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Macromolecular structure changes in polyethylene during reorientation
Maeromoleeular structure changes in polyethylene during reormntation
2659
(2) Both these linear and rubber-like polymers, with M>>M0, behaved hke crosslinked rubbers and their behavlour could be qualitatively described by the Bueche theory in the low frequency range of mechanical dispersion (3) A second, high frequency, range of mechanical dispersion was found in the studied rubber-like polymers and the most probable time lags were here shorter by 4-5 powers of ten than m the low frequency range (4) The Bueche theory does not take mto account the presence of a high frequency range of mechanical dispersion Translated by K. A. ALLEN REFERENCES l. J. FERRY, Vyazkouprugle svomtva pohmerov (Vlsco-elastm Propertms of Polymers). Izd. mostr, hr., 1963 2. A. TOBOL'SKII, Struktura 1 svolstva pohmerov (The Structure and Propertms of Polymers). Izd. " K h l m l y a " , 1964 3 K . S . COLE and R. H. COLE, J. Chem Phys 9: 34, 1941 4. E. R. FITZGERALD, L. D. GRANDINE and J. D. FERRY, J Appl. Phys 24: 650, 1964 5. L. TRELOAR, Flzlka uprugostl kauchuka (Physms of R u b b e r Elastmlty) ITd mostr hr., 1953 6 F. BUECHE, J. Chem. Phys. 22: 603, 1954 7 W. P. MASON, N. O. BACKER, H. T. McSKIMIN and J. H. HEISS, Phys Rev 75: 936, 1949 8 S . P . I(ABIN and G. P. MIKHAILOV, Zhur tekhn fiz 26: 511, 1956 9. K. PYUSUKE, J. Phys. Soe J a p a n 16: 1580, 1961
MACROMOLECULAR STRUCTURE CHANGES IN POLYETHYLENE DURING REORIENTATION* V S. KUKSENKO, S NIZAMIDINOV a n d A. I SLUTSKER A. F. Ioffe Physmo-technologleal Institute
(Recewed 25 July 1966)
A~ increasing amount of attention is given to the study of structural rearrangements occurring during the extension of crystalhne polymers, because the orientation process is of scientific and practical interest. The earlier, widely accepted theory about the "theological" nature of ormntation, according to which the structural elements of the previously unorientated polymer move and rotate during extension, is being reexamined using X-ray diffraction [1-5], electron micro* Vysokomol. soyed A9" No. 11, 2352-2357, 1967.
2660
V
S. KUKSE~KO et al.
scopic [6, 7], and optical [7, 8] methods of study, amongst others. The orientation of crystallizing solid polymers is, in the extreme, connected with the destruction of the original structure and the formation of a different one. The observable complexity of the macromolecular structure of crystallizing polymers prior to orientation makes it desirable to study orientation under conditions in which the initial structure would be relatively simple and could be characterized by means of a small number of parameters. This condition is fulfilled also during reorientation of an already orientated polymer, which consists of stretching the latter at right angles to the previous axis of orientation. It is Well known that macromolecular structures of different crystallizing polymers, or even of previously unorientated ones, will differ quJte a lot from those present in orientated polymers. Those already in an orientated state will show fairly similar types of simple structure. A typical features will be the fairly regular alternatmn, along the axis of orientation, of crystalline with amorphous regions (long periods). The axes of the molecular chains will be more or less aligned in the same direction, in the crystalline and amorphous zones, as the axis of orientation [9]. The study of the effect of stress applied at right angles to the previous orientation axis, leading to reorientation, could lead to a better understanding of the structural changes in general. There are few publications dealing with the study of reorientation, and one [1] in which X-ray diffraction was used with either a n a r r o w - o r a wide-angle beam, the polymer examined having initially had a relatively small degree of orientation (200% elongation); the axis of reorientation was not exactly at right angles to the previous ormntatioll. An earlier study by one of the authors of this paper [4] did not examine reorientation by stages, but analyzed the structure only in its initial and final stages. This paper reports the results of tracing the consecutive stages of reorientation. High pressure polyethylene was used in the shape of a 3 mm thick plate. This was subjected to mono-axial extension at 100°C up to the limit (about 5-fold stretching) A sample was cut out of this orientated plate in the shape of a spatula with a flat part at each end of a rod, and this was also stretched to the limit at room temperature. The different temperatures selected for the initial orientation and the reorientation aimed at obtaining a clearer picture of the changes, since the orientated structures of polymers extended at different temperatures would be similar, a difference was found in the macromolecular characteristics of structure (larger regions, larger dimensions of crystallites) [9, 10, 4]. The reorientation of the sample thus produced gave rise to a neck and was thought suitable for the study of the consecutive stages of reorientation by examining the structure present in different parts of the sample, moving from the primarily orientated part to the transitional, and finally the neck part. The structural exammatmn was made by X-ray narrow- and w,de-angle dlflractmn. T h e d i f f r a c t i o n p i c t u r e s were p r o d u c e d o n a n a p p a r a t u s h a w n g a s h a r p l y focused v a l v e a n d a " p r e c i s e " c o l h m a t m n o f t h e p r i m a r y b e a m , as d e s c r i b e d e a r h e r [11]. T h e l a t e r a l sec-
Macromolccular structure changes in polyethylene during reorientation
266I
tion of the beam had a diameter of 100/~. The X-ray pictures produced at the two angles were obtained b y placing a fiat sheet with an aperture at a short distance from the sample between it and the rachation source when using small angle scattering (Fig. 1). Thin type of diffraction study (and also the c x a m i n a t m n method of the ormntatmnal changes of structure b y taking pictures of different parts of the same sample) was similar to that described elsewhere [5].
