Effect of a lateral substituent on the mesomorphic properties of ferroelectric side chain liquid crystalline polysiloxanes

The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and ( S )-2-methylbutyl 4-\[(4-oxybiphenyl-4-yl)carbonyloxy]-3-fluorobenzoate mesogenic side groups is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurements reveal liquid crystalline properties for all synthesized monomers and polymers. All three precursor olefinic monomers reveal cholesteric and smectic A phases. The olefinic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral fluoro substituent. The results seem to demonstrate that incorporating a lateral fluoro substituent in the mesogenic cores of the monomers affects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral fluoro substituent in the mesogenic cores of the polymers affects only the thermal stability of the mesophases formed. The lateral fluoro substituent has a more profound effect on the mesomorphic behaviour for the monomers than that for the polymers.     

Differences between smectic homo‐ and co‐polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC‐homo‐ and LC‐co‐polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo‐ and co‐polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC‐elastomers formed from homo‐ and co‐polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Liquid crystalline copolymers containing electron donor and acceptor mesogens with a chiral group in the spacer unit

Side-chain liquid crystalline polymers containing both mesogenic (carbazolylmethylene)aniline and (4′-nitrobenzylidene)aniline units with various spacer groups were prepared to examine effects of the structure of spacer groups on the liquid crystalline properties. The copolymer containing (R)-(+)-2-methylpropylene as a chiral group in the spacer unit induced a smectic phase; the copolymer with a trimethylene spacer of similar length to the chiral spacer exhibited a nematic phase. Smectic phases were observed for the copolymer containing the chiral spacer group when the proportion of the carbazolyl group was in the range of 0.55–0.88. For example, the copolymer with the proportion of the carbazolyl group of 0.68 expressed the smectic phase from 88° to 167°C. This isotropic temperature was 37° higher than the calculated value (130°C) based on an assumed copolymer composition without the electron donor–acceptor interaction. Thus, it is assumed that for the chiral copolymer containing both electron donor and acceptor groups, the thermal stability and the induction of the smectic phase were caused by both the electron donor–acceptor interaction and the existence of the chiral group in the spacer unit. © 1995 John Wiley & Sons, Inc.     

Differences between smectic homo- and co-polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the 'de Vries' type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the 'de Vries' type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC-elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Optically-active comb-branch liquid crystalline polymers having the chiral center in the flexible spacer unit. III. Copolymers of (S)-2-(4′-cyano-4-biphenylyloxy)-1-methylethoxy ethyl methacrylate and di[6-(4-methoxy-4′-oxybiphenyl) hexyl]-2-methylene butane-1,4-dioate

A new series of copolymers with optically-active liquid crystalline side chain units has been synthesized from the comonomers (S)-2-[2-(4′-cyano-4-biphenylyloxy-1-methylethoxy]ethyl methacrylate ( 1 ) and di[6-(4-methoxy-4′-oxybiphenyl)hexyl]-2-methylene butane-1,4-dioate ( 4 ). Chiral nematic phases were exhibited by two members of the series, rich in monomer 1 , while a smectic phase was exhibited in copolymers rich in 4 . While it was thought possible that ordered chiral liquid crystalline phases may be induced by copolymerizing chiral mesogenic monomers with mesogenic derivatives of itaconic acid where the high side chain density encourages greater ordering in the system, no evidence of smectic C^* phases could be found in the present systems. © 1993 John Wiley & Sons, Inc.     

New SmCG phases in a hydrogen-bonded bent-core liquid crystal featuring a branched siloxane terminal group

