Manfred Stamm

Characterization of perfluorosulfonic acid membranes by conductivity measurements and small-angle x-ray scattering

Abstract Three perfluorosulfonic acid membranes, Nafion 120, Nafion 117 and Dow, pretreated in water at 100°C were characterized by conductivity measurements in the temperature range of 20–90°C and by Small-Angle X-Ray Scattering (SAXS) at room temperature. The results demonstrate that conductivity is determined by the apparent proton equivalent conductivity and not the proton volume concentration. The apparent equivalent conductivities increase in the sequence Nafion 120

Hairy tubes: Mesoporous materials containing hollow self-organized cylinders with polymer brushes at the walls

In this chapter, a route to prepare “hairy tubes” is presented. Hydrogen-bonded supramolecules, based on PS-block-P4VP, self-organize into cylinders in a glassy rigid PS-matrix. Annealing the material and applying oscillatory shear flow increases the macroscopic order of the cylinders. Part of the supramolecular complex, i.e. PDP, can be conveniently washed out using methanol after the structure has been formed. Thus, empty tubes with P4VP “hairs” on the walls are obtained. This simple method allows tailoring the tubes and the transport properties, for example by tuning the block lengths or by chemical modification of the hairs.

Temperature Controlled Encapsulation and Release Using Partially Biodegradable Thermo-magneto-sensitive Self-rolling Tubes

We suggest a new approach for controlled encapsulation and release of microparticles, cells and drugs using thin bilayer films of thermoresponsive and biodegradable polymers, which are able to form self-rolling tubes. The magnetic nanoparticles are incorporated in the thermoresponsive layer in order to provide the microtubes sensitivity to magnetic field. We demonstrate reversible rolling and unrolling of the polymer films, reversible capture and release of microparticles in response to change of temperature as well as manipulation of particle-loaded microtubes using external magnetic field. The suggested approach can be successfully implemented for controlled delivery of drugs and cells in living organisms as well as to design scaffolds for tissue engineering.

Switchable photoluminescence of CdTe nanocrystals by temperature-responsive microgels.

In the present study, we report a method for preparing a fluorescent thermosensitive hybrid material based on monodisperse, thermosensitive poly( N-isopropyl acrylamide) (PNIPAM) microgels covered with CdTe nanocrystals of 3.2 nm diameter. The CdTe nanocrystals were covalently immobilized on the surface of PNIPAM microgels. The chemical environment around the CdTe nanocrystals was modified by changing the temperature and inducing the microgel volume-phase transition. This change provoked a steep variation in the nanocrystal photoluminescence (PL) intensity in such a way that when the temperature was under the low critical solution temperature (LCST) of the polymer (36 degrees C) the PL of the nanocrystals was strongly quenched, whereas above the LCST the PL intensity was restored.

Switching of friction by binary polymer brushes

Surface force studies on polystyrene-poly(2-vinylpyridine) (1 : 1) mixed polymer brushes and corresponding monobrushes were carried out in dried state under a controlled environment. The aim was to explore possibilities to control adhesion and friction between inorganic or polymeric surfaces by use of polymer brushes. The effect of switching of chemical composition of binary brush surfaces (on treatment with suitable solvents) on wettability, surface roughness, and hence the adhesion and friction properties of the surfaces were determined. Atomic force microscopy (AFM) with silicon tips, silicon nitride tips, and colloidal probes with silica particles were employed to investigate the interactions between inorganic surfaces and polymer brushes. To study the interactions between different polymer brush surfaces colloidal probes were covered with polystyrene and poly(acrylic acid) brushes on the surface. For all the dry polymer brushes samples, surface roughness values were in the range of 0.35-1.0 nm only. Adhesion and friction forces of polymer brush samples were reduced in comparison to the silicon wafer and were correlated with each other (except for the silicon tip). Switching in adhesion and friction forces up to a factor of 4.5 was possible by switching of the conformation of mixed brushes on treatment with selective solvents. The friction force on a PS + P2VP gradient polymer brush layer varied laterally and increased with increase in the P2VP content. Friction depends on wettability, scan velocity of the AFM tip, grafting density, and composition gradient of polymer brushes. Moreover, for PS and P2VP monobrushes, friction forces were shown to increase with increasing grafting density. Polymer brush layers thus may be used to control the adhesion and friction behavior of solid surfaces in different ways.

