrylates [CH 2 C(Me)(COO[CH 2 ] 6 OPHCOOPhR)] n (R= CN, OMe) with different molecular weightg were studied by a combination of dielectric relaxation spectroscopy and measurements of the rotational viscosity by director reorientation experiments. The rotational diffusion constant obtained by relaxation frequency determinations allows the estimation of the contribution γ 1 m of the mesogenic groups to the total rotational viscosity γ 1 t on the basis of Marrucci's theory. γ 1 m then is used to separate the contribution Δγ 1 of the friction due to rearrangements of anisotropic ordered polymer backbones to γ 1 t .? The molecular weight dependence of Δγ 1 can be described by a power law, as predicted by Brochard's theory

Ferroelectric liquid-crystalline side group polymers Spacer length variation and comparison with the monomers

Abstract The phase behaviour and the ferroelectric properties of liquid-crystalline side group polyacrylates containing an (R)-4″(1-methylheptyloxy)-4′-biphenylyloxycarbonyl-4-phenoxy moiety are compared with the properties of their monomers. A S*C phase polymorphism including monolayer and bilayer structures was found for the polymers. Some of the S*C subphases, consisting of smectic mono- or bi-layers, do not exhibit ferroelectric properties. These unusual properties do not exist for the corresponding monomeric acrylates. Therefore, one has to take into account that the polymer chain exerts a strong influence on the phase behaviour and the ferroelectric properties.

Unusual phase behaviour of a new liquid-crystalline side group polymer

Abstract The phase structure of a series of liquid-crystalline side group polymethacrylates with azo-groups in the mesogenic units was investigated by X-ray diffraction and differential scanning calorimetry. The influence of the spacer and tail length on the nature of the liquid crystal phase is discussed. For one polymer, depending on the annealing temperature, a phase transition from a metastable monolayer SA phase to a bilayer SC phase and to an unidentified phase, possibly crystalline, was found. This behaviour is discussed considering steric hindrance of the mesogenic side groups and the anisotropic conformation of the backbone.