Author: Naa Larteokor McFarlane

Publisher:

Published: 2010

Total Pages:

ISBN-13: 9781109671759

The phase behavior of model polymer - colloid mixtures is measured in the "protein limit", i.e., when the radius of gyration of the polymer (R g) is greater than or approximately equal to the radius of the colloid (R) and in the "colloid limit" (R> R g). In this work, alumina-covered silica nanoparticles are mixed with poly (ethylene oxide) (PEO) or poly (vinyl pyrolidone) (PVP) at asymmetry ratios of R g /R = 0.7 and 1.8. The adsorption of the two polymers onto the cationic nanoparticles was measured using isothermal titration calorimetry (ITC), gravimetric methods, and dynamic light scattering. Addition of PEO to stable nanoparticle dispersions leads to phase separation by depletion flocculation in both deionized water and buffer solutions. The phase separation mechanism for the PVP - nanoparticle system depends on the suspension medium. In water, bridging induced separation occurs below the saturation adsorption of PVP; above surface saturation, mixing leads to depletion-induced separation. In acidic buffer, phase separation results from depletion-induced interactions. ITC measurements of the heats of adsorption unambiguously determine the effects of polymer type and added buffer solution on the stability of nanoparticle dispersions upon the addition of adsorbing polymer. We find weak segmental adsorption energies of ~0.2 k B T for PEO in water and buffer, consistent with the observed phase separation. For PVP in water, segmental adsorption energies of order ~1.6 k B T support bridging flocculation in water, whereas a weaker adsorption energy of ~0.7 k B T in buffer is consistent with a lack of significant bridging flocculation. The difference between bridging and depletion is distinguished by visual appearance, rheological measurements, and small-angle neutron scattering (SANS). SANS measurements of PVP phase separated samples show a loss of the fractal region at low wavevector with increasing polymer concentration in moving from bridge flocculated to the depletion phase separation regime. There was also a concurrent shift in the interaction peak to lower Q values. These two effects signify a decrease in the density of the fractal aggregates with changing phase separation mechanism, consistent with a shift from bridging flocculation to depletion attraction. The ratio of polymer concentration to polymer entanglement concentration (c/c*) required to induce phase separation increases with increasing R g /R in agreement with theoretical predictions of the polymer reference interaction site model (PRISM). This trend opposes classical depletion theories because the classical theories do not account for polymer entanglement, amongst others, by assuming non-interacting polymers that interact as hard spheres. This assumption is clearly violated when R g> R when the nanoparticles can interpenetrate the polymer coils. Cationic nanovesicles are formed by sonication and characterized by viscometry, dynamic light scattering, and small-angle neutron scattering. The phase behavior of PVP - nanovesicle mixtures are measured and compared to the cationic nanoparticle system. Unlike the colloids, the nanovesicles do not phase separate on a short time scale, but rather, become unstable and revert back to the birefringent lamellae structure due to electrostatic repulsion of the charged heard groups. The rate of this coalescence is enhanced by the addition of polymer. This work provides a complete data set exploring bridging flocculation and depletion induced phase separation in the protein limit. As such it can be used to test theoretical work and to provide guidance in formulating polymer - nanoparticle mixtures. Extension to systems of nanovesicles highlights the differences inherent in selfassembled systems as compared to nanoparticles. The results can help guide industrial formulations containing mixtures of polymer, nanoparticles, and surfactant mesophases.