pH and Salt Response of Mixed Brushes Made of Oppositely Charged Polyelectrolytes Studied by in Situ AFM Force Measurements and Imaging

The response of mixed brushes made of poly­(acrylic acid) and poly­(2-vinyl pyridine) with a mixing ratio of about 60:40 was studied using atomic force microscopy (AFM) force measurements with colloidal probes and AFM imaging with a sharp tip in the pH range between 2.5 and 8 and at varying KCl concentrations up to 1 M. It was found that under all conditions a dense polyelectrolyte complex layer coexists with excess polyelectrolyte chains in varying swelling states depending on pH and salt concentration. The mixed brush thus combines typical features of polyelectrolyte brushes and complexes. So, the increase of the salt concentration not only led to a transition from osmotic to salted brush regime but also to salt-induced softening or partial decomposition of the complex layer. Attractive forces at high salt concentrations indicated the presence of P2VP chains in the swollen layer even at high pH values

Swelling and Surface Interactions of End-Grafted Poly(2-vinylpyridine) Layers in Acidic Solution: Influence of Grafting Density and Salt Concentration

In previous studies, the authors found that end-grafted layers of the weak polybase poly­(2-vinylpyridine) (P2VP) in aqueous solutions do not only swell and collapse if the pH value and salt concentration are varied but also exhibit a pH- and salinity-dependent adhesion to microsized silica spheres. For a better understanding of these effects, in situ force measurements using the AFM colloidal probe technique were applied to end-grafted P2VP layers of different grafting densities in NaCl solutions at pH 2.5. Although a mushroom-to-brush transition could be seen in the dry state, the layers were in the brush regime in aqueous solutions at all NaCl concentrations and grafting densities. We observed an increase of the brush height with increasing grafting density and a salinity-dependent collapse and reswelling of the brushes. The adhesion between the P2VP layer and a silica sphere depended on both grafting density and salinity. At low salt concentrations, the adhesion reached its highest value at the intermediate grafting density and disappeared with denser brushes. Maximum adhesion was obtained for high NaCl concentrations and the lowest grafting density. From a detailed analysis of the experiments, we gained insight into chain stretching and density profiles under complex ionic conditions and into the mechanism of adhesion of polyelectrolytes to solid surfaces

How do immobilised cell-adhesive Arg–Gly–Asp-containing peptides behave at the PAA brush surface?

<p>Bio-engineered surfaces that aim to induce normal cell behaviour <i>in vitro</i> need to ‘mimic’ the extracellular matrix in a way that allows cell adhesion. In this computational work, several model cell-binding peptides with a minimal cell-adhesive Arg–Gly–Asp sequence are investigated in the bulk as well as immobilised on a soft surface. For this reason, a combination of density functional theory and all-atom MD simulations is applied. The major goal of the modelling is to characterise the accessibility of the cell-recognition motif on the functionalised soft polymer surface. As a reference system, the behaviour of three peptide sequences is preliminarily studied in explicit water simulations. From the analysis of the MD trajectories, the solvent accessible surface area, the distribution of water molecules around peptide groups, the secondary structure and the thermodynamics of hydration are evaluated. Furthermore, each peptide is immobilised on the surface of a homopolymer poly(acrylic acid) brush. During MD simulations, all three peptides approach closely toward PAA brush, and their surface accessibility is characterised. Although the peptides are adsorbed onto the brush, they are not hidden by the polymer strands, with RGD unit accessible on the surface and available for guided cell adhesion.</p

Facile Approach to Grafting of Poly(2-oxazoline) Brushes on Macroscopic Surfaces and Applications Thereof

This study reports on a facile and versatile approach for modification of macroscopic surface via grafting of multifunctional poly­(2-oxazoline) molecules in brush-like conformation. For this purpose, carboxyl-terminated poly­(2-isopropyl-2-oxazoline) molecules have been synthesized by ring-opening cationic polymerization and subsequently grafted on underlined substrates by exploiting the ”grafting to” approach. A systematic variation in thickness of the grafted poly (2-isopropyl-2-oxazoline) brushes has been demonstrated. Polymer-modified surfaces have been characterized by means of a number of analytical tools including ellipsometry, X-ray photoelectron spectroscopy, ultraviolate spectroscopy, attenuated total reflection infrared spectroscopy and atomic force microscopy. Interestingly, poly­(2-isopropyl-2-oxazoline) molecules have been found to retain their physical properties even after grafting on macroscopic surfaces. Finally, fabricated polymer brushes have been used as platform for stabilization of inorganic nanoparticles on macroscopic surfaces

