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X-ray diffraction study of the reorientation of polyethylene
96
YA. V. GENIlV e$ ed.
3. A, N. PRAVEDNIKOV, I. Ye. KARDASH, Ye. N. TELESHOV and B. V. KOTOV,
Vysokomol. soyed. A13: 425, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 2, 483, 1971) 4. S. G. ENTELIS and O. V. NESTEROV, Bold. AN SSSR 148: 1323, 1963 5. H. ADKINS and Q. E. THOMPSON, J. Amer. Chem. Soe. 71: 2242, 1949 6. G. S. KOLESNIKOV, O. Ya. FEDOTOVA and E. I. KHOFBAUER, Vysokomol. soyed. AI0: 1511, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 7, 1748, 1968) 7. I. Ye. KARDASH, A. Ya. ARDASHNIKOV and A. N. PRAVEDNIKOV, Vysokomol. soyed. Al1: 1996, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2276, 1969) 8. G. T. MORGEN and R. W. THOMASON, J. Chem. Soc., 2691, 1926 9. A. BARKER and C. C. BARKER, J. Chem. See., 870, 1954 10. Ch. COURTOT and J. MOREAUS, Compt. rend. 217: 453, 1943
X-RAY DIFFRACTION STUDY OF THE REORIENTATION OF POLYETHYLENE* YA. V.
Gv.~uw, V.
I. GERASI~OV, :B. !V[. GIlVZBURG, N. SULTAI~OV a n d
D. YA. TSVAIVKIIV Heteroorganic Compounds Institute,U.S.S.R. Academy of Sciences
(Received 17 May 1971) THE reorientation of p o l y m e r samples is of m a j o r interest in t h a t it provides t h e possibility o f s t e p - b y - s t e p analysis of t h e f o r m a t i o n of oriented structures. The processes in question h a v e been a n a l y s e d b y several different m e t h o d s [1-5], including low- a n d wide-angle X - r a y s [2, 4, 5] which h a v e revealed certain features of the s t r u c t u r a l changes i n v o l v e d as well as t h e f a c t t h a t t h e reorientat i o n processes m a y differ according t o the n a t u r e of the original s t r u c t u r e a n d t h e d e f o r m a t i o n conditions. Our aim in this i n v e s t i g a t i o n was to determine t h e s t r u c t u r a l changes i n v o l v e d in t h e reorientation of annealed p o l y e t h y l e n e samples. A s t u d y was m a d e of s t r u c t u r a l t r a n s f o r m a t i o n s a p p e a r i n g as a result of def o r m a t i o n a t r o o m t e m p e r a t u r e , using samples t h a t had first of all u n d e r g o n e prolonged annealing.
EXPERIMENTAL Low-density polyethylene (LDPE) samples were used in this investigation, as was also the case in refs. [2, 4, 5]. The isotropie film samples with a thickness of 2 mm underwent 600 ~o extension at room temperature, which was followed by annealing in the stretched state at 100° for 2 hr. The annealed samples were reoriented at room temperature across the direction of elongation. When the reorientation had taken place, the samples were removed from the stretching apparatus, and were then kept for one day to allow stress * Vysokomol. soyed. A15: No. 1, 85-90, 1973.
X-ray diffraction study of reorientation of polyethylene
95
relaxation to take place. Low- and wide-angle X-ray photographs were taken of parts of the same sample with different degrees of deformation. Both the low- and wide-angle pictures were obtained simultaneously with the aid of a special low-angle c&mere [6] at room temperature, the distance between film and sample being 200 and 15 mm for low- and wide-angles respectively. DISCUSSION
OF RESULTS
Figure 1 shows the low- and wide-angle X-ray photoTwo jibrillar subgroups. graphs characterizing structural changes occurring during the reorientation of LDPE
samples.
All the regions appearring in these X-ray
photographs
are situ-
FIG. 1. Low-angle (I) and wide-angle (II) X-ray patterns obtained for different parts of an oriented sample: a-original sample; b-e-intermediate stages; f-reoriented sample. Direction of initial reorientation is vertical; the reorientation direction is horizontal.
ated along the axial line of the sample (marked by the small circles in the schematic diagram shown in Fig. 2). The X-ray patterns in Fig. l&‘, d” were obtained for the d’ and cl” regions (Fig. 2) located above and below the axial line.
