Handbook of Modern Coating Technologies

Polymer brushes

A polymer brush is a unique type of surface functionalization, consisting of long polymer chains covalently attached with its one terminus to a substrate, while the rest of the chain remains mobile. When the brush is immersed in a good solvent, it tries to dissolve but can­not go very far because of its tethered end, so the thin 10—100 nm swollen brush layer is nei­ther a liquid, nor a solid, but a very peculiar organization of soft matter.

End-tethered polymer decorated surfaces are getting increasingly important in many cur­rent technologies [93], including (1) biology—brushes are important in reducing friction in mammalian joints [94]; (2) manufacturing—brushes are involved in flow instabilities [95] which limit the extrusion speed of polymer filaments used to make our clothes and fiber- reinforced construction materials; and (3) energy—brush formation takes part in advanced oil recovery [96] from porous shale rock.

Due to its atomic resolution, its noninvasive nature and the possibility of enhancing the contrast between solvent and polymer by isotopic replacement NR is a powerful tool for the structural and dynamical investigation of polymer brushes. Monomer-density profiles of end-tethered chains in solution were mainly revealed by NR on PS brushes in the beginning of the 1990s [97,98]. In the case of brush/melt interfaces NR was uniquely used to get the monomer-density profile at high resolution due to the possibility of labeling and its high spa­tial resolution [99].

More recent studies have been focusing on polydimethylsiloxane (PDMS) melts in contact with irreversibly adsorbed PDMS (pseudobrush) [100] or one-end-tethered PDMS [101] or to study the functionalization of poly(2-hydroxyethyl methacrylate) brushes [102], while others went on to more complicated brushes like star-polymers end-tethered to a substrate [103]. With the advance of chemistry the monomer-density profiles of ATRP brushes were uncov­ered by NR [104,105]. Other NR studies could even resolve the chain end distribution in grafted films in a good solvent [106] by appropriate labeling. Brush structure under confine­ment became recently accessible by NR in the case of two brushes in close contact [107—109] or in close contact to a polymer membrane [110—112].

Recently a big body of work including NR was used to characterize the structure of bio­compatible brushes, especially made of poly(ethylene glycol) [113—116], and its interaction with proteins [39,117,118] important in antifouling applications, for example.

Another advantage of NR is the fact that most engineering materials such as aluminum or titanium are transparent for the neutrons which allows to access buried interfaces or samples embedded in complex environments like shearing devices [119]. Structural investigations of brushes under shear load have been performed by NR measurements on PS brushes in sol­vents [120—122]. These report no change of the grafted layer density or thickness with shear.

However, a grafted layer in a pure solvent is a rather academic case and not directly related to polymer flow past surfaces. To the best of our knowledge only two shear studies of PS melt in contact with a PS brush exists. The first one was also performed by NR with in situ shear [123]. In this study no reproducible result could be obtained and this was explained by metastable states of the brush. However, very high torques were applied in this study and the brushes were not characterized after the shear experiments. It could be shown by NR as well that grafted PS brushes can be destroyed by high torque shear [124] and thus this sce­nario is likely in the aforementioned study. Desorption of PS brushes physisorbed on silicon by a PEO end block under shear was studied as well for one chain length [125] or even bimodal brushes [126] showing that for these brushes the applicable shear rate is even lower than for covalently grafted brushes. The second study used ex situ-sheared PS brushes in a deuterated PS matrix [127] and here a reproducible retraction of the brush from the melt could be observed with shear. This study was performed more recently in situ as well, in contact with a semi-dilute PS solution [128].

Another sample environment frequently used in NR investigations of polymer brushes are pressure cells. Some examples include brush structure investigation under confinement and pressure in the case of two brushes in close contact [107,129] or brushes solved in supercritical carbon dioxide under pressure [130]. As a more detailed example we will review a recent NR study on the temperature and pressure dependence of a poly-2-(dimethylamino) ethyl methacrylate (PDMAEMA) brush which exhibits a LCST when exposed to water [131]. By using a recently designed pressure cell for NR [132] it was possible to investigate this sys­tem under hydrostatic pressures up to 1200 bar. The neutron beam was impinging on the brush/water interface through a single crystal silicon block to which the polymer chains were physisorbed. To enhance the contrast between polymer and solvent, heavy water was used as solvent. Fig. 4—18 shows the NR profiles and the deduced volume fractions of PDMAEMA monomers as a function of temperature and pressure. As can be seen from the figure a positive temperature difference can be perfectly counterbalanced by an increased pressure, which makes this system interesting for pressure sensors or as lubrication coatings in artificial joints in the human body. Interdiffusion dynamics of polymer melts into polymer brushes were recently performed by NR on PS. The diffusion kinetics close to the glass tran­sition temperature of the matrix melt is on the time scale of hours and could be well resolved with consecutive TOF-NR measurements [133].

• Magnetic nano-particle assembly

Recently, the self-assembly of magnetic nano-partciles at solid boundaries was more and more intensiviely studied with NR. These systems take ideal benefit from the specific proper­ties of neutrons. The large penetration power allows the study of buried interfaces, while the mangetic moment makes the neutron sensitive to the magnetic induction of the particles. Isotope contrast variation allows in addtion the investigation of the coatings stabilising the partciles in the solvent. For the purpose of this review we limit ourselfs in pointing a very

recent review on this topic [134] summarising results on the self-assembly of magnetic nano­particles at chemically and magnetically templated substrates. The main massage of the review is that the self-assembly process is dominated by the magnetic dipol interaction of the particles