Some aspects of the crystallization of high polymers

SOME ASPECTS OF THE

CRYSTALLIZATION OF HIGH

POLYMERS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECH-NISCHE HOGESCHOOL TE DELFT, KRACHTENS ARTIKEL 2 VAN HET KONINKLIJK BESLUIT VAN 16 SEPTEMBER 1927 STAATSBLAD No. 310 EN OP GEZAG VAN DE RECTOR MAGNIFICUS DR O. BOTTEMA. HOOGLERAAR IN DE AFDELING DER ALGEMENE WETENSCHAPPEN, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 6 APRIL 1955 DES NAMIDDAGSTE 2 UUR,

DOOR

GERRIT SCHUUR,

9^

Prof Dr Ir A. van Rossem

1

Introduction 5

CHAPTER 1: Some general remarks about crystallization.

1. Crystallization of monomeric substances 7

2. Liquid crystals 10 3. Crystallization of high polymers 11

4. The second-order transition point 13 CHAPTER 2: Survey of the literature about spherulites in high

polymers.

1. The discovery of spherulites 16 2. The structure of spherulites 17 3. The growth and the melting of spherulites . . . . 2 1

4. Former explanations of the formation of spherulites . 23 CHAPTER 3: The mechanism of crystallization.

1. The "auto-orientation" of macro-molecules during

crystallization 29 2. The formation of normal spherulites 31

3. The structure of special spherulites 35 4. The absence of spherulites in high polymers . . . . 45

CHAPTER 4: The continuity of the crystal-lattice in high

polymers.

1. Some preliminary remarks 50

2. The continuity of the crystal-lattice within a

spherulite 51 3. The continuity of the crystal-lattice in oriented fibres 54

4. The continuity of the crystal-lattice in stretched

rubbers 59 5. The occurrence of crystallites 62

CHAPTER 5: The rate of crystallization and the melting range

of high polymers.

1. The crystallization curves 64

2. The melting of natural rubber 67 3. The crystallization of emulsions of polymers . . . 70

4. The prospects of a mathematical formulation of the crystallization and melting of high polymers . . . . 72 5. Suggestions for a further investigation into the

crystallization of high polymers 73

Summ,ary 75 References 80 Samenvatting 83

Many crystalline high polymers are found in nature. In more recent times the number of crystalline high polymers has been increased considerably by the chemical modification of natural high pwlymers and by polycondensation or polymerization of synthetic monomers. High polymers, such as cellulose, natural rubber, polyamides and polyethylene which are crystalline or able to crystallize under certain circumstances have an enormous economical importance. The crystal-lization has a great influence on the mechanical, optical, electrical and chemical properties of these high polymers.

These factors provided the impetus for a tremendous amount of work on these various aspects of the crystallization and a review of it is beyond the scope of this treatise.

The importance of the crystallization can readily be demonstrated by a few examples. A fibre must be strong in its longitudinal direct-ion, which is achieved by an orientation of the molecules. Oriented fibres of non-crystalline polymers, such as polystyrene or polymethyl-methacrylate are strong, but the molecules tend to lose their orientation when the fibres are flexed rep)eatedly. The tensile strength then decreases considerably. As a result the wear resistance of non-crystalline fibres is very low.

However, in crystalline high polymers the molecules are stabilized in their oriented positions by the crystal-lattice. They do not lose their orientation when the fibre is subjected to a repeated deformation and the strength of the fibre is preserved. A high polymer must there-fore be strongly crystalline to be of value as a raw material for fibres.

Another example is provided by crystalline rubbers, such as natural rubber and Neoprene. They crystallize when stretched, resulting in an increased cohesion between the molecules and consequently in a high tensile strength. Some synthetic rubbers do not crystallize and the molecules slip, due to the low cohesion forces, when the rubber is stretched considerably. A very low tensile strength is then found. This can be improved by active fillers, but sometimes it is undesirable or impossible to incorporate these fillers in the rubber. On the other hand crystallization of a rubber in the unoriented state is undesirable because it results in a stiffening and a loss of the elastic properties.

The author, working on new applications of natural rubber and rubber derivatives at the Research Department of the

Rubber-Stich-ting, found that the knowledge about crystallization was so in-complete that it hampered his work considerably.

Rapid progess in the development of new applications of rubber hydrochloride especially seemed possible only if more basic know-ledge was available. Consequently, this investigation was started.

One aspect of the crystallization particularly, the formation of spherulites, seems to have been more or less overlooked until recently. These spherulites have therefore been investigated together with other aspects of the crystallization of high polymers. This thesis is a report of the work, which is of a general theoretical interest.

The principal results are a theory about the crystallization of high polymers and definite evidence that the crystal-lattice of high poly-mers is usually continuous, that is a refutation of the micellar theory.

Finally, it should be remarked that no complete review has been given about the spherulitic structures in high polymers so far. The author has tried, therefore, to discuss, or, at least, to mention, all the work which has appeared in this field up to October 1954.

The author wishes to thank the Managing Directors of the Rubber-Stichting for their permission to publish this work. He is also indebted for their financial contribution to the cost of printing this thesis.

S O M E G E N E R A L R E M A R K S A B O U T C R Y S T A L L I Z A T I O N .

§ I. Crystallization of Monomeric S u b s t a n c e s .

Most m o n o m e r i c s u b s t a n c e s in t h e solid s t a t e a r e c r y s t a l l i n e , t h e a t o m s , ions or m o l e c u l e s of w h i c h t h e y a r e composed b e i n g p a c k e d t o g e t h e r in a r e g u l a r t h r e e - d i m e n s i o n a l p a t t e r n . I n l i q u i d s a n d some a p p a r e n t l y solid m a t e r i a l s , s u c h as pitch a n d glass, t h e a r r a n g e m e n t is n o t r e g u l a r a n d t h e s e s u b s t a n c e s a r e called a m o r p h o u s . H o w e v e r , e v e n in a m o r p h o u s s u b s t a n c e s a c e r t a i n r e g u l a r i t y m a y be p r e s e n t for s h o r t distances. Therefore, monomeric substances are only

crystalline if the orderly arrangement prevails for distances which are large compared to the dimensions of the atoms, ions or molecules.

This t h r e e - d i m e n s i o n a l p a t t e r n , a c c o r d i n g to w h i c h t h e r e g u l a r o r d e r i n g of t h e m o l e c u l e s t a k e s place, is called t h e space lattice. T h e r e a r e only f o u r t e e n different w a y s in w h i c h similar p o i n t s can b e a r r a n g e d in a r e g u l a r p a t t e r n ; only f o u r t e e n different space lattices t h e r e f o r e exist. T h e s e space lattices consist of a l a r g e n u m b e r of r e p e t i t i o n s of a basic p a t t e r n of a t o m s , t h e unit-cell.

T h e p o i n t s in a space lattice can b e a r r a n g e d in an infinite n u m b e r of w a y s in a series of p a r a l l e l a n d e q u i d i s t a n t p l a n e s , t h e net-planes or lattice-planes. I n c r y s t a l l i n e s u b s t a n c e s , p a r t i c l e s w i t h a m o r e or less s y m m e t r i c a l s h a p e a r e e v i d e n c e of t h e o r d e r l y i n t e r n a l s t r u c t u r e . T h e s e c r y s t a l s h a v e a fixed n u m b e r of p l a n e surfaces, w h i c h a r e p a r a l l e l to l a t t i c e - p l a n e s a n d c o n s e q u e n t l y inclined t o o n e a n o t h e r at c h a r a c t e r i s t i c angles. O n a c c o u n t of t h e v a r i o u s d e g r e e s of s y m m e t r y t h e c r y s t a l s a r e c o n v e n i e n t l y classified i n t o s e v e n symmetry systems, w h i c h m a y b e s u b d i v i d e d i n t o t h i r t y - t w o classes of crystal symm,etry.

