Self-organized ordered microporous thin films from grafting copolymers

Highly ordered porous films of cellulose-based graft copolymers were prepared by the breath figure method upon solvent evaporation. The morphology of the microporous films was investigated by scanning electron microscopy (SEM) and atom force microscopy (AFM). The influences of the preparing conditions, the length and the type of the side chains of copolymers on the morphology of the porous films were investigated. It was found that the average pore size is decreased with increasing the concentration of copolymer solutions and with increasing the side chain length. Moreover, it was confirmed that both the present aggregation in solution and the timely precipitation of copolymer are beneficial for the formation of the ordered microstructure by comparison of solvent and the property of side chains of copolymers. The porous films can be used as the template for the preparation of the micropatterning of fluorescence materials and have the potential applications in many fields such as templates, devices, nanocontainer, photonic and bandgap materials.     

Porous polyurethane film fabricated via the breath figure approach for sustained drug release

The breath figure (BF) method is an effective process for fabricating porous polymeric films. In this study, we fabricated porous polymer films from thermoplastic polyurethane (PU) through static BF with CHCl₃ as a solvent under 55–80% relative humidity. The porous PU films were prepared within various pore structures and sizes, which were adjustable, depending on the fabrication conditions. The humidity and exposure time were examined as variable parameters affecting the surface morphology, wettability, and cytotoxicity. Atorvastatin calcium, a hyperlipidemic agent, was loaded into the porous films during the casting process, and the drug-loading and drug-releasing behaviors of the porous PU membranes were evaluated. Approximately 60–80% of the drug was released in 14 days. The films exhibited sustained drug-release performances because of the hydrophobicity and nonbiodegradable nature of PU for perivascular drug administration. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47658.     

Fabrication of honeycomb polyvinyl butyral film under humidity provided by super saturated salt solutions

We introduce a novel method for making breath figure arrays (BFAs) on polyvinyl butyral (PVB) films under humidity by putting the dip coated samples inside a chamber filled with supersaturated salt solutions. With the increase in humidity or retention time, multiple porous films with bigger pore sizes and wider size distributions are generally obtained, resulting from the coalescence of the following‐up water drops. The presence of nitrogen flow accelerates the evaporation of solvents and vapor, thus increases the elastic interaction among water droplets such that films with smaller pores can be obtained. Compared to PVB/ethyl acetate solution, it is easier to obtain a film with regular honeycomb pattern by using PVB/chloroform solution. Its lower boiling point makes it evaporate faster to increase temperature gradient between film surface and the atmosphere; while its higher density makes it provide stronger support to maintain the shape of water droplets. In this study, the solubility of polymer in solvents is initially proposed to explain the BFAs formation. Higher mobility of polymer chains in a good solvent increases chances of its polar groups to interact with water droplets, thus increases the possibility of regular arrangements of water droplets on the film. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of casting conditions on the performance of porous cellulose acetate membranes in reverse osmosis

Several sets of porous cellulose acetate membranes were made using the same casting solution composition and gelation conditions but varying the casting solution temperature and solvent evaporation conditions. The films were tested in reverse osmosis experiments at 250 psig using aqueous feed solutions containing 3500 ppm NaCl. The results show that the product rate obtained at a given level of solute separation is independent of evaporation time in the range tested and, for a given casting solution composition, the temperature of the casting solution and conditions of solvent evaporation during film formation together constitute an important interconnected variable governing the porous structure of the resulting membranes. These results offer a new approach to the problem of developing more productive reverse osmosis membranes and have led to a new class of porous cellulose acetate membranes capable of giving product rates 100% to 150% higher than those of the best membranes reported, at any given level of solute separation under the experimental conditions used. These results are of practical importance in low-pressure reverse osmosis applications.     

Porous poly(2-hydroxyethyl acrylate) hydrogels prepared by radical polymerisation with methanol as diluent

Dynamic-mechanical and water sorption properties of porous and non-porous hydrogels have been studied as a function of their porosity and crosslinking density. Porous hydrogels with different crosslinking densities were prepared by co-polymerisation of 2-hydroxyethyl acrylate and ethyleneglycol dimethacrylate in solution in methanol. Pores were formed due to the segregation of the solvent from the polymer network during the polymerisation process. The porosity of the samples was observed by scanning electron microscopy. The pores collapse during the drying process after polymerisation but they reopen when the xerogel is immersed in liquid water. Bulk polymer networks with varying crosslinking densities were also synthesised and used as a reference in the analysis of the porous hydrogels. Water sorption from the gas phase and from the liquid phase was studied by means of equilibrium sorption isotherms and immersion experiments, respectively. Dynamic-mechanical spectroscopy conducted on the xerogels shows that the elastic modulus in the rubber-like state highly depends on the amount of solvent used in the polymerisation what is attributed to the presence of discontinuity surfaces in the xerogel although the pores are closed.     

