Cylindrical brushes

Hand washing typically involves the appropriate soap and a brush. For burets, a cylindrical brush with a long handle (buret brush) is used to scrub the inner wall. With flasks, a bottle or test tube brush is used to clean the neck. Also, there are special bent brushes available to contact and scrub the inside of the base of the flask... 

Fig. 20 a-d. Four types of cylindrical brushes with different flexibility of the side chain and the main chain a stiff backbone - stiff side chains b stiff backbone - flexible side chains c flexible backbone - stiff side chains d flexible backbone - flexible side chains... 

Tapping mode SFM was employed to visualize cylindrical brushes of polymethyl-methacrylate as single species laying flat on a solid substrate (Fig. 23 a). Light scattering studies in solution allowed a complementary characterization of the size and conformation of the molecules in the diluted state. Two important observations have been made, that corroborate the theoretical predictions ... 

Table 1. Molecular properties of cylindrical brushes of polymethylmethacrylate ...

It must be noted that the end-to-end distance of the brushes is also affected by adsorbing them flatly on a hard wall. In monolayers (d = 2) the molecules are constrained to two dimensions which favors extension and parallel aUgnment [ 163 ]. This is particularly pronounced if the side chains get tightly adsorbed and will be discussed in detail below. Yet even in the case of a weak interaction with a substrate, the chains are extended. In contrast, in thick films the individual chain can adopt a less straight conformation by transition between layers in the z-direction. Figure 24 clearly demonstrates the difference in the ordering of cylindrical brushes of polystyrene depending on the film thickness [78]. 

Fig. 24a,b. Micrographs of a a monolayer b thick film of cylindrical brushes of polystyrene on mica. The order is significantly higher in the monolayer film [78]... 

Fig. 27a-c. Conformations of cylindrical brushes upon adsorption on a surface a extended coil b two-dimensional helix c globule... 

This way it has been possible to observe the anticipated conformational transition by experiment. Cylindrical brushes of polybutylacrylate-graft polymethacrylate were adsorbed on the surface of water from a CHCI3 solution on a Langmuir trough and studied by recording the tt-A (surface pressure vs molecular area) isotherm. At different stages of compression, the monolayer was... 

Fig. 45 When cylindrical brushes possessing a gradient grafting density along the backbone are adsorbed on a surface, one can observe a transition from a rod-like to a tadpole conformation upon partial desorption of side chains. The end with a higher grafting density, and thus with a greater extension of the side chains, is predicted to collapse more readily than the loose end (Reproduced with permission from [173])...

The following Sect. 2 deals with the association of diblock copolymers and discusses micelles, vesicles, micellar aggregation and interaction. In Sect. 3 recent work on micelle and vesicle formation of triblock copolymers is presented, whereas in Sect. 4 more complicated architectures such as graft copolymers, cylindrical brushes and Janus micelles are discussed. 

So far, there have been only few reports about the synthesis of amphipolar polymer brushes, i.e. with amphiphilic block copolymer side chains. Gna-nou et al. [115] first reported the ROMP of norbornenoyl-endfunctionalized polystyrene-f -poly(ethylene oxide) macromonomers. Due to the low degree of polymerization, the polymacromonomer adopted a star-like rather than a cylindrical shape. Schmidt et al. [123] synthesized amphipolar cylindrical brushes with poly(2-vinylpyridine)-block-polystyrene side chains via radical polymerization of the corresponding block macromonomer. A similar polymer brush with poly(a-methylstyrene)-Wocfc-poly(2-vinylpyridine) side chains was also synthesized by Ishizu et al. via radical polymerization [124]. Using the grafting from approach, Muller et al. [121, 125] synthesized... 

Lyotropic liquid-crystalline ordering was also observed for cylindrical brushes formed by the homopolymerization of macromonomers, i.e. macromolecules with polymerizable endgroups (Figure 34) [403]. The extended conformation of these cylindrical brushes is controlled by the coiling and steric repulsion of the side chains and is independent of the unperturbed flexibility of the backbone segments [402]. 

Cylindrical Brushes with Anionic Side Chains. 136... 

Table 3 Comparison of different chemical pathways to prepare cylindrical brushes...

Since the pioneering work of Tsukahara on the homopolymerization of methacryloyl end-functionalized polystyrene macromonomers to polymacromonomers several chemically different polymacromonomers were successfully synthesized [106-118]. Also different routes to cylindrical brush polymers by grafting from [119-121] and grafting on to [122, 123] techniques were reported, each of which exhibits certain advantages and disadvantages, as outlined in Table 3. 

In the following text we focus on cylindrical brushes prepared by polymerization of macromonomers, in particular on two systems. First, cylindrical brushes consisting of poly-2-vinylpyridine side chains and a poly-methacrylic main chain are discussed these were converted into cationic polyelectrolyte brushes by quaternization of the pyridine units by alkyl halides. The second system comprises sulfonated anionic cylindrical brushes, i.e. polymacromonomers with polystyrene side and main chains and polymacromonomers with polymethacrylic main and polymethacrylic acid side chains. 

Table 7 Fraction of free counterions, fc (normalized to the number of chemically quater-nized monomers), the effective charge density, / (normalized to the total number of monomers), the effective charge density per main chain monomer fma the cross sectional radius of gyration RgjC) the mean concentration of counterions cc and the mean inverse Debye screening length Xb l within the volume of a cylindrical brush molecule due to condensed counterions...

The hydrolysis/elimination reaction of the f-butyl methacrylate brushes is not easily driven to full conversion. Whereas linear f-butyl methacrylate chains are fully converted to polyacrylic acid in dioxane with catalytic amounts of toluene sulfonic acid the cylindrical brush structures precipitated at about 50% conversion (sample PMA-PBMA31-MAS50). Complete cleavage of the ester groups was achieved, however, by dissolving the partial-... 

In a second series of experiments barium hydroxide was added in order to monitor the effect of bivalent counter-ions on R sulfonate group and Ba2+ (BaS04 is insoluble in water) a counter-ion included cross-linking within the cylindrical brush should be observable in dilute solution. A slight decrease of R experimental error (Table 11). Above 10% Ba2+ relative to SO3- the brushes precipitate. 

Neither the main nor the side-chain dimensions of ionic cylindrical brushes differ significantly from their uncharged counterparts. Obviously, the osmotic swelling is not significant for the present small side-chain lengths, i.e. Psc<50. The intermolecular structure factor of ionic cylindrical brushes are difficult to interpret. The intermolecular distances derived from the peak of the structure factor is different for H+ and Cs+ counter-ions and is always significantly smaller than the mean distance calculated from the concentration of the particles and the known molar mass. Thus, a two state model may also be postulated for the present cylindrical polyelectrolyte structures. 

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