Mesogenic Side Groups Research Articles

Gas-influenced surface and bulk properties of a liquid crystalline side-group polymer

A preliminary study on the influence of gas sorption on the surface and the bulk properties of a liquid crystalline (LC) side-group polyacrylate PAC-6 is reported. For the first time the gas-induced enrichment of mesogenic side groups at the surface is observed, while the reproducible temperature-scanning surface tension in the high viscous smectic phase is enabled through the plasiticization of gas. Selected experiments demonstrate that the gas-influenced surface and bulk properties of the LC polymer are widely comparable with those of low-molecular-weight ones. The surface tension and the phase transitions depend strongly on the LC phase state, temperature, gas type and gas pressure. At higher pressures, the CO2-induced pretransitional surface excess order is confirmed in the nematic and isotropic phases of PAC-6. Furthermore, our experiments imply a reversible, physical gas sorption in PAC-6.

A comparative study of photo-optical behaviour of photosensitive chiral copolymers with cholesteric mesophases induced in nematogenic and smectogenic matrices

The photo-optical behaviour of two series of chiral photochromic acrylic copolymers with a chiral nematic phase has been studied. These copolymers contain identical chiral photochromic units, but have different structures of the phenyl benzoate mesogenic side groups which are responsible for the development of LC phases. This approach allowed us to examine specific features of the photo-optical behaviour of the copolymers as a function of the nature of the LC 'matrix' in which the cholesteric phase was induced. The action of UV irradiation was shown to lead to the E-Z isomerization of the chiral side groups and, as a consequence, to untwisting of the cholesteric helix of the copolymers. For copolymers of both series, the effective quantum yields of this photochemical process were calculated. In the case of copolymers in which the cholesteric mesophase is induced in a smectogenic matrix, the corresponding values of the quantum yield are lower and depend slightly on temperature. A plausible explanation of the above phenomena is suggested.

Photoreaction of Photo-cross-linkable Methacrylate Polymer Films Comprising 2-Cinnamoyloxyethoxybiphenyl Side Group by Linearly Polarized Ultraviolet Light and Liquid Crystal Alignment on the Resultant Films

This paper presents photoreaction of methacrylate polymer films comprising a 2-cinnamoyloxyethoxybiphenyl mesogenic side group by a linearly polarized (LP) ultraviolet (UV) light and an alignment b...

Atomic force microscopic studies of the influence of stretching on thin oriented films of ferroelectric liquid crystalline elastomers

To analyze the impact of the molecular structure on the network formation in ferroelectric liquid crystalline polymers, two copolymers are studied, which are identical except for the molecular position of the cross-linkable group: (i) Polymer P1 with cross-linkable groups attached to the backbone via a short spacer and (ii) Polymer P2 with the cross-linkable group in terminal position of a mesogenic side group. When mechanical stress is imposed on thin films in homeotropic orientation through stretching, the two elastomers react differently to the deformation, as seen by AFM imaging of the surface topology: For Polymer P1, “intra-layer” cross-linking results in two dimensional networks in each backbone layer, separated by liquid-like FLC sidegroup layers. As there are practically no vertical connections in this intralayer network, no vertical distortions occur. In Polymer P2 a three dimensional, “inter-layer” network is formed; the system reacts with a distortion of the smectic layering.

Flow Behavior of Two Side-Chain Liquid Crystal Polymers Studied by Transient Rheology

Transient stresses in shear-rate-controlled experiments were measured for two nematic side-chain liquid crystal polymers (side-chain LCPs), a flow-aligning one and a non-flow-aligning one, over a range of different temperatures. The chemical structures of the LCPs differ in the number of methylene groups in the spacer linking the mesogenic side groups to the polysiloxane backbone. The LCP with the shorter spacer forms only a nematic liquid crystalline phase and is flow-aligning over the whole temperature range of this phase. In contrast, the LCP with the longer spacer has an additional low-temperature smectic phase and is a non-flow-aligning nematic. Damped time-dependent oscillations in both the first normal stress difference and the shear stress were found for the non-flow-aligning side-chain LCP, whereas a rheological response characteristic of a flow-aligning system was found for the other one. A comparison of the oscillating transients of the non-flow-aligning LCP with Ericksen's transversely isotropic fluid model shows good agreement for the shear stress but not for the first normal stress difference. The intensity of the stress oscillations in the non-flow-aligning system increases spectacularly after prior squeezing or stretching of the sample. Its rheological response after flow reversal is in agreement with the previously observed “log-rolling” orientation of the mesogens. In the range of shear rates studied, both polymers are slightly shear thinning and show positive steady-state values of the first normal stress difference, which increases almost linearly with increasing shear rate.

