On the shape of bottle-brush macromolecules: Systematic variation of architectural parameters

We measured the form factor of bottle-brush macromolecules under good solvent conditions with small-angle neutron scattering and static light scattering. The systems under investigation are brushes, synthesized via the grafting-from route, built from a poly(alkyl methacrylate) backbone to which poly(n-butyl acrylate) side chains are densely grafted. The aim of our work is to study how the systematic variation of structural parameters such as the side chain length and backbone length change the conformation of the polymer brushes in solution. All spectra can be consistently described by a model, considering the bottle-brush polymers as flexible rods with internal density fluctuations. Parameters discussed are (1) the contour length per main chain monomer l(b), (2) the fractal dimension of the side chains D-s, as well as (3) the fractal dimension D, and (4) the Kuhn length lambda(k) of the overall brush. l(b)=0.253 +/- 0.008 nm is found to be independent of the side chain length and equal to the value found for the bare main chain, indicating a strongly stretched conformation for the backbone due to the presence of the side chains. The fractal dimension of the side chains is determined to be D-s=1.75 +/- 0.07 which is very close to the value of 1/0.588 approximate to 1.70 expected for a three-dimensional self-avoiding random walk (3D-SAW) under good solvent conditions. On larger length scales the overall brush appears to be a 3D-SAW itself (D=1.64 +/- 0.08) with a Kuhn-step length of lambda(k)=70 +/- 4 nm. The value is independent of the side chain length and 46 times larger than the Kuhn length of the bare backbone (lambda(k)=1.8 +/- 0.2 nm). The ratio of Kuhn length to brush diameter lambda(k)/d >= 20 determines whether lyotropic behavior can be expected or not. Since longer side chains do not lead to more persistent structures, lambda(k)/d decreases from 8 to 4 with increasing side chain length and lyotropic behavior becomes unlikely.