- Polyelectrolyte brush (1) (remove)
- Synthesis and Characterization of Cationic Spherical Polyelectrolyte Brushes (2005)
- In this thesis, the preparation of cationic spherical polyelectrolyte brushes by photo-emulsion polymerization has been presented for the first time. The structure and properties of the cationic SPB, have been comprehensively investigated by dynamic light scattering and atomic force microscopy, and compared with anionic SPB. Moreover, both of them were tested as dual retention-aid in conjunction with cationically modified polyacrylamide in papermaking. The interaction of SPB with negatively charged surfaces was investigated by AFM operated in Tapping Mode. It was demonstrated that the negative SPB forms two-dimensional aggregates of densely packed polymer particles, which can be explained by a particle-particle interaction dominating the repulsive interaction of the particles with the mica substrate. The positively charged SPB exhibits a completely different particle-surface interaction behaviour from that of negatively charged SPB. Here network-like structure films of dried particles without long-range 2D order are formed, which is due to the strong attractive particle-surface interaction of positively charged polyelectrolyte chains in the shell of the cationic SPB particles. These chains spread over the negatively charged mica surface and anchor the particles. The swelling behaviors of cationic and anionic SPB as a function of ionic strength in the system were studied by DLS. Adding more and more salt leads to a strong shrinkage of the surface layer as expected for polyelectrolyte brushes. For some ions, however, high salt concentrations may lead to a re-swelling of the brush layer in case of the cationic systems. This points to specific interactions of the counterions with the PATAC chains. This strong specific interaction between the counterions and the attached polyelectrolyte may even lead to flocculation of the particles at intermediate salt concentration. Surprisingly, for sodium iodide and magnesium sulfate the solubility increases again if the salt concentration is raised to 1 mol/l. At lowest ionic strength electrostatic interaction prevails and the brush layer is swollen in all cases by the osmotic pressure of the counterions. Intermediate salt concentrations lead to a partial screening of the electrostatic interaction and to a shrinkage of the brush layer. This effect can be well captured by the theory of Hariharan et al.. In case of cationic brushes, however, the shrinkage becomes very pronounced around salt concentrations of 0.1 mol/l. In some cases there is even a collapse of the surface layer due to specific interactions between the polyion and the counterions. Cationic systems re-swell if immersed in concentrated salt solutions. This is observed for monovalent as well as for divalent counterions. The analysis of the reduced excluded-volume parameter v/(lKl2) suggests that there is an adsorption of the counterions at high salt concentrations. The salting-in behavior thus finds an explanation in the increase of v due to the adsorption of salt ions. All data demonstrate that specific effects of different counterions lead to a behavior of the brush layer not expected from a purely electrostatic model. All specific effects seen at high concentrations of added salt can be explained by the increase of the reduced excluded-volume parameter which is due to the adsorption of salt ions. Cationic and anionic SPB were tested as dual-component retention system in combination with cationically modified polyacrylamide, and compared with the traditional "microparticle" system in which bentonite acts as secondary flocculant. The anionic SPB-based system shows high flocculation efficiency as tested under sheared conditions using a dynamic drainage jar. The high retention level of the SPB dual-component retention system can be explained by the enlarged specific surface area and cation exchange capacity of SPB, which are caused by grafted flexible polyelectrolyte chains. Furthermore, chemically grafted polyelectrolyte brushes show more advantages than bentonite, which may further delaminate upon dilution and cause CaCO3 fillers partially detach from the fiber. Images from field emission scanning electron microscopy of difference stages of retention testes suggest that the flocculation mechanism of anionic SPB and CPAM as dual-component retention system can be summarized in three steps: at first, excessive CPAM were added to bind cellulose fibers and CaCO3 fillers and formed loose macroflocs with positive charges; then the agglomerations were broken into microflocs by strong shear strength; at last, anionic SPB with negative charges were added and caused much finer and denser flocs, thus creating paper sheets with better optical appearance due to higher homogeneity. Images from FESEM images and AFM support the model of anionic SPB’s acting as a particle bridge between fibers and CaCO3 fillers.