Polymer Brush Studies

The silicon wafer surface was prepared in the same manner as that for the cooperativity studies (see section 3.1.2.1). 

0% solution by weight was prepared by dissolving 50 mg of copolymer in 5.0 mL toluene. A clean silicon wafer was set onto the spin coater and four drops of solution were placed on the surface. Each polymer brush sample was rotated at 1000 rpm for 1 minute. 

To chemically bond the polymer chains to the silicon surface the polymer brush samples required heat well above their Tg s. All samples were placed in a vacuum oven (25 Torr) at 170°C immediately following spin coating. The terminal OH groups reacted with the native oxide layer of the silicon wafer as demonstrated in literature. The reaction time was varied by removing samples from the high temperature environment at 10, 20, 40, 100, and 200 hours. (Because the 200 hour sample results duplicated the 100 hour sample results, only results for the 100 hour sample are presented.) To remove any unreacted polymer, the samples were immersed in toluene and placed in a sonicator for a total of 34 hours at approximately 40°C. 

After the unreacted polymer was removed, the films were washed with toluene and chloroform to alter the chain comformations. Analysis was performed to observe 

Three major variables were investigated in this polymer brush study the initial reaction time of the sample, the solvent last used to wash the sample, and the effect of a hydrolysis reaction. As the samples are discussed, the number depicts the reaction time, the next letter represents the solvent (t, toluene c, chloroform), and a h indicates a hydrolyzed polymer. For example, 40ht means the sample has a reaction time of 40 hours, was last washed in toluene, and has been hydrolyzed. A complete listing of all the polymer brush samples prepared appears in Table 3.2-3. 

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Yamamoto, S., Ejaz, M., Tsuji, Y., Matsumoto, M., and Fukuda, T. 2000. Surface Interaction forces of well-defined, high-density polymer brushes studied by atomic force microscopy. 2. Effect of graft density. Macromolecules 33 5608-5612. 

Surface interaction forces of well-defined, high-density polymer brushes studied by atomic force microscopy. 1. Effect of chain length. 33 ... 

A polystyrene/polyZ-butyl methacrylate (PS/PtBMA) di-block copolymer was previously synthesized for the polymer brush studies. The molecular weight of the copolymer is approximately 50,000 g/mol with the m n ratio approximately 1 1. The chemical structure of the di-block copolymer appears in Figure 3.2-1. 

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

There have been several studies on the use of RAFT to form polymer brushes by polymerization or copolymerization of macromonomers 348-350. 

He, G. L., Merlitz, H., Sommer, J. U. and Wu, C. X. (2007) Static and dynamic properties of polymer brushes at moderate and high grafting densities A molecular dynamics study. Macromolecules, 40, 6721-6730. 

Salt effects in polyelectrolyte block copolymer micelles are particularly pronounced because the polyelectrolyte chains are closely assembled in the micellar shell [217]. The situation is quite reminiscent of tethered polymer brushes, to which polyelectrolyte block copolymer micelles have been compared, as summarized in the review of Forster [15]. The analogy to polyelectrolyte brushes was investigated by Guenoun in the study of the behavior of a free-standing film drawn from a PtBS-PSSNa-solution [218] and by Hari-haran et al., who studied the absorbed layer thickness of PtBS-PSSNa block copolymers onto latex particles [219,220]. When the salt concentration exceeded a certain limit, a weak decrease in the layer thickness with increasing salt concentration was observed. Similar results have been obtained by Tauer et al. on electrosterically stabilized latex particles [221]. 

By studying the properties of polymer layers on soHd surfaces it soon became obvious that not only is the chemical composition of the immobihzed polymer cmcial for the performance of the material, but so is its morphology. This has been recognized in various fields of applications e.g. stabihzation of small particles suspensions by attached polymer brush-type layers [159, 160], control of adhesion [161] or friction [162] and tailored stationary phases for chromatography [163-165]. 

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