Electron Paramagnetic Resonance Studies of Polymer Chain Dynamics in Solution

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  • March 19, 2019
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  • Sim, Sooyeon
    • Affiliation: College of Arts and Sciences, Department of Applied Physical Sciences, Materials Science Graduate Program
Abstract
  • The temperature and side-chain structure dependencies of the time-resolved electron paramagnetic resonance (TREPR) spectra of acrylic polyhedral oligosilsesquioxane (POSS)-based copolymers and acrylic block copolymers are presented. The carbon-centered radicals within the main chain of an acrylic polyhedral oligosilsesquioxane (POSS)-based copolymers and acrylic block copolymers are produced in dilute solution at temperatures ranging from 25 °C to over 120 °C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I α-cleavage of the side chain ester moiety and provides a means to investigate the conformational energy landscape using a highly localized, minimally perturbative spin probe. The observed photochemical degradation mechanism of those copolymers is quite similar to that previously observed in photolysis of homo poly (methacrylate)s. As expected, chromophores with POSS group side chains do not participate in the photodegration process, which suggests that POSS utilize a beneficial effect against UV light photodegradation in these copolymers. The resulting experimental spectra suggest since all of the investigated methacrylate-POSS copolymers show the same photodegradation phenomena, that POSS could play an important roles in preventing UV degradation and improving photoresistance in such materials. The recent development of several controlled and free radical polymerization mechanisms have allowed the synthesis of many types of block copolymers, especially acrylics. Thus, the photophysical and photochemical behavior of acrylic blocks in dilute solution, especially the site selective photochemistry was studies here by TREPR. It is interesting note that alternating line broadening effects due to the modulated hyperfine constant cause by a bulky ester side chain are observed in high temperature TREPR spectra of both t-butyl methacrylate-POSS block copolymer and poly t-butyl methacrylate rich block copolymers. The excellent fit of the simulations allow for identification of the signal carriers. The spectra show a strong signal from the tert-butyl radical after decarboxylation of the oxo-acyl radical. This process is rarely observed in high temperature fast limit TREPR spectra. The temperature dependence of TREPR spectra of all block copolymers presented here shows similar conformational dynamics to those reported in our previous papers. Radical-triplet pair interactions are used to investigate the dynamics of acrylic polymers in dilute solution. Methyl methacrylate was randomly copolymerized with a small amount of an amine-containing monomer to create the polymers. The amine subunits were then oxidized to nitroxide moieties (stable free radicals). Using TREPR spectroscopy on the sub-microsecond time scale, competition is observed between two deactivation processes of the ester side chain triplet state: 1) Norrish I -cleavage, leading to a main chain free radical studied previously in our laboratory, and 2) spin polarization transfer or quenching by a nearby stable nitroxide radical. The two processes have TREPR spectral signatures that are easily distinguished by their widths, number of transitions, and chemically induced electron spin polarization (CIDEP) mechanisms. The main chain polymeric radical signal dominates the TREPR spectrum if the probability of radical-triplet pair encounters is low. These competing reactions show a strong dependence on nitroxide incorporation (mol %), temperature, solvent, and acrylic polymer ester side chain structure. A comparison of steady-state EPR and TREPR signals from inter- vs. intramolecular processes clearly demonstrates the influence of polymer chain dynamics on the observed phenomena.
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  • In Copyright
Advisor
  • Forbes, Malcolm
  • Wu, Yue
  • Sheiko, Sergei
  • Samulski, Edward T.
  • Moran, Andrew
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2014
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  • Chapel Hill, NC
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  • This item is restricted from public view for 2 years after publication.
Date uploaded
  • April 22, 2015

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