Main-chain Liquid Crystalline Research Articles

Flexible films of main-chain luminescent liquid crystalline polymers containing renewable biobased bifuran moieties

Luminescent liquid crystalline polymers (LLCPs) have attracted great attention due to their potential applications in liquid crystal display, optical memory storage, organic light-emitting diode, and other fields. However, the fluorescence signals of LLCPs bearing conventional luminophores showing aggregation-caused quenching behavior are strikingly diminished or even vanish in the aggregated state. Herein, we have synthesized three main-chain LLCPs with α-cyanostilbene (CS) and bifuran units connected by different flexible spacer lengths via melt polycondensation. On the one hand, the CS unit acts as a rigid rod-like mesogen with aggregation-induced emission (AIE) characteristics. On the other hand, the renewable biobased bifuran moiety has been chosen because of two main reasons. One is it is a perfect alternative to the rigid fossil-based biphenyl group. The other reason is the addition of bifuran moiety is beneficial for the polycondensation and the extension of the chain length, leading to the enhancement of the toughness of LLCPs. Upon 365 nm UV light irradiation, these polymers undergo an irreversible Z/E isomerization in solution, while the [2 + 2] cycloaddition of CS chromophores occurs in the solid film. Importantly, the polymers display a good film-forming ability, so that the flexible thin films can be achieved by ordinary compression molding. Moreover, the fluorescent patterns can be directly printed on the flexible films on the basis of the [2 + 2] cycloaddition behavior, which shows their promising applications ranging from information storage to high-tech anti-counterfeiting.

Fully Room Temperature Reprogrammable, Recyclable, and Photomobile Soft Actuators from Physically Cross-Linked Main-Chain Azobenzene Liquid Crystalline Polymers.

Fully room temperature three-dimensional (3D) shape-reprogrammable, recyclable, and photomobile azobenzene (azo) polymer actuators hold much promise in many photoactuating applications, but their development is challenging. Herein, we report on the efficient synthesis of a series of main-chain azo liquid crystalline polymers (LCPs) with such performances via Michael addition polymerization. They have both ester groups and two kinds of hydrogen bond-forming groups (i.e., amide and secondary amino groups) and different flexible spacer length in the backbones. Such poly(ester-amide-secondary amine)s (PEAsAs) show low glass transition temperatures (Tg ≤ 18.4 °C), highly ordered smectic liquid crystalline phases, and reversible photoresponsivity. Their uniaxially oriented fibers fabricated via the melt spinning method exhibit good mechanical strength and photoinduced reversible bending/unbending and large stress at room temperature, which are largely influenced by the flexible spacer length of the polymers. Importantly, all these fibers can be easily reprogrammed under strain at 25 °C into stable fiber springs capable of showing a totally different photomobile mode (i.e., unwinding/winding), mainly owing to the presence of low Tg and both dynamic hydrogen bonding and stable crystalline domains (induced by the uniaxial drawing during the fiber formation). They can also be recycled from a solution at 25 °C. This work not only presents the first azo LCPs with 3D shape reprogrammability, recyclability, and photomobility at room temperature, but also provides some important knowledge of their structure-property relationship, which is useful for designing more advanced photodeformable azo polymers.

Versatile Mechanochromic Sensor based on Highly Stretchable Chiral Liquid Crystalline Elastomer.

A mechanochromic strain sensor that is capable of distinguishing the orientation, the location, and the degree of deformation based on the highly stretchable membrane of main-chain chiral liquid crystalline elastomer (MCLCE) is proposed. The MCLCE film is designed to exhibit uniform and significant color shift upon the small strain by using step-growth polymerization of liquid crystal (LC) oligomer and its phase-stabilization in solvent mesogen. As conformally placed on the bottom elastomer sheet, the MCLCE film shows multimodal, instantaneous color change for sensing arbitrary in-plane deformation, out-of-plane bending, and nonzero Gaussian deformation. Based on high freedom in the device design, it is also demonstrated that this sensor can display color patterns or encrypted images in response to the localized weight or strain. The simple and straightforward concept proposed here can be applicable in the fields of wearable devices, displays, and soft robotics.

