Chain Packing Research Articles

High-performing anion exchange membranes enabled by diversifying the polymer backbone of quaternized poly(arylene alkylene)s

Anion-conducting poly(arylene alkylene)s are one of the most encouraging anion exchange membrane candidates with high alkaline stability for clean energy power generation. However, optimization of their conductivities often relies on the improvement of ion contents. Herein, a simple design strategy of backbone modification is reported, which can modulate the chain packing behavior and increase the free volume by introducing dimethylfluorenylene (DMeF) units into the poly(terphenylene alkylene) backbone, so as to realize the conductivity improvement of the membrane. The average chain spacing of DMeF-incorporated PTFPA membrane is extended to over 5.1 Å from the original poly(terphenylene alkylene) with the value of 4.2 Å. Accordingly, the membrane properties can be effectively modified by varying the contents of DMeF units, and PFTPA-81 containing 81 mol% of DMeF units particularly demonstrates a combination of high ion conductivity (177 mS cm−1), proper water absorption (<90 wt% at 80 °C), and uncompromised alkaline stability (>1000 h in 1 M aq. at 80 °C). Moreover, a H2–O2 fuel cell prototype fabricated from PFTPA-81 achieved a high peak power density of 1.33 W cm−2, and the cell voltage decreased less than 8% (from 0.75 V to 0.69 V) for an in-situ durability test of 30 h.

Crystallization of Star-Shaped Poly(l-lactide)s with Arm Chains Aligned in the Same Direction in Two-Dimensional Crystals in a Langmuir Monolayer

Polylactide (PLA) crystallizes to form extended-chain crystals in a Langmuir monolayer because crystallization is accelerated on the water surface. This is a unique situation where chain packing can be analyzed by simply measuring the lamellar thickness. Herein, star-shaped poly(l-lactide)s (PLLAs) with 2-12 arms were synthesized through the polymerization of l-lactide with various polyols as initiators, and their crystallization behavior in a monolayer was studied via atomic force microscopy. The PLLAs comprising 2-4 arms crystallized with all arms aligned in the same direction and being folded at the central polyol unit. Meanwhile, the PLLAs comprising 6 and 12 arms crystallized with both halves of the arms extended from the center to the opposite directions, most likely due to the steric hindrance of the crowded arms. Considering that the PLLAs crystallized from a once-formed condensed amorphous state during compression, they have a strong tendency to crystallize with the arms aligned in the same direction. The crystallization rate of star-shaped PLAs is known to reduce compared with that of a linear PLA even if the number of arms is as few as 2. This should be closely related to the unique crystallization behavior of the star-shaped PLLAs with the arms aligned in the same direction.

Boosting membrane carbon capture via multifaceted polyphenol-mediated soldering

Advances in membrane technologies are significant for mitigating global climate change because of their low cost and easy operation. Although mixed-matrix membranes (MMMs) obtained via the combination of metal-organic frameworks (MOFs) and a polymer matrix are promising for energy-efficient gas separation, the achievement of a desirable match between polymers and MOFs for the development of advanced MMMs is challenging, especially when emerging highly permeable materials such as polymers of intrinsic microporosity (PIMs) are deployed. Here, we report a molecular soldering strategy featuring multifunctional polyphenols in tailored polymer chains, well-designed hollow MOF structures, and defect-free interfaces. The exceptional adhesion nature of polyphenols results in dense packing and visible stiffness of PIM-1 chains with strengthened selectivity. The architecture of the hollow MOFs leads to free mass transfer and substantially improves permeability. These structural advantages act synergistically to break the permeability-selectivity trade-off limit in MMMs and surpass the conventional upper bound. This polyphenol molecular soldering method has been validated for various polymers, providing a universal pathway to prepare advanced MMMs with desirable performance for diverse applications beyond carbon capture.

