Phase Behavior of Nematic Liquid Crystal/Star‐Polymer Systems

We investigated mixed nematic liquid crystal/star‐polymer systems and derived a new model based on Freed's lattice cluster theory to describe their phase behavior. To account for the highly oriented interactions between segments, the proposed model requires an additional parameter (δε/k) related to the energy of the oriented interaction. The nematic–isotropic transition temperature of p‐ethoxy‐benzylidene‐p‐n‐butylaniline (EBBA)/star‐polystyrene systems were determined by thermo‐optical analysis technique. The proposed model gives very good agreement with experimental data.     

Tailoring Linear and Nonlinear Optical Properties of a Side‐Chain Liquid Crystalline Azo‐Polymethacrylate

The molecular orientation in a side‐chain liquid crystalline azopolymer with push–pull chromophores ( Pol‐PZ‐CN ) is studied by UV‐vis spectroscopy, refractive index and second harmonic generation measurements. Fresh films show homeotropic arrangement, but irradiation with polarized light induces “in‐plane” birefringence, which is enhanced by subsequent thermal annealing up to values as high as 0.34. The nonlinear response of Corona poled films with “in‐plane” anisotropy is governed by three independent NLO d_ij coefficients, evidencing symmetry different from the usual (C_∞v). For irradiated films, stable values d₃₁ = 1.4, d₃₂ = 0.9 and d₃₃ = 11 pm·V^?1 are measured at 1.9 µm. A noticeable higher d₃₁/d₃₂ ratio is obtained for preirradiated and annealed films. The proposed two‐step method consisting of light irradiation and heating is an effective strategy to tailor the polar molecular orientation of nonlinear liquid crystalline azopolymers.

     

Two‐Step Smectic CA Phase Formation from Isotropic Liquid upon Supercooling in Main‐Chain Liquid‐Crystalline BB‐5(1‐Me) Polyester

The mesophase formation process of a main‐chain BB‐5(1‐Me) polyester in the supercooled isotropic‐liquid state has been studied using DSC, WAXD, and optical microscopy. The supercooled isotropic liquid follows two different SmCA formation processes. Above 85 °C, it forms the expected SmCA phase. Below, it yields a metastable SmA phase that displays a layer spacing almost twice the one of the SmCA phase. The SmA phase develops from a homogeneous nucleation followed by 3D growth and thereafter transforms to the SmCA phase. This two‐step SmCA formation involves a change in polymer chain configuration: first, the chains are folded in each two units to form the SmA phase and then they stretch to achieve the characteristic zigzag arrangement of the mesogens in the SmCA phase.

     

Phase Behaviour of New Side‐Chain Liquid Crystalline Polyoxetanes

Summary: The synthesis and thermal behaviour of two new polyoxetanes with side chains having a dimer structure mesogen‐spacer‐mesogen and a long terminal alkyl group are reported. The two polymers differ in the chemical structure of the central spacer. The polyoxetanes have been obtained by cationic ring‐opening polymerisation of an appropriately substituted oxetane ring. Both polyoxetanes studied exhibit liquid crystalline properties showing a similar sequence of phases. DSC and synchrotron experiments indicate that a highly ordered phase with a probable orthorhombic packing is observed at low temperature. On heating, this phase melts into a mesophase with hexagonal order within the smectic layers, which, on further heating, is transformed into a disordered smectic mesophase.

Synchrotron X‐ray diffraction patterns of one of the poyoxetanes in a heating experiment.     

Directing the Smectic Layer Orientation by Shear Flow in Hierarchical Lamellar‐within‐lamellar Liquid Crystalline Diblock Copolymers

A self‐assembled lamellar‐within‐lamellar structure of a side chain liquid crystalline diblock copolymer was shear aligned to induce overall alignment and to direct the smectic layer orientation within the copolymer lamellae. The copolymer consisted of a polystyrene block and a poly(methyl methacrylate) block bearing cholesteryl mesogens with only short oxycarbonyloxyethyl spacers separating the mesogens from the backbone. Upon shearing, the copolymer lamellae exhibited uniaxial alignment whereas the smectic layers of the mesogens showed coexisting perpendicular and parallel orientations with respect to the copolymer lamellae. The fraction of the parallel oriented domains could be systematically increased by tuning the oscillation frequency and strain amplitude.

