Crystalline Polymer Films Research Articles

Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate.

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

Time‐Dependent Information Encryption in Liquid Crystalline Polymer with Programmable Glass Transition Temperature

AbstractTime‐dependent dynamic information encryption technology is a promising approach to enhancing the security and complexity of information transmission. Herein, a time‐dependent information encryption model from a liquid crystalline polymer (LCP) film with a programmable glass transition temperature (Tg) and gradually adjustable fluorescence is demonstrated. The programmable Tg is achieved by adjusting the degree of order of the LC molecules via a configuration interconversion of spiropyran‐based materials (SPBMs), which can convert between a V‐shaped colorless spiro (SP) and a rodlike dark‐colored merocyanine (MC) form. An LCP film obtained by visible light polymerization exhibits a lower Tg than UV light, because the SPBM molecules keep different configurations in the two films. By adjusting the ratio of two isomerization forms of SPBM molecules during the polymerization process, the Tg values of LCP films can change from 11.6 °C to 31.1 °C. Based on the isomerization rate of SPBM in the LCP films with different Tg, time‐dependent information encryption is successfully achieved.

The directed self-assembly of reflective liquid crystalline polymer films to form polarization-independent diffractive optical elements

The directed self-assembly of liquid crystal polymers enables significantly enhanced optical properties compared to conventional isotropic optical materials. The transmissive properties of these thin polymeric films have been extensively studied, but the exploration of reflective liquid crystalline polymeric materials films has received little attention due to challenges in producing large, high-quality self-assembled films. We explore here for the first time stacked polymer cholesteric liquid crystal thin films as a pathway to generate reflective diffractive optics that act on both hands of circularly polarized light independently. We present 3 unique exemplar cases with increasing design complexity to communicate the widespread potential of these unique thin polymer films.

Improving OFF-State Bias-Stress Stability in High-Mobility Conjugated Polymer Transistors with an Antisolvent Treatment.

Conjugated polymer field-effect transistors are emerging as an enabling technology for flexible electronics due to their excellent mechanical properties combined with sufficiently high charge-carrier mobilities and compatibility with large-area, low-temperature processing. However, their electrical stability remains a concern. ON-state (accumulation mode) bias-stress instabilities in organic semiconductors have been widely studied, and multiple mitigation strategies have been suggested. In contrast, OFF-state (depletion mode) bias-stress instabilities remain poorly understood despite being crucial for many applications in which the transistors are held in their OFF-state for most of the time. Here, a simple method of using an antisolvent treatment is presented to achieve significant improvements in OFF-state bias-stress and environmental stability as well as general device performance for one of the best performing polymers, solution-processable indacenodithiophene-co-benzothiadiazole (IDT-BT). IDT-BT is weakly crystalline, and the notable improvements to an antisolvent-induced, increased degree of crystallinity, resulting in a lower probability of electron trapping and the removal of charge traps is attributed. The work highlights the importance of the microstructure in weakly crystalline polymer films and offers a simple processing strategy for achieving the reliability required for applications in flexible electronics.

Non-volatile optical memory based on cooperative orientation switching: improvement of recording speed and contrast by utilizing out-of-plane orientation mode.

Herein, we report a novel strategy toward non-volatile optical memory with high-contrast, high-speed recording, and non-destructive readout capability based on the cooperative out-of-plane orientation of a fluorescent dye doped into azobenzene liquid crystalline polymer film. By employing the out-of-plane orientation switching upon irradiation with UV light and thermal heating, high-contrast turn-on fluorescence switching was successfully achieved and the optical recording was demonstrated with non-destructive fluorescence readout capability. Furthermore, the recording speed and the fluorescence on/off contrast in the present system were dramatically improved compared to the previous in-plane orientation mode.

