高分子结晶理论的新概念与新进展

回顾了传统的高分子结晶成核与生长模型,指出了该模型在应用中遇到的一些问题;同时总结了Strobl根据近年小角X射线散射结果提出的高分子结晶新机理-中介相机理.介绍了Strobl等构建的热动力学图解对熔体、中介相和片晶的转变过程,阐述了各相间的平衡转变温度、潜在的转变热以及表面自由能,说明了处于熔体和晶体之间的中介相的热动力学性质是理解高分子结晶过程的重要依据.

THE NEW CONCEPTS AND NEW DEVELOPMENTS IN HTE THEORY OF POLYMER CRYSTALLIZATION

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.