Polymerization-induced Phase Separation Research Articles

Preparation of reverse mode polymer dispersion liquid crystals with vertically oriented layers and the electro-optical properties

ABSTRACT The reverse-mode polymer dispersed liquid crystals (re-mode PDLC) are feasible to be prepared by polyimide vertically oriented layers. The indium tin oxide (ITO) glass coated with polyimide-oriented layer was prepared by centrifugal spin coating and high temperature curing, and the re-mode PDLC film was prepared by polymerisation-induced phase separation method. After that the effects of diluent, negative liquid crystal and gadolinium oxide (Gd2O3) nanoparticle content on the morphology and electro-optical properties of re-mode PDLC were comprehensively investigated. The results show that the re-mode PDLC with appropriate diluent and negative liquid crystal content has stable contrast ratio and driving voltage, and the re-mode PDLC samples doped with Gd2O3 nanoparticles can further increase the contrast ratio to improve the problem of low contrast of ordinary re-mode PDLC samples. In the thermal ageing life test, the driving voltage ageing degree of the nanoparticle-doped re-mode PDLC samples can be stabilised, which has some significance for use of re-mode PDLC.

Superior electro-optical switching properties in polymer dispersed liquid crystals prepared with functionalized carbon nanotube nanocomposites of LC for switchable window applications

In this work, we are reporting the results on electro-optical switching properties of polymer dispersed liquid crystal (PDLC) devices for switchable window related applications. The PDLC cells are prepared by mixing octadecylamine functionalized single-walled carbon nanotubes (ODA-SWCNT) nanocomposites of liquid crystals (LC) and acrylate-based monomer by polymerization induced phase separation method. The important switchable device parameters such as threshold voltage, switching voltage, response time, contrast ratio and effective refractive index are investigated. The PDLC cells prepared with 0.05 wt% ODA-SWCNT nanocomposites of the LC, 7CB show almost zero threshold field and 36% less driving field compared to PDLC prepared with pure 7CB. A distinct optical response (frequency doubling) phenomenon is observed for PDLC prepared with 0.03 and 0.05 wt% ODA-SWCNT nanocomposites of 7CB compared to PDLC cells prepared with pure 7CB and lower concentrations (<0.03 wt%) of ODA-SWCNT. The relaxation of optical transmittance is observed at low frequency even when the applied electric field (Eapp) is ON which is due to the strong induced internal electric field (due to free ions) which nullifies the Eapp. The physical mechanism of this unique optical behavior is discussed in detail. The PDLC prepared with 0.01 wt% ODA-SWCNT show 84% faster response time compared to PDLC prepared with pure 7CB. The working of prototype PDLC devices for use in switchable window related applications is reported. The results indicate that, the PDLC prepared with 0.05 wt% ODA-SWCNT nanocomposites of 7CB show high ON state clarity and high OFF state opacity compared to PDLC prepared with pure LC and lower concentrations of ODA-SWCNT.

Highly Damping and Self-Healable Ionic Elastomer from Dynamic Phase Separation of Sticky Fluorinated Polymers.

Shock-induced low-frequency vibration damage is extremely harmful to bionic soft robots and machines that may incur the malfunction of fragile electronic elements. However, current skin-like self-healable ionic elastomers as the artificial sensing and protecting layer still lack the ability to dampen vibrations, due to their almost opposite design for molecular frictions to material's elasticity. Inspired by the two-phase structure of adipose tissue (the natural damping skin layer), here, a highly damping ionic elastomer with energy-dissipating nanophases embedded in an elastic matrix is introduced, which is formed by polymerization-induced dynamic phase separation of sticky fluorinated copolymers in the presence of lithium salts. Such a supramolecular design decouples the elastic and damping functions into two distinct phases, and thus reconciles a few intriguing properties including ionic conductivity, high stretchability, softness, strain-stiffening, elastic recovery, room-temperature self-healability, recyclability, and most importantly, record-high damping capacity at the human motion frequency range (loss factor tan δ>1 at 0.1-50Hz). This study opens the door for the artificial syntheses of high-performance damping ionic skins with robust sensing and protective applications in soft electronics and robotics.

