Swelling‐Induced Chain Stretching Enhances Hydrolytic Degrafting of Hydrophobic Polymer Brushes in Organic Media

Hydrophilic polymer brushes grown via surface‐initiated polymerization from silicon oxide surfaces can detach or degraft in aqueous media. Degrafting of these chain end‐tethered polymers is believed to involve hydrolysis of bonds at the polymer–substrate interface. Degrafting so far has not been reported for hydrophobic polymer brushes in non‐aqueous media. This study has investigated the degrafting and swelling properties of poly(tert‐butyl methacrylate) (PtBMA) brushes in different water‐miscible, organic solvents, viz. DMF, acetone and THF. In the presence of a sufficient quantity of water in the organic solvent, degrafting was also observed for PtBMA brushes. More importantly, however, the rate of degrafting depended on the nature of the organic solvent and the apparent initial rate constant of the degrafting reaction was found to correlate with the swelling ratio of the polymer brush in the different solvents. This correlation is first, direct evidence in support of the hypothesis that degrafting is facilitated by a tension that acts on the bond(s) that tether the polymer chains to the surface and which is amplified upon swelling of the polymer brush.     

Mesoscopic description of solvent effects on polymer dynamics

Solvent effects on polymer dynamics and structure are investigated using a mesoscopic solvent model that accounts for hydrodynamic interactions among the polymer beads. The simulation method combines molecular dynamics of the polymer chain, interacting with the solvent molecules through intermolecular forces, with mesoscopic multiparticle collision dynamics for the solvent molecules. Changes in the intermolecular forces between the polymer beads and mesoscopic solvent molecules are used to vary the solvent conditions from those for good to poor solvents. Polymer collapse and expansion dynamics following changes in solvent conditions are studied for homopolymer and block copolymer solutions. The frictional properties of polymers are also investigated.     

The morphology and structure of PS‐b‐P4VP block copolymer films by solvent annealing: effect of the solvent parameter

Lamellae (symmetric) forming polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) block copolymers (BCPs) were used to produce nanostructured thin films by solvent (toluene) casting (spin‐coating) onto silicon substrates. As expected, strong micellization of PS‐P4VP in toluene results in poorly ordered hexagonally structures films. Following deposition the films were solvent annealed in various solvents and mixtures thereof. A range of both morphologies including micelle and microphase separated structures were observed. It was found that nanostructures typical of films of regular thickness (across the substrate) and demonstrating microphase separation occurred only for relatively few solvents and mixtures. The data demonstrate that simple models of solvent annealing based on swelling of the polymer promoting higher polymer chain mobility are not appropriate and more careful rationalization is required to understand these data. Analysis suggests that regular phase separated films can only be achieved when the copolymer Hildebrand solubility parameter is very similar to the value of the solvent. It is suggested that the solvent anneal method used is best considered as a liquid phase technique rather than a vapor phase method. The results show that solvent annealing methods can be a very powerful means to control structure and in some circumstances dominate other factors such as surface chemistry and surface energies. Copyright © 2009 John Wiley & Sons, Ltd.     

SELECTIVE NEURITE OUTGROWTH ON SILVER NEGATIVE ION (Ag~- )-IMPLANTED POLYSTYRENE SURFACES

INTRODUCTIONA negative ion-implantation technique[1,2] developed by us for use in semiconductors, integrated circuits (IC),and large scale-integrations (LSI) has been studied in its application on biotechnology. The implantation of highion energy beams causes the impairment of insulators such as common synthetic polymers. Our negative ion-implantation, however, is considered to result in less charge-up problems compared with positive ion-implantation. That is, negative ionic species can…     

Evolution of fractal-like surface structures in layers of polyacrylonitrile solutions by interfacial dynamic processes

Solvent evaporation causes concentration and temperature gradients at the free surface of a coating polymer solution. Thereby surface-tension-driven flows can result in a surface instability with regular structures within the layer. Different types of surface structures can occur at spots with thickness differences. One can find fractal-like structures especially in the edge zone of a layer. The evolution of such fractal-like structures at the surfaces of coating solutions of polyacrylonitrile (PAN) in dimethylformamide (DMF) during layer hardening is examined. Condensation of water on the surface of the fluid layer exerts a strong influence on the formation of these structures. Solutions of PAN in DMF are important for the production of ultrafiltration membranes. Such membranes are produced by precipitation in a water bath after a short open time.     

