Dynamic alternation between inter- and intra-polymer hydrogen bonds in a polymer complex as studied by solid-state C-13 2D exchange NMR

Solid-state C-13 two-dimensional (2D) exchange NMR has been applied to inve stigate dynamic alternation among three forms of hydrogen bonds in the poly (acrylic acid)/poly(ethylene oxide) (PAA/PEG) complex. The C-13 CPMAS NMR s pectrum of the signal for the PAA carboxyl carbon in the complex is split i nto three peaks corresponding to three forms of hydrogen bonds, namely, the complex form, the dimeric form, and the free form. These three peaks coale sce into a single peak at temperatures above 346 K, showing that dynamic al ternation of hydrogen bonds occurs in the complex. 2D exchange NMR spectra directly show that the dissociation-association of hydrogen bonds occurs at temperatures above 329 K, which is 7 deg higher than the glass transition temperature (T-g). The exchange rates are on the order of Hz and have a wid e distribution. At the same time, it was found that PAA conformational tran sitions occur at a rate comparable to that of the hydrogen bond alternation . On the other hand, the PEO motion shows no apparent correlation to the hy drogen-bonding dynamics. It is concluded that the hydrogen-bonding dynamics is coupled to the segmental motion of PAA in the complex.