Polymer Alignment Research Articles

Surface-anchored alkylated graphene oxide as a two-dimensional homeotropic alignment layer for nematic liquid crystals

Graphene oxide (GO) is one of the thinnest two-dimensional (2D) organic materials and has been investigated for application in various fields due to its excellent mechanical and optical properties. The unique planar structure and hydrophilic functional groups allow GO to spontaneously form various nanoarchitectures. Herein, we report an unprecedented methodology to achieve the homeotropic alignment of nematic liquid crystals (NLCs) using surface-anchored alkylated graphene oxide (AGO). The GO is functionalized by amino reactions with dodecylamine to impart amphiphilic characteristics. This AGO spontaneously forms a 2D-alignment layer on indium tin oxide surface via polar affinity, and the hydrophobic alkyl chains of AGO guide the constant direction of the NLCs. Furthermore, a study on the effect of the alkylation ratio of AGO on the NLC orientation was conducted, and the achievement of a 2D-alignment layer based on the optimal AGO successfully resulted in the initial homeotropic orientation of the NLCs without any defects. Compared to a conventional polymer alignment layer, it showed a comparable alignment ability, and exhibited an improved optical transmittance due to the ultrathin nature of the AGO alignment layer. This new modus of NLC vertical alignment based on the use of AGO can expand the utility of functionalized GO.

Solid-state rigid-rod polymer composite electrolytes with nanocrystalline lithium ion pathways.

A critical challenge for next-generation lithium-based batteries lies in development of electrolytes that enable thermal safety along with the use of high-energy-density electrodes. We describe molecular ionic composite electrolytes based on an aligned liquid crystalline polymer combined with ionic liquids and concentrated Li salt. This high strength (200 MPa) and non-flammable solid electrolyte possesses outstanding Li+ conductivity (1 mS cm-1 at 25 °C) and electrochemical stability (5.6 V versus Li|Li+) while suppressing dendrite growth and exhibiting low interfacial resistance (32 Ω cm2) and overpotentials (≤120 mV at 1 mA cm-2) during Li symmetric cell cycling. A heterogeneous salt doping process modifies a locally ordered polymer-ion assembly to incorporate an inter-grain network filled with defective LiFSI and LiBF4 nanocrystals, strongly enhancing Li+ conduction. This modular material fabrication platform shows promise for safe and high-energy-density energy storage and conversion applications, incorporating the fast transport of ceramic-like conductors with the superior flexibility of polymer electrolytes.

Out-of-Plane Alignment of Conjugated Semiconducting Polymers by Horizontal Rotation in a High Magnetic Field.

The effective control of film morphology and molecular packing in the out-of-plane direction of semiconductor polymers plays a critical role in governing charge carrier transport in the direction perpendicular to the substrate. In this study, a highly out-of-plane alignment of the n-type polymer P(NDI2OD-T2) film has been successfully achieved by horizontal rotation in a high magnetic field (HR-HMF). The out-of-plane alignment of the P(NDI2OD-T2) film has showed a change from 72% face-on to 98.2% face-on lamellar texture as well as a 1.6-fold increase of the π-π stacking crystalline correlation length compared with that of as-cast polymer films without HR-HMF-induced alignment. Meanwhile, the film with near-perfect face-on molecular packing exhibited more than 18-fold enhancement of electron mobility compared to the unaligned film. The excellent electrical performance achieved with the HR-HMF process indicates its application potential for fabricating high-performance sandwich-type organic electronic devices, such as solar cells and light-emitting diodes.

Molecular Origin of Strain‐Induced Chain Alignment in PDPP‐Based Semiconducting Polymeric Thin Films

AbstractDonor–acceptor (D–A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)‐based D–A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side‐chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear‐chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side‐chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field‐effect transistors. This study deconvolutes the alignment of different components within the thin‐film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.

A Review on Bio-Polymers Derived from Animal Sources with Special Reference to their Potential Applications

Biopolymers are naturally found material and most of the materials are made in nature during the life cycles of plants, animals, fungi and bacteria. For any pharmaceutical formulation the two main ingredients are active pharmaceutical ingredient and excipients. As excipients in any kind of dosage form, the biopolymers play a vital function. Biopolymers are pharmacologically inert ingredients formulated along with the active ingredient to increase the volume; they help in the formulating dosage form and also simultaneously can improve the physicochemical parameters of the dosage form and so are widely used in the development of new drug delivery system. The biopolymers which are obtained from animal sources are usually non-toxic, biocompatible, stable and economic; and can control the release pattern of the drug. Natural polymers have more preponderant effects on fast dissolving tablets than synthetic polymers. Now-a-days, because of many problems regarding drug release and adverse effects of synthetic polymers, manufacturers are going towards using natural polymers. In this review article we mainly discussed about types of polymer, different alignment of natural polymer, advantages of natural polymers over synthetic polymers, drawbacks of natural polymers, mechanism of drug release of polymer, different methods of preparation of biopolymers, role of polymer in pharmaceutical industry and drug delivery systems.
 Keywords: Natural polymers, Chitin, Collagen, Sources, Preparation.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Summary Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1.