a FIG. 1
b FIO. 2
FIO. 1. Scheme of the instrument with a micro beam for taking X-ray pictures at smalland wide-angles: 1--sharp-focusing X-ray tube, 2--diaphragms, J--sample, 4--photographic plate for taking pictures from a wide angle, 5--mask (iris) for the primary beam, 6--photo-film for small-angle photography. FTG. 2. Scheme of the X-ray diffraction picture taken after reormntatmn: I - - a x m of mltml ormntation, I I - - r e o r m n t a t i o n axm, III--pr]mary beam. The points on the samples denote the positrons whmh were photographed; a and b--sample pomtmns.
All the pictures were taken under vacuum using CuKa radiation (wavelength of 1.54 A). The samples were fixed onto swallow-tad cross wires and this made it possible to move the different parts within the same sample "smoothly" into the field of the beam. As the regmn of the transition zone between the part of initial ormntatmn and the neck had an axial distance of about 2 ram, the hght beam of 0.1 m m width ensured sufflcmnt "resolut m n " to observe structural variations. The sample was fixed in a direction at mght angles to the axm of reormntatlon m the beam and was moved in stages along thin axm. While holding the reormntatlon axis all the time at mght angles to the original, pmtures were taken at 2 different sample positions (a) the plane formed b y the axes of the first a n d second ormntatlon was held at right angles to the beam (Fig. 2a), and (b) the same plane being parallel with the beam (Fig. 2b). The pictures of the sample in positions (a) and (b) gave a certain idea of "space" of the structure.
EXPERIMENTAL
The small-angle X-ray diffraction pictures shown in Fig. 3a are of the sample in position (a) (Fig. 2) and were taken in a sequence from the part of the sample with the initial orientation to the reorientated part. Figure 3b illustrates the same sequence, but was taken in position (b) (Fig. 2) Figure 4 is a schematic illustration of the same small-angle reflections showing approximately the same position on the sample as taken by X-ray diffraction; it also gives the schemes of the wider-angle reflections obtained simultaneously
2662
V. S. K~-KS~r~KO et al.
with the others. The wider-angle diffractior~ pictures give only the very intense reflections [110] obtained at an angle of 21040 ' The following can be deduced from the shown X-ray diffractions and their schemes about the development of reorientation in the sample Posstion a Small angle, the layer reflections along the axis of first orientation had an intensity maximum at angle ~m__26' (which is equivalent to d _ 2 0 5 A according to the known formula d=2/~,,, in which d is tim value of the larger
a
m tl mm m m 1°
b
1
2
3
4
FIG. 3. Small-angle X-ray diffraction pictures of vamous stages of reomentatlon" a and b-sample positions (Fig. 2); l - 4 - - n u m b e r of X-ray pmture (showing the approximate place whmh was photographed; see Fig. 4). Arbitrary scale shown on X-ray diffraction picture 4a, the same for all. Vertical ax]s of reorientatlon.
distance). They started to change by becoming smaller along the length of the line of plane, taking on a sort of "roof-hke" form to some extent. Together with this change and the intensity decrease of these reflections went the appearance of new ones, which were already situated on the axis of reorientation having its peak at an angle of Cm-~ 45' (d ~- 120 A). The initial reflections started to disappear at the end of the reorientation zone and new ones became more intense; these had the shape of the usual meridian plane reflections typical for orientated polyethylene, of monotypical quality (i e. identical with the originally observed) before reorientation, b u t at a 90 ° angle to the initially observed reflections Larger angle. The equatorial reflections obtained for the initial orientation (with fairly narrow azimuthal scatter, a sign of a high degree of orientation) started to split and to move gradually to a "new equator" which was equivalent to the reorientated state. They became elongated in shape towards the end and also sufficiently narrow, so that a high degree of orientation must have existed also in the reorientated part. Position b. Small angle, an almost circular diffraction ring was found for the part of initial orientation and its maximum intensity was at an angle of 32'.
