September 19-22, 2023 • Boulder, Colorado
Session Chair:
Session Chair:
RadTech International North America | The Association for Ultraviolet and Electron Beam Technologies
6935 Wisconsin Ave, Suite 207, Chevy Chase, MD 20815 | (240) 497-1242 | uveb@radtech.org
Allan Guymon, University of Iowa
Microgel formation during early stages of photopolymerization leads to heterogeneous polymer networks which significantly limits material properties. In this study, we use controlled radical photopolymerization, including reversible addition-fragmentation chain transfer (RAFT) agents and nitroxide-mediated photopolymerization (NMP2) initiators, in cross-linked resins to mediate the polymerization process and network formation. RAFT agent addition has shown a significant impact on polymerization behavior and ultimate polymer properties in model systems. When comparing to conventional free radical polymerization, reaction rate is slightly decreased with RAFT containing formulations due to the formation of RAFT-adduct radicals during chain propagation that enable radical deactivation and reinitiation. RAFT-modified films exhibit narrower half width of tan delta peaks, indicating more homogeneous networks. In addition, RAFT structure-based network adaption is also investigated by comparing to networks polymerized from thiol chain transfer agent. Although incorporating thiol increases strain to break values and overall toughness, the RAFT containing polymer networks enable further network rearrangement through the adaptive nature of bond formation and breaking, leading to 2-fold increase in toughness. Additionally, we have designed and synthesized a series of NMP2 initiators with different chromophores to tune light absorbance characteristics and material properties. In linear polymeric systems, incorporating NMP2 initiators leads to controlled molecular weight evolution and low polydispersity. When incorporating specific NMP2 into cross-linked resins, similar polymerization kinetics are achieved to those using conventional photoinitiators. However, gelation is delayed, and significantly decreased shrinkage stress is observed. NMP2 polymerized cross-linked polymer films also exhibit enhanced flexibility and increased toughness.
Jason Killgore, NIST
Coming Soon.
Takeo Suga, Waseda University
UV-curing process based on photo-induced radical polymerization has been widely utilized for inks, paints, adhesives, and photo-resist materials. Especially, facile tuning of UV intensity, wavelength, irradiation area etc, has enabled their industrial applications, however the precise control of photo-polymerization is not trivial due to their too rapid reaction time (within seconds). Delicate balance of reaction kinetics, deformation (shrinkage), and phase-separation associated to polymer network formation needs to be considered. Reversible deactivation radical polymerization (RDRP) techniques such as atom-transfer controlled radical polymerization (ATRP) have greatly impacted the advancement of polymer synthesis in the last 20 years, allowing well-defined polymers with precise molecular weight distribution and segment blocks. Recently, further temporal (on/off) control of polymerization via external stimuli such as photo-excitation has proposed and gained increased attention. Here we focus on organo-catalyzed iodine-transfer controlled radical polymerization, which allows reversible photo-activation of polymeric dormant without metal catalysts. In this study, we synthesized polymeric dormant with C-I endgroup and utilized to the UV-curing process to challenge the precise control of photo-polymerization and phase-segregation simultaneously. The obtained coatings were optically clear, but internal nanostructure of the coating exhibited unprecedented, bicontinuous nanodomains with gradient size distribution. The nanodomains were evolved via polymerization-induced microphase separation (PIMS), and kinetically trapped by crosslinking. The domain size was tunable with catalysts (10-30 nm for PTH and 50-80 nm for PPh3), UV intensity, crosslinker content, and other processing aids. In addition to iodine endgroup, we also compared the utility of bromine or RAFT endgroups for precise UV-curing. Post-functionalization of the evolved nanostructures will be also discussed.
Christopher Bowman, University of Colorado
Comin Soon.
Thomas Griesser, Leoben
Orthogonal photoreactions have gained increased attention in polymer chemistry over the last decade.1 The selective activation of chromophores with light of different wavelengths enables the spatial and temporal control of the material properties in lithography based additive manufacturing.2,3,4 In this contribution, several photo crosslinkable groups were investigated and introduced in thiol-ene reactive systems. The sequence-dependent λ-orthogonality of network formation and crosslinking reaction allows for the spatially resolved alteration of thermo-mechanical properties during or after 3d printing. Furthermore, the selective activation of bond exchange reactions in photopolymers by sequence-dependent λ-orthogonality was studied.5 Therefore, a temperature-resistant (> 200 °C) organic phosphate with a photolabile o-nitrobenzyl protecting group was introduced into a visible light-curable thiol-ene resin. Once cured, irradiation at 372 nm spatiotemporally activates the phosphate, which catalyzes transesterifications at elevated temperature. The formed catalyst has no effect on the thermal stability of the polymeric network and allows the activation of bond exchange reactions in selected domains of printed 3D objects. The use of sequence-dependent λ-orthogonality easily enhances the versatility of lithography-based additive manufacturing and is an important step towards establishing this technology in our daily lives.