Protein adsorption on and swelling of polyelectrolyte brushes: A simultaneous ellipsometry-quartz crystal microbalance study

With a coupled spectroscopic ellipsometry-quartz crystal microbalance with dissipation (QCM-D) experimental setup, quantitative information can be obtained about the amount of buffer components (water molecules and ions) coupled to a poly(acrylic acid) (PAA) brush surface in swelling and protein adsorption processes. PAA Guiselin brushes with more than one anchoring point per single polymer chain were prepared. For the swollen brushes a high amount of buffer was found to be coupled to the brush-solution interface in addition to the content of buffer inside the brush layer. Upon adsorption of bovine serum albumin the further incorporation of buffer molecules into the protein-brush layer was monitored at overall electrostatic attractive conditions [below the protein isolectric poimt (IEP)] and electrostatic repulsive conditions (above the protein IEP), and the shear viscosity of the combined polymer-protein layer was evaluated from QCM-D data. For adsorption at the “wrong side” of the IEP an incorporation of excess buffer molecules was observed, indicating an adjustment of charges in the combined polymer-protein layer. Desorption of protein at pH 7.6 led to a very high stretching of the polymer-protein layer with additional incorporation of high amounts of buffer, reflecting the increase of negative charges on the protein molecules at this elevated pH.

The control of thin film morphology by the interplay of dewetting, phase separation and microphase separation

In thin film geometry, the interplay between dewetting and phase separation or microphase separation controls the morphology of the polymeric structures resulting on a solid support. For the model system of polystyrene, polyparamethylstyrene and the diblock copolymer of the two homopolymers, the regime of ultrathin films is addressed experimentally. Evolving structures are probed with real and reciprocal space analysis techniques such as the optical microscopy, phase measuring interference microscopy, scanning force microscopy, neutron or x-ray reflectivity and grazing incidence small angle neutron or x-ray scattering approaches. The effective interface potential of the solid support is tuned by means of a change of the silicon substrate coating. Coating layers of silicon oxide, polyamide and polyimide are under investigation. A power law behaviour describing the most prominent in-plane length as a function of the initially prepared film thickness is observed. All reported structures have been prepared on large scale surfaces, such as typical Si wafers with 100 mm diameter.

Biocompatible polymeric materials with switchable adhesion properties

We report on the design, investigation and comparison of the adhesive properties of poly-(N-isopropylacrylamide) and biocompatible polyethylene glycol-based thermoresponsive brushes. Specifically, a poly-(N-isopropylacrylamide), PNIPAM, poly-(oligo(ethylene glycol) methyl ether methacrylate-co-2-(2-methoxyethoxy)ethyl methacrylate), P(OEGMA-MEO2MA), and poly-(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(propylene glycol) methacrylate), P(OEGMA-OPGMA) brushes were synthesised on planar silicon wafers and silica particles via the surface-initiated atom transfer radical polymerisation. Switching of swelling and surface charge were investigated using spectroscopic ellipsometric and zeta-potential measurements, respectively. Adhesion properties were studied in situ in aqueous solutions at different temperatures using three kinds of AFM probes with well defined tips geometries and radii. We found that: (1) adhesion properties of all polymer brushes reversibly switch with temperature—polymer brushes are completely non-adhesive below the Low Critical Solution Temperature (LCST) and become sticky above the LCST; (2) adhesive force scales nonlinearly with the probe radius; (3) adhesion energy, obtained using Derjaguin/Muller/Toporov approaches, decreases with the increase of the tip radius that is attributed to different depth of penetration of the probes in the polymer layer; (4) adhesion decreases in the sequence PNIPAM – P(OEGMA-MEO2MA) – P(OEGMA-OPGMA). Based on dynamic light scattering and cryo-TEM experiments, the latter effect is attributed to the formation of a thin oligoethylene glycol shell around the hydrophobic polymer core in the case of P(OEGMA-OPGMA). We expect that the P(OEGMA-OPGMA) system, which demonstrates less sticky adhesion properties, could be particularly promising for cell adhesion experiments and tissue engineering.

Nano-dewetting: Interplay between van der Waals- and short-ranged interactions

The wetting behavior of a polymer liquid on a coated substrate is investigated via numerical self-consistent field calculations and experiments. The polymer does not wet the coated substrate, but the substrate might stabilize a mesoscopic film. Experiments observe a stable mesoscopic film of nanometer thickness in coexistence with macroscopic drops at high temperatures. Upon cooling, this mesoscopic polymer film breaks up into droplets (nano-dewetting). Our self-consistent field calculations suggest that the stability of the mesoscopic film is determined by a subtle interplay between van der Waals forces and short-ranged forces due to the distortion of the profile in the vicinity of the substrate.

Mixed silane self assembled monolayers and their in situ modification

Mixed self assembled silane monolayers were prepared by coadsorption of Br and methyl terminated trichlorosilanes. By subsequent in situ modification the Br groups were converted into amino groups which can be used to tether polyglutamates to the surface. Here we describe the characterization of the monolayers by means of contact angle measurements and X-ray photoelectron spectroscopy (XPS). Both methods clearly indicate that the Br terminated silane is preferentially adsorbed from the solution. Following the in situ modification by XPS a yield of approximately 80% was determined for the substitution of the bromine by azide, whereas the yield for subsequent reduction to the amine is close to 100%.