The Effect of a Topcoat with Amorphous Polymer Layers on the Mesogen Orientation and Photoalignment Behavior of Side Chain Liquid Crystalline Polymer Films

The mesogen orientations of liquid crystals are sensitive to the nature of the contacting surface. For side chain liquid crystalline polymer (SCLCP) films, most investigations have been conducted for thin films formed on a solid substrate surface such as glass, quartz and metal oxides, and little knowledge has been accumulated for SCLCP films whose top surface is covered by amorphous polymers. This work presents the effect of a topcoat with amorphous polymers placed on SCLCP films on the mesogen orientation and photoalignment behavior. When an SCLCP film that adopts a homeotropic mesogen orientation is covered with a glass plate or polymer layer, the mesogens turns to a random planar orientation. This planar orientation is favorable for efficient in-plane photoalignment by irradiation with linear polarized light. An in-plane order parameter exceeding 0.5 is readily obtained. Unexpectedly, a significant stabilization of the liquid crystal phase by over 10 °C is observed above the isotropization temperature of the SCLCP. These fundamental sets of knowledge should be significant in the fabrication of various polymer LC devices.

Bending Behaviors in Photoresponsive Liquid Crystalline Polymer Films Derived from a Hockey Stick-Shaped Reactive Mesogen

Bending Behaviors in Photoresponsive Liquid Crystalline Polymer Films Derived from a Hockey Stick-Shaped Reactive Mesogen

Photoindeuced bending behavior of uniaxial aligned crosslinked NBA liquid crystalline polymer films

Crosslinked liquid crystal polymers (CLCPs) have recently attracted much attention because they exhibit unique properties in stimuli responsiveness. We have recently focused on the photoinduced deformation behavior of CLCP containing N-benzylideneaniline (NBA) as a photoresponsive moiety. We prepared two types uniaxially aligned NBA-containing crosslinked liquid crystalline polymer film, homogeneous and homeotropic, and evaluated its photomechanical properties. The initial alignment of photoactive mesogens significantly affects the bending behavior of the CLCP films: the homogeneous films bent toward the irradiation direction of the actinic UV light, while the homeotropic films bent away from the light source.

Specific deformation behavior of isotactic polypropylene films under a multiaxial stress field.

The specific deformation behavior of crystalline polymer films, namely unoriented crystallized isotactic polypropylene (it PP) films, was investigated under a multiaxial stress field. Changes in the aggregation structure of the films were investigated during the bulge deformation process using in situ small-angle X-ray scattering, wide-angle X-ray diffraction (WAXD) measurements, and polarized high-speed-camera observations. The films had a thickness of approximately 10 μm. The it PP films were fixed at the hole of a plate, then bulge deformation was applied using N2 or He gas pressure, and stress-strain curves were then calculated from the applied pressure and bulge height. Yielding was observed in the stress-strain curves. Below the yield point, in situ WAXD measurements revealed that the crystal lattice expanded isotropically at the center, edge, and bottom of the bulge hole. Above the yield point, a craze started to form slightly near the center, and crazes formed in various directions with a further increase in strain, while the crystal lattice expanded uniaxially along the circumference at the edge and bottom. Crazes oriented in various directions merged and lost birefringence, indicating a change to the isotropic orientation. The different directions of the crazes indicated several directions of stress. In other words, even if multiaxial deformation is applied to a crystalline it PP film, the string-shaped crystalline polymer chain structure produces local anisotropic uniaxial stress.

Photo-triggered surface relief formation of polystyrene films based on the Marangoni flow driven by a surface photoresponsive skin layer

Surface relief gratings (SRGs) are widely investigated in azobenzene-containing polymer films. We have recently demonstrated that a photoresponsive azobenzene layer existing only at the surface of a non-photoresponsive liquid crystalline polymer film can induce mass migration by patterned UV light irradiation, resulting in the SRG structure. This process is explained by the Marangoni flow triggered by the photoinduced surface tension change of the surface layer. In this work, we newly propose that the same strategy is applicable to an amorphous polymer material of polystyrene. The results obtained here indicate that photoinduced SRG formation is expanded for general amorphous polymers.

Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals.

To visualize and simultaneously quantify the necking behavior of crystalline polymer films during uniaxial stretching, tetraarylsuccinonitrile (TASN) moieties were introduced into polymers at the center of the main chain. TASN can produce a relatively stable radical that emits yellow fluorescence in response to mechanical stress. During the uniaxial elongation test of the TASN-centered crystalline polymers, the yellow fluorescence derived from the dissociated TASN radicals was used for microscale observations that showed the orientation of the polymer chains in the stretching direction. Furthermore, by comparing the radical generation in linear and star-shaped TASN-centered crystalline polymers during their tensile deformation, we found that the TASN dissociation ratio is higher in the star-shaped polymer, which has more chains connected to the lamellar crystal. Thus, the microforces generated in the amorphous region during uniaxial stretching were probed via the use of TASN, which allowed a direct visualization of the necking initiation and propagation processes as well as a quantification via electron paramagnetic resonance spectroscopy.

