Structural relaxation and penetrant diffusion in polymer networks

Lin, Tsai-Wei

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https://hdl.handle.net/2142/125569 Copy

Description

Title

Structural relaxation and penetrant diffusion in polymer networks

Author(s)

Lin, Tsai-Wei

Issue Date

2024-07-09

Director of Research (if dissertation) or Advisor (if thesis)

Sing, Charles E

Doctoral Committee Chair(s)

Sing, Charles E

Committee Member(s)

• Schweizer, Kenneth S
• Evans, Christopher M
• Kuenstler, Alexa S

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Penetrant diffusion
• Crosslinked polymer networks
• Glass transition
• Alpha relaxation time
• Molecular dynamics
• Membrane separation

Abstract

Controlling the permeation of atomic, molecular, and nanoparticle “penetrants” through dense polymeric media (liquid, glass, crosslinked permanent or dynamic rubber network, thermoset) is a crucial scientific problem broadly relevant in materials, biological chemistry, and energy sciences. In particular, understanding how the many physicochemical factors (e.g., size, shape, and chemistry) and thermodynamic state control penetrant activated mass transport is critical for membrane separations, which serve as promising alternatives to distillation or extraction for chemical separations. A remaining challenge for membrane design is the trade-off between permeability and selectivity described by an empirical upper bound of membrane performance, which the scientific community has been thriving to shift higher through structure/property optimization of membranes. Therefore, we are motivated to explore a new paradigm of using highly-crosslinked networks near glass transition temperature Tg for the selective transport of small molecules. Polymer networks hold promise in membrane separation because dramatic differences in molecular diffusion can result from subtle changes in the network structure. We constructed a coarse-grained molecular dynamics simulation model that reflects the specific polymer chemistry of the network studied, along with experiment and theory done by collaborators, to understand the role that crosslinking has in governing the structural relaxation and the transport of penetrants in polymer networks. We observed a very large increase in segmental relaxation time and glass transition temperature Tg as a result of tight crosslinking and provided a mechanistic microscopic understanding of the observations by showing that structural relaxation involves a coupled local cage and nonlocal collective physics, with the latter becoming more dominant upon cooling. Our investigation of penetrant diffusion reveals that permanent crosslinking can regulate penetrant transport through (1) the near-Tg coupling between penetrant hopping and the structural relaxation of polymer networks (2) the confining mesh that obstructs penetrant motion. We found that the main effect of permanent crosslinking is to slow down polymer structural relaxation and greatly suppress the elementary penetrant hopping event, while signatures of mesh confinement, though of secondary importance, are observed at certain conditions depending on penetrant size and thermodynamic state. The good agreement between experiment, simulation, and theory demonstrated size ratio (penetrant diameter to Kuhn length) as a key variable determining penetrant diffusivity, though the role of chemistry-specific effects (e.g, shape, penetrant-polymer interaction) are non-negligible, as shown in simulation and theory. Finally, we study the bond exchange dynamics and structural relaxation in associative dynamics covalent network (i.e., vitrimers) over wide ranges of crosslink densities fcross, temperatures T, and bond exchange rate. Simulations unravel the intricate interaction between these two dynamic processes and reveal that only when the bond exchange time scale is comparable to Kuhn segmental alpha time will the dynamic crosslinking show noticeable acceleration of segmental relaxation and change in Tg. Overall, we have developed models to connect experimental findings and theoretical predictions for specific polymer networks. These models elucidate the effect of crosslinking on structural relaxation in both permanent and dynamic polymer networks. By combining simulation, experiment, and theory, we identify the mechanisms of penetrant diffusion in crosslinked networks. These insights can not only guide the engineering of polymer membranes for separation but also expand our understanding of the dynamics of polymers and penetrants in crosslinked networks.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125569

Copyright and License Information

Copyright 2024 Tsai-Wei Lin

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