Effect of fluorinated nematic mesogens on phase behaviors and optical properties of chiral liquid crystalline polysiloxanes

A series of new chiral side-chain liquid crystalline polymers (P₁–P₇) have been synthesized with poly(methylhydrogeno)siloxane, two chiral liquid crystalline monomers, cholesteryl-4-allyloxybenzoate (M₁) and cholesteryl 4-(10-undecylen-1-yloxy) benzoate (M₂), and a nematic liquid crystalline monomer, 4-(trifluoromethyl)phenyl 4-(undec-10-enoyloxy)benzoate (M₃). The chemical structures and liquid crystalline properties of the synthesized polymers have been investigated by FTIR, ¹H-NMR, differential scanning calorimetry (DSC), polarizing optical microscopy (POM), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). All chiral polymers show wide mesophase temperature ranges and a high thermal stability with decomposition temperatures (T _d) at 5 % weight loss greater than 300 °C. P₁–P₄ display a single cholesteric phase, but P₅–P₇ containing more fluorinated units show a smectic A (SA) phase besides a cholesteric phase. The optical properties of the polymers have been characterized by circular polarization spectra and optical rotation analysis. The cholesteric polymers P₃ and P₄ exhibit different colors at room temperature, and the color can remain over 24 months. The maximum reflection bands of polymers P₁–P₄ shift to long wavelength with increasing the content of M₃ in the polymer systems. For P₅–P₇, the reflection wavelengths change sharply around the temperature of the SA–Ch phase transition. The specific rotation value of P₂ smoothly decreases from ?8.2° to ?0.29° when it is heated, but the specific rotation value of polymer P₇ changes from negative value to positive value on heating cycle. The optical properties of the polymers offer tremendous potential for various optical applications.     

Ferroelectric liquid crystalline polymers based on mesogens with halogen-containing terminal groups

A new series of halogen-containing side chain ferroelectric liquid crystal polymers was synthesized. Mesophases were characterized by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and molecular simulation. The behaviour of the liquid crystalline phase was investigated with variation of chiral centres, spacer units and grafted ratios. It was found that the thermal stability and temperature range of the chiral smectic C phase decreased with increasing length of the oligo-oxyethylene spacer, and decreasing mesogenic group content. The bulky substituent attached to the chiral centre reduces molecular packing in smectic liquid crystal phases, which disturbs the orientation of the side chain liquid crystal polymer. Furthermore, the influence of molecular structure on electrooptical properties of FLCPs has been studied by broad band dielectric spectroscopy (from 0.1 to 1 ×10 6 Hz).     

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).     

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).     

Synthesis and liquid crystalline behavior of side chain liquid crystalline polymers containing triphenylene discotic mesogens with different length flexible spacers

A series of side chain liquid crystalline polymers(SCLCPs) containing triphenylene(Tp) units in the side chains, denoted as PMTS(without spacer) and PMTn S(n = 2, 3, 4, 6, which is the number of the methylene units between the main chain and Tp moieties in the side chains), with different lengths of spacers were synthesized through conventional free radical polymerization. The chemical structures of the monomers were confirmed by 1H/13C-NMR, and the phase behaviors were examined by differential scanning calorimetry(DSC), polarized optical microscopy(POM) and wide-angle X-ray diffraction(1D and 2D WAXD). The molecular characterization of the polymers was performed with 1H-NMR, gel permeation chromatography(GPC) and thermogravimetric analysis(TGA). The phase behaviors of the polymers have been investigated by a combination of techniques including DSC, POM, 1D and 2D WAXD. The results showed that the length of spacer has significant effects on the LC phase behavior of polymers. For PMTS and PMT2 S, they displayed the columnar phase developed by the Tp moieties and the main chain as a whole due to the strong coupling effect of the Tp moieties and the main chain. For the PMT3 S, PMT4 S and PMT6 S, they formed the symmetry hexagonal columnar(ΦH) phase owing to the decoupling effect. All of these indicated that the "decoupling effect" or "coupling effect" depended on the length of spacers, leading to the different LC phase formation mechanism.     

