Water Vapor-induced Phase Separation Research Articles

Recyclable and leakage-suppressed microcellular liquid–metal composite foams for stretchable electromagnetic shielding

The metallic conductivity and high liquidity of liquid metal (LM) make LM-polymer composites exhibit high electrical conductivity, large stretchability, and excellent mechanical–electrical decoupling, which provide significant advantages in the fabrication of stretchable electromagnetic interference (EMI) shielding materials. However, problems such as undesired LM leakage, high density, and poor recyclability due to the frequent use of chemically cross-linked polymers limit the applications of LM-polymer composites. Herein, we report the fabrication of stretchable and recyclable microcellular thermoplastic polyurethane (TPU)/LM composite foams (TLFs) with controllable LM distribution through a water–vapor-induced phase separation (WVIPS) process. The sedimentation of LM droplets helps to improve the shielding effectiveness (SE) of TLF, while the microcellular structure enables TLF with more uniform LM dispersion to achieve high leakage resistance. Moreover, the stretch-activated TLF shows high electrical stability with only slight resistance changes during stretching and exhibits a strain-insensitive shielding behavior. As a stretchable joule heater, the sample has outstanding Joule-heating performance, with stable temperature dropping only slightly under large tensile strains. More importantly, the excellent solubility of TPU, combined with a solution-based WVIPS process, allows our TLFs to be easily recycled into new products with very similar structure and performance to those before recycling, indicating the ideal recyclability of such materials.

Nanosphere-structured hierarchically porous PVDF-HFP fabric for passive daytime radiative cooling via one-step water vapor-induced phase separation

Growing demand for efficient and economical cooling for indoor as well as outdoor applications, especially personal cooling in outdoor environments, is a major global challenge today. Currently, tailored optical structures with spectral selectivity are being used as cooling strategies. However, developing these photonic structures generally requires sophisticated multi-step manufacturing processes and use of multiple solvents which limits their acceptability for large-scale production and cost-effectiveness. Therefore, for the first time, herein we report the fabrication of nanosphere-structured hierarchically porous poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) fibrous fabric via a facile one-step electrospinning process based on water vapor-induced phase separation (WVIPS) technique using single solvent. The fabricated fabric with interconnected nanospheres structure and hierarchically porous fibers possessed large roughness and high specific surface area. Cooling performance of the fabricated PVDF-HFP fabrics was evaluated using skin simulators to mimic human body. The as-prepared PVDF-HFP fabric exhibits a superior average solar reflectance (∼93.7 %) and infrared emittance (∼91.9 %), yielding a temperature drop of ∼19.8 °C and ∼13.2 °C compared to the bare skin simulator and the one covered with cotton fabric, respectively, under solar intensity of ∼950 W m−2. Additionally, excellent hydrophobicity and adsorption make designed fabrics potentially suitable materials for not only radiative cooling but also for waterproof and filtration applications.

The role of solvent solubility parameters in the formation of intermittent low and high molecular weight polystyrene rich structures in a thin film resulting from water vapor induced phase separation

The water vapor-induced phase separation of self-similar polymer chains to form highly organized polystyrene thin films was investigated. The Hansen solubility of a set of polystyrenes was determined as a function of molecular weight. The solvent choice for the phase separation of polystyrene in thin films was investigated. Solvents with high hydrogen bonding promote the formation of intermittent phases of polystyrene film. Methyl ethyl ketone and 1,4-dioxane were found to form highly ordered structures.

Nanosphere-Structured Hierarchically Porous Pvdf-Hfp Fabric for Passive Daytime Radiative Cooling Via One-Step Water Vapor-Induced Phase Separation

Nanosphere-Structured Hierarchically Porous Pvdf-Hfp Fabric for Passive Daytime Radiative Cooling Via One-Step Water Vapor-Induced Phase Separation

Full-Textile Human Motion Detection Systems Integrated by Facile Weaving with Hierarchical Core-Shell Piezoresistive Yarns.