Polymer dispersed ionic liquid electrolytes with high ionic conductivity for ultrastable solid‐state lithium batteries

AbstractSolid polymer electrolytes (SPEs) have emerged as one of the most promising candidates for building solid‐state lithium batteries due to their excellent flexibility, scalability, and interfacial compatibility with electrodes. However, the low ionic conductivity and poor cyclic stability of SPEs do not meet the requirements for practical applications of lithium batteries. Here, a novel polymer dispersed ionic liquid‐based solid polymer electrolyte (PDIL‐SPE) is fabricated using the in situ polymerization‐induced phase separation (PIPS) method. The as‐prepared PDIL‐SPE possesses both outstanding ionic conductivity (0.74 mS cm−1 at 25°C) and a wide electrochemical window (up to 4.86 V), and the formed unique three‐dimensional (3D) co‐continuous structure of polymer matrix and ionic liquid in PDIL‐SPE can promote the transport of lithium ions. Also, the 3D co‐continuous structure of PDIL‐SPE effectively accommodates the severe volume expansion for prolonged lithium plating and stripping processes over 1000 h at 0.5 mA cm−2 under 25°C. Moreover, the LiFePO4//Li coin cell can work stably over 150 cycles at a 1 C rate under room temperature with a capacity retention of 90.6% from 111.1 to 100.7 mAh g−1. The PDIL‐SPE composite is a promising material system for enabling the ultrastable operation of solid‐state lithium‐metal batteries.

Multicomponent oxide microspheres with designed macroporosity (MICROSCAFS®): a customized platform for chemicals immobilization.

A novel versatile, easily recoverable, and recyclable material platform is herein presented, consisting of multicomponent oxide microspheres, of silica-titania and silica-titania-hafnia composition, with tailored interconnected macroporosity (MICROSCAFS®). When functionalized or loaded with desired species, they are potential enablers of emerging applications in environmental remediation, among other fields. We combine emulsion templating for the spherical shape of the particles, with an adapted sol-gel technique involving polymerization-induced phase separation by spinodal decomposition. An advantage of our method regards the employed mix of precursors, which prevents the use of specific gelation additives and porogens and allows a high reproducibility of MICROSCAFS®. We present insight into their formation mechanism using cryo-scanning electron microscopy, and a systematic study of the effect of multiple synthesis parameters on the MICROSCAFS® size and porosity. The composition of the silicon precursors has the most significant effect on fine-tuning the pores size, ranging from the nanometer to the micron scale. Mechanical properties are correlated with morphological features. Larger macroporosity (68% open porosity, estimated by X-ray computed tomography) leads to less stiffness, higher elastic recovery, and compressibility values up to 42%. We believe this study creates a base for consistent custom MICROSCAFS® production, with a design for various future applications.

Shape‐Controlled Synthesis of Covalent Organic Frameworks Enabled by Polymerization‐Induced Phase Separation

The shape and morphology modulations of covalent organic frameworks (COFs) are both difficult, but are of significanceto tackle to realize high-performance and practical applications. Here, a two-step method is reported that separates the phase separation and crystallization processes for the shape-controlled synthesis of COFs. The insight into the polymerization-induced phase separation (PIPS) allows for the flexible shaping of COFs into column, rod, film and others, as well as for constructing hierarchically porous structure. The as-synthesized COF monoliths are crack-free, no powder detaching, and show 214MPa of compressive modulus. The resulting good permeability and mechanical flexibility enable COF films to apply for flow-through adsorption and extraction of pollutants at high flow rates. This work successfully resolves the contradiction between PIPS and crystallization, offering a general approach for scalable production of COFs with desired shapes, sizes, and morphologies.

