Mesomorphic Phase Research Articles

Ferroelectric and antiferroelectric chiral multilactate liquid crystalline materials with negative dielectric anisotropy

In this work, the mesomorphic, electro-optic and dielectric properties have been discussed in the light of molecular structure-property correlations of two chiral multilactate liquid crystalline materials possessing the orthogonal paraelectric Smectic A* phase, tilted ferroelectric Smectic C* phase and the tilted antiferroelectric Smectic CA* phase, over a substantially broad temperature range. Interestingly, a reasonably rare re-entrant Smectic C* phase (SmCre*) has also been identified in one of the investigated materials. These materials differ in their linkage groups (keto or ether) and an additional chiral unit in the terminal chain. The phase transition temperatures and transition enthalpies were determined from Polarizing Optical Microscopy and Differential Scanning Calorimetry (DSC) measurements. The compounds exhibit negative dielectric anisotropy (Δε) throughout the mesomorphic range (maximum −18 and −4 in SmA* for both the compounds) and moderately high values of spontaneous polarization (∼153 nC/cm2 in SmCre* phase). The temperature dependence of the response time (τ), bulk viscosity (η) and the activation energies (Ea) throughout the mesomorphic phase have been determined from the spontaneous polarization measurements. To emphasize the structure-property correlations in more detail, dielectric spectroscopy measurement has also been performed to measure the dielectric strength, dielectric loss, frequency dependent permittivities, relaxation time and relaxation frequencies. The clear evidence of the relatively rare SmCre* phase has also been confirmed from the temperature and frequency dependence of the dielectric permittivity. These results shed important light on the emergence of these materials as a smart alternative for their application in multicomponent mixtures targeted for advanced electro-optic and photonic devices.

Understanding the Impact of Chain Mobility on Conformational Evolution and Kinetics of Mesophase Formation in Poly(ʟ-lactide) under Low-Pressure CO2.

Although the mesomorphic phase as an intermediate state has been introduced to understand polymer crystallization, the understanding of the mesomorphic phase is far from complete. Here, the effect of chain mobility on the mesophase structuring in melt-quenched poly(ʟ-lactide) (PLLA) treated in low-pressure CO2 at 1.6-2.0 MPa and 0 °C was investigated using infrared (IR) spectroscopy, differential scanning calorimetry (DSC), and atomic force microscopy (AFM). The IR and AFM results demonstrated that the final degree of order and the kinetics of structural evolution during the CO2-induced mesophase formation were critically dependent on the CO2 pressure. This was attributed to the distinct dynamics of conformational evolution (gg to gt conformer transition) due to the different CO2 pressures. The thermal behavior from the DSC results showed that CO2 pressure dominated both the scale and dynamics of the chain motion of PLLA. At a lower CO2 pressure of 1.6 MPa, smaller-scale segmental motion was not replaced by the larger-scale cooperative motion that occurred at a relatively higher CO2 pressure of 2 MPa, which was favorable for faster mesophase formation. Consequently, by inhibiting direct crystallization under limited mobility conditions, it was demonstrated that different chain mobility controlled by CO2 pressure and thus CO2 solubility impacted the dynamics of the mesophase formation of PLLA. The present results have implications for understanding the role of chain mobility in determining the intermediate structural phases in semicrystalline polymers.

Dynamic change of molecular conformation generating a variation of liquid crystalline phases for triphenylene mesogens with peripheral alkoxyazobenzenes

ABSTRACT A triphenylene mesogen which possesses six dodecyloxyazobenzene units at the periphery linked by propylene-ether chain (2-C12-3) was synthesised and its mesomorphic phase transition behaviour was investigated. It was found that 2-C12-3 exhibits six mesophases including one monotropic phase. The calamitic-discotic bi-mesomorphic phase transition was also observed. As cooling the compound at the temperature of iso phase, SmA phase comes up and further cooling gives Col ob phase. In comparison to the corresponding ester-linked derivative (1-C12-3), which shows only two mesophases (SmA and Colr), the linkage part connecting triphenylene unit and the peripheral alkoxyazobenzenes significantly affect the molecular ordering in mesophase, though the bi-mesomorphic property is maintained in the case of dedecyloxy chains.