Actuation performances of catkin fibers reinforced thiol-acrylate main-chain liquid crystalline elastomer

ABSTRACT Liquid crystalline elastomers (LCEs) have been utilized as an important class of smart actuator materials. However, the modest actuation mechanical and robustness performances remain a challenge. Inspired by the specific structures, well mechanical properties and physico-chemical characteristics of some natural plant fibers, a composite of thiol-acrylate main-chain LCE matrix incorporated with catkin fibers is designed and developed. The catkin fibers build a network as reinforcement phase, and demonstrate effective compatibility and integration property with the matrix, their high flexibility can be adapted to the large deformational performance of LCE matrix. The prepared LCE composite demonstrates strong mechanical actuation properties. The modulus and driving force triggered by the stimuli are obviously increased. The tensile strength and fatigue failure resistant property under high loadings and repeated cycles of thermal actuation or photothermal actuation are greatly enhanced. While the stimulus response deformation rate, phase transition temperature and liquid crystal phase structure of the LCE matrix, and so on, do not weaken or change. This work promotes the LCE materials’application potential and broadens the application value of natural plant fibers.

Main‐chain liquid‐crystalline polymers bearing different anion

ABSTRACT In this paper, d-camphoryl chloride, terephthaloyl chloride, hydroquinone and 2,3-dibromosuccinyl chloride were used as raw materials to synthesise the main-chain liquid crystalline polymers by condensation polymerisation. A series of main-chain ionic liquid crystalline polymers with triphenylphosphine as the cationic group were successfully prepared by grafting triphenylphosphine onto the liquid crystalline polymer. By characterisation, we found that the main chain ionic liquid crystal polymer containing quaternary phosphonium ions had low glass transition temperature, high ionic conductivity and strong antibacterial properties. The quaternary phosphonium ions-containing main-chain ionic liquid-crystalline polymers show Smectic A phase, and the main-chain liquid crystalline polymers show nematic phase. The antibacterial experiments showed that the quaternary phosphonium ions-containing main-chain ionic liquid-crystalline polymers had good antibacterial properties, and the antibacterial ability of the polymer was also enhanced with the increase of phosphorus content. In addition, the orientation structure of liquid crystal also had a significant influence on its antibacterial properties.

Competition between Circularly Polarized Light and Molecular Chirality in the Assembly of Main-chain Liquid Crystalline Polymers

Abstract Polymeric systems responding to chiral information are actively pursued for applications in optoelectronics and photonics. Here we report on the diastereomeric relationship between chiral liquid crystal polymers and circularly polarized light (CPL). We synthesize a chiral copolymer incorporating photoisomerizable diazobenzene moieties as well as isosorbide-based monomers. The circular dichroism of the film can be inverted by irradiation with CPL indicating that the influence of the chiral subunits on the helicity of the cholesteric arrangement can be overruled by the action of CPL. By following the out-of-equilibrium photo-switching process over time, we demonstrate that the formation of helical ordering in the chiral liquid crystal phases is much faster by CPL irradiation than by heat.

Main-chain side-chain combined liquid crystalline polymers bearing azobenzene and biphenyl-based mesogens – segregation of the mesogens within two distinct layers

ABSTRACT Two different mesogenic units, namely biphenyl and azobenzene, were installed centrally within the polymer backbone and pendant alkylene segments; the purpose was to examine if chain folding of the backbone occurs so as to segregate the azobenzene and biphenyl units in adjacent layers of a smectic-type ordering. Two sets of isomeric main-chain side-chain liquid crystalline polymers (MCSC-LCPs), namely SC-AzoCx-MC-BPCx and SC-BPCx-MC-AzoCx (x=6 or 10), were synthesised; the former series has biphenyl in the backbone and azobenzene in the side-chain, while their locations are switched in the latter. It was observed that all four polymers were semicrystalline at room temperature and exhibited a smectic mesophase upon melting. DSC and X-ray scattering studies revealed that switching the locations of the azobenzene and biphenyls had little effect on; the phase transition temperatures and smectic layer spacings. A second series, namely SC-AzoCx-MC-BPCy and SC-BPCx-MC-AzoCy, where the length of alkylene spacers in the side-chain (Cx) and backbone (Cy) is different, was also studied; this was done to examine the influence of unequal alkylene segment lengths on the mesophase structure and interlayer spacing, and importantly, whether the zigzag folding-induced formation of a layered structure is retained, despite their unsymmetrical nature.

Unconventional Approaches to Prepare Triazine-Based Liquid Crystal Dendrimers.