Isodimorphic crystallization and thermally-induced crystal transitions in poly(octamethylene-ran-decamethylene carbonate): Critical role of comonomer defects

Minor comonomer units can enter the crystalline phase of isodimorphic copolymers as defects, and affect their chain conformation and packing. However, the effects of minor comonomer inclusion on the crystalline structure and crystal transitions of aliphatic copolycarbonates are still unclear. Herein, we synthesized the poly(octamethylene-ran-decamethylene carbonate) with various compositions and systematically studied their crystallization behavior and crystal transitions upon cooling and heating. The random copolycarbonates exhibit isodimorphic crystallization behavior, showing the pseudo-eutectic region at octamethylene carbonate fraction (fC8) of 0.64–0.79. The octamethylene carbonate (C8)-rich copolycarbonates (fC8 ≥ 0.79) show reversible solid-solid crystal transitions as poly(octamethylene carbonate) (PC8) homopolymer upon cooling and heating, which are temperature- and composition-dependent. With fC8 decreasing, the δ-α transition is observed at a high temperature with more inclusion of comonomer defects, and the crystals are conformational disordered with development of more gauche conformation in methylene segments at a low temperature when compared to PC8 homopolymer. At a high temperature, the chain conformation in methylene segments of C8-rich copolycarbonates are analogous as that of PC8 homopolymer, thus the α-β transition temperature remains unchanged.

Quantifying the Contribution of London Dispersion Interaction and Adjacent Chain Packing on the Polymer Stiffness: A DFT Study

Linear polymers often have the role of sustaining tension along their chains, and thus, the longitudinal modulus of such polymers has been of great interest. To quantify the contribution of individual chain properties and their association with crystals, we studied the deformation behavior of three major classes of linear polymers, polyethylene, poly(p-phenylene terephthalamide), henceforth abbreviated as PPTA, and cellulose, using density functional theory (DFT). Polyethylene, the simplest polymer, elongates by simply widening its bond angle and stretching its C–C bonds. The single-chain spring constant along the chain is not affected by packing, and the longitudinal Young’s modulus is proportional to the number of chains per cross-sectional area. In contrast, the torsion around chemical bonds plays an important role in the deformation of PPTA and cellulose. Finally, lateral packing contributes to the stiffening of individual chains, the effect of which can be modeled by either removing the correction for London dispersion interactions or removing adjacent chain packing. This finding highlights the different structure–property relationship between crystalline polymers with and without hydrogen-bonding networks and will provide molecular insights that may assist the molecular design of high-performance polymeric materials.

Bacterial cellulose: A highly versatile nanomaterial.

Bacterial cellulose: A highly versatile nanomaterial.

Development of polyamide/polyacrylonitrile thin film composite RO membranes by interfacial polymerization assisted with an aromatic/aliphatic organic solvent mixture

AbstractThe proper control of polymerization is a prerequisite in fabrication of thin film composite reverse osmosis (RO) membranes. But it not trivial when hydrophilic substrates are used. Herein, we report an innovative approach which involves using an aromatic/aliphatic solvent mixture of toluene and n‐hexane as the organic phase for interfacial polymerization. Membranes were tuned by using a customized developed polyacrylonitrile (PAN) substrate and exploring the impacts of organic solvent and curing temperatures on their morphology and performance characteristics. Results revealed that increasing the temperature of organic phase to 20°C improved salt rejection significantly to 98.6% with the flux of 16.1 LMH. However, its further increase to 30°C was not beneficial due to formation of a looser chain packing which led to rejection drop. In addition, raising curing temperature to 90°C was not favorable due to transformation of surface morphology from ridge‐and‐valley to nodular structure, accompanied with defective sites at the selective layer and insufficient degree of crosslinking evidenced by declines in both flux and rejection to 11.1 LMH and 83.4%, respectively. Overall, the findings suggested that an optimal performance could be obtained by using a solvent mixture of toluene/n‐hexane (1,1) and setting organic solvent and curing temperatures to 20 and 70°C, respectively.

Highly porous ultrathin polyamide membranes for fast separation of small molecules from organic solvents

Membranes with permselective performance as well as high stability in harsh solvents are urgently desired for energy-efficient precise chemical separations. In this work, we utilize an adamantane diamine as a molecular building block to fabricate thin-film composite polyamide membranes for high-performance organic solvent nanofiltration (OSN). The bulky, cage-like adamantane structure acts as a molecular-level steric hindrance unit to inhibit polymer chain packing which generating interconnected micropores within the interfacially polymerized polyamide layer. This translates into high membrane methanol permeance of 20.6 L m-2 h-1 bar-1 and 94.7% retention to organic dye molecules with molecular weight down to 327 g mol-1. Furthermore, the membrane demonstrates high stability in different organic solvents taking advantage of the crosslinked rigid structure, which shows constant solute retention throughout a 180 h continuous filtration experiment with a DMF solution containing methyl orange as feed solution. Employing rigid and bulky molecular features in selective layer is thus demonstrated to significantly promote the polyamide membrane permeability and solvent resistance without compromising the overall selectivity.

