Electromechanical effects in ferroelectric liquid crystalline polymers

Electric fields can induce mechanical vibrations in planar aligned sandwich cells of ferroelectric liquid crystals. Measurements on a polysiloxane and a polyacrylate side chain polymer proved that the electromechanical effect also exists in ferroelectric liquid crystalline polymers. The main characteristics of the electromechanical responses of these polymers are described and compared to the response of low molecular weight ferroelectric liquid crystals. According to the proposed interpretation, the vibrations in the direction parallel to both the smectic layers and cell substrates are due to coupling between the director rotation and the flow, while the resonances in the vibrations normal to the plates are connected to layer deformations.     

Time-dependent nonlinear optical properties of pyroelectric liquid crystalline polymers

Highly oriented pyroelectric liquid-crystalline polymers were prepared by photopolymerization under the influence of a static electric field from binary mixtures of two acrylate monomers exhibiting chiral smectic C mesomorphism. Both monomers contained nitro groups to yield second order nonlinear optical properties (second harmonic generation) and one of the monomers had two functional groups to yield a crosslinked polymer. The room temperature second order nonlinear susceptibility of the polymers showed during the first two hours a 10 % decrease after which it remained constant during the next 48 days. At elevated temperatures there was a significant difference in the nonlinear optical properties over time between crosslinked and uncrosslinked polymers. The uncrosslinked polymer showed a pronounced loss of second order nonlinear optical activity with time at ≥38°C. The crosslinked polymer showed a much smaller and basically a temperature independent decrease rate in the second order nonlinear optical properties at all the ageing temperatures (23-130°C). Both the loss in mesogen order parameter, very evident for the uncrosslinked polymer, and conformational changes occurring within the mesogens (β mechanism), may account for the observations made.     

Synthesis and properties of mesogen‐jacketed liquid crystalline polymers containing bistolane mesogen

On the basis of the concept of mesogen‐jacketed liquid crystalline polymers, a series of new methacrylate monomers, (2,5‐bis[2‐(4′‐alkoxyphenyl) ethynyl] benzyl methacrylate (MACn, n = 4, 6, 8, 10, and 12) and 2,5‐bis[2‐(6′‐decanoxynaphthyl) ethynyl] benzyl methacrylate (MANC10), and their polymers, PMACn (n = 4, 6, 8, 10, and 12) and PMANC10 were synthesized. The bistolane mesogen with large π‐electron conjugation were side‐attached to the polymer backbone via short linkages. Various characterization techniques such as differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized light microscopy were used to study their mesomorphic phase behavior. The polymer PMACn with shorter flexible substituents (n = 4) forms the columnar nematic (?_N) phase, but other polymers with longer flexible tails (n = 6, 8, 10, and 12) can develop into a smetic A (S_A) phase instead of a ?_N phase. The PMANC10 containing naphthyl can also form a well‐defined S_A phase. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Molecular and collective relaxations of ferroelectric side chain liquid crystalline polysiloxanes

The influences of mesogenic group chemical structures on dielectric relaxation behavior were investigated for ferroelectric side chain liquid crystalline polymers (FLCPs). The relaxation time and activation energies of the Goldstone mode, α‐, and β‐relaxations decrease with increasing spacer length because of the plasticizer effect of the spacer. Moreover, the relaxation intensity increases with increasing spacer length for FLCPs. An FLCP with a longer spacer length exhibits a higher mesogenic group mobility, and subsequently leads to easier reorientation toward the alternating electrical field. An increase in mesogenic core rigidity results in an increase in the relaxation time and activation energies, and a decrease in the relaxation intensities for the Goldstone mode, α‐, and β‐relaxations. Moreover, the β‐relaxation is suppressed and cannot be observed in the glassy state for FLCPs containing naphthyl biphenylcarboxylate as the mesogenic group. Shorter relaxation time, smaller activation energies, and higher intensity of the α‐, and β‐relaxations were obtained for FLCPs containing chiral moiety with a flexible heptyl alkyl chain. However, the relaxation intensity of the Goldstone mode for FLCPs containing this chiral moiety was smaller than that for FLCPs containing the chiral moiety with a butyl alkyl chain. For FLCPs containing a chiral moiety with two asymmetrical centers, their Goldstone mode relaxation showed larger amplitude. The α‐ and β‐relaxations are suppressed for these FLCPs because of the dense packing and memory effect of the smectic phase. The relationship between the chemical structure of the mesogenic group and dielectric relaxations is discussed in great detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2035–2049, 2006     