Liquid crystalline polymers containing heterocycloalkane mesogens. 2. Side-chain liquid crystalline polysiloxanes containing 2,5-disubstituted-1,3-dioxane mesogens

Polysiloxanes and copolysiloxanes containing 2-(p-hydroxyphenyl)-5-(p-methoxyphenyl)-1,3-dioxane and 5-(p-methoxyphenyl)-1,3-dioxan-2-yl as mesogenic units and an aliphatic spacer containing 11 and 10 methylene units, respectively, were synthesized. Their phase behavior was studied by differential scanning calorimetry and optical polarization microscopy, and compared with the phase behavior of the polysiloxanes and copolysiloxanes containing 4-methoxy-4′-hydroxybiphenyl and 4-cyano-4′-hydroxybiphenyl mesogens attached to the polymer backbone through an aliphatic spacer containing 11 methylene units. All synthesized polymers present smectic mesomorphism. The polymers containing 4-methoxy-4′-hydroxybiphenyl and 4-cyano-4′-hydroxybiphenyl are also crystalline, while the polymers containing 1,3-dioxane based mesogens do not crystallize.     

Cinnamate ester containing liquid crystalline side chain polymers

The synthesis of methacrylate esters of 4-cyanophenyl-(4-(ω-hydroxyalkyloxy)) cinnamates, with spacer lengths of 2 and 6 methylene units and the synthesis of the corresponding acrylate ester with a spacer of 2 methylene units are described. The methacrylate monomers were polymerized by free radical polymerization, both as homopolymers and as copolymers with the analogous benzoate monomer of spacer length 6. The acrylate ester could not be polymerized successfully under the same reaction conditions. Polymers were characterized by NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermo-optic observations. Of the monomers prepared, only the cinnamate with a hexamethylene spacer shows a mesophase, seen on supercooling of the melt. All of the polymers prepared were liquid crystalline, with smectic behavior predominating in the polymethacrylates with the longer spacer group. A narrow nematic region is seen just below the clearing temperature with a range of 3–9°C, nematic character is increased in the copolymer series with the degree of incorporation of the cinnamate monomer with the spacer group of length 2.     

Luminescent lanthanide-containing chiral liquid crystalline polymers in the side chain

Luminescent lanthanide-containing chiral liquid crystalline polymers are graft-copolymerised using poly(methylhydrogeno)siloxane (PMHS), crosslinking agent, liquid crystalline monomer and lanthanide complexes. The chemical structures of the monomers are characterised by FTIR, ^1?H NMR and elemental analyses. The mesomorphic properties and phase behaviour are investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy and X-ray diffraction. The polymers containing less than 9 mol% of the crosslinking units reveal reversible mesomorphic phase transition, wide mesophase temperature ranges and high thermal stability. With the introduction of lanthanide complex units, the polymers are enabled with the significant luminescent properties. The temperature dependence of fluorescence intensity was studied in the liquid crystalline phase. The IR imaging shows that the lanthanide complex units evenly distribute in polymers.     

Structural characterization of the smectic polymorphism of chiral liquid crystalline polysiloxanes containing biphenyl mesogens

A structural characterization of chiral side chain siloxanes with different average degrees of polymerization, DP_n, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DP_n = 35) and oligomers (DP_n = 4) contained the 4,4'-biphenylene unit with either an (S)-2-methylbutoxy (An, Bn) or an (S)-2-chloro-3-methylbutanoyloxy substituent (Cll). The spacer segment connected to the siloxane backbone had a variable number, n, of methylene groups (n = 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-S_F1 (or S_I1)-S_C1-S_A1-I, whereas in the oligosiloxanes the sequence C-S_B1-S_A1-I (B11) or C-S_F1-S_C1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection of the electron density profiles along the layer normal, p(z), and deduction of the most physically acceptable electron density profiles from among the numerous possibilities for each smectic phase. The electron density profiles were in agreement with monolayer smectic phases presenting a microphase separation between the siloxane backbones and the side chains, so constraining the polymer backbones within a thin layer.     