Development and optimization of whey protein- Lepidium perfoliatum gum packaging films: An approach towards antimicrobial and biodegradable films

The use of plastic materials as the main packaging components in the food industry has caused widespread environmental and economic problems, so the use of edible films and coatings with diverse capabilities can be considered as a good choice. This study aimed to optimize a composite edible film based on whey protein concentrate (WPC) and Lepidium perfoliatum L. gum (LPG) using transglutaminase enzyme through Response Surface Methodology and loaded with oregano (Origanum vulgare L.) essential oil. The optimum value of independent factors with regard to minimum thickness, moisture, and solubility of films was LPG concentration 0.5 % w/v, WPC/LPG ratio 2.9, enzyme content 10 % and glycerol concentration 15 % v/v. The optimum responses for thickness, moisture, and solubility of films were 130 µm, 8.13 %, and 53.61 %, respectively. The physicochemical properties, X-ray diffraction analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, mechanical and thermal properties of optimized films were also studied. Microscopic images showed that the prepared films had a rough and irregular surface, while the cross-section micrograph indicated a smooth and crack-free surface. The physicochemical properties revealed a denser packing of polymer chains and semi-crystalline structure of films. The water vapor permeability of the film was 1.21 × 10−10 ± 0.03 g/m s Pa. The obtained results confirmed that preparation of composite films based on WPC and LPG was successful and the loaded film with oregano essential oil can be used for biodegradable food packaging.

Nanohybrid Pebax/PEGDA-GPTMS membrane with semi-interpenetrating network structure for enhanced CO2 separations

Polymeric membranes containing polyethylene oxide (PEO) groups are promising CO2 separation materials, but suffer from the trade-off between separation performance and mechanical strength. To overcome this challenge, we combine the merits of semi-interpenetrating polymer network and nanohybrid to maintain a high PEO loading and robust membrane structure. The membranes were prepared by introducing in-situ formed PEO networks with silica nodes into Pebax1657 to produce a semi-interpenetrating network structure. The introduced network and in situ formed silica nanoparticles reduce the chain packing efficiency of Pebax matrix to enhance the CO2 diffusivity, while the increased PEO content enhances CO2 solubility. Under feed gas pressure of 25 bar, nanohybrid membranes containing 40 wt% polymer network display excellent gas separation performance with CO2 permeability of 272.8 Barrer and CO2/N2 selectivity of 79.6 under mixed gas separation tests, surpassing 2008 Robeson upper bound. Moreover, the hybrid membranes showed high mechanical strength, the Young's modulus increases by 1.95 times. This strategy provides a simple route for the design of nanohybrid membranes with high separation performance as well as excellent durability.

Occurrence of gradual resonance in a finite-length granular chain driven by harmonic vibration

This study presents numerical simulations of the resonance of a finite-length granular chain of dissipative grains driven by a harmonically vibrated tube. Multiple gradual resonant modes, namely non-resonance mode, partial-resonance mode, and complete-resonance mode, are identified. With a fixed vibration frequency, increasing vibration acceleration leads to a one-one-one increase in the number of grains participating in resonance, which is equal to the number of grain-wall collisions in a vibration period. Compared with the characteristic time of the grain–grain and the grain–wall collisions, the time of free flight plays a dominant role in grain motion. This situation results in the occurrence of large opening separation gaps between the grains and independent grain–grain and grain–wall collisions. A general master equation that describes the dependence of the system energy on the length of the granular chain and the number of grain–wall collisions is established, and it is in good agreement with the simulation results. We observe a gradual step-jump increase in system energy when the vibration acceleration is continuously increased, which is dedicated to an individual energy injection. The phase diagrams in the spaces of packing density and vibration acceleration, chain length and vibration acceleration show that shorter granular chain and larger packing density favor the occurrence of complete-resonance mode.