T h e c r y s t a l s a r e h o m o g e n e o u s b u t anisotropic, t h e p h y s i c a l p r o p e r -ties, s u c h a s t h e r e f r a c t i v e i n d e x , a b s o r p t i o n of Ught a n d c o n d u c t i o n of h e a t a n d electricity, b e i n g in g e n e r a l u n e q u a l in different c r y s t a l directions. A t h r e e d i m e n s i o n a l r e p r e s e n t a t i o n of a n y physical p r o p e r t y a s a function of t h e d i r e c t i o n in t h e c r y s t a l is not, e x c e p t i n special cases, a s p h e r e , b u t a l e s s s y m m e t r i c a l figure.

T h e r e f r a c t i v e i n d e x is one of t h e s e p r o p e r t i e s . O n l y in c r y s t a l s possessing cubic s y m m e t r y is t h e r e f r a c t i v e i n d e x c o n s t a n t ; in all o t h e r c r y s t a l s t h e r e f r a c t i v e i n d e x d e p e n d s on t h e d i r e c t i o n s of

vibration and propagation of the light. These birefringent crystals resolve light into two components vibrating in planes at right angles to each other, the refractive indices in these planes being unequal. With plane polarized light, the refractive index in both planes can be determined separately.

When a crystal is observed between crossed nicols, the crystal will f'ppear dark if the light from the polarizer vibrates in the direction r i one of these planes. The light passes through the crystal unchanged and it cannot therefore pass the analyser. This is called the extinction

position of the crystal. If the crystal is rotated, it will be found that

the extinction positions for all crystals are 90° apart. In intermediate positions light from the polarizer is resolved into two components vibrating in the crystal's own vibration directions. When this light, consisting of two separate components, passes through the analyser, each component is again resolved into the vibration direction of the analyser, so that light emerges from the analyser vibrating in one plane. The crystal, therefore, transmits light in the intermediate positions. The two components in the crystal have different velocities. Entering the crystal they start in phase with each other, but on leaving the crystal, they are in general no longer exactly in phase. On reaching the analyser, they are both resolved into one plane of vibration and interfere with each other. The difference of the refractive indices is dependent on the wave-length. In polychromatic light, therefore, the difference in phase will cancel out completely light of only one wave-length. For other wave-lengths there will only be a reduction of intensity. For a given wave-length, the phase difference depends on the thickness of the crystal and the difference between the two refractive indices. If blue light is entirely cancelled out, the crystal will appear yellowish. If red light is cut out, a greenish colour will be seen. For increasing thickness of a crystal the observed colours are in the same order as that of the interference colours given by very thin films. This order is called "Newton's scale".

The principal method of studying the arrangement of the molecules ;-nd atoms is the interpretation of X-ray diffraction patterns. Electron diffraction patterns may provide the same information.

Crystals from monomeric substances usually give quite sharp diffraction spots. It may b e concluded, therefore, that crystals are usually regular for considerable distances. However, perfect crystals are never encountered and a slight broadening of the X-ray reflect-ions, which is observed, may be due to several causes. It may arise

because some crystals have slightly different unit-cell dimensions, due possibly to the presence of strain. Also, extremely small crystals

will cause a certain broadening. Structural irregularities on a very small scale and thermal motions of the molecules also give rise to a certain diffuseness.

Under certain conditions, some substances are able to form crystals of quite different internal structure. This phenomenon is called

poly-morphism. One of the structures is most stable under a given set of

conditions, but sometimes a meta-stable structure can exist for geological periods. Different atoms or ions of approximately the same size can sometimes replace each other in the lattice. An example is given by a mixed solution of ammonium sulphate and potassium sulphate, which deposits crystals containing any proportions of the two substances. This phenomenon is called isomorphism or mixed

crystal formation. The unit-cell dimensions of mixed crystals are

intermediate between those of the pure components.

The reason for the formation of a regular lattice is that atoms, ions or molecules tend to settle down in positions of minimum free energy. A particular arrangement has a lower free energy content than any other and this pattern is therefore taken on everywhere. Crystallization

liberates a certain amount of energy, the heat of crystallization. During the crystallization process the thermodynamic properties, such as enthalpy and entropy, change discontinuously. It is a phase transition of the first order.

Crystallization is regarded as originating in certain nuclei. These nuclei grow and form crystals. The rate of crystallization is dependent on the rate at which nuclei are formed and on the rate at which they grow. Although many phenomena are not yet fully understood, there exists a considerable amount of knowledge concerning the formation of nuclei and the mechanism of crystal growth in monomeric substances. The penomena in polymers are, however, very different; hence most of this knowledge cannot be applied to polymeric sub-stances and is therefore beyond the scope of this work.

The cohesion of crystals is not the same in all directions. Most crystals break almost exclusively along certain planes, usually those with a high reticular density of atomic or molecular packing and large interplanar spacing. The tendency to cleavage is also increased by small intermolecular forces between different planes. If a crystal is subjected to shear or tension, the initial deformation is purely elastic, following Hooke's law, but as soon as the stress reaches a certain value, the yield-value, the deformation is no longer proport-ional to the applied stress and the crystal starts to deform in a non-reversible way, by a process of slip along definite lattice planes. This slip creates irregularities, which hinder a further deformation. A

certain value of stress, therefore, causes a limited amount of slip. To continue the deformation, a larger stress is necessary. This is called work-hardening. It is very important in relation to the rheological behaviour of metals. The theories concerning these phenomena, however, cannot be applied to polymers, and are also beyond the scope of this work.

§ 2. Liquid Crystals.

A number of organic substances melt sharply to a turbid liquid and on further heating an equally sharp change into a normal liquid occurs. On cooling the reverse takes place at the same temperatures. The turbid liquid is double refracting and gives interference patterns in polarized light. The name "liquid crystals" was therefore proposed.

Apart from double refraction, the liquid crystals possess no crystalline properties and various other names were therefore suggested. The best is probably the term "mesomorphic state". Only long linear molecules are capable of forming a mesomorphic state. The molecules tend to arrange themselves parallel to each other, so that a number of groups or "swarms" is obtained in each of which there is a more or less definite orientation, but the arrangement in one swarm is not necessarily parallel to that in another. In a sense, the mesomorphic liquid may be regarded as a polycrystalline material. In each crystal, the orientation is definite, but the distribution of the crystal axes is random. The turbidity of the mesomorphic liquid is the result of the scattering of light by the swarms. The size of a given swarm is not constant and the molecules in the swarm are not held rigidly.

Sometimes the molecules in the swarms are arranged in parallel layers. This arrangement is called the "smectic state". The molecules are roughly perpendicular to the layers, but within each layer the separation between the molecules is not uniform.

In the "nematic state" the molecules are also parallel, but no longer in definite layers. There is an additional freedom of movement in the direction of the length of the molecules.

Most substances capable of liquid crystal formation have a transition point at which the smectic or the nematic phase are formed and a melting point at some higher temperature when formation of a true liquid phase takes place. A few substances, however, have more than one transition point. A good example is ethylanisol-p-aminocinnamate. From a meta-stable crystalline phase it transforms

to a smectic phase at 83° C. At 91° another smectic phase is formed, which in turn changes to a nematic phase at 118". Finally, a true melting point is reached at 139° C.