Synthesis and characterization of side chain cholesteric liquid crystalline polymers containing isosorbide as a chiral centre

A series of new side chain cholesteric liquid crystalline polysiloxanes was synthesized by grafting copolymerization of a mesogenic monomer (M₁) and a chiral monomer (M₂). The chemical structures of the monomers and polymers obtained were confirmed by FTIR, and ¹H and ¹³C NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the content of the chiral unit on phase behaviour of the polymers is discussed. Monomer M₁ showed nematic and smectic phases on cooling. The polymers P₁ and P₂ showed a nematic phase, P₃-P₅ showed cholesteric Grandjean texture, and P₆ and P₇ exhibited smectic short-rod texture. The polymers containing more than 7.2 mol % and less than 28.6 mol % of the chrial unit showed an induced cholesteric phase. Experimental results demonstrated that the glass transition, melting and clearing temperatures decreased with increasing content of the chiral unit.     

Structural variations of liquid crystalline polymers with phasmidic-type mesogens

The fixation of phasmidic-type mesogens through spacer groups to polysiloxane backbones provided a method of obtaining liquid crystalline polymers with phasmidic mesogens as side groups. The polymer 6b showed a bilayer structure in the mesophase range. The tendency to form liquid-crystalline phase depends on the number and the length of alkoxy substituents.     

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.     

Synthesis and properties of novel ferroelectric liquid crystalline polyacetylenes containing terphenyl mesogens with chiral groups

Abstract  

Novel ferroelectric liquid crystalline (FLC) polyacetylenes, namely, “side-end-fixed” type of PAM ₆ OTPhOR* and “side-on-fixed” type of PAM ₃ OCO(TPh)OR* were designed and synthesized in high yields (89.3 and 62.0%), respectively, where the terphenyl was linked at the different positions. The thermal stability of PAM ₃ OCO(TPh)OR* is better than PAM ₆ OTPhOR* owing to the “jacket effect” from terphenyl pendant linked at the waist position well protecting the main chain from the perturbations. The PAM ₃ OCO(TPh)OR* shows enantiotropic chiral smectic A phase (SmA*), but it is noteworthy that the PAM ₆ OTPhOR* exhibits enantiotropic chiral smectic C phase (SmC*) responsible for ferroelectric liquid crystallinity. Compared to “side-end-fixed” type of polymer, the “side-on-fixed” type shows better light emitting property, ascribed to the mesogen linked at the waist position has stronger tendency to enhance the main-chain coplanarity. Furthermore, the circular dichroism (CD) spectra demonstrate that the asymmetric force field generated by the chiral center affects the secondary structure of PAM ₃ OCO(TPh)OR*.     

Photochromic liquid-crystalline polymers. Main chain and side chain polymers containing azobenzene mesogens

Abstract

Two classes of thermotropic polymers were synthesized containing the trans-azobenzene unit as both a mesogenic and a photochromic group. In the former class (I) the azobenzene unit is incorporated into the main chain of substituted polymalonates, while in the latter class (II) it is appended as a side chain substituent to a polyacrylate backbone. The liquid-crystalline properties of the polymers were studied as a function of the chemical structure. All of the prepared polymers I have smectic phases. Polymers II are nematic and/or smectic, or cholesteric when including a chiral residue R'. Polymers I and II when radiated at 348 nm in chloroform solution undergo trans-to-cis isomerization of the azobenzene moiety. The calculated rate constants are comparable with those of low molar mass model compounds, and indicate that the macromolecular structure does not significantly affect the photoisomerization rate.     

Studies on thermotropic liquid crystalline polyphosphates containing photoreactive dual mesogens

A series of thermotropic liquid crystalline polyphosphates, containing photochromic molecules of stilbene in the backbone and substituted azobenzene in the side chain, were synthesized respectively. The inherent viscosity measurements were determined for all the polymers. The stability and char yield, T_g, T_m and T_i were investigated by thermal analysis. Polarizing optical microscopy of all the polymers exhibited birefringent melts with liquid crystalline behaviour. Variable temperature powder X-ray diffraction techniques were performed for confirmation of textures. UV-visible photolysis studies investigated the simultaneous behaviour of reactivity rates of crosslinking of stilbene units and isomerization, caused by azobenzene units in the main chain/side chain LCPs. Photoisomerization kinetics demonstrated the switching time rates for the trans-cis conversion of the azobenzene unit. A model polymer was synthesized and compared for the effect of stilbene in the main chain. Dipole moment values were calculated for the simulated low molecular mass of the pendant substituents to predict the polarity using MOPAC 3D Pro.     

