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

Solid-state C-13 two-dimensional (2D) exchange NMR has been applied to investigate dynamic alternation among three forms of hydrogen bonds in the poly(acrylic acid)/poly(ethylene oxide) (PAA/PEG) complex. The C-13 CPMAS NMR spectrum of the signal for the PAA carboxyl carbon in the complex is split into three peaks corresponding to three forms of hydrogen bonds, namely, the complex form, the dimeric form, and the free form. These three peaks coalesce into a single peak at temperatures above 346 K, showing that dynamic alternation 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 wide distribution. At the same time, it was found that PAA conformational transitions 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 hydrogen-bonding dynamics. It is concluded that the hydrogen-bonding dynamics is coupled to the segmental motion of PAA in the complex.