Naphthalene-based symmetrical liquid crystalline dimers: Synthesis, characterization, mesomorphic behaviour and DFT studies

The reactions of different α,ω-dibromo alkanes with (E)-4-(naphthalen-2-yldiazenyl)phenol and (E)-4-(naphthalen-2-ylimino)methyl)phenol, respectively, led to the synthesis of two new homologous series of liquid crystalline symmetrical dimers with azobenzene or azomethine central linkage and containing the naphthalene moiety as a rigid core. Spectroscopic techniques like FT-IR, 1H NMR, 13C NMR and elemental analysis confirmed their structure. These dimers exhibited mesomorphic phases, studied through POM and DSC. Their photophysical behaviour was explored using UV–visible spectroscopy. Remarkably, dimers with even spacer units (n = 2, 4, 6, 8, 10, 12) displayed excellent liquid crystalline properties as nematic phases, with extended mesophase lengths and thermal stability for both series. The study revealed the vital role of spacer length in determining liquid crystalline behaviour. The computational studies employed the B3LYP method and electron density method for optimization.

High-Δn tolane-liquid crystal diluters with low melting point and low rotational viscosity

High birefringence (Δn) and low melting point (Tm), high Δn and low rotational viscosity (γ1) are two pairs of contradictory performance parameters. In this work, two high-Δn LC diluters 3PTPF2V and 3PTPFOV are proposed to achieve a fine balance between them. These target compounds containing a rigid tolane core, a lateral fluorine substituent and a buty-3-enyl or allyloxy terminal group were further synthesized. Some experimental instruments, such as POM, DSC, Abbe refractometer, multi-channel liquid crystal evaluation system, were adopted to investigate their properties of mesomorphic phase behavior, Tm, Δn and γ1 by comparing with classical LC diluters 3HHV and V2PTP2V. Meanwhile, density functional theory (DFT) calculations were employed to correlate the experimental results, and their comprehensive effects on two high-Δn LC mixtures were further evaluated. Research results indicated that both target compounds possess low Tm (∼20 °C), low γ1 (∼6.5 mPa·s), and high Δn (>0.26). Performance in high-Δn LC mixture demonstrated that they could effectively decrease Tm and γ1, meanwhile maintain high Δn and Tc values, meaning their potential application prospects in fast-response LC optical devices.

Morphology of impact polypropylene copolymer extruded cast film revealed by confocal Raman imaging.

An impact polypropylene copolymer (IPC), composed of polypropylene (PP) and ethylene-propylene copolymer (EPC), was synthesized through two-stage in-reactor polymerization. A systematic investigation of the crystalline structure, thermal behavior, morphology, and tensile properties of the IPC extruded cast film was conducted. Specifically, the morphology of EPC was obtained by confocal Raman imaging by depicting the spatial distribution of the Raman band located at 1064 cm-1. The EPC phase exhibits fibrous morphology with the long axis aligning along the machine direction (MD). A three-dimensional (3D) heterogeneous structure of the IPC cast film obtained by confocal Raman imaging confirms that the fibrous EPC phase is dispersed in a 3D framework of the PP matrix. The mesomorphic phase in the as-prepared cast film transforms to a stable α-form crystal after annealing at 130 °C, which improves the yield strength but decreases the elongation of the cast film. The WAXD and SAXS results indicate that there is no obvious orientation of the crystallites. Thus, the anisotropy of tensile properties in the MD and transverse directions is closely related to the anisotropic phase morphology at the micrometer scale. The results reveal that the mechanical performances of IPC films are determined by the crystalline structure of the PP matrix and the morphology.

