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896 Macromolecules, Structure

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H, and H NMR are most useful for synthetic polymers,
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whereas H, N, P, and C are often used for materials
of biological origin.
Studies of the degree of polymer or biopolymer orien-
tation with respect to a strain direction take advantage of
the orientation-dependent NMR chemical shift or dipole–
dipole interaction in the solid state. NMR spectra of ori-
entedmaterialscanprovideinformationaboutthearrange-
ment of individual chain segments with respect to the draw
direction. Often, the amount of orientation in amorphous
regions can also be determined.
NMR quantification of the fraction of crystalline and
amorphous components in a solid relies upon differences
in molecular motion between these regions. The crys-
talline regions have very little molecular motion and thus
FIGURE 39 Chain conformations of (a) nylon 66 and (b) nylon
have very long relaxation times. The amorphous regions,
6. [From Holmes, D. R., Bunn, C. W., and Smith, D. J. (1955).
on the other hand, are generally quite mobile and have
J. Polym. Sci. 17, 159.]
much shorter relaxation times. These differences in relax-
ation times are generally sufficient for spectral discrimi-
nation, thereby affording quantification of the fraction of
in the amorphous state. One school maintained that the
crystalline and amorphous components.
chainsdidnotminglewiththeirneighborsbutrathercoiled
Solid-state NMR spectra of polyethylene provide a par-
back on themselves. Another school insisted instead that
ticularly clear example of the differences between crys-
the chains interacted with their neighbors forming mi-
talline and amorphous components. We have already seen
crobundles or lamellae too small to be seen by X-ray
that polyethylene in the crystalline solid state exists in a
diffraction. Flory, in 1953, postulated that neither of these
planar zigzag conformation. The amorphous regions are
ideas is correct, and that in the solid state macromolecules
expected to have some gauche character and according
exhibit their unperturbed, θ-condition dimensions, since
to the γ -gauche model (see Section IV.C) would be ex-
a polymer chain surrounded by itself and like chains has
pected to have a different chemical shift. We see from
nothing to gain by expanding.
Fig. 40 that this is the case. The peaks arising from the
The advent of small-angle neutron scattering has sup-
crystalline and the amorphous regions are separated by
pressed essentially all further controversy in this regard. In
approximately 2.4 ppm, which is what we expect for an
this method, deuterated polymer is diluted in a protonated
equilibrium amount of gauche bonds in the amorphous
polymermatrix.Thedeuteratedpolymerhasagreaterscat-
region.
tering cross section than the corresponding protonated
Solid-state NMR chemical shifts are also sensitive to
materials, thereby providing contrast. The information packing effects. The C spectra of isotactic and syn-
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from neutron scattering provides ¯ r and s , as is the
diotactic polypropylene (Fig. 41) illustrate this point. As
case for light scattering. Results from neutron scattering
we recall, isotactic polypropylene adopts a 3 l helix com-
experiments show that polymer chains in the amorphous
posed of alternating gauche and trans conformations. The
solid state exhibit their unperturbed dimensions.
methyl groups occupy positions on the surface of the coil,
Neutron scattering experiments are also particularly
and the methine and methylene groups are stacked one
useful for determining copolymer morphology (see
over the other in alternating fashion. These three types
Section V). The scattering profile can be analyzed to
of carbons give rise to the three resonances observed in
provide information about the degree of phase mixing and
Fig. 41(a).
also about the interfacial gradient.
Syndiotactic polypropylene, on the other hand, adopts
a repeating (gg)(tt)(gg)(tt) conformation, with four mono-
mer units per repeat. In this conformation the methylene
4. Solid-State NMR Spectroscopy
groups can reside in one of two distinct environments
NMRspectroscopyofsolidpolymersisapowerfulmethod [Fig. 41(b)]. These groups are magnetically nonequiva-
for establishing the degree of orientation in stretched poly- lent and produce two peaks. The separation between these
mers, the degree of crystallinity in bulk samples, and the two resonances can be explained in terms of the γ -gauche
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conformation in the solid state. Generally, C, Si, F, shielding effect. The external CH 2 experiences two trans