Theoretical Predictions of Temperature-Induced Gelation in Aqueous Dispersions Containing PEO-Grafted Particles

Abstract

In this work we utilize classical polymer density functional theory (DFT) to study gelation in systems containing colloidal particles onto which polymers are grafted. The solution conditions are such that the corresponding bulk system displays a lower critical solution temperature (LCST). We specifically compare our predictions with experimental results by Shay et al., who investigated temperature response in aqueous dispersions containing polystyrene particles (PS), with grafted 45-mer poly(ethylene oxide) (PEO) chains. Our DFT treatment is based on a model for aqueous PEO solutions that was originally developed by Karlström for bulk solutions. In this model, monomers are assumed to be in either of two classes of states, labelled A and B, where B is more solvophobic than A. On the other hand, the degeneracy of B exceeds that of A, causing the population of solvophobic monomers to increase with temperature. In agreement with experimental findings by Shay et al., we locate gelation at temperatures considerably below T Θ , and far below the LCST for such chain lengths. This gelation occurs * To whom correspondence should be addressed 1 also without any dispersion interactions between the PS particles. Interestingly, the polymerinduced interaction free energy displays a non-monotonic dependence on the grafting density.At high grafting densities, bridging attractions between grafted layers take place (considerably below T Θ ). At low grafting densities, on the other hand, the polymers are able to bridge across to the other particle surface. Shay et al. conducted their experiments at very low ionic strength, using de-ionized water as a solvent. We demonstrate that even minute amounts of adsorbed charge on the surface of the particles, can lead to dramatic changes of the gelation temperature, especially at high grafting densities. Another interesting prediction is the existence of elongated (chain-like) equilibrium structures, at low particle concentrations. We emphasize that our model does not rely upon any temperature-dependent interactions.

From Attraction to Repulsion to Attraction: Non-monotonic Temperature Dependence of Polymer-Mediated Interactions in Colloidal Dispersions

“…Note that a simplifying aspect of these systems is that the presence of a grafted PEG layer will allow us to neglect Hamaker interactions between the PS particles. 41 Our findings from CLSM studies, combined with diffusion analyses, were collected at various particle and polymer concentrations, and the results are summarized in Figure 2. The temperature was kept constant at 20 °C.…”

Interactions between Sterically Stabilized Nanoparticles: The Effects of Brush Bidispersity and Chain Stiffness

“…As such, these systems have received significant experimental [ 5 , 6 ] and theoretical [ 7 , 8 ] attention. One way of controlling the properties of sterically stabilized systems is by tuning either the solvent quality [ 9 , 10 , 11 , 12 ] or the properties of the (ungrafted) polymer matrix [ 13 , 14 ]. However, this method does not apply to the technologically important solvent-free (matrix-free) nanocomposites consisting of inorganic cores grafted with polymeric brushes [ 15 ].…”

“…While MD simulations provide exact results (with inevitable statistical noise) for a given microscopic model, they can also be time-consuming for systems involving long polymer chains and high grafting densities [ 8 ]. An appealing alternative is provided by mean-field techniques, such as self-consistent field theory [ 10 , 29 , 30 ], integral equation theory [ 31 , 32 , 33 , 34 ], and density functional theory (DFT) [ 9 , 12 , 35 ]. The latter method has been already applied to study solvent-free polymer brushes [ 36 ].…”

Linear and Ring Polymer Brushes: A Density Functional Theory Study

“…[4,5] As such, these systems have received significant experimental [4][5][6][7][8][9] and theoretical [10][11][12][13][14][15][16][17][18][19] attention. These extensive studies have shown that thermodynamic and microstructural properties of sterically stabilized systems can be controlled either by tuning the solvent quality [20][21][22][23][24] or by adjusting the properties of the (ungrafted) polymer matrix. [25][26][27] Yet another useful control variable involves the grafted polymer topology.…”