Side-chain liquid crystalline polymers with silphenylene-siloxane main chains. III. Synthesis and characterization of polymers with phenyl benzoate mesogenic groups

Side-chain liquid crystalline (SCLC) silphenylene-siloxane polymers with a phenyl benzoate mesogenic group and polymethylene spacers were prepared and characterized, and their properties were compared with those of equivalent SCLC polymers, SCLCPs, with a biphenyl mesogenic group. With identical spacers and terminal substituents, the melting temperatures of the former were much lower, but the isotropization temperatures were lowered to a lesser extent, than those of the latter, and, consequently, a more thermally stable nematic phase was obtained for the former. Both types of SCLCPs formed nematic phases, while polymethylsiloxanes with the same side-chain mesogens exhibited smectic phases with wider temperature ranges. The lower thermal stability of the mesophases in the silphenylene-siloxane SCLCPs compared to those of the SCLC polymethylsiloxanes can be attributed to both the rigidity of the backbone and the greater separation of the side-chains along the main chains of the former.     

Main chain liquid crystalline polymers with laterally linked organometallic mesogens

The synthesis and characterization of the phase behaviour of a homologous set of polymers of 1,m-diamino-n-alkanes with bis[4-(4-alkoxybenzoyloxy)-2-hydroxybenzaldheyde]copper(II) is reported. The structural peculiarity of these polymers, which is indicated in the title, stems from the presence of metal-complexed salicylideneaminato segments connected via the nitrogen atoms. Calorimetric analysis, polarizing optical microscopy, X-ray diffraction and dynamic viscosity measurements indicate the occurrence of nematic mesomorphism for all the polymers-including two structural analogues containing Ni(II) or Pd(II) in place of Cu(II).     

Influencing factors on liquid crystalline properties of cholesterol side-chain liquid crystalline polymers without spacer: molecular weight and copolymerisation

ABSTRACT

It is a challenge to tailor the phase behavior and phase structure of side-chain liquid crystalline polymers carrying targeted ordered structures and functional properties. In this work, liquid crystalline (LC) properties of cholesterol side-chain polymers without spacer were controlled by molecular weight (M_n) and copolymerization. On the one hand, two series of homopolymers without the spacer, poly (methacrylic acid) cholesterol esters (PCholMC_n) and poly (acrylic acid) cholesterol esters (PCholAC_n) with different M_n and low polydispersity, were achieved by reversible addition-fragmentation chain transfer polymerization. The experiment results indicated that the M_n had little effect on the LC properties of PCholMC_n and all homopolymers formed the smectic A phase. However, the phase structures of PCholAC were found to be strongly M_n dependent. The polymers PCholAC_n were amorphous when the M_n was lower than a critical value of approximately 1210₃ g/mol. But when the M_n exceeded the critical value, the polymers exhibited smectic A phase. On the other hand, two kinds of random copolymers, poly(cholest-5-en-3-methacrylate)-co-polymethyl acrylate (PCholMC-co-MA) and poly(cholest-5-en-3-acrylate)-co-polymethyl acrylate (PCholAC-co-MA) were synthesized with various composition. The findings suggested that the steric effect of main-chain and the interaction of mesogens would promote the formation of LC phase.     

Thermotropic liquid crystalline polymers with low thermal transitions. II. Low melting thermotropic liquid crystalline homo- and co-polyesters

Thermotropic liquid crystalline (TLC) polymers with low melt transitions are useful for imaging technologies. This is the first report describing thermotropic liquid crystalline copolyesters of low melt transitions comprised of a mesogen with up to three different spacer moieties. We have noted that the smectic mesophase range decreased with increasing amounts of different spacer moieties, without altering the isotropic transition and thereby leading to a broader nematic range. © 1995 John Wiley & Sons, Inc.     