The effect of molecular structure on the thermal properties of ferroelectric liquid crystalline cyclic siloxane tetramers

A series of liquid crystalline cyclic siloxane tetramers was prepared to investigate, in a systematic manner, the role of molecular structure of (a) the spacer group, (b) the mesogenic side group and (c) the chiral end group, on the liquid crystalline behaviour of these novel tetramers. The results from this systematic structure/property correlation study clearly showed the effects of the structure of the chiral end group and the mesogenic side group on the thermal stability and temperature range of the SmC* phase (ferroelectric) exhibited by these materials. For a given chiral end group, the effect of the length of the spacer group on the thermal stability and temperature range of the SmC* phases depended greatly on the structure of the mesogenic side group. By appropriate choice of spacer group, mesogenic side group and chiral end group, a number of tetramers exhibiting wide SmC* ranges (ferroelectricity) from below room temperature were synthesized.

Cooperative modes of molecular mobility in liquid crystalline comb-like oligoacrylates and oligomethacrylates with 4-cyanoazobenzene side groups

Cooperative modes of molecular mobility in comb-like liquid crystalline oligoacrylates and oligomethacrylates containing mesogenic 4-cyanoazobenzene side groups and spacers of 3, 5, 7, 9, 10, and 11 CH2 groups were studied by means of dielectric spectroscopy. Near Tg the existence of two cooperative relaxation processes denoted as α and δ was revealed. The δ-relaxation is connected to the motion of mesogenic groups with respect to the short mesogenic axis and the α-relaxation is connected to the motion of mesogenic groups with respect to the long axis and with the segmental mobility of the main chains. The temperatures of α- and δ-transitions decrease with increasing spacer length. The liquid-crystalline structure formation increases main chain rigidity and temperatures of the α- and δ-transitions. For these oligomers aligned by an external electric field or by mechanical shear, the planar or homeotropic orientation of mesogenic groups is established. The choice of orientation conditions makes it possible to obtain the predetermined order parameters.

Cooperative modes of molecular mobility in liquid crystalline comb-like oligoacrylates and oligomethacrylates with 4-cyanoazobenzene side groups

Cooperative modes of molecular mobility in comb-like liquid crystalline oligoacrylates and oligomethacrylates containing mesogenic 4-cyanoazobenzene side groups and spacers of 3, 5, 7, 9, 10, and 11 CH2 groups were studied by means of dielectric spectroscopy. Near Tg the existence of two cooperative relaxation processes denoted as α and δ was revealed. The δ-relaxation is connected to the motion of mesogenic groups with respect to the short mesogenic axis and the α-relaxation is connected to the motion of mesogenic groups with respect to the long axis and with the segmental mobility of the main chains. The temperatures of α- and δ-transitions decrease with increasing spacer length. The liquid-crystalline structure formation increases main chain rigidity and temperatures of the α- and δ-transitions. For these oligomers aligned by an external electric field or by mechanical shear, the planar or homeotropic orientation of mesogenic groups is established. The choice of orientation conditions makes it possible to obtain the predetermined order parameters.

Conformational mobility in a polymer with mesogenic side groups: Dielectric and DSC studies

The relaxation times of the conformational rearrangements of the main-chain segments of a liquid crystalline side-chain polymer was determined from differential scanning calorimetry experiments in the temperature interval around and below its glass transition. Phenomenological models with fitting parameters were used to evaluate the temperature dependence of the relaxation times and the form parameter of the relaxation times distribution. These parameters were compared with its counterparts in the dielectric α relaxation process which appear in the temperature interval immediately above the calorimetric glass transition. For the temperature interval below the calorimetric glass transition the differential scanning calorimetry (DSC) results were compared with the dielectric results obtained by the thermally stimulated depolarisation technique.

Side-On Fixed Liquid Crystalline Polymers: New Stationary Phases for High Performance Liquid Chromatography

New stationary phases are obtained by coating on a silica support liquid crystalline polysiloxanes with mesogenic side groups laterally attached to the polymer backbone through the flexible spacer. They show very good chromatographic performances in HPLC for the separation of polycyclic aromatic hydrocarbons. Solute/mesogenic group interactions are evaluated through molecular mechanics calculations and correlated to the chromatographic results.

