Probing molecular-level surface structures of polyethersulfone/pluronic F127 blends using sum-frequency generation vibrational spectroscopy

Surface Segregation Methods toward Molecular Separation Membranes

As a highly promising approach to solving the issues of energy and environment, membrane technology has gained increasing attention in various fields including water treatment, liquid separations, and gas separations, owing to its high energy efficiency and eco-friendliness. Surface segregation, a phenomenon widely found in nature, exhibits irreplaceable advantages in membrane fabrication since it is an in situ method for synchronous modification of membrane and pore surfaces during the membrane forming process. Meanwhile, combined with the development of synthesis chemistry and nanomaterial, the group has developed surface segregation as a versatile membrane fabrication method using diverse surface segregation agents. In this review, the recent breakthroughs in surface segregation methods and their applications in membrane fabrication are first briefly introduced. Then, the surface segregation phenomena and the classification of surface segregation agents are discussed. As the major part of this review, the authors focus on surface segregation methods including free surface segregation, forced surface segregation, synergistic surface segregation, and reaction-enhanced surface segregation. The strategies for regulating the physical and chemical microenvironments of membrane and pore surfaces through the surface segregation method are emphasized. The representative applications of surface segregation membranes are presented. Finally, the current challenges and future perspectives are highlighted.

Strong Hydration at the Poly(ethylene glycol) Brush/Albumin Solution Interface

Albumin molecules are extensively used as biocompatible coatings, and poly(ethylene glycol) (PEG) materials are widely used for antifouling. PEG materials have excellent antifouling property because of their strong surface hydration. Our previous research indicates that hydration at the PEG/bovine serum albumin solution interface is stronger than that at the PEG/water interface. This research shows that this observation is general for different types of albumin molecules. Different albumins including bovine, porcine, rat, rabbit, and sheep serum albumins were studied in this research. It was found that the hydration at the PEG methacrylate (pOEGMA)/albumin solution interface is always stronger than that at the pOEGMA/water interface. Here, we define "strong interfacial hydration" as "ordered strongly hydrogen-bonded interfacial water". We believe that such a strong hydration is because of the strong hydration on the albumin surface, leading to its biocompatible property. All of the albumin molecules demonstrated stronger hydration on the pOEGMA surface compared to other protein molecules such as lysozyme and fibrinogen. The strong hydration on albumin molecules is related to the high surface coverage of glutamic acid and lysine with similar amounts.

Membrane-based separation technologies: from polymeric materials to novel process: an outlook from China

During the past two decades, research on membrane and membrane-based separation process has developed rapidly in water treatment, gas separation, biomedicine, biotechnology, chemical manufacturing and separation process integration. In China, remarkable progresses on membrane preparation, process development and industrial application have been made with the burgeoning of the domestic economy. This review highlights the recent development of advanced membranes in China, such as smart membranes for molecular-recognizable separation, ion exchange membrane for chemical productions, antifouling membrane for liquid separation, high-performance gas separation membranes and the high-efficiency hybrid membrane separation process design, etc. Additionally, the applications of advanced membranes, relevant devices and process design strategy in chemical engineering related fields are discussed in detail. Finally, perspectives on the future research directions, key challenges and issues in membrane separation are concluded.

Absolute Orientations of Water Molecules at Zwitterionic Polymer Interfaces and Interfacial Dynamics after Salt Exposure

Nonfouling zwitterionic polymers have wide applications ranging from the naval industry to biomedical engineering. Strong hydration at polymer surfaces has been proven to be crucial to their nonfouling property, but the absolute orientations of water molecules on the polymers and the competition between water and salt binding have not been elucidated. In this work, the absolute orientations of water molecules on two zwitterionic polymer brushes, poly(carboxybetaine methacrylate) (pCBMA) and poly(sulfobetaine methacrylate) (pSBMA), were measured using regular and phase-sensitive sum frequency generation (SFG) vibrational spectroscopy. The pH-dependent studies in a pH range from 2 to 12 showed that at a pH of 7, the water absolute orientations are different on the pCBMS and pSBMA surfaces. Phase-sensitive SFG studies confirmed the results obtained from the pH-dependent measurements. Salt effects on the hydration of zwitterionic polymers were examined as a function of time, which indicated that the pCBMA surface and the associated interfacial water exhibit a slow restructuring process after salt binding (likely due to the strong binding of pCBMA with water), whereas the surface of pSBMA and the associated water have a fast change after salt binding.