YE. V. GENIN et al.
98
Let us consider the wide-angle has crystallites
patterns shown in Fig. 1. The original sample
of c texture type, and the texture
original direction
of stretch.
axis of the latter follows the
During the course of reorientation
situated on the equator split up into two subgroups of reflections. crystallitea
is preserved
in each of these subgroups,
a
a
b
b
the reflections The c texture of
but the texture
axes are
d
d
e
FIU. 2. Schematic drawings of the sample for the low- and wide-angle X-ray patterns, and structural models illustrating the reorientation process. The regions in whioh the photographing was done are marked in the diagram by circleswith letters oorrespondingto those in Fig. 1.
inclined at an angle to one another in the different subgroups. As the deformation degree increases, the axes diverge from one another, the angle between them increasing until it becomes 90”. In the process each of the axes turns symmetrically to an angle of 45’ to both the old and the new axes of orientation. The angle between the texture axes is reduced during further orientation, and their direction now approximates to that of the new axis of orientation (Fig. le).
X-ray diffraction study of reorientation of polyethylene
99
I n the end the two subgroups merge to form a single system having crystallites with ~ type texture and a texture axis t h a t coincides with the direction of elongation. So far we have been dealing with the X-ray patterns obtained for the different regions of the sample located along its axial line. However, if the pictures are taken instead at the points d' and d" (see Fig. 2), it then appears that, in the regions situated above and below this line, there is now only one of the two subgroups of fibrils with crystallites of c texture type, wMch means t h a t ~he reorientation process results in two separate groups of crystallites, each with its own type of c texture, in the regions above and below the axial line. Near this line the two groups of crystallites are superposed one on the other, and a structure consisting, as it were, of two cross c textures results. The wide-angle X-ray photographs obtained in this investigation for the reoriented annealed samples mainly coincide with the corresponding photographs characterizing the reorientation process occurring with unannealed samples [4]. This means t h a t the process of change in the orientation of crystallites and the reorientation of the axes of the macromolecules from the original axis of elongation to a perpendicular direction proceed in much the same manner, whether or not the originally oriented sample has undergone annealing. Changes in the long period. I t is seen t h a t the low-angle X-ray photograph of the original sample (Fig. la) has the usual striated reflection of the fibrillar type with d r = 175 A. As the process of reorientation develops, this reflection gradually contracts towards the meridian, and becomes thicker (Fig. lb, c), its shape being now roughly spherical, which is probably characteristic for layer structures [7] (Fig. 1). In the course of further stretching the intensity of this reflection is reduced, and all that remains of the latter in the completely stretched sample is a thin weak line appearing on the equator of the low-angle X-ray photograph of the reoriented sample (Fig. l f). Starting at the stage marked by :Fig. lc, additional reflections (apart from the one last-mentioned) appear in the X-ray photographs, forming the rhomboid pattern that is clearly seen in Fig. ld. These new reflections t u r n round gradually and contract towards the meridian, which coincides with the direction of reorientation, and finally form a four-spot fibrillar reflection with dr~--125 A and ~ 3 5 ° (Fig. 2). In the course of investigation of the wide-angle photographs in the previous section it was found t h a t in the process of elongation all the macromolecules split up into two groups, each having its own texture axis. Low-angle X-ray photographs of each subgroup were therefore obtained and analysed individually to elucidate the general pattern of low-angle scattering. The X-ray patterns for the regions above, below and near the axial line are shown in Fig. ld, d', d". I t should be noted t h a t similar observations were reported in [5] in regard to annealed L D P E samples t h a t had been reoriented at various angles to the direction of stretch, and in actual fact the authors were observing the behaviour of a single fibrillar subgroup. As is apparent from the X - r a y patterns in Fig. ld', d", and from the data
100
Y~.. V. G ~
et al.
