Recently a debate about the initial crystallization process which has not been the hotspot for a long time since the theory proposed by Hoffman-Lauritzen (LH) dominated the field arose again.For a long time the Hoffman-Lauritzen model was always confronted by criticism

and some of the points were taken up and led to modifications

but the foundation remained unchanged which deemed that before the nucleation and crystallization the system was uniform.In this article the classical nucleation and growth theory of polymer crystallization was reviewed

and the confusion of the explanations to the polymer crystallization phenomenon was pointed out.LH theory assumes that the growth of lamellae is by the direct attachment of chain sequences from the melt onto smooth lateral sides.Whereas

more and more experimental results show that before the crystallite appears polymer chains go through conformation adjustment and orientation transformation which favor the formation of crystallites

i.e. a preordering structure exists before or at the early stage of polymer crystallization.This is a deviation from the simple process of nucleation and growth at the early stages of melt crystallization which can be gone back to Ostwald’s “rule of stages".Strobl’s multi-step crystallization mechanism based on recent SAXS results was summarized in the paper.The theory assumes for the formation of polymer crystallites

a passage through a transient mesomorphic phase.It is proposed that the initial step is always the creation of a mesomorphic layer which starts with an attachment of chain sequences from the melt onto a growth face of a mesomorphic layer with minimum thickness and spontaneously thickens

up to a critical value

where it solidifies through a cooperative structural transition.The transition produces a granular crystalline layer

which transforms in the last step into homogeneous lamellar crystallites and this granular substructure is observed for many semicrystalline polymers.Crystallization of polymers is controlled by many factors which comprise of the variation of the main structural parameters such as lamellar thickness

long spacing

crystallinity with the crystallization temperature

the structural changes during recrystallization processes and the temperature dependence of growth rates etc. To expatiate upon the transforming process among melting

mesophase and lamellae a thermodynamics scheme was introduced.Many experiments indicate the existence of mesophase

and the thermodynamic framework by Strobl is generally feasible for many situations.Indeed

it is the interference of mesophase which generally dominants polymer crystallization by the selection of the lamellar thickness.Sirota argued that the granular structure of the lamellae was itself a fingerprint of the transient mesophase into which a lamella originally grew

and it was then converted to the higher-density stable form;and the lower interfacial energy of the mesophases is connected with their lower density.Based on Strobl’s suggestion two crystallization modes of mesophase forming polymers have been offered which were partitioned by crystallization rates and only the second case by high rates was deemed to apply to the Ostwald’s rule of stages.In conclusion

it is very important to newly understand the polymer crystallization process for the thermodynamical properties of the mesophase between melting and crystallite.