The Role of Polymer Polydispersity in Phase Separation and Gelation in Colloid−Polymer Mixtures
Entry by Emily Redston, AP 225, Fall 2011
Work in progress
The Role of Polymer Polydispersity in Phase Separation and Gelation in Colloid−Polymer Mixtures by J. J. Lietor-Santos, C. Kim, M. L. Lynch, A. Fernandez-Nieves, and D. A. Weitz. Langmuir 26 3174–3178 (2010)
Mixtures of non-adsorbing polymer and colloidal particles exhibit a very rich range of different morphologies. These microstructures depend on the particle and polymer concentrations as well as the relative size of the particles and polymer. The addition of the polymer to a colloidal suspension leads to a depletion attraction that is capable of inducing a fluid-solid transition (i.e. forming a gel). A gel is defined as a connected network that spans space and can support weak stresses. Gels are extensively used in commercial applications, such as personal care or food products, where they are able to help stabilize the system against sedimentation. In this manner, gels can reduce phase separation, which will increase product shell life. The nature of the fluid-solid transition depends on the range of the depletion attraction, which, in turn, depends on the ratio of the size of the polymer to the colloidal particle. For short-range interactions, gelation is induced by spinodal decomposition; the system undergoes a gas-liquid phase separation which is interrupted by the dynamical arrest of the particles in the colloid-rich region, leading to formation of the gel. By contrast, for larger ranges of the attraction, phase separation can proceed to completion, without being interrupted by dynamic arrest, leading to gelation.
However, in the presence of gravitational effects, the structure may no longer be capable of sustaining its own weight; instead, it collapses, disrupting the phase separation process or rupturing the gel that was previously formed. This is an undesirable effect technologically, as it can dramatically shorten the shelf life of a commercial product. This suggests that the use of shorter polymers at sufficiently high concentration is most likely of greatest practical use. However, technological polymers are very rarely monodisperse, and thus the microstructures and their behavior may be drastically modified. However, despite the practical importance, the gravitational behavior of colloid- polymer mixtures using polydisperse polymers has never been investigated.
In this paper, we investigate in detail the behavior of model colloidal particles mixed with nonadsorbing polymer with a poly- disperse size distribution, similar to that often found in commer- cial samples. We use a predominantly short-range interaction to induce gelation. However, we show that the presence of a small concentration of much larger polymers can have dramatic con- sequences on the behavior. We find that only those mixtures in the proximity of the liquid-gel boundary collapse under the influence of gravitational effects. Interestingly, however, we find that the gravitational stress imposed on the network is not the ultimate cause for this behavior. Instead, it results from coarsening of the particle network due to spinodal decomposition, which results from the long-range attraction induced by the fraction of the polymer distribution with long chain lengths. We verify this by