th  - 1987 - 51st ENC Conference

WF36
A PROTON MAS NMR METHOD FOR DETERMINING INTIMATE MIXING IN POLYMER BLENDS
D.L. VanderHart" and W.F. Manders, National Bureau of Standards, Gaithersburg,
MD 20899, and R.S. Stein and W. Hermans, Polymer Science and Engineering
Department, University of Massachusetts, Amhearst, MA 01002
Intimate mixing (on a scale less than i nm) in polymers can be probed
effectively by laC CP-MAS techniques as has been shown by Schaefer, et al.
(Macromolecules, 1981, 14, 188). The basic idea is that if one mixes
deuterated and protonated polymers, cross polarization (CP) signals from the
carbons on the deuterated polymers will arise only if dipolar-coupled protons
are close by, say, 0.5 nm or less. A non-trivial problem in this method,
however, is that perdeuterated polymers usually contain 1 - 2% residual
protons, and these 'Impurity' protons, as well as protons on the prononated
chains, generate deuterated carbon signals.
In this poster, we show that similar information is available by observing
the impurity protons directly. The impurity protons are different from the
protons on the protonated chains since their average interaction with other
protons is much weaker. In a relatively rigid polymer, at low magic angle
spinning (](AS) frequencies, the impurity proton signals in the deuterated
homopolymer will be broken up into well-defined sidebands and centerbands (see
spectra A and B, non-spinning and 2.2kHz spinning, respectively, of deuterated
atactic polystyrene (aPS)). In contrast, a relatively rigid protonated
homopolymer will produce no narrow centerbands or sidebands because of the
stronger proton dipolar interactions and the concurrent fast spin-exchange
behavior. In a mixture of relatively rigid polymers, one deuterated and the
other protonated, there will be an attenuation of the narrowed centerband and
sidebands from the impurity protons when mixing is intimate, i.e. when protons
from the protonated polymer lie within, say, 0.8 nm of the impurity protons.
Spectrum C is that of a protonated aPS mixed with the deuterated aPS using 2.2
kHz spinning and a spectral display where the impurity protons contribute
equal signals to those in B. An 85% attenuation, compared to B, of the
aromatic proton centerband at 7 ppm is seen in the blend spectrum. As
expected in this blend of similar polymers with similar tacticities, mixing is
quite intimate. Interference from the broad and strong protonated aPS
resonances is limited to a rolling baseline. Because of M.AS, and the
resonance distinctions which ensue, this
A analysis can be carried out in the presence
of varying amounts of mobile solvent residues
which, in our case, contribute to the
aliphatic bands between 0 and 3 ppm. This
technique offers an important sensitivity
advantage over the 13C CP technique. The
requirement for two relatively rigid polymers
is common to both techniques, so low
temperature operation is sometimes mandated.
Perhaps the best news is that this technique
represents a minor comeback for the
beleaguered single-pulse ID experiment.
Bandwidth requirements should allow one to
carry out this experiment using high-
resolution electronics.
I I I I I I I l I
40 20 0 -20 PPM