Christopher Kloxin, University of Delaware
Interpenetrating polymer networks (IPNs) consist of two, or more, polymer networks that are chemically distinct but physically entangled. Simultaneous IPNs are formed using two orthogonal polymerization chemistries to create distinct networks in the same volume; in contrast, sequential IPNs are formed in a two-step process, where the first network scaffold is formed followed by the polymerization of a second network-forming monomer. During the formation of a simultaneous IPN, the network chains tend to undergo some extent of phase separation before being topologically constrained by the formation of network crosslinks. Thus, one may anticipate that higher rates of polymerization will lead to less phase separation (i.e., more network interpenetration) and conclude that the kinetics of the two network forming monomers will have a significant effect on IPN properties.
In this talk, we will explore the property development of simultaneous IPNs formed using the photoinitiation of the copper-catalyzed azide–alkyne cycloaddition (CuAAC) and radical-mediated methacrylate polymerizations in a single pot. Photoinitiation intensity was observed to affect the evolution of network formation, with the methacrylate-based network forming first at higher intensities and the CuAAC-based network forming first at lower intensities. We found that differences in the IPN evolution led to differences in local phase separation behavior, evidenced by dynamic mechanical analysis and atomic force microscopy. Surprisingly, at higher light intensity (and, therefore, at greater polymerization rates), a greater degree of phase separation was observed. We attribute this result to molecular weight evolution differences between step and chain polymerization mechanisms in the CuAAC and methacrylate network forming monomers, respectively. These studies offer new insights into how IPN kinetics can influence phase separation and property formation and how the inherent simplicity of using photocontrol over property selection can lead to new materials with distinct thermomechanical properties and morphologies from the same resin.
Xavier Allonas, University de Haute Alsace
Photopolymerization reaction is developed and studied for a long time and has led to important industrial achievements. Although that many photoinitiating systems have been proposed over the years and that the photopolymerization mechanism has been understood since a long time, one can quite often observe a real difficulty to link the primary processes occurring in the excited states and the actual properties of the materials created. This paper aims to give some examples on how the photochemical processes control the properties of the photopolymers.
Christopher Barner-Kowollik, Queensland University of Technology
Over the last decade, our laboratory has employed monochromatic tunable laser systems to reveal a fundamental mismatch between the absorptivity of a chromophore and its photochemical reactivity in the vast majority of covalent bond forming reaction as well as specific bond cleavage reactions. Our data overturns the long-held paradigm that effective photochemical reactions are obtained in situations where there is strong overlap between the absorption spectrum and the emission wavelength. However – as we explore herein – the absorption spectrum of a molecule provides only information about singlet excitation and remains entirely silent on the accessibility of the critical triplet states, which dictate photochemical reactivity. We propose future avenues of enquiry on how action plots can be understood and explore how they are of key importance for tailoring photochemical applications in soft matter materials design with never before seen precision.
Zach Page, The University of Texas at Austin
Light as an energy source to create polymers has enabled transformative technologies in imaging, lithography, adhesives, and 3D printing. Its broad utility arises from the unparalleled spatiotemporal control over chemical transformations that it offers. However, contemporary methods rely on high energy UV light (< 400 nm), which limits material compatibility due to degradation and attenuation that occurs upon absorption and/or scattering. Excitingly, the recent commercialization of inexpensive light emitting diodes has opened up an avenue to examine mild visible-to-near infrared induced reactions in the preparation of polymers. This presentaton will focus on how the ZAP Research Group has contributed to the development of rapid photopolymerization chemistry from a fundamnetal perspective. Specifically, design principles in visible light photo-redox catalysis, -uncaging, and -initiation will be described. The ultimate objectives include driving photopolymerzations on projection-based 3D printing relevant timescales (<60 seconds to cure) and light intensities LEDs (<50 mW/cm2), while maintaining good resin shelf-stability under ambient conditions in the absence of light.
Jun Nie, Beijing University of Chemical Technology
Ultrafast curing efficiency of free radical photopolymerization endows photocurable materials with drawbacks like low storage stability and high volume shrinkage. To overcome this issue, a new concept of “Photo-isomer induced free radical polymerization” was proposed and it focuses on achieving photopolymerization with controllable curing efficiency based on chalcone photoinitiator, of which initiating efficiency could be adjusted by their thermal/photo isomerization. This research was divided into three progresses flowing the representative chalcone photoinitiator designed: (1) (2E,6E)-2,6-bis(furan-2-ylmethylidene) cyclohexan-1-one(E,E-BFC), of which initiating efficiency was enhanced via additives triethanolamine(TEOA); (2) (2E,6Z)-2,6-bis((1H-pyrrol-2-yl)methylene)cyclohexan-1-one(E,Z-BPC)/TEOA realizes thermally activated photopolymerization while it was seriously interfered by thermal polymerization; (3) (Z)-3-((1H-pyrrol-2-yl)methylene)chroman-4-one(Z-PCO)/TEOA achieved photopolymerization with controllable curing efficiency utilizing 465nm light with high intensity to transform Z→E isomerization. In general, “Photo-isomer induced free radical polymerization” leads to soft curing efficiency of photocurable materials making their transportation and application convenient.