- Structure, Dynamics and Association of Thermosensitive Core-Shell Particles (2009)
- This thesis reports the synthesis, characterization, dynamics and association of thermosensitive core-shell particles. The particles consist of a solid core of poly(styrene) with a thin layer of poly(N-isopropylacrylamide) (PNIPAM) onto which a network of PNIPAM is affixed. The degree of crosslinking of the PNIPAM shell effected by the crosslinker N,N-methylenebisacrylamide. Cryo-TEM, SAXS and DLS have been used to investigate the structure and swelling of the particles. The micrographs show directly inhomogeneities of the network. Moreover, a buckling of the shell from the core particle was observed. The buckling increases with decreasing degree of crosslinking. A comparison of the overall size of the particles determined by DLS and Cryo-TEM demonstrates that the hydrodynamic radius provides a valid measure for the size of the particles. The phase transition in the PNIPAM network has been for the first time directly imaged by cryo-TEM. A quantitative method was developed to access quantitative information about the average and local structure of colloidal latex particles in dilute suspension at room temperature by cryo-TEM. The density profile derived from the cryo-TEM micrographs by image processing for the core and core-shell particles was compared to the results obtained by SAXS. Full agreement was found for the core particles. The discrepancy between the two methods in case of the core-shell particles was attributed to the buckling of the network affixed to the surface. The present analysis shows that SAXS is only sensitive to the average radial structure. The phase diagram and the colloidal crystallization of the different systems was investigated by a combination of direct observation, polarized optical microscopy and rheology. The effective volume fraction of the particles as derived from their hydrodynamic radius provides the base for all further analysis. After addition of salt all dispersions crystallize at volume fractions above 0.5. The core-shell microgels can be treated as hard spheres up to volume fractions of at least 0.55. We measured the dynamics of these model dense colloidal suspensions at the glass transition with a combination of rheometric instruments (piezoelectric axial vibrator, conventional mechanical rheometer and torsional resonator) over seven decades of frequencies. The application of the time-temperature superposition principle was not necessary. The connection between equilibrium stress fluctuations as measured in the frequency dependent linear shear moduli and the shear stresses under strong flow conditions far from equilibrium viz. flow curves was investigated. Data over an extended range in shear rates and frequencies were compared to theoretical results from integrations through transients and mode coupling approaches developed by Fuchs and coworkers. The connection between non-linear rheology and glass transition was clarified. For the first time in the rheology of suspensions we achieved a semi-quantitative description of both regimes with the same model. Additional processes like ageing or hopping are still not fully described by the theory which implies further developments of the model in the future. In presence of salt the composite core-shell particles reversibly aggregate above the LCST. The kinetics of reversibility of the phenomenon was investigated by DLS. The rheological measurements in the semi-dilute and concentrated regime the onset of the attractive interactions above the LCST leads to a strong thickening of the solution followed by a phase separation. This effect was applied locally for solutions, maintained close to LCST, after irradiation with a focused laser. Reversible micro-aggregates of a few micrometers diameter were obtained in the irradiated area under this local heating. The tunability and reversibility of the system presents a great advantage to extend the present investigations to the understanding of complex colloidal solutions in the attractive regime. Cationic gold nano-particles were synthesized and adsorbed onto the anionic core-shell particles. All analysis corroborates the association between the two kinds of particles. Microscopy demonstrates the formation of defined and separated electrostatic dipoles and DLS shows the reorganization in solution into larger structures. The novelty of this dissertation relies on a detailed characterization of thermosensitive colloidal core-shell particles and a new way to quantitatively characterize colloids by cryo-TEM. As a main feature, the rheology of these colloidal suspensions and the understanding of the glass transition are discussed. The experimental results supported by the application and comprehension of the theory provide an extended contribution to the dynamics of colloidal dispersions. To conclude the investigation of the aggregation in various colloidal systems enlarges the scope of the thesis to new interesting applications.