crystal planes, and the minimum lattice energy can be obtained from the growth on
this crystal plane. Therefore, its advancing speed will be the lowest to survive as the
observable facets. At the beginning of lateral growth of the single-layer lamellar
crystal, many crystal faces may compete with each other. At the later stage, only the
facets with the slowest growth front can survive, which determine the symmetric
shape of the single crystal. Those faster growing facets will be removed by the
merging of their slower neighbors. For instance, the simplest orthorhombic poly-
ethylene crystals exhibit lozenge shapes in toluene, because the plane spacing along
110 diagonals appears larger than that alongaandblattice axes. As illustrated in
Fig.10.14, because the adjacent chain folding spreads along the separate facets,
separate sectors with various orientations of fold ends develop, which suggests that
the single-layer lamellar crystals are actually a multiple twin crystal. If the single
crystal grown from the bulk thin film is decorated by the sprayed paraffin wax, the
paraffin crystallites will orient to follow the surface orientations of each sector,
revealing the existence of fold-end sectors with various orientations in parallel with
growth facets, as shown in Fig.10.15. Dynamic Monte Carlo simulations also
reproduced four 110 sectors of fold-end orientations in the single lamellar crystal
grown from semi-dilute solution, as illustrated in Fig.10.16.
- Axialites
When polymers crystallize at high temperatures, due to the difficulty of sponta-
neous crystal nucleation, the practical crystal nucleation often utilizes the foreign
surfaces provided by the impurities. The sizes of the heterogeneous nuclei are
normally large, so they can induce the growth of multi-layer lamellar crystals.
One can see that the multi-layer lamellar crystals spread from the same center, just
like an opened book. Such morphology of multiple stacking of lamellar crystals is
calledaxialites, as illustrated in Fig.10.17.
Fig. 10.14 Illustration of fold-end sectorization in the single polyethylene crystals (Bassett et al.
1959 )
204 10 Polymer Crystallization