Fibre Alignment and Void Assessment in Thermoplastic Carbon Fibre Reinforced Polymers Manufactured by Automated Tape Placement.

Automated Tape Placement (ATP) technology is one of the processes that is used for the production of the thermoplastic composite materials. The ATP process is complex, requiring multiple melting/crystallization cycles. In the current paper, laser-assisted ATP was used to manufacture two thermoplastic composites (IM7/PEEK and AS4/PA12). Those specimens were compared to specimens that were made of thermoset polymeric composites (IM7/8552) manufactured while using a standard autoclave cycle. In order assess the quality, void content, fibre distribution, and fibre misalignment were measured. After manufacturing, specimens from the three materials were assessed using optical microscopy and computed tomography (CT) scans. The results showed that, as compared to the thermoset composites, thermoplastics that are manufactured by the ATP have a higher amount of voids. On the other hand, manufacturing using the ATP showed an improvement in both the fibre distribution inside the matrix and the fibre misalignment.

Additive Manufacturing of Polymer Matrix Composite Materials with Aligned or Organized Filler Material: A Review

The ability to fabricate polymer matrix composite materials with continuous or discontinuous filler material, oriented in a user‐specified direction, enables implementing designer material properties, such as anisotropic mechanical, thermal, and electrical properties. Conventional fabrication methods rely on a mold, which limits specimen geometry and is difficult to implement. In contrast, additive manufacturing, including fused filament fabrication or fused deposition modeling, direct ink writing, or stereolithography, combined with a method to align filler material such as a mechanical force or an electric, magnetic, shear force, or ultrasound wave field, enables 3D printing polymer matrix composite material specimens with complex geometry and aligned filler material, without the need for a mold. Herein, we review the combinations of fabrication and filler material alignment methods used to fabricate polymer matrix composite materials, in terms of operating and design parameters including size, resolution, print speed, filler material alignment time, polymer matrix and filler material requirements, and filler manipulation requirements. The operating envelope of each fabrication method is described and their advantages, disadvantages, and limitations are discussed. Finally, different combinations of 3D printing and filler material alignment methods in the context of important engineering applications, such as structural materials, flexible electronics, and shape‐changing materials, are illustrated.

Rich phase transitions in strongly confined polymer-nanoparticle mixtures: Nematic ordering, crystallization, and liquid-liquid phase separation.

We investigate the rich phase behavior of strongly confined semi-flexible (SFC) polymer-nanoparticle (NP) systems using the graphics processing unit accelerated Langevin dynamics simulation. Hard nanoparticles (HNP) that repel each other and ideal nanoparticles (INP) that do not interact with the same species are used as model additives to a strongly confined semiflexible polymer fluid. Both types of NPs exclude the monomer beads in the same way, but they have qualitatively different effects on the SFC isotropic-nematic (I-N) transition. For the total volume fraction ϕtot < 0.16, adding a low volume fraction of HNPs (ϕp) disrupts the long range nematic order of the polymers, whereas adding HNPs in a moderately packed system (0.16 < ϕtot < 0.32) facilitates polymer alignment due to the restricted polymer orientational degree of freedom. For dense packing (ϕtot > 0.32), polymers and NPs separate into layers along the slit height and the NPs form crystalline microdomains. In contrast, INP additives always promote inter-polymer alignment for low to moderate monomer volume fractions (ϕm). Furthermore, we found that INPs form a droplet-like fluid domain in dense nematic polymer systems.

High thermal conductivity states and enhanced figure of merit in aligned polymer thermoelectric materials

Control of morphology and chain alignment in polymer thermoelectric materials is critical to their performance. A x25 improvement in zT is reported for chain aligned films.

Manipulating Air-Gap Electrospinning to Create Aligned Polymer Nanofiber-Wrapped Glass Microfibers for Cortical Bone Tissue Engineering.

