Electron paramagnetic resonance measurement of trapped radical concentrations in frontally polymerized and bulk‐polymerized multifunctional (meth)acrylates

Thoma, Anna; Dahal, Ashutosh; Regmi, Aayush; Valencia, Alecia; Viner, Veronika; Cueto, Rafael; Baker, Blane; Bunton, Patrick

doi:10.1002/pola.24869

Cited by 3 publications

(2 citation statements)

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“…Crosslinking of polymer chains causes this effect to occur at lower conversions, which means that monomers with higher functionality will autoaccelerate at lower conversion. Thoma et al measured by EPR the concentration of the trapped radicals as high as 8.7 × 10 −3 mol kg −1 . This explains the large difference in front velocity from monofunctional acrylate, in which there is no crosslinking, to diacrylate, in which there is extensive crosslinking as well as the smaller difference between difunctional and trifunctional acrylates.…”

Section: Resultsmentioning

confidence: 99%

The effect of acrylate functionality on frontal polymerization velocity and temperature

Bynum

Tullier

Morejon‐Garcia

et al. 2019

J. Polym. Sci. Part A: Polym. Chem.

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Frontal polymerization is a method of converting monomer(s) to polymer via a localized reaction zone that propagates from the coupling of thermal diffusion with the Arrhenius kinetics of an exothermic reaction. Several factors affect front velocity and temperature with the role of monomer functionality being of particular interest in this study. Polymerizing a di and triacrylate of equal molecular weight per acrylate revealed that as the proportion of triacrylate was increased the velocity and temperature increased. This is attributed to increased crosslinking and autoacceleration. Comparing several different acrylate monomers, both neat and diluted with dimethyl sulfoxide (DMSO) so as to maintain constant acrylate group concentration, shows that velocity increases with increased functionality from mono to difunctional monomers. This trend breaks when applied to tri-and tetraacrylates, with fronts containing trifunctional monomer being the fastest. Acrylates containing hydroxyl functionality, as in the case of pentaerythritolbased triacrylates, are slower than acrylates without. This is attributed to a chain-transfer event and was tested using octanol and a hydroxyl-free acrylate. It has also been shown that small amounts of water cause a lowering of front velocity due to energy lost via vaporization, which lowers the front temperature.We are particularly interested in the effect of functionality on the front velocity, because the front velocity determines how Additional supporting information may be found in the online version of this article.

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Section: Resultsmentioning

confidence: 99%

The effect of acrylate functionality on frontal polymerization velocity and temperature

Bynum

Tullier

Morejon‐Garcia

et al. 2019

J. Polym. Sci. Part A: Polym. Chem.

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“…Evidence of a well-cured surface of a photopolymer does not guarantee sufficient or homogeneous through-cure, which is also challenging to verify. Although trapped radicals can persist in high-conversion, vitrified regions within structurally heterogeneous photopolymer networks, − the low mobility necessary for such a radical persistence also restricts free monomer migration from adjacent domains of lower conversion, thus limiting significant postcuring potential . More critically, any low-conversion regions present at the end of the irradiation interval will persist because of very efficient biradical termination that effectively arrests postcuring.…”

Section: Introductionmentioning

confidence: 99%

High-Efficiency Radical Photopolymerization Enhanced by Autonomous Dark Cure

et al. 2020

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Radical photopolymerization (RPP) has grown into a multibillion-dollar technology for reduced energy consumption and waste with increased productivity. However, radical-mediated polymerization ceases almost immediately following discontinuation of irradiation because of rapid termination of reactive centers. This restricts the wider use of RPP in applications that involve light attenuation or irregular surfaces because uniform polymerization is not guaranteed for these challenging exposure conditions and the resultant undercuring leads to compromised material properties and harmful leachable monomers. Herein, we developed a unique radical dark-curing photoinitiator (DCPI) that continues its polymerization beyond the cessation of irradiation. The DCPI achieved a remarkable 25–60% additional conversion over a 1 h period, when light was shuttered at 20% conversion, compared to the 1–3% additional conversion achieved by a Norrish type II control photoinitiator. We elucidated the origin of the high photon efficiency using computational studies and experiments, which suggest that the DCPI may be the most-photon-efficient photoinitiator to date. We also demonstrated that the mechanical properties of dark-cured polymer are similar to and even exceed those of the corresponding polymer obtained by extended photocuring. In particular, the initial 0.1 MPa storage modulus continuously developed to 4.3 MPa without further irradiation, while also exhibiting ∼30% less shrinkage stress than the full exposure control. With its superior photo- and dark-polymerization efficiency, the DCPI enhances the performance of many existing RPP processes while extending RPP to heretofore unattainable applications. The DCPI accomplishes such a performance through an inherent automatic rectification of initially undercured regions and defies the RPP paradigm that light dosage directly correlates to conversion.

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