Acrylic polymer/silica organic–inorganic hybrid emulsions for coating materials: Role of the silane coupling agent

Acrylic polymer/silica organic–inorganic hybrid emulsions were synthesized by a simple method, that is, a conventional emulsion polymerization and subsequent sol–gel process, to provide water‐based coating materials. The acrylic polymer emulsions contained a silane coupling agent monomer, such as methacryloxypropyltriethoxysilane, to form highly solvent‐resistant hybrid films. On the other hand, the hybrid films from the surface‐modified polymer emulsions, in which the silane coupling agent was located only on the surface of the polymer particles and the particle core was not crosslinked, did not exhibit high solvent resistance. A honeycomblike array structure, which was derived from the polymer particles (diameter ≈ 50 nm) and the silica domain, on the hybrid film surfaces was observed by atomic force microscopy. The crosslinked core part and silane coupling agent containing the shell part of the polymer particles played important roles in attaining high solvent resistance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4736–4742, 2006     

Surface characterization of the extracellular matrix remaining after cell detachment from a thermoresponsive polymer

The temperature-responsive behavior of poly(N-isopropyl acrylamide) (pNIPAM) directly affects the attachment and detachment of cells cultured on these surfaces. At culture temperatures, cells behave similarly to those on tissue culture polystyrene (TCPS), while at room temperature, cells cultured on pNIPAM spontaneously detach as a confluent sheet. In comparison, cells grown on TCPS remain attached indefinitely after the temperature drop, requiring enzymatic or mechanical removal. In this work, we present an examination of the response of bovine aortic endothelial cells (BAECs) and extracellular matrix (ECM) proteins to plasma polymerized NIPAM (ppNIPAM) surfaces using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and immunostaining. Immunoassay results reveal that, although fibronectin, laminin, and collagen closely associate with the cell sheet, some collagen may be associated with the surface, as well. Our XPS results indicate that ppNIPAM surfaces after cell liftoff differ from their blank counterparts, the primary distinction being the presence of amide and alcohol species on ppNIPAM surfaces used for cell culture, possibly owing to the presence of a proteinaceous film. Finally, a comparison between ppNIPAM-treated surfaces used for cell culture versus control surfaces by principal component analysis of the ToF-SIMS data confirms that the surfaces differ; the presence of molecular ion fragments from amino acids (e.g., alanine, glycine, and proline) is the chief reason for this difference. Therefore, from our surface characterization of ppNIPAM-coated TCPS after cell liftoff, we conclude that although low-temperature liftoff of the BAEC monolayer is accompanied by the majority of the components of the ECM, some of the ECM proteins still remain at the surface.     

Surface morphology of spin‐coated molecularly imprinted polymer films

The surface morphology of thin molecularly imprinted polymer films has been studied using atomic force microscopy (AFM). The films were produced by spin coating onto glass substrates and examined as a function of host polymer, imprinting template, casting solvent, spin‐coater rotation speed and post‐production treatment. It was observed that the gross features of such films are template controlled. The fine structure is determined by parameters such as solvent, spin speed or subsequent treatment. The relationship between these observations, polymer–template interactions and the mechanism of film formation in spin coating is discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Malonic Acid Diazoesters for C−H Insertion Crosslinking (CHic) Reactions: A Versatile Method for the Generation of Tailor‐Made Surfaces