Macromolecular structure changes in polyethylene during reorientation
2663
The origin of this ring is so far unclear. Some ideas will be given in the section of "Results". This ring split with progressing reorientation, changed shape and its intensity weakened, until it completely disappeared at the end Band reflections started to appear on the axis of reorientation and their peak was at angle ~m~- 45' (d ~- 120 A); their positions, final shape and development agreed well with those of the final reflections obtained in position ~. Wider angle: the initial, almost circular, uniform intensity reflection for the initial orientation part is evidence of a practically isotroplc azimuthal distribution of crystallites around the axis of initial ormntation (axis C of crystallites, i.e. of the molecular chains in them, which are approximately parallel with the axis of initial orientation). The ring disintegrated to a "cross" system of reflections, which were elongated towards the equator and represented the state of reorientation. This progress and the final shape of the reflectmns (narrow azimuthal arcs on the equator) was in good agreement with the shape of the wider-angle reflections obtained in position a. The general nature of the changes of the wider-angle diffractions on progressing to reorientation was brought about by the transition from the initial state of orientation (mono-axial) to the state existing after reorientation. RESULTS
We believe t h a t the obtained results permit certain conclusions to be drawn about the structural changes occurring during the reorientation of polyethylene. The small- and large-angle diffraction pictures showed a "unidimenslonal" macromolecular structure to exist as a result of initial orientation; this was typical of orientated crystalline polymers. The initial low-angle band reflections were apparently caused by the string-like arrangement, along the axis of orientation, of the crystallites and the layers of amorphous zones between them; the average alternation distance (long period) was 205 A. These beads or bars, giving the uniform band in small-angle diffraction, were formed by the fibrillar elements of the macromolecular structure. The presence in the small-angle diffraction of a ring when the primary beam passed parallel to the axis of primary orientation (Figs. 3b and 4) is evidence of a more or less regular heterogeneity of density of the sample in a direction at right angles to the axis of orientation. I t is possible that this heterogeneity is due to the separateness of these bands, i.e. fibrils within the amorphous interspaces. This presented a good chance of assessing the diameter of the fibrils, but so far there is no promising interpretation of the particular diffraction, and a special study will be necessary to do so. A similar "beam"-like diffraction at small-angles was observed in the case of orientated polyethylene terephthalate [ 12], but was not analyzed. Whatever the case, the sum of all the diffraction observations on polyethylene with initial orientation agreed well enough with that of a structure consisting of fibrillar elements having the same orientation as the sample. These elements are
2664
V. S. KUKSENKO
et al
internally non-uniform because of the alternation of crystalline with amorphous zone along their axis The axial dmtance between alternating zones will be determined, as pointed out earlier [13, 2, 4], by the temperature of elongation during orientation. The stretching orientation of polymers not so treated previously causes a structural transition from the disorientated to the orientated state, with the respective structural change from spherulites and bands, to fibrillar. The conclusion of the disintegration of the spherulites and bands, and the formation of new macromolecular elements during orientation is thus quite obvious, and was proved by direct methods [6, 7]. The position is quite different in the case of reorientation. Fully "formed" elements already exist from the initial orientation, namely the fibrillar rods consisting of crystallites. I t was therefore expected that these elements would turn during reorientation and "arrange themselves" in the direction of the new orientation axis. The fact that reorientation cannot be considered as a rheological movement (viscous flow) of complete and stal~le fibrillar elements of the macromolecular structure was already shown [4] elsewhere and the results given indicated changes in the dimension of the larger axis of these elements during reorienration; polyethylene and capron were used to show t h a t these changes of dimension were a function of the reorientation temperature, compared with that of the primary orientation The results of the stepwise study of the changes of macromolecular structures by the methods of small-angle diffraction during reorientation reported m this paper are, in our opinion, confirmation of the theory of a non-rheological nature of the structural changes at the macromolecular level. A simple turning of the "complete" crystallite structures would mean that the progress of the changes in the small-angle diffraction picture would be approximately as shown in Fig. 5 (top row) It was found to be different, and like that in the bottom row of Fig. 5. There was a certain amount of bending of the band reflections at the start and a few cross-like forms appeared, which suggest that some rotatmn of the bars has taken place This process did not develop to any extent. The slightly changed initial reflections then weakened and this is taken to indicate the disintegration of the original fibrillar elements. The developing new reflections of meridian bands (along the new axis, of reorientation) show that a new orientated fibrillar structure is produced and has smaller periods than the original ( d - 120 A), because the reorientation temperature (room temperature) was lower t h a n that of the primary orientation (100°C). The impression is also gained that this new structure is "depositing" itself in the axis of reorientation, because the new reflections are of a band-like, not "cross-like" shape even at an early stage of reorientation The analysis of the small-angle diffraction changes during reorientation shows generally that the original bands of crystallites disintegrate and t h a t new shapes, fibrils, are produced. An interesting feature in this process are the crystallites
Macromolecular structure changes m polyethylene during reormntatmn
2665
themselves. A substantial difference is noticeable in the changes on comparing wide-angle with small-angle diffraction. I t follows from Fig. 4 t h a t t h e r e is a certain c o n t i n u i t y in the change of wide-angle reflections from an equatorial original line to a new one (position a), or of the s m o o t h contraction of reflections to a new equatomal line (position b). This b e h a v i o u r of the wider-angle reflections during
/
i M //J /
\i/, '~1/ I
/
/ '
!
--. 2"1
I m
Fro. 4
W-!