Honeycomb Surface with Shape Memory Behavior Fabricated via Breath Figure Process

A bio‐inspired honeycomb pattern that exhibits shape memory behavior is successfully fabricated via the breath figure process. By the surface modification along with a chemical crosslinking process to enhance the recoverability, this honeycomb‐like structure with shape memory behavior is realized. The surface wettability is dependent on the surface topography controlled by the deformation of temporary shape (elliptical circle) and recovery of the permanent shape (round circle) at the microscopic scales without the need of micromolding or expensive lithography strategy. This approach opens a facile route to an efficient, inexpensive, and versatile method to prepare films with switchable wettability.     

Breath figure templated semifluorinated block copolymers with tunable surface properties and binding capabilities

Patterning of functionalized polymeric surfaces enables the adjustment of their characteristics and use in novel applications. We prepared breath figure (BF) films from three semifluorinated diblock copolymers, which all are composed of a polystyrene block and a semifluorinated one to compare their surface properties. “Click” chemistry was employed to one of the polymers, containing a poly(pentafluorostyrene) block to incorporate hydrophilic sugar or carboxylic acid moieties. The structure of the polymer alters the obtained porous morphology of the films. Contact angle (CA) analyses of the BF films reveals that the surface porosity increases water CAs compared with solvent cast films, and, in the case of hydrophobic polymers, leads to significant increase in the CAs of dodecane. The hydrophobicity of the BF films is further amplified by the removal of the topmost layer which leads in some cases to superhydrophobic surfaces. BF films containing glucose units are hydrophilic exhibiting water CAs below 90°. These glycosylated porous surfaces are shown to bind lectin Con A‐FITC or can be labelled with isothiocyanate marker. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41225.     

Direct-Ink-Writing of Electroactive Polymers for Sensing and Energy Storage Applications

Considering the high levels of materials used in the fields of electronics and energy storage systems, it is increasingly necessary to take into consideration environmental impact. Thus, it is important to develop devices based on environmentally friendlier materials and/or processes, such as additive manufacturing techniques. In this work, poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are prepared by direct-ink-writing (DIW) by varying solvent evaporation temperature and fill density percentage. Different morphologies for both polymers are obtained, including dense films and porous membranes, as well as different electroactive β-phase content, thermal and mechanical properties. The dielectric constant and piezoelectric d₃₃ coefficient for dense films reaches up to 16 at 1 kHz and 4 pC N⁻¹, respectively for PVDF-HFP with a fill density of 80 and a solvent evaporation temperature of 50 °C. Porous structures are developed for battery separator membranes in lithium-ion batteries, with a highest ionic conductivity value of 3.8 mS cm⁻¹ for the PVDF-HFP sample prepared with a fill density of 100 and a solvent evaporation temperature of 25 °C, the sample showing an excellent cycling performance. It is demonstrated that electroactive films and membranes can be prepared by direct-ink writing suitable for sensors/actuators and energy storage systems.     

An efficient preparation of porous polymeric microspheres by solvent evaporation in foam phase

This paper reports an efficient method of preparing porous polymeric microspheres by solvent evaporation in foam phase, in which phase separation between polymer and porogen occurs in foam phase instead of that in water phase by using the traditional solvent evaporation method. The method provides outstanding features, including being time-saving, of high-yield and able for continuous production, in which formation of porous polymeric microspheres finished within 3 min with a high production yield up to approximate 95 wt% and the process was able to be developed into a continuous process for production of porous polymeric microspheres. It was also universal to non-crosslinked polymers since the method is a development on the traditional emulsion solvent evaporation method. The new method is efficient and can be used potentially on the industrial scale for continuous production of porous polymeric microspheres.     