The tremendous progress of the wearable intelligent system has brought an urgent demand for flexible pressure sensors, especially for those possessing high sensing performances, simple manufacture technology, and efficient integration. In this work, hierarchical core-shell piezoresistive yarns (HCPYs), which contain internal silver-plated nylon electrodes and surface microporous structured carbon nanotubes (CNTs)/thermoplastic polyurethane (TPU) sensing layer, are designed and manufactured via facile wet-spinning accompanied by a water vapor coagulating bath. The obtained HCPY can either be inserted into traditional textiles to assemble a single-pressure sensor, or be woven into a textile-based flexible pressure sensors array with expected size and resolution, without compromising their comfort, breathability, and three-dimensional (3D) conformability. Simultaneously, to further enhance the sensing performance, the surface microporous structures of HCPYs are optimized by altering the treatment humidity and exposure time during the process of water vapor-induced phase separation. The wearable pressure sensors assembled by the optimal HCPY achieved a high sensitivity up to 84.5 N-1, a good durability over 5000-cycle tests, a fast response time of 2.1 ms, and a recovery time of 2.4 ms. Moreover, the wearable pressure sensors have been successfully used to monitor physical signals and human motions. The textile-based flexible pressure sensors array has also been seamlessly integrated with sportswear to detect movements of the elbow joint and map spatial pressure distribution, which makes HCPY a promising candidate for constructing next-generation wearable electronics.

Bifunctional ZIF-8 Grown Webs for Advanced Filtration of Particulate and Gaseous Matters: Effect of Charging Process on the Electrostatic Capture of Nanoparticles and Sulfur Dioxide.

Metal-organic framework (MOF), an emerging class of porous hybrid inorganic-organic crystals, has been applied for various environmental remediation strategies including liquid and air filtration. In this study, the role of the zeolite imidazole framework-8 (ZIF-8) was explored on the charge trapping ability and its contribution to capturing the targeted pollutants of NaCl nanoparticles and SO2 gas. Poly(lactic acid) fibers with controlled surface pores were electrospun using water vapor-induced phase separation, and the fiber surface was uniformly coated with ZIF-8 crystals via an in situ growth method. As a novel process approach, the corona charging process was applied to the ZIF-8 grown webs. The ZIF-8 promoted the charge trapping in the corona process, and the charged ZIF-8 web showed a significantly improved electrostatic filtration efficiency. Also, the charged ZIF-8 web showed an enhanced SO2 capture ability, both in the static and dynamic air flow states, demonstrating the applicability as a bifunctional filter for both particulate and gaseous matters. The approach of this study is novel in that both particulate and gas capture capabilities were associated with the charge trapping ability of ZIF-8, implementing the corona charging process to the ZIF-8 webs.

The Effect of Microcellular Structure on the Dynamic Mechanical Thermal Properties of High-Performance Nanocomposite Foams Made of Graphene Nanoplatelets-Filled Polysulfone.

Polysulfone nanocomposite foams containing variable amounts of graphene nanoplatelets (0–10 wt%) were prepared by water vapor-induced phase separation (WVIPS) and supercritical CO2 (scCO2) dissolution. WVIPS foams with two ranges of relative densities were considered, namely, between 0.23 and 0.41 and between 0.34 and 0.46. Foams prepared by scCO2 dissolution (0.0–2.0 wt% GnP) were obtained with a relative density range between 0.35 and 0.45. Although the addition of GnP affected the cellular structure of all foams, they had a bigger influence in WVIPS foams. The storage modulus increased for all foams with increasing relative density and GnP’s concentration, except for WVIPS PSU-GnP foams, as they developed open/interconnected cellular structures during foaming. Comparatively, foams prepared by scCO2 dissolution showed higher specific storage moduli than similar WVIPS foams (same relative density and GnP content), explained by the microcellular structure of scCO2 foams. As a result of the plasticizing effect of CO2, PSU foams prepared by scCO2 showed lower glass transition temperatures than WVIPS foams, with the two series of these foams displaying decreasing values with incrementing the amount of GnP.

A simple strategy to prepare functional PI microspheres by constructing coating on PAA microspheres followed thermal imidization reaction

With the development of downstream applications, functionalized polyimide (PI) composites have gradually become a research hotspot. The traditional approaches to prepare functional PI are complex and time-consuming, which are not suitable for large-scale production. In this paper, a three-step strategy for PI microspheres functionalization through PAA microsphere preparation, functional material cladding and thermal imidization is proposed. First, polyamide acid (PAA)-the precursor of polyimide, was prepared in the form of microspheres with a mean particle size of about 2 μm by water vapor induced phase separation (WVIPS) method. Subsequently, the functional fillers like TiO2 and graphene oxide have been successfully coated on the surface of PAA microspheres by hydrogen bonding and electrostatic adsorption force, respectively. It is worthy to note that the coated microspheres exhibiting well stability after the thermal imidization process. This work provided a facile and efficient strategy for the fabrication of functionalized PI composites.