"-*
l
2
/
I
3
4
FIG. 5
Fro. 4. Scheme of X-ray diffraction pmtures at different stages of reormntatlon: a and b--sample positions (Fig. 2), 1-4--number of X-ray picture; I--initial orientation axis; II--reormntat]on axis; M--small-angle reflections, B--w~der-angle reflecttons ([110] reflectnons). FIG. 5. Scheme of the X-ray chffractlon pmtures of different stages of reorlentatlon, eqmvalent to position a of sample, as m F~g 4. Numbers 1-4, I and II as m Fig. 4. Top row: Hypothetical hanges of small-angle diffraction for the turmng of ttm bands--fibrils without dlsmtegratton. Bottom. Observed small-angle chffractton changes. r e o r i e n t a t i o n agrees with the t h e o r y t h a t the crystallites r o t a t e b y 90 ° in this process. T h e observed radial widening of the reflections indicates a disintegration of the crystallitos into smaller parts, b u t n o t a p p a r e n t l y into separate molecules T h e last conclusion is subject t o a f u r t h e r careful s t u d y , as it is of great import a n c e to the e x p l a n a t i o n of o r i e n t a t i o n in crystallizing p o l y m e r s Should the orystallites r e t a i n a certain stability during orientation, t h e y could c a r r y over to
2666
V . S . KUKS~.NKO et aJ.
the new state of orientation some of their structural features which they had before. If an isotropic (not orientated) polymer would show a more or less smooth turning of the crystals during orientation (which is indicated by numerous X-ray data) [14, 5, 3], those in the orientated state are certainly very "folded", which is an established fact in the case of erystallites in unorientated polymers [15]. There also arises the problem of the relationship between crystallites which allows them to rotate during orientation. A further investigation of the structural changes during orientation, possibly making greater use of informative X-ray diffraction methods, together with other methods, should certainly result in a better understanding of the orientation process. The authors wish to express their sincere thanks to S. N. Zhurkov for his interest in this work, and to D. ¥ a . Tsvankin and Yu. A. Zubov for advice an d discussion. CONCLUSIONS (1) X - r a y b e a m diffraction w i t h small- a n d wide-angle b e a m s was used to s t u d y t h e s t r u c t u r a l changes during t h e r e o r i e n t a t i o n of p o l y e t h y l e n e . ~2) I t was concluded on t h e basis of t h e results of small-angle results t h a t t h e fibril-like s t r u c t u r e , w i t h fairly large gaps, is d e s t r o y e d d u r i n g r e o r i e n t a t i o n a n d t h a t a new, m o n o t y p i c a l s t r u c t u r e f o r m s parallel w i t h t h e o r i e n t a t i o n axis (3) T h e s m o o t h changes of t h e wider-angle reflections during r e o r i e n t a t i o n p e r m i t t h e t h e o r y t h a t a t r a n s f o r m a t i o n of t h e crystallites t a k e s place in which t h e " b a n d s " of these (larger gaps) g r a d u a l l y f o r m in a direction parallel to t h e reo r i e n t a t i o n a~is. T r a n s l a ~ by K. A. ALLEl~ REFERENCES 1. H. HENDUS, Kollmd-Z. 165: 32, 1959 2. R. HOUWINK and A. STAVERMAN, Khlmlya : tekhnologtya polunerov (The Chemistry and Technology of Polymers). Izdat. " K h u m y a " 1: 426, 1965 3. A. Ye. GROMOV and A. I. SLUTSKER, Sbormk: Karbotsepnye vysokomolekulyarnye soyedmemya (In. Carbon-chum High Polymers). Izdat. Akad. Nauk SSSR, p. 247, 1963 4. A. Ye. GROMOV and A. I. SLUTSKER, Vysokomol. soyed. 7: 546, 1965 (Translated m Poly. Sel. U.S.S.R. 7: 3, 60, 1965) 5. N. KASAI and M. KAKUDO, J. Polymer ScL A2: 1955, 1964 6. S. N. ZHURKOV, V. A. MARIKHIN, L. P. MYASNIKOVA and A. I. SLUTSKER, Vysokomol, soyed. 7: 1041, 1965 (Translated in Poly. Sci. U.S.S.R. 7: 6, 1151, 1965) 7. I. L. HAY and A. KELLER, Kollmd-Z. and Z. fur Polymere 204: 43, 1965 8. K. SASAGURI, S. HOSCHINO and R. S. STEIN, J. Appl. Phys. 35: 47, 1964 9. R. HOSEMANN, Polymer 3: 349, 1962 10. B. BELBEOCH and A. GUINIER, Makromol. Chemm 31: 1, 1959 11. A. Ye. GROMOV and A. I. SLUTSKER, Prlbory : tekhn, eksper., No. 3, 165, 1964 12. W. STATTON and G. GODARD, J. Appl. Phys. 28: 1111, 1957 13. E. W. FISCHER and G. F. SMIDT, Angew. Chemm 74: 551, 1962 14. A. BROWN, J. Appl. Phys. 20: 552, 1949 15. P. H. GEIL, Polymer Single Crystals, Intersclenee Pubhsh., 1963
2659
(2) Both these linear and rubber-like polymers, with M>>M0, behaved hke crosslinked rubbers and their behavlour could be qualitatively described by the Bueche theory in the low frequency range of mechanical dispersion (3) A second, high frequency, range of mechanical dispersion was found in the studied rubber-like polymers and the most probable time lags were here shorter by 4-5 powers of ten than m the low frequency range (4) The Bueche theory does not take mto account the presence of a high frequency range of mechanical dispersion Translated by K. A. ALLEN REFERENCES l. J. FERRY, Vyazkouprugle svomtva pohmerov (Vlsco-elastm Propertms of Polymers). Izd. mostr, hr., 1963 2. A. TOBOL'SKII, Struktura 1 svolstva pohmerov (The Structure and Propertms of Polymers). Izd. " K h l m l y a " , 1964 3 K . S . COLE and R. H. COLE, J. Chem Phys 9: 34, 1941 4. E. R. FITZGERALD, L. D. GRANDINE and J. D. FERRY, J Appl. Phys 24: 650, 1964 5. L. TRELOAR, Flzlka uprugostl kauchuka (Physms of R u b b e r Elastmlty) ITd mostr hr., 1953 6 F. BUECHE, J. Chem. Phys. 22: 603, 1954 7 W. P. MASON, N. O. BACKER, H. T. McSKIMIN and J. H. HEISS, Phys Rev 75: 936, 1949 8 S . P . I(ABIN and G. P. MIKHAILOV, Zhur tekhn fiz 26: 511, 1956 9. K. PYUSUKE, J. Phys. Soe J a p a n 16: 1580, 1961
