Liquid Crystal Polymer Fibers Research Articles

Production and Investigation of Cholesteric Liquid Crystal-Polymer Fibers by Electrospinning Method

Sıvı kristallerin ekran teknolojisi, sensör, esnek devre elamanları, ışık modülatörleri gibi uygulamalarda kullanılabildiği bilinmektedir. Kolesterik mezofaz sıvı kristaller ışığın seçici geçirgenlik özelliği ile ön plana çıkmaktalardır. Bu çalışmada, ilk defa tek-iğneli elektro-eğirme kullanılarak kolesterik sıvı kristaller-polimer kompozit liflerinin üretilmiş ve incelenmiştir. Bu bağlamda ilk olarak, mavi, yeşil ve kırmızı renkte ışığı yansıtabilecek sırasıyla 470 nm, 550 nm ve 640 nm spiral adım uzunluklarına sahip kolesterik sıvı kristaller hazırlanmıştır. Konak nematik mezofaz olarak E-7 ve yüksek burma gücüne sahip R-5011 kiral katkı maddesi kullanılmıştır. Bu kolesteriklerin yaklaşık istenilen adım uzunluklarına sahip oldukları UV-VIS spektrofotometre ölçümleri ile gösterilmiştir. Ayrıca, hazırlanan kolesteriklerin karakteristik bir tekstür olan oily-streak desenlere sahip olduğu görülmektedir. Bu kolesterikler poliakrilonitril ile karıştırılarak dimetilformamid içerisinde çözdürülmüştür. Bu çözeltiler tek-iğneli elektro-eğirme sistemi ile 16 kV, 18 kV, 20 kV, 22 kV ve 24 kV uygulama voltajları altında eğirilerek sıvı kristal polimer kompozit lifleri haline getirilmişlerdir. Eğirilen liflere katkılanan kolesterik sıvı kristallerin varlığı optik ve yapısal analizler geçekleştirilerek belirlenmiştir. Polarize optik mikroskop ile yapılan görüntülemelerde çapraz polarizörler arasındaki lifler boyunca parlamalar gözlenmiştir. Bu parlamalar lif boyunca sıvı kristallerin yerleştiğini göstermektedir. Ayrıca, yapısal analiz sonucu her bir lifte üretim parametreleri ve katkı maddelerine göre lif yapısının değişimi üzerine çalışma yapılmıştır. Tüm kolesterik örnekleri için düşük uygulama voltajlarında eğirilen lifler boyunca uzanan küresel boncuk yapılar tespit edilmişken daha yüksek uygulama voltajlarında eğirilen liflerde boncuk yapılar oldukça seyrek gözlenmiştir. Kızılötesi spektroskopisi analizleri sonucu liflerin sıvı kristallerin sergilediği titreşim pikleri ile aynı dalga sayılarında pikler vermesi lif yapılarında sıvı kristallerin varlığını göstermiştir.

Implication of Freeze-Thaw Erosion and Mechanism Analysis of High-Performance Aromatic Liquid Crystal Fibers.

According to the demand for high-performance fibers for high-latitude ocean exploration and development, this paper selects representative products of high-performance liquid crystal fibers: thermotropic liquid crystal polymer fibers (TLCP) and poly p-phenylene terephthalamide (PPTA) fibers. Through a series of freeze-thaw (F-T) experiments for simulating a real, cold marine environment, we then measure the retention of mechanical properties of these two kinds of fibers. Before that, due to the difference in their chemical structures, we tested their Yang-Laplace contact angle (YLCA) and water absorption; the results suggested that PPTA fibers would absorb more moisture. Surprisingly, then, compared with thermotropic liquid crystal polymer (TLCP) fibers, the retention of the mechanical properties of poly p-phenylene terephthalamide (PPTA) fibers decreased by around 25% after the F-T experiments. The Fourier-transformed infrared (FT-IR) analysis, the attenuated total reflection (ATR) accessory analysis and the degree of crystal orientation measured by two-dimensional wide-angle X-ray diffraction (2D-WAXD) confirm that no changes in the chemical and the orientation structure of the crystal region of the fibers occurred after they underwent the F-T cycles. However, as observed by scanning electron microscopy (SEM), there are microcracks of various extents on the surface of the PPTA fibers, but they do not appear on the surface of TLCP fibers. It is obvious that these microcracks will lead to the loss of mechanical properties; we infer that the moisture absorbed by the PPTA fibers freezes below the freezing point, and the volume expansion of the ice causes the collapse of the microfibrillar structure. The two sorts of fibers subjected to the F-T experiments are immersed in a sodium chloride solution, and the amount of water infiltrated into the PPTA microfibrillar structure is evaluated according to the content of sodium ions in the fiber surface and subsurface layers through X-ray spectroscopy (EDS) elemental analysis. From the above analysis, we found that TLCP fibers can more effectively meet the operating standards of the severe and cold marine environment.