Novel photo-crosslinkable liquid crystalline polymers containing vanillylidene cycloalkanones and azobenzene units

Two series of novel liquid crystalline photo-crosslinkable bis(vanillylidene-azobenzene) cycloalkanone containing polymers, namely poly(vanillylidene alkyloxy-4,4'-azobenzenedicarboxylic ester)s, have been synthesised from bis[m-hydroxyalkyloxy(vanillylidene)cycloalkanone] (m = 6, 8, 10) with azobenzene dicarbonylchloride by solution polycondensation method at ambient temperature. Polymers with varying spacer lengths have been synthesised and characterised by spectroscopic techniques. These variations have been correlated with the thermal properties and transition temperatures. Thermal transitions were analysed by differential scanning calorimetry (DSC) and the mesophases were identified by hot stage optical polarised microscopy (HOPM). All of the polymers were found to exhibit liquid crystalline properties. Transition temperatures were observed to decrease with increasing spacer length. The thermogravimetric analysis reveals that all of the polymers were stable up to 280°C undergo two-stage decomposition. Using the UV-visible photolysis studies we investigated the simultaneous behaviour of reactivity rates of crosslinking in the vanillylidene unit and isomerisation caused by the azobenzene unit in the photo-crosslinkable main chain liquid crystalline polymers. The photolysis of liquid crystalline bis(vanillylidene)cycloalkanone-based polymers reveals that there are two kinds of photoreactions in these systems: the EZ photoisomerisation of azobenzene unit and 2p+2p addition by vanillylidene units. The EZ photoisomerisation in the liquid crystal phase disrupts the parallel stacking of the mesogens, resulting in the transition from the liquid crystal phase to isotropic phase. The photoreaction involving 2p+2p addition of the bis(vanillylidene)cycloalkanone units in the polymers results in the conjoining of the chains. The cyclopentanone polymers exhibited a faster rate of photolysis than the cyclohexanone polymers.     

Luminescent Iridium-containing liquid crystalline polymers in the side chain

Luminescent liquid crystalline polymers consisting of Iridium attached to polysiloxanes are prepared. 4-Cyanophenyl 4-(allyloxy) benzoate (M₁) and an Iridium complex (Ir-M₂) grafted to poly(methylhydrogeno)siloxane are used for the preparation of the Iridium-containing liquid crystalline polymers. The chemical structures are characterised by Fourier transform infrared spectroscopy and ¹H NMR. The mesomorphic properties and phase behaviour are investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy and X-ray diffraction. The polymers containing <1.2 mol% of the Iridium ions reveal reversible mesomorphic phase transition, wide mesophase temperature ranges and high thermal stability. The introduction of the Iridium ions does not change the liquid crystalline state of polymer systems; on the contrary, the polymers are enabled with the luminescent properties. With the Iridium ion contents ranging between 0.3 and 1.2 mol%, luminescent intensity of polymers gradually increased. The temperature dependence of luminescent intensity was studied in the liquid crystalline phase.     

Synthesis and properties of liquid crystalline polyurethanes

Liquid crystalline polyurethanes were prepared from 4,4′-bis(2-hydroxyethoxy)biphenyl (BHBP) and 2,4-tolylene diisocyanate (TDI). The effect of partial replacement of BHBP by 25–75 mol % poly(oxytetramethylene) diol (PTMO, M _n = 250) on the liquid crystalline properties was studied. The BHBP/TDI/PTMO polyurethanes were obtained by one- and two-step polyaddition. The polyurethanes were investigated by DSC, polarizing microscopy, x-ray, and IR spectroscopy. The molecular weight distribution was determined by GPC. The morphology of the polymers was investigated by the SALS method. Thermogravimetric investigations of the polyurethanes were also performed. All polyurethanes containing BHBP units have liquid crystalline properties. Partial replacement of BHBP by PTMO-250 considerably changes the phase transition temperatures and the range of mesophase occurrence. More homogeneous polyurethanes were obtained, if the two-step polyaddition method was applied. The polyaddition method affects the phase transition temperatures. © 1993 John Wiley & Sons, Inc.     

Synthesis and properties of mesogen‐jacketed liquid crystalline polymers containing biphenyl mesogen with asymmetric substitutions

This work focuses on the design, synthesis, and characterization of a series of mesogen‐jacketed liquid crystalline polymers (MJLCPs), poly(alkyl 4′‐(octyloxy)‐2‐vinylbiphenyl‐4‐carboxylate) (pVBP(m,8), m = 1, 2, 4, 6, 8, 10, 12). For the first time, we realized asymmetric substitutions in the mesogens of MJLCPs. The polymers obtained by conventional free radical polymerization were investigated in detail by a combination of various techniques, such as differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized light microscopy. Our results showed that all the polymers were thermally stable, and their glass transition temperatures decreased when m increased. The liquid crystalline (LC) phases that developed at high temperatures and disappeared at low temperatures were strongly dependent on the difference in lengths of alkyl groups on the 4 and 4′ substitution positions of the side‐chain biphenyl. While polymer pVBP(1,8) was not liquid crystalline, columnar liquid crystalline phases were observed for all other pVBP(m,8) (m = 2, 4, 6, 8, 10, 12) polymers. Polymer pVBP(8,8) showed a tetragonal columnar nematic liquid crystalline phase, and the other LC polymers exhibited columnar nematic phases. In additions, the smaller the difference in the lengths of the terminal alkyls, the easier the development of the liquid crystalline phase. Birefringence measurements showed that solution‐cast polymer films exhibited moderately high positive birefringence values, indicating potential applications as optical compensation films for liquid crystal displays. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity K_par; and the drastic drop in the axial expansivity α_∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide-angle x-ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, K_par; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, α_par; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large-scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.     