Simultaneously Enhancing the Mechanical Strength and Ionic Conductivity of Stretchable Ionogels Enabled by Polymerization-Induced Phase Separation

Stretchable ionogels have been considered as ideal materials for constructing flexible electronics. However, current ionogels suffer from the well-known trade-off between mechanical strength and conductivity. Here, we develop a simple strategy based on polymerization-induced phase separation to simultaneously enhance the mechanical performance and conductivity of the ionogels by randomly copolymerizing a hydrophobic and a hydrophilic monomer in a hydrophobic ionic liquid (IL). The polymerization process induced the formation of a bicontinuous network containing a polymer-rich phase and a solvent-rich phase. The polymer-rich domains with hydrogen bonds can bear loading, greatly improving the mechanical strength; meanwhile, the solvent-rich domains form conductive nanochannels to enhance the conductivity. The resulting copolymer ionogel is highly stretchable (500% strain), and the optimal fracture stress and conductivity are 0.29 MPa and 3.4 mS/cm, achieving 7.8- and 2.3-fold enhancements compared with that of the prepared homogeneous (pure PMEA) ionogel at the same IL content, respectively. Moreover, the ionogels also exhibit anti-swelling properties in various liquids and self-adhesiveness. Potential applications of this ionogel as a wearable sensor in a complex environment are further demonstrated.

4-Amino-TEMPO-Immobilized Polymer Monolith: Preparations, and Recycling Performance of Catalyst for Alcohol Oxidation.

Continuous flow reactors with immobilized catalysts are in great demand in various industries, to achieve easy separation, regeneration, and recycling of catalysts from products. Oxidation of alcohols with 4-amino-TEMPO-immobilized monolith catalyst was investigated in batch and continuous flow systems. The polymer monoliths were prepared by polymerization-induced phase separation using styrene derivatives, and 4-amino-TEMPO was immobilized on the polymer monolith with a flow reaction. The prepared 4-amino-TEMPO-immobilized monoliths showed high permeability, due to their high porosity. In batch oxidation, the reaction rate of 4-amino-TEMPO-immobilized monolith varied with stirring. In flow oxidation, the eluent permeated without clogging, and efficient flow oxidation was possible with residence times of 2-8 min. In the recycling test of the flow oxidation reaction, the catalyst could be used at least six times without catalyst deactivation.

The first space-filling polyhedrons of polymer cubic cells originated from Weaire-Phelan structure created by polymerization induced phase separation

The Weaire–Phelan structure is a three-dimensional structure composed of two different polyhedra having the same volume, i.e., pyritohedron and truncated hexagonal trapezohedron. It was proposed by Weaire and Phelan in 1993 as a solution of the Kelvin problem of filling space with no gaps with cells of minimum surface area and equal volume. It was found in physical systems including liquid foam and a metal alloy while it has never been constructed as organic materials. We report herewith the first polymeric Weaire–Phelan structure constructed through phase-separation of a single polymer species that is synthesized by simple polyaddition between tetrakis(3-mercaptopropionate) and 1,6-diisocyanatohexane. The structure has the order of micrometers and is amorphous unlike reported crystal structures similar to the Weaire–Phelan structure.

In Situ Polymerization of Ionic Liquid with Tunable Phase Separation for Highly Reversible and Ultralong Cycle Life Zn-Ion Battery.

Severe Zn dendrite growth and side reactions greatly limit the application of aqueous zinc-ion batteries. Herein, we design a layer of polyionic liquid (PCAVImBr) film with a tunable pore size and charge density on the Zn anode to endow homogenized distribution of an electronic field, acerated Zn2+ permeation, and inhabitation of water entry. Such an optimal combination is achieved via a polymerization induced phase separation strategy, where the enhanced cross-linking density arrests the phase separation in a shallow depth and vice versa. Furthermore, the Zn@PCAVImBr electrode has good plating/stripping reversibility, which retains a 99.6% CE efficiency after 3000 cycles. The symmetric cells can achieve a cycle life of more than 2400 h at different current densities. It is worth mentioning that the NVO//Zn@PCAVImBr full cell can still reach a 91.2% capacity retention after nearly 4000 cycles at a high current of 10 A g-1, and provides new insights for the future research of zinc-ion battery anodes.

Electro-Optical Characteristics of Polymer Dispersed Liquid Crystal Doped with MgO Nanoparticles.

