One- and Two-Dimensional MAS 13C NMR Analyses of Molecular Motions in Poly(2-hydroxypropyl Ether of Bisphenol-A)

Abstract

The dynamics of amorphous poly(2-hydroxypropyl ether of bisphenol-A) (PHR), quenched from the melt, has been investigated by one- and two-dimensional solid-state 13C NMR spectroscopy. CP/MAS and dipolar decoupled/MAS 13C NMR spectra from −150 to 180 °C give two specific features: (1) below 23 °C, resonance lines for C−H carbons of phenylene rings split into two lines; (2) line widths of resonance lines become broad at 110−140 °C (30−60 °C above the glass transition temperature). Feature 1 indicates that phenylene C−H carbons exist in two magnetically different sites at low temperatures. These two sites are associated with asymmetric conformational states, which may be produced by OH···π hydrogen bond formation. The coalescence of the resonance lines at elevated temperatures is caused by the π flip motion of phenylene rings, which corresponds to the γ relaxation for PHR. The correlation time of the π flip motion is analyzed by the two-site exchange model and is found to follow the Arrhenius equation. The apparent activation energy is 51 kJ mol-1 by assuming an inhomogeneous correlation time distribution described by a Kohlrausch−Williams−Watts (KWW) function with an exponent of 0.2. Feature 2 is caused by the so-called motional broadening, which is originated by enhanced segmental motions. This dynamics corresponds to the α relaxation for PHR and can be described by the William−Landel−Ferry (WLF) equation. Two-dimensional (2D) CP/MAS 13C exchange NMR experiments confirm the existence of flip angle distribution as well as the distribution of correlation times of the phenylene ring π flip motion. The 2D experiments at −120 °C confirm the KWW exponent of 0.2.