PMAA brush

Figure 21 shows the segment density profiles of such PMAA brushes as obtained by Fresnel-modeling, and, more directly (i.e. model free ), by FT analysis of ellipsometric spectra at different solution pH [68]. Figure 22 shows the swollen brush height L obtained from these experiments. 

Fig. 23 Viscoelastic parameters measured during approach to a PMAA brush under various conditions of pH. The friction coefficient is increased at intermediate pH...

When the brushes are brought into contact with solutions containing multivalent ions the picture described above changes dramatically. To demonstrate the effect the same PMAA brushes as described above were exposed to solutions containing multivalent ions. Figure 25 shows as examples changes of brush thickness with increasing ion concentration when the brushes are exposed to sodium, calcium, and aluminium nitrate solutions [74]. 

Fig. 25 Swollen thickness of PMAA brushes as a function of the external salt concentration for aqueous sodium, calcium, and aluminium nitrate solutions. Values at a concentration of 10 6 mol L 1 correspond to the swelling in pure MilliQ water. The dry film thicknesses were 42 nm (stars), 45 nm (circles) and 46 nm (squares)...

Fig. 26 Representative ellipsometric spectra (A as a function of the incidence angle F is omitted for clarity) of PMAA brushes on LaSFN9 prisms swollen in aqueous solutions of NaN03 and Ca(N03)2. The concentrations are 10 5, 10 4, 10-3, 10 2, 10 1 and 10° mol L-1 from the top downwards and the offset in A by which the curves are shifted for clarity is 20. The solid lines represent model calculations using a complementary error function to describe the segment density profile...

Fig. 27 Representative segment density profiles of the collapse of a PMAA brush on a LaSFN9 prism swollen in aqueous solutions of Ca(N03)2 having different external salt contents as denoted in the figure. The profiles were obtained from model calculations based on the ellipsometric spectra shown in Fig. 26...

Last, PMAA brushes in contact with aluminium solutions show similar behavior to the calcium case however, the collapse concentration is found to be lower by roughly two orders of magnitude. If an aluminium salt solution is added to the brush the collapse starts at concentrations of <10-5 mol L 1 (Fig. 25). Furthermore, it is interesting to note that the film still contains rather large amounts of water in the collapsed state and remains much more swollen, compared to the same film which has been collapsed by exposure to calcium. 

A similar approach was adapted to prepare poly(methacrylic acid) (PMAA) brushes on silicon using TEM grids as masks. 2-Bromo-2-dimethyl-A-[3-(trimethoxysilyl)propyl]propanamide was employed as initiator. Patterned PMAA brushes with a resolution in the range of 100 pm were obtained, which were then used for templating the formation of microstructured calcite films (Figure 3.3) [2]. 

Figure 5.10 AFM images of a nanopatterned PMAA brush in height and firiction mode. Images...

In this work, we have adapted the PMP method originally developed by Otsu et al. to prepare polyfmethacrylic acid) (PMAA) brushes on silicon surfaces, which are covered with native Si02. The PMP method is based on dithiocarbamate... 

In this work, a silane-derivatized dithiocarbamate iniferter was utilized to prepare PMAA brushes on Si/Si02 surfaces under UV irradiation. The combination of the photoiniferter-mediated photopolymerization with a UV-LED source appears to be ideally suited to the direct preparation of polyelectrolyte brushes with minimal free polymer formation during brash synthesis. Following characterization of the PMAA brushes by means of surface-analytical techniques, such as quartz crystal micro-balance with dissipation monitoring (QCM-D), spectroscopic ellipsometry, and static contact-angle measurements, the PMAA brushes were demonstrated to enhance aqueous lubrication of Si/ Si02 under low-contact-pressure conditions. 

Static water contact angles were determined by the sessile-drop method employing a Rame-Hart goniometer (Rame-Hart Instrument Co., Model-100, Netcong, NJ) at all stages of the surface-modification process, i.e., after cleaning of the Si02 substrates, after vapor deposition of the photoiniferter, and after photopolymerization of the PMAA brushes. 

Figure 3. Characterization of the dry PMAA brush thickness as a function of irradiation time. The monomer solution consisted of 10 vol % MAA in water, and the 365 nm UV-LED intensity at the sample surface was 12 mW/cm. The lines serve as a guide to the eye.

Table 1 shows the calculated minimum molecular weights of the PMAA brushes that were obtained from average dry ellipsometry thickness data of the photoiniferter monolayer and the PMAA brushes. Since the fraction of photoiniferter molecules that induces simultaneous polymer growth is... 

Table 1. Minimum Molecular Weights of the PMAA Brushes Prepared in This Work Based on Calculations from Eqs 1—3...

Investigation of the in Situ PMAA Brush Growth by Means... 

The in situ growth characteristics of PMAA brushes, determined by means of QCM-D experiments, are shown in Figure 6. After the UV-LED was switched on, the resonance frequency (black circles) increased slightly before a continuous decrease was observed. The onset of the negative frequency shift is believed to mark the start of PMAA brush polymerization since the decrease in resonance frequency can be attributed to an increase in the mass that is coupled to the QCM crystal. Up to 25 min of brush growth, i.e., until ca. 40 min in Figure 6, the frequency of the third... 

Figure 6. Frequency shift (black circles) and dissipation changes (gray circles) observed in QCM-D experiments during sinface-initiat, in situ polymerization of PMAA brushes from 10 vol % monomer solutions in water. After a first polymerization step, the QCM cell was rinsed with fresh monomer solution (after 100 min) before the UV-LED was switched on again (after 290 min) to induce a second polymerization step. After the UV source was switched off (after 325 min), the cell was rinsed with fresh monomer twice before pure water was continuously pumped through the system in order to remove unbound polymer.

Figure 8. Long-term pin-on-disk experiments involving PMAA brushes with two distinct brush heights (30 and 240 mn). The shorter brush (gray circles) did not sustain the applied tribological stress (1 N normal load, 1 mm/s sliding speed) for 1000 rotations, while the 240 mn PMAA samples displayed very low friction coefEcients throughout the entire experiment.

When the identical long-term pin-on-disk experiment was performed with the longer PMAA brush (240 nm dry thickness), the coefficient of friction decreases from initially... 

Figure 9. Comparison of pi vs sliding speed plots between 15 nm PMAA brushes and 3 nm PEG (5000) monolayers, which have been prepared via grafting from and grafting to methods, respectively. The molecular weight of individual polymer chains was expected to be of the same order of magnitude for both brushes.

Figure 10. Presumed tribological interface between a highly hydrated PMAA brush and an ox-PDMS slider in an aqueous environment of neutral pH.

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