The smectic phase gives an X-ray diffraction pattern, but only in the direction perpendicular to the layers. The nematic phase only gives diffuse X-ray haloes like true liquids. Although the arrangement of the molecules is not at random, some orientation being present, there is no rigid three-dimensional lattice in liquid crystals. The mesomorphic state cannot be considered therefore as crystalline.

§ 3. Crystallization of High Polymers.

The first indication of crystallization in high polymers was given by the optical investigations of Ambronn •), which made it clear that submicroscopic, crystalline, oriented particles must be assumed to be present in cellulose fibres.

Definite evidence of this crystallization was procured by Scherrer 2), who obtained an X-ray diffraction pattern from ramie fibres. Nishi-kawa and Ono •^) and Hull *} had made the same discovery previously, but their work had attracted no attention.

Bunschoten'') observed an increase of the density of raw rubber at low temperatures. This phenomenon was investigated more ex-haustively by Van Rossem •>) and on the basis of this work Pickles'') suggested that the increase in density could be explained by a crystallization of the rubber. Nevertheless, the discovery of the X-ray diffraction pattern of natural rubber by Katz ') in 1925 was un-expected, because u p to that time rubber had been considered a typical amorphous colloid.

A large number of natural and synthetic linear polymers is now known to display this sudden change in volume, enthalpy, entropy and other thermodynamic properties, when they are cooled below a certain temperature. The X-ray diffraction patterns reveal a three-dimensional lattice after this change has taken place. Hence, we are justified in calling this first order transition, crystallization. Apart from this rough similarity, however, many profound differences exist between the crystallization of monomeric substances and the crystallization of polymers.

Linear polymers can only crystallize with the molecule in parallel positions. However, the molecules vary in length and therefore crystals with sharp boundaries cannot exist. The longer molecules will extend into the amorphous surroundings and a sharp isolation of the crystalline phase from a melt or solution is impossible. Besides,

the molecules are so strongly entangled that a complete disentangle-ment cannot occur during the crystallization process, resulting in a lattice with many irregularities. These irregularities are the principal reason why high polymers have a melting range instead of a melting point. Whatever the interpretation of the X-ray diffraction, the distances along which an orderly arrangement prevails are certainly not large in comparison to the length of the molecules. Consequently

it may be concluded that not molecules but individual members of the chain are the crystallizing units.

The crystallizing units in monomeric substances, the atoms, ions or molecules, can move separately. All theories concerning the formation of nuclei, the growth of crystals and the deformation of these crystals suppose this freedom of movement for the crystallizing units. In high polymers, however, the crystallizing units cannot move separately and hence none of these theories is applicable to crystalline polymers.

The X-ray photographs of crystalline polymers reveal not only a rather diffuse diffraction pattern, but also a diffuse background as is found in amorphous materials. As the positions of the reflections do not shift when the polymers are swollen by liquids **), it has been concluded that crystalline polymers consist of a two-phase system. The chain molecules are considered to align for short distance, forming

crystalline regions, alternating with less well-ordered amorphous regions. A crystalline region is called a micel or a crystallite.

The molecules are much longer than the crystallites and a molecule therefore passes through several crystalline and amorphous regions. According to this concept, the crystallite or micel has no sharp boundaries but the transition from a crystalline into an amorphous region is gradual. This "fringed-micel" has been postulated by Her-mann and Gerngross"). In this way, the simultaneous presence of X-ray diffraction patterns and diffuse haloes may be accounted for. Liquids only penetrate the amorphous regions and swelling therefore has no influence on the diffraction pattern of a crystalline polymer. The mechanical properties are also attributed to this structure, the hard and brittle crystallites providing the strength, and the amorphous regions the extensibility and flexibility of the polymeric substance. A historical review of the development of the micellar theory has been given by P. H. Hermans *").

It follows from the diffraction theory that the width of the X-ray interferences of a crystalline material increases with decreasing size of the crystals. Hengstenberg and Mark ' ' ) calculated the size of the crystallites in ramie and rubber from the width of the interferences. This width, however, is also influenced by the perfection of the

crystal lattice in the crystallites and therefore these calculations may not be considered valid, and certainly do not constitute proof of the existence of the crystallites.

It is also a doubtful question whether the small-angle interference maxima are to be interpreted as a measure of mean distances between the crystallites. To do this Wallner '^) has to resort to assuming that the crystallites are unstable, whereas, on the contrary, it is presumed on the evidence of the mechanical properties of the high polymers that a crystallite is stable and pjermanent.

Moreover, attempts to determine the size of the crystallites by means of electron microscopy did not lead to definite results. Ranby and Ribi '•') obtained particles from hydrolised cellulose, showing a structure and size strongly reminiscent of the crystallites. However, P. H. Hermans " ) , measuring the degree of crystallization of these particles, found the same value as in the original cellulose. If these particles were to be indentified with the crystallites, a much higher value should have been found.

Finally, in some synthetic polymers very high degrees of crystal-lization have been found. For example, Hoffmann '•'') found 82 zt 7 % crystalline material in polychlorotrifluoroethylene and Buckley et al. '^) found as much as 95 % in polymethylene. Such high percentages make it doubtful whether the crystalline phase can be discontinuous. It appears that the existence of crystallites is purely hypjothetical and definite evidence has never been found.

§ 4. The Second-Order Transition Point.

At high temperatures, dependent on the type of polymer, the molecules can move as a whole. This is the so-called macro-Brownian

movement or macroflow. It is accompanied by a p)ermanent

deform-ation of the material, when an external force is applied. At lower temperatures, or when the molecules are cross-linked, this macroflow is hindered. Small segments of the molecules, however, may still be able to move. This is the micro-Brownian movement or microflow. This microflow enables a deformation of the substance to occur without a displacement of the molecules as a whole. The deformation only results in an orientation of the molecules, which decreases the entropy of the system. As soon as the deforming force is removed, the segments return into a configuration having a larger entropy. The deformation is only tempwrary and the material returns to its original shape. The visco-elastic properties of high polymers can thus be explained.

At still lower temperatures, the thermal movements of the molecules are decreased to such an extent that the positions of the segments become more or less fixed by cohesive forces and steric hindrances. The molecules can no longer reach their equilibrium positions within times which are normally used when conducting experiments. A change in slope is then manifested when any of the primary thermo-dynamic properties, such as volume, is plotted against temperature. It is only a discontinuity in the first derivative of the primary thermo-dynamic properties, and consequently the temperature at which this discontinuity occurs is called the second-order transition point. The value of the second-order transition temperature depends on the speed of the test with which it is observed. It is a non-equilibrium phenomenon and not a real thermodynamic singularity. Some poly-mers display a rather sharp second-order transition point, but, in general, the transition occurs over a range of temperatures.

This hindrance of the microflow by lowering the temperature causes also a change in the rheological properties. The molecules can no longer orientate when an external force is applied. The substance hardens gradually, when it is cooled, until finally a point is reached where the rubber-like properties have disappeared completely. The sample breaks when a force is applied suddenly. This is called the

brittle point of the polymer. The brittle point is usually somewhat

higher than the second-order transition point, because the speed of the tests with which the brittle point is determined is higher than the speed of the dilatometric measurements with which the second-order transition point is usually determined.