Influence of a Mechanical Field on the Liquid Crystalline to Isotropic Phase Transformation

The influence of external mechanical stress on the liquid crystalline to isotropic phase transformation of polysiloxane networks with mesogenic side groups is investigated by stress optical measurements. The hypothetical second order phase transformation temperature T* and the critical exponent γ, as described by the phenomenological Landau — de Gennes theory, are determined. Different systems having nematic and smectic behaviour, respectively, are investigated. Furthermore, the crosslinking density is varied for the nematic system. While in previous investigations of side-on nematic polymer networks, the phase transformation temperature depends on the applied stress, this effect is not found for end-on nematic systems. For smectic elastomers a hypothetical phase transformation temperature T* close to Tlc,i is observed. In contrast to previous experiments, however, no smectic fluctuation could be identified above Tlc,i.

Molecular Dynamics of Liquid Crystalline Side-Chain Oligoacrylates and Oligomethacrylates with 4-Cyanazobenzene Side Groups. Influence of External Fields on Dielectric Properties

Dielectric behavior of several smectic side-chain oligoacrylates and Oligomethacrylates with 4-cyanazobenzene mesogenic side groups has been investigated. Below the glass transition temperature two relaxation regions of dipole relaxation (the γ1 and the [Btilde] processes) caused by local mobility were detected. The molecular mobility of the γ1 process increases with spacer length but in the case of the [Btilde] process the mobility is not changed. In the LC state, near Tg transition, two cooperative processes, α- and β, related to the reorientation of transverse (μ⊥) or longitudinal (μ|) components of the dipole moment of the mesogenic group, respectively, were observed. The orientation in external electric or mechanic fields leads to the establishment of planar or homeotropic orientation of mesogenic side groups.

Temperature Dependent AFM on Ultrathin Oriented Films of FLC Elastomers

Crosslinked ferroelectric liquid crystalline polymers are studied by atomic force microscopy. Polysiloxane copolymers have been synthesized with mesogenic and photo-crosslinkable side groups, the latter connected either directly to the backbone via a short spacer, or as terminal group on a part of the mesogens. These elastomers are prepared as thin freely suspended films in homeotropic orientation. The topography consists of plateaus separated by steps of characteristic height, corresponding to the surfaces and edges of smectic layers. From the temperature behavior as well as from the reaction to mechanical deformation, a model for the network architectures is proposed: In the first case (“intra-layer crosslinking”), a predominantly two dimensional network is formed within the backbone layers separating the smectic layers, in the second case (“inter-layer crosslinking”) a primarily three dimensional network is established which is dependent on the mesophase of crosslinking.

Side-Group Liquid Crystal Copolymers Containing Nonmesogenic Units: Dependence of the Thermal Behaviour upon Composition

The synthesis and properties of a new mesogenic/nonmesogenic copolymer system, namely the poly[6-(4′-methoxyazobenzene-4′-oxy)hexyl methacrylate]-co-poly[methyl methacrylate]s, are described. Copolymers containing 0.59 or greater mol fraction of the mesogenic side-groups exhibit nematic behaviour while those containing 0.39 or less mol fraction mesogen are amorphous. The glass transition temperatures exhibit a negative deviation from linear behaviour on varying the copolymer composition. In contrast, the nematic-isotropic transition temperatures decrease in an essentially linear fashion on increasing the nonmesogen content. This behaviour is compared with that of a smectogenic series and differences are accounted for in terms of a microphase separated smectic phase.

Mutual orientation of main chains and side chains in nematic liquid-crystalline side-chain polysiloxanes

The orientation of main-chain segments of some polysiloxanes containing mesogenic side groups was estimated by 29 Si NMR under high-power proton decoupling. The second-order orientation degree related to the orientation distribution of chemical shift tensors was calculated from the spectra. From this conclusions were drawn about the degree of segment orientation for which a discussion of the transverse orientation distribution was necessary. It turns out that most of the samples investigated here have a structure in which the main-chain segments are perpendicular to the director field; however, if this conformation involves sterical hindrance, a parallel arrangement can arise. The apparent contradiction to SANS data concerning backbone conformation of such polymers is discussed; thereby it is noted that in some cases the two methods take a somewhat different point of view. Thus, without further structural information, there is no unambiguous connection between mutual orientation of main and side chains on the one hand and the average shape of the whole molecule on the other.