Syntheses and properties of liquid crystalline N-substituted pyrroles and their polymers

Three liquid crystalline N-substituted pyrroles were synthesized from 6-(1-pyrrolyl)hexanol with phenolic derivatives having a mesogenic core of cyclohexylbenzene or biphenyl by Mitsunobu reaction. These pyrroles had two anodic peaks at 1.4 and 1.8 V (vs. SCE). The former was due to an oxidation of the pyrrole moiety and the latter was due to an oxidation of the mesogenic moiety. These pyrrole monomers were polymerized by electrochemical and chemical methods. The potentiostatic method and the chemical method using FeCl₃ gave a soluble and fusible polymer, respectively. A polymer having a mesogenic core of cyclohexyl benzene obtained by the chemical method and a polymer having a mesogenic core of biphenylketone obtained by the potentiostatic method had a liquid-crystalline phase. The phase was identified as smectic A by polarizing microscopy and XRD analysis. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2691–2698, 1998     

Studies on the synthesis and properties of ferroelectric side chain liquid crystalline polyoxetanes

Two series of novel ferroelectric liquid crystalline (FLC) monomers were derived from 3-(hydroxymethyl)-3-methyloxetane, used as the backbone unit, and 2-(S)-[2-(S)-methylbutoxy]propionic acid, as a chiral moiety. The corresponding polyoxetanes were prepared by ring-opening polymerization using BF₃ · OEt₂ as an initiator. In addition to the structure identification, their liquid crystal phase behavior and electrical properties are also studied. Before their connection to the chiral molecular moiety, two series of carboxylic acids, 4-(6-[(3-methyloxetan-3-yl)methoxy]alkoxy)-benzoic acids and 4,4′-[6-(3-methyloxetan-3-yl)alkoxy]biphenylcarboxylic acids, show the phase sequence K Sc I and K Sc N I, respectively. After connection, the phase behavior of the corresponding chiral monomers is changed from K Sc I to K Sc* N* I as well as from K Sc N I to K Sc* Sa I. Only the phase sequence K Sc* Sa I is observed in both series of polyoxetanes. All of the synthesized monomers exhibited enantiotropic chiral smectic C(Sc*) phase. The monomers, with the biphenyl unit linked directly with a chiral center, possessed higher spontaneous polarization (P_s) values. Polyoxetanes possess a wide temperature range for the liquid crystal phase, about 120°C, and the Sc* phase range can be up to 95°C. However, the position of the biphenyl unit will not affect the spontaneous polarization of the synthesized side chain FLC polyoxetanes. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2843–2855, 1997     

Synthesis and nonlinear optical properties of side chain liquid crystalline polymers containing azobenzene push–pull chromophores

Azobenzene monomeric precursors bearing piperazine as donor moiety with different withdrawing groups and derived side chain polymethacrylates have been prepared and characterized. Monomers having terminal cyano or nitro groups, and the corresponding polymers, exhibited smectic A phases. Linear and nonlinear optical properties of every monomer and thin films of the cyano polymer ( pol‐PZ‐CN ) have been also studied. UV‐vis spectroscopy revealed out‐of‐plane orientation in the as prepared films, as confirmed by waveguide refractive index measurements. Moreover, absorption spectra indicated the presence of azo aggregates in these films. The initial molecular arrangement has been modified by applying thermal annealing within the mesophase range and UV‐blue irradiation. Although thermal annealing resulted in a significant amplification of the out‐of‐plane optical anisotropy due to thermotropic self‐organization of side chain azo moieties, irradiation with 440 nm light induced some disruption of aggregates. The nonlinear optical response of Corona poled films has been studied by second harmonic generation measurements, and the influence of the molecular arrangement on the nonlinear d_ij coefficients has been analyzed. The more efficient poling corresponded to preirradiated films. In any case, a noticeable degree of polar order (70% of the initial d₃₃ value) remained for several months after the poling in films kept at RT. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 232–242, 2010     