Most triazine-based liquid crystalline (LC) dendrimers reported thus far are the main-chain LC macromolecules with long flexible chains at their periphery and attached to internal rigid or semi-rigid frameworks. Their formation of mesogenic phases often depends on the intermolecular face-to-face π–π interactions between dendritic molecules, which are unusual. Their mesogenic phases can also be formed by incorporation of mesogenic units to the dendritic skeletons through long flexible chains, as most side-chain LC dendrimers, in which the peripheral mesogenic units generally play the important roles. For main-chain triazine-based dendrimers, their morphology is maintained by restricted freedom of rigid or semi-rigid connecting units, and their formations of LC phases are therefore not straightforward to be controlled. In this review, we thus describe modulating of the intermolecular face-to-face π–π interactions between the triazine-based dendritic molecules, with the aim of forming LC phases through molecular design.

Simulation of main chain liquid crystalline polymers using a Gay-Berne/Lennard-Jones hybrid model

Main chain liquid crystalline polymers (MCLCP) offer strong potential to exhibit auxetic properties. However, the first synthesized molecules did not show the expected results. To address this issue, simulation provides an interesting advantage as it can grasp the molecular reasons why the auxetism is not reached, and thus it can guide the synthesis of successful candidates. However, the crucial step of validation is mandatory. In this study, we report the simulation of MCLCP at the coarse-grained level. Interactions are represented by an hybrid Gay-Berne/Lennard Jones (GB/LJ) potential which is particulary adapted in the simulation of anisotropic systems. Energies stemming from different configurations of pairs of ellipsoids are computed at the MP2 level, and are fitted using the GB potential. The methylene groups are represented by spheres whose interactions are depicted by the LJ potential. Molecular dynamics in the NPT ensemble (number of particles, pressure and temperature are kept constant) are thus carried out beginning at 800 K to 375 K, by cooling the system by 25 K steps. It is shown that the smectic A phase emerges at about 500 K, and the smectic B phase at about 425 K. Such mesophases occur without the use of any external stimulus. These findings pave the way to simulate LCP which have been specifically designed to potentially exhibit auxetic properties.

Main-Chain Liquid Crystalline Hydrogels that Support 3D Stem Cell Culture.

Hydrogels are frequently utilized as three-dimensional matrices for the culture and regeneration of soft tissues, but one challenge with the existing hydrogels is that, though the natural extracellular matrix of tissues may be ordered, there are few biocompatible ways to incorporate anisotropy within hydrogels. Liquid crystalline (LC) polymers are well suited for this because of their combination of molecular ordering and polymer elasticity; however, the hydrophobic nature of LC monomers has hindered their polymerization into hydrogels under cytocompatible conditions. This work reports on the generation of main-chain LC hydrogels in aqueous media and the ability of LC phases to affect mesenchymal stem cell behavior. The synthesis results in high gel fraction materials, and calorimetry, thermomechanical analysis, and X-ray scattering show that the networks organize into LC phases in the dry and hydrogel states. Human mesenchymal stem cells (hMSCs) cultured within the hydrogels show excellent viability, and hMSC proliferation proceeds at a faster rate in LC hydrogels compared to non-LC hydrogels. TThe result is a new synthetic approach for calamitic liquid crystalline hydrogels, which support the encapsulation and culture of human stem cells and are expected to enable applications as anisotropic and responsive substrates for tissue engineering and regenerative medicine.

Reversible photo-responsive gel-sol transitions of robust organogels based on an azobenzene-containing main-chain liquid crystalline polymer.

Stimuli-responsive supramolecular gels have been widely investigated, but the construction of a liquid crystalline gel with a high mechanical property and reversible photo-response still remains a challenge. This is due to the difficulty of designing gelators with liquid crystal properties and gelation abilities in organic solvents simultaneously. In this study, an azobenzene-containing main-chain polyester (Azo-mLCP) with a pendant amide group was synthesized. The organogel of Azo-mLCP via a hydrogen bond in dioxane possessed reversible thermal- and photo-responsive behaviours. The organogel exhibited a good self-supporting ability when the concentration of the gelator was more than 7.5 wt%. The rapid trans-to-cis isomerization of Azo-mLCP in solution was studied via UV-Vis absorption spectra. In addition, the gel-to-sol transition of the organogel could be triggered efficiently by an incomplete trans-to-cis conversion strategy. This study opens a way for the main-chain liquid crystalline polymers to serve in potential applications in photo-responsive robust actuators, electro-optical devices, and so on.

