Main-chain Liquid Crystalline Polymers 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.

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.

Inside Back Cover: Mechanochromic, Shape‐Programmable and Self‐Healable Cholesteric Liquid Crystal Elastomers Enabled by Dynamic Covalent Boronic Ester Bonds (Angew. Chem. Int. Ed. 9/2022)

Reprogrammable elastomers with dynamic covalent bonds are of paramount significance for emerging applications such as adaptive optics and soft robotics. In their Research Article (e202116219), Ling Wang, Wei Feng, and co-workers describe mechanochromic, shape-programmable, and self-healable cholesteric liquid crystal elastomers obtained by introducing dynamic covalent boronic ester bonds into the main-chain chiral liquid crystalline polymer networks.

Photo-responsive liquid crystalline polymer from a renewable furfural derivative of dimethyl 2,5-furandicarboxylate via catalytic carbonylative esterification

A main chain liquid crystalline polymer has been prepared via melting polycondensation of a bifunctionalized furan monomer (DMFD) from inedible biomass with a photo-responsive azobenzene unit, which can be used in information storage.

Main-chain liquid crystalline polymers bearing periodically grafted folding elements

Inclusion of non-mesogenic pendant segments, such as alkyl, PEG and fluoroalkyl, at periodic intervals along the backbone of main-chain LCPs, that contain flexible spacers, provides an interesting alternative to control the mesophase structure and the layer periodicity.

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.

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.

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.

Spontaneous Tilting Transition in Liquid-Crystalline Polymer Brushes

The equilibrium phase behavior of main-chain liquid-crystalline polymer brushes in good solvent is investigated using self-consistent field theory. The calculation assumes semidilute brushes, where the grafting density is sufficient to cause overlap among the polymers but low enough to allow the solvent to be treated implicitly. The polymers are modeled as semiflexible wormlike chains with Maier–Saupe interactions. Previous calculations based on freely jointed chains have predicted that extended brushes collapse into folded nematic brushes, as the liquid-crystalline interaction strength is increased. For the more sophisticated model of wormlike chains, which penalizes hairpin folds, we find that extended brushes become unstable with respect to tilting prior to the onset of folding. This implies that the brushes spontaneously collapse by a symmetry-breaking transition, where the chains tilt rather than fold.

Remolding of Photoresponsive Polymer Materials by Means of Dynamic Covalent Bonds In a Main Chain

We prepared crosslinked azobenzene main-chain liquid-crystalline polymers (MCLCPs), in which the exchange of crosslinks can be brought about by transesterification reactions. The exchangeable crosslinks enable reshaping and self-healing of the MCLCPs upon heating. Furthermore, the stress generated by stretching the MCLCP film is completely relaxed at 180 °C through rearrangement of networks. The film after hot-drawing shows reversible bending behavior upon irradiation with UV and visible light.

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.

Friction-induced surface textures of liquid crystalline polymer evaluated by atomic force microscopy, spectroscopy and nanoindentation

The main-chain thermotropic liquid crystalline polymer (LCP), Vectra A950, has been reported to be able to simultaneously lower dynamic friction coefficient and wear rate when introduced into poly(vinylidene fluoride) (PVDF) as lubricant. In this investigation, we evaluate the friction surfaces of Vectra A950 through atomic force microscopy (AFM), spectroscopy and nanoindentation. By AFM, various frozen nanometer-level textures are observed at friction surfaces of LCP particles after cessation of friction. Besides, orientation and microscopic hardness of textures for Vectra A950 are unveiled via polarized Raman spectroscopy and nanoindentation tests, respectively. It is worth noting that as an important hint of orientation for LCPs under shear, observation of LCPs textures is conventionally conducted at molten state by polarized-light optical microscope and shear apparatus, however, which hardly detects textures morphology at solid state. Overall, this investigation not only proposes a new approach to evaluate textures of LCPs, but also provides guide for further exploring tribological enhancement mechanism of LCPs.

Friction-induced lubricating nanocoatings of main-chain thermotropic liquid crystalline polymer

The main-chain thermotropic liquid crystalline polymer (LCP) has been reported to be able to simultaneously lower friction coefficient and wear rate when introduced into poly (vinylidene fluoride) (PVDF) as the lubricant. In this investigation, we aim to obtain some insight into the lubricating mechanism of LCP. Through atomic force microscopy (AFM) and Raman spectroscopy, friction-induced in-situ nanocoatings of LCP are observed on friction PVDF domains, which arises from increasing surface temperatures and high shear thinning properties of LCP. Consequently, LCP nanocoatings obviously enhance microscopic hardness of friction PVDF domains. In other words, LCP nanocoatings could be treated as a type of protective lubricant to prevent the direct contact between counterface and PVDF domains, promote wear resistance and lower friction coefficient. To our best knowledge, such an interesting lubricating mechanism is rarely reported. Overall, this investigation not only proposes the novel lubricating mechanism of LCP, but also provides support for further tribological applications in the polymeric systems.

Tribological enhancement effect of main-chain thermotropic liquid crystalline polymer

Although main-chain thermotropic liquid crystalline polymer (LCP) has been extensively investigated as self-reinforcing composites, tribological application is rarely reported. This paper explores tribological enhancement effect of LCP on representative matrix poly (vinylidene fluoride) (PVDF). Moreover, structures of PVDF/LCP blends are assessed in detail. Due to immiscibility, LCP in blends is uniformly dispersed in the form of particles or microfibrils, dependent on LCP content. Besides, LCP improves storage modulus of PVDF. Most importantly, tribological performance of PVDF is obviously promoted. Compared with neat PVDF, the wear rate, coefficient of friction and oscillation amplitude of blend containing 20 wt% LCP simultaneously decrease by 97.7%, 80.7% and 80.2%, respectively. In particular, frequency-enhancement phenomenon is observed. By probing worn PVDF domains with nanoindentation test and atomic force microscopy (AFM), preliminary tribological enhancement mechanism of LCP is unveiled. Overall, this finding provides a new, but promising lubrication route of polymer.

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.

Synthesis of novel and regioselectively mesogen-incorporated thermotropic liquid crystals from cellulose derivatives

ABSTRACTRegioselectively mesogen-incorporated cellulose derivatives, in which the hydroxy group at C-6 is displaced by a bulky and rigid mesogenic group such as {4-[(4-methoxyphenoxy)carbonyl]phenoxy}acetate or [(4′-cyanobiphenyl-4-yl)oxy]acetate, and the C-2 and C-3 groups are displaced by octanoyl or lauroyl groups, were synthesized. Thermogravimetric and differential scanning calorimetric analyses of the final products, along with polarized optical microscopic observations, revealed that the obtained cellulose derivatives have a thermotropic liquid-crystalline nature and transition from the mesomorphic to the isotropic phase over a wide range of temperatures. These derivatives could be considered to be main-chain liquid-crystalline cellulosic polymers.