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894 Macromolecules, Structure

C. Experimental Observation of Chain
Conformation in the Solid State
Polymers in the solid state can generally assume both crys-
talline and amorphous states, oftentimes together in the
same sample. The fraction of crystallinity in a sample is
generally dependent on both the chemical structure and
microstructure of the polymer chain and on the thermal
history of the sample. For example, the tetrahedral bond-
ing of the quaternary carbon in polycarbonate introduces a
“bend” in the backbone and prevents efficient crystal pack-
ing. On the other hand, the regular structure of polyethy-
lene in the planar zigzag form enhances its tendency to
crystallize:

CH 3 O
( C O C O n ( ( CH 2 CH 2 n (
CH 3

Polymer chains in the solid state adopt primarily gauche
and trans conformations, in close analogy to the situa-
FIGURE 36 Top: conformations of a fragment of a polypropylene
chain: (a) trans,C α C β :t,C α CH 3 : g; (b) gauche +,C α C β :g, tion we have already encountered in the solution state.
C α CH 3 : t, (c) gauche −,C α C β :g.C α CH 3 : g. Bottom: 90-MHz The chief physical methods for determining solid-state
13 C NMR spectrum of the methyl region of atactic polypropylene. polymer chain conformations include vibrational spec-
The “stick” spectrum shows the RIS-predicted chemical shifts for troscopy, X-ray diffraction, neutron scattering, and NMR
the 36 heptad sequences, based on the γ -gauche effect. spectroscopy. We shall discuss representative results from
each of these methods.

appropriate populations of the trans and gauche con-
1. Vibrational Spectroscopy
formations about the central bonds of each heptad.
The γ -gauche model can be successfully extended to IR and Raman spectroscopy (Section III.A) can be used to
other vinyl polymers including poly(vinyl chloride) and determine chain conformations. The CH 2 rocking band is
polystyrene. sensitive to the interaction of chains with their neighbors.
Using the method of normal coordinate analysis, it is pos-
sible to establish interchain and intrachain force fields.
3. Kerr Effect and Dipole Moment Measurements
Furthermore, once conformationally sensitive bands are
of Chain Conformation in Solution
identified, they can be used to identify the fraction of
Kerr effect (electric birefringence) measurements are gauche bonds. Using Fourier transform IR techniques and
exceptionally sensitive to solution state conformations. digital subtraction, difference spectra can be generated.
Results from Kerr effect measurements can be used to Thus, a spectrum of an unfavorable conformer can be ob-
provide a critical test of a particular RIS model. Likewise, tained, even if it is present as only a small fraction of the
dipole moments are conformationally sensitive and can total sample.
also be predicted by RIS methods.
Taken together, characteristic ratios; UV, IR, and NMR 2. X-Ray Diffraction
spectra; fluorescence measurements; molar Kerr con-
stants; and dipole moments provide a fairly clear and X-ray diffraction is one of the primary methods for deter-
consistent view of polymer chain conformation in so- mining macromolecular conformations in the crystalline
lution. The fact that the experimental results can be solid state. The intramolecular conformational consider-
predicted from the RIS model and other semiempiri- ations we have already discussed for polymer chains in
cal treatments suggests that our understanding of solu- solution appear to be the dominant forces for determin-
tion state chain conformations is at a fairly advanced ing solid-state conformations. However, in the solid state
state. we must also consider the intermolecular requirements of