Hydrogen bond driven self-supporting organogels from main-chain liquid crystalline polymers

Main-chain liquid crystalline polymers (LCPs) have been widely investigated due to the excellent thermal stability and mechanical properties. In addition, hydrogen bond has been utilized to construct peptide supramolecular organogels by bottom-up approach but rarely to fabricate other supramolecular polymeric gels. In this work, some main-chain polymers with different proportion of pendent amide group have been designed and synthesized by melt polycondensation. The relationship between the liquid crystalline property and the proportion of amide group has been investigated by DSC and WAXD. Interestingly, among these polymers, the smectic main-chain LCP P100 with the highest molar ratio of amide group displays the best gelation ability with a minimum gelation concentration (MGC) of 2 wt% in THF. The hydrogen bond has been proved to be one of the most important driving forces for the gelation by NMR technique. The organogels exhibit good self-supporting ability with a storage modulus higher than 104 Pa when the gelator concentration is more than 4 wt%. At last, a hairpin model has been proposed to illustrate the microstructure for these organogels. These robust polymeric organogels that have been synthesized by bottom-up approach using hydrogen bond broaden the applications for main-chain LCPs ranging from actuators to electro-optical devices.

Flexible and electro-induced shape memory Poly(Lactic Acid)-based material constructed by inserting a main-chain liquid crystalline and selective localization of carbon nanotubes

To realize electro-induced shape memory effect for poly (lactic acid) (PLA)-based material, a liquid crystalline, poly [4,4’-bis(6-hydroxyhexyloxy) biphenyl phenylsuccinate] (PBDPS), was introduced in its main chain, which can not only provide high flexibility but also make carbon nanotubes (CNT) distribute selectively in the polymer matrix via π-π interaction. The presence of CNT resulted in the obvious enhancement of tensile strength as well as highly electrical conductivity. Notably, the as-prepared nanocomposites showed good shape memory performance with high recovery rate, high shape fixation and recovery ratios.

Facile In-situ Polymerization of Thermotropic Liquid Crystalline Polymers as Thermally Conductive Matrix Materials

Although thermally conductive composites that can efficiently dissipate the heat generated from electronic devices are in high demand, most neat polymers used as matrix materials are problematic because they have poor thermal conductivities. The low thermal conductivity of polymeric materials is caused by structural defects; therefore, it can be improved by increasing the orientational regularity of the polymer chains. Here, main-chain liquid crystalline polymers (LCPs) were designed and synthesized to investigate the effects of liquid crystallinity-induced structural regularity on the thermal conductivity of the polymers. In addition, an in-situ polymerization method was devised for commercial applicability, and the thermal conductivity of the obtained polymer was compared to that of a conventionally polymerized polymer having the same structure. The designed polymers exhibited thermotropic liquid crystalline characteristics, and the polymer with longer spacers between the rigid segments showed relatively higher thermal conductivity exceeding 0.5 W·m-1· K-1 after sample preparation by injection molding. In addition, X-ray diffraction analysis revealed that the differences in the thermal conductivity, depending on the molding temperature during specimen preparation, were caused by variations in chain orientation within the same polymer. Based on the obtained results, it was concluded that LCPs are strong candidate matrix materials for thermally conductive composites; the suggested in-situ polymerization method could be applied practically to the polymerization of polyester-type LCPs.

Molecular-Weight Dependence of Nucleation Effect of a Liquid Crystalline Polyester β-Nucleating Agent for Isotactic Polypropylene

Unlike low-molecular-weight inorganic or organic nucleating agents (NAs), the physical properties, including phase transition and rheological behaviors, of polymeric NAs are greatly influenced by molecular weight. In this paper, the molecular-weight dependence of the β-nucleation behaviors of a polymeric NA (a main-chain liquid crystalline (LC) polyester, PBDPS) was studied in detail. The nucleation effect of PBDPS on isotactic polypropylene (iPP) shows a strong molecular-weight dependence. Low-molecular-weight PBDPS tended to induce the formation of α-iPP, whereas high-molecular-weight PBDPS tended to induce β-iPP. Moreover, the relative content of β-iPP crystals (kβ) first increased and then decreased as the molecular weight of PBDPS increased, with the maximum value (86.1%) obtained when 2 wt % PBDPS21.2k was incorporated. Furthermore, the stiffness, toughness, and heat distortion temperature (HDT) of iPP were simultaneously improved by incorporating PBDPS.

