Directed network structure through controlled radical photopolymerization
Creators: name: Fang, Huayang
Publisher: name: University of Iowa
Year: 2022
Subjects: Controlled radicals polymerization
Nitroxide-mediated photopolymerization
Photopolymerization
Polymer network rearrangement
RAFT polymerization
public abstract

description: 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.
description: 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.
description: 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.
description: 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.
description: 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.
description: 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.