Investigating the effect of dynamic covalent bonds on the crystallization, rheology and molecular relaxation in ethylene vitrimers

Soman, Bhaskar

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https://hdl.handle.net/2142/117819 Copy

Description

Title

Investigating the effect of dynamic covalent bonds on the crystallization, rheology and molecular relaxation in ethylene vitrimers

Author(s)

Soman, Bhaskar

Issue Date

2022-12-02

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

Schweizer, Kenneth S

Doctoral Committee Chair(s)

Evans, Christopher M

Committee Member(s)

• Leal, Cecilia
• Sing, Charles E

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Dynamic bonds
• Crystallization
• Rheology
• Dielectric spectroscopy

Abstract

Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. Vitrimers are a class of polymer networks with properties intermediate to thermosets and thermoplastics. Their properties are a consequence of the dynamic crosslinks which undergo reversible exchange reactions and allow for local rearrangements in network structure resulting in stress relaxation and flow. This thesis aims to investigate properties that will be critical to the design, processing, and use of vitrimers as sustainable commodity polymers and functional materials. A series of precise ethylene based vitrimers were synthesized and investigated for their (i) rheological response, (ii) dielectric relaxation behavior, (iii) crystallization and crystal evolution, and (iv) the effect of topological defects on shear modulus and crystallization kinetics. The ethylene vitrimers are also shown to be easy to recycle. Networks are dissolved in water and alcohol-based solvents and can be recovered by repolymerization while preserving their material properties. Over a broad temperature window, the viscosity of the vitrimer shows a positive deviation from its characteristic Arrhenius trend, indicating an increasing role of segmental dynamics on the flow. From dielectric spectroscopy, three distinct relaxation modes are identified - the alpha process, the beta process, and a normal mode relaxation that is interpreted as the time taken for the exchange of network strands between crosslink junctions. At high temperatures, a strong correlation between the rheological flow timescale and the normal mode relaxation shows that flow is controlled by bond exchange, while at lower temperatures an increase in alpha relaxation time correlates with the positive deviation from Arrhenius behavior in the networks. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role in crystallization has not been systematically explored. In this report, a comparison between permanent and dynamic networks with the same linker length, suggests that the incorporation of dynamic junctions allows for local rearrangement of the network structure which facilitates crystallization. The ethylene vitrimers also show an unexpected evolution in melting temperature, crystal structure, and morphology over time that is also attributed to the dynamic bond exchange. The incorporation of dangling chain defects into the ethylene vitrimers results in a decrease in shear modulus that can be modeled by the rubbery network theory. X-ray scattering patterns reveal that increasing defect concentration results in the formation of imperfect crystals. A counterintuitive trend in crystallization kinetics is observed in the defected networks and is speculated to be a consequence of an increase in the entropic barrier to crystallization coming from the dangling chains.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Bhaskar Soman

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