Dense grafting

The deformation of densely grafted chains reflects a balance between interaction and elastic free energies. In this discussion of structure, we assume that a dense... 

Liang, D., Zhou, S., Song, L., Zaitsev, V. S., and Chu, B., Copolymers of poly(N-isopropylacrylamide) densely grafted with poly(ethylene oxide) as high-performance separation matrix of DNA, Macromolecules, 32, 6326, 1999. 

The plots of h/h vs. copolymer concentration also reveal differences in the micropolarity of the hydrophobic domains created upon association of the various copolymers in water. A qualitative assessment of this property is given by the h/h value determined in the copolymer solutions of highest concentration when the plateau value is attained (Fig. 25). This value depended significantly on the grafting level the solution of the most densely grafted copolymer yielded the lowest h/h value (1.40) and the pure homopolymer the highest. In all cases, this value is higher than the value (1.20) recorded for micellar solutions of the macromonomer. It can be concluded... 

Multiple detection 2+ detectors, proper calibrants No segment-segment interactions, No neighbor-group eflFects Bulk composition and composition distribution. Broad applicability, Intrinsic generation of calibration curve. No additional sample preparation work Statistical copolymers. Densely grafted chains 2, 3, 5, 8... 

One system with these desirable features is a suspension of small a 50 nm) silica particles onto which octadecyl chains have been densely grafted (Stober et al. 1968 van Helden et al. 1981 Woutersen and de Kruif 1991). The octadecyl chains have a theta point near room temperature in various solvents, including benzene, dodecane, and hexadecane. 

Figure 7-6 shows the viscosity of thermoreversible dispersions (discussed in Section 7.2.4) of a 50-nm silica particles onto which octadecyl chains have been densely grafted in benzene at particle volume fractions 4> — 0.088-0.133, as a function of temperature T (Woutersen and de Kruif 1991). For T > 9 — 316 K, the viscosity relative to that of the solvent, rir = r /f]s, is independent of temperature, and its dependence on volume fraction 0 is exactly as expected for hard spherical particles without attractive interactions. As T is lowered below 9, however, the viscosity rises rapidly, because of the onset of attractive interactions. 

In a first example, for a densely grafted PEL brush system positively charged quaternized poly-4-vinylpyridine brushes have been prepared by following a two step approach [2, 63, 64, 66]. In the first step a neutral poly-4-vinylpyridine monolayer is prepared and, subsequently, charges are introduced by a second, polymer-analogous quaternization step. The grafting density of the parental neutral brush is adjusted by varying the polymerization time (Fig. 12) [63, 64]. The substrates were planar silicon substrates in... 

The detailed analysis156 demonstrated that the onset of the side chain collapse is shifted toward the range of poorer solvent conditions compared to that in an individual chain of length N. Both the side chains and the backbone remain extended at z < z with respect to the Gaussian dimensions for densely grafted brushes, and the length of a dense brush is... 

Comb or densely grafted polymers are defined as polymers that have at least one polymeric chains per monomer unit of the main chain, and Figure 29 shows examples obtained by metal-catalyzed living radical polymerization. Comb polymers possess physical properties similar to those of star polymers in solution. 

The field of densely grafted copolymers has received considerable attention in recent years. The materials (also called bottle-brush copolymers) contain a grafted chain at each repeat unit of the polymer backbone. As a result, the macromolecules adopt a more elongated conformation. Examples of brush copolymers have been provided within the context of ATRP [307-309]. Synthesis of the macroinitiator was achieved through one of two approaches. One method used conventional radical polymerization of 2-(2-bromopropionyloxy)ethyl acrylate in the presence of CBr4 to produce a macroinitiator with Mn=27,300, and a high polydispersity of Mw/Mn=2.3 (Scheme 46A) The alternative involved the ATRP of 2-trimethylsilyloxyethyl methacrylate followed by esterification of the protected alcohol with 2-bromopropionyl bromide. While synthetically more challenging, the latter method provided a macroinitiator of well-defined structure... 

Scheme 46 A,B. Synthesis of densely grafted brush copolymers via A a conventional radical polymerization followed by ATRP B wholly by ATRP [307]...

Mn=55,500, Mw/Mn< 1.3) leading to a brush synthesized entirely by a controlled process (Scheme 46B). From either macroinitiator, the ATRP of St and nBA was conducted, leading to the desired densely grafted structures. The grafting reactions were found to be very sensitive to reaction conditions additional deactivator, high concentrations of monomer, and reduced temperatures were all necessary to produce the desired materials. 

Prucker O, Konradi R, Schimmel M, Habicht J, Riihe J. Photochemical strategies for the preparation and microstructuring of densely grafted polymer brushes. In Advincula RC, Brittain WJ, Caster KC, Ruhe J, editors. Polymer Brushes. Weinheim Wiley—VCH 2004. pp. 449—69. 

Fig. 5 Typical result for dewetting of a PDMS film (92 nm, = 308 kg/mol) on top of a densely grafted brush of end-functionalized PDMS molecules (6.3 nm, = 8.8 kgAnol). The temperature was 50°C. Dewetted distance d and the width w of the rim are represented in the inset. The lines are best fits of y = A(t — t f to the data, whae y stands for d or w. zl is a constant prefactor, and to is a time offset, a was foimd to be 0.65 for d and 0.32 for w [146, 147]...

---