Solvent and relative humidity effect on highly ordered polystyrene honeycomb patterns analyzed by Voronoi tesselation

Highly ordered honeycomb‐patterned polystyrene surfaces are efficiently prepared by static breath figure method. The structured arrays can be obtained by casting a dilute solution polymer on glass substrates under various conditions. Tetrahydrofuran and chloroform are used as solvent to form cavities of several micrometers. The analysis of the surfaces indicates nonlinear relation between concentration and pore size in this system. Voronoi tessellations of the polystyrene surfaces at different relative humidity (RH) are achieved, and each conformational entropy determined. Optimum parameters of concentration and RH are obtained for both solvents. Analysis of hole size distribution and conformational entropy demonstrates the high order of the films obtained. This is a promising method for the fabrication of homogeneous and highly porous films from polystyrene. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44004.     

Vapor pressure over stressed coating films

Summary During formation of films from solutions of reactive polymers, crosslinking and solvent evaporation occur simultaneously but with different rates. If the film adheres to the substrate, the formed network shrinks only in the direction normal to the film surface and lateral strains develop. It was demonstrated theoretically that the lateral strain reduces the activity of the solvent in the film and thus lowers the evaporation rate. Received: 25 February 2000/Revised version: 26 June 2000/Accepted: 26 June 2000     

Preparation of porous silicon rubber membranes by breath figure method

A series of porous silicon rubber membranes with regularity and uniform size had been prepared by breath figure method form polydimethylsiloxane and hydrogen silicon oil (Si-H1341). Hydrophilicity and hydrophobicity of polymer, different methods of cast films, co-solvents, solid particle dispersants, temperature, solution concentration, and molecular weight of silicon rubber are considered and discussed in the article to analyze effects of experiment situations on porosity of porous silicon rubber membranes. Remarkably, using the method of blending silicon rubber with branch hydrophilic silica-gel, large area hydrophilic modification of the silicon rubber can be achieved, and it is easy to form pores with regular structure. With synergies of co-solvents, the excellent porosity with more rounded pore shape and better regularity is acquired. The results indicate that the pore size of the silicon were affected greatly by the addition of co-solvent, the boiling point of solvent, molecular weight of raw materials, and reaction temperature. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47912.     

Controlled mesoporous film formation from the deposition of electrosprayed nanoparticles

Thin films of controlled morphology were fabricated by electrospray drying a colloidal nanoparticle suspension using a conductive and volatile solvent and impacting the nanoparticles on a substrate. Three parameters were used for control: impact velocity, size of the nanoparticles or nanoparticle agglomerates, and solvent volatility. The impact velocity was controlled by charging nanoparticles through electrospray dispersion and varying the electric field driving the particle impaction. It was found that the structure is governed by the relative importance of charged particle drift imposed by the external electric field and the thermal velocity due to Brownian motion. Peclet number correlates with the morphology of the deposit where columnar structures result from high Pe, corresponding to ballistic deposition and porous, fractal-like structures result from small Pe. These patterns match predictions based on Monte Carlo simulations in the literature. For dispersions with higher nanoparticle concentrations, droplet evaporation causes densification of the particle ensemble to form a spherical aggregate that deposits in a predominantly ballistic manner, with smaller aggregates forming denser films. If the droplet evaporation lifetime is altered for the aggregates to be partially wet upon impacting the substrate, the subsequent rapid evaporation of the remaining solvent on the substrate leads to formation of films with high interconnectivity. Films formed by the electrospray technique have large-scale uniformity and their structure is independent of thickness. The interpretation of the observed morphologies in terms of Peclet number and Damkhöler number provides a conceptual framework for a rational design of film structures as required by many applications.

Copyright © 2017 American Association for Aerosol Research     

Single Solvent‐Based Film Casting Method for the Production of Porous Polymer Films

A single solvent‐based film casting process for fabricating porous polymer films is developed in this study. The porous film is produced by mixing concentrated polylactic acid (PLA)/chloroform solution (20 wt%) and fresh chloroform solvent is followed by film casting. The average pore sizes of the films produced are seen to increase from 2.1 (±0.1) µm to 6.4 (±0.2) µm with increasing ratio of concentrated PLA solution and fresh solvent from 1:2 to 1:4. Functional groups of PLA after casting into porous film are confirmed via Fourier transform infrared spectroscopy analysis. Cytocompatibility studies (via Alamar Blue assessment) utilizing MG‐63 cells on the porous PLA films reveal an increase in cell metabolic activity up to 8 d postseeding. In addition, these direct cell culture studies show that the porous membranes support cell adhesion and growth not only on the surface but also through the porous structures of the membrane, highlighting the suitability of these porous films in tissue engineering applications.     