A quantitative theory, which enables a prediction of the second-order transition pxjint or the brittle point from the chemical and physical structure of the polymer, has not yet been evolved. Never-theless many qualitative relationships between the properties of the molecules and the position of the second-order transition point have been established: The second-order transition point is raised by strong secondary bonds between the chain molecules and is lowered by increased flexibility of the chains. The flexibiUty is stimulated by the presence of double bonds and decreased by steric hindrance, due to bulky side-groups. The second-order transition point increases with increasing molecular weight u p to a certain value. Plasticizers may lower the second-order transition point considerably.

More detailed information concerning second-order transitions in high polymers can be found in the comprehensive reviews by Boyer and Spencer ' ' ) and by Boyer i**).

second-order transition point and in the crystalline state. Hence, the differences between crystalUne and amorphous material are more easily observed above the second-order transition point. Moreover, all kinds of non-equilibria may easily occur below the second-order transition point, and complicate investigation. That is why, in the course of this work, polymers with a low second-order transition point have always been used to investigate crystallization phenomena.

SURVEY OF THE LITERATURE ABOUT SPHERULITES IN HIGH POLYMERS.

§ 1. The Discovery of Spherulites.

A spherulite is an assemblage of crystals radiating from a point in all directions. Usually it is due to the growth of a large number of crystals from a certain centre, all with the same crystallographic axis pointing outwards. When seen in the polarizing microscope between crossed nicols the spherulites display a dark Maltese cross in an illuminated field. The arms of the cross are parallel to the vibration directions of polarizer and analyser. This is due to the fact that the crystals are here at or near their extinction positions. In intermediate positions the crystals are illuminated.

The occurrence of spherulitic structures is not uncommon among monomeric substances. They have been observed in sulfur •") and selenium ^o) and in some organic substances which can form hydrogen-bonded chains ^i).

A thin film of a polymer can easily be made by melting a small quantity between two coverslides. When such a film is cooled, it has been observed that spherulites grow outwards from certain points, the boundary at any one time being circular and quite sharp. Event-ually they meet and form linear boundaries. Fig. 1 gives an example of the ultimate result of this process with gutta percha. When the film is too thick, overlapping of the si)erulites may result in a confused mottled appearance.

The spherulites can even be seen in unpolarized Hght ^'^). It is possi-ble to improve the visibility in unpolarized light with dyestuffs ^3). Spherulites have also been noticed in the surfaces of nylon and poly-ethylene, by the uneven reflection of incident light ^'^, ^^, ^^).

In very thin films of polyethylene ^^), polyamides ^^) and poly-caprolactam ^s), the spherulites are visible with the electron microscope. The observed structures have very small diameters and can therefore only correspond with the centres of the spherulites seen in the polarizing microscopje. Finally it should be mentioned that the spherulites are readily observable by means of phase-contrast microscopy.

The spherulites in high polymers have already been known for a considerable time. Tschirch ^^), while investigating gutta percha,

Fig. 1. Normal spherulite of gutta percha between crossed nicols. Crystallized at 18° C. Magnified 320 X.

mentions certain spherulitic structures as far back as 1905. The first photographs of spherulites were published in 1929 by Kirchhof^i), who obtained several spherulitic structures when crystallizing gutta percha from solutions. A few years later Smith, Saylor and Wing observed spherulites in natural rubber •*-, *3).

So far spherulites have been seen in gutta percha, natural rubber, Neoprene •'•*), polyethylene -''), polychlorotrifluoroethylene -^, '"o) polytetrafluoroethylene '^^), polyamides ^'', **'), polyurethanes ^^), linear condensation products of urea and formaldehyde •'^) and poly-esters •*", *"). SpheruUtic structures have also been observed in solutions of synthetic polypeptides 9») and amylose'"'). Recently the author has found spherulites in rubber hydrochloride and poly-vinylidene chloride. According to B u n n ' " ) , the spherulitic structures have been observed in a sufficient number of polymers to give the impression that it is the normal manner of crystallization in high polymers.

§ 2. The Structure of the Spherulites.

The crystallization of polyethylene terephthalate can be interrupt-ed at any time by quenching the polymer. When the partially crystallized polymer is treated with suitable solvents, the spherulites remain unchanged, while the embedding amorphous material dissol-ves. In this way the spherulites can be isolated. They prove to be

spheres with a fibrous fine structure ^s). Isolated spherulites of poly-ethylene have been obtained by slowly cooling a solution in xylene *'^). Isolated spherulites of rubber hydrochloride were made by the author by crystallizing amorphous rubber hydrochloride dispersed in water. Both kinds of spherulites are also spheres. From this evidence it may be concluded that the spherulites in high polymers are spheres.

According to Champ)etier and Bonnet ^s), cold drawing causes the spherulites in polyhexamethylene adipamide to disappear, but they came to this conclusion after only observing the specimens per-pendicular to the direction of stretch. The behaviour of the spherulites on cold drawing has also been observed by Langkammerer and Catlin 36). Hexamethylene adipamide filaments with spherulites embedded in optical clear material were extended by 400 %. After this cold drawing, the spherulites were no longer visible in longitudinal sections, but alternating light- and dark streaks could be seen between crossed nicols. In the cross-section, however, although there was some distortion, many of the crosses still appeared to be perfectly formed. On drawing, the spherulites change from spheres into elongated cylinders, maintaining an orderly arrangement about the axis of the cylinder.

The way in which the molecules are arranged in spherulites can be determined from their optical properties. Spherulites of poly-ethylene are optically negative, the refractive index being lower for light vibrating along the radius than for the tangential vibration direction. From the properties of drawn fibres in which the molecules are parallel to the fibre axis, it is known that the refractive index is lower for the vibration direction perpendicular to the molecules than along the chain axis. Hence, the molecules in the spherulites of polyethylene are oriented perpendicular to the radius ^s).

The spherulites of natural rubber, gutta percha, rubber hydro-chloride, polyurethanes and polychlorotrifluoroethylene are also negative. The spherulites of px)lyvinylidene chloride are optically positive as was shown by the author. However, oriented fibres of polyvinylidene chloride have their lowest refractive index for light vibrating along the direction of the chain axis. Hence, it may be

concluded that the molecules in the spheTulites of high polymers are tangentially oriented.

In some polyamides both positive and negative spherulites have been seen, sometimes side by side in the same specimen *3). Positive spherulites do not necessarily mean radial orientation of the chain molecules in these polyamides, for the optical proi)erties of

pxjlyhexa-methylene adipamide are such that a positive spherulite would also be formed if the direction of the radius were along the hydrogen bond direction, which is normal to the chain axis. However, Herbst '•'') succeeded in passing a narrow X-ray beam through portions of large spherulites of polyhexamethylene adipamide and polycaprolactam. The diffraction patterns showed that the hydrogen bond plane is roughly normal to the direction of the radius. It seems therefore that the explanation mentioned above is not correct. According to Herbst, the molecules in spherulites should be p)erpendicular to the radius, but at random in the other directions. This is in accord with the fact that the double refraction of spherulites is low compared to that of highly oriented fibres. Another possible cause, which has been suggested by Bryant *^) and Brenschede *^) is "form birefringence", where a system of regions with a small refractive index, such as radial cracks, may account for the existence of these positive spheru-lites. The actual birefringence of a spherulite would be the net result of an intrinsic birefringence contribution due to the orientation of the molecules and a positive form birefringence due to regions of low density. Bunn *>) suggests that a cause of positive spherulite bire-fringence is radial strain birebire-fringence due to contraction on crystallization. A further possible explanation is that the polyamides crystallize in different modifications, one modification showing the normal negative spherulites and the other modification the positive ones. In oriented fibres this modification should have the lowest refractive index for light vibrating in the direction of the molecules. In view of the multiplicity of possible causes, the interpretation of the optical properties of these spherulites should be approached with caution until more evidence is available.

