High-Pressure CO2 Sorption in Polymers of Intrinsic Microporosity under Ultrathin Film Confinement

Penetrant-induced plasticization in microporous polymer membranes

Penetrant-induced plasticization has prevented the industrial deployment of many polymers for membrane-based gas separations. With the advent of microporous polymers, new structural design features and unprecedented property sets are now accessible under controlled laboratory conditions, but property sets can often deteriorate due to plasticization. Therefore, a critical understanding of the origins of plasticization in microporous polymers and the development of strategies to mitigate this effect are needed to advance this area of research. Herein, an integrative discussion is provided on seminal plasticization theory and gas transport models, and these theories and models are compared to an exhaustive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas permeation tests and mixed-gas permeation tests for pure polymers and composite films. Finally, an evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided along with suggestions for future directions of fundamental and applied research on the topic.

Gas Adsorption and Diffusion Behaviors in Interfacial Systems Composed of a Polymer of Intrinsic Microporosity and Amorphous Silica: A Molecular Simulation Study

We investigate the adsorption and diffusion behaviors of CO₂, CH₄, and N₂ in interfacial systems composed of a polymer of intrinsic microporosity (PIM-1) and amorphous silica using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. We build model systems of mixed matrix membranes (MMMs) with PIM-1 chains sandwiched between silica surfaces. Gas adsorption analysis using GCMC simulations shows that gas molecules are preferentially adsorbed in microcavities distributed near silica surfaces, resulting in an increase in the solubility coefficients of CO₂, CH₄, and N₂ compared to bulk PIM-1. In contrast, diffusion coefficients obtained from MD simulations and then calibrated using the dual-mode sorption model show different tendencies depending on gas species: CO₂ diffusivity decreases in MMMs compared to PIM-1, whereas CH₄ and N₂ diffusivities increase. These differences are attributed to competing effects of silica surfaces: the emergence of larger pores as a result of chain packing disruption, which enhances gas diffusion, and a quadrupole-dipole interaction between gas molecules and silica surface hydroxyl groups, which retards gas diffusion. The former has a greater impact on CH₄ and N₂ diffusivities, whereas the latter has a greater impact on CO₂ diffusivity due to the strong quadrupole-dipole interaction between CO₂ and surface hydroxyls. These findings add to our understanding of gas adsorption and diffusion behaviors in the vicinity of PIM-1/silica interfaces, which are unobtainable in experimental studies.

Nano-Confinement Effects on Structural Development and Organic Solvent-Induced Swelling of Ultrathin Carbon Molecular Sieve Films

Successful implementation of carbon molecular sieve (CMS) membranes in large scale chemical processes inevitably relies on fabrication of high performance integrally skinned asymmetric or thin-film composite membranes. In principle, to maximize separation efficiency the selective CMS layer should be as thin as possible which requires its lateral confinement to a supporting structure. In this work, we studied pyrolysis-induced structural development as well as ethanol vapor-induced swelling of ultrathin CMS films made from a highly aromatic polyimide of an intrinsic microporosity (PIM-PI) precursor. Utilization of a light polarization-sensitive technique, spectroscopic ellipsometry, allowed for the identification of an internal orientation within the turbostratic amorphous CMS structure driven by the laterally constraining support. Our results indicated a significant thickness dependence both in the extent of pyrolytic collapse and response to organic vapor penetrant. Thinner, substrate-confined films (∼30 nm) collapsed more extensively leading to a reduction of microporosity in comparison to their thicker (∼300 nm) as well as self-supported (∼70 μm) counterparts. The reduced microporosity in the thinner films induced changes in the balance between penetrant-induced dilation (swelling) and filling of micropores. In comparison to thicker films, the initial lower microporosity of the thinner films was accompanied by slightly enhanced organic vapor-induced swelling. The presented results are anticipated to generate the fundamental knowledge necessary to design optimized ultrathin CMS membranes. In particular, our results reinforce previous findings that excessive reduction of the selective layer thickness in amorphous microporous materials (such as PIMs or CMS) beyond several hundred nanometers may not be optimal for maximizing their fluid transport performance.

