Chemistry Reference
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(b)
(a)
(c)
FIGURE 4.4
Possible conformations of polymer chains at the surfaces of chain-folded single crystals.
(a) Adjacent reentry model with smooth, regular chain folds, (b) adjacent reentry model
with rough fold surface, and (c) random reentry (switchboard) model.
regularity of chain folding and reentry is very likely much less under these condi-
tions than in the single crystals produced by slow crystallization from dilute
solution.
Another major difference in crystallization from the melt and from dilute solu-
tion is that neighboring growing lamellae will generally be close together under
the former conditions. Segments of a single molecule are thus likely to be incor-
porated in different crystallites in bulk crystallized polymer. These “tie mole-
cules” bind the lamellae together and make the resulting structure tough. The
number of tie molecules increases with increasing molecular weight and with fas-
ter total crystallization rates. The crystallization rate is primarily a function of the
extent of supercooling. Cooler crystallization temperatures promote more nuclei
but retard the rates of conformational changes required for segmental placement
on growing nuclei. (It is observed empirically that the maximum rate of isother-
mal crystallization occurs at about 0.8
T
m
, where the maximum crystal melting