However, not all spherulites of high polymers display the normal Maltese cross as a result of the (normal) tangential arrangement of the molecules. A number of exceptions have been found recently.

For the sake of brevity all spherulites with the molecules arranged tangentially will be called normal spherulites, whereas the term

special spherulites will be used for all other spherulites displaying

various peculiarities.

Campbell and Allen 34) have noticed special spherulites in Neo-prene. In a certain position the spherulites showed a cross, the arms of which were of unequal thickness. When the specimen was rotated through 45°, the cross separated into two hyperbolas. This phenome-non was observed in a fihn, which was thick relative to the diameter of a spherulite. Hence some care must be exercised in explaining the structure, since a simple sphere of unannealed glass may give

quite complicated effects under the polarizing microscope *^).

According to Keller -'s), polyethylene terephthalate crystallizing at 100° C. to 130° C. gives normal spherulites. Crystallization at higher temperatures transforms the arms of the Maltese crosses into zig-zag lines, the angular extension of the zig-zag lines increasing with temp)erature of crystallization. At 239° C. the zig-zags extend over an arc of 90° and their ends meet, resulting in a spherulite with concentric black circles and apparently a black cross at 45° to the directions of vibration. Above 239° C. the patterns become even more complicated until at 260° C. no more spherulites are formed, but only fibre bundles. Spherulites with a zig-zag cross have also been observed by Allen ^s) in polyamides crystallizing from solution.

On crystallization at 70° C. polyethylene produces spherulites which are manifested by dark concentric circles (Fig. 2). These spherulites have also been observed by Keller 3»). It appears that the rings are also visible in unpolarized light. With polarized light only those sectors of the rings are visible which are more or less parallel to the vibration direction of the light. The exposures of this phenomenon were too indistinct to be reproduced and a sketch will have to suffice

(Fig. 3).

Spherulites of polyglycol adipate with black concentric rings have been seen by Jenckel et al. *") and Point*").

Fig. 2. Polyethylene (alkathene 20), crystallized at 70° C, between crossed nicols. Magnified 900 x .

It would seem that special spherulites are formed preferentially at temperatures near the melting point of the polymer and also when these special spherulites originate from a solution.

Under these conditions gutta pjercha forms several kinds of special spherulites which will be dealt with in detail in Chapter 3.

Fig. 3. Sketch of the sectors of the rings in Fig. 2 which remain visible if the light is polarized in the direction of the arrows.

§ 3. The Growth and the Melting of Spherulites.

Spherulites only grow in unoriented px)lymers. When an orientation of the molecules exists, crystallization results in a parallel orientation of the crystallites and spherulites cannot be observed.

When an unoriented amorphous film of polyethylene, polychloro-trifluoroethylene or gutta percha is observed through the polarizing microscope between crossed nicols, it can be seen that an overall grayness develops suddenly from the completely dark background, and that the spherulites appear much more slowly out of this general gray field. Price ^2) explained this phenomenon as the sudden develop-ment of many small randomly oriented crystalline regions, having dimensions of about 500—1000 A° and too small to scatter the polarized light appreciably. This view was substantiated by an X-ray examination of polychlorotrifluoroethylene, which indicated that an appreciable amount of crystallinity had occurred before any spheru-lites were present. However, this sudden development of random crystallites has not been observed in rubbers. Also, Keller ^9) has been unable to observe crystallinity by X-rays in the clear regions surrounding the spheruhtes in partially crystalUzed polyethylene terephthalate.

No overall grayness, or X-ray crystallinity, could be observed in quenched polychlorotrifluoroethylene. Still, this quenched polymer

develops no spherulites when it is heated. This same phenomenon has been observed in polyethylene and gutta percha. Quenching apparently results in the formation of nuclei too small to scatter the polarized light appreciably or to show X-ray interferences. Annealing, however, enables these nuclei to grow and to attain a size big enough to give X-ray reflections ^^^ 49)^ but even after prolonged heating, the reflections are more diffuse than the reflections of a normally crystallized polymer containing spherulites.

The growth of a spherulite starts from a certain centre as may be seen in Fig. 4, which shows the birth of a spherulite in gutta pjercha at room temperature, as observed by the author. Initially a small illuminated point is seen between crossed nicols. This point grows normal to the direction of the molecules into a streak (1), which develops into a sheaf-like structure (2). Thus the molecules in this sheaf are oriented in the transverse direction. On both sides of this sheaf illuminated regions develop (3). With a gypsum plate of red of the first order, addition is seen in the sheaf and subtraction in the side regions. Hence the molecules in the patches are perpendicular to the molecules in the sheaf. The sheaf and the patches grow out-wards until the spherulite is completed (4, 5, 6). The radius of a spherulite is usually about 50 micron. However, small spherulites with a radius of a few microns and very large spherulites visible with the naked eye have been observed.

The sheaf-like structures have also been observed with the electron

Fig. 4. The growth of a normal spherulite of gutta percha at 18° C, observed between crossed nicols. Magnified 930 X.

microscope 2^, ^^, ^^). It is, however, not certain that the sheaf is present in all kinds of spherulites. Jenckel and Klein -'^) have reported the occurrence of small spheres, having no pronounced internal struc-ture, in the centre of spherulites from polyurethanes, but these spheres could not be reproduced afterwards *"). Brooks and Mat-thews 27) J however, have shown that no sheaf-like structures can be observed in an electron-micrograph of polymethylene.

From the fact that meeting spherulites form linear boundaries, Brenschede "'') came to the conclusion that all spherulites in a given specimen have an equal growth-rate.

Price 22) was able to ascertain that the diameter of the spheruhtes of polychlorotrifluoroethylene depends linearly on time, indicating that the growth process is not diffusion controlled, for, if it were, the dependence should be on the square root of time. The radial rate of growth of polyethylene terephthalate is 1.6 microns/min. at 220° C. and 7 microns/min. at 185° C. •>''). The same measurements have been made with gutta percha. At 29° C. the radius increases at a rate of about 7 microns/min. The rate of growth is strongly depiendent on temperature.

The spherulites in polymers arise only under conditions of appre-ciable supercooling.

When a specimen containing spherulites is heated, the spherulites disappear gradually and as a whole. The separate melting of certain segments of spherulites has never been observed. The general outline of the spherulites remains until the last crystallites have melted. When a specimen is heated a few degrees above its melting point for a short time and then cooled, the spherulites reappear in the original positions. This phenomenon has been observed in poly-ethylene, polychlorotrifluoropoly-ethylene, polyamides and gutta percha. Hence it may be concluded that the spherulites persist even a few degrees above their melting point as zones of imperfect radial orientation •'>"). It could be observed that under these circumstances the spherulites in gutta percha reappear as a whole. If the tem-perature has been raised a few degrees more above the melting point, recrystallization gives a mottled appearance without spherulites. The zones of radial orientation are disintegrated, but some parts seem to persist. If still higher temperatures are used, recrystallization results in a completely new set of spherulites.

§ 4. Former Explanations of the Formation of Spherulites.