     

Self‐Assembly Behavior and Optical Properties of Poly(3‐thiopheneacetate)/Dialkyldimethylammonium Complexes

Poly(3‐thiopheneacetate)/dialkyldimethylammonium complexes (PTA‐Cn) were prepared by mixing regiorandom poly(3‐thiopheneacetic acid) with dialkyldimethylammonium bromide to study the effect of their self‐assembled structures on the optical properties. The lengths of the alkyl groups in the ammonium salts were varied from decyl (C10) to octadecyl (C18) groups. These complexes showed fully developed layer structures with ordered mesophases. PTA‐C16 and PTA‐C18 with longer alkyl groups showed side‐chain crystalline phases, and PTA‐C10, PTA‐C12, and PTA‐C14 with shorter alkyl groups showed side‐chain crystalline and liquid crystalline phases at room temperature. When PTA‐C18 was heated, a melting transition from side‐chain crystalline to smectic liquid crystalline phase was observed at 41.5 °C. The maximum absorption and emission wavelengths of these complexes increased with increasing alkyl chain length indicating that the π‐conjugated structure of the regiorandom thiophene backbone changes according to the alkyl groups.

     

Lamellar Morphology of an ABA Triblock Copolymer with a Main‐Chain Nematic Polyester Central Block

A liquid crystal (LC) ABA triblock copolymer with poly(styrene) (PS) A blocks and a main‐chain nematic LC polyester B block is synthesized by atom‐transfer radical polymerization of styrene with an LC polyester macro‐initiator. The nematic LC and PS amorphous phases are segregated from each other to form lamellae with a spacing of 27 nm. The 16 nm‐thick nematic LC lamellae are significantly smaller than the contour length of the LC segment (63 nm), and the nematic director is parallel to the lamellae. The central LC segment is primarily more extended in the lamellar direction, but folds so as to meander through the LC lamella and bridges adjacent PS domains. The lamellar microstructure exhibits a reversible spacing increase of up to 31 nm with increasing temperature, suggesting a corresponding increase in the probability of the chain folding.

     

Dielectric Relaxation of Side‐Chain Liquid Crystalline Ionomers Containing Alkaline Metal Ions

Summary: Two sets of side‐chain, liquid‐crystalline ionomers based on copolymers of butyl acrylate and 4‐{[6‐(acryloyloxy)hexyl]oxy}benzoic acid containing 2.5–20 mol‐% of lithium or rubidium ions were studied. As the content of ions of alkali metals increases, segmental mobility markedly decreases. As compared with rubidium‐containing ionomers, ionomers containing lithium ions are characterized by a far more efficient decrease in their dielectric relaxation times because ionic radius of lithium ions is lower than that of rubidium ions.

     

Structure and Phase Transitions of Poly(heptamethylene p,p′‐bibenzoate): Time‐Resolved Synchrotron WAXS and DSC Studies

Time‐resolved wide‐angle X‐ray scattering (WAXS), as well as differential scanning calorimetry (DSC) and polarisation microscopy studies, were applied to investigate the structure and phase transitions of poly(heptamethylene p,p′‐bibenzoate). Temperature dependencies of several structural parameters were determined. Complete transformation from an isotropic melt to a smectic phase was suggested whereas the transition from a smectic to crystalline phase is only partial (around 30%), although it takes place from the ordered SCA phase. Crystals are formed within the SCA domains with nearly the same coherent length. On the basis of the analysis of the position and the profile of the diffuse wide‐angle X‐ray scattering and mesogenic layer spacing, it was assumed that either crystallisation modifies the smectic structure, or mesophase losses its positional order because of the lack of mobility of the spacers at low temperatures.

WAXS scattering profiles corresponding to P7MB: a) cooling from the isotropic melt at 2 °C · min^?1; b) subsequent melting at 12 °C · min^?1.     

Cholesteric Liquid‐Crystalline Side‐on Polysiloxanes: Effects of Biaxiality on the Cholesteric Structure

Summary: Biaxial side‐on cholesteric copolysiloxanes with laterally attached mesogenic groups were investigated in mixtures with a low molar mass liquid crystal. While mixtures with a low concentration of the polymer exhibit the conventional fingerprint texture, for concentrations above about 60 mol‐% characteristic irregularities appear in the cholesteric structure that are obviously due to phase biaxiality. Besides the absence of pseudo‐isotropic lines, irregular patterns perpendicular to the helix axis emerge and the periodic distance of regions with similar optical properties along the helix axis is strongly disturbed. This suggests that the photonic band gap width of a uniaxial cholesteric phase becomes strongly affected when a phase transformation into a biaxial cholesteric phase occurs.

Cholesteric fingerprint texture of a mixture of uniaxial nematic monomer and biaxial cholesteric polymer.     