PAEK- and PES-like perarylated phosphonium ionenes: Synthesis, thermal properties, and conductivity

Perarylated phosphonium salts are known to exhibit high degrees of thermal stability, often in excess of 400 °C. Here, the thermal and conductive properties of a series of perarylated phosphonium ionenes, prepared utilizing a PAEK-/PES-like synthetic approach, is described. All of the ionenes were found to display Td5% values in excess of 365 °C. Structural changes in the polymer backbone were found to have a prominent impact on Tg, with the meta-analogs exhibiting values ∼45 °C below the para-analogs, emphasizing the significant impact that solid-state packing can have on thermal transitions. All of the phosphonium ionenes were found to be resistant to decomposition under caustic conditions (NaOH), even at elevated temperature (60 °C) over a period of 48 h. Ionic conductivities were on the order of ∼10−7 S/cm at 120 °C, with the meta-analogs exhibiting a 1-2 orders-of-magnitude enhancement compared to their para-analogs, indicating that disruption of chain packing can lead to improvements in ionic conductivity while maintaining relatively high thermal and chemical resistance.

Molecular simulation of the structural formation of mono- and bidisperse polyethylene upon cooling from the melts

Monte Carlo (MC) simulation of the coarse-grained polyethylene (PE) model on the high coordination lattice was employed to investigate the structural formation of mono- and bidisperse PE chains upon stepwise cooling from the melts. The evolution of the ordered structure was analyzed in terms of the fraction of trans state, the length of trans sequence, the orientation correlation function at bond and molecular scale, the intermolecular pair correlation function, and the anisotropic change of chain dimension. Simulation data suggest that the rate of structure formation and the degree of crystallinity were decreased for the bidisperse PE mixture. In general, polymers in the mixture form a less-ordered structure, exhibit slower crystallization rates and have lower melting temperatures. For the monodisperse systems, the shorter chains exhibit an increase of the local bond properties including a fraction of trans conformation, intra- and intermolecular order parameters, but show a decrease in molecular properties such as chain orientation and chain packing. For mixed molecular weight systems, short-chain components are perturbed by the long chains resulting in a lower fraction of trans conformation, less dense structures, and a significant decrease in the degree of the bond orientation. Both short and long polymers can co-crystallize initiated at the same simulation time and finally reach the same magnitude of the chain order parameter.

Effect of Anionic Lipids on Mammalian Plasma Cell Membrane Properties.

The effect of lipid composition on models of the inner leaflet of mammalian cell membranes has been investigated. Grazing incidence X-ray diffraction and X-ray and neutron reflectivity have been used to characterize lipid packing and solvation, while electrochemical and infrared spectroscopic methods have been employed to probe phase behavior in an applied electric field. Introducing a small quantity of the anionic lipid dimyristoylphosphatidylserine (DMPS) into bilayers of zwitterionic dimyristoylphosphatidylethanolamine (DMPE) results in a significant change in the bilayer response to an applied field: the tilt of the hydrocarbon chains increases before returning to the original tilt angle on detachment of the bilayer. Equimolar mixtures, with slightly closer chain packing, exhibit a similar but weaker response. The latter also tend to incorporate more solvent during this electrochemical phase transition, at levels similar to those of pure DMPS. Reflectivity measurements reveal greater solvation of lipid layers for DMPS > 30 mol %, matching the greater propensity for DMPS-rich bilayers to incorporate water. Taken together, the data indicate that the range of 10-35 mol % DMPS provides optimum bilayer properties (in flexibility and function as a barrier), which may explain why the DMPS content of cell membranes tends to be found within this range.