Effects of Pluronic TR–702 on Chlorinated Poly(vinyl chloride) Flat–sheet Membranes Prepared by Water Vapor Induced Phase Separation

Membrane properties such as porosity and surface morphology significantly affect fouling in membrane bioreactors (MBRs). Amphiphilic Pluronic block–copolymers have attracted attention in recent years as their use during membrane preparation has resulted in improved membrane properties. This study focused on establishing a technique to improve membrane properties by adding Pluronic TR–702 to prepare a chlorinated poly(vinyl chloride) (CPVC) flat–sheet membrane suitable for use in an MBR. Deterioration of Pluronic TR–702 was expected during long–term MBR applications. Therefore, Pluronic TR–702 was removed from the membranes prior to evaluation. The influence of the additive on the membrane thickness, pure water permeability, pore size and distribution, surface morphology, and surface porosity was investigated. Excessive addition of Pluronic TR–702 to the dope solution led to the preparation of membranes with larger pore sizes that were unsuitable for MBR. On the other hand, appropriate addition increased the pore size and thickness of the membranes and decreased the surface porosity with increasing additive content. When the concentration of Pluronic TR–702 was 50 wt/wt%, the membrane with the suitable pore size for the use in MBR was prepared. This method is a useful technique for improving the properties of membranes used for MBRs.

Electrical Conduction Behavior of High-Performance Microcellular Nanocomposites Made of Graphene Nanoplatelet-Filled Polysulfone.

Graphene nanoplatelet (GnP)-filled polysulfone (PSU) cellular nanocomposites, prepared by two different methods—namely, water vapor-induced phase separation (WVIPS) and supercritical CO2 dissolution (scCO2) foaming—were produced with a range of densities from 0.4 to 0.6 g/cm3 and characterized in terms of their structure and electrical conduction behavior. The GnP content was varied from 0 to 10 wt%. The electrical conductivity values were increased with the amount of GnP for the three different studied foam series. The highest values were found for the microcellular nanocomposites prepared by the WVIPS method, reaching as high as 8.17 × 10−2 S/m for 10 wt% GnP. The variation trend of the electrical conductivity for each series was analyzed by applying both the percolation and the tunneling models. Comparatively, the tunneling model showed a better fitting in the prediction of the electrical conductivity. The preparation technique of the cellular nanocomposite affected the resultant cellular structure of the nanocomposite and, as a result, the porosity or gas volume fraction (Vg). A higher porosity resulted in a higher electrical conductivity, with the lightest foams being prepared by the WVIPS method, showing electrical conductivities two orders of magnitude higher than the equivalent foams prepared by the scCO2 dissolution technique.

Highly permeable polyethersulfone substrates with bicontinuous structure for composite membranes in CO2/N2 separation

Substrate morphology has a significant effect on the CO2 transport performance of a thin-film composite (TFC) membrane. In this study, a new mixed solvent comprising 2-pyrrolidone (2-PD) and 2-methoxyethanol (2-ME) was used to prepare polyethersulfone (PES) substrates with bicontinuous structure by water vapor-induced phase separation, followed by immersion in water as nonsolvent. Compared with the commonly used solvent, N-methyl-2-pyrrolidone (NMP), the 2-PD/2-ME mixed solvent is more hydrophilic and can decrease the thermodynamic stability of the casting solution significantly. As a result, the phase separation was induced via the spinodal decomposition mechanism. The open morphology bestowed the optimized substrate with a very high CO2 permeance of 133,226 ± 3870 GPU (1 GPU = 10–6 cm3(STP) s–1 cm–2 cmHg–1 = 3.3464 × 10-9 kg-mol s–1 m–2 Pa–1), which was 5 times more permeable than the baseline substrate synthesized with NMP. By using this new substrate, the prepared TFC membrane containing amines showed an increased CO2 permeance of 908 ± 8 GPU at 57 °C, which was 48 GPU higher than that with the baseline substrate. Meanwhile, the CO2/N2 selectivity was retained at 162 ± 4 at 57 °C. In addition, the good scalability of the new PES substrate was demonstrated by roll-to-roll fabrication of a 50′ long scale-up PES substrate with a width of 21ʺ.

