Nematic liquid crystalline polymer films for gas separation

The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO2, CH4 and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH4 and He over CO2 decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO2).</p

Self-assembling liquid crystals as building blocks to design nanoporous membranes suitable for molecular separations

The transition from the current “linear” economy to a “circular” economy with a strong focus on the recovery and reuse of materials and resources undoubtedly necessitates efficient and effective separation technologies. Membrane technology will play an important role in this transition to a circular economy. In that perspective, separation at the molecular level to separate and fractionate e.g. individual ions and small molecules for reuse is especially essential. Unfortunately, conventional membrane materials and their fabrication methods mostly lack design and control over pore size and selectivity at a true molecular level. In view of this challenge, nanostructured polymer membranes based on self-assembled materials are gaining more and more interest. Using the self-assembly properties of polymerizable liquid crystal molecules ensures control at a molecular level and gives rise to narrow pore size distributions, high pore densities and control of pore size and functionality. In this review, the potential of liquid crystal materials and their self-assembly properties to fabricate nanoporous membranes for water purification, desalination and selective recovery is presented. The basic principles of liquid crystals, the self-assembling characteristics and methods to control pore size and functionality are discussed in the perspective of membrane properties and applications. Efforts reported in the literature highlighting advances and pointing out important limitations for different pore morphologies are discussed. The versatility of liquid crystal based membranes is highlighted by exploring approaches for post-modification of the nanopores to further tune the pore size and control the pore functionality after polymerization of the liquid crystals. The work provides readers with a thorough understanding of the design and fabrication of nanoporous liquid crystal membranes combined with a perspective on the potential of liquid crystal membranes. Next to recent advances, future challenges are presented as well, with the most crucial two: 1) The formation of thin, defect-free nanoporous liquid crystal layers supported on a microporous support; 2) Large-scale production combined with alignment control over longer length scales

Switchable gas permeability of a polypropylene-liquid crystalline composite film

The development of functionalized polyolefins for use as stimuli-responsive commodity polymers has recently received much attention. In this work, a microporous polypropylene (PP) scaffold is used to align and fortify a smectic liquid crystalline network (LCN) which can switch its gas permeability upon pH changes. The LCN is a photopolymerized liquid crystalline mixture of a dimerized benzoic acid derivative monoacrylate and a diacrylate crosslinker. In the hydrogen-bonded state, the composite membrane shows a high-molecular order and a low permeability for He, N2, and CO2 gases. By pH switching from the hydrogen-bonded state to the salt form, the molecular order is reduced, and the gas permeability is increased by one order of magnitude. This increase is mainly attributed to a loss in order of the system, increasing the free volume, resulting in an increased diffusibility. By exposing the composite film to basic or acidic environments, reversible switching between low and high gas permeability states is observed, respectively. The physical constraints imposed by the PP scaffold strengthens the membrane while the reversible switching inside the liquid crystalline polymer is maintained. This switching of gas permeation properties is not possible with the fragile freestanding LCN films alone

Systematic investigation of methods to suppress membrane plasticization during CO2 permeation at supercritical conditions

The suppression of CO2-induced plasticization in polyimide membranes at supercritical conditions up to 120 bar is investigated. Three approaches (polymer blending, thermal treatments and chemical crosslinking) known from relatively low-pressure applications are applied and their effectiveness to suppress membrane plasticization at high CO2 pressures and under supercritical conditions is systematically identified. CO2 sorption measurements reveal that especially Henry sorption promotes plasticization and that the corresponding Henry sorption parameter (kD) correlates with the d-spacing and Tg of the membranes. A lower d-spacing and higher Tg results in a reduced kD parameter and thus a higher resistance to plasticization. A high interchain rigidity is required to suppress plasticization at the highly plasticizing liquid-like CO2 densities. Chemical and thermo-oxidative crosslinking results in the largest decrease in interchain mobility and therefore shows the highest resistance to plasticization, but also a significantly lower permeability. Thermally treating the membranes in N2 retains a high permeability, while still displaying significant plasticization resistance. Polymer blending does increase the plasticization resistance, but strongly reduces the permeability. All three methods manage to suppress plasticization at supercritical conditions, but crosslinking offers superior plasticization resistance. However, proper tailoring strategies are required to combine a high plasticization resistance with a high permeability

Humidity-gated, temperature-responsive photonic infrared reflective broadband coatings