MACROMOLECULAR STRUCTURE CHANGES IN POLYETHYLENE DURING REORIENTATION* V S. KUKSENKO, S NIZAMIDINOV a n d A. I SLUTSKER A. F. Ioffe Physmo-technologleal Institute
(Recewed 25 July 1966)
A~ increasing amount of attention is given to the study of structural rearrangements occurring during the extension of crystalhne polymers, because the orientation process is of scientific and practical interest. The earlier, widely accepted theory about the "theological" nature of ormntation, according to which the structural elements of the previously unorientated polymer move and rotate during extension, is being reexamined using X-ray diffraction [1-5], electron micro* Vysokomol. soyed A9" No. 11, 2352-2357, 1967.
2660
V
S. KUKSE~KO et al.
scopic [6, 7], and optical [7, 8] methods of study, amongst others. The orientation of crystallizing solid polymers is, in the extreme, connected with the destruction of the original structure and the formation of a different one. The observable complexity of the macromolecular structure of crystallizing polymers prior to orientation makes it desirable to study orientation under conditions in which the initial structure would be relatively simple and could be characterized by means of a small number of parameters. This condition is fulfilled also during reorientation of an already orientated polymer, which consists of stretching the latter at right angles to the previous axis of orientation. It is Well known that macromolecular structures of different crystallizing polymers, or even of previously unorientated ones, will differ quJte a lot from those present in orientated polymers. Those already in an orientated state will show fairly similar types of simple structure. A typical features will be the fairly regular alternatmn, along the axis of orientation, of crystalline with amorphous regions (long periods). The axes of the molecular chains will be more or less aligned in the same direction, in the crystalline and amorphous zones, as the axis of orientation [9]. The study of the effect of stress applied at right angles to the previous orientation axis, leading to reorientation, could lead to a better understanding of the structural changes in general. There are few publications dealing with the study of reorientation, and one [1] in which X-ray diffraction was used with either a n a r r o w - o r a wide-angle beam, the polymer examined having initially had a relatively small degree of orientation (200% elongation); the axis of reorientation was not exactly at right angles to the previous ormntatioll. An earlier study by one of the authors of this paper [4] did not examine reorientation by stages, but analyzed the structure only in its initial and final stages. This paper reports the results of tracing the consecutive stages of reorientation. High pressure polyethylene was used in the shape of a 3 mm thick plate. This was subjected to mono-axial extension at 100°C up to the limit (about 5-fold stretching) A sample was cut out of this orientated plate in the shape of a spatula with a flat part at each end of a rod, and this was also stretched to the limit at room temperature. The different temperatures selected for the initial orientation and the reorientation aimed at obtaining a clearer picture of the changes, since the orientated structures of polymers extended at different temperatures would be similar, a difference was found in the macromolecular characteristics of structure (larger regions, larger dimensions of crystallites) [9, 10, 4]. The reorientation of the sample thus produced gave rise to a neck and was thought suitable for the study of the consecutive stages of reorientation by examining the structure present in different parts of the sample, moving from the primarily orientated part to the transitional, and finally the neck part. The structural exammatmn was made by X-ray narrow- and w,de-angle dlflractmn. T h e d i f f r a c t i o n p i c t u r e s were p r o d u c e d o n a n a p p a r a t u s h a w n g a s h a r p l y focused v a l v e a n d a " p r e c i s e " c o l h m a t m n o f t h e p r i m a r y b e a m , as d e s c r i b e d e a r h e r [11]. T h e l a t e r a l sec-
Macromolccular structure changes in polyethylene during reorientation
266I
tion of the beam had a diameter of 100/~. The X-ray pictures produced at the two angles were obtained b y placing a fiat sheet with an aperture at a short distance from the sample between it and the rachation source when using small angle scattering (Fig. 1). Thin type of diffraction study (and also the c x a m i n a t m n method of the ormntatmnal changes of structure b y taking pictures of different parts of the same sample) was similar to that described elsewhere [5].