Effects of Graphene Nanoplatelets and Cellular Structure on the Thermal Conductivity of Polysulfone Nanocomposite Foams.

Polysulfone (PSU) foams containing 0–10 wt% graphene nanoplatelets (GnP) were prepared using two foaming methods. Alongside the analysis of the cellular structure, their thermal conductivity was measured and analyzed. The results showed that the presence of GnP can affect the cellular structure of the foams prepared by both water vapor induced phase separation (WVIPS) and supercritical CO2 (scCO2) dissolution; however, the impact is greater in the case of foams prepared by WVIPS. In terms of thermal conductivity, the analysis showed an increasing trend by incrementing the amount of GnP and increasing relative density, with the tortuosity of the cellular structure, dependent on the used foaming method, relative density, and amount of GnP, playing a key role in the final value of thermal conductivity. The combination of all these factors showed the possibility of preparing PSU-GnP foams with enhanced thermal conductivity at lower GnP amount by carefully controlling the cellular structure and relative density, opening up their use in lightweight heat dissipators.

Water vapor induced phase separation: a simple and efficient method for fabricating polyetherimide microspheres

In this paper, a simple and efficient method coupled with water vapor induced phase separation (WVIPS) technique has been successfully applied for the fabrication of Polyetherimide (PEI) microsphere, exhibiting well spherical shape with a relatively uniform size. The formation mechanism of water vapor induced phase separation process is discussed in detail. Furthermore, the effects of the PEI solution concentration, preparation temperature and relative humidity (RH) on the structure and size of PEI samples were investigated. Lower PEI solution concentration is much more favorable to fabricate PEI microspheres. The SEM images and the particle size analysis show that adjusting the preparation temperature and relative humidity during the fabrication process can control the average size of PEI microspheres, ranging from 725 nm to 1.9 μm based on the 3 wt% PEI solution system. This WVIPS method is simple and efficient, and could broaden the applications area of PEI composite materials.

Construction of gradient structure in polyetherimide/carbon nanotube nanocomposite foam and its thermal/mechanical property

MWNTs-COOH@Fe3O4 hybrid with ferromagnetism was prepared and polyetherimide (PEI)/MWNTs-COOH@Fe3O4 nanocomposite foams with continuously gradient structure were fabricated via water vapor-induced phase separation (WVIPS) method under a magnetic field. Along the magnetic field direction, a continuously gradient distribution of MWNTs-COOH@Fe3O4 formed in PEI matrix, while the average cell size gradually decreased, the cell wall thickness and apparent density increased, forming gradient cell structure for the foam. Both mechanical property and thermal stability of the foam were improved by introducing MWNTs-COOH@Fe3O4. For the composite foam, the tensile strength, elongation at break and storage modulus decreased along magnetic field direction, due to enrichment and aggregation of MWNTs-COOH@Fe3O4 in strong magnetic field. Meanwhile, tanδ decreased and thermal degradation temperature increased, revealing formation of gradient distribution of thermal and mechanical property in the foam, which showed potential application prospective in aerospace and defense area.

Superhydrophobic and Low‐k Polyimide Film with Porous Interior Structure and Hierarchical Surface Morphology

AbstractPorous interior structured polyimide (PI) films with a hierarchical surface are fabricated from 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride and 4,4′‐oxydianiline by a water vapor induced phase separation process under a humid environment. Superhydrophobic properties with a water contact angle of 161° are obtained using the hierarchical surface morphology, which can be adjusted from flower‐like to wrinkle‐shape particles facilely by changing the relative humidity. The dielectric constant (k) of the PI film decreases sharply from 2.8 (film prepared under dry conditions) to ≈1.9 (film prepared under humid conditions) because of the interior porous structure and fluorine‐containing framework. Both a low‐k and superhydrophobicity are very important parameters for PI films in microelectronic and insulating applications.

Studies on thermal degradation kinetics and dielectric properties of polyether imide foam/nanosilica-based nanocomposites

ABSTRACTIn this paper, polyether imide (PEI) having properties such as a high glass transition temperature of 216°C, high heat resistance, high flame resistance, low smoke generation and a high melting point within the range of 400°C, having low thermal conductivity and low dielectric constant was chosen to be a polymeric foam. Water vapor-induced phase separation method was used to prepare PEI foams. PEI foams were reinforced with nano-silica (weight 1, 3 and 5%) in order to alter the dielectric properties, thermal conductivity and degradation kinetics of foamed polymer. The tested samples showed a reduction in dielectric constant than that of solid PEI but at a higher loading, it showed a higher value due to threshold percolation and a reduction in thermal conductivity was observed for foamed PEI. From thermogravimetric analysis, we can conclude that PEI with 3% filler loading showed better thermal stability compared to other PEI foam compositions.