In this study, we synthesized three analogous bent-core molecules, a hydrogen-bonded complex and a covalent-bonded compound with branched siloxane units (H-SiO and C-SiO, respectively) and a hydrogen-bonded complex with an alkyl unit (H-Alk), and investigated the effects of the hydrogen bonding and branched siloxane terminal units on their mesomorphic properties. The covalent-bonded compound C-SiO and the hydrogen-bonded complex H-Alk exhibited typical SmCP phases; in contrast, the hydrogen-bonded complex H-SiO exhibited a series of general tilt smectic (SmCG) phases with highly ordered layer structures (i.e., SmC?G(2)P(F)-USmCG(2)P(A)-SmCG(2)P(F)-SmCGP(F) upon cooling). During the SmCG-type phase transition process, a 2D-modulated ribbon structure transferred into highly ordered layers via undulated layers, as the hydrogen-bonding strength increased with reduced temperatures. As the SmCG domains were aligned under dc electric fields, a gradual decrease in the leaning angle from ca. 60° to 50° (while the tilt angle kept at ca. 31°) could be determined by in situ wide-angle X-ray scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer structure of the hydrogen-bonded complex H-SiO. In addition, the ferroelectric-(antiferroelectric)-ferroelectric transitions proven by the switching current responses in the SmCG-type phases of H-SiO reveal that the polar switching occurred through collective rotations around the long axis of H-SiO. Therefore, novel SmCG phases with a series of highly ordered 2D-structures were induced by the effects of the hydrogen bonding and branched terminal siloxane unit in the bent-core hydrogen-bonded LC complex H-SiO.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

Liquid crystal properties of a mesogenic polyacetylene, poly(11-[(4'-heptoxy-4-biphenylyl)carbonyloxy]-1-undecyne)

The liquid crystalline properties of a mesogenic poly(1-alkyne) and the corresponding monomer were studied using transmission electron microscopy, X-ray diffraction, polarizing optical microscopy and differential scanning calorimetry. The monomer exhibits a monotropic smectic A phase and a metastable crystalline phase. The rigid polymer backbones do not prevent the mesogenic moieties from packing into smectic A and B phases in the temperature ranges 127.6-74.1°C and 74.1°C-room temperature, respectively, on cooling from the isotropic melt.     

Study on new chiral liquid crystalline monomers and polymers containing menthyl groups

A series of new chiral monomers (M1–M4) and the corresponding siloxane polymers (P1–P4) containing menthyl groups were synthesised to establish the relationship between their structure and liquid crystalline properties. The effect of the mesogenic core rigidity and the spacer length on the phase behaviour of the monomers and polymers obtained in this study was discussed. The selective reflection of light for the chiral monomers was studied with UV-Vis spectrometer. Polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and thermogravimetric analysis were used to characterise the phase behaviour and thermal stabilities. It was found that these chiral monomers and polymers were beneficial for the formation of the mesophases when a flexible spacer was inserted between the mesogenic core and terminal menthyl groups. M1–M3 showed enantiotropic chiral smectic C phase and cholesteric phase, and monotropic cubic blue phase on cooling cycle. M4 only showed cholesteric phase. P1–P4 showed a smectic A phase. With increasing the mesogenic core rigidity or decreasing the spacer length, the corresponding melting temperatures, glass transition temperatures and isotropic temperatures all increased.     

Functional liquid-crystalline polyesters based on twin nonamethylene spaced p-oxybenzoate dyads and dithiols

A new series of thermotropic liquid-crystalline poly-(ester-β-sulphide)s 9Sn was prepared by reacting the mesogenic nona-methylene bis[4-(4-acryloyloxybenzoyloxy)benzoate] 1 with α,ω-alka-nedithiols of varying length. The polymers exhibit nematic and smectic mesophases, which are enantiotropic or monotropic in character, depending on the number n of methylene units in the dithiol precursor (n = 2–10). The phase transition parameters were determined and compared with those of closely related poly(ester-β-sulphide)s 6Sn. It is suggested that these systems may be suitable model polymers to test current theory of the existence of highly anisotropic or weakly anisotropic mesophases in polymers.     

Synthesis and characterization of chiral liquid‐crystalline polysiloxanes containing fluorinated units and sulfonic acid groups

Chiral side‐chain liquid‐crystalline polysiloxanes ( PS‐1 , PS‐2 , PS‐3 , PS‐4 , PS‐5 , PS‐6 ) bearing fluorinated units and sulfonic acid groups were synthesized with poly(methylhydrogeno)siloxane, cholest‐5‐en‐3‐ol(3β)‐4‐(2‐propenyloxy)benzoate, and 3‐trifluoromethyl‐phenyl 3‐sulfo‐4‐undec‐10‐ enoyloxy‐benzoate. The effects of fluorinated units and sulfonic acid groups on characteristic of liquid‐crystalline properties were studied. PS‐1 , PS‐2 , and PS‐3 exhibited both smectic and cholesteric mesophases, while PS‐4 , PS‐5 , and PS‐6 exhibited only cholesteric mesophase. As the polymers contained more fluorinated units and sulfonic acid groups, segregation of the fluorinated segment to the surface and aggregation of hydrogen bonding should occur. Therefore, the highly ordered lamellar mesogen–siloxane matrix systems should be disturbed severely, suggesting that PS‐4 , PS‐5 , and PS‐6 show no smectic phase. The maximum reflection bands become broad and shifted slightly to long wavelength from PS‐1 to PS‐6 . Copyright © 2008 John Wiley & Sons, Ltd.     

