Chemistry Reference
In-Depth Information
Fig. 2.13
Spherulitic growth (from ref [
50
])
morphology. It contains lamella growth on long fibrils. Drawing of a crystalline polymer forces the
spherulites to rearrange into parallel arrangements known as
drawn fibrilar morphology
.
In order for the ordered phase to crystallize from an amorphous melt a nucleation barrier must be
overcome. This barrier is a result of interfacial energy between the ordered phase and the melt that
causes super cooling. Sirota [
44
] suggested that in order for the nucleation barrier of the stable phase
to be sufficiently high to form out of the melt, another phase with a lower nucleation barrier and a free
energy intermediate between that of the stable phase and the melt must form. This, he points out, is
implied by Oswald's rule [
45
] and evidence presented by Keller [
35
] that crystallization in semicrys-
talline polymer systems is mediated by a transient metastable phase [
47
,
48
].
Stroble and coworkers demonstrated that lamellar thickness is determined by a transition between
the metastable phase and the stable crystal phase [
46
-
49
]. In addition, by relating the crystallization
temperature, melting temperature, and crystalline lamellar thickness, he suggested that lamellar
growth fronts are thin layers of a mesomorphic phases. He feels that these phases thicken until such
thickness is reached that stable crystal phases are favored. The conversion occurs in a block wise
fashion but results in granular structures that were observed in many polymers [
46
-
49
]. This conver-
sion is a stabilization process that lowers the free energy of the newly formed crystallites and prevents
them from returning to the mesomorphic phase upon subsequent elevation of the temperature [
46
-
49
].
Such a concept of crystallization, however, is not universally accepted. Sirota [
49
], pointed out,
however, that this picture and the thermodynamic framework are generally correct in many cases.
Sirota [
49
] believes that the origin of granular structures, mentioned above, can be understood in
the following way. The initial nucleation and growth take place by stem addition. into mesophases.
Lamellae thickness occurs while the chains are in the more mobile mesophase. When the thicknesses
grow large enough to allow conversion from mesophases to crystals, the average densities in the
lamellae have been set and the crystals break up into blocks. The transitions from mesophases to
crystals also involves increases in lateral packing densities. In semicrystalline polymers, the
entanglements in the amorphous regions, as well as the folding of the chains and the lamellar
spanning the chains, will also have an effect. These effects limit the allowable lateral displacements
