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Using molecular simulations to understand polymer entanglements and coacervate phase behavior

Author
Bobbili, Sai Vineeth
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2021.
Physical Description
1 electronic document
Additional Creators
Gómez Jiménez, Enrique
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Open Access.
Summary
Advancements in computer simulations have led to their application in understanding structure-property relations of polymer melts and solutions. Friction coefficient and entanglement length are two fundamental parameters in modern tube-based theories. In this work, we use molecular dynamics simulations to study the impact of chain architecture and orientation on these two properties. Multiple scaling arguments have been proposed to describe how the entanglement molecular weight depends on polymer architecture and concentration. Such scaling arguments are well supported either by experiments or through simulation data. Each of these arguments makes certain assumptions, which limits their range of validity. Here, we use simulations to explore a wide range of entangled bead-spring ring chains, to find out how entanglement properties vary with chain stiffness and concentration. We quantify entanglement using three techniques: chain shrinking to find the primitive path, measuring the tube diameter by the width of the "cloud" of monomer positions about the primitive path, and directly measuring the plateau modulus. As chain stiffness varies, we observe three distinct scaling regimes, suggestive of the Lin-Noolandi scaling, semiflexible chains, and stiff chains. The packing length p figures prominently in scaling predictions of the entanglement length and bulk modulus for polymer melts and solutions. p has been argued to scale as the ratio of chain displaced volume V and mean square end-to-end distance R^2. This scaling works for several cases; however, it is not obvious how to apply it to chains with side groups, and the scaling must fail for sufficiently thin, stiff chains. In this work, we measure the packing length in simulations, without making any scaling assumptions, as the typical distance of closest approach of two polymer strands in a simulated bead-spring melt using inter-molecular radial distribution functions. While predicting entanglement length has been the focus of several scaling arguments and simulation studies, understanding the behavior of friction coefficient has received less attention. The monomer friction coefficient [zeta] is known to vary with monomer structure, solvent, and concentration; its variation with chain conformation is less well known and appreciated. We explore the decrease of friction coefficient in extensional flow of polymer liquids, during which chains become partially stretched and aligned. In the second half of this dissertation, we use molecular dynamics simulations to investigate the phase behavior of polyelectrolyte complex coacervates. When oppositely charged polyelectrolytes mix in an aqueous solution, associative phase separation gives rise to coacervates. Experiments reveal the phase diagram for such coacervates, and determine the impact of charge density, chain length and added salt. We propose an idealized model and a simple simulation technique to investigate coacervate phase behavior and show the impact of added salt using a phase diagram. Most studies understanding the phase behavior of polyelectrolyte complex coacervates focus on symmetric mixtures of oppositely charged polymers. This is very rare in biological coacervates. Mixing ratio plays an important role in stability of complexes and applications of such coacervates. We use our idealized simulation model to study the impact of charge-asymmetry on the phase behavior of coacervates.
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Dissertation Note
Ph.D. Pennsylvania State University 2021.
Reproduction Note
Microfilm (positive). 1 reel ; 35 mm. (University Microfilms 29049948)
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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