Osteons are the repeating unit throughout cortical bone, consisting of canals filled with blood and nerve vessels surrounded by concentric lamella of hydroxyapatite-containing collagen fibers, providing mechanical strength. Creating a biodegradable scaffold that mimics the osteon structure is crucial for optimizing cellular infiltration and ultimately the replacement of the scaffold with native cortical bone. In this study, a modified air-gap electrospinning setup was exploited to continuously wrap highly aligned polycaprolactone polymer nanofibers around individual 1393 bioactive glass microfibers, resulting in a synthetic structure similar to osteons. By varying the parameters of the device, scaffolds with polymer fibers wrapped at angles between 5–20° to the glass fiber were chosen. The scaffold indicated increased cell migration by demonstrating unidirectional cell orientation along the fibers, similar to recent work regarding aligned nerve and muscle regeneration. The wrapping decreased the porosity from 90% to 80%, which was sufficient for glass conversion through ion exchange validated by inductively coupled plasma. Scaffold degradation was not cytotoxic. Encapsulating the glass with polymer nanofibers caused viscoelastic deformation during three-point bending, preventing typical brittle glass fracture, while maintaining cell migration. This scaffold design structurally mimics the osteon, with the intent to replace its material compositions for better regeneration.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π-Conjugated Polymers by Light.

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities.Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively.Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.

Persistence–of–straining and polymer alignment in viscoelastic turbulence

Drag reduction in turbulent flows by the addition of polymers is a phenomenon that has a variety of applications in the engineering. The highly nonlinear interactions between the turbulent flow and the polymers in the polymeric solution still impose challenges to the understanding of this phenomenon. Efforts to address this problem come mainly from two sources: experiments and direct numerical simulations. In the present work we investigate through direct numerical simulations the roles of the non–persistence–of–straining tensor and the covariant strain acceleration tensor from the statistical and their alignment with the conformation tensor. In this regard, we provide a better picture of the relative orientation of this quantity.

Rheology-Assisted Microstructure Control for Printing Magnetic Composites-Material and Process Development.

Magnetic composites play a significant role in various electrical and electronic devices. Properties of such magnetic composites depend on the particle microstructural distribution within the polymer matrix. In this study, a methodology to manufacture magnetic composites with isotropic and anisotropic particle distribution was introduced using engineered material formulations and manufacturing methods. An in-house developed material jetting 3D printer with particle alignment capability was utilized to dispense a UV curable resin formulation to the desired computer aided design (CAD) geometry. Formulations engineered using additives enabled controlling the rheological properties and the microstructure at different manufacturing process stages. Incorporating rheological additives rendered the formulation with thixotropic properties suitable for material jetting processes. Particle alignment was accomplished using a magnetic field generated using a pair of permanent magnets. Microstructure control in printed composites was observed to depend on both the developed material formulations and the manufacturing process. The rheological behavior of filler-modified polymers was characterized using rheometry, and the formulation properties were derived using mathematical models. Experimental observations were correlated with the observed mechanical behavior changes in the polymers. It was additionally observed that higher additive content controlled particle aggregation but reduced the degree of particle alignment in polymers. Directionality analysis of optical micrographs was utilized as a tool to quantify the degree of filler orientation in printed composites. Characterization of in-plane and out-of-plane magnetic properties using a superconducting quantum interference device (SQUID) magnetometer exhibited enhanced magnetic characteristics along the direction of field structuring. Results expressed in this fundamental research serve as building blocks to construct magnetic composites through material jetting-based additive manufacturing processes.

Tunable Polarization Gratings Based on Nematic Liquid Crystal Mixtures Photoaligned with Azo Polymer-Coated Substrates

Liquid crystal polarization gratings are of great interest for optical communications as elements performing beam steering, splitting, multiplexing or beam combining. Material birefringence, cell thickness or a period of the liquid crystal director pattern influence, among other features, spectroscopic and electro-optical characteristics of fabricated devices, determining thus their functionality and applicability. Here, we report on liquid crystal polarization gratings that allow for complete maximization of the first-order diffraction efficiency (resulting in total elimination of the zeroth-order diffraction) for any wavelength of an incident beam from green to the near-infrared spectral region by applying a low electric voltage. The gratings with periods as small as 10 μm were obtained by holographic exposure of the cell substrates coated with light-sensitive azo polymer alignment layers, and then filled with three different liquid crystal mixtures. The influence of gold nanoparticle dopants in the liquid crystalline mixtures on spectroscopic and electro-optical properties of the devices is presented. Moreover, on the basis of the measured transmittance spectra of the fabricated gratings, the unknown birefringence of liquid crystal mixtures as well as their effective birefringence due to molecular reorientation in the electric field in the visible and near IR region were determined.

Autonomous MXene-PVDF actuator for flexible solar trackers

Abstract We report a novel flexible solar tracking system based on a photothermal-thermomechanical (PT-TM) actuator comprised of Ti3C2Tx MXene and polyvinylidene fluoride (PVDF) bilayer. The actuation function of the proposed device originates from photothermal and surface plasmon-assisted effects in MXenes, coupled with thermomechanical deformation of in-plane aligned PVDF polymer. Two types of solar tracking modes are evaluated based on the experimental deformation behavior of the PT-TM actuator. We find that the uniaxial East-West solar tracking option increases the overall energy intensity reaching the solar module by over 30%, in comparison with the optimized tilting-controlled mode. We also demonstrate the thermally driven self-oscillation of the MXene-PVDF device, which may have promising potential for optically and thermally driven soft robotics. The PT-TM actuator devices display robust mechanical strength and durability, with no noticeable degradation in their performance after more than 1000 cycles.