Thermal,Optical and Electrochemical Properties of Side‐Chain Azopyridine Polymers Complexed with Metalloporphyrins

Summary: New multifunctional supramolecular polymers were obtained by complexing a side‐chain azopyridine polymer with ZnTPP and CoTPP via axial coordination between the pyridyl group and the transition metals of the metalloporphyrins. Thermal, optical and electrochemical properties of the complexes in the solid state at various molar ratios of metal/pyridine were characterized by means of DSC, TGA, UV‐vis spectroscopy and CV. On one hand, the results show that linking ZnTPP or CoTPP to the azopyridine polymer results in significant changes of its properties, such as an increase in the glass transition temperature and the thermal stability of the polymer, and a decrease in the trans‐cis photoisomerization of azopyridine moieties. On the other hand, both ZnTPP and CoTPP complexed with the polymer exhibit changes in their redox behaviors.

Complexation via axial coordination between PAzPy and ZnTPP or CoTPP.     

Self‐Assembly of a Hydrogen‐Bonded Association Chain Liquid Crystalline Polymer (LCP)

A hydrogen‐bonded association chain liquid crystalline polymer (LCP) based on tetraethyleneglycoxy‐bis(2,6‐dimethyl‐4‐benzoic acid) and 4,4′‐(p‐phenylenedi‐1,2‐ethenediyl)bipyridyl in a 1 : 1 stoichiometric ratio was synthesized using melting mixing method. X‐ray scattering from fiber drawn from its nematic phase exhibits preferred orientation, indicating that hydrogen bonding association chain polymer can also be strong enough to withstand the shear or tensile force and could be macroscopically oriented under shear or tensile force. Two complimentary components are likely to form a co‐crystallized structure to form a monoclinic crystal.     

X‐Ray Diffraction Studies on the Phase‐Transformational Behavior of a Smectic Liquid‐Crystalline Elastomer Composed of Chiral Mesogens

Structural changes through successive phase transformations of a chiral smectic liquid‐crystalline elastomer are investigated by X‐ray scattering technique. In uniaxially deformed elastomers, the smectic layer seemingly tilts even in the SmA phase, in which an in‐plane chevron structure formed in the tilted smectic phase. On the basis of an analysis of the layer reflection peaks, the layer correlation length in the tilted smectic phases is shorter than that in the non‐tilted SmA phase, though smectic layers in the tilted smectic phases are better ordered than those in SmA.

Experimental arrangement for X‐ray measurements of the uniaxially deformed elastomer in the tilted smectic phase at room temperature.     

Shape‐Memory Effect Controlled by the Crosslinking Topology in Uniaxially‐Deformed Smectic C* Elastomers

We have investigated the shape‐memory effects of uniaxially‐deformed, chiral, smectic C (SmC^*) elastomers for two different types of crosslinker, namely, a hydroquinone‐type crosslinker and a rod‐like crosslinker. Mesogens tilt with decreasing temperature from the SmA phase to the SmC^* phase in SmC^* elastomers synthesized with the hydroquinone‐type crosslinker. As for SmC^* elastomers with the rod‐like crosslinker, however, not mesogens but smectic layers are tilted in the smectic phases, because the crosslinker is sufficiently rigid to hinder mesogens from tilting. Because the shape change of such elastomers is coupled to the transformation of molecular alignment, SmC^* elastomers synthesized with the hydroquinone‐type crosslinker elongate with increasing temperature in SmC^* because of the decrease in molecular‐tilt angle, whereas those with the rod‐like crosslinker have an almost constant sample length in the temperature range of the SmC^* phase, despite the rearrangement of the layer structure. Both types of elastomer exhibit a reversible shape change that corresponds to the reversible change in molecular alignment during a heating and cooling process, within successive phase transitions between the isotropic phase and the smectic C^* phase.

     

Amorphous‐Smectic Glassy Main‐Chain LCPs, 1. Poly(ether esters) Derived from Hydroxybibenzoic Acid and (R,S)‐ and (R)‐2‐Methylpropane‐1,3‐diol

The synthesis and the characterization of main‐chain liquid‐crystalline poly(ether esters), derived from hydroxybibenzoic acid and (R,S)‐ and (R)‐2‐methylpropane‐1,3‐diol, are reported. These polymers show an interesting thermal behavior. They develop mesophases with a slow rate of formation, allowing the easy quenching of the melt into: a) the glassy amorphous state, b) the glassy liquid‐crystalline state, or c) a mixture of both, depending on the thermal treatment. The extent of the transformation and the symmetry of the different phases have been determined by means of calorimetric and X‐ray diffraction methods. Dielectric spectroscopy results provide additional evidence for the detection of distinct glass transitions. The results show that the racemic polymer forms a low‐ordered SmC_alt mesophase, while a more ordered phase is obtained in the case of the enantiomerically pure polymer. The comparison of the properties of the different states evidences the special behavior and properties of the glass transition (T_g) in these polymers. Emphasis is paid to the location of the T_g of the liquid‐crystalline state in comparison to the T_g of the amorphous state. It is found that the glass transition of the SmC_alt glass in R,S‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R,S)‐2‐methylpropane‐1,3‐diol) appears at lower temperatures than the glass transition of the amorphous state. However, in R‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R)‐2‐methylpropane‐1,3‐diol), where the more ordered phase is present, the glass transition follows the classical tendency of semicrystalline polymers.