The monomeric substances in which spherulites have been observed, such as sulfur, selenium and certain organic substances, have a

tendency to form linear aggregates. Spherulitic structures have also been observed in organic substances capable of forming liquid crystals, such as azoxy-phenetol'''). This, together with the fact that the spherulites are so frequently observed in high polymers, suggests that a linear molecule favours the formation of spherulites, but a theory to explain the occurrence of spherulites in monomeric sub-stances has so far not been presented. However, many suggestions have been made to explain the spherulites in high polymers, and they will now be discussed:

The first tentative explanation was given by Rosevaere et al. 52). Their reasoning is as follows:

Fig. 5. T h e growth of a crystallite according to Rosevaere, Waller and Wilson.

"The initial crystallites tend to grow in their longitudinal direction by reason of the fact that elementary units of the macro-molecules engage on the principle of a zip fastener. This process produces an organization similar to that which would be obtained by passing a comb through the disordered material in one direction as far as possible before the entanglement being pushed ahead of it prevented further movement of the comb, and then pushing it as far as possible in the opposite direction. Crystallization by this mechanism would produce a locahzed accumulation of disorder at the ends of the crystallites, greater than that which is present in the original material. Crystallite growth produces changes in the surrounding regions through which the crystallizing chains pass. A chain crystallizing in a manner similar to the closing of a zip fastener pulls the uncrystallized part towards

the point of crystalhzation, and this pulling at portions of the chains, changes their orientation near the crystallite. As crystallization progresses chain A of Fig. 5 is pulled into the position shown by the dotted line. On the other hand the slack or disorder in the chains, which is for the most part directly forward from the point of crystal-lization, is pushed ahead, giving the high disorder at the ends of the crystallites. Other regions near the crystallite will be affected to a degree intermediate between these two extremes. The increased orientation beside the first crystallite increases the probability that a crystal nucleus will form in that region and that the orientation will be parallel to that of the first one. This effect tends to produce one crystallite beside another, thus forming a chain of holes and crystal-lites, with the crystallites arranged as the rungs in a ladder. The concentration of disorder at the ends of the crystallite spreads out much wider than the crystallites themselves and these spreading parts inhibit adjacent crystallites from growing longer than the first one. This effect tends to make the crystallites of a given row more nearly the same size. The pulling of chain A during the formation of the

; >^' . ^ > -•'nil M i n i w/^ _ - z Cr 3 i ' ' / / I II in III 111

s^

first crystallite in Fig. 5 may produce a small amount of orientation in the region Y, parallel to the original nuclei, thus promoting the tendency to form a second row of crystallites parallel to the first one. Fluctuation in the ordering of the polymer just before crystallization will tend to cause deviations of these rows from straight lines and the growth of several rows of initially parallel crystallites will eventu-ally lead to the spherulitic structure shown in Fig. 6."

Several objections may be made to this explanation. It is presumed that the macro-molecules engage on the principle of a zip fastener, but any such mechanism has never been proved. Secondly the explanation why the crystallites in a given row have approximately the same size sounds somewhat naive and the same may be said about the reasoning to explain how several rows of parallel crystallites come into being. Also, the photographs of Fig. 4 do not agree with this theory. Moreover the existence of special spherulites is in flat contradiction to this explanation.

Jenckel and Wilsing 2») made different assumptions in an endeavour to account for the existence of spherulites. They assumed that the molecules are oriented parallel to the radius of the spherulite, but as the molecules are usually oriented tangentially, their argument falls to the ground.

That is why Jenckel and Klein 2:!) have proposed a new explanation, based on the assumption that the molecules extending from an original crystallite should transfer an activation energy preferentially along the chains to the surrounding regions. In this way, these authors can account for the spherical shape of the spherulites. It could also be explained why spherulites do not occur in very thin films of poly-urethanes, the molecules giving their activation energy to the micro-scope slides of the specimen. However, this absence of spherulites can also be explained from the fact that a certain tendency exists for the molecules to crystallize parallel to the surface of the film. This has been denominated trans-crystallization by Jenckel, Teege and Hinrichs *''). Thin films of polyethylene, which show no trans-crystallization, have indeed the normal spherulitic structure.

If the molecules conduct the activation energy preferentially along the chain, one might exp>ect that the thermal conductivity of oriented polymers would be exceptionally high in the direction of orientation and very low in the perpendicular directions. Dauphinee, Ivey and Smith 54) found a slight increase of the thermal conductivity in the direction of extension of natural rubber, stretched 100 %. GR-S, how-ever, showed under these conditions a pronounced decrease of the thermal conductivity of 30 %. Thus the experiments so far reported

are not conclusive on this point and do not substantiate the view that thermal conductivity should be exceptionally high in the direction of stretch. Whatever results an extensive investigation of the thermal conductivity yield, the theory of Jenckel and Klein is unable to explain why the molecules in normal spherulites are oriented perpendicular to the radius, let alone the quite complicated structures of the special spherulites.

Price 22) also assumes that the elementary units of the chain engage on the principle of a zip fastener. Consequently the crystallization in the direction parallel to the molecules should be very fast. Thus when a number of crystallites starts to grow out from some centre, if their direction of slow growth is radial, the spaces between these crystallites will fill in very rapidly and a sphere will be formed. The centre may be a heterogeneity or an assemblage of some nuclei having by chance a favourable structure. A separated nucleus, which does not have the opportunity to form a sphere with neighbouring nuclei, should grow in the longitudinal direction into a very thin plate or needle and become unobservable.

If the above reasoning is correct, it is difficult to see why a crystalline polymer does not consist completely of these thin plates and needles. It is even more difficult to visualize how the spherulites could grow through this irregular network of plates or needles to the regular structures which they actually possess. Moreover the experi-mental evidence presented in Fig. 4 is at variance with this theory, as it can be observed that the direction of rapid crystallization is pjerpendicular to the direction of the molecules. Finally, it is also contradicted by the existence of special spherulites.

In view of the spjecial spherulites observed in polyethylene tereph-thalate, Keller 39) supposed that they consist of spirals, without describing the structure in any detail, or explaining how this struc-ture came into being.

The most probable explanation so far has been given by Bunn * ' ) : Crystal nucleus formation starts at those comparatively rare places where sections of a number of molecules happen to be lying parallel to each other. Crystal growth occurs longitudinally by progressive straightening along the same molecules, and laterally by accretion of sections of other molecules. Owing to the tangled character of the mass of chain molecules, growth along the chain molecules cannot proceed very far; nevertheless, as soon as portions of some molecules are locked in crystals, neighbouring sections of these same molecules being to some extent constrained, are the more ready to act as centres for the formation of new crystals, which would be oriented

different-ially from the original crystal. Growth in lateral directions would also be hampered by the molecular tangle; distortion seems inevitable, and as a result, a particular lateral crystallographic direction, if it could be traced, would be expected to follow a meandering path through the mass. Both processes lead to the formation of variously oriented crystal regions around the original centre. Natural selection completes the process of spherulite formation and determines which crystal direction becomes the radius of the spherulite, the most rapidly growing direction becoming predominant along radii. Thus, Bunn presumes that crystallization is faster in lateral directions than in the longitudinal direction of the molecules.

Although this theory is not directly at variance with Fig. 4, it is still somewhat difficult to explain the formation of the centre of a spherulite on this ground. The occurrence of special spherulites, however, is experimental evidence that the growth of spherulites is not simply governed by a natural selection process.

Summarizing, it may be concluded that all the explanations so far proposed are at variance with some part of the experimental facts. Also, none of these explanations has been co-ordinated with other aspects of the crystallization process.