In this paper, inorganic oxide MgO nanoparticles-doped polymer dispersed liquid crystal (PDLC) films were made from a mixture of the prepolymer, SLC1717 liquid crystal, and MgO nanoparticles by the polymerization induced phase separation (PIPS) process. To observe the effect of MgO concentration, PDLC was dispersed with 0.2, 0.4, 0.6, and 0.8 wt.% MgO. Electro-optical properties of the films have been investigated using LCD parameter meter and Scanning Electron Microscope (SEM) at room temperature. It is established that MgO nanoparticles affect the microstructure of PDLC films significantly because of the formed agglomerates of MgO nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the driving voltage for doped systems with MgO nanoparticles. When the doping amount of MgO is 0.8 wt.%, the threshold voltage (Vth) is reduced to about 7.5 V. Therefore, MgO-doped PDLC is expected to become an excellent choice in the field of energy-saving.

Microporous Structure Formation of Poly(methyl methacrylate) via Polymerization-Induced Phase Separation in the Presence of Poly(ethylene glycol).

It has been demonstrated that nano- or micro-structured polymeric materials have huge potential as advanced materials. However, most of the current fabricating methods have limitations either in cost or in size. Here, we investigate the bulk polymerization of methyl methacrylate in the presence of poly(ethylene glycol) (PEG). We found that phase separation occurs during bulk polymerization. After removal of PEG via sonication, microscopic structures of poly(methyl methacrylate), including porous structures, co-continuous monolith structures, or particle aggregation structures, are formed. These structures can be controlled by the amount of PEG added and the reaction temperature. The results are summarized in phase diagrams. The addition of PEG significantly affects the reaction kinetics. Phase separation is coupled with the reaction acceleration known as the Trommsdorff effect. As a result, the reaction completes in a shorter time when the PEG amount is higher. We demonstrate surface coating to fabricate an amphiphobic surface, repelling both water and oil. The methods presented here have the potential to fabricate microscopic structures in large areas cost-effectively.

Ionic liquid-doped liquid crystal/polymer composite for multifunctional smart windows

In this work, a facile approach to fabricating multifunctional smart windows by employing an ionic liquid-doped liquid crystal (LC)/polymer composite was reported. The composite was prepared via an established method of polymerization-induced phase separation while a commercial ionic liquid was doped into the composite as a supporting electrolyte. The smart window, which was prepared by sandwiching the as-prepared composite in two indium tin oxide (ITO) glasses, can exhibit a multifunctional mode. Specifically, the light transmittance and the coloration of the windows were regulated by different external stimuli. The electrically switchable transmittance of the window was achieved by an alternating current (AC) electric field and the window transformed into a transparent state from an opaque state under the AC field. The temperature was also used to realize the heat controllable transmittance, in which the window was turbid at a lower temperature and transparent at a high temperature. The electrically induced coloration of the window was manipulated by a direct current (DC) electric field and the window was changed into a green state from a colorless state under the DC field. This work is expected to open avenues to achieve a promising candidate for multifunctional smart windows with practical applications.

(Invited) Multifunctional Lithium Ion Battery Separators through Polymerization-Induced Phase Separation