Molecular Dynamics of Swollen Poly(2-vinylpyridine) Brushes

The influence of swelling on the molecular dynamics of poly­(2-vinylpyridine) (P2VP) brushes is measured by broadband dielectric spectroscopy (BDS) in a broad temperature (350–420 K) and spectral (0.1 Hz–1 MHz) range with nanostructured, highly conductive silicon electrodes, separated by 35 nm high insulating silica spacers. A “grafting-to” method is applied to prepare P2VP brushes with a grafting density σ = 0.12 nm^–2 and a film thickness <i>d</i> = 7.3 nm as measured by ellipsometry. Swelling of the P2VP brushes is realized with tetrahydrofuran (THF) vapor using a flow cell. In the dry state, the segmental dynamics of the P2VP brushes coincides with the dynamic glass transition of the bulk system while in the swollen state it becomes faster by up to 1–2 decades due to the plasticizing effect of THF

Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes

We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phase-transition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity

Biofunctionalization of Titanium Substrates Using Nanoscale Polymer Brushes with Cell Adhesion Peptides

The grafting of polymer brushes to substrates is a promising method to modify surface properties such as wettability and the affinity toward proteins and cells for applications in microelectronics, biomedical devices, and sensors. Poly­(acrylic) acid (PAA) brushes are of high interest because of their stimuli-responsive behavior and the presence of carboxy (COOH) groups, which allow for immobilization of bioactive molecules. The “grafting-to” approach results in homogeneous and well-defined polymer brushes, but, although grafting-to has been demonstrated with PAA brushes on silicon (Si) substrates, it has not been performed on biocompatible materials such as titanium (Ti). Here, we have described a facile method to modify biocompatible Ti substrates with PAA brushes to amplify their substrate functionality. The grafting-to PAA “pseudo” brushes were successfully grafted to Ti substrates and retained their pH-dependent swelling behavior. An RGD peptide was covalently bound to COOH groups of the PAA brushes (PAA–RGD) as a model bioactive group. While NIH/3T₃ cell adhesion was significantly decreased on PAA-functionalized Ti substrates, PAA–RGD on Ti had cell adhesion comparable to that of flat Ti at 24 and 48 h, with significantly more cells adhered to PAA–RGD compared to PAA on Ti at 48 h

Adsorption of Poly(vinylformamide-<i>co</i>-vinylamine) Polymers (PVFA-<i>co</i>-PVAm) on Copper

The adsorption of poly­(vinylformamide) (PVFA) and its derivative statistical copolymer poly­(vinyl-formamide-<i>co</i>-vinylamine) (PVFA-<i>co</i>-PVAm) on metallic copper and copper oxide particles as well as planar copper surfaces was studied as a function of the degree of hydrolysis of PVFA, the pH, and the polymer concentration in solution. The chemical composition and molecular structure of the PVFA-<i>co</i>-PVAm layers were investigated by surface-sensitive spectroscopic methods such as XPS, DRIFT spectroscopy, and ellipsometry. The findings allowed us to explain the adsorption mechanisms and the forces driving the PVFA-<i>co</i>-PVAm adsorption. It was shown that PVFA-<i>co</i>-PVAm layers thicker than 30 nm are able to protect the planar copper surface against corrosive attack

Tunable Hydrophilic or Amphiphilic Coatings: A “Reactive Layer Stack” Approach

Thin films with tunable properties are very interesting for potential applications as functional coatings with, for example, anti-icing or improved easy-to-clean properties. A novel “reactive layer stack” approach was developed to create covalently grafted mono- and multilayers of poly­(glycidyl methacrylate)/poly­(<i>tert</i>-butyl acrylate) diblock copolymers. Because these copolymers contain poly­(glycidyl methacrylate) blocks they behave as self-cross-linking materials after creation of acrylic acid functionalities by splitting off the <i>tert</i>-butyl units. The ellipsometrically determined coating thickness of the resulting hydrophilic multilayers depended linearly on the number of applied layers. Amphiphilic films with tunable wettability were prepared using triblock terpolymers with an additional poly­(methyl methacrylate) block. The mechanism of the formation of the (multi)­layers was investigated in detail by studying the acidolysis of the surface-linked <i>tert</i>-butyl acrylate blocks by infrared reflection absorbance spectroscopy, accompanied by surface analysis using atomic force microscopy and contact angle measurements. In the case of the amphiphilic and switchable terpolymer layers this reaction was very sensitive to the used acidic reagent