Page 75 - Carrahers_Polymer_Chemistry,_Eighth_Edition

38 Carraher’s Polymer Chemistry

nature of the R group varies as expected. For the furthest left
R = –CH , –CH CH , –CH=CH , –OCH , –CH CH CH(CH ) , –O–CH CH CH(CH ) and
3 2 3 2 3 2 2 3 2 2 2 3 2
cyclohexyl. For the next R = –CH CH(CH )CH CH , –CH CH(CH ) . For the third from
2 3 2 3 2 3 2
the left R = –CH(CH ) , and for the extreme right R = a variety of substituted cyclohexyls,
3 2
including 2-methylcyclohexyl and 4-fl uorocyclohexyl.
2.3 POLYMER CRYSTALS

Before 1920, leading scientists not only stated that macromolecules were nonexistent, but they also
believed, if they did exist, they could not exist as crystals. However, in the early 1920s Haworth used
X-ray diffraction techniques to show that elongated cellulose was a crystalline polymer consisting
of repeat units of cellobiose. In 1925, Katz in jest placed a stretched natural rubber band in an X-ray
spectrometer and to his surprise observed an interference pattern typical of a crystalline substance.
This phenomenon may be shown qualitatively by the development of opacity when a rubber band
is stretched (try it yourself) and by the abnormal stiffening and whitening of unvulcanized rubber
o
when it is stored for several days at 0 C. The opacity noted in stretched rubber and cold rubber is
the result of the formation of crystallites or regions of crystallinity. The latter was fi rst explained
by a fringed micelle model that is now found not consistent with much of the current experimental

findings (Figure 2.14).
Amorphous polymers with irregular bulky groups are seldom crystallizable, and unless special
techniques are used even ordered polymers are seldom 100% crystalline. The combination of amor-
phous and crystalline structures varies with the structure of the polymer and the precise conditions
that have been imposed on the material. For instance, rapid cooling often decreases the amount of

crystallinity because there is not sufficient time to allow the long chains to organize themselves into
more ordered structures before they become frozen in place. The reason linear ordered polymers
fail to be almost totally crystalline is largely kinetic, resulting from an inability of the long chains
to totally disentangle and perfectly align themselves during the time the polymer chain is cooling
and mobile.
Mixtures of amorphous and mini-crystalline structures or regions may consist of somewhat random
chains containing some chains that are parallel to one another forming short-range mini-crystalline
regions. Crystalline regions may be formed from large-range ordered platelet-like structures, includ-
ing polymer single crystals, or they may form even larger organizations such as spherulites as shown
in Figures 2.15 and 2.16. Short- and long-range ordered structures can act as physical cross-links.
In general, linear polymers form a variety of single crystals when crystallized from dilute solu-
tions. For instance, highly linear PE can form diamond-shaped single crystals with a thickness
on the order of 11–14 nm when crystallized from dilute solution. The surface consists of “hairpin

FIGURE 2.14 Schematic two-dimensional representation of a modified micelle model of the crystalline-
amorphous structure of polymers.

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