Effects of Smectic Layer Deformation on Mechanical Properties of Glassy Liquid Crystal Polymer Fibers

AbstractIn this work, it is demonstrated that the stress response of glassy smectic liquid crystal (LC) polymer fibers is largely dependent on layer deformations. Fibers with smectic layers stacked along the fiber axis (fiber A), prepared by stretching a molten main‐chain LC polyester, deform the layers by tensile deformation in the LC state, yielding fibers with layers tilted from the fiber axis (fiber B) and those with divided layers (fiber C). These fibers in the glassy LC state differ in Young's modulus (E), yield stress and strain (σy, εσy), and strain at break (εb). Fiber A has εb = 30%; however, fiber B is much more ductile (εb > 290%). Fiber C has higher E = 0.97 GPa and σy = 82 MPa than fiber A (E = 0.33 MPa and σy = 33 MPa). Annealing fiber C in the LC state yields fiber D with large‐area layers, similar to fiber A. Fiber D has E = 0.74 GPa and σy = 25 MPa, comparable to those of fiber A. Thus, dividing layers improves E and σy. This strengthening of glassy smectic LC by dividing layers applies to other polymers with layered structures, including lamellar block copolymers and semi‐crystalline polymers.

Increase of Thermal Stability Caused by High Thermal Conduction Fiber and Sheet in Conduction-Cooled HTS Coils

We have examined thermal stability in high-temperature superconducting (HTS) coils, focusing on thermal properties of coil's structural components. As the component materials, high thermal conduction fibers and sheets made from a liquid crystal polymer (LCP) were used. First, we fabricated an LCP fiber reinforced plastic (LCP-FRP), and experimentally estimated thermal conductivity of the LCP-FRP round rod at cryogenic temperature, and made sure the conductivity was high. Next, thermal strain of the LCP-FRP rod from room temperature to 77 K was measured. From the results, the LCP-FRP bar expanded to a longitudinal direction of the bar with the cool down. Finally, we made an experimental apparatus which simulated a part of the HTS coil, and inserted the LCP sheet or a GFRP sheet in the HTS coil model. When the model was heated by a local heater, temperature rise of the model having the LCP sheet was lower than that having the GFRP sheet. From those experimental results, we think that thermal stability of the HTS coil becomes better by using the LCP sheet for non-metallic structural component in the coil.

A Novel Manufacturing Concept of LCP Fiber-Reinforced GPET-Based Sandwich Structures with an FDM 3D-Printed Core.

The presented research was focused on the development of a new method of sandwich structure manufacturing involving FDM-printing (fused deposition modeling) techniques and compression molding. The presented concept allows for the preparation of thermoplastic-based composites with enhanced mechanical properties. The sample preparation process consists of 3D printing the sandwich’s core structure using the FDM method. For comparison purposes, we used two types of GPET (copolymer of polyethylene terephthalate)-based filaments, pure resin, and carbon fiber (CF)-reinforced filaments. The outer reinforcing layer “skins” of the sandwich structure were prepared from the compression molded prepregs made from the LCP (liquid-crystal polymer)-fiber fabric with the GPET-based matrix. The final product consisting of an FDM-printed core and LCP-based prepreg was prepared using the compression molding method. The prepared samples were subjected to detailed materials analyses, including thermal analyses (thermogravimetry-TGA, differencial scanning calorimetry-DSC, and dynamic thermal-mechanical analysis-DMTA) and mechanical tests (tensile, flexural, and impact). As indicated by the static test results, the modulus and strength of the prepared composites were slightly improved; however, the stiffness of the prepared materials was more related to the presence of the CF-reinforced filament than the presence of the composite prepreg. The main advantage of using the developed method is revealed during impact tests. Due to the presence of long LCP fibers, the prepared sandwich samples are characterized by very high impact resistance. The impact strength increased from 1.7 kJ/m2 for pure GPET samples to 50.4 kJ/m2 for sandwich composites. For GPET/CF samples, the increase is even greater. The advantages of the developed solution were illustrated during puncture tests in which none of the sandwich samples were pierced.