Epoxidized poly(decenyloxazoline) in thermoset coatings

The side chain double bonds of poly(decenyloxazoline) were epoxidized using standard techniques. The epoxidation was facile and a high percentage of the double bonds were converted to oxirane groups. Films of the epoxidized resin were cured at an elevated temperature with either diamine or anhydride crosslinking agents. Both types of crosslinking agents yielded cured films with good solvent resistance, high hardness, and high gloss. The films cured with the anhydride crosslinkers and the higher molecular weight diamine had better properties than the films cured with the low molecular weight diamine. This could be because some of the low molecular weight diamine was lost due to evaporation during the cure cycle, which would result in lower crosslink density.     

Pore collapse during the fabrication process of rubber‐like polymer scaffolds

Rubbery polymer scaffolds for tissue engineering were produced using templates of the pore structure. The last step in the fabrication process consists of dissolving the template using a solvent that, at the same time, swells the scaffolding matrix that was a polymer network. Sometimes the polymer matrix is stretched so strongly that when the solvent is eliminated, i.e., the network is dried, it shrinks and is not able to recover its original shape and, consequently, the porous structure collapses. In this work we prepared, using the same fabrication process (the same template and the same solvent), a series of polymer scaffolds that results in collapsed or noncollapsed porous structures, depending on the polymer network composition. We explain the collapse process as a consequence of the huge volume increase in the swelling process during the template extraction due to the large distance between crosslinking points in the scaffolding matrix. By systematically increasing the crosslinking density the porous structure remains after network drying and the final interconnected pores were observed. It is shown that this problem does not take place when the scaffolding matrix consists of a glassy polymer network. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1475–1481, 2007     

Porous biodegradable polyesters. I. Preparation of porous poly(L‐lactide) films by extraction of poly(ethylene oxide) from their blends

Porous poly(L ‐lactide) (PLLA) films were prepared by water extraction of poly(ethylene oxide) (PEO) from solution‐cast PLLA and PEO blend films. The dependence of blend ratio and molecular weight of PEO on the porosity and pore size of films was investigated by gravimetry and scanning electron microscopy. The film porosity and extracted weight ratio were in good agreement with the expected for porous films prepared using PEO of low molecular weight (M_w = 1 × 10³), but shifted to lower values than expected when high molecular weight PEO (M_w = 1 × 10⁵) was utilized. The maximum pore size was larger for porous films prepared from PEO having higher molecular weight, when compared at the same blending ratio of PLLA and PEO before water extraction. Differential scanning calorimetry of as‐cast PLLA and PEO blend films revealed that PLLA and PEO were phase‐separated at least after solvent evaporation. On the other hand, comparison of blend films before and after extraction suggested that a small amount of PEO was trapped in the amorphous region between PLLA crystallites even after water extraction and hindered PLLA crystallization during solvent evaporation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 629–637, 2000     

Effect of UV crosslinking and physical aging on the gas permeability of thin glassy polyarylate films

The effect of crosslinking by UV irradiation on the gas permeation properties of thin films (thickness ≤1 μm) made from two benzophenone-based polyarylates were examined. In addition to the permeation response to UV crosslinking in these two polymers, the effects of crosslinking on the rate of physical aging was also explored. The sequence of physical aging and crosslinking, as well as reversal of the aging process was studied in order to separate the similar effects of aging and crosslinking. The results show that crosslinking very thin films can greatly improve the long-term performance of membranes when compared to noncrosslinked films of similar thickness.     

Effect of solvent evaporation rate on the crystalline state of electrospun Nylon 6

The role of solvent evaporation on the crystalline state of electrospun Nylon 6 fibers was examined by electrospinning into a closed chamber filled with different concentrations of solvent vapor. It was found that the thermodynamically stable α form became increasingly dominant in Nylon 6 fibers electrospun out of both 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and formic acid as the vapor phase solvent concentration increased. It is believed that the formation of the metastable γ form is due in part to the fast solvent evaporation kinetics associated with the electrospinning process. By varying the vapor phase concentration and thus the rate of solvent evaporation during electrospinning, we were able to vary the resulting crystal structure of the electrospun Nylon 6, as shown by XRD, Raman and FTIR.