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HAL Id: jpa-00214666

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Submitted on 1 Jan 1971

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CRYSTALLIZATION OF CHAIN MOLECULES

A. Keller

To cite this version:

A. Keller. CRYSTALLIZATION OF CHAIN MOLECULES. Journal de Physique Colloques, 1971,

32 (C5), pp.C5a-29-C5a-31. �10.1051/jphyscol:1971503�. �jpa-00214666�

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JOURNAL DE PHYSIQUE Colloque C5a, supplment au no 10, Tome 32, Octobre 1971, page C5a-29

CRYSTALLIZATION OF CHAIN MOLECULES

A. KELLER

H. H. Wills Physics Lab., University of Bristol.

Abstract. - A

very brief survey on the crystallization of macromolecules is presented. It includes the essential features of lamellar crystals which contain the chains in a folded configuration which is recognized as the basic mode of crystallization of unstressed macromolecules and the formation of fibrous crystals which arise when the chains have been previously oriented by stress of flow.

Some specifi; issues of current topicality are outlined.

All sufficiently regular chain molecules can form a crystallographic facets having thicknesses in the 100 A

crystal lattice by appropriate parallel alignment of the range (Fig. 2). In view of the fact that the molecules chains. The atomic positions in this lattice can be which are much larger than the lamellae are thick were determined by the traditional methods of structural found to be perpendicular or at a specified large angle crystallography. The determination of size, shape, to the lamellar surface a regularly folded structure was connectedness of the crystalline regions, however, is not

such a straightforward matter. According to traditional concepts a crystalline polymer ought to contain some amorphous material in order to account for its proper- ties and the crystals themselves should be of elongated fibrillar character in keeping with the long chain nature of the molecules. These expectations are incorporated in the traditional fringed micelle concept (Fig. 1).

FIG. 2. -

Typical solution-grown crystal

of

polyethylene.

Electron micrograph.

FIG. l. -

The fringed micelle representation of crystalline polymers (after Bunn).

Nevertheless when individual polymer crystals could finally be obtained by crystallization from solution these were found to be regular lamellae bounded by

postulated as the only molecular arrangement which could account for the observations (Fig.

3). This

introduces a new molec~llar concept, chain folding, and a quantity characterizing this property the fold lenght.

The latter corresponds to the lamellar thickness and can be determined most conveniently by low angle X-ray scattering.

It was observed that the fold length is uniquely determined by the supercooling at which the polymer has crystallized. This forms the basis of most theoretical treatments. Accordingly, it is visualized that chain folding is of kinetic origin. Crystals formed from fully

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971503

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C5a-30 A. ICELLER

FIG. 3. - Diagrammatic representation of chain folding in polymer crystals (the folds are drawn sharp and regular).

extended chains should be stablest the chains never- theless fold because in this way they can crystallize faster. The theories demonstrate that at a given super- cooling there is a fold length at which the crystallira- tion is the fastest and this will be the fold length which determines the lamellar thickness. Quantitative agree- ment between theory and experiment is very satisfac- tory.

At this stage we may turn back to the

a priori

expectations of fibrous crystals which appears to be violated by the discovery that the crystals have a lamellar habit. In fact the contradiction is not as acute as it may first seem. It has turned out that crystals with fibrous characteristics are in fact obtained if the chains are stretched out prior to crystalli~ation. The fibrous products obtained in this way contain the chains at least partly in an extended chain type configuration. As in an assembly of molecules under an orienting influence some molecules will be stretched more than others, the more highly oriented chains will crystallize first giving rise to the fibrous crystals. The chains which are not stretched, or only to a lesser extent, may use these fibres as nuclei for chain folded lamellar crystallization.

Accordingly, we would have a dual structure

:

a fibrous backbone with a transversely growing chain folded lamellar overgrowth (Fig. 4). These kind of structures are important constituents of materials which form during extrusion or spinning and hence have important

technological and biological implications. The stretch- ing of the molecules may occur statically as in a stretch- ed cross-linked network or dynamically as by flow of solutions and melts. In the case of flow a longitudinal velocity gradient (as opposed to the more usual shear flow resulting from a transverse velocity gradient) is required.

