One- and two-dimensional MAS C-13 NMR analyses of molecular motions in poly(2-hydroxypropyl ether of bisphenol-A)

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 C-13 NMR spectroscopy. CP/MAS and dipolar decoupled/MAS C-13 NM R spectra from -150 to 180 degreesC give two specific features: (1) below 2 3 degreesC, resonance lines for C-H carbons of phenylene rings split into t wo lines; (2) line widths of resonance lines become broad at 110-140 degree sC (30-60 degreesC above the glass transition temperature). Feature 1 indic ates that phenylene C-H carbons exist in two magnetically different sites a t low temperatures. These two sites are associated with asymmetric conforma tional states, which maybe produced by OH . . . pi hydrogen bond formation. The coalescence of the resonance lines at elevated temperatures is caused by the pi flip motion of phenylene rings, which corresponds to the gamma re laxation for PHR. The correlation time of the pi 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 inhomogeneo us 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. Th is dynamics corresponds to the a relaxation for PHR and can be described by the William-Landel-Ferry (WLF) equation. Two-dimensional (2D) CP/MAS C-13 exchange NMR experiments confirm the existence of flip angle distribution a s well as the distribution of correlation times of the phenylene ring pi fl ip motion. The 2D experiments at -120 degreesC confirm the KWW exponent of 0.2.