Trial Manufacture of Nanofibers Made from a Main-Chain Liquid-Crystalline Elastomer Composed of Bibenzoate Mesogens

Trial Manufacture of Nanofibers Made from a Main-Chain Liquid-Crystalline Elastomer Composed of Bibenzoate Mesogens

Sustained heparin release actuator achieved from thermal and water activated shape memory hydrogels containing main-chain LC units

In this work, liquid crystalline (LC) monomer with two n-decenoyl terminated groups was synthesized and applied into shape memory hydrogels containing main-chain LC unit by Michael addition polymerization. The liquid crystalline textures of LC monomer and hydrogels were determined by polarized optical microscopy. Thermal and water activated shape memory properties were investigated. And the shape memory properties were improved by incorporating main chain LC unit into hydrogels. The biocompatibility was evaluated by the cytotoxicity test and all these hydrogels illustrated no cytotoxicity. The good biocompatibility and reinforced shape memory properties with shape fixity ratio higher than 96% and shape recovery ratio higher than 93%, indicating a new idea to prepare shape memory hydrogels by introducing main-chain mesogenic units into hydrogels in the safe application of smart biomedical areas. Finally, water-activated actuators were prepared and could change themselves into “C” or “S” shape in water at 25 °C with about 20 min. And through in vitro heparin release of hydrogel and actuator investigation, more sustained heparin release performance of actuator was observed in our study. This indicated that the heparin loaded actuator would have potential application in anticoagulant scaffold by combining the sustained heparin delivery character and the shape self-changing character of water-activated actuators.

Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders

A promising way to induce shape transformation in soft materials is via spatial variation in the orientation of the alignment of liquid crystalline elastomers (LCEs). Here, we improve the nascent thermomechanial shape transformation in main-chain LCEs prepared via aza-Michael addition reactions. Specifically, increasing the alkyl length in the n-alkylamine chain extender effectively reduces the actuation temperature by destabilizing the nematic phase as well as reduces the glass transition temperature (Tg) by increasing the free volume. In addition, incorporating a hydroxyl end-group in the amine chain extender (i.e., n-alkanolamine) increases the actuation strain and improves the film quality by preventing side-chain aggregates of n-alkylamine-functionalized LCEs. Interestingly, uniaxially aligned n-alkanolamine-functionalized LCEs exhibit an unprecedentedly large elongation and an enhanced toughness even along the loading direction likely due to hydrogen bonding between chains. Thus, our study highlight...

Phase behavior of main-chain liquid crystalline polymer networks synthesized by alkyne-azide cycloaddition chemistry.

Liquid crystalline polymer networks (LCNs) couple polymer chain organization to molecular ordering, the switching of which has been shown to impart stimuli-responsive properties, including actuation and one-way shape memory, to the networks. While LCNs have long been proposed as artificial muscles, recent reports have also suggested potential as dynamic biomaterial substrates. In contrast to many existing LCNs synthesized using hydrophobic spacers, this work investigates networks synthesized using more hydrophilic spacers to promote interaction with water. A challenge with such materials is liquid crystalline phases could be disrupted in hydrated networks. This work thus investigates the impact of polyether spacers and mesogen composition on the phase behavior of LCNs. Main-chain LCNs were synthesized using alkyne-azide cycloaddition ("click" chemistry), where two different mesogens (5yH and 5yMe) and a non-LC monomer (5yTe) were coupled with one of two different polyether spacers, poly(ethylene glycol) and poly(propylene glycol), and a crosslinker. The chemistry led to high gel fraction materials, the workup of which resulted in networks that displayed no difference in cellular toxicity due to leachable components compared to tissue culture plastic control. Calorimetric analysis, dynamic mechanical analysis, and X-ray scattering revealed the LC microstructure and temperature-responsive properties of the networks. The use of low molecular weight polyether spacers was found to prevent their crystallization within the LC network, and adjusting mesogen composition to enhance its LC phase stability allowed the use of spacers with larger molecular weights and pendant groups. Hydrated networks were found to rearrange their structure compared to dry networks, while maintaining their LC phases. Like other crosslinked LC materials, the networks display shape changes (actuation) that are tied to changes in LC ordering. The result is a new synthetic approach for polydomain networks that form stable LC phases that are tailorable using polyether spacers and may enable future application as hydrated, stimuli-responsive materials.

Supramolecular main-chain liquid crystalline polymers with competitive hydrogen bonding: Inclusion of structurally analogous hydrogen bond acceptors and the effects on liquid crystallinity

ABSTRACTA series of liquid crystalline supramolecular copolymers formed through hydrogen bonding has been created. These polymers composed of bis-benzoic acids and two of pyridyl species- a small rigid bispyridyl capable of forming a mesophase and a small, flexible bis pyridyl non-mesogenic disruptor species. It was found that the polymers display monotropic nematic character at high concentrations of disruptors – up to 75% inclusion. Smectic phases were observed in loadings up to 50%. It is believed that the structural similarities between the mesogenic and non- mesogenic acceptors contributes significantly to the enhanced stability of the liquid crystalline phases at high composition.