Surfactant-assisted-water-exposed versus surfactant-aqueous-solution-exposed electrospinning of novel super hydrophilic Polycaprolactone-based fibers: Cell culture studies

Blend electrospun scaffolds composed of Polycaprolactone and Pluronic are suitable for bone tissue engineering due to their excellent biocompatibility and hydrophilicity. However, exceeding from certain amounts of Pluronic, surface enrichment of this polymer leads to segregation of Pluronic chains within the fiber, endangering the integrity and mechanical properties of the scaffold. In this article, a novel method of blend electrospinning has been employed using a parallel water supply, positioning the Pluronic chains on the surface, thus enhancing the miscibility within the fibers. Water uptake test revealed the super hydrophilicity of obtained scaffolds. Atr-FTIR and X-ray photoelectron spectroscopy verified a higher percentage of Pluronics on the surface in comparison with conventional blend electrospinning. Tensile test demonstrated improved mechanical properties of the modified scaffolds. The results of cytocompatibility tests have also revealed enhanced viability of cells on these scaffolds confirming their great promise for clinical applications. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1204-1212, 2019.

Surfactant-assisted-water-exposed versus surfactant-aqueous-solution-exposed electrospinning of novel super hydrophilic polycaprolactone based fibers: Analysis of drug release behavior

Surface hydrophilicity and scaffold integrity determine the drug release behavior of drug loaded electrospun fibrous mats. When mixture miscibility is acceptable, blend electrospinning of hydrophobic with hydrophilic polymers can improve scaffold hydrophilicity while the hydrophobic polymer maintains the mechanical strength of scaffold. Polycaprolactone (PCL) and Pluronic P123 (P123) blend electrospinning has been investigated. In routine blend electrospinning, surface enrichment of Pluronic sets a limit for P123 weight ratio in which exceeding from that limit causes the excess P123 to be accumulated within the electrospun fiber core. To overcome this setback, a method named surfactant assisted water exposed (SAWE) electrospinning was introduced which was proven to be effective for increasing the surface enrichment of Pluronic. In order to test the validity of this method, the electrospinning of solution containing PCL which is exposed to aqueous solution of P123 was investigated. This new method was named surfactant aqueous solution exposed (SASE) electrospinning. Myelin formation at the contact interface of aqueous solution and chloroform solution was studied and it was found that this layer can effectively barricade the migration of Pluronic chains between immiscible phases. For SASE, fiber surface coverage by P123 was uneven and loose. Electrospun scaffolds from SAWE and SASE were loaded with drug to investigate the effect of the exposure time during electrospinning on in vitro drug release. By increasing the exposure time, the abnormal two-stage phased release profile of SAWE became normal with moderate initial burst. Longer exposure time increased the initial burst of the drug loaded SASE fibers. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 597-609, 2019.

Electrospinning of Polycaprolactone/Pluronic F127 dissolved in glacial acetic acid: fibrous scaffolds fabrication, characterization and in vitro evaluation

Abstracts The Polycaprolactone (PCL) fibrous scaffolds in nano to micro scale have been considered as excellent templates for cell culture and tissue growth. The hydrophobic nature of the PCL, however, yields low initial cell seeding density, heterogeneous cell spreading and slow cell growth rate. Therefore, in this study the surface hydrophilic fibrous scaffolds were directly fabricated by the electrospinning of PCL solutions with small quantities (0.5-5%) of Pluronic F127 (PEO₁₀₀-PPO₆₅-PEO₁₀₀) dissolved in benign solvent of glacial acetic acid. The clear and miscible solutions were achieved by controlling the proper F127 content in the blend solutions. The continuous and smooth fibers with average diameters from 0.71 to 1.43 μm made up the fibrous scaffolds in non-woven mode. Then the water wetting angle of the scaffolds could be adjusted from 126° to 0° by varying F127 content owing to its hydrophilic PEO chains presented on surface the blended fibers. Finally, it was demonstrated that the blended fibrous scaffolds with the F127 content less than 1% exhibited better cell attachment, proliferation and spreading performance than those of pure PCL scaffolds.

