Aromatic ring flips in a semicrystalline polymer.

We report a complete analysis of the phenyl ring motions that occur in the semicrystalline polymer, [aromatic-d4] poly(butylene terephthalate). Solid state NMR results show that there are three distinct motional regimes, consisting of (1), the crystalline regions, where the phenyl rings are static; (2) the interphase regions, in which the aromatic groups undergo slow 180 degree ring flips; and (3) the amorphous part, in which the rings undergo 180 degree flips (tau(c)=1.8 x 10(-6)s at 70C) superimposed on rapid, low angle librational motions.