The fabrication of temperature responsive photonic polymers remains a challenge. Here, we report the fabrication of humidity-gated temperature-responsive infrared reflective photonic coatings using an easy-to-process bar-coating technique. At high humidity the hydroscopic cholesteric liquid crystalline polymer is able to absorb water vapour from the air causing swelling of the photonic coating. By increasing the temperature, water is desorbed from the coating, resulting in a reversible 420 nm shift of the photonic reflection band. In particular, it is shown that temperature-responsive single-layered broadband IR reflective coatings, prepared by creation of a pitch gradient of the cholesteric liquid crystals, might be suitable for smart window applications in high relative humidity environments such as greenhouses

Molecular Order Determines Gas Transport through Smectic Liquid Crystalline Polymer Membranes with Different Chemical Compositions

Amorphous polymers are often used for gas separation but have a trade-off between gas permeability and selectivity. Here, the effect of chemical composition and temperature on gas permeability and solubility in well-ordered LC polymer membranes is investigated. Membranes with various compositions of a monomethacrylate LC (M1) with a crown ether functionality to enhance CO2 solubility and a smectic diacrylate (M2) cross-linker were fabricated, while all having the same order (smectic C) and alignment (planar). Single gas sorption and permeation data show for the membranes with 30 wt % M1 a higher CO2 solubility coefficient compared to membranes without M1, which results in a higher CO2 permeability and selectivity. For membranes that contain more than 30 wt % M1 decreasing layer spacings lead to reduced gas solubilities that result in lower gas permeabilities without additional selectivity gain toward CO2. The effect of temperature is demonstrated by comparing single gas sorption and permeation data below and above the Tg of the membranes. The diffusion coefficient increases above the Tg of the membranes with increasing M1 content leading to higher CO2 permeabilities and selectivities. These results show that not only the chemical composition but also the layer spacing of the smectic structures determines the gas separation performance of smectic LC polymer membranes

About Gas Barrier Performance and Recyclability of Waterborne Coatings on Paperboard

For preserving food packed in environmentally friendly and recyclable paperboard packages, it is important to have sufficient gas barrier performance of the paperboard container. Paperboard has poor intrinsic barrier properties and to overcome this deficiency, so a barrier coating is needed that does not hinder the recycling of the paperboard substrate. However, the gas barrier properties and the recyclability of such coatings have been rarely studied. Here, both the gas barrier performance and the removal of an alkali-soluble resin (ASR)-stabilized waterborne barrier coatings from paperboard are investigated. For barriers for gases, such as nitrogen, carbon dioxide, and oxygen, defect-free coatings are needed which is achieved by applying three coating layers. The oxygen transmission rate (OTR) of the three-layered coating on paperboard was 920 cm 3/(m 2∙day). For water vapor barriers, two coating layers already show a strong improvement, as water follows a different penetration mechanism than the other tested gases. The water vapor transmission rate WVTR of double coated paperboard was 240 g/(m 2∙day). Preliminary results show that the coating is removed by immersion of the coated paperboard in an aqueous alkaline solution at room temperature. This causes de-protonation of the carboxylic acids of the ASR and subsequent re-dispersion of the coating in water. Removing double-layer coatings from the paperboard is more challenging, possibly due to the coating/coating interface between the two coating layers and enhanced adhesion between coating and paperboard

Tuning the Gas Separation Performances of Smectic Liquid Crystalline Polymer Membranes by Molecular Engineering

The effect of layer spacing and halogenation on the gas separation performances of free-standing smectic LC polymer membranes is being investigated by molecular engineering. LC membranes with various layer spacings and halogenated LCs were fabricated while having a planar aligned smectic morphology. Single permeation and sorption data show a correlation between gas diffusion and layer spacing, which results in increasing gas permeabilities with increasing layer spacing while the ideal gas selectivity of He over CO2 or He over N2 decreases. The calculated diffusion coefficients show a 6-fold increase when going from membranes with a layer spacing of 31.9 Å to membranes with a layer spacing of 45.2 Å, demonstrating that the layer spacing in smectic LC membranes mainly affects the diffusion of gasses rather than their solubility. A comparison of gas sorption and permeation performances of smectic LC membranes with and without halogenated LCs shows only a limited effect of LC halogenation by a slight increase in both solubility and diffusion coefficients for the membranes with halogenated LCs, resulting in a slightly higher gas permeation and increased ideal gas selectivities towards CO2. These results show that layer spacing plays an important role in the gas separation performances of smectic LC polymer membranes