Biophotonic Films: Biomimetic Polymer Film with Brilliant Brightness Using a One‐Step Water Vapor–Induced Phase Separation Method (Adv. Funct. Mater. 23/2019)

Biophotonic Films: Biomimetic Polymer Film with Brilliant Brightness Using a One‐Step Water Vapor–Induced Phase Separation Method (Adv. Funct. Mater. 23/2019)

Microcellular polyetherimide/carbon nanotube composite foam: Structure, property and highly reinforcing mechanism

Microcellular polyetherimide (PEI)/carbon nanotube (CNT) composite foams were prepared via water vapor-induced phase separation (WVIPS) process. The hydroxylated multi-wall nanotubes (MWNTs-OH) were applied, and strong interfacial interaction formed with PEI molecules by high grafting ratio. Much PEI resin adhered to MWNTs-OH surface, resulting in an obvious increase of MWNTs-OH diameter and formation of coating structure. For the composite foams, the homogenous spherical-like cell structure can be observed. With increasing MWNTs-OH content, the tensile strength of the composite foam first increased, and then decreased, reaching maximum at 4 wt% MWNTs-OH, a roughly 53% increase as a result of reinforcing effect of MWNTs-OH on matrix. Meanwhile, the storage modulus of the composite foam was much higher than that of pure PEI foam over the temperature range tested and still maintained above 610 MPa at 200 °C, suggesting a remarkable improvement of thermal mechanical stability. MWNTs-OH distributed uniformly in the matrix, and under stress it was pulled out with surface completely wrapped by PEI resin, confirming that the failure mode was the deformation and destruction of non-interface resin. By incorporation of MWNTs-OH, the thermal decomposition temperature, char yield and LOI values of the composite foam increased, while the UL-94 tests were classed as a V-0 rating.

Biomimetic Polymer Film with Brilliant Brightness Using a One‐Step Water Vapor–Induced Phase Separation Method

AbstractThe scales of the white Cyphochilus beetles are endowed with unusual whiteness arising from the exceptional scattering efficiency of their disordered ultrastructure optimized through millions of years of evolution. Here, a simple, one‐step method based on water vapor–induced phase separation is developed to prepare thin polystyrene films with similar microstructure and comparable optical performance. A typical biomimetic 3.5 µm PS film exhibits a diffuse reflectance of 61% at 500 nm wavelength, which translates into a transport mean free path below 1 µm. A complete optical characterization through Monte Carlo simulations reveals how such a scattering performance arises from the scattering coefficient and scattering anisotropy, whose interplay provides insight into the morphological properties of the material. The potential of bright‐white coatings as smart sensors or wearable devices is highlighted using a treated 3.5 µm film as a real‐time sensor for human exhalation.

Enhancing the electrical conductivity of polyetherimide‐based foams by simultaneously increasing the porosity and graphene nanoplatelets dispersion

Significant improvement in electrical conductivity of graphene nanoplatelets‐filled polyetherimide (PEI) foams was achieved by simultaneously increasing the porosity and graphene nanoplatelets dispersion. Foams were prepared by means of water vapor‐induced phase separation using a concentration of graphene nanoplatelets (GnP) between 1 and 10 wt%. To obtain two sets of foams having different density and porosity, PEI's concentration in N‐methyl pyrrolidone (NMP) solvent prior to foaming was set at 15 and 25–30 wt%, respectively. High‐power sonication was applied to GnP‐NMP suspension before PEI's addition for the foam series with higher porosity (15 wt% PEI). All foams were later characterized in terms of cellular structure, thermal stability, dynamic‐mechanical properties, and electrical conductivity. A notable enhancement in electrical conductivity was observed with foaming, especially when increasing the porosity and applying sonication, with foams reaching values as high as 1.7 × 10−1 S/m while maintaining the thermal stability and mechanical performance. POLYM. COMPOS., 40:E1416–E1425, 2019. © 2018 Society of Plastics Engineers