Ultralarge Contraction Directed by Light‐Driven Unlocking of Prestored Strain Energy in Linear Liquid Crystal Polymer Fibers

AbstractAnisotropic 1D contraction motion of polymeric actuating materials has drawn growing interests in fields ranging from soft robotics to biomimetic muscles. Although light‐driven liquid crystal polymers (LCPs) represent promising candidates to realize contraction (<20%) triggered remotely and spatially, there remain multitudes of challenges to develop an LCP system possessing ultralarge contraction rate. Here, a novel strategy combining shape memory effect and photochemical phase transition is presented to realize light‐driven contraction as large as 81% in a newly designed linear liquid crystal copolymer, where the eutectic mesogens of azobenzene and phenyl benzoate self‐organize into the smectic B phase. Importantly, this highly ordered structure as the switching segment firmly locks the stress‐induced strain energy, which is rapidly released by reversible trans–cis photoisomerization that destroys the lamellar liquid crystal phase, therefore leading to such ultralarge contraction. Fibers serve as light‐driven building blocks to achieve precise origami, to mimic the recovery of a “broken” spider web and to screen objects in different sizes, laying new ground for advanced applications of light‐driven LCPs from biomimetic robots to human assists.

Design of thermal hybrid composites based on liquid crystal polymer and hexagonal boron nitride fiber network in polylactide matrix

Development of high thermally conductive and electrically insulative composites is of interest for electronic packaging industry. Advancements in smaller and more compact electronic devices required improvements in packing materials, including their weight, thermal conductivity, and electrical resistivity. In addition, with the increasing environmental awareness, the usage of green (bio‐based) alternatives was equally important. In the present study a hybrid based on fibers of highly concentrated hexagonal boron nitride (hBN) in liquid crystal polymer (LCP) matrix were fabricated. These hybrids were formed by arranging hBN platelets into LCP fiber form to reach high filler concentration and then randomly mix it in polylactide (PLA) matrix. With appropriate filler interaction within the hybrid, thermal conductivity similar to that of pure fiber could be achieved. Filler interaction may be tailored by optimizing the fibers aspect ratio. This study demonstrated the effect of random fillers in fibers shape in increasing the overall thermal conductivity of PLA polymeric hybrid using hBN and LCP fibers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 457–464

Preparation of poly(γ-benzyl-L-glutamate) nanofibers by electrospinning from isotropic and biphasic liquid crystal solutions

Liquid crystal polymer (LCP) nanofibers were prepared by electrospinning from isotropic and biphasic poly(γ-benzyl-L-glutamate) (PBLG)/dichloromethane-pyridine solutions. The as-spun PBLG fiber from the isotropic solutions had an undulating orientation of closed packed α-helices like a serpentine trajectory. The as-spun fibers from the biphasic solutions containing the LC phase, on the other hand, had uniaxially oriented hexagonal lattices of α-helices along the fiber axis. These results indicated that the orientation of the ordered structure of the electrospun lyotropic LCP fibers can be controlled by the initial microstructure of the spinning solution.

Strengthening of highly heat-resistant liquid-crystal polymer fibers upon heat treatment

Fibers of fully aromatic thermotropic liquid-crystal polymers containing mesogenic groups in the backbone are prepared on laboratory machines that make it possible to vary the conditions of jet spinning and subsequent heat treatment. The thermal and mechanical properties of the polymer fibers are investigated using differential thermal and thermomechanical analyses and strength measurements. It is found that, after spinning under optimum conditions, the strength of the fibers at room temperature is approximately equal to 1 GPa. After heat treatment under specially chosen conditions, the strength increases by a factor of two or three due to an increase in the activation energy of fracture. The role of crystallization and cross-linking in strengthening of liquid-crystal polymer fibers is elucidated.