Antifouling membranes for sustainable water purification: strategies and mechanisms

One of the greatest challenges to the sustainability of modern society is an inadequate supply of clean water. Due to its energy-saving and cost-effective features, membrane technology has become an indispensable platform technology for water purification, including seawater and brackish water desalination as well as municipal or industrial wastewater treatment. However, membrane fouling, which arises from the nonspecific interaction between membrane surface and foulants, significantly impedes the efficient application of membrane technology. Preparing antifouling membranes is a fundamental strategy to deal with pervasive fouling problems from a variety of foulants. In recent years, major advancements have been made in membrane preparation techniques and in elucidating the antifouling mechanisms of membrane processes, including ultrafiltration, nanofiltration, reverse osmosis and forward osmosis. This review will first introduce the major foulants and the principal mechanisms of membrane fouling, and then highlight the development, current status and future prospects of antifouling membranes, including antifouling strategies, preparation techniques and practical applications. In particular, the strategies and mechanisms for antifouling membranes, including passive fouling resistance and fouling release, active off-surface and on-surface strategies, will be proposed and discussed extensively.

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

Sum frequency generation vibrational spectroscopy (SFG-VS) has become one of the most appealing technologies to characterize molecular structures at interfaces. In this focal point review, we focus on SFG-VS studies at buried polymer interfaces and review many of the recent publications in the field. We also cover the essential theoretical background of SFG-VS and discuss the experimental implementation of SFG-VS.

Wetting of Hydrophilic Electrospun Mats Produced by Blending SEBS with PEO-PPO-PEO Copolymers of Different Molecular Weight

The interaction of electrospun mats with water is critical for many possible applications, and the water contact angle on the surface is the parameter usually measured to characterize wetting. Although useful for hydrophobic surfaces, this approach is limited for hydrophilic mats, where wicking also has to be considered. In this case, it is still unclear how the fiber surface chemical composition and morphology will affect the wetting behavior of electrospun mats. In this work, wetting was studied with different hydrophilic membranes produced by blending thermoplastic elastomer poly(styrene)-b-poly(ethylene-butylene)-b-poly(styrene) (SEBS) with amphiphilic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) molecules. Three different types of PEO-PPO-PEO, with different molar masses, PEO content, and physical form were used. The effect of these differences on the wetting behavior of the electrospun mats was evaluated by contact angle goniometry, wicking measurements, and different imaging techniques. X-ray photoelectron spectroscopy was used to characterize the surface chemical composition. The smaller molecules quickly saturated the surface at low concentrations, making the mats hydrophilic. The sheath of PEO-PPO-PEO also resulted in fast absorption of water, when comparing the saturated and nonsaturated surfaces. Longer PEO chain-ends seemed to hinder complete segregation and also led to a higher activation time when in contact with water. Liquid PEO-PPO-PEO was easily leached by water.