A simple and versatile method for the modification of a broad spectrum of surfaces with thin polymer films through the thermally or photochemically induced generation of surface‐attached polymer networks is reported. The system is based on copolymers containing diazomalonate groups, which can be activated by heat or light. To this end, the copolymers are deposited from solution onto solid substrates by standard techniques of thin‐film deposition (spin coating, dip coating). Upon activation the diazomalonate group decomposes and forms a carbene, which induces C−H insertion crosslinking (CHic) reactions. In the course of this process network formation and covalent surface attachment occur at the same time. The crosslinking process proceeds very rapidly, especially when the carbenes generated in the activation process cannot undergo Wolff‐rearrangement. The presented system can be used for the generation of a wide range of polymer layers and microstructures on a broad spectrum of surfaces.     

Formation and characterization of polymersomes made by a solvent injection method

In this article a solvent injection method is described for vesicle formation using poly(butadiene)‐ b‐poly(acrylic acid) diblock copolymers as the amphiphilic molecules. Vesicles composed of polymer bilayers are commonly referred to as polymersomes. Solvent injection is shown to be a rapid method for polymersome formation suitable to make large volumes of polymersome solution. The method can be manipulated to obtain a wide range of vesicle sizes depending on the polymer concentration and preparation conditions. Polymersome solutions in this study are characterized using dynamic light scattering (DLS), fluorescent microscopy, and electron microscopy. Polymersome sizes range from tens of nanometers to several microns. The membrane thickness of smaller polymersomes is found to lie between 14–20 nm. Larger polymersomes are found to have somewhat thicker membranes. The procedure involves the addition of polymers dissolved in an organic solvent to a stirred aqueous solution. The formation of polymersomes by this method is proposed to be governed by the limited mutual solubility of the two solvents and the simultaneous diffusion of solvent and water out of and in to initially formed organic solvent droplets. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface tension evolution during early stages of drying of sol–gel coatings

Striation defects in spin-coated thin films have been blamed on unfavorable capillary forces that occur due to solvent evaporation commonly experienced during coating deposition. Solvent evaporation during spinning causes predictable composition changes at the surface and these can either stabilize or de-stabilize the surface with respect to convective motions within the coating solution. The present work examines the surface tension changes while adding the most volatile component rather than removing it. This is then a “reverse drying” process, but it provides us with the slope of the surface tension change during normal coating drying. We have examined coating solutions for a case where a specific solvent addition has previously been shown to prevent the formation of striation defects. By measuring both the starting solution (one that produces bad striation defects) and the co-solvent-modified solution (that produces much flatter coatings), we are able to demonstrate the correlation between surface tension changes during spinning and the striation defect formation (or prevention). For the present case, an aluminum-titanate sol–gel recipe, the solvent that eliminated the striation defects is also responsible for a continuous, gradual, reduction in surface tension during the spin-on process, consistent with a model proposed earlier (D. P. Birnie, J Mater Res 16:1145–1154, 2001).     

Surface Reconstruction by a Coassembly Approach

Materials with switchable surfaces, capable of changing surface properties under external stimuli, are playing a pivotal role in many applications, such as tissue engineering, biosensors, and drug/protein delivery. In this research silica particles with patterned and switchable surfaces are fabricated. Surface micelles on silica particles are formed by coassembly of polymer brushes and “free” block copolymer chains in a selective solvent. The cores of the surface micelles are crosslinked by anthracene photodimerization. After quaternization of the coronae, amphiphilic surface micelles are prepared. The surface micelles are able to rearrange in different media. After treatment with an organic solvent, the surfaces of silica particles are occupied by hydrophobic polymer components; in aqueous solution, the positively charged polymer chains are on the surfaces. The switching of the surface micelles results in changes in surface composition and wetting behaviors.     