Molecular Dynamics and Strengthening of Liquid-Crystal Polymers

The role of large-scale molecular motion in the self-organization and strengthening of liquid-crystal polymer fibers is discussed. It is shown that, at high temperatures, these objects are oriented liquid-crystal melts in which macromolecules remain extended but execute high-frequency conformational motions without leaving the tube approximately 20 A in diameter. This large-scale motion is referred to as quasi-segmental motion. During annealing, the chains involved in quasi-segmental motion can accomplish longitudinal displacements (reptate) over considerable distances. It is this reptation that favors spontaneous self-organization and, consequently, strengthening of liquid-crystal polymer fibers upon heat treatment. The role played by the quasi-segmental motion of rigid macromolecules in the strengthening of polymers of different types is compared with the role played by the segmental motion of flexible chains in this process.

Hot topics in architectural acoustics: New materials

While both new metrics for measuring sound and analytical software programs for predicting sound performance have proven to be powerful tools of architectural acoustics, new acoustical materials have allowed us to further engineer these sensitive soundscapes. These developments embrace several areas of contemporary acoustical materials. HVAC liners have seen the addition of fiber-free alternatives to traditional linings. New solutions include coated polyimides and melamine foams. In electroacoustics, materials that have improved performance are now replacing some old standbys. PEN materials such as Kaladex are finding more acceptance for use in tweeter domes and microphone elements. Liquid crystal polymer fibers, such as Vectran, which can be used instead of aramid and carbon fibers, offer similar performance improvement for wet lay-up direct radiator transducer cones. Absorptive panels have moved beyond fabric wraps. They are now paintable and can mimic typically reflective treatments. Finally, diffusers have shown advances in both acoustical performance and geometries that better blend with aesthetic goals. These new surfaces consist of a wide range of surface type including modulated reflection phase gratings, binary amplitude gratings, and most recently boundary-element optimized surfaces of constant or variable impedance. Each of these exciting developments are reviewed in this presentation.

Thermotropic liquid crystal polymer fabric reinforced polyimide composite materials

An aromatic copolyeste thermotropic liquid crystal polymer (LCP) fabric was used to reinforce a polyimide (PI) matrix to produce composites. The LCP/PI composite was made to eliminate directional difference in the mechanical properties of LCP by controlling the individual reinforcement fabric alignment in different directions without losing any mechanical properties. As a result, a transversely isotropic LCP composite material could be obtained from the highly anisotropic LCP fiber. Interfacial adhesion between the LCP and polyimide was greatly improved by a NH3/H2 plasma treatment on the LCP fabric, thereby the significantly improving mechanical properties of the composite. Moreover, inter-laminar shear strength of the LCP/PI composite was further increased after heat treatment at 220°C for 1 hour. The LCP/polyimide composite retains stable mechanical properties up to 250°C.

Fiber stability analysis for in‐situ liquid crystalline polymer composites

AbstractWe present a simple mathematical model based on classical theories of macroscopic elasticity that provides a fundamental description of the liquid crystal polymer (LCP) fiber break‐up instability that is observed during the annealing of in‐situ liquid crystal‐thermoplastic polymer composites. The thermodynamic model is solved using a number of consistent simplifying assumptions, and is restricted to thin liquid crystal polymer fibers. A significant feature of the liquid crystal surface energy incorporated in the model is the elastic storage due to deviations of the macroscopic orientation and of the orientational order from the preferred equilibrium values. The model predicts that thin LCP fibers are unstable to periodic surface perturbations that eventually would produce fiber break‐up and lead to an array of LCP droplets, as in the case of Rayleigh's fiber instability. The additional liquid crystalline elastic storage modes incorporated in the model, but not present in isotropic melts, tend to increase the magnitude of the critical wave‐length for fiber break‐up over that given by the Rayleigh criteria (i.e., fiber circumference) by a relatively small factor that depends on the ratio of the interfacial tension and the nematic anchoring energy.

