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Construction of a Universal Gel Model with Volume Phase Transition
1Department of Chemistry and Chemical Biology, Rutgers University, 123 Bevier Road, Piscataway, NJ 08854-8087, USA.
Gels (Basel, Switzerland)
|March 4, 2020
Summary
This study reveals that gel volume phase transitions share similarities with fluid condensation, introducing a universal chemical potential that incorporates network elasticity, leading to novel isotherms and density profiles not predicted by van der Waals theory.
Area of Science:
- Soft Matter Physics
- Polymer Science
- Physical Chemistry
Background:
- The condensation of vapor to liquid is driven by competing energetic attraction and entropic dispersion forces.
- Van der Waals' equation of state explains this discontinuous transition by accounting for molecular attractions and finite volume.
- Gel volume phase transitions also involve competing polymer segment attraction and entropic dissolution, but network cross-links introduce solid-like properties.
Purpose of the Study:
- To investigate if the solid-like nature of gels (intrinsic volume and shape) adds fundamental physics to their volume phase transitions beyond van der Waals theory.
- To construct a universal chemical potential for gels that captures the volume transition universally.
- To explore new thermodynamic descriptions for gel behavior, particularly the role of network elasticity.
Main Methods:
- Developed a universal chemical potential for gels, analogous to van der Waals' universal equation of state for fluids.
- Incorporated a network elasticity component based on a nonlinear Langevin model, restricting the swollen gel's radius to a universal value.
- Analyzed the resulting isotherms and density profiles as a function of gel location and temperature.
Main Results:
- A new family of isotherms and gel density profiles emerged, distinct from those predicted by a purely van der Waals analysis.
- Observed an abrupt onset of large-amplitude density fluctuations at a critical temperature.
- Identified a second critical temperature at which the entire swollen gel collapses to a high-density phase.
Conclusions:
- The solid-like elasticity of gels introduces fundamental physics to their volume phase transitions, leading to phenomena not captured by classical fluid condensation theories.
- The constructed universal chemical potential and nonlinear Langevin model provide a novel framework for understanding gel behavior.
- The study reveals distinct critical phenomena in gels, including density fluctuations and a sharp collapse transition.
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