Crystallization Regulation by Self‐Assembling Liquid Crystal Template Enables Efficient and Stable Perovskite Solar Cells

AbstractIt is found that the disordered growth of bottom perovskite film deteriorates the buried interface of perovskite solar cells (PSCs), so developing a new material to modify the buried interface for regulating the crystal growth and defect passivation is an effective approach for improving the photovoltaic performance of PSCs. Here, we developed a new ionic liquid crystal (ILC, 1‐Dodecyl‐3‐methylimidazolium tetrafluoroborate) as both crystal regulator and defect passivator to modify the buried interface of PSCs. The high lattice matching between this ILC and perovskite promotes preferential growth of perovskite film along [001] direction, while the oriented ILC with mesomorphic phase has a strong chemical interaction with perovskite to passivate the interface defect, as a result, the modified buried interface exhibits suppressed defects, improved band alignment, reduced nonradiative recombination losses, and enhanced charge extraction. The ILC‐modified PSC delivers a power conversion efficiency of 24.92 % and maintains 94 % of the original value after storage in ambient for 3000 h.

Crystallization Regulation by Self-Assembling Liquid Crystal Template Enables Efficient and Stable Perovskite Solar Cells.

It is found that the disordered growth of bottom perovskite film deteriorates the buried interface of perovskite solar cells (PSCs), so developing a new material to modify the buried interface for regulating the crystal growth and defect passivation is an effective approach for improving the photovoltaic performance of PSCs. Here, we developed a new ionic liquid crystal (ILC, 1-Dodecyl-3-methylimidazolium tetrafluoroborate) as both crystal regulator and defect passivator to modify the buried interface of PSCs. The high lattice matching between this ILC and perovskite promotes preferential growth of perovskite film along [001] direction, while the oriented ILC with mesomorphic phase has a strong chemical interaction with perovskite to passivate the interface defect, as a result, the modified buried interface exhibits suppressed defects, improved band alignment, reduced nonradiative recombination losses, and enhanced charge extraction. The ILC-modified PSC delivers a power conversion efficiency of 24.92 % and maintains 94 % of the original value after storage in ambient for 3000 h.

Identifying a Mesomorphic Boundary Zone within Polybutene-1 Single Crystals Emerging During a Crystal–Crystal Transformation

We present experimental evidence that the transformation from one solid phase to another solid phase within a faceted polymer single crystal passes through a transient mesomorphic phase. This phase exists in the boundary zone separating the two crystalline phases of isotactic polybutene-1 (PB-1), i.e., Form II and Form I of PB-1. Employing real-time atomic force microscopy in tapping mode (TM-AFM), we followed the crystal–crystal transformation in time. TM-AFM measurements performed at about 20 °C under a nitrogen atmosphere with high humidity have allowed through differences in viscoelastic properties and associated changes in thickness to distinguish the two crystalline phases as well as the boundary zone. Measurements at an elevated temperature of 90 °C (which was far below the melting temperatures of both Form II and Form I crystals) further revealed that this boundary zone shows a transient and diffuse character and a distinctly increased thickness. Using high-resolution phase imaging of TM-AFM, we observed that within this rather extended boundary zone of up to some hundred nanometers at elevated temperatures, Form I crystals with a diameter of around 30 nm were growing in a softer and featureless matrix phase. The boundary zone and so the growth front of the Form I crystal advanced at an almost constant rate, suggesting that chains in the mesomorphic phase were considerably mobile. Our observation of an extended boundary zone probably indicates that compared to a direct crystal–crystal transformation, the corresponding free energy barrier for this transformation is lower within such a zone. Hence, our here presented model study sheds light on the poorly understood and complex pathways involved in a solid–solid transformation within a polymer single crystal.