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.

Facile way of dynamically tailoring microporous structures in polyvinylidene fluoride films prepared by thermally induced phase separation

AbstractPolyvinylidene fluoride (PVDF) films were prepared via thermally induced phase separation (TIPS) using diphenyl carbonate as the diluter in an attempt to disclose the competitive relationship between crystal growth and droplet growth during phase separation process. By varying the quenching temperature different temperature gradient fields were established, which were theoretically evaluated via enthalpy transformation method. The effects of polymer concentration and quenching temperature on evolution of hierarchical morphologies in TIPS films were systematically investigated. According to the morphological characteristics, the cross‐sectional morphology of the films with lower polymer concentration (ΦP = 25%) could be divided into three layers; while that of higher polymer concentration counterpart (ΦP = 55%) only presented a bi‐layered structure. The reason for this could be ascribed to the effect of cooling rate on both crystal growth and droplet growth during TIPS process, which further determined the formation of the hierarchical structure in microporous films. With an increasing quenching temperature, both pore size and porosity of PVDF films increased, accompanied by an improvement on both thermal stability and dynamic mechanical thermal property. The present study could insightfully supply a facile route to fabricate structure‐controllable microporous films of crystalline polymers via an appropriate regulation of the TIPS quenching parameters.

Photo-Luminescence Characteristics of Aligned Organic Fluorescent Dye in Liquid Crystalline Polymer Films for Photosynthesis Promotion

Photo-Luminescence Characteristics of Aligned Organic Fluorescent Dye in Liquid Crystalline Polymer Films for Photosynthesis Promotion

Enantiomeric Switching of the Circularly Polarized Luminescence Processes in a Hierarchical Biomimetic System by Film Tilting.

Circularly polarized luminescence (CPL) switching has attracted great attention due to the potential applications in chiral photonics and electronics. However, the lack of examples to achieve switchable CPL within a single material in the dry solid state hampers the scope of applications. Herein, we demonstrate a crystalline chiral polymer film as a polarizing medium consisting of radially assembled twisted crystallites, where achiral aggregation-induced emissive luminogens (AIEgens) are confined between the twisted crystalline stacks, eventually yielding handedness-switchable CPL by simple film tilting. Hierarchically organized twisted crystallites create the selective reflection activity of the polarizing medium. Upon film tilting, enantiomeric switching is realized by selectively collecting transmitted and reflected CPL components. The confined AIEgens in the crystalline polarizing system show a great enhancement of the luminescence efficiency. Moreover, the approach is general with broadband activity, and various AIEgens could be applied to generate full-color-tunable CPL. Additionally, the rigid and continuous nature of this polarizing system affords enhanced optical stability and facile modulation, developing a general route for designing chiroptical materials.

Photo-triggered large mass transport driven only by a photoresponsive surface skin layer

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films. Various mechanisms have been proposed to account for these motions. This study explores light-inert side chain liquid crystalline polymer (SCLCP) films with a photoresponsive polymer only at the free surface and reports the key effects of the topmost surface to generate surface relief gratings (SRGs) for SCLCP films. The top-coating with an azobenzene-containing SCLCP is achieved by the Langmuir–Schaefer (LS) method or surface segregation. A negligible amount of the photoresponsive skin layer can induce large SRGs upon patterned UV light irradiation. Conversely, the motion of the SRG-forming azobenzene SCLCP is impeded by the existence of a LS monolayer of the octadecyl side chain polymer on the top. These results are well understood by considering the Marangoni flow driven by the surface tension instability. This approach should pave the way toward in-situ inscription of the surface topography for light-inert materials and eliminate the strong light absorption of azobenzene, which is a drawback in optical device applications.