THE MECHANISM OF CRYSTALLIZATION.

§ 1. The "Auto-orientation" of Macromolecules during Crystalli-zation.

It is common knowledge that melting of a highly oriented crystalline polymeric specimen (such as a monofilament) results in a shrinkage in the longitudinal direction, which may amount to several hundred per cent. The dimensions in the other spacial directions increase correspondingly.

To a less extent, the reverse takes place when oriented polymeric materials crystallize. Thiessen and Wittstadt 57) observed a length-ening when an oriented strip of natural rubber crystallized. This phenomenon was also investigated by Smith and Saylor **). Strips of natural rubber were stretched by different amounts and nailed to a board. The stretched samples were placed in a cabinet at —25° C. After a few hours these samples had increased in length by 2—4 per cent. Samples of the product from the reaction between dichloro-ethylether and sodiumpolysulfide, a thiokol, also exhibited this secondary extension after two weeks at —25° C , whether tested in the vulcanized or the unvulcanized condition. According to P a r k 5»), the maximum secondary extension (approx. 4 per cent) of natural rubber is found when the specimens are stretched 200—300 per cent. At 600 per cent extension the secondary extension is very small.

Reinhardt •>'•>) has found that if a supercooled filament of poly-vinylidene chloride is stretched to an elongation of 150—250 per cent and allowed to crystallize further, it will elongate slightly without the application of load. A secondary extension of 23 p)er cent has been reported by Brenschede •"•) on annealing of quenched fibres of poly-urethane, which had been stretched 60 per cent. It was found by the author that the presence of large amounts of plasticizers does not hamper the secondary extension of rubber hydrochloride. If super-cooled amorphous fibres of rubber hydrochloride, containing approxi-mately 40 per cent tricresylphosphate, are elongated from 20 to 80 per cent and allowed to crystallize at 20° C , a secondary extension takes place. The maximum of about 2 per cent is obtained after an elongation of 40 per cent. During the crystallization part of the plasticizer sweats out.

such as natural rubber, thiokol, polyvinylidene chloride, polyurethane and plasticized rubber hydrochloride, it is safe to assume that all strongly crystalline high polymers will display this same tendency in varying degrees.

It has already been stated in Chapter I that there is no experimental evidence for the existence of very small crystallites as the micellar theory assumes. For didactical reasons a new definition for crystallites will be used. Any amount of material in which the molecules He parallel is called a crystallite. The direction in which the molecules are oriented is termed the longitudinal direction of the crystallite. It is immaterial to the argument whether this crystallite in fact consists of several crystallites aligned in parallel, separated by a small amount of amorphous material, or of a single crystallite containing large irregularities. For the purpose of explaining the mechanism of crystallization, a crystallite will now be defined as a small quantity of strongly oriented polymer. Just as a monofilament, this crystallite will shorten considerably when it is melted. But, also, the reverse takes place during crystallization. To elucidate this point it is supposed that the cube in Fig. 7 represents a small quantity of amorphous material somewhere in a polymer. This quantity is so small that it can be a part of only one crystallite after crystallization has taken place. Let it be supposed that a nucleus develops in this cube, as the results of which the cube crystallizes with the molecules parallel to edge a. This orientation of the molecules must result in a stretching of the cube in this direction. This is called here the "auto-orientation" of the polymer. The elongation of the cube causes deformation of the surrounding material, which consequently continues to crystallize parallel to edge a. During the growth of this crystallite a considerable shrinkage takes place in the directions b and c. As material from the

a

V \

Fig. 7. Diagram of a cube of amorphous material which stretches on crystallizing. The arrows point to amorphous material which has to be supplied

surrounding has to be supplied, an orientation perpendicular to edge a is initiated, and as the result, growth parallel to edge a comes to a standstill. There remains then a residual orientation in the surrounding regions of this crystallite.

Treloar «i) thoroughly investigated the crystallization of natural rubber and found that the rate of crystallization of an oriented rubber far exceeds the rate of an unoriented one. The crystallites are then formed parallel to the direction of stretch. As all other crystalline polymers behave similarly, it may safely be assumed that the residual orientation at some distance from the first crystallite will result in the formation of nuclei, for the rate of crystallization in these regions exceeds the crystallization rate of unoriented material elsewhere in the polymer. Moreover, these nuclei are formed parallel to the direction of stretch, which means that these nuclei are perpendicular to the first crystallite. The new nuclei, generated by the orientation of the surrounding regions, will likewise grow and the process is thus repeated. In this way crystal growth and nucleus formation in poly-mers are interrelated. As a result the overall crystallization has an autocatalytic character.

§ 2. The Formation of Normal Spherulites.

For the purpose of explaining, on the basis of the auto-orientation mechanism described in the foregoing section, how spherulites come

i n t o e x i s t e n c e , it will b e c o n v e n i e n t to c o n s i d e r crystallization in a t h i n film, in w h i c h t h e c r y s t a l l i t e s lie in one p l a n e (Fig. 8 ) . C r y s t a l l i t e A l e a d s t o t h e f o r m a t i o n of c r y s t a l l i t e B , after w h i c h C a n d D c o m e into e x i s t e n c e . T h e s h r i n k a g e of t h e c r y s t a l l i t e s A a n d C in t h e h o r i z o n t a l d i r e c t i o n is at least p a r t l y c o m p e n s a t e d b y t h e e x p a n s i o n of B and D in t h i s direction. H e n c e t h e s e c r y s t a l l i t e s can n o w c o n t i n u e to g r o w and, b y reciprocity, will l a r g e l y c o m p e n s a t e t h e s t r e s s e s set u p b y t h i s g r o w t h . A s p h e r u l i t e f o r m e d in this w a y consists of at least four s e c t o r s w h i c h m u t u a l l y form angles of a b o u t 90°. I n a given position, a s p h e r u l i t e of t h i s k i n d is e n t i r e l y d a r k b e t w e e n crossed nicols: t h e c r y s t a l l i t e s a r e t h e n p a r a l l e l or p e r p e n d i c u l a r to t h e d i r e c t i o n of t h e p o l a r i z e r a n d t h e a n a l y s e r . W h e n t h e s p e c i m e n is r o t a t e d t h r o u g h a n a n g l e of 45° t h e s p h e r u l i t e b e c o m e s completely i l l u m i n a t e d . W i t h a g y p s u m p l a t e of r e d of t h e first o r d e r , a d d i t i o n is s e e n in t w o opposite sectors a n d s u b t r a c t i o n in t h e t w o o t h e r s . T h e s e s i m p l e " s q u a r e " s p h e r u l i t e s h a v e a c t u a l l y b e e n seen in g u t t a p e r c h a on crystallization at 45° C. (Fig. 9 ) . Their optical behaviour is in full a g r e e m e n t w i t h t h e a b o v e - m e n t i o n e d suggestion.

Fig. 9. Gutta percha crystallized at 45° C. between crossed nicols. When the specimen is rotated through 45° the "square" spherulite becomes dark in its

entirety. Magnified 740 x .

