Modeling polymerization-induced self-stratifying coatings

Jeong, Hyeonmin

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https://hdl.handle.net/2142/127473 Copy

Description

Title

Modeling polymerization-induced self-stratifying coatings

Author(s)

Jeong, Hyeonmin

Issue Date

2024-11-26

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

Sing, Charles E

Doctoral Committee Chair(s)

Sing, Charles E

Committee Member(s)

• Peters, Baron G
• Guironnet, Damien S
• Statt, Antonia

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)

• Polymerization-induced phase separation
• polymerization-induced self-stratification
• polymer phase separation dynamics
• continuum modeling
• Cahn-Hilliard formalism

Abstract

Self-stratifying coatings represent a cutting-edge advancement in materials science, offering a unique solution for creating multi-layered structures in a single application. These coatings have broad and varied applications across industries such as automotive, aerospace, marine, and construction, where multi-functional surfaces are essential. They address key challenges in coating technology, including reducing processing time, minimizing waste, and lowering costs. However, engineering these coatings by leveraging principles of polymer chemistry and phase separation is challenging, as it depends on the natural separation of different components within the coating during curing. Achieving the right balance of materials with varying surface energies, viscosities, and densities is particularly difficult, as these properties must be precisely tuned for effective stratification. This challenge motivates us to explore the various factors influencing phase separation and the self-stratification process, aiming to identify the optimal conditions and pathways for designing advanced coatings with tailored properties. The dynamics of polymerization-induced phase separation (PIPS) for polymer blends is important in determining the final morphology and properties of polymer materials. The phase-separated structures formed through PIPS are of significant interest due to their potential to tailor material properties and complex morphology, including porosity, mechanical strength, and chemical functionality, making it highly relevant in various applications, from filtration systems and nano-structured materials to advanced coatings. To gain a deeper understanding of the fundamental processes underlying PIPS process in a mixture of two species independently undergoing linear step-growth polymerization, we develop a polymerizing Cahn-Hilliard (pCH) formalism as continuum modeling for a binary polymer melt. In our method, we explicitly model polydispersity in these systems to consider different molecular-weight components that will diffuse at different rates. We first show that this pCH model predicts results consistent with the Carothers predictions for step growth polymerization kinetics, the Flory-Huggins theory of polymer mixing, and the classical predictions of spinodal decomposition in symmetric polymer blends. The model is then used to characterize (i) the competition between phase separation dynamics and polymerization kinetics, and (ii) the effect of unequal reaction rates between species. For large incompatibility between the species (i.e. high $\chi$), our pCH model demonstrates that the strength for phase separation directly corresponds to the kinetics of phase separation. We find that increasing the reaction rate $\tilde{k}$, first induces faster phase separation but this trend reverses as we further increase $\tilde{k}$ due to the competition between molecular diffusion and polymerization. In this case, phase separation is delayed for faster polymerization rates due to the rapid accumulation of slow-moving, high molecular weight components. Surface-directed phase separation (SDPS) is a specialized phenomenon in materials science where phase separation processes are influenced or guided by the presence of a surface or interface. In practical terms, SDPS is a sophisticated process that plays a crucial role in the design and fabrication of advanced materials with precisely controlled micro-structures. In the framework of polymerization-induced phase separation, we investigate how polymer-substrate interactions impact the dynamics of phase separation in polymer blends, as these interactions are critical in determining the final morphology and properties of the materials. Specifically, a preferential surface can act as an additional driving force for phase separation, complementing the effects of polymerization. To explore this, we modify the existing polymerizing Cahn-Hilliard (pCH) method by adding a surface potential to model the phase separation behavior under linear step-growth polymerization with the presence of surface. Our approach explicitly accounts for polydispersity by considering different molecular-weight components, each with its own diffusion constant, while the surface potential preferentially influences only one species. We first show that the surface with potential induces faster phase separation of smaller molecules at early stages before the degree of polymerization becomes large enough to drive bulk phase separation. We then use this model to examine the degree of anisotropic ordering in the direction perpendicular to the surface over various polymerization rate $\tilde{k}$ and strength of the potential $\tilde{V}_0$. We find that at low $\tilde{k}$, smaller molecules have sufficient time to diffuse and accumulate at the potential surface, resulting in the richer production of heavier polymers at the surface without the need for the larger polymers to diffuse on their own toward the surface. Conversely, at high $\tilde{k}$, larger polymers first evenly accumulate throughout the system before undergoing phase separation; The concentration wave initiated from the potential surface then propagates into the bulk, resulting in anisotropic phase separation. The dynamics of depolymerization is increasingly recognized as crucial in the innovative field of polymer upcycling, where the goal is to transform waste polymers into valuable monomers or other useful chemicals. This process not only extends the lifecycle of plastic materials but also significantly contributes to the advancement of a circular economy. Depolymerization—the breaking down of polymers into their constituent monomers—stands as a fundamental mechanism, offering the potential to recover raw materials of high purity. These materials can then be repurposed in the production of new polymers or other products, thereby closing the loop on plastic waste. In our $ongoing$ research, we have developed a depolymerizing Cahn-Hilliard model to explore the intricate competition between phase separation driven by polymer incompatibility and the mixing behavior induced by random scission depolymerization. We find a sequential phase behavior where phase separation is initially governed by polymer incompatibility, followed by a subsequent mixing behavior driven by depolymerization. Through this work, we aim to provide a deeper understanding of the underlying physics of depolymerization dynamics. Such insights are vital for the development of more effective and sustainable polymer upcycling processes, ultimately enhancing the efficiency and environmental impact of polymer recycling.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127473

Copyright and License Information

Copyright 2024 Hyeonmin Jeong

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