In situ tailoring the hierarchical microstructures in polyethylene films fabricated by thermally induced phase separation via temperature gradient fields

In this study, a series of polyethylene thin films were prepared by thermally induced phase separation (TIPS) method with di(2-ethylhexyl)phthalate as the diluent. Morphological observations showed that the microstructures at different locations were fairly distinct and could generally be divided into three layers. When the polymer concentration was lower than the monotectic point (Φm), both pore size and porosity in TIPS films were heavily determined by growth time and growth rate of the diluent droplets. At relatively higher polymer concentration, cooling rate heavily affected the pore size in the films primarily through its influence on polymers’ crystallization process. The relationship between the film microstructure and cooling rate was quantitatively investigated (especially the effect of quenching conditions on the evolution of film morphology) with the aid of an enthalpy transformation method, which established the three-dimensional temperature profiles and supplied an insight into the competition between phase separation kinetics and polymer crystallization. In addition, our findings showed that the pore size, porosity and thermal stability of films all increased with an increasing quenching temperature, which was significantly different from the trends of variation in water vapor transmission rate as well as the $$ E^{\prime}_{ \hbox{max} } $$ and $$ E^{\prime\prime}_{ \hbox{max} } $$ values of dynamic mechanical analysis as the quenching temperature increased. The present work presents a facile way of in situ tailoring hierarchical microstructures in TIPS films of crystalline polymers, which will be very useful to the design of polymer porous films with specific properties.

Highly crystalline and efficient red-emissive π-conjugated polymer film: tuning of macrostructure for light-emitting properties

π-Conjugated alternating polymer of bithiophene and tetrafluorophenylene gave highly crystalline and efficient red-emissive film. Interestingly, significant tuning of the light-emission of the films was realized by controlling the crystallinity.

Temperature-Dependent Gas Transport Behavior in Cross-Linked Liquid Crystalline Polyacrylate Membranes

Stable, cross-linked, liquid crystalline polymer (LCP) films for membrane separation applications have been fabricated from the mesogenic monomer 11-(4-cyanobiphenyl-4′-yloxy) undecyl methacrylate (CNBPh), non-mesogenic monomer 2-ethylhexyl acrylate (2-EHA), and cross-linker ethylene glycol dimethacrylate (EGDMA) using an in-situ free radical polymerization technique with UV initiation. The phase behavior of the LCP membranes was characterized using differential scanning calorimetry (DSC) and X-ray scattering, and indicated the formation of a nematic liquid crystalline (LC) phase above the glass transition temperature. The single gas transport behavior of CO2, CH4, propane, and propylene in the cross-linked LCP membranes was investigated for a range of temperatures in the LC mesophase and the isotropic phase. Solubility of the gases was dependent not only on the condensability in the LC mesophase, but also on favorable molecular interactions of penetrant gas molecules exhibiting a charge separation, such as CO2 and propylene, with the ordered polar mesogenic side chains of the LCP. Selectivities for various gas pairs generally decreased with increasing temperature and were discontinuous across the nematic–sotropic transition. Sorption behavior of CO2 and propylene exhibited a significant change due to a decrease in favorable intermolecular interactions in the disordered isotropic phase. Higher cross-link densities in the membrane generally led to decreased selectivity at low temperatures when the main chain motion was limited by the lack of mesogen mobility in the ordered nematic phase. However, at higher temperatures, increasing the cross-link density increased selectivity as the cross-links acted to limit chain mobility. Mixed gas permeation measurements for propylene and propane showed close agreement with the results of the single gas permeation experiments.

Photoinitiated Marangoni flow morphing in a liquid crystalline polymer film directed by super-inkjet printing patterns

Slight contaminations existing in a material lead to substantial defects in applied paint. Herein, we propose a strategy to convert this nuisance to a technologically useful process by using an azobenzene-containing side chain liquid crystalline (SCLCP) polymer. This method allows for a developer-free phototriggered surface fabrication. The mass migration is initiated by UV-light irradiation and directed by super-inkjet printed patterns using another polymer on the SCLCP film surface. UV irradiation results in a liquid crystal-to-isotropic phase transition, and this phase change immediately initiates a mass migration to form crater or trench structures due to the surface tension instability known as Marangoni flow. The transferred volume of the film reaches approximately 440-fold that of the polymer ink, and therefore, the printed ink pattern acts as a latent image towards the amplification of surface morphing. This printing-aided photoprocess for surface inscription is expected to provide a new platform of polymer microfabrication.