Surface Plasticization of Poly(ether ether ketone)

This paper describes a study of the surface plasticization and antiplasticization of an amorphous and a semicrystalline poly(ether ether ketone) (PEEK) in solvent environments using nanohardness method. A range of solvents (octane, chloroform, tetrachloroethane, acetone, dichlorobenzene, polyethyleneglycol (PEG), methanol and water) based on the Hilderbrand’s Solubility Parameter were selected as solvent environments. The results of the nanoindentation hardness experiments performed on the virgin and the solvent immersed polymeric surfaces are reported. The surface plasticization or antiplasticization is reported on the basis of the softening or the hardening of the near surface layers (?1 μm) after immersion of the polymeric surfaces in the solvent environments. Surface plasticization of the amorphous PEEK has been observed in organic solvents. The chlorine containing solvents have severely degraded the hardness of the amorphous polymer. A surface hardening of the amorphous PEEK has been observed after immersion in water. Semicrystalline PEEK was seen to exhibit a considerable inert behaviour to common organic solvents but chlorinated organic solvents and water have caused a decrease in the surface mechanical properties.     

Surface Skin Development and Rupture During Sol-Gel Spin-Coating

Spin coating is one of the standard methods for depositing sol-gel, nanocomposite, or polymer coatings onto flat substrates (silicon wafers, glass plates for displays, sensor substrates, etc.). Our recent research has been focused at understanding a wide variety of defect formation mechanisms and looking for ways to prevent these defects. A key aspect of spin-coating is the solvent evaporation that happens at the top surface of the coating fluid during spinning--even while the fluid is moving rapidly, radially, outward on the substrate. As the fluid gets physically thinner, concentration gradients can develop within the fluid as a result of the evaporation. The enrichment of the precursors at the top surface can cause premature gelation there, in other words a “skin” may form. This skin layer is also more resistant to stretching than the underlying fluid and can retard radial fluid motions as well as further solvent evaporation. It is even possible for the stretching forces to be large enough that fracture or tearing of the skin occurs. These surface ruptures create wispy locations of significantly thinner coating or small randomly located linear rips, but they do not typically result in complete penetration through the coating. The occurrence of these defects depends to a large degree on the volatility of key solvents used in the sol-gel process, with less volatile solvents being less likely to result in surface ruptures.     

Surface characterization of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl-polyhedral oligomeric silsesquioxane) by inverse gas chromatography

The surface properties of poly(methyl methacrylate-co-n-butyl acrylate-co-cyclopentylstyryl polyhedral oligomeric silsesquioxane) (poly(MMA-co-BA-co-styryl-POSS)) were studied by means of inverse gas chromatography (IGC) using 10 non-polar and polar solvents as the probes. Thermodynamic parameters of adsorption, e.g., specific retention volume, the dispersive component of the surface free energy, the specific interaction contribution to the free energy of adsorption and the acid/base constants were obtained to investigate the interactions between the surfaces of the copolymers and different solvents. It was found that incorporation of styryl-POSS into polymer resulted in increasing interactions between polymers and solvents, dispersive component of surface free energy of polymer and acidity of the surfaces of the polymers. The more the styryl-POSS were embedded, the stronger the interaction between the polymer surface and solvent, the dispersive component of the surface free energy and the acidity of the polymer surface were.     

Dispersible Carbon Nanotubes

A method is proposed to produce nanoparticles dispersible and recyclable in any class of solvents, and the concept is illustrated with the carbon nanotubes. Classically, dispersions of CNTs can be achieved through steric stabilization induced by adsorbed or grafted polymer chains. Yet, the surface modification of CNTs surfaces is irreversible, and the chemical nature of the polymer chains imposes the range of solvents in which CNTs can be dispersed. To address this limitation, supramolecular bonds can be used to attach and to detach polymer chains from the surface of CNTs. The reversibility of supramolecular bonds offers an easy way to recycle CNTs as well as the possibility to disperse the same functional CNTs in any type of solvent, by simply adapting the chemical nature of the stabilizing chains to the dispersing medium. The concept of supramolecular functionalization can be applied to other particles, for example, silica or metal oxides, as well as to dispersing in polymer melts, films or coatings.     