Returning to the chain folded lamellar crystals obtained under unstressed and quiescent conditions the question of the nature of the fold surface and the structure of the fold presents itself. This is,closely linked with the overriding issue whether we have any amor- phous component (usually expected in a crystalline polymer) within such crystals

;

and if yes, how is this to be envisaged. By methods used to determine the degree of crystallinity in general, a notable crystallinity deficiency is observed even in the most regular lamellae and by a variety of considerations this crystallinity deficiency is visualized as an amorphous layer on the crystal surface constituted by irregular loose folds. On the other hand there are numerous experimental indications that the folds are adjacently reentrant and possess at least a certain amount of regularity. This is a complex and controversial issue which cannot be analysed in brief. Here merely two lines of work will be sketched which together with many others, not mentioned here, bear on this issue. The main purpose of presenting them here is to illustrate how entirely diffe- rent methods can be applied in aid of the same quest.

The first is a basically chemical approach and so far restricted to polyethylene. Chain folded lamellar crystals are reacted with strong oxidising agents such as ozone and nitric acid. These are found to thin the layers from surface downwards in the process of digesting the chains. The molecular weight distribution is determined in the course of this thinning process. The development of a quantized molecular weight distribu- tion is registered which obviously corresponds to single, double and multiple chain traverses resulting from a random cutting of the fold. Detailed analysis of these discrete peaks in the molecular weight distribu- tion can provide further information on the fold structure. Thus it can tell us that the folds are not all at the same level, but some are buried below the lamellar surface and that at least those which are at some depth below the surface must be adjacently reentrant and sharp. Those which are close to the layer surfacecan be looser while the uppermost crystal layer can contain large non-adjacently reentrant loops and loose hairs.

The latter features contribute to the partial amorphous characteristic of the crystals referred to previously.

Even so at least the existence of crystallographic regu- larity traversing the full layer thickness could be ascertained. In ultimate generality the purpose of these studies is to elucidate the crystalline-amorphous characteristics of these layers in specific molecular terms.

FIG. 4.

-

Fibrillar crystals with lamellar overgrowth (shish-

kebabs) of polyethylene grown in an agitated xylene solution.

The specialized topic to be included here

Electron micrograph (Pennings).

relates to the study of polyamides. These are chemi-

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CRYSTALLIZATION OF CHAIN MOLECULES C5a-31

caily more complex than polyethylene but in view of

the greater specificity of the molecules they are capable of providing some detailed information on the central issue of how a crystal layer is to be divided into crystal core and a fold. It could be demonstrated that specific interchain forces, such as the hydrogen bonds reduce the fold length, a point of obvious interest, As the chemical repeat length of these molecules is compara- tively large there will only be a very few such repeats within a lamella. Hence such a lamella will act as a very small crystal, in the case of the presently studied polyhexamethylene adipamide (Nylon

66) consisting

only of 4 repeating units. This gives rise to detectable

subsiduary maxima in the diffraction pattern which promises to offer a powerful new diffraction approach to the structure of such lamellae and to the nature of the fold. Certain specific conclusions e. g. that the straight chains traverse the full lamellar thickness and that the chains prefer to fold in the acid group has already been established, hopefully setting the pattern of further developments to follow.

Finally some connections between chain folded crystallization of synthetic polymers and biomolecules are pointed out. Work on the polyamides in particular appears to provide a bridge between these two hitherto disconnected subjects.

LA FIN DE L'EVOLUTION DES ETOILES ET LES PULSARS

E. SCHATZMAN

Resume. -

Une esquisse est donnh de notre connaissance actuelle des dernikres etapes de 1'Bvolution des Btoiles. de I'explosion des supernova1 a la formation des pulsars. De brkves indica- tions sont donnkes sur la structure des Btoiles &neutrons.

Abstract. - A

sketch is given of our knowledge of the last stages of stellar evolution, from

supernova1 outbursts to pulsar formation. Brief indications are given on the structure of neutron

stars.

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