Polymeric membrane formation by wet-phase separation; turbidity and shrinkage phenomena as evidence for the elementary processes

Investigating microstructure evolution in block copolymer membranes

Block copolymer self-assembly in conjunction with nonsolvent-induced phase separation (SNIPS) has been increasingly leveraged to fabricate integral-asymmetric membranes. The large number of formulation and processing parameters associated with SNIPS, however, has prevented the reliable construction of high performance membranes. In this study, we apply dynamical self-consistent field theory to model the SNIPS process and investigate the effect of various parameters on the membrane morphology: solvent selectivity, nonsolvent selectivity, initial film composition, and glass transition composition. We examine how solvent selectivity and concentration of polymers in the film impact the structure of micelles that connect to form the membrane matrix. In particular, we find that preserving the order in the surface layer and forming a connection between the supporting and surface layer are nontrivial and sensitive to each parameter studied. The effect of each parameter is discussed, and suggestions are made for successfully fabricating viable block copolymer membranes.

Phase separation in surfactant-containing amorphous solid dispersions: Orthogonal analytical methods to probe the effects of surfactants on morphology and phase composition

Amorphous-amorphous phase separation (AAPS) is an important phase transition process for amorphous solid dispersion (ASD) performance both in terms of drug release as well as physical and chemical stability during storage. Addition of surfactants to ASD systems can impact both of these processes. One possible mechanism through which surfactants affect ASD performance is via their impact on AAPS. Unfortunately, despite their increasing usage in ASD formulations, the effect of surfactant on AAPS is still poorly understood, and there are limited analytical techniques that provide microstructural and composition information about phase separated ASDs. In this study, the impact of four surfactants (sodium dodecyl sulfate, Tween 80, Span 20 and Span 85) on water-induced phase separation in ASDs formulated with ritonavir and polyvinylpyrrolidone/vinyl acetate (PVPVA) was investigated using a variety of orthogonal analytical methods. Transparent films of ASDs with different compositions were prepared by spin coating. Fluorescence confocal microscopy in combination with an in situ humidity chamber was used to monitor the kinetics and morphology of phase separation following exposure to high relative humidity. Optical photothermal IR analysis of phase separated films enabled characterization of domain composition and surfactant distribution. Liquid-liquid phase separation concentration, zeta potential and solution nuclear magnetic resonance spectroscopy measurements enabled interpretation of interaction with and partition of surfactants into the drug-rich phase. It was found that phase separation kinetics and morphology were notably changed by the surfactants. Further, the surfactants showed different affinities for the drug-rich versus the aqueous/polymer-rich phases. The employed analytical techniques were found to be complementary in providing insight into surfactant location in phase separated systems. This study highlights the complexity of phase separation, especially in the presence of surfactants, and provides a foundation to understand the impact of AAPS on ASD performance.

Hyperbranched polyethylenimine functionalized silica/polysulfone nanocomposite membranes for water purification

Hyperbranched polyethyleneimine functionalized silica (PEI-SiO₂) nanoparticles with considerable hydrophilicity were synthesized and incorporated into a polysulfone (PSF)/dimethylacetamide (DMA)/polyvinylpyrrolidone (PVP) membrane casting solution in five different ratios to fabricate PEI-SiO₂/PSF nanocomposite membranes using nonsolvent-induced phase separation. The hydrophilic PEI-SiO₂ nanoparticles were characterized by TEM, FTIR, TGA, and XPS analyses. Morphology, water contact angles, mean pore sizes, overall porosity, tensile strengths, water flux, antifouling and the dye separation performances of the PEI-SiO₂/PSF membranes were also studied. The PEI-SiO₂ nanoparticles were uniformly dispersed in the PSF-based membranes, where a fall in the water contact angle was observed from 65.4° to 49.7° by addition of 2 wt% nanoparticles. The fouling resistance parameters of the PEI-SiO₂/PSF membranes were declined with an increase in the nanoparticle concentration, suggesting the superior hydrophilic nature of the PEI-SiO₂ nanoparticles. The permeability of the nanocomposite membranes was increased from 38.5 to 70 L m^-2 h^-1 bar^-1 by incorporation of 2 wt% PEI-SiO₂. Finally, improvements were observed in the flux recovery ratio (95.8%), Reactive Green 19 dye rejection (99.6%) and tensile strengths of the PEI-SiO₂/PSF membranes over the neat PSF and SiO₂/PSF membranes, which were used as controls. The results of this study demonstrate the promising application of PEI-SiO₂ nanoparticles in improving the separation and antifouling performances of the PSF membranes for water purification.