In state-of-the-art lithium ion batteries, separators (microporous membranes) play a passive yet critical role – hosting liquid electrolyte and maintaining physical separation of the electrodes. However, as the demands on lithium ion batteries increase, with an emphasis on greater energy density, longevity (cycle/calendar life), and safety, engineering separators to take a on more active role in the cell (electro)chemistry is expected to be an important strategy. Myriad membrane materials and separator designs have been developed to impart additional functionality, for example, acid and/or transition metal scavenging, temperature responsiveness, enhanced thermal stability, increased ion dissociation, combustion suppression, and mechanical strength.In this talk, I will preset my group’s approach to additive manufacturing of next-generation lithium ion battery separators. Our approach is based on polymerization induced phase separation (PIPS), wherein polymerizable monomers (or prepolymer resins) are mixed with porogen. Through rapid, low-cost, readily scalable photopolymerization, the monomers are converted to a crosslinked polymer network, which results in the porogen becoming immiscible and phase separating through spinodal decomposition. By tuning the thermodynamics of the polymer-porogen mixture and photopolymerization kinetics, the porosity and pore size of the resulting polymeric phase can be tuned. We have shown that ethylene carbonate (EC) mixed with common acrylate monomers, such as 1,4-butanediol diacrylate, is an effective porogen. Most importantly because EC is an indispensable component in liquid electrolytes, it does not need to be extracted from the separator prior to incorporation into the electrochemical cell.By controlling the ratio of the 1,4-butanediol diacrylate (BDDA) monomer to EC, monolithic microporous membranes are readily prepared with 25 µm thickness and pore sizes and porosities ranging from 6.8 to 22nm and 15.4% to 38.54%, respectively. The optimal poly(1,4-butanediol diacrylate) (pBDDA) separator has a porosity of 38.5% and average pore size of 22 nm; uptakes 127% liquid electrolyte by mass, and has an ionic conductivity of 1.98 mS/cm, which is higher than that of Celgard 2500. Lithium ion battery half cells consisting of LiNi0.5Mn0.3Co0.2O2 cathodes and pBDDA separators were shown to undergo reversible charge/discharge cycling with an average discharge capacity of 142 mAh/g and a capacity retention of 98.4% over 100 cycles - comparable to cells using state-of-the-art separators. Furthermore, the pBDDA separators were shown to be thermally stable to 400°C, lack low temperature thermal transitions that can compromise cell safety, and exhibits no thermal shrinkage up to 150°C. I will also discuss my group’s efforts to engineer separators with additional functionality to improve cell performance under abuse conditions.

Phase-field simulation for the formation of porous microstructures due to phase separation in polymer solutions on substrates with different wettabilities

The porous microstructure has been widely observed in a variety of polymer solutions that have been broadly applied in many industry fields. Phase separation is one of the common mechanisms for the formation of the porous microstructure in binary polymeric mixtures. Previous studies for the formation of porous microstructures mostly focus on the separation of the bulk phase. However, there is a paucity of investigation for the phase separation of polymer mixtures contacting the solid substrate. When the polymeric liquid mixtures interact with the solid substrate, the wetting boundary condition has to be taken into account. In this work, we present a phase-field model which is coupled with the wetting boundary condition to study the phase separation in binary polymer solutions. Our consideration is based on the polymerization-induced phase separation, and thermally induced phase separation by using the Flory–Huggins model. By taking the wetting effect into account, we find that polymer droplets spontaneously occur in the microstructure, even though the bulk composition is outside the spinodal region. This phenomenon is caused by the surface composition resulting from the wetting effect that was often overlooked in literature. For the phase separation in the binary polymer mixture, we also study the impact of the temperature gradient on the microstructural evolution. The porosity, the number of droplets, and the mean radius of the droplets are rationalized with the temperature gradient.

Mechanically and thermally robust microporous copolymer separators for lithium ion batteries

Next generation, multifunctional separators can enhance energy storage, power, and safety performance of lithium ion batteries but must be simple to fabricate and incorporate with existing roll-to-roll manufacturing. This study presents a strategy to facilely prepare these separators using UV-initiated polymerization-induced phase separation (PIPS), wherein microporous polymer separators are fabricated directly from constituent monomers and ethylene carbonate (EC) porogen. This enables a wide compositional design space as co-monomers with specific chemical functionality can be readily incorporated into the PIPS precursor mixture. Herein, 1,4-butanediol diacrylate (BDDA) was copolymerized with poly(ethylene glycol) diacrylate (PEGDA) to increase the acrylate conversion in the photopolymerization and improve mechanical properties. By tuning the ratio of PEGDA and EC, separators with high porosity (41.3%) and effective ionic conductivity (2.09 mS cm−1) were prepared. Inclusion of PEGDA was essential to increasing the elastic modulus to > 345 MPa, which is required for cell assembly by roll-to-roll manufacturing. All separators prepared were shown to enable reversible cycling of lithium metal/LiNi0.5Mn0.3Co0.2O2 half-cells for 100 cycles. Unlike conventional polyolefin separators, which were shown to melt at 160 °C and shrink by up to 29.8% at elevated temperatures, the PIPS separators possess exceptional, safety-enhancing thermomechanical properties, undergoing no phase transitions or thermal shrinkage.