Liquid-crystalline properties of chiral malolactonate monomers and their comb-shaped polymers

A series of optically pure mesogenic ester precursors, and the malolactonate monomers containing mesogenic alcohols obtained from them were prepared and polymerized to homopolymers and copolymers. The liquid-crystalline properties of the precursors, monomers and polymers were investigated by differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction. The liquid-crystalline properties of the chiral mesogenic precursors and monomers were only slightly influenced by the chemical structure, configuration and the alkyl spacer length of the mesogenic alcohol substituent of the ester. The precursor, 1-[6-(4'-hexyloxy-4-biphenylyl)oxyhexyl] hydrogen (S)-O-mesylmalate (V-(S)-6) formed smectic E and smectic A phases on melting, while the monomer, 6-(4'-hexyloxy-4-biphenylyl)oxyhexyl (R)-malolactonate (II-(R)-6) showed only a crystal-isotropic phase transition. In contrast, the homopolymer of the latter exhibited a chiral smectic C phase. Copolymers from 2-(4'-hexyloxy-4-biphenylyl)oxyethyl (R)-malolactonate (II-(R)-2) also formed chiral smectic C phases.     

Mesophase formation in a system of top-shaped hard molecules: density functional theory and Monte Carlo simulation

We present the phase diagram of a system of mesogenic top-shaped molecules based on the Parsons-Lee density functional theory and Monte Carlo simulation. The molecules are modeled as a hard spherocylinder with a hard sphere embedded in its center. The stability of five different phases is studied, namely, isotropic, nematic, smectic A, smectic C, and columnar phases. The positionally ordered phases are investigated only for the case of parallel alignment. It is found that the central spherical unit destabilizes the nematic with respect to the isotropic phase, while increasing the length of the cylinder has the opposite effect. Also, the central hard sphere has a strong destabilizing effect on the smectic A phase, due the inefficient packing of the molecules into layers. For large hard sphere units the smectic A phase is completely replaced by a smectic C structure. The columnar phase is first stabilized with increasing diameter of the central unit, but for very large hard sphere units it becomes less stable again. The density functional results are in good agreement with the simulations.     

STUDY ON CHIRAL LIQUID CRYSTALLINE POLYMERS WITH BIPHENYLENE AND AZOBENZENE AS THE MESOGENS

Chiral liquid crystalline polymers containing biphenylene and azobenzene as the mesogensand S(-)-2-methyl-1-butanol as the chiral end group were synthesized and characterized by DSC,POM and X-ray diffraction. These polymers show crystalline or glassy liquid crystalline phase atroom temperature. Most polymers show smectic A or highly ordered smectic phases abovemelting temperature.     

Frustrated molecular packing in highly ordered smectic phase of side-chain liquid crystalline polymer with rigid polyacetylene backbone

Using poly(5-{[(4'-heptoxy-4-biphenylyl)carbonyl]oxy}-1-pentyne) as an example, we demonstrate the incorporative accommodation of the rigid polyacetylene backbones and the mesogenic pendants, which leads to a highly ordered smectic (Sm) phase with a frustrated structure. The polymer exhibits a recognizable sheetlike molecular shape due to its rigid backbone and relatively short spacer (three methylene units), and the building block of the liquid crystalline (LC) phase is the whole molecule. In the LC phase, five layers of the molecules stack as a smectic A (SmA) block, and adjacent SmA blocks glide halfway of the molecular width from one to another. In scanning tunneling microscopy (STM) experiments, the STM tip scrape is found to generate a regular nanopattern with periodic electron conductivity, of which the spacing is determined by the side-chain length.     