Control of conductive filament growth in flexible organic memristor by polymer alignment

Abstract This study investigates the dynamics for conductive filament growth in organic memristors from the viewpoint of the ionic transport by controlling the ordering of polymer molecules. Furthermore, we propose a simplistic and novel approach to improving the reliability of the devices by confining conductive filament growth. It is observed that conductive filament growth is varied with the polymer molecular ordering that governs the ion pathways. In the device with the aligned polymer electrolyte, conductive filament growth is realized only in a localized region due to the confinement of the ion pathways by the aligned polymer molecules. Our flexible organic memristor consisting of the homeotropically aligned polymer shows the reliable resistive switching characteristics and stable mechanical flexibility. This observed principle of conductive filament growth in the polymer electrolyte will provide a physical understanding of conductive filament growth in organic memristors, and useful guidelines for developing a new class of flexible neuromorphic electronics.

Graphene assisting magnetic alignment of a high-performance semiconducting polymer for improved carrier transport

We report the fabrication of large-area highly aligned films of a naphthalene dicarboximide-based copolymer P(NDI2OD-T2) via solvent vapor annealing of as-spun films under a high magnetic field. Structural characterization reveals that the incorporation of a small amount of graphene nanosheets via solution mixing remarkably improves the degree of chain alignment and ordering of the semiconducting polymers. Field-effect transistors based on the magnetically aligned polymer/graphene composites exhibit a dramatic enhancement of electron mobility as well as extraordinarily high mobility anisotropy of 81, compared to pristine P(NDI2OD-T2)-based devices. We further proposed a mechanism to explain the enhancement of molecular orientation and charge transport, which is based on the assembling of polymer aggregates on the π-plane of graphene to facilitate magnetic alignment.

Unprecedented dipole alignment in α-phase nylon-11 nanowires for high-performance energy-harvesting applications.

Dipole alignment in ferroelectric polymers is routinely exploited for applications in charge-based applications. Here, we present the first experimental realization of ideally ordered dipole alignment in α-phase nylon-11 nanowires. This is an unprecedented discovery as dipole alignment is typically only ever achieved in ferroelectric polymers using an applied electric field, whereas here, we achieve dipole alignment in as-fabricated nanowires of 'non-ferroelectric' α-phase nylon-11, an overlooked polymorph of nylon proposed 30 years ago but never practically realized. We show that the strong hydrogen bonding in α-phase nylon-11 serves to enhance the molecular ordering, resulting in exceptional intensity and thermal stability of surface potential. This discovery has profound implications for the field of triboelectric energy harvesting, as the presence of an enhanced surface potential leads to higher mechanical energy harvesting performance. Our approach therefore paves the way towards achieving robust, high-performance mechanical energy harvesters based on this unusual ordered phase of nylon-11.

Shear-Enhanced Stretchable Polymer Semiconducting Blends for Polymer-based Field-Effect Transistors

Solution-sheared field-effect transistors based on the blend of the high-mobility donor-acceptor conjugated copolymer poly(diketopyrrolo[3,4-c]pyrrole-co-thieno[3,2-b]thiophene) (PDBT-co-TT) and the elastic polymer poly(styrene-butadiene-styrene) (SBS) are demonstrated for stretchable electronics. In this study, PDBT-co-TT serves as a charge transport layer, and the insulating polymer SBS with double bonds is used for improving elasticity. Through combination with solution shearing, the phase separation and charge transport properties of the PDBT-co-TT/SBS blends can be manipulated. Compared with the spin-coated PDBT-co-TT/SBS blends showing lower charge mobilities (~10−3 cm2 V−1 s−1), the solution-sheared polymer-blend films with the PDBT-co-TT content of 20% maintain high mobility (>0.1 cm2 V−1 s−1). The films with 60% PDBT-co-TT can even achieve mobility as high as 2 cm2 V−1 s−1, which is higher than the pristine conjugated polymer. Furthermore, as the SBS content increases, the dichroic ratios of the solution-sheared blends increase, indicating improved alignment of the conjugated polymer chains. The PDBT-co-TT/SBS blends exhibit great stretchability and high charge mobilities even under 100% strain due to their mesh-like morphology. Moreover, solution shearing not only improves polymer alignment but also controls surface morphology to enhance stretchability. This work reveals the importance of solution shearing in high-mobility stretchable polymer semiconductor blends.