Thermodynamic Stability Analysis of Liquid-Crystalline Polymer Fibers

Classical theories of liquid-crystalline materials are used to develop a new model that describes the thermodynamic stability of nematic liquid-crystal cylindrical fibers that arise in the fabrication of in-situ liquid crystal polymer (LCP) composites. The thermodynamic model identifies the new contributions to elastic storage due to the nematic orientational order present in LCP fibers. It is shown that the additional nematic surface and bulk elastic storage mechanisms tend to promote fiber stability when compared with isotropic fibers. The theory predicts that elastic storage due to orientational deformations within the fiber may be able to overcome the classical capillary (Rayleigh) instability present in isotropic fibers. The parametric conditions that lead to fiber stability are smaller fibers, low interfacial tensions, and large nematic elastic constants. Nevertheless, using estimates typical of the actual in situ LCP−polymer composites, it is found that LCP fibers are unstable and will break up, in agreement with existing experimental studies on the stability of liquid-crystalline polymer fibers in a thermoplastic elastomeric matrix when subjected to annealing at high temperatures.

Fluctuations and light scattering in a liquid crystal polymer fiber.

The spatial correlation function is calculated for thermal fluctuations of the liquid crystal director field in the core of an optical fiber composed of a main chain liquid crystal polymer in the smectic-C phase. Due to steric constraints on the liquid crystal polymer, the smectic layers orient perpendicular to the fiber axis, the liquid crystal director lies flat against the core-cladding interface, and a line disclination appears along the fiber axis. The analysis is based on a Landau\char21{}de Gennes expression for free energy, which includes ordinary smectic-C elastic terms as well as terms added to model the disclination. The correlation function is used to relate the scattering distribution and attenuation of guided light to the elastic constants of the liquid crystal polymer. \textcopyright{} 1996 The American Physical Society.

Strength of fibers from wholly aromatic polyesters

A theory of the strength (or the tenacity) of highly oriented Liquid Crystal Polymer (LCP) fibers was developed, and its results were compared with existing tensile strength data of fibers of a copolymer of 1,4-oxybenzoate and 6,2-oxynaphthoate. A basic premise of the theory is that the mechanical load transfer between polymer chains is through intermolecular interaction which acts in a manner similar to that of shear stress, and that the fiber strength is primarily governed by the intermolecular adhesion strength. The theory also incorporates the effects of MW, MW distribution, and the chain orientation distribution. Analysis of the experimental tenacity data demonstrates that the present theory can quantitatively describe the variation of the tenacity of LCP fibers with MW both in the as-spun and in the heat-treated states. The theory further predicts that the predominant factor governing the tenacity of LCP fibers is primarily due to MW increase due to solid-state polymerization. It is also demonstrated that the intermolecular adhesion between LCP chains is relatively weak and does not improve with heat treatment. The absence of factors that limit the MW increase (i.e, imbalanced end-groups and side reactions of end groups) is a prerequisite for fast heat treatment of a LCP fiber to a high tenacity.

A method of forming composite structures using in situ‐formed liquid crystal polymer fibers in a thermoplastic matrix

AbstractA new high speed and potentially economical method of creating a composite material and structures therefrom is tested. The method consists of spinning composite fibers from a melt blend of a thermoplastic with a liquid crystal polymer (LCP). Discontinuous fibrils of the LCP are formed in situ during the spinning process. These composite fibers are aligned and placed in a mold and heated to melt the thermoplastic matrix, but not the fibrils. A finished composite structure reinforced by the LCP fibrils is obtained when the thermoplastic phase is consequently consolidated. Our experiments show the proposed process is reasonable for an easily processed polystyrene matrix. High modulus fibrils with essentially infinite L/D ratios are readily produced in the extrusion process using 40 wt% of a wholly aromatic poly(ester‐co‐amide) LCP from Celanese. The integrity and alignment of the LCP fibrils is retained in the molding step. Mechanical tests show that the fibers produced by high shear rate processing have a stiffness approaching 23 GPa and match an axial rule‐of‐mixtures theory. The use of polystyrene resulted in brittleness. Molded composite plates exhibit slightly lower stiffness and significantly lower strength than individual fibers.