Abstract
In this work, we have synthesized polystyrene particles that carry short end-grafted polyethylene glycol (PEG) chains. We then added dissolved 100 kDa PEG polymers and monitored potential flocculation by confocal microscopy. Qualitative predictions, based on previous theoretical developments in this field (Xie, F.; et al. Soft Matter 2016, 12, 658), suggest a non-monotonic temperature response. These theories propose that the "free" (dissolved) polymers will mediate attractive depletion interactions at room temperature, with a concomitant clustering/flocculation at a sufficiently high polymer concentration. At high temperatures, where the solvent is poorer, this is predicted to be replaced by attractive bridging interactions, again resulting in particle condensation. Interestingly enough, our theoretical framework, based on classical density functional theory, predicts an intermediate temperature regime where the polymer-mediated interactions are repulsive! This obviously implies a homogeneous dispersion in this regime. These qualitative predictions have been experimentally tested and confirmed in this work, where flocs of particles start to form at room temperature for a high enough polymer dosage. At temperatures near 45 °C, the flocs redisperse, and we obtain a homogeneous sample. However, samples at about 75 °C will again display clusters and eventually phase separation. Using results from these studies, we have been able to fine-tune parameters of our coarse-grained theoretical model, resulting in predictions of temperature-dependent stability that display semiquantitative accuracy. A crucial aspect is that under "intermediate" conditions, where the polymers neither adsorb nor desorb at the particle surfaces, the polymer-mediated equilibrium interaction is repulsive.
Highlights
Colloidal dispersions have been the subject of many studies, with focus ranging from stability and drug delivery, to selfassembly, patch formation, and so on.[1−5] The stability of such dispersions can often be controlled by pH and temperature variation, or with additives, such as salt or polymers.[6−10] One advantage in a system where polymers are used is their ability to mediate attractive, as well as repulsive, interactions between the particles, depending on parameters such as polymer length, solvent quality, or the presence of grafting bonds to the particles
This may facilitate a controlled stability.[11−16] Crystallization of colloidal particles has been investigated in many soft matter studies, and the coexistence between “gas” and ”liquid” phases can often be regulated by polymer properties.[17,18]
We reiterate that recent work[30] has demonstrated that in the intermediate regime, that is, for intermediately adsorbing surfaces, the polymer-mediated interaction is repulsive at equilibrium. These predictions were admittedly based on approximate density functional theory (DFT), but for the special case of ideal chains composed of bonded point-like monomers, DFT results are exact, and the predictions were shown to be valid for those model systems
Summary
Colloidal dispersions have been the subject of many studies, with focus ranging from stability and drug delivery, to selfassembly, patch formation, and so on.[1−5] The stability of such dispersions can often be controlled by pH and temperature variation, or with additives, such as salt or polymers.[6−10] One advantage in a system where polymers are used is their ability to mediate attractive, as well as repulsive, interactions between the particles, depending on parameters such as polymer length, solvent quality, or the presence of grafting bonds to the particles. The degeneracy of the B class exceeds that of A; that is, the population of solvophobic monomers increases with temperature This can lead to an LCST.[27−29] Adding solvophobic colloidal particles to such a solution generates a system that displays the same qualitative temperature response as was observed by Feng et al That is, at low temperatures, type A monomers predominate and one observes depletion interactions, whereas polymer bridging dominates at higher temperatures due to the attraction between type B monomers and the colloidal surface. The intermediate temperature regime is characterized by a polymer-mediated potential of mean force (PMF) between colloidal particles, which is repulsive This is a crucial and general theoretical observation, which was verified for systems for which one can obtain exact solutions (theta solvent conditions, with polymers modeled by connected point-like monomers). We believe that our suggested combination of experiments and modeling will guide us toward a better understanding of interactions and temperature dependence in polymer/particle dispersions
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