PROTON SPIN-DIFFUSION STUDIES OF POLYMER BLENDS HAVING MODEST MONOMERSIZE .2. BLENDS OF CELLULOSE WITH EITHER POLY(ACRYLONITRILE) OR POLY(4-VINYLPYRIDINE)

Blends of cellulose (CELL) with either poly(4-vinylpyridine) (P4VPy) o r poly(acrylonitrile) (PAN) have been examined by solid-state proton N MR. Multiple-pulse techniques combined with the phenomenology of spin diffusion allow one to obtain information about average domain size al ong the thinnest dimension of the domains. In addition, one can obtain information about the stoichiometry of the phases. CELL/P4VPy blends were examined over a composition range from 30 to 70% CELL; CELL/PAN b lends spanned a composition range from 32 to 88% CELL. It was found th at average overall repeat distances for the CELL/P4VPy blends were in the 8-12-nm range, while for the CELL/PAN blends the range was ca. 16- 24 nm. As a function of overall composition, there were no sudden chan ges in dimensions or, by implication, in miscibility. As to stoichiome try, very little mixing took place; typically <5 % of polymer A would be found in a polymer B-rich phase. Given this weak mixing, it is surm ised that the thermodynamics of mixing are quite unfavorable for these pairs; i.e., the kinetics of phase separation are important in defini ng the morphology. The importance of kinetics is also manifested in a rather wide dispersion of domain size in the CELL/P4VPy blends; disper sion is smaller in the CELL/PAN blends. Small-angle X-ray scattering w as also applied to a representative sample of each blend. The SAXS res ults corroborated the NMR results regarding domain size and size dispe rsion. Finally, the fact that dynamic mechanical analysis (at 11 Hz) h ad previously been reported on these blends enabled us to comment on t he critical domain size below which the molecular motion (or T(g)) for PAN would be influenced by the rigidity of the surrounding CELL phase . That critical size was found to be in the 10-13-nm range.