     

Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

Summary: A new series of side‐chain liquid‐crystal polymers, the poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane], has been synthesized in which the spacer length is varied from 2 to 8 methylene units. The synthesis used for the chemical modification of polyepichlorohydrin (PECH) involved the phase transfer catalyzed etherification of the chloromethyl groups with sodium 4‐cyano‐4′‐biphenoxide and lithium n‐(4‐cyano‐4′‐oxybiphenyl)‐alkanoates. All the resulting polymers (except polymers 7 and 8), including that without spacer, characterized by ¹H NMR, and IR, exhibit thermotropic liquid crystalline mesomophism. The thermal behavior of the polymers has been characterized using differential scanning calorimetry (DSC) and polarized light microscopy (POM) equipped with hot‐stage. A more pronounced odd‐even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display slightly higher values, which were also dependent on the substitution degree of PECH. The flexible PECH chain assists nematic LC formation compared with other more rigid backbone polymers where a smectic phase is often encountered with the same mesogen. A comparison of the thermal properties of the cyanobiphenyl based series reported here. Polyacrylate (PA) and poly(methacrylate)‐based (PMA) SCLCPs containing 4‐cyanobiphenyl as the mesogenic unit are consistent with the general rule that increasing backbone flexibility for a given mesogenic unit and spacer length enhance the clearing temperature. This was not found from the PECH‐based series, which show the lower clearing temperature than the PMA and PA series, even though they have more flexible backbone than PA and PMA series. So the clearing temperature is not solely determined by the flexibility of the backbone.

Schlieren textures of PECHOC2‐B taken at 110 °C (cooling from isotropic phase after annealing 1 h).     

Molecular‐Recognition‐Induced Phase Transitions of Two Thermo‐Responsive Polymers with Pendent β‐Cyclodextrin Groups

PNIPAM‐based thermo‐responsive polymers with pendent β‐cyclodextrin groups were synthesized and the molecular‐recognition‐induced phase‐transition behavior of fabricated polymers was investigated. The results showed that the thermo‐sensitive PNG‐ECD and PNG‐HCD polymers could significantly recognize the guest ANS molecules, and their LCSTs in ANS aqueous solutions were lower than those in blank aqueous solution. The more ANS could be recognized by the polymer, the lower was the LCST of the polymer. The guest NS molecules had an opposite influence on the LCSTs of the PNG‐ECD and PNG‐HCD polymers, because the complexation between CD and NS slightly enlarged the hydrophilic moiety of the polymers.

     

Influence of Smectic Liquid Crystallinity on Lamellar Microdomain Structure in a Main‐Chain Liquid Crystal Block Copolymer Fiber

The microlamellar and smectic liquid crystal (LC) structures of a block copolymer of a main‐chain LC polyester connected at both ends with poly(ethyl methacrylate) are investigated by fiber X‐ray scattering. In the as‐spun fiber, the lamellae are parallel to the fiber axis, while the smectic layers are perpendicular to it. Annealing the as‐spun fiber at a temperature higher than the isotropization temperature (T_i) of the LC segment preserves the lamellae, but the LC structure disappears. Further annealing the fiber at T < T_i improves the lamellar stacking coherence and aligns the smectic layers parallel to the lamellae. In contrast, annealing the as‐spun fiber at T < T_i conserves the smectic layers and arranges the lamellae in parallel to the smectic layers. Thus, the liquid crystallinity affects the lamellar ordering and orientation.

     

Orientation of the Main Chain in Liquid‐Crystalline Side‐Chain Polyesters with Variation of Main‐Chain Spacer Length

Previous X‐ray investigations on liquid‐crystalline side chain polyesters with variable spacer length in the side chain as well as in the main chain led to the assumption that the structure will change qualitatively if the main chain spacer length exceeds a certain value. We obtained indications supporting this assumption by applying two‐dimensional ¹³C NMR. It was shown that for a sample with main chain spacer shorter than the critical length, the main chain segments align perpendicular to the side chains (〈P₂〉 = ?0.46), and for spacer length larger than critical length, the main chain segments preferentially orient themselves parallel to the side chain (〈P₂〉 = 0.28).

Visualization of the main‐chain order as revealed by the values of Table 2 . The opening angle of the cones corresponds to the average angle of deviation as defined in the text.