Slow ester side-group flips in glassy poly(alkyl methacrylate)s characterized by centerband-only detection of exchange nuclear magnetic resonance

Slow side-group dynamics in a series of five poly(alkyl methacrylate)s with various side-group sizes [poly(methacrylic acid) (PMAA), poly(methyl metha crylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(isobutyl methacrylat e) (PiBMA), and poly(cyclohexyl methacrylate), with -H, -CH3, -CH2CH3, -CH2 CH(CH3)(2), and -cyclohexyl alkyl substituents (CODEX), respectively] were studied quantitatively by centerband-only detection of exchange nuclear mag netic resonance (NMR). Flips and small-angle motions of the ester groups as sociated with the P relaxation were observed distinctly in the CODEX NMR da ta, and the fraction of slowly flipping groups was measured with a precisio n of 3%. In PMMA, 34% of the side groups flipped on a 1-s timescale, wherea s the fraction was 31% in PEXa at 25 degreesC. Even the large isobutylether and cyclohexylester side groups flipped in the glassy state, although the flipping fraction was reduced to 22 and about 10%, respectively. In PMAA, n o slow side-group flips were detected on the 1-s timescale. A striking diff erence in the temperature dependence of the flipping fraction in PMMA versu s PEMA and PiBMA was observed. In PMMA, the flipping fraction was temperatu re-independent between 25 and 80 degreesC, whereas in PEMA, it increased co ntinuously from 31 to 60% between 25 and 60 degreesC. A similar doubling wa s also observed in PiBMA. (C) 2001 John Wiley & Sons, Inc.