a FIG. 1
b FIO. 2
FIO. 1. Scheme of the instrument with a micro beam for taking X-ray pictures at smalland wide-angles: 1--sharp-focusing X-ray tube, 2--diaphragms, J--sample, 4--photographic plate for taking pictures from a wide angle, 5--mask (iris) for the primary beam, 6--photo-film for small-angle photography. FTG. 2. Scheme of the X-ray diffraction picture taken after reormntatmn: I - - a x m of mltml ormntation, I I - - r e o r m n t a t i o n axm, III--pr]mary beam. The points on the samples denote the positrons whmh were photographed; a and b--sample pomtmns.
All the pictures were taken under vacuum using CuKa radiation (wavelength of 1.54 A). The samples were fixed onto swallow-tad cross wires and this made it possible to move the different parts within the same sample "smoothly" into the field of the beam. As the regmn of the transition zone between the part of initial ormntatmn and the neck had an axial distance of about 2 ram, the hght beam of 0.1 m m width ensured sufflcmnt "resolut m n " to observe structural variations. The sample was fixed in a direction at mght angles to the axm of reormntatlon m the beam and was moved in stages along thin axm. While holding the reormntatlon axis all the time at mght angles to the original, pmtures were taken at 2 different sample positions (a) the plane formed b y the axes of the first a n d second ormntatlon was held at right angles to the beam (Fig. 2a), and (b) the same plane being parallel with the beam (Fig. 2b). The pictures of the sample in positions (a) and (b) gave a certain idea of "space" of the structure.
EXPERIMENTAL
The small-angle X-ray diffraction pictures shown in Fig. 3a are of the sample in position (a) (Fig. 2) and were taken in a sequence from the part of the sample with the initial orientation to the reorientated part. Figure 3b illustrates the same sequence, but was taken in position (b) (Fig. 2) Figure 4 is a schematic illustration of the same small-angle reflections showing approximately the same position on the sample as taken by X-ray diffraction; it also gives the schemes of the wider-angle reflections obtained simultaneously
2662
V. S. K~-KS~r~KO et al.
with the others. The wider-angle diffractior~ pictures give only the very intense reflections [110] obtained at an angle of 21040 ' The following can be deduced from the shown X-ray diffractions and their schemes about the development of reorientation in the sample Posstion a Small angle, the layer reflections along the axis of first orientation had an intensity maximum at angle ~m__26' (which is equivalent to d _ 2 0 5 A according to the known formula d=2/~,,, in which d is tim value of the larger
a
m tl mm m m 1°
b
1
2
3
4
FIG. 3. Small-angle X-ray diffraction pictures of vamous stages of reomentatlon" a and b-sample positions (Fig. 2); l - 4 - - n u m b e r of X-ray pmture (showing the approximate place whmh was photographed; see Fig. 4). Arbitrary scale shown on X-ray diffraction picture 4a, the same for all. Vertical ax]s of reorientatlon.
distance). They started to change by becoming smaller along the length of the line of plane, taking on a sort of "roof-hke" form to some extent. Together with this change and the intensity decrease of these reflections went the appearance of new ones, which were already situated on the axis of reorientation having its peak at an angle of Cm-~ 45' (d ~- 120 A). The initial reflections started to disappear at the end of the reorientation zone and new ones became more intense; these had the shape of the usual meridian plane reflections typical for orientated polyethylene, of monotypical quality (i e. identical with the originally observed) before reorientation, b u t at a 90 ° angle to the initially observed reflections Larger angle. The equatorial reflections obtained for the initial orientation (with fairly narrow azimuthal scatter, a sign of a high degree of orientation) started to split and to move gradually to a "new equator" which was equivalent to the reorientated state. They became elongated in shape towards the end and also sufficiently narrow, so that a high degree of orientation must have existed also in the reorientated part. Position b. Small angle, an almost circular diffraction ring was found for the part of initial orientation and its maximum intensity was at an angle of 32'.