Room-temperature triphenylene-based discotic liquid crystal dimers: effects of amide linkage and central fluoromethylene spacer

ABSTRACT Two series of triphenylene-based dimers both having a central fluoromethylene spacer of four or six carbon atoms and ester or amide linkages were synthesised and their phase behaviour investigated. All the synthesised dimers are room-temperature discotic liquid crystals and exhibit hexagonal columnar mesophases that supercool into a stable anisotropic columnar glass. The length of the fluoromethylene spacer has a minor effect on the phase transition temperatures of these dimers. However, dimers 1a and 2a with a (CF2)4 spacer exhibit approximately 40°C higher clearing points and wider mesomorphic phase ranges compared with their hydrocarbon analogs 1c and 2c with a (CH2)4 spacer respectively. Compared to the corresponding ester-linked dimers 1a-c, three amide-linked dimers 2a-c exhibit 20°C higher clearing points upon cooling. The observation indicates not only that the introduction of the fluoromethylene spacer stabilised the mesophase in these discotic dimers, but also that the two interactions have synergistic effects on columnar packing in this system. Moreover, only amide-linked dimers exhibit good gelation property in a mixture of toluene and petrol ether, with a lower critical gelation concentration of 1.5 wt/vol%, thus indicating that gel formation occurred under the collaboration of hydrogen bonding and intrinsic π interactions in these dimers.

GROWTH, STRUCTURE, AND PHASE BEHAVIOR OF DI-TERT-BUTYL-PARA-TERPHENYL CRYSTALS

The results of studying the growth of para-terphenyl derivative (4,4"-di-tert-butyl-para-terphenyl (tBu-3P-tBu)) crystals are presented. The solubility of this compound in toluene at 20°С has been established by spectrophotometry. Using the techniques of growth from solutions and physical vapor transport,tBu-3P-tBu single crystals up to 1 cm long have been obtained for the first time. Their structure at 85 K has been interpreted in the triclinic system, sp. gr. P1 (Z = 8), using single-crystal X-ray diffraction. Flat rectangular crystals with the best morphological characteristics have been grown from vapor. The developed face of these crystals exhibits elementary growth steps 1.4 nm high, corresponding to molecular monolayers oriented in the (001) plane. The presence of a polymorphic transition at 229.2°C and mesomorphic liquid crystal phase above the melting temperature (255.6°С) is found.

Synthesis of some new Heterocyclic Compounds and Schiff bases of 4-phenyl-3-ene-butanone and study their liquid crystal properties

A new heterocyclic compounds(II-V)were synthesized through a reaction of compound(I) with urea,thiourea,hydrazine hydrate, and hydroxylamine hydrochloride and series of Schiff bases(VIa-d) and (VIIa-d) were synthesized from( II) and (III).The prepared compounds were characterized by (FT-IR) spectrum. Physical properties of the prepared compounds were recorded and their mesomorphic phases were investigated.

Impact of ferroelectric nanoparticles on the ordering behavior and electric response of a liquid crystal near the isotropic-smectic A transition

ABSTRACT We present a theoretical study of the electric behaviour of a ferroelectric liquid crystal (FLC) doped with ferroelectric nanoparticles (FNPs). According to the recent reports on industrial developments based on liquid crystal materials, FLCs are increasingly used not only in displays but also in preparing new materials with unconventional properties. In this work, we focus on the anomalies caused by the nanoparticles on ordering behaviour, dielectric permittivity, and polarization of the system, as well as the pre-transitional effects near the Isotropic-Smectic A (I-SmA) phase transition. We obtain the polarization of a nano-doped system under an applied electric field and discuss the influence of the concentration of nanoparticles on the phase transition for both isotropic and mesomorphic phases. The calculations are based on the phenomenological Landau theory. Our results show weak induced orientational order in the isotropic phase of the I-SmA phase transition. Considering the Kerr effect in the nano-doped system under the influence of macroscopic polarization, we find that the FNPs significantly affect such induced orientational behaviour. Our results confirm a pre-transitional anomaly of dielectric permittivity for the I-SmA phase transition. The strong influence of nanoparticles on the pre-transitional behaviour in both isotropic and mesomorphic phases is also revealed.

