“Grafting Through”: Mechanistic Aspects of Radical Polymerization Reactions with Surface-Attached Monomers

In this paper, we investigate the influence of selected reaction parameters on the formation of surface-attached polymer monolayers. The process is based on the use of self-assembled monolayers containing a polymerizable group and the performance of a bulk free radical polymerization reaction (“grafting through polymerization”). To this, methacryl moieties were immobilized on silica gel surfaces via a silane linker. During the polymerization reaction in a conventional way, free polymer is formed in solution. However, every now and then during chain growth also surface-attached monomers become integrated in the polymer chains, leading instantaneously to covalent linking of the growing polymer molecules to the surfaces. As more and more polymer chains become attached, this leads to the formation of a surface-attached polymer layer on the silica surface. Various sets of polymerization reactions were performed and the influence of a variation of temperature, reaction time and concentration of monomer, initiator, and immobilized monomer onto the layer formation are investigated. We propose a model of the layer formation process and the grafting-through process is compared to grafting-to and grafting-from techniques

Binding of Functionalized Polymers to Surface-Attached Polymer Networks Containing Reactive Groups

To study diffusion and binding of polymers into surface-attached networks containing reactive groups, surface-attached polymer networks bound to oxidized silicon surfaces are generated, which contain succinimide ester groups. The surface-attached polymer layers are brought into contact with poly­(ethylene glycol)­s (PEG), which carry terminal amine end groups and which have systematically varied molecular weights. The coupling reaction between the active ester groups in the polymer networks and the amine groups in the incoming chains are studied by ellipsometry, surface plasmon spectroscopy, AFM, and Fourier transform infrared spectroscopy (FTIR). The degree of functionalization of the reactive layers by the PEG-NH₂ depends strongly on the cross-link density of the network, the active ester content, and the molecular weight of the amine-terminated polymer. A model for the attachment reaction is proposed which suggests that the incoming polymer chains bind only at the outer periphery of the network in a narrow penetration zone. According to this model, when the incoming polymers are rather short, penetration into the layer and binding are prohibited by the high segment density and the anisotropic stretching of the surface-attached networks (“entropic shielding”). For incoming chains with a higher molecular weight and/or networks with a small mesh sizes, size exclusion effects determine diffusion and binding

Reduced Lateral Confinement and Its Effect on Stability in Patterned Strong Polyelectrolyte Brushes

The stability of strong polyelectrolyte brushes (PEBs) was studied in bulk and in patterned structures. Thick PEBs of poly­([(2-methacryloyloxy)­ethyl]­trimethylammonium chloride) with thicknesses >100 nm were synthesized using single electron transfer living radical polymerization. Brush patterning was identified using deep-ultraviolet photolithography by means of either a top–down (TD) or bottom–up (BU) method, with features as small as 200 nm. The brushes were soaked in water under a range of pH or temperature conditions, and the hydrolysis was monitored through dry-state ellipsometry and atomic force microscopy measurements. BU patterns showed reduced degrafting for smaller patterns, which was attributed to increased stress relaxation at such dimensions. In contrast to the already relaxed BU-patterned brush, a TD-patterned brush possesses perpendicular structures that result from the use of orthogonal lithography. It was found that the TD process induces cross-linking on the sidewall, which subsequently fortifies the sidewall materials. This modification of the polymer brushes hindered the stress relaxation of the patterns, and the degrafting trends became irrelevant to the pattern sizes. With proper tuning, the cross-linking on the sidewall was minimized and the degrafting trends were again relaxation-dependent