Liquid crystalline ionomers. II. Main chain liquid crystalline polymers with terminal sulfonate groups

Liquid crystalline ionomers containing sulfonate groups on the terminal unit of the chain were synthesized by an interfacial condensation reaction of 4,4′-dihydroxy-α,α′-dimethyl benzalazine, the monofunctional dye fast yellow (FY), and a 50/50 mixture of sebacoyl and dodecanedioyl dichlorides. The weight-average molecular weights were estimated from inherent viscosity measurements to be between 6000–11,000 and the sodium sulfonate concentrations ranged from 0–18.4 meq/100 g polymer. Elemental analyses, however, indicated much higher molecular weights, which suggested that there was a distribution of chains with one, two, or no FY endgroups. The polymers were semicrystalline and melted at ca. 140°C to form nematic mesophases that were stable over a temperature range of ca. 80°C. They were thermally stable to about 350°C. The ionomeric nature of the polymers was confirmed by the presence of intermolecular associations in nonpolar solvents, as demonstrated by dilute solution viscosity measurements.     

Side-chain liquid crystalline polymers with silphenylene-siloxane main chains. II. Preparation and characterization of polymers containing biphenyl mesogens

Side-chain liquid crystalline polymers with poly(silphenylene-siloxane) backbones and 4,4′-biphenyl-containing pendant mesogenic groups have been prepared and characterized. Polymers with spacers having only three methylene groups were not liquid crystalline, LC, but those with spacers having eight methylene groups or more and a long terminal substituent were LC as indicated by an isotropization peak on the shoulder of the melting peak in the DSC thermogram and the appearance of a Schlieren texture on examination by polarized light microscopy. However, the LC behavior could not be confirmed by wide-angle X-ray diffraction, WAXD, because the crystalline pattern apparently remained up to the isotropization temperature, presumably because the melting transition and isotropization are too close. In contrast, polymethylsiloxanes with the same mesogenic side-chains revealed the presence of well-defined smectic phases by WAXD as well as by polarized light microscopy.     

Synthesis and characterization of liquid crystalline polysiloxanes containing benzyl ether mesogens

Side chain liquid crystalline polysiloxanes were synthesized by the hydrosilation of poly(methylhydrosiloxane) with p-(1-undecenyl-11-oxy) benzyl ethers of 4-cyanophenol (IM), 4-methoxyphenol (IIM), 4-cyano-4′-hydroxybiphenyl (IIIM), 4-methoxy-4′-hydroxybiphenyl (IVM), and 2-cyano-6-hydroxynaphthalene (VM). The phase behavior of both monomeric and polymeric liquid crystals was characterized by differential scanning calorimetry and optical polarization microscopy. IM is a monotropic liquid crystal, IIM is crystalline, and IIIM and IVM are enantiotropic liquid crystals, whereas VM presents two virtual liquid crystalline transitions and crystalline polymorphism. All the synthesized polysiloxanes present enantiotropic smectic mesomorphism.     

Preparation and characterization of side chain cholesteric liquid crystalline polysiloxanes containing two chiral groups

A series of liquid crystalline (LC) polysiloxanes containing diosgeninyl and menthyl groups (from monomers M ₁ and M ₂, respectively) were synthesized. The chemical structures of the monomers and polymers obtained were confirmed by elemental analysis, Fourier transform infrared spectroscopy, proton NMR and carbon‐13 NMR. The LC properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction. Monomer M ₁ showed cholesteric oily‐streak and spiral textures. Copolymers P ₂–P ₅ exhibited cholesteric phases. With increasing concentration of M ₂ units, the glass transition and clearing temperatures decreased. Experimental results demonstrated that a flexible polymer backbone and a long flexible spacer tended to favour a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.