Normal spherulites in gutta percha a r e f o r m e d at r o o m tempjerature, w h e n there is a greater tendency to crystallize and the developing structures are not so ideal. Sometimes a sheaf-like crystallite is formed first, as m a y be seen from Fig. 4. At the sides of this sheaf n e w crystallites develop with their molecules perpendicular to the

mole-cules in the sheaf. The explanation of Fig. 4 on the basis of the auto-orientation mechanism is given in Fig. 10. First a single crystallite develops (1). At both ends of this crystallite too much material is present, whereas on both sides a shortage exists. Hence the material starts to flow as the arrows in Fig. 10 indicate (2). The crystallite continues to grow in the lateral directions, but as the flow of the

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Fig. 10. Diagram of the growth of a normal spherulite. Compare with Fig. 4. The dotted lines indicate the direction of orientation of the molecules.

material causes a pre-orientation of the molecules, the boundaries of the crystallite become more or less curved (3). The reproduction of this phenomenon in Fig. 4 is unfortunately not very clear. Finally so much material has flowed from the regions at the ends of the first crystallite that new nuclei are formed here with their molecules perp)endicular to the molecules in the sheaf (4). Now the crystalhtes are able to comptensate each other's stresses and grow radially into a normal spherulite (5). It is clear also that a number of crystallites, by chance arising in the right positions, can act as the centre of a spherulite. Thus the formation of normal spherulites is in entire agreement with the mechanism of crystallization suggested.

The only condition for the formation of spherulites is that a number of crystallites are formed at large angles until a favourable configurat-ion has developed at the periphery, enabling the crystallites at the site to compensate each other's stresses in their further growth. This compensation may take place at a considerable distance, for the spherulites easily attain a size amounting to several tens of microns.

Films of strongly crystalUne polymers, such as polyethylene, gutta percha, polyamides and polychlorotrifluoroethylene, show an overall grayness before the spherulites develop. There is apparently very active nucleus formation in these polymers and many randomly oriented small crystals are formed. They can, however, only continue to grow in relative positions favourable to the mutual compensation of stresses and it is here that the centres of the spherulites are generated. The random crystallites will either be destroyed or be oriented in the correct direction and absorbed in the spherulites. The above explanation of the formation of a spherulite falls into line with the fact that the growth is not governed by diffusion. It also explains why melting causes a spherulite to disappear as a whole. A single sector cannot melt separately; in that case the whole structure would collapse.

The spherulites are the result of the flow of the molecules, enabling a maximum number of individual members of the chains to be arranged in the crystal lattice. Hence the viscosity of the system

during the crystallization process is of great importance. When a polymer is quenched to below a certain temperature, it becomes too viscous to permit this flow and consequently no spherulites are formed. If the quenched polymer is heated, the development of spherulites is still prevented. Price 22) made it clear that this is due to the formation of too many or too stable nuclei. X-ray exposures indicate that under these circumstances the crystallites remain small and randomly oriented. There are evidently two temperature limits

for t h e f o r m a t i o n of s p h e r u l i t e s . T h e u p p e r limit is s o m e d e g r e e s b e l o w t h e m e l t i n g point of t h e p o l y m e r , b e c a u s e t h e s p h e r u l i t e s arise only u n d e r conditions of a p p r e c i a b l e supercooling. T h e l o w e r limit is t h e t e m p e r a t u r e at w h i c h t h e m o l e c u l e s a r e n o l o n g e r able to flow. T h i s t e m p e r a t u r e will b e s o m e w h a t a b o v e t h e s e c o n d - o r d e r t r a n s i t i o n point.

T h e e s s e n t i a l f e a t u r e of t h e c r y s t a l l i z a t i o n of h i g h p o l y m e r s is t h e a u t o - o r i e n t a t i o n of t h e m o l e c u l e s . A p a r t from a few e x c e p t i o n s , w h i c h will be m e n t i o n e d a f t e r w a r d s , all p o l y m e r s will e x h i b i t t h i s m e c h a n i s m ; h e n c e all crystallizing p o l y m e r s will h a v e a s t r o n g t e n d e n c y to g e n e r a t e s p h e r u l i t e s . A l t h o u g h o u r a r g u m e n t d e a l s w i t h s p h e r u l i t e s i n t h i n films, it is o b v i o u s t h a t in p r i n c i p l e , t h e s a m e t h i n g h a p p e n s in space. T h e r e t h e d e f o r m a t i o n s of t h e v a r i o u s sectors c o m p e n s a t e e a c h o t h e r in t h e s p h e r i c a l configuration.

§ 3. The Structure of Special Spherulites.

Normal spherulites are usually obtained when a polymer crystallizes far below the melting point, but with crystallization at temperatures in the neighbourhood of the melting point, spherulites are found which differ from each other in a marked degree according to the conditions and the nature of the polymer. In Chapter 2, section 2, many of these special spherulites have already been mentioned. Knowledge about high polymers is still too imcomplete to explain why under certain conditions a given type of spherulite should come into existence. Nevertheless, these special spherulites will be considered here to see whether their formation is compatible with the general mechanism. Firstly several types of special spherulites of gutta percha will be treated:

a) Even after crystallization at room temperature, spherulites with

Fig. 11. Diagram of a spherulite which is formed if there is some degree of orientation in the amorphous material. Only the cross and the hyperbolas are

visible between crossed nicols. The arrows show the directions of analyser and polarizer.

a cross, the arms of which were of unequal thickness, have been seen in gutta percha and polyethylene. When the specimen is rotated through 45°, the cross separated into two hyperbolas because the crystalUne material is here in its extinction position (Fig. 11). Assuming that some orientation existed in the polymer prior to crystallization, the sectors will tend to develop in the direction of orientation. Thus the spherulites take on a configuration exhibiting the behaviour described above. In a specimen within which some degree of orientation prevails, all the "spherulites" are arranged with their longitudinal axis parallel to the direction of orientation. The exposures of this phenomenon were too indistinct to be reproduced and a diagrammatic sketch will therefore have to suffice. The cross and the hyperbolas in Fig. 11 are actually visualized. The direction of orientation of the crystallites is indicated by the dotted lines.

Spherulites of this same kind have been observed by Campbell and Allen 34) in Neoprene. However, it has been mentioned already that these spherulites in Neoprene have been obtained in specimens which were thick in comparison to the diameters of the spherulites, and that therefore some care must be exercised in explaining the structure, because simple unannealed spheres of glass may give quite complicated optical effects under the polarizing microscope ^5).

b) When gutta percha crystallizes at 48° C , spherulites of dendrite-like structure are formed (Fig. 12). There is no Maltese

Fig. 12. Gutta percha crystallized at 48° C, between crossed nicols. Magnified 150

X-Fig. 13. Growing spherulite of gutta percha, crystallizing at 48° C, between crossed nicols. Magnified 410 x .

cross a n d w i t h a g y p s u m p l a t e of r e d of t h e first o r d e r , t h e p i c t u r e p r e s e n t e d is one of a l t e r n a t i n g s m a l l c o n t i g u o u s s e c t o r s w i t h s u b -t r a c -t i o n a n d addi-tion. T h i s s -t r u c -t u r e c a n b e o b s e r v e d e v e n b e -t -t e r in Fig. 13. A n a r r a n g e m e n t of t h e c r y s t a l l i t e s in a c c o r d a n c e w i t h Fig. 14 w o u l d a c c o u n t for s p h e r u l i t e s of t h i s k i n d . T h e c r y s t a l l i t e s r e c i p r o c a l l y form rigUt a n g l e s a n d a r e a r r a n g e d in typical " h e r r i n g b o n e s " . T h e s e h e r r i n g b o n e s h a v e also b e e n o b s e r v e d v e r y c l e a r l y in n a t u r a l r u b b e r crystallizing a t 0° C. (Fig. 15). H o w e v e r , all c r y s t a l l i t e s w h i c h a r e in