The simultaneous control over the alternating sequence and the inner-core position of the mesogenic segments in side-chain liquid crystal polymers

Hydrosilylated 1,1-diphenylethylene (DPE-SiH/2SiH) are herein quantitatively and alternatingly incorporated during the living anionic copolymerization of isoprene with DPE-SiH/2SiH, which can efficiently control the Si-H densities and positions. The diversified mesogenic moieties containing azobenzene (Azo) or trifluoromethyl (F3C) groups, respectively, with the tuned inner-core position of the mesogenic segments are rationally designed and then fabricated into multiple alternatingly-arranged side-chain liquid crystal polymers (SCLCPs) along the alternating backbones. This study presents the novel SCLCPs with the simultaneous control over the sequence and the inner-core position of the mesogenic segment, providing the promising route and study model to develop the mechanistic investigations. The temperature range of mesomorphic phase formation (ΔT) and polarized optical morphologies are proved to be synergistically dependent on the diversified mesogenic moieties and the exactly controlled densities of the mesogenic segment.

Synthesis and properties of fluorinated terphenyl liquid crystals utilizing 5,6-dihydro-4H-cyclopenta[b]thiophene as core unit

5,6-dihydro-4H-cyclopenta[b]thiophene was incorporated to fluorinated terphenyl and six novel liquid crystals were designed and synthesized by Suzuki coupling reactions in high yields. These mesogens possessed wide smectic A or nematic phase ranges, which proved the usefulness of this core in the formation of liquid crystal phase as 1,4-disubstituted phenylene-based reference compound have no mesomorphic phase. Besides, the birefringence and dielectric anisotropy values of liquid crystals were also increased, making this core a practical candidate for the modification of terphenyl-typed mesogens. The density functional theory calculations were then used and whose reliability was ensured by comparison of the experimental and calculated absorption spectrum. The calculated length/width ratios of mesogens were improved after the introduction of this core unit, which might promote the formation of mesomorphic phases. The orders of calculated polarizability and dipole moment of two representative mesogens and their references were in agreement with the sequences of their birefringence and dielectric anisotropy values, respectively, providing some relationship information between microscopic molecular structures and macroscopic physical properties.

Multistage Structural Ordering and Crystallization of Poly(trimethylene terephthalate) during Sub-Tg Stretching: Synergetic Effects of Chain Orientation and Conformational Transition

Crystallizable glassy polymers can undergo multistage structural evolutions and form multiple types of ordered phases (e.g., mesomorphic and crystalline phases) during stretching below their glass transition temperatures (Tg). However, the structural evolution of polymers during sub-Tg stretching and the associated molecular mechanisms have not been well understood. In this work, we investigated the multistage structural ordering of glassy polymers and the related mechanisms during stretching below Tg by studying poly(trimethylene terephthalate) (PTT) as a model system. Roles of chain orientation and conformational transition on the multistage structural evolution of melt-quenched PTT were elucidated through a loading–unloading experiment. When stretched below Tg (e.g., 25 °C), the orientation of polymer chains and the stacking of phenylene rings lead to an intermolecular ordering, which induces the generation of a smectic C-like mesomorphic phase. The orientation degree of chains in such a mesomorphic phase increases with further stretching. This highly oriented mesomorphic phase transforms into a triclinic crystalline phase when the applied stress is unloaded. Stretching at a higher temperature (e.g., 40 °C) results in the formation of a more ordered mesomorphic phase and facilitates the induced crystallization due to the enhanced chain mobility. Fourier transform infrared results demonstrated that the unloading-induced crystallization of stretched PTT is originated from the trans-gauche conformational transition.

