Novel and Accurate Method for Determination of Glass Transition Temperature of Spin-Labeled Polymer by ESR Microwave Power Saturation

A novel method to determine a glass transition temperature (T(g)) of spin-labeled polymer applying the continuous-wave electron spin resonance (CW-ESR) has been developed. Poly(cyclohexyl acrylate) (PCHA), polystyrene (PS), poly(methyl acrylate) (PMA), poly(propylene glycol) (PPG), and poly(ethylene glycol) (PEG) were labeled by attachment of nitroxide radicals; the temperature-dependent microwave power saturation measurements were carried out. The saturation factor, S, determined from the microwave power saturation measurement, had different temperature dependence above and below the T(g) of the spin-labeled polymer because of the different temperature dependence of the spin-lattice relaxation time. The inflection point in the temperature dependence of S was defined as a T(g), (ESR). The T(g), ESR was in good agreement with the T(g), (DSC) determined by differential scanning calorimetry (DSC) for the PCHA, PS, PMA, and PPG. The T(g), (ESR) of the PCHA labeled at the chain end was about 3 K lower than that of the one labeled at the midchain segments, which indicated that the local T(g) around chain ends is slightly depressed. Furthermore, the T(g), (ESR) of highly crystallized PEG was determined to 221 K even though the DSC was unable to detect the glass transition. This value agreed well with the T(g) of PEG determined by thermally stimulated current.