Synthesis by ATRP and effects of molecular weight on photomechanical properties of liquid crystalline polymers containing side-chain azobenzene chromophores

The synthesis of a series of azobenzene containing liquid crystalline methacrylic homopolymers, poly(4-ω-methacryloyloxy-hexyloxy-4′-ethoxyazobenzene) [Poly(M6A)], with distinct average chain lengths and low polydispersity has been achieved by Atom Transfer Radical Polymerization (ATRP) in THF solution using allyl 2-bromoisobutyrate as initiator and Cu(I)Br as catalyst. Under the adopted conditions the living centers concentration is found to be constant throughout the polymerization process and well defined chain end-groups are obtained. All the obtained polymeric samples, having average molecular mass ranging from 3300 to 14000 g/mol, exhibit smectic and nematic liquid-crystalline phases on heating, with transition temperatures strongly dependent on polymerization degree, as characterized by differential scanning calorimetry and polarized optical microscopy.The photomechanical effects (i.e. the dependence of volume and density) exhibited upon trans-to-cis and cis-to-trans photoisomerization of the azobenzene mesogenic groups have been investigated by ellipsometry and related to molecular weight, with particular attention to important parameters for potential applications such as the relative variation of total volume, response time, stability and reproducibility.     

Annealing effects on the thermal properties of liquid crystalline polyurethanes

This work is a continuation of our earlier investigations of liquid crystalline polyurethanes prepared from 4,4′-bis(2-hydroxyethoxy) biphenyl (BHBP), 2,4-tolylene diisocyanate (TDI), and poly (oxytetramethylene) diols (PTMO). The annealing effects on the thermal properties of the investigation polyurethanes are presented for three samples with the same BHBP content, different flexible spacer length, and different molecular weight of the polyurethanes. The annealed polyurethanes were investigated by means of DSC, and polarizing microscopy. The results of the thermal analysis show that the temperatures of phase transitions depend on the annealing temperature and time. These dependences are different for different molecular weights. © 1994 John Wiley & Sons, Inc.     

Phase behaviour of lyotropic liquid crystalline side chain polymers in aqueous solutions

Amphiphilic lyotropic liquid crystalline surfactants are synthesized displaying 10-undecenoic acid as hydrophobic and ethyleneglycol units as hydrophilic parts of the molecules. By addition reaction of the monomeric surfactants with poly [oxy(methylsilylene)], the surfactants are attached as side chains to the siloxane main chain. The phase behaviour of a polymer-water system and the corresponding monomer-water system is investigated by polarizing microscopy. The monomeric surfactant exhibits a liquid crystallineM ₁-phase of hexagonally packed, rod-like micelles in a concentration range of 49 to 70% surfactant. The liquid crystalline state of the polymeric surfactant is more stable, which is indicated by a broader temperature- and concentration range (35%–90% polymer surfactant). At lower concentration aM ₁-phase exists, which is separated by a cubic phase from a lamellarG-phase at higher concentration of the polymer surfactant. Compared to the monomeric system, the increased stability of the polymeric mesophase can be understood by the restriction of motions of the amphiphiles due to the linkage to the polymer main chain.Dedicated to Prof. Dr. F. H. Müller.The authors are greatly indebted to Wacker Chemie, D-Burghausen, FRG for kindly delivering the poly(hydrogensiloxane).     

SAXS investigations on chiral liquid crystalline polymers: antiferroelectric and ferrielectric structures?

The structure and phase behavior of liquid crystalline polymers (LCPs) having a common chiral side chain mesogen but different main chain structures have been investigated using small-angle X-ray scattering (SAXS). While the low molecular weight chiral side chain mesogen by itself exhibits ferroelectricity, the SAXS data of the side chain LCP with a flexible polyacrylate backbone contains a bilayered superstructure peak that is indicative of antiferroelectric order. The combined LCP with a nonpolar main chain mesogen also has a bilayered superstructure, but has a different structural organization in the proposed antiferroelectric phase compared to the side chain LCP. Further changes in the phase behavior and structural organization occur when a polar group is introduced into the main chain mesogen. A ferrielectric phase has been proposed to explain the observation of a trilayered superstructure in the corresponding SAXS data. The influence of the chemical structure and connectivity on the phase behavior and superstructure formation in the chiral LCPs is discussed.