Macromolecular structure changes in polyethylene during reorientation
2663
The origin of this ring is so far unclear. Some ideas will be given in the section of "Results". This ring split with progressing reorientation, changed shape and its intensity weakened, until it completely disappeared at the end Band reflections started to appear on the axis of reorientation and their peak was at angle ~m~- 45' (d ~- 120 A); their positions, final shape and development agreed well with those of the final reflections obtained in position ~. Wider angle: the initial, almost circular, uniform intensity reflection for the initial orientation part is evidence of a practically isotroplc azimuthal distribution of crystallites around the axis of initial ormntation (axis C of crystallites, i.e. of the molecular chains in them, which are approximately parallel with the axis of initial orientation). The ring disintegrated to a "cross" system of reflections, which were elongated towards the equator and represented the state of reorientation. This progress and the final shape of the reflectmns (narrow azimuthal arcs on the equator) was in good agreement with the shape of the wider-angle reflections obtained in position a. The general nature of the changes of the wider-angle diffractions on progressing to reorientation was brought about by the transition from the initial state of orientation (mono-axial) to the state existing after reorientation. RESULTS
We believe t h a t the obtained results permit certain conclusions to be drawn about the structural changes occurring during the reorientation of polyethylene. The small- and large-angle diffraction pictures showed a "unidimenslonal" macromolecular structure to exist as a result of initial orientation; this was typical of orientated crystalline polymers. The initial low-angle band reflections were apparently caused by the string-like arrangement, along the axis of orientation, of the crystallites and the layers of amorphous zones between them; the average alternation distance (long period) was 205 A. These beads or bars, giving the uniform band in small-angle diffraction, were formed by the fibrillar elements of the macromolecular structure. The presence in the small-angle diffraction of a ring when the primary beam passed parallel to the axis of primary orientation (Figs. 3b and 4) is evidence of a more or less regular heterogeneity of density of the sample in a direction at right angles to the axis of orientation. I t is possible that this heterogeneity is due to the separateness of these bands, i.e. fibrils within the amorphous interspaces. This presented a good chance of assessing the diameter of the fibrils, but so far there is no promising interpretation of the particular diffraction, and a special study will be necessary to do so. A similar "beam"-like diffraction at small-angles was observed in the case of orientated polyethylene terephthalate [ 12], but was not analyzed. Whatever the case, the sum of all the diffraction observations on polyethylene with initial orientation agreed well enough with that of a structure consisting of fibrillar elements having the same orientation as the sample. These elements are
2664
V. S. KUKSENKO
et al
internally non-uniform because of the alternation of crystalline with amorphous zone along their axis The axial dmtance between alternating zones will be determined, as pointed out earlier [13, 2, 4], by the temperature of elongation during orientation. The stretching orientation of polymers not so treated previously causes a structural transition from the disorientated to the orientated state, with the respective structural change from spherulites and bands, to fibrillar. The conclusion of the disintegration of the spherulites and bands, and the formation of new macromolecular elements during orientation is thus quite obvious, and was proved by direct methods [6, 7]. The position is quite different in the case of reorientation. Fully "formed" elements already exist from the initial orientation, namely the fibrillar rods consisting of crystallites. I t was therefore expected that these elements would turn during reorientation and "arrange themselves" in the direction of the new orientation axis. The fact that reorientation cannot be considered as a rheological movement (viscous flow) of complete and stal~le fibrillar elements of the macromolecular structure was already shown [4] elsewhere and the results given indicated changes in the dimension of the larger axis of these elements during reorienration; polyethylene and capron were used to show t h a t these changes of dimension were a function of the reorientation temperature, compared with that of the primary orientation The results of the stepwise study of the changes of macromolecular structures by the methods of small-angle diffraction during reorientation reported m this paper are, in our opinion, confirmation of the theory of a non-rheological nature of the structural changes at the macromolecular level. A simple turning of the "complete" crystallite structures would mean that the progress of the changes in the small-angle diffraction picture would be approximately as shown in Fig. 5 (top row) It was found to be different, and like that in the bottom row of Fig. 5. There was a certain amount of bending of the band reflections at the start and a few cross-like forms appeared, which suggest that some rotatmn of the bars has taken place This process did not develop to any extent. The slightly changed initial reflections then weakened and this is taken to indicate the disintegration of the original fibrillar elements. The developing new reflections of meridian bands (along the new axis, of reorientation) show that a new orientated fibrillar structure is produced and has smaller periods than the original ( d - 120 A), because the reorientation temperature (room temperature) was lower t h a n that of the primary orientation (100°C). The impression is also gained that this new structure is "depositing" itself in the axis of reorientation, because the new reflections are of a band-like, not "cross-like" shape even at an early stage of reorientation The analysis of the small-angle diffraction changes during reorientation shows generally that the original bands of crystallites disintegrate and t h a t new shapes, fibrils, are produced. An interesting feature in this process are the crystallites
Macromolecular structure changes m polyethylene during reormntatmn
2665
themselves. A substantial difference is noticeable in the changes on comparing wide-angle with small-angle diffraction. I t follows from Fig. 4 t h a t t h e r e is a certain c o n t i n u i t y in the change of wide-angle reflections from an equatorial original line to a new one (position a), or of the s m o o t h contraction of reflections to a new equatomal line (position b). This b e h a v i o u r of the wider-angle reflections during
/
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3
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FIG. 5
Fro. 4. Scheme of X-ray diffraction pmtures at different stages of reormntatlon: a and b--sample positions (Fig. 2), 1-4--number of X-ray picture; I--initial orientation axis; II--reormntat]on axis; M--small-angle reflections, B--w~der-angle reflecttons ([110] reflectnons). FIG. 5. Scheme of the X-ray chffractlon pmtures of different stages of reorlentatlon, eqmvalent to position a of sample, as m F~g 4. Numbers 1-4, I and II as m Fig. 4. Top row: Hypothetical hanges of small-angle diffraction for the turmng of ttm bands--fibrils without dlsmtegratton. Bottom. Observed small-angle chffractton changes. r e o r i e n t a t i o n agrees with the t h e o r y t h a t the crystallites r o t a t e b y 90 ° in this process. T h e observed radial widening of the reflections indicates a disintegration of the crystallitos into smaller parts, b u t n o t a p p a r e n t l y into separate molecules T h e last conclusion is subject t o a f u r t h e r careful s t u d y , as it is of great import a n c e to the e x p l a n a t i o n of o r i e n t a t i o n in crystallizing p o l y m e r s Should the orystallites r e t a i n a certain stability during orientation, t h e y could c a r r y over to
2666
V . S . KUKS~.NKO et aJ.