Blending and Morphology Control To Turn Hydrophobic SEBS Electrospun Mats Superhydrophilic

Thermoplastic elastomer SEBS, a triblock copolymer composed of styrene (S) and ethylene-co-butylene (EB) blocks, can be dissolved and processed by electrospinning to produce flexible nonwoven mats that can be interesting for applications like filtration or separation membranes. Controlling surface properties such as hydrophobicity/hydrophilicity is critical to achieving a desired performance. In this study, hydrophobic electrospun SEBS mats were obtained, following which an amphiphilic molecule (Pluronic F127) was solution-blended with SEBS prior to electrospinning, in a bid to produce a hydrophilic membrane. The result was a fast-spreading superhydrophilic mat with thinner fibers that preserved the flexibility of the SEBS. The morphologies of nonwoven mats, flat films (prepared by dip-coating using identical solutions) and of the surface of individual fibers were characterized using different microscopy techniques (optical, scanning electron microscopy and atomic force microscopy). Chemical analysis by X-ray photoelectron spectroscopy (XPS) revealed a large F127 concentration in the outermost surface layer. In addition, an analysis of dip-coated flat films revealed that for 20 wt % of F127, there was a change in the blend morphology from dispersed F127-rich regions in the SEBS matrix to an interconnected phase homogeneously distributed across the film that resembled grain boundaries of micellar crystals. Our results indicated that this morphology change at 20 wt % of F127 also occurred to some extent in the electrospun fibers and this, combined with the large surface area of the mats, led to a drastic reduction in the contact angle and fast water absorption, turning hydrophobic electrospun mats superhydrophilic.

Using coaxial electrospinning to fabricate core/shell-structured polyacrylonitrile–polybenzoxazine fibers as nonfouling membranes

We used coaxial electrospinning to produce core/shell polyacrylonitrile (PAN)–benzoxazine (BA) fibers. The PAN–PBA core/shell fibers that we obtained after curing exhibited low surface energies and excellent biononfouling properties.

Structural and functional characterization of proteins adsorbed on hydrophilized polylactide-co-glycolide microfibers

Background:

Hydrophobic biopolymers such as polylactide-co-glycolide (PLGA, 85:15) have been extensively explored as scaffolding materials for tissue engineering applications. More recently, electrospun microfiber-based and nanofiber-based scaffolds of PLGA have received increased attention because they act as physical mimics of the fibrillar extracellular matrix. However, the hydrophobicity of the PLGA microfiber surface can limit its use in biomedical applications. Therefore, in a previous study, we fabricated Pluronic^® F-108 (PF-108)-blended PLGA microfibrous scaffolds that alleviated the hydrophobicity associated with PLGA by enriching the surface of microfibers with the ethylene oxide units present in PF-108.

Methods:

In this study, we report the influence of the extent of surface enrichment of PLGA microfibers on their interaction with two model proteins, ie, bovine serum albumin (BSA) and lysozyme. BSA and lysozyme were adsorbed onto PLGA microfiber meshes (unmodified and modified) and studied for the amount, secondary structure conformation, and bioactivity of released protein.

Results:

Irrespective of the type of protein, PF-108-blended PLGA microfibers showed significantly greater protein adsorption and release than the unblended PLGA samples. However, in comparison with BSA, lysozyme showed a 7–9-fold increase in release. The Fourier transform infrared spectroscopy studies for secondary structure determination demonstrated that irrespective of type of microfiber surface (unblended or blended), adsorbed BSA and lysozyme did not show any significant change in secondary structure (α-helical content) as compared with BSA and/or lysozyme in the free powder state. Further, the bioactivity assay of lysozyme released from blended PLGA microfiber meshes demonstrated 80%–85% bioactivity, indicating that the process of adsorption did not significantly affect biological activity. Therefore, this study demonstrated that the decreased hydrophobicity of blended PLGA microfibrous meshes not only improved the amount of protein adsorbed (lysozyme and BSA) but also maintained the secondary structure and bioactivity of the adsorbed proteins.

Conclusion:

Modulating the hydrophobicity of PLGA via blending with PF-108 could be a viable strategy to improve its interaction with proteins and subsequent cell interaction in tissue engineering applications.