Finely tuning the microporosity in phosphoric acid doped triptycene-containing polybenzimidazole membranes for highly permselective helium and hydrogen recovery

High-performance polymer membranes with well-defined microporosity and size-sieving ability are especially attractive for helium and hydrogen recovery. Here we report novel macromolecular engineering of polybenzimidazole (PBI) membranes that integrate hierarchical triptycene units for high permeability and polyprotic acid doping for size sieving via controllable manipulation of microporous architecture. The triptycene moieties disrupt chain packing and introduce additional configurational free volume, leading to significantly boosted He and H2 permeabilities compared to previously reported PBI membranes. The acid doping resulted in crosslinked PBI membranes via hydrogen bonding and proton transfer with dramatically enhanced gas selectivities. Via adjusting the H3PO4-doping level, triptycene-based polybenzimidazole (TPBI) composite membranes (TPBI-(H3PO4)x) exhibit the highest gas selectivities for He enrichment (i.e., α(He/CH4) = 7052 ± 156) and H2 purification (i.e., α(H2/CH4) = 5128 ± 110) among existing polymeric gas separation membranes. Additionally, under mixed-gas conditions at 150 °C, the TPBI-(H3PO4)0.98 membrane displays a H2 permeability of 46.7 Barrer and a H2/CO2 selectivity of 16, far beyond the Robeson's 2008 upper bound for H2/CO2 separation. The facile and diverse tunability and excellent gas separation performance make TPBI-(H3PO4)x membranes highly attractive for helium and hydrogen separation.

Investigating the role of membrane composition on the stability of decorated nanoparticle aggregates

Gold nanoparticles are a ubiquitous photosensitizer with a broad range of applications in microscopy and targeted drug delivery. Vesicles photosensitized with gold nanoparticles are promising as targeted drug delivery vehicles due to their non-invasive rupture mechanism. Controlling the vesicle properties increases control of the spatial-temporal release of cargo and drug dosage. The aggregation of gold nanoparticles affects the photoporation of these vesicles by interfering with peak SPR wavelengths. These nanoparticles cause leaflet bending deformations on the order of the nanoparticle size, while ligand chains may also disrupt local packing of lipid chains. Aggregation may be driven by the need to minimize one or both effects, but their relative contributions are unknown. In order to test the contributions of these two perturbations to membrane structure, we simulated multi-nanoparticle systems in lipid membranes of varying compositions. We used coarse-grained molecular dynamics simulation via the MARTINI forcefield to simulate simple spherical nanoparticles with a decorated ligand exterior. We found that large-scale nanoparticle aggregation depends more on ligand chain length than nanoparticle size, suggesting that aggregation of gold nanoparticles is dominated by microscopic perturbations to lipid packing.

Cholesterol promotes similar chain packing in n-3 PUFA-containing membranes.

Cholesterol's organization in biomembranes has great influence on membrane function. Omega-3 polyunsaturated fatty acid (n-3 PUFA)-containing lipids are believed to have a myriad of health benefits and are known to modulate membrane organization in model membranes via their aversion for cholesterol. 2H NMR has observed that different n-3 PUFA-containing phosphatidylcholine (PC) lipids modulate membrane organization to different degrees. In ternary lipid mixtures with sphingomyelin and cholesterol, docosahexaenoic acid (DHA, 22:6)-containing PC infiltrates lipid rafts more than eicosapentaenoic acid (EPA, 20:5)-containing PC. Additionally, molecular dynamics (MD) simulations have shown that n-3 PUFA-containing membranes differ in their stereospecific interactions with cholesterol. The mean-torque model provides a method of relating lipid disorder to a thermodynamic parameter (potential of mean torque) which can be used to quantify lipid-lipid interactions. Here we apply this approach to compare 1-stearoyl-2-eicosapentaenoylphosphatylcholine (EPA-PC, 18:0-20:5PC), 1-stearoyl-2-docosahexaenoylphosphatylcholine (DHA-PC, 18:0-22:6PC), 1-stearoyl-2-docosapentaenoylphosphatylcholine (DPA-PC, 18:0-22:5PC) and, as a monounsaturated control, 1-stearoyl-2-oleoylphosphatidylcholine (OA-PC, 18:0-18:1PC) bilayers and their interactions with cholesterol. Analysis of data from 2H NMR and MD simulations shows that all three n-3 PUFA-containing phospholipids display similar chain packing in the presence of cholesterol, which is illustrated by the projected chain length of the sn-1 chains.