Liquid crystalline polyaniline and phthalocyanine-based polysiloxanes bearing lateral fluoro-substituted benzoic acid groups

A series of novel liquid crystalline polymers (PI, PII, PIII and PIV) containing lateral fluoro-substituted benzoic acid groups was synthesised using cholesteryl 4-(2-propenyloxy)-benzoate, 4-(allyloxy)-3-fluorobenzoic acid and poly(methylhydrogeno)siloxane. PI and PII showed smectic phase, but PIII and PIV showed chiral nematic phase due to more lateral fluoro-substituted benzoic acid groups in the polymer systems. Liquid crystalline polyaniline (PAN) and phthalocyanine (Pc)-based polysiloxanes showing chiral nematic phase were prepared by use of Pc, PAN and these liquid-crystalline polymers via hydrogen bond. PAN-based polysiloxanes showed different liquid crystalline behaviours from Pc-based polysiloxanes due to the difference of molecular structure. PAN-based polysiloxanes showed greater d-spacings between the side mesogenic groups than Pc-based polysiloxanes due to long rod-like geometrical shapes. Hydrogen bond based on lateral fluoro-substituted benzoic acid groups was formed to different geometrical shapes (strip or roundness) between PAN and Pc-based polysiloxanes.     

Carborane-containing liquid crystals: a comparison of 4-octyloxy-4-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl with its hydrocarbon analogues

4-Octyloxy-4-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl ( 1BC ) has been synthesized along with three hydrocarbon analogues in which the 1,12-dicarbadodecaborane is replaced by a phenyl ( 1PH ), trans -cyclohexyl ( 1CH ) or bicyclo\[2.2.2]octyl ( 1BO ) ring. The mesogenic properties of these materials have been compared and contrasted in both their pure states and as binary mixtures. The binary phase diagrams for the liquid crystal 1BC , with its hydrocarbon analogues 1CH and 1BO exhibit excellent miscibility of the smectic A phase while the more highly ordered smectic phases (SmB and SmE) for the hydrocarbons are suppressed by 1BC . In contrast the binary mixture of 1BC with the terphenyl analog ( 1PH ) exhibits complex behaviour in which the thermal stability of the smectic E phase is enhanced. X-ray diffraction data for the 1PH - 1BC binary mixture suggest a strong in-plane molecular ordering which might be attributed to intermolecular associations stabilizing the smectic E phase in preference to other smectic modifications.     

Liquid crystalline behavior of comb–branch copolymers of di-[6-(4-methoxy-4′-biphenyloxy)hexyl]-2-methylene butane-1,4-dioate with two non-mesogenic chiral monomers

A mesogenic monomer di-[6-(4-methoxy-4′-biphenyloxy) hexyl]-2-methylene butane-1,4-dioate, has been copolymerized with non-mesogen monomers, one racemic and one optically pure, with a view to examining the potential for using these to induce chiral liquid crystal line phases in the copolymer. This proved to be ineffective, with the copolymer exhibiting only smectic A or smectic B phases. Transition temperature–copolymer composition diagrams have been constructed for both copolymer series and their features have been contrasted. Monomer reactivity ratios have been derived for both copolymer systems.     

Liquid-crystalline side chain polymers containing a chiral spacer unit exhibiting chiral smectic phases

The synthesis and mesomorphic properties of two liquid-crystalline side chain polymers with a chiral centre in the α or β position of the α-hydroxy acid representing the spacer unit are described. The chiral α branching leads to a dramatic decrease in the transition temperatures and a strong narrowing of the smectic mesophase (compared with the unbranched model compound I). The chiral β branching results in a chiral smectic phase, a pronounced contraction of the S_c phase, and the loss of the higher ordered S_f phase. The S*_c phase was confirmed by X-ray investigations of oriented samples. Depending on the polymerization conditions samples were obtained which were oriented in melt drawn fibres either with their smectic layers or their mesogenic units in the direction of stress.     

Chiral liquid-crystalline copolyacrylates with biphenyl and phenyl benzoate side groups

The phase behavior and structure of liquid-crystalline phases of three series of copolymers with mesogenic units belonging to the families of biphenyls and phenyl benzoates have been studied, with the former mesogenic units containing a chiral center in a flexible spacer. A variation in the isotropization temperature as a function of composition is well described by the additivity rule, whereas the tilt angle of mesogenic groups in smectic layers changes nonlinearly. This angle decreases beginning from homopolymers and attains the minimum value for the copolymers of the equimolar composition, which show the highest ferroelectric activity among the polymer systems under study.