Protein fouling resistant membrane prepared by amphiphilic pegylated polyethersulfone

A mild and facile grafting of poly(ether glycol) methyl ether methacrylate (PEGMA) monomers onto polyethersulfone (PES) was carried out. Then, the PES-g-PEGMA membranes with integrally anisotropic morphology were fabricated through the coupling of non-solvent induced phase inversion and surface segregation. Compared with PES control membrane, the surface hydrophilicity of PES-g-PEGMA membranes was remarkably enhanced due to the drastic enrichment of poly(ethylene glycol) (PEG) segments on the membrane surface; protein adsorption was significantly inhibited due to the hydrogen bonding interactions between hydrophilic groups and water molecules. Ultrafiltration experiments were used to assess the permeability and protein fouling resistance of the PES-g-PEGMA membranes. It was found that the PES-g-PEGMA membranes with higher surface coverage of PEG segments displayed stronger antibiofouling property. Moreover, the stable antibiofouling property for PES-g-PEGMA membranes was acquired due to covalent bonding interactions between hydrophilic PEGMA side chains and PES main chains.

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.

Surface orientation of phenyl groups in poly(sodium 4-styrenesulfonate) and in poly(sodium 4-styrenesulfonate):poly(3,4-ethylenedioxythiophene) mixture examined by sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy was used to investigate the surface phenyl ring orientations of spin-coated poly(sodium 4-styrenesulfonate) (PSSNa) and of PSSNa in the spin-coated polymer material Baytron P. Baytron P is a commercially available conducting polymer suspension with poly(3,4-ethylenedioxythiophene) (PEDOT) and PSSNa in water, which is widely used in organic electronic devices. SFG spectra collected using different polarization combinations showed that the twist angles for the phenyl groups on the spin-coated PSSNa and Baytron P surfaces were not random; therefore, both the twist and tilt angles needed to be deduced in order to determine their respective phenyl group orientations. Results indicated that on the PSSNa surface, the para-substituted phenyl ring has a tilt angle of 47.5 degrees +/- 5.3 degrees and a twist angle of 58.8 degrees +/- 8.2 degrees. The tilt and twist angles, 48.2 degrees +/- 3.6 degrees and 65.2 degrees +/- 4.0 degrees, respectively, were determined for the Baytron P phenyl ring, which were not very different from those of PSSNa phenyl groups on the surface.

Surface structures of an amphiphilic tri-block copolymer in air and in water probed using sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate surface structures of an amphiphilic surface-active block copolymer (SABC) film deposited on a CaF(2) substrate, in air and in water in situ. Developed as a surface-active component of an antifouling coating for marine applications, this amphiphilic triblock copolymer contains both hydrophobic fluorinated alkyl groups as well as hydrophilic ethoxy groups. It was found that surface structures of the copolymer film in air and in water cannot be probed directly using the SFG experimental geometry we adopted because SFG signals can be contributed from the polymer/air (or polymer/water) interface as well as the buried polymer/CaF(2) substrate interface. Using polymer films with varied thicknesses, structural information about the polymer surfaces in air and in water can be deduced from the detected SFG signals. With SFG, surface restructuring of this polymer has been observed in water, especially the methyl and methylene groups change orientations upon contact with water. However, the hydrophobic fluoroalkyl group was present on the surface in both air and water, and we believe that it was held near the surface in water by its neighboring ethoxy groups.

Modification of poly(lactic/glycolic acid) surface by chemical attachment of poly(ethylene glycol)

Biodegradable polyesters such poly(lactic acid) and poly(lactic/glycolic acid) (PLGA) copolymers are preferred biomaterials and used among others as drug delivery systems, although their surface hydrophobicity limits their application. In this work, chemical modification of the PLGA surface was developed by coupling of either linear or starlike poly(ethylene glycol) (PEG) molecules via chemical bonds to the PLGA surface following amino functionalization as a first step to improve its biocompatibility. The chemical attachment was followed by detailed X-ray photoelectron spectroscopy (XPS) studies. It was shown that substantial modification can be achieved by linear PEG, but even higher surface coverage with hydrophilic groups can be obtained when the six-armed PEG is applied with the additional advantage of possible further functionalization via free amino groups available on the surface of the latter. As a final goal, a significant increase of water wettability together with reduced protein adsorption was achieved on PEG-coupled PLGA surfaces.