Poly(ionic liquid) nanovesicles via polymerization induced self-assembly and their stabilization of Cu nanoparticles for tailored CO2 electroreduction

Herein, we report a straightforward, scalable synthetic route towards poly(ionic liquid) (PIL) homopolymer nanovesicles (NVs) with a tunable particle size of 50 to 120 nm and a shell thickness of 15 to 60 nm via one-step free radical polymerization induced self-assembly. By increasing monomer concentration for polymerization, their nanoscopic morphology can evolve from hollow NVs to dense spheres, and finally to directional worms, in which a multilamellar packing of PIL chains occurred in all samples. The transformation mechanism of NVs’ internal morphology is studied in detail by coarse-grained simulations, revealing a correlation between the PIL chain length and the shell thickness of NVs. To explore their potential applications, PIL NVs with varied shell thickness are in situ functionalized with ultra-small (1 ∼ 3 nm in size) copper nanoparticles (CuNPs) and employed as electrocatalysts for CO2 electroreduction. The composite electrocatalysts exhibit a 2.5-fold enhancement in selectivity towards C1 products (e.g., CH4), compared to the pristine CuNPs. This enhancement is attributed to the strong electronic interactions between the CuNPs and the surface functionalities of PIL NVs. This study casts new aspects on using nanostructured PILs as new electrocatalyst supports in CO2 conversion to C1 products.

NMR Studies of Aromatic Ring Flips to Probe Conformational Fluctuations in Proteins.

Aromatic residues form a significant part of the protein core, where they make tight interactions with multiple surrounding side chains. Despite the dense packing of internal side chains, the aromatic rings of phenylalanine and tyrosine residues undergo 180° rotations, or flips, which are mediated by transient and large-scale "breathing" motions that generate sufficient void volume around the aromatic ring. Forty years after the seminal work by Wagner and Wüthrich, NMR studies of aromatic ring flips are now undergoing a renaissance as a powerful means of probing fundamental dynamic properties of proteins. Recent developments of improved NMR methods and isotope labeling schemes have enabled a number of advances in addressing the mechanisms and energetics of aromatic ring flips. The nature of the transition states associated with ring flips can be described by thermodynamic activation parameters, including the activation enthalpy, activation entropy, activation volume, and also the isothermal volume compressibility of activation. Consequently, it is of great interest to study how ring flip rate constants and activation parameters might vary with protein structure and external conditions like temperature and pressure. The field is beginning to gather such data for aromatic residues in a variety of environments, ranging from surface exposed to buried. In the future, the combination of solution and solid-state NMR spectroscopy together with molecular dynamics simulations and other computational approaches is likely to provide detailed information about the coupled dynamics of aromatic rings and neighboring residues. In this Perspective, we highlight recent developments and provide an outlook toward the future.

New insights into contribution of aromatic ring versus aliphatic ring to thermal transition temperatures of heat resistant polyamides: A comparison study of PA 10T and t-PA 10C

In order to understand the differences in the contribution of aromatic (p-phenylene) ring versus aliphatic (trans-1,4-cyclohexylene) ring to the thermal transition temperature of heat-resistant polyamides, a comparison study is both experimentally and computationally implemented on a typical aromatic-heat-resistant polyamide PA 10T containing p-phenylene moieties and a typical alicyclic-heat-resistant polyamide t-PA 10C containing trans-1,4-cyclohexylene moieties. DSC, WAXD, solid state-NMR, computational simulations, and in-situ FTIR were utilized in sequence to reveal the differences in thermal performance, crystal structure, chain conformation, cohesive energy, interchain interaction, and melting process of PA 10T and t-PA 10C. t-PA 10C displays 30 °C-higher Tm and 50 °C-higher Tg than PA 10T. As indicated by the hydrogen bonding dissociation process, the enthalpy change dominates the difference in Tm as the gap in the entropy change between PA 10T and t-PA 10C is negligible. Computational simulation indicates that the energy-favorable hydrogen bonding conformation and dense trans-1,4-cyclohexylene-associated chain packing endow the formation of stronger interchain interactions in t-PA 10C which results in larger cohesive energy and greater cohesive energy density. The higher melting enthalpy change in the melting process derived from in-situ FT-IR experiment also verifies the simulated energy results. Henceforth, the question that WHY can the building block change, from rigid planar aromatic unit (PA 10T) to conformation-switchable nonplanar aliphatic unit (t-PA 10C), in the two polyamides induce so large improvement in Tg and Tm is systematically answered.