Untapped potentials of acrylonitrile-butadiene-styrene/polyurethane (ABS/PU) blend membrane to purify dye wastewater

Production of acrylonitrile-butadiene-styrene/polyurethane (ABS/PU) blend membrane with high rejection efficiency for disperse and vat dyes, is introduced as a facile and cost effective technique to purify textile wastewater. In this respect, membranes are produced using commercially available polymers, i.e. ABS and PU, with different compositions (ABS/PU: 100/0, 80/20, 70/30, 60/40 and 50/50 w/w) through wet casting. Casting solutions with concentration of 30 wt% are prepared using two different solvents, i.e. dimethylformamide (DMF) and N-methyl-2- pyrrolidone (NMP). The prepared membranes are characterized using a variety of analytical techniques including SEM imaging, FTIR spectroscopy, dry and wet gas permeation, evaluation of reusability, antifouling and mechanical properties, photostability, surface hydrophilicity and pure water permeability (PWP) of the produced membranes. According to the results, irrespective of solvent type, ABS/PU membranes with higher PU content have lower porosity and smaller pore size both of which contribute to enhanced dye rejection efficiency. This is while the impact of PU content on the photostability of ABS/PU membranes was found to be negligible. Additionally, the produced ABS/PU membranes exhibit good reusability and antifouling properties. However, the mechanical properties of ABS/PU membranes with higher PU contents are inferior to those with lower PU contents. This contrast highlights the prominence of optimum PU content to make a trade-off between dye rejection efficiency and mechanical properties. In this regard, ABS/PU (60/40 w/w) membrane is recognized as the one with optimum composition. Furthermore, it was found that regardless of PU content, membranes cast from DMF-based solutions exhibit superior rejection performance over those cast from NMP-based solutions. Overall, one can witness that employing ABS/PU membranes provides a meritorious and clean approach to refine disperse and vat dye wastewaters, a great threat to the environment and human health.

Polymer membrane with a mineral coating for enhanced curling resistance and surface wettability

Zirconia-wrapped membranes were fabricated via a mineralization process on polydopamine/polyethyleneimine-deposited surfaces. The rigid and hydrophilic mineral coating simultaneously endows the membranes with enhanced curling resistance and surface wettability, enabling the membranes to separate oil-in-water emulsions.

Preparation of microporous silicone rubber membrane with tunable pore size via solvent evaporation-induced phase separation

Silicone rubber membrane with ordered micropores in the surface was prepared by means of the solvent evaporation-induced phase separation. A ternary solution including liquid silicone rubber precursor, liquid paraffin, and hexane was cast to form a film with a two-phase structure after the hexane was evaporated. The micropores were generated by removing liquid paraffin phase in the cured silicone rubber film. The effects of the liquid paraffin concentration, casting temperature, initial casting solution thickness, air circulation, and addition of surfactant Span-80 on the pore structure in the membrane surface were investigated. The average pore size increases with increasing liquid paraffin concentration or the initial casting solution thickness. The formation of pore structure in the membrane surface is related to the phase separation and thus the phase separation process of the casting solution surface was in situ observed using the digital microscope. The formation mechanism of pore is attributed to a nucleation, growth, and coalescence process of liquid paraffin phase in the membrane surface.

Simultaneous Tuning of Chemical Composition and Topography of Copolymer Surfaces: Micelles as Building Blocks

A simple method is described for controlling the surface chemical composition and topography of the diblock copolymer poly(styrene)-b-poly(dimethylsiloxane)(PS-b-PDMS) by casting the copolymer solutions from solvents with different selectivities. The surface morphology and chemical composition were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively, and the wetting behavior was studied by water contact angle (CA) and sliding angle (SA) and by CA hysteresis. Chemical composition and morphology of the surface depend on solvent properties, humidity of the air, solution concentration, and block lengths. If the copolymer is cast from a common solvent, the resultant surface is hydrophobic, with a flat morphology, and dominated by PDMS on the air side. From a PDMS-selective solvent, the surface topography depends on the morphology of the micelles. Starlike micelles give rise to a featureless surface nearly completely covered by PDMS, while crew-cut-like micelles lead to a rough surface with a hierarchical structure that consists partly of PDMS. From a PS-selective solvent, however, surface segregation of PDMS was restricted, and the surface morphology can be controlled by vapor-induced phase separation. On the basis of the tunable surface roughness and PDMS concentration on the air side, water repellency of the copolymer surface could be tailored from hydrophobic to superhydrophobic. In addition, reversible switching behavior between hydrophobic and superhydrophobic can be achieved by exposing the surface to solvents with different selectivities.

