Page 83 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

898 Macromolecules, Structure

carbons in the γ positions, whereas the internal CH 2 expe- casional vacancies and dislocations found in crystals of
riences two trans carbons in the γ . Therefore, the internal small molecules. The nature and significance of the amor-
carbon chemical shift is expected to occur upfield by two phous phase is not entirely straightforward, but the notion
γ -gauche interactions, or 8 ppm (2 × 4 ppm). The peaks of a degree of crystallinity is useful and well established.
are actually separated by 8.7 ppm, in good agreement with It is commonly estimated by comparing the density and
the γ -gauche model. heat of fusion to those of an appropriate standard. Defin-
NMR methods have been evolving rapidly in recent ing the degree of crystallinity as χ, it is found that most
years and much progress has been made in developing polymers vary in χ from less than 0.5 to 0.95. Even for
methods to measure the distances and relative orientations single crystals of linear polyethylene, χ is usually no more
between nearby atoms in polymer chains. These studies than 0.9. For high pressure branched polyethylene, χ is
typically involve introducing isotopic labels into the poly- about 0.5.
mers chain. In polycarbonate, for example, the distances For the crystallization of linear polyethylene, typical
between the carbonate groups depend strongly on chain conditions involve preparation of an approximately 0.01%
◦
conformation. Thechemical shifts in solidsareanisotropic solution at 130–140 C, which is then held at a fixed tem-
◦
and depend on how an atom is oriented relative to the perature of 70–80 C until a crop of crystals is obtained.
external magnetic field. From such effects it is possible (Since the crystal habit of polymers varies markedly with
to measure the relative orientation of neighboring atoms, crystallization temperature, it is not desirable to simply
such as those in polyethylene. The fraction of trans and let the solution cool to room temperature.) The crystals so
gaucheconformationscanbedirectlymeasuredfromthese obtained take the form of thin, flat platelets or lamellae a
spectra. few micrometers in long dimensions and approximately
10 nm in thickness. The latter dimension increases with
the temperature of crystallization and in fact crystals once
formed at lower temperatures will increase in thickness
V. SOLID-STATE MORPHOLOGY
upon annealing at a higher temperature. The chain or c
axis of the molecules is oriented across the thickness of
A. Crystalline Polymers and Chain Packing
the platelet rather than in its plane. Since the molecules
In order for crystallinity to occur, synthetic polymer may be 1000 nm or more in length, it follows that the
chains must be capable of packing closely together in a chains must be folded many times. These features are
regular, parallel array. Some natural polymers, notably shown schematically in Fig. 42. The nature of the sur-
globular proteins, may crystallize even though their chains face at which the chains emerge, fold, and reenter is a
are folded into complex spheroidal shapes. But such poly- matter of some controversy and no doubt varies from case
mer molecules are usually—for a given type—all iden- to case, particularly as the degree of crystallinity varies.
tical in chain length and manner of folding and can be On the one hand, the appearance of the seemingly reg-
stacked like tennis balls. For synthetic polymers, this is ular and planar surfaces of lamallae, with well-defined
not the case, and crystallinity requires packing side by side edges and corners, suggests that the chains fold back with
in extended form either as planar zigzags or helices (see minimum loops and probably with reentry into the crys-
Section IV.C.2). To do this, the chain must be at least fairly tal at adjacent positions, particularly when the extent of
regular in structure. Unless the chain is predominantly iso- crystallinity is high. It has been proposed, on the other
tactic or (much more rarely) syndiotactic, it usually cannot hand, that at least in some cases the fold surface has
be fittedsufficientlywellto itsneighbors to crystallize.Ex-
ceptions are polychlorotrifluoroethylene ( CFCICF 2 ) n ,
poly(vinyl fluoride), and poly(vinyl alcohol). On the other
hand, natural rubber, although perfectly regular in cova-
lent structure (Section III.D), crystallizes only on very
long standing or on stretching.
The conformations of representative polymer chains in
the crystalline state have been discussed in Section IV.C.
Our concern here is with the morphology of polymer crys-
tals at the level of the optical and electron microscope.
Polymers never crystallize completely even though the
chains may be entirely regular in structure. The degree FIGURE 42 Schematic representation of crystalline lamellae
of disorder far surpasses that corresponding to the oc- showing folded chains, crystal faces, and axes.