Non-monotonic temperature response of polymer mediated interactions

Area of Science:

• Soft Matter Physics
• Colloid Science
• Polymer Physics

Background:

• Re-entrant solidification in colloidal systems with polyethylene oxide (PEO) shows transitions from crystalline to fluid to flocculated phases with increasing temperature.
• Previous models treated polymers as hard spheres with temperature-dependent potentials, but a more detailed approach is needed.
• The observed behavior involves polymer depletion at low temperatures and bridging attractions at high temperatures, with repulsion in between.

Purpose of the Study:

• To develop a detailed polymer density functional theory for aqueous polyethylene oxide (PEO) solutions to explain re-entrant solidification.
• To investigate the role of polymer-colloid interactions in driving colloidal phase transitions.
• To demonstrate that polymer-mediated interactions, rather than temperature-dependent potentials, can explain the observed re-entrant behavior.

Main Methods:

• Developed a polymer density functional treatment based on Karlström's model for aqueous PEO solutions.
• Modeled monomers in two states (A and B) with differing solvophobicity and degeneracy, influencing their population with temperature.
• Accounted for all polymer configurations using a mean-field Boltzmann weight.

Main Results:

• The model successfully reproduces the qualitative temperature response observed by Feng et al., including depletion interactions, polymer bridging, and intermediate repulsive interactions.
• The homogeneous fluid phase is predicted to form due to polymer-mediated repulsion, even without electrostatic interactions.
• Colloid interactions mediated by neutral polymers show a trend of attraction-repulsion-attraction based on surface affinity.

Conclusions:

• A detailed polymer density functional approach can explain re-entrant solidification in colloidal systems driven by polymer-colloid interactions.
• The model highlights the importance of polymer state (solvophobicity and degeneracy) in determining colloidal phase behavior.
• Repulsive interactions mediated by polymers play a crucial role in the formation of the fluid phase.