One-step fabrication of porous micropatterned scaffolds to control cell behavior

This paper reports a one-step method to fabricate highly porous micropatterned 2-D scaffold sheets. The scaffold sheets have high glucose diffusion, indicating that the porosity and pore morphology of the scaffolds are viable with respect to nutrient transport, and a micropattern for cell alignment. HUVEC culturing proved that the scaffold sheets are suitable for cell culturing. More extensive culturing experiments with mouse myoblasts, C2C12, and mouse osteoblasts, MC3T3, showed that tissue organization can be controlled; the micropattern design affects the extent of cell alignment and tissue formation. Cells are favorably settled in the micropattern and even at higher confluence levels, when the cells start to overgrow the ridges of the micropattern, these cells align themselves in the direction of the micropattern. Preliminary multi-layer stacking experiments indicate that the 2-D scaffold sheets are very promising as basis for building 3-D scaffolds.

Phase separation micromolding: a new generic approach for microstructuring various materials

Phase separation micromolding (PSmicroM) is a versatile microfabrication technique that can be used to structure a very broad range of polymers, including block copolymers and biodegradable and conductive polymers without the need for clean-room facilities. By incorporating a subsequent process step, carbon, ceramic, and metallic microstructures can also be fabricated from a polymeric or hybrid precursor. The replication process is straightforward and cost-effective. It relies on phase separation of a polymer solution while in contact with a structured mold. Intrinsic shrinkage during the phase separation facilitates the release of the replica from the mold, which increases the reliability of the process even at small feature sizes, thin polymer films, or high aspect ratios. Under suitable circumstances perforation of the polymer film can be obtained, resulting in completely open "through" microstructures. Furthermore, porosity can be introduced in a microstructure, which may result in unknown functionalities.

Fabrication of microporous thermoplastic polyurethane for use as small-diameter vascular graft material. I. Phase-inversion method

Fabrication conditions of microporous thermoplastic polyurethane to be used in small-diameter vascular grafts are studied. Porosity variations due to various factors such as concentration of PU solution, composition of coagulation bath, effect of coagulant temperature, and effect of dissolved air in PU solution are discussed. The liquid-liquid phase-inversion process used for preparation of PU films and the mechanism for the formation of microporous films are discussed. Surface and cross-section morphologies of PU films are studied with the use of scanning electron microscopy (SEM), and porosity value and mechanical strength of PU films are also determined. SEM photomicrographs show that PU films prepared in an alcoholic coagulant have uniform porous structure compared to films prepared in water coagulant. Increasing the polymer concentration and coagulant temperature (>23 degrees C) decreases the macrovoid formation, as seen in cross sections of PU films. This enhances the tensile modulus of PU films. By using this process adjustment may be made on the morphology and compliance, as they are important factors in design and fabrication of small-diameter vascular grafts.

Molecular design of the alpha-keratin composite: insights from a matrix-free model, hagfish slime threads

We performed mechanical tests on a matrix-free keratin model-hagfish slime threads-to test the hypothesis that intermediate filaments (IFs) in hydrated hard alpha-keratins are maintained in a partly dehydrated state. This hypothesis predicts that dry IFs should possess mechanical properties similar to the properties of hydrated hard alpha-keratins, and should swell more than hard alpha-keratins in water. Mechanical and swelling measurements of hagfish threads were consistent with both of these predictions, suggesting that an elastomeric keratin matrix resists IF swelling and keeps IF stiffness and yield stress high. The elastomeric nature of the matrix is indirectly supported by the inability of matrix-free IFs (i.e. slime threads) to recover from post-yield deformation. We propose a general conceptual model of the structural mechanics of IF-based materials that predicts the effects of hydration and cross-linking on stiffness, yield stress and extensibility.