in ref. [5], there is a low-angle X-ray pattern, with four reflections, for each fibrillar subgroup, and these four reflections m a y be split up into two pairs of two-spot patterns. To describe the shape of these reflections we have first to examine the wide-angle structural diagram in order to determine the direction of t h e texture axis, which is the fibril axis (Fig. 3). I t can be seen from Fig. ld' that one pair of reflections is a tilting two-spot pattern [7] with dr-----175 A (Fig. 3),
F
Fie. 3. Schematic diagrams of the low- and wide-anglo X-ray patterns, and structural models corresponding to the patterns in Fig. ld, d', d". a n d this pair evidently corresponds to the low-angle reflection of the original structure, seeing that the size of the long period is the same. The other pair of reflections is in the form of a straight two-spot, but here df----125 A, i.e. the size of the long period differs. Schematic diagrams of the structures corresponding to these two two-spot patterns appear in Fig. 3. I t should be noted that both structures have a common texture axis, and moreover the direction of the macromolecular axis is the same, and the crystallites of these structures are of a planar type, as m a y be surmised from the two-spot form of the low-angle reflections [7]. However, the inclinations of the crystallite faces, as well as the sizes of the long periods differ in the structures corresponding to the two different two-spots. Previous investigations showed [7] t h a t a tilting two-spot with d-----175A corresponds approximately to bt/a-~l.0-1.5, and for t ~ 1.5 b/a:0.7-1.O, where a, b and t are the transverse and longitudinal dimensions, and the tangent of the angle of taper, respectively. For a straight two-spot bt/a~0.4-0.6 and t-~0.76, while blab--0.5-0.8, i.e. the transverse dimensions of the crystallites also differ in the two structures. As may be seen from the patterns in Fig. ld', d", the
X-ray diffraction study of reorientation of polyethylene
101
structure of the subgroups located above and below the axial line are altogether identical, apart from the direction of the texture axis. Reverting once again to the low-angle X-ray pattern obtained from the middle line (Fig. ld) this m a y be interpreted as consisting of two X-ray p a t t e r n s pertaining to different subgroups placed together near the middle line of the sample. I t will be seen that the reflection is approximately spherical, results from t h e original low-angle reflection (Fig. ld), consists of two tilting two-spots with d ~ 1 7 5 A, and is in fact a merged tilting four-spot. The other (rhomboid) intensity distribution consists of two straight two-spots with df----125 A. The stretching of the annealed sample is responsible for the difference in the size of the long periods of the two reflections. The long period of the annealed sample ( d ~ 175 A) is greater than that formed (d-~125/~) in the course of stretching at room temperature. In the end reflections corresponding to a period with d----125/~ remain in the reoriented structure, whilst those with d ~ 1 7 5 A weaken gradually, and disappear.
a
b
d
1~io. 4. Schematic diagram of the transformation of a low-angle reflection of fibrillar type into a layer one with increasing skewing of the erystallites. The letters a, b and v under the diagrams correspond to the notation in Fig. 1.
Transformation of low-angle reflections of the fibrillar type into larger ones. L e t us now consider the process whereby the original striated low-angle reflection (Fig. la) is transformed into the spherical reflection seen in Fig. ld..The transformation takes place gradually, without any alteration in the size of the long period. It is of major interest, in view of the change in crystallite shape involved in the process. In the original sample the long period is positioned along the texture axis, the direction of which coincides with that of the macromolecular axes. As the degree of deformation increases, the maeromolecular axes, and accordingly the fibrillar axes, diverge gradually from the original direction, and are finally turned through an angle of 90 ° . At the same time the centre of the reflection seen in Fig. la remains constantly in the same place, i.e. on an axis
102
YE. V. G~.~r~ et a/.