Switching and structuring of binary reactive polymer brush layers

Switchable binary polymer brushes grafted to Si-wafers were prepared from hydrophilic and hydrophobic polymer components. When exposed to solvents, either the hydrophobic or the hydrophilic component extends in to the liquid phase, depending on the polarity of the solvent. The hydrophilic component was poly-2-vinylpyridine; the hydrophobic component was made photocrosslinkable in that a polystyrene copolymer containing a photodimerizing chromophore was used. In this system surfaces differing in water contact angle between 60° and 100° can be produced by variation of the solvent. The chromophore was phenylindene, which forms crosslinks upon direct UV-irradiation. Therefore, the polystyrene component can be fixed in the extended or collapsed state. It will be shown that by irradiation through an appropriate mask, surfaces can be structured and the structures fixed. In both the systems structural patterns differing in surface properties were produced and fixed photochemically.     

Polymer coating of carboxylic acid functionalized multiwalled carbon nanotubes via reversible addition‐fragmentation chain transfer mediated emulsion polymerization

In this work, successful polymer coating of COOH‐functionalized multiwalled carbon nanotubes (MWCNTs) via reversible addition fragmentation chain transfer (RAFT) mediated emulsion polymerization is reported. The method used amphiphilic macro‐RAFT copolymers as stabilizers for MWCNT dispersions, followed by their subsequent coating with poly(methyl methacrylate‐co‐butyl acrylate). Poly(allylamine hydrochloride) was initially used to change the charge on the surface of the MWCNTs to facilitate adsorption of negatively charged macro‐RAFT copolymer onto their surface via electrostatic interactions. After polymerization, the resultant latex was found to contain uniform polymer‐coated MWCNTs where polymer layer thickness could be controlled by the amount of monomer fed into the reaction. The polymer‐coated MWCNTs were demonstrated to be dispersible in both polar and nonpolar solvents. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Thermoresponsive Poly(N‐isopropylacrylamide)‐Based Block Copolymer Coating for Optimizing Cell Sheet Fabrication

Thermoresponsive surfaces are prepared via a spin‐coating method with a block copolymer consisting of poly(N‐isopropylacrylamide) (PIPAAm) and poly(butyl methacrylate) (PBMA) on polystyrene surfaces. The PBMA block suppresses the removal of deposited PIPAAm‐based polymers from the surface. The polymer coating affects the temperature‐dependent cellular behavior of the surfaces with respect to protein adsorption. By adjusting layer thicknesses, PBMA‐b‐PIPAAm‐coated surfaces are optimized to regulate the adhesion/detachment of cells by temperature changes. Thus, thermoresponsive polymer‐coated surfaces are able to harvest contiguous cell sheets with their basal extracellular matrix proteins.

     

Three‐dimensional reference interaction site model self‐consistent field analysis of solvent and substituent effects on the absorption spectra of Brooker's merocyanine

Solvent and substituent effects on the absorption spectra of Brooker's merocyanine (BM) are investigated using the three‐dimensional reference interaction site model self‐consistent field method and time‐dependent density functional theory. The π–π* excitation energies are computed for BM and its derivative 2,6‐di‐tert‐butyl (di‐t‐Bu) BM. The behaviors of the computed excitation energies with increasing solvent polarity are in good agreement with those of the corresponding experimental measurements. In addition, analysis of the solute–solvent interaction energies and spatial distribution functions reveals that the effects of the solvent on the absorption spectra are reduced by the steric hindrance of the t‐Bu groups. Furthermore, from the difference in the solute–solvent interaction energies of BM and di‐t‐Bu BM, it is shown that the effect of the t‐Bu substituents on the absorption spectrum is greater in high‐polarity solvents. © 2015 Wiley Periodicals, Inc.