Improving PDLC electro-optical performance using a photomask multi-step polymerisation method

ABSTRACT Polymer-dispersed liquid crystals (PDLCs) have been intensively studied for their light scattering properties and are of great use in electro-optical display devices. In this paper, high performance PDLCs were prepared by a multi-step polymerisation method using a photomask through a polymerisation-induced phase separation (PIPS) process. The effect of the photomask on the polymerisation of polymerisable monomers was investigated and it was confirmed that the photomask could effectively polymerise the polymerisable monomers in the light-transmitting fraction preferentially to form polymer-dominated spacer columns. In the subsequent complementary polymerisation process, the liquid crystal content of the previously unpolymerised fraction increases, which leads to a decrease in threshold and drive voltage and an increase in contrast of the PDLC film. Compared to conventional polymerisation methods, the multi-step polymerisation method can reduce the threshold voltage to 38.5 V, the drive voltage to 55.8 V and the contrast to 42, which greatly improves the overall electro-optical properties of PDLC, and the formation of spacer columns improves the bonding of the film, offering new possibilities for achieving energy efficient PDLC films and flexible films.

Dye-Sensitized Solar Cell Based on TiO2 Anode Thin Film with Three-Dimensional Web-like Structure.

TiO2 films with a three-dimensional web-like porous structure were prepared using the photo polymerization-induced phase separation method integrated with the pulling coating process. By adjusting the ratio of the substance in the precursor sol and the coating times, the relationships between the sol ratio, the coating times, the film structure, and the performance of the DSC were studied. The optimal film structure was found and a detailed description is given. The performance of the DSC was further improved by introducing the barrier layer and the surface-modified layer of the TiO2 coating. This promoted the short-circuit current density and the photoelectric conversion efficiency of the DSC, the mechanism of which was also investigated. Ultimately, the photoelectric conversion efficiency of the DSC based on the TiO2 anode films with a three-dimensional web-like structure was stabilized at a higher level as a result of the structural improvement.

Hard block length distribution of thermoplastic polyurethane determined by polymerization-induced phase separation

Three thermoplastic polyurethanes (TPUs) based on the same monomers and composition were produced in bulk by hand and machine casting methods. The soft segment was PTHF® 1000 and the hard segment was made of 1,4-butanediol and 4,4′-methylene diphenyl diisocyanate (MDI). The hard segment content amounted to 30%. The hand cast TPUs were prepared at 80 and 125 °C. The temperature conditions for the machine prepared sample were comparable to that of the hand casting process at 125 °C. The samples prepared at high temperature were transparent, whereas the sample produced at low temperature was translucent. The latter showed some crystallinity and in crystallization studies it crystallized more readily than the transparent samples. The distinctions in transparency and crystallization behavior were ascribed to differences in the hard block length distribution. Using thermal fractionation – a series of successive self-nucleation and annealing steps – the minor differences in hard block length distribution could be shown and quantified. The TPU synthesized at low temperatures possessed the longer hard blocks. These facilitated crystallization by enhancing nucleation and crystal growth. Superstructures, induced by the longer hard blocks, scattered visible light and reduced the transparency.

The role of nanomesh fibres loaded with BaTiO3 nanoparticles on the electro-optical performance of PDLC devices

In this paper, polymer-dispersed liquid crystal (PDLC) films were prepared from a blend of prepolymer, liquid crystal and BaTiO3 (BTO) nanoparticle-laden nanofibres films by a polymerisation-induced phase separation (PIPS) method. By contrasting the microstructure and electro-optical curves of various proportions of monomer types, it was demonstrated that the hole size of the polymer network had a significant influence on the electro-optical nature of the PDLC films. nanofibres films loaded with different BTO nanoparticle contents were introduced into the liquid crystal mixture specimens, and the alterations in the polymer network, electro-optical nature of these specimens were analysed using SEM images, EDS and electro-optical curves. The electro-optical nature of the nanofibres films doped with PDLCs were found to be improved and the ageing resistance was greatly enhanced. Therefore, the development of PDLC films with faster response times and better electro-optical nature of reticular nanofibres films doped with PDLCs will significantly improve the promising technological applications of PDLC-based equipment for optical windows, displays, energy storage and flexible devices.