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

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.

Depletion-Driven Assembly of Polymer-Coated Nanocrystals

“…This materials platform is amenable to (1) employing NC building blocks with different optical, magnetic, or catalytic properties, (2) adding new functionalities to NCs at their surfaces via copolymers to introduce additional, complementary properties in the assemblies, and (3) developing new methods to dynamically tune NC–NC attractions. In addition to changing surface polymer graft length, density, and composition, changes in solvent quality and consequently Flory–Huggins coefficients for the surface polymer as well as the depletant polymer may be used to fine-tune the resulting NC morphology. , Polymers at the surface of NCs may be designed to mediate new properties in NC assemblies, and the choice of depletion as the source of attractions enables the exchange of these surface polymers independently of the assembly methodology. Importantly, understanding which individual contributions are critical to the formation of the resulting morphology on the nanoscale establishes the foundation to design assemblies with targeted, structure-dependent properties.…”

Depletion-driven assembly of polymer-coated nanocrystals

“…In addition to changing surface polymer graft length, density, and composition, changes in solvent quality and consequently Flory Huggins coefficients for the surface polymer as well as the depletant polymer may be used to fine tune the resulting NC morphology. 120,121 Polymers at the surface of NCs maybe be designed to mediate new properties in NC assemblies, and the choice of depletion as the source of attractions enables the exchange of these surface polymers independently of the assembly methodology.…”

Assembling Inorganic Nanocrystal Gels

“…The strength of the gel and its corresponding properties may be tuned via the concentration of bridging molecules within a range before re-entrant fluid behavior or precipitation occurs. Bridging gels formed with polymer bridges may offer additional tunability due to polymer conformational dependence on temperature (or other stimuli), which can modulate phase behavior and enable switchable, structure-dependent properties …”

“…Bridging gels formed with polymer bridges may offer additional tunability due to polymer conformational dependence on temperature (or other stimuli), which can modulate phase behavior and enable switchable, structure-dependent properties. 43…”