Kinetic of crystallization from the melt and chain folding in polyethylene fractions revisited: theory and experiment

This article reviews the various theories of polymer crystallization which have been proposed over the last thirty years. Their background in thermodynamics and crystal growth theories of simple substances is laid out in some detail. This provides the basis for a discussion of the application of surface nucleation and surface roughening concepts to polymer crystallization. The widely used nucleation model of Lauritzen and Hoffman is examined in depth with emphasis on its consistency with experimental evidence and including its recent modifications and extensions. Alternative approaches are also analyzed, in particular Point's ‘multipath’ model and the ‘roughness/pinning’ model by Sadler and Gilmer, but also ‘molecular nucleation’ and ‘chain sliding’. The aim of this article is to lead to a better understanding of the basic hypotheses of the various models and to give guidance to the theorist in improving the models as well as to the experimentalist in interpreting his/her data. It will hopefully make the subject more accessible and stimulate discussion and further research directed at resolving the controversial issues.

Polymer crystallization theories

The diversity of monomers available for synthesis of high polymers makes it possible to prepare a wide variety of long-chain macromolecular compounds. It is instructive to consider a hierarchical organization of structure in polymers at four successive levels--the molecular, nano-, micro-, and macrolevels--and to examine how interactions at and between these various levels of structure have important and often quite specific influences. Examples are drawn from semicrystalline polymers with flexible chains, liquid-crystalline polymers composed of rigid macromolecules, and amorphous polymers. Structural hierarchies in biocomposite systems are also discussed, particularly in soft connective tissues such as tendon and intervertebral disk.

Hierarchical structure in polymeric materials

Analyzing and understanding the structure and morphology of crystalline polymers has been and remains to be a major challenge. Some of the issues have resisted analyses for decades. The present account illustrates how individual contributions help build a body of knowledge that must cover length scales ranging from submolecular features to morphology and correlates them with bulk properties. Emphasis is put on structures and morphologies as formed spontaneously. Possible extension of the research area to connected fields is illustrated with a development on supramolecular crystals.

50th Anniversary Perspective: Polymer Crystals and Crystallization: Personal Journeys in a Challenging Research Field

Single crystals of polyethylene have been grown from solution in various solvents at temperatures between 70 and 120° C. This represents an overlap in crystallization conditions with those used for melt growth, where substantial isothermal thickening is known to occur during growth. The crystal thicknesses have been measured by Raman spectroscopy. Values of the equilibrium dissolution temperature and fold surface free energy are calculated for each solvent and the results analysed using the kinetic theory. Variations in crystal properties with time of crystallization are also investigated. A specific dependence of fold length on supercooling has been found to apply over the whole temperature range, consistent with predictions by the kinetic theories of crystallization in spite of changes in morphology which are incompatible with assumptions underlying the theoretical model. No evidence for isothermal thickening has been observed, except possibly for a small marginal effect at the highest temperature of 120° C investigated, over the same temperature range where melt crystallized material shows the effect prominently. Crystals grown at all temperatures displayed a rise in dissolution temperature with time which could be associated with an increase in surface perfection. All these findings have wider implications for our picture of polymer crystallization and crystal structure which are discussed here. A further, explicit, correlation with melt crystallization is deferred to a subsequent publication.

D.M. Sadler

Papers

A model for chain folding in polymer crystals: rough growth faces are consistent with the observed growth rates