Frustrated liquid crystalline side group polymers for optical storage

The phase formation and thermodynamic properties of side group copolymers composed of different concentrations of mesogenic azobenzene side groups and non-mesogenic side groups were investigated. Such kinds of polymers can be used for optical data storage. The storage process is caused in this case by a light-induced reorientation of the chromophores. The basic idea was that the introduction of non-mesogenic units should lead to frustration effects in the sense that amorphous polymers are obtained, which can be handled more easily than liquid crystalline polymers yet which still possess strong orientational interactions between the side groups allowing cooperative switching processes. The results of thermodynamic, structural and dynamical investigations indicate that this goal can be achieved on the basis of such copolymers.

SANS Experiments on Swollen Mesomorphous Networks

The structure of mesomorphous elastomers, made of cross-linked side-chain liquid crystal polymers, is investigated by small-angle neutron scattering. Three types of samples are synthesized in the presence of deuterated toluene, one without mesogenic groups (C) and two with different proportions of pending mesogenic side chains (A and B). SANS is measured at the preparation concentration state and at maximum swelling, in equilibrium with deuterated solvent. These experiments give a test of the homogeneity of cross-linking. Samples carrying bulky mesogenic side groups (A and B) appear to be less heterogeneous than the corresponding mesogen-free network (C). Results are discussed in terms of mesomorphous interactions which are still present at the preparation swelling ratio and in terms of a steric effect due to the side chains, which prevents chains from interpenetrating and entangling.

Poly(p-phenylene)s with Mesogenic Side Groups: A Potential Class of NII Side Chain Liquid Crystalline Polymers?

The synthesis and characterization of a series of n-[(4-cyano-4‘-biphenylyl)oxy]alkyl-2,5-dichlorobenzoate monomers containing from n = 2 to 12 methylenic groups in the alkyl spacer and their Ni(0)-catalyzed polymerization to produce poly(p-phenylene)s (PPP) containing mesogenic side groups are described. The resulting side chain liquid crystalline polymers (SCLCPs) have both a rigid rodlike mesogenic backbone and rigid rodlike mesogenic side groups. All polymers display only a broad enantiotropic nematic phase and no side-chain crystallization. The isotropization temperature of these SCLCPs exhibits a steeper increase with the degree of polymerization (DP) than that observed for polymers with flexible or semiflexible backbones. Therefore, above plateau DPs their isotropization temperature is higher than that of polymers with flexible or semiflexible backbones, although it displays a classic odd−even dependence on the spacer length. These unusual trends may indicate that the director of the nematic phase ...

Synthesis and characterization of novel ferroelectric liquid crystals and copolymers containing biphenyl azobenzene and / or phenyl biphenyl carboxylate mesogenic groups

Two new ferroelectric liquid crystals, 4-[(S)-1-(ethoxycarbonyl)]ethyl 4-[4-(9-decenyloxy)biphenyl]azobenzoate (10) and its ester analogue 4-[(S)-1-(ethoxycarbonyl)]ethyl {[4-(9-decenyloxy)]biphenyl-4'-yl}carbonyloxybenzoate (17), and side chain polysiloxane copolymers and terpolymers containing various amounts of 10 and 17 as mesogenic side groups have been synthesized. Their mesomorphic and physical properties have been characterized. Both 10 and 17 exhibit isotropic, SmA*, SmC*, and Sm(I or F)* phases. The copolymer with 17 as the mesogenic group and terpolymers having low content of the azo compound 10 show a SmC* phase over a large temperature range down to room temperature. The copolymer with the azo compound 10 as the mesogenic group is amorphous.

Thermal and dielectric properties of oriented liquid crystalline polysiloxane doped with azo‐dye

Investigations of dielectric relaxation and thermal properties of mixtures composed of liquid crystalline side-chain polysiloxane and low molecular mass azo-dye have been carried out. The dyes have been chosen to solublize well in the polymer matrix at concentrations up to 0.08 mol fraction. The dielectric relaxation experiments have shown the presence of separate processes attributed to reorientational motions of mesogenic side groups and dye molecules. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 369–377, 1999