In contrast, in 1966, Colson and Eby assumed that the comonomer B could enter
the crystalline phase of A, with the mole fractionXB, and caused a lattice defect
energyDHB(Colson and Eby 1966 ). They derived that
Tm¼T^0 mð 1 XBDHB
DHuÞ (10.19)
Equation (10.19) fits well for the experimental results on the parallel system of
copolymers with small comonomer units.
10.3 Crystalline Structures of Polymers
10.3.1 Hierarchical Crystalline Structures
Most of the crystallizable polymers contain good sequence regularities along the
backbones, such as HDPE, iPP, Nylon, PET and POM. Only a few of the crystal-
lizable polymers contain random sequences but with strong hydrogen bonding or
strong interactions between polar groups, such as PVA and PAN. So far, we have
devoted much more efforts on the study of the former systems than that of the latter.
Most of crystalline polymer materials exhibit multi-scale hierarchical structures.
At the scale of 0.1 nm, the polymer chains contain regular sequences. At the scale of
0.5 nm, they form stable helical conformations, which then pack together in a
compact parallel fashion to make the periodic lattice structure, with the unit cell at
the scale of 1 nm. At the scale of 10 nm, the folded-chain lamellar crystals are formed
for the flexible polymer chains. At the scale of micrometers or larger, the lamellae
further assemble into spherulites. Such hierarchical structural characteristics at
varying length scales of polymer morphologies are illustrated in Fig.10.7.
Fig. 10.7 Illustration of multi-scale hierarchical structure of crystalline polymers (For instance,
polyethylene) forming folded-chain lamellae and spherulites
10.3 Crystalline Structures of Polymers 197