Modeling of Gas Transport through Polymer/MOF Interfaces: A Microsecond-Scale Concentration Gradient-Driven Molecular Dynamics Study

Membrane-based separation technologies offer a cost-effective alternative to many energy-intensive gas separation processes, such as distillation. Mixed matrix membranes (MMMs) composed of polymers and metal-organic frameworks (MOFs) have attracted a great deal of attention for being promising systems to manufacture durable and highly selective membranes with high gas fluxes and high selectivities. Therefore, understanding gas transport through these MMMs is of significant importance. There has been longstanding speculation that the gas diffusion behavior at the interface formed between the polymer matrix and MOF particles would strongly affect the global performance of the MMMs due to the potential presence of nonselective voids or other defects. To shed more light on this paradigm, we have performed microsecond long concentration gradient-driven molecular dynamics (CGD-MD) simulations that deliver an unprecedented microscopic picture of the transport of H₂ and CH₄ as single components and as a mixture in all regions of the PIM-1/ZIF-8 membrane, including the polymer/MOF interface. The fluxes of the permeating gases are computed and the impact of the polymer/MOF interface on the H₂/CH₄ permselectivity of the composite membrane is clearly revealed. Specifically, we show that the poor compatibility between PIM-1 and ZIF-8, which manifests itself by the presence of nonselective void spaces at their interface, results in a decrease of the H₂/CH₄ permselectivity for the corresponding composite membrane as compared to the performances simulated for PIM-1 and ZIF-8 individually. We demonstrate that CGD-MD simulations based on an accurate atomistic description of the polymer/MOF composite is a powerful tool for characterization and understanding of gas transport and separation mechanisms in MMMs.

Thickness Changes in Temperature-Responsive Poly(N-isopropylacrylamide) Ultrathin Films under Ambient Conditions

In this paper, we report detailed experimental observations of unusual changes in the thickness of solid poly(N-isopropylacrylamide) (PNIPAM) ultrathin films, which are well known to have temperature-responsive hydrophilic-hydrophobic switching properties. To date, a number of studies have been carried out on the bulk and the brush forms of PNIPAM in contact with liquid water, as well as in highly humid environments, and, recently, these ultrathin films have been preliminarily shown to exhibit temperature responses even under low-humidity, ambient conditions. In this work, the thicknesses of ultrathin PNIPAM films in a temperature/moisture-controlled sample stage were monitored continuously using multichannel X-ray reflectometry. At room temperature, the sample thickness showed an unexpected increase after thermal treatment at 70 °C for 3 h. In the temperature cycle between 15 and 60 °C, heating and cooling resulted in some clear differences. During cooling, initially, the thickness was almost constant but began to increase when the temperature exceeded 33 °C, which corresponds to the lower critical solution temperature (LCST). This observation indicates that the PNIPAM ultrathin film is sensitive to the small amounts of water contained in the air, even under ambient, low-humidity conditions. On the other hand, during heating run from 15 to 60 °C, the humidity dependence was monotonic, and no specific changes in the PNIPAM films were observed at around the LCST. By studying the humidity dependence, we found that the hydrophilic and hydrophobic states of the PNIPAM ultrathin film exhibit different temperature dependence behaviors. In addition, we found that swelling takes place even under low-moisture conditions. To understand the difference in the thickness changes observed on cooling and heating further, some models considering the effect of the boundary conditions in the polymer ultrathin film system were considered. In the case of the ultrathin film, the hydrophilic/hydrophobic switching property occurred only in the surface layer, which dominated the absorption of water molecules from air. In contrast, the interface layer was time-stable and provided an escape route for water molecules during heating.

Thin Composite Carbon Molecular Sieve Membranes from a Polymer of Intrinsic Microporosity Precursor

Ultra-thin composite carbon molecular sieve (CMS) membranes were fabricated on well-defined inorganic alumina substrates using a polymer of intrinsic microporosity (PIM) as a precursor. Details of the pyrolysis-related structural development were elucidated using focused-beam, interference-enhanced spectroscopic ellipsometry (both in the UV-vis and IR range), which allowed accurate determination of the film thickness, optical properties as well as following the chemical transformations. The pyrolysis-induced collapse of thin and bulk PIM-derived CMS membranes was compared with CMS made from a well-known non-PIM precursor 6FDA-DABA. Significant differences between the PIM and non-PIM precursors were discovered and explained by a much larger possible volume contraction in the PIM. In spite of the differences, surprisingly, the gas separation properties did not fundamentally differ. The high-temperature collapse of the initially amorphous and isotropic precursor structure was accompanied by a significant molecular orientation within the formed turbostratic carbon network guided by the laterally constraining presence of the substrate. This manifested itself in the development of uniaxial optical anisotropy, which was shown to correlate with increases in gas separation selectivity for multiple technologically important gas pairs. Reduction of CMS skin thickness significantly below ∼1 μm induced large losses in permeability coefficients with only small to moderate effects on selectivity. Remarkably, skin thickness reduction and physical aging seemed to superimpose onto the same trend, which explains and strengthens some of the earlier fundamental insights.