Phase Behavior in Model Homopolymer/CO2 and Surfactant/CO2 Systems: Discontinuous Molecular Dynamics Simulations.
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Date
2004-11-14
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Abstract
Discontinuous molecular dynamics simulation is used to explore phase behavior in homopolymer/solvent and surfactant/solvent systems. This research provides insight into how homopolymer and surfactant geometry and intermolecular forces affect performance in supercritical CO₂ (scCO₂).
We first simulate surfactant/scCO₂ systems and explore the effect of the surfactant volume fraction, packing fraction and temperature on the phase behavior. The phase diagram and the density dependence of the critical micelle concentration (CMC) are in qualitative agreement with experimental observations. Then we explore how the interaction parameters affect the types of phase behavior in homopolymer/scCO₂ systems. Increasing the packing fraction of homopolymers in both hard-sphere solvents and square-well solvents serves to increase the solvent's ability to dissolve homopolymers only when the segment-solvent interaction strength exceeds a critical value. For homopolymer/scCO₂ systems, we find that it is necessary to account for the solvent-solvent attraction to model lower critical solution temperature (LCST) behavior. Finally we explore how the interaction strengths affect phase behavior and micellization in surfactant/scCO₂ systems. We find that it is necessary to account for the interactions experienced by both the head and tail blocks in order to capture the essential features of real surfactant/scCO₂ systems. The CMC increases with increasing packing fraction only when the head-solvent interaction strength exceeds a critical value.
Next we explore the physical conditions and parameters that give rise to the appearance of LCST behavior in homopolymer/supercritical solvent systems. In hard-sphere solvents, homopolymers always exhibit upper critical solution temperature (UCST) behavior. In square-well solvents, LCST behavior is observed at high solvent-solvent interaction strengths but not at low ones, indicating that the strong solvent-solvent attractions contribute to the appearance of LCST behavior in homopolymer/solvent systems. Homopolymers exhibit LCST behavior at lower solvent-solvent interaction strength with increasing solvent-solvent interaction width, indicating that the solvent-solvent interaction range plays an important role in the appearance of LCST behavior. Finally we find that by increasing the chain length for solvent-philic homopolymers, one can switch from UCST to LCST behavior in square-well solvents.
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phase behavior, polymer, supercritical CO<sub>2</sub>, surfactant, computer simulation
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Degree
PhD
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Chemical Engineering
