Dissertation
Directed network structure through controlled radical photopolymerization
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2022
DOI: 10.25820/etd.006832
Abstract
Photopolymerization is regarded as an environmentally friendly technique that combines a solvent-free system with fast reaction rates that can occur at ambient temperatures with low energy costs. Because of these inherent advantages, photopolymerization has experienced rapid growth in a wide variety of applications. However, early-stage microgel formation during polymerization is often seen during photopolymerization. These microgels result in heterogeneous polymer networks which significantly limit material properties. Mechanical properties of polymer materials are largely dependent on the network structure as inhomogeneous cross-linked networks are formed. In this study, we explore the usage of controlled radical photopolymerization, including reversible addition-fragmentation chain transfer (RAFT) agents and nitroxide-mediated photopolymerization (NMP2) initiators, in cross-linked formulations to mediate the polymerization process and network formation.
Initially, we utilize the RAFT agent as a reactive additive to polymerize with photocurable cross-linked formulations. RAFT agent addition has a significant impact on polymerization behavior and ultimate polymer properties due to the reversible degenerative process of formed RAFT-intermediate radicals. The mediation of formed RAFT-intermediate also leads to a slow polymerization rate in RAFT-containing formulations.
In order to increase polymerization rate while maintaining the increased thermomechanical properties, the effect of RAFT agents with different R-groups on reaction rate and thermomechanical properties are investigated. Altering the R-group of the RAFT agent modifies fragmentation rate and chain reinitiation, allowing a tunable overall reaction rate. Two trithiocarbonate RAFT agents which share the same Z-group but different R-groups, i.e., cyanomethyl dodecyl trithiocarbonate (CDT) and 2-(dodecylthiocarbonothioylthio) propionic acid (DPA), are incorporated and compared in model systems. Faster photopolymerization rates were observed in DPA-modified polymerization as its attached R-group facilitates rapid fragmentation and chain reinitiation. Polymer network formation and corresponding mechanical properties are changed significantly by incorporating different concentrations of CDT and/or DPA RAFT agents, especially for systems that have medium and high chain mobilities, DPA mediated polymerizations not only show fast reaction rate but also greater modifications to thermomechanical properties.
Additionally, combining the advantages of nitroxide-mediated polymerization with the photoactivity of alkoxyamine compounds, three alkoxyamine-based photoinitiators enabling a reversible termination mechanism for nitroxide-mediated photopolymerization (NMP2) were designed and synthesized. Altering chromophore compounds attached to NMP2 initiators changes the light absorbance properties and stability of dissociated nitroxide radicals. Under UV light exposure, NMP2 initiators photolyze to carbon-centered propagating radicals and nitroxide-based persistent radicals. When applying an NMP2 initiator to initiate monofunctional monomer (hexyl acrylate), a significantly increased molecular weight with narrow polydispersity is observed, In addition, a linear relationship with conversion of molecular weight is also shown, indicating a controlled polymerization through NMP2.
When incorporating NMP2 initiators into cross-linked epoxy acrylate resins, a similar monomer conversion is achieved to the conventional FRP. However, NMP2 initiation has a significant impact on polymerization behavior and corresponding mechanical properties. When compared to conventional photoinitiated systems, NMP2-initiated formulations show a reduced polymerization rate with delayed gelation. These slower polymerization rates provide controlled chain propagation that delays gelation and decreases shrinkage stress in polymer networks, contributing to significantly increased flexibility, elongation, and enhanced toughness in final polymer materials.
By investigating the fundamental effects of NMP2 initiators and RAFT agents on crosslinked model systems, we aim to increase the uniformity of photopolymer networks and mitigate shrinkage stress to produce photopolymer materials with enhanced toughness. This study also expands the applications of controlled radical polymerization techniques to many other applications, including thin-film coatings, medical devices, and additive manufacturing.
Details
- Title: Subtitle
- Directed network structure through controlled radical photopolymerization
- Creators
- Huayang Fang
- Contributors
- C Allan Guymon (Advisor)David G Rethwisch (Committee Member)Beth Rundlett (Committee Member)Syed Mubeen (Committee Member)Jon Scholte (Committee Member)Joe Gomes (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemical and Biochemical Engineering
- Date degree season
- Summer 2022
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.006832
- Number of pages
- xxi, 189 pages
- Copyright
- Copyright 2022 Huayang Fang
- Grant note
- Financial support for this research from the Industry/University Cooperative Research Center (IUCRC) for Fundamentals and Applications of Photopolymerizations at the University of Iowa and the University of Colorado
- Language
- English
- Description illustrations
- illustrations
- Description bibliographic
- Includes bibliographical references.
- Public Abstract (ETD)
Photopolymerization is a light-induced polymerization process that uses high-energy light to link monomers/oligomers together to become polymers. It is an environmentally friendly reaction with low energy consumption, higher productivity, and reduced waste. Due to these advantages, photopolymerization has been widely used to create common polymers such as dental restorative materials, photoresists, and medical filters and masks. Although the rapidity of photopolymerization is beneficial, it may result in inevitable defects within polymer networks, limiting the properties and applications of photopolymers. In this research, by adding regulating agents, we are able to control polymer network formation to produce tougher photopolymers. By controlling the concentration of regulating agent, we are able to manipulate the physical and mechanical properties of final polymer materials. Changing structure of the regulating agent also shows great promise in producing photopolymers with greater stiffness and elasticity while maintaining faster reaction rates. Our results show more homogeneous networks can be produced with fewer defects, leading to significantly increased physical and mechanical properties. In summary, this work aims to apply the regulating agents to control photopolymerization behavior and network formation, thereby creating tougher photopolymer materials.
- Academic Unit
- Chemical and Biochemical Engineering
- Record Identifier
- 9984285050302771
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