Local Segmental Dynamics of Poly(2-hydroxyethyl Methacrylate) in Methanolic Solution by Spin Label X-Band ESR

• Published in: The journal of physical chemistry. B 2004-07, Vol.108 (27), p.9482-9490
• Main Authors: Marek, Antonín, Czernek, Jiří, Steinhart, Miloš, Labský, Jiří, Štěpánek, Petr, Pilař, Jan
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp036083y

A copolymer of 2-hydroxyethyl methacrylate (HEMA) with spin-labeled methacrylic acid units distributed randomly along the main chain (∼3 mol %) was synthesized. ESR spectra of methanolic solutions of the copolymer at concentrations ranging from 2 to 50 wt % were measured at X-band (9 GHz) over a broad temperature range. Temperature dependence of the parameter R S, characterizing the local segmental dynamics in PHEMA chain, and of the other parameters characterizing local dynamics and conformation of the copolymer were determined by fitting experimental ESR spectra to the theoretical spectra calculated using the MOMD model. Arrhenius plots of the rotational parameters exhibit atypical nonlinear behavior characterized by two breaks at particular temperatures in the R S plot. It is concluded that, similarly to PMMA in a number of solvents, PHEMA in methanol undergoes a conformational transition from a less compact to more compact conformation of the polymer chain when the temperature increases in a particular temperature range. Results of dynamic light scattering and X-ray scattering experiments support this conclusion. Analysis of the shape of the ordering potential, which constrains the preferred orientation of the effective axis of internal rotation of the tethered nitroxide, revealed the occurrence of two different conformations of the nitroxide moiety. Based on the results of quantum chemical calculations, two conformations of the model system (nitroxide plus tether) are proposed, which are in a good agreement with the geometry required by the ordering potentials.