Abstract
When crystallizing macromolecules, one needs to recognize that they are sufficiently long to connect
neighboring phases. This coupling between the phases is the reason that on ordering, practically all
polymers develop a globally metastable, semicrystalline structure, consisting of multiple phases with
different degrees of mobility. The ordered and amorphous phases are separated by nanophases of strained
segments of molecule of nanometer dimensions, consisting of rigid-amorphous fractions, RAFs, which
soften, above, within, or below the often very broad melting range of the ordered phases. Thus, ordering
of a melt of macromolecules causes significant changes in the remaining amorphous phases. A summary
of the various phases is given in terms of their structures, molecular vibrations, and large-amplitude
motion as they influence the glass and order-disorder transitions. This is followed by a description of the
limits of the classical nucleation theory of crystals when applied to macromolecules. Discussed in detail
are the problems in primary and secondary nucleation which arise from the presence of RAFs. Special
macromolecular nucleation processes that needed to be considered are the self-nucleation on cooling
from above the melting temperature or on heating from above the glass transition temperature, and the
molecular nucleation causing molar mass segregation on ordering. Finally, the glass transitions of the
phases of various sizes and degree of order are discussed on hand of selected, pertinent examples.