the new state of orientation some of their structural features which they had before. If an isotropic (not orientated) polymer would show a more or less smooth turning of the crystals during orientation (which is indicated by numerous X-ray data) [14, 5, 3], those in the orientated state are certainly very "folded", which is an established fact in the case of erystallites in unorientated polymers [15]. There also arises the problem of the relationship between crystallites which allows them to rotate during orientation. A further investigation of the structural changes during orientation, possibly making greater use of informative X-ray diffraction methods, together with other methods, should certainly result in a better understanding of the orientation process. The authors wish to express their sincere thanks to S. N. Zhurkov for his interest in this work, and to D. ¥ a . Tsvankin and Yu. A. Zubov for advice an d discussion. CONCLUSIONS (1) X - r a y b e a m diffraction w i t h small- a n d wide-angle b e a m s was used to s t u d y t h e s t r u c t u r a l changes during t h e r e o r i e n t a t i o n of p o l y e t h y l e n e . ~2) I t was concluded on t h e basis of t h e results of small-angle results t h a t t h e fibril-like s t r u c t u r e , w i t h fairly large gaps, is d e s t r o y e d d u r i n g r e o r i e n t a t i o n a n d t h a t a new, m o n o t y p i c a l s t r u c t u r e f o r m s parallel w i t h t h e o r i e n t a t i o n axis (3) T h e s m o o t h changes of t h e wider-angle reflections during r e o r i e n t a t i o n p e r m i t t h e t h e o r y t h a t a t r a n s f o r m a t i o n of t h e crystallites t a k e s place in which t h e " b a n d s " of these (larger gaps) g r a d u a l l y f o r m in a direction parallel to t h e reo r i e n t a t i o n a~is. T r a n s l a ~ by K. A. ALLEl~ REFERENCES 1. H. HENDUS, Kollmd-Z. 165: 32, 1959 2. R. HOUWINK and A. STAVERMAN, Khlmlya : tekhnologtya polunerov (The Chemistry and Technology of Polymers). Izdat. " K h u m y a " 1: 426, 1965 3. A. Ye. GROMOV and A. I. SLUTSKER, Sbormk: Karbotsepnye vysokomolekulyarnye soyedmemya (In. Carbon-chum High Polymers). Izdat. Akad. Nauk SSSR, p. 247, 1963 4. A. Ye. GROMOV and A. I. SLUTSKER, Vysokomol. soyed. 7: 546, 1965 (Translated m Poly. Sel. U.S.S.R. 7: 3, 60, 1965) 5. N. KASAI and M. KAKUDO, J. Polymer ScL A2: 1955, 1964 6. S. N. ZHURKOV, V. A. MARIKHIN, L. P. MYASNIKOVA and A. I. SLUTSKER, Vysokomol, soyed. 7: 1041, 1965 (Translated in Poly. Sci. U.S.S.R. 7: 6, 1151, 1965) 7. I. L. HAY and A. KELLER, Kollmd-Z. and Z. fur Polymere 204: 43, 1965 8. K. SASAGURI, S. HOSCHINO and R. S. STEIN, J. Appl. Phys. 35: 47, 1964 9. R. HOSEMANN, Polymer 3: 349, 1962 10. B. BELBEOCH and A. GUINIER, Makromol. Chemm 31: 1, 1959 11. A. Ye. GROMOV and A. I. SLUTSKER, Prlbory : tekhn, eksper., No. 3, 165, 1964 12. W. STATTON and G. GODARD, J. Appl. Phys. 28: 1111, 1957 13. E. W. FISCHER and G. F. SMIDT, Angew. Chemm 74: 551, 1962 14. A. BROWN, J. Appl. Phys. 20: 552, 1949 15. P. H. GEIL, Polymer Single Crystals, Intersclenee Pubhsh., 1963