Oxadiazolyl-Pyridinium as Cationic Scaffold for Fluorinated Ionic Liquid Crystals

The synthesis and characterization of a new class of 1,2,4-oxadiazolylpyridinium as a cationic scaffold for fluorinated ionic liquid crystals is herein described. A series of 12 fluorinated heterocyclic salts based on a 1,2,4-oxadiazole moiety, connected through its C(5) or C(3) to an N-alkylpyridinium unit and a perfluoroheptyl chain, differing in the length of the alkyl chain and counterions, has been synthesized. As counterions iodide, bromide and bis(trifluoromethane)sulfonimide have been considered. The synthesis, structure, and liquid crystalline properties of these compounds are discussed on the basis of the tuned structural variables. The thermotropic properties of this series of salts have been investigated by differential scanning calorimetry and polarized optical microscopy. The results showed the existence of an enantiotropic mesomorphic smectic liquid crystalline phase for six bis(trifluoromethane)sulfonimide salts.

Synergistic Effect of Pressurization Rate and β-Form Nucleating Agent on the Multi-Phase Crystallization of iPP.

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and β-form nucleating agent (β-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and β-NA enables the preparation of a unique multi-phase crystallization of iPP, including β-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6–1.9 MPa/s, a significant competitive formation between β- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both β- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, β- and γ- iPP coexist with each other. Moreover, with different β-NA contents, the best pressurization rate for β-iPP growth is the same as 1.9 MPa/s, while more β-NA just promotes the content of β-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of β-iPP, it shows that β-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

Photodeformable Liquid Crystalline Polymers Containing Functional Additives: Toward Photomanipulatable Intelligent Soft Systems

Deformable liquid crystalline polymers (LCPs) have been an attractive topic for years because of their great potential in intelligent soft systems such as artificial muscles, soft robotics, and smart optical devices. However, the application of conventional thermal‐responsive LCPs is usually limited by the low thermal conductivity of the polymer matrix and the high dependency on external heating device. In comparison, light‐controlled methods have many advantages, including being noncontact, remote in situ, and the ability to manipulate the spatial resolution of LCPs, which facilitate the development of diverse untethered and remotely manipulatable intelligent soft devices. Recently, remarkable progress has been made in the development of photodeformable LCPs by incorporation of organic or/and inorganic photoresponsive components as functional additives because the diverse functions of the incorporated components can be facilely combined with the directional shape‐morphing behaviors of LCPs in these systems via the interplay among the functional building blocks, mesomorphic phases, and polymer matrices. This review focuses on the design strategies, manufacturing methods, and working principles of photomanipulatable LCPs with incorporated photosensitive organic dyes or/and inorganic nanocomponents. Their possible applications and future developments are also briefly summarized.

Electrically switchable and optically tunable liquid crystal/ silicone oil based non-aqueous emulsions for electro-optical device applications

Liquid crystal/silicone oil based emulsions have been prepared and characterized for interfacial interactions, optical and electro-optical behaviour. Emulsions are stabilized without any surface-active agent via non-covalent interactions, surface alignment and anchoring conditions. Emulsions are characterized by polarizing optical microscopy. Distinct mesomorphic phases, droplet geometries (radial, hedgehog, bipolar) and transitions viz. radial to bipolar and bipolar to broken bipolar are observed as a function of varying silicone oil concentration in liquid crystal. These emulsions are quite stable and show reversible characteristics as a function of time and temperature. The stability of these emulsions has been explained considering hydrophobic interactions, decrease in internal surface tension and anchoring energy etc. Further, the liquid crystal/silicone oil devices have been fabricated and tested for switching and electro-optical behaviour. The emulsion contains 25% silicone oil in liquid crystal matrix shows the best switching characteristics at low deriving voltage 5 V, about 87% enhancement in optical transmission, 8.3 dB contrast and lower Frederiks threshold ~65%. Such switchable and optically tuneable surfactant-free emulsions with controlled morphological dynamics can provide a new door to design novel functional soft material platform for better optical, electro-optical, photonics, sensing and liquid crystals elastomer applications. • Surfactant-free, electrically switchable and optical tuneable liquid crystals/silicone oil emulsions. • 87% enhancement in transmission, 8.3 dB contrast and lower Fredrick's threshold ~65% has been achieved in emulsion systems. • Tuning of optical and electro-optical parameters depends on the boundary conditions, topological deformations.