We report on the radical-chain polymerization of styrene using self-assembled monolayers of azo initiators covalently bound to high surface area silica gels. In this process monolayers of poly(styrene) molecules terminally attached to the surface of the inorganic substrate are obtained. As the initiator molecules are immobilized at the surfaces in a one-step reaction, well-reproducible layers can be prepared and the surface concentration of the initiator can be adjusted in a wide range between the limit of detection and full surface coverage. In the subsequent polymerization reactions polymer monolayers with high, controlled graft density can be obtained. The synthesis of the attached layers and the characterization by X-ray photoelectron spectroscopy, diffuse reflectance infrared spectroscopy (DRIFT), and elemental analysis are described. After cleavage of an ester group that connects the polymers to the surface, the molecular weights of the polymers were determined. The results of the study show that this “grafting from” technique can be used for the preparation of polymer layers with controlled, high graft densities.

We report a simple and yet effective way to photochemically attach thin polymeric layers to solid surfaces. The system is based on a photoreactive benzophenone derivative that is bound to SiO2 surfaces via a silane anchor. This substrate is then covered with a polymer film that is reacted with the benzophenone moieties by illumination with UV light (λ > 340 nm). As a result of the photochemical reaction, a thin layer of the polymer is covalently bound to the surface. Nonattached polymer is removed by extraction. As examples, we have successfully attached thin layers of poly(styrene) and poly(ethyloxazoline). The thickness of the layer is a function of the illumination time and the molecular weight of the polymer. The film thickness increases linearly with the radius of gyration of the polymers used for attachment. Using this system, we were able to photochemically attach up to 16 nm thick films of poly(styrene).

The kinetics and mechanism of a radical chain polymerization reaction initiated from a self-assembled monolayer of an azo initiator attached to the surfaces of silica particles are investigated. The rate of the decomposition of the surface-attached initiator is followed by differential scanning calorimetry and volumetry. The kinetics of formation of the terminally attached polymer is studied by dilatometry. After polymerization the polymer is removed from the surfaces and the molecular weight averages and molecular weight distribution of the degrafted polymer are studied as a function of reaction parameters during polymerization. From the molecular weight, the mass of the attached polymer, and the specific surface area, the number of polymer molecules per area (or the distance between the anchoring sites) can be calculated and compared to the corresponding values of the initiator monolayers. The mechanism of a polymerization with surface-attached radicals is compared to that of conventional radical chain polymerization in solution.

The swelling behavior of thin, surface-attached cross-linked dimethylacrylamide (DMAAm) films in contact with water has been characterized with multiple-angle nulling ellipsometry and compared to the swelling of nonattached, bulk DMAAm networks. The polymer networks are fabricated by crosslinking thin films of statistical copolymers composed of DMAAm and a photoreactive benzophenone derivative monomer. Covalent attachment of the film to the surface is achieved by means of a benzophenone-based silane monolayer. UV illumination simultaneously cross-links and chemically links the layer and chemically links the layer to the surface, forming stable networks that do not delaminate upon swelling. It is observed that the surface-attached networks swell less than the nonattached, bulk networks at the same cross-link density; however, the swelling of surface-attached networks is larger than that suggested by simple geometric considerations for swelling in one dimension. The results are in qualitative agreement with Flory-Rehner theory extended to one-dimensional swelling.

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A novel concept for the generation of molecularly thin polymer layers attached to a solid surface is described. In contrast to commonly used techniques, where functional groups of the polymers are reacted with appropriate sites on the surfaces of a substrate the polymer layers are formed in situ by using self-assembled monolayers of an initiator. As an example, the formation of polystyrene monolayers terminally attached to silicon oxide surfaces through radical chain polymerization which has been started by a self-assembled monolayer of an azo initiator is described. The thickness of the attached polymer films can be adjusted over a wide range up to values of several hundred nanometers by variation of polymerization parameters such as temperature and azo conversion. When suitable conditions are chosen, monolayers with thicknesses inaccessible by other techniques of preparation can be obtained.

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Steric forces between polymer brushes and atomic force microscope tips were investigated. We studied two systems:  polystyrene (PS) grafted to silicon in cyclohexane and poly(ethylene oxide)/poly(methacrylic acid) (PEO/PMAA) diblock copolymer adsorbed with the PMAA block to aluminum oxide in aqueous medium. On approach exponentially decaying repulsive forces were observed in both systems. With a homemade heat stage we could adjust the temperature. Increasing the temperature between 19 and 53 °C led to a linear increase of the decay length for PS in cyclohexane. Also the work required to bring the tip to a certain distance increased roughly linearly with temperature. This supports the view that the repulsion is of entropic origin. At the same time this demonstrates that the temperature dependence of surface forces could be routinely measured. For PEO in water the repulsive force was not significantly affected by a change in temperature. Approaching and retracting parts of force curves measured with PS in cyclohexane were in most cases indistinguishable. In contrast, for PEO in water a significant hysteresis was observed. This might be caused by an escape of polymers underneath the tip of the atomic force microscope. When retracting the tip in some cases the stretching of individual polymers was observed in both systems. Stretching force vs distance curves could be described by a wormlike chain model with typical persistence lengths of 1 nm.