corresponding to the original direction of the macromolecular axes. This means that the position of a crystallite face, the normal to which is directed towards the centre of a reflection, will likewise remain unaltered. Owing to the constant rotation of the fibrillar axis, there will be a corresponding constant increase in the angle between the macromolecular axes and the normal to the face of a crystallite. Therefore, as the process of contraction continues, a crystallite will be transformed from a rectangular parallelepiped into an oblique-angled parallelepiped, and the degree of bevelling, characterized b y the angle fp between the normal the face of the erystallite and the axis of the fibril will be still further increased. The process involved is like the shear deformation of a crystallite if the direction of shear coincides with that of the macromolecules. As a result of this "shearing" a crystallite is gradually transformed into a plate where the axes of macromolecules pass approximately along the plane of the plate (Fig. 4). I t was found in ref. [7] that the value of bt/a is a general parameter influencing the shape of a low-angle reflection. With increased bevelling the value of ~ rises, and t = t a n ~, hence the magnitude of bt/a will likewise be increased. This increase governs the change in the shape of a reflection from the striated to spherical type. Consequently the gradual change in the shape of the reflection with d = 1 7 5 4 during the reorientation process marks a smooth transition during which a erystallite is transformed from a parallelepiped into a plate. The process of transition from a reflection of fibrillar t y p e to a layer reflection (analysed above) differs from the case considered in ref. [7] involving the shrinkage of oriented polyethylene sampies. In the process of shrinking the fibrillar structure is transformed into a layer one owing to a change in the size of the crystallites, while in the case of reorientation only a change in crystallite shape is involved. It was pointed out above that during the reorientation process the other pair of reflections with d-----125 A has the appearance of a straight two-spot, and therefore characterizes a fibrillar structure with crystallites in the form of parallelepipeds. One may assume that during the deformation process the plates m a y split up into separate blocks forming a second system of fibrils with d = 1 2 5 4. In view of the X-ray data obtained b y investigating the reorientation of oriented L D P E samples the structural changes occurring in the polymer could be described as follows. The reorientation process results in the formation of two regions on either side of the axial line. Each of these regions consists of fibrillar subgroups, the crystallites of the latter having a planar type c texture, and these subgroups turn in the direction of elongation. Moreover there is another region close to the axial line of the sample, and in this region there are fibrillar subgroups superposed on one another. The direction of bevelling of crystallites in the subgroups shows that in the regions on either side of the axial line the effect of what would appear to be shear deformations is conductive to more rapid slipping of the regions located closer to the "neck". The reorientation of the P E samples, whether or not t h e y have been annealed, proceeds in accordance with this mechanism. I t is in respect to changes in the size of the
X-ray diffraction study of reorientation of polyothyleno
103
long period t h a t the behaviour of the annealed and unannealed P E samples differs. I n the case of the annealed samples the long period of the original films is determined by the annealing temperature, and during the process of reorientation, by the temperature of the medium, i.e. room temperature; in the case of unannealed P E samples the initial stretching and reorientation take place at the same temperature, and so there is no change in the size of the long periods in this case. CONCLUSIONS
(1) I t has been shown t h a t during the reorientation of low-density polyethylene (LDPE) films two regions with dissimilarly oriented texture axes are formed on either side of the axial line; near this line the two regions in question are superposed on one another. The stretching process is accompanied by gradual turning of the fibrils and the macromolccules. (2) The transition from low-angle reflections of fibrillar type to layer ones takes place, in the case under consideration, through rectangular crystaUites undergoing shear deformation, and changing into elongated plates. (3) The difference in the behaviour of the annealed and unannealed L D P E samples is due to the fact t h a t in the case of the former the size of the long period changes, whilst for the latter it remains constant. REFERENCES 1. H. HENDUS, Kolloid-Z. 165: 32, 1959 2. V. S. KUKSENKO, S. NIZAMIDINOV and A. I. SLUTSKER, Vysokomol. soyed. Ag: 2352, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 11, 2659, 1967) 3. A. KELLER and J. G. RIDER, ft. Mater. Sei. 1: 389, 1966 4. V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. BI2: 653, 1970 (Not translated in Polymer Sei. U.S.S.R.) 5. N. SULTANOV, B.M. GINZBURG and S. Ya. FRANKEL', Vysokomol. soyed. 13: 2691, 1971 (Not translated in Polymer Sci. U.S.S.R.) 6. V. I. GERASIMOV, D. Ya. TSVANKIN, Pribory i tekhnika eksperimenta, No. 2, 204, 1968 7. V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. A12: 599, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 11, 2944, 1970)
YA. V. GENIlV e$ ed.
3. A, N. PRAVEDNIKOV, I. Ye. KARDASH, Ye. N. TELESHOV and B. V. KOTOV,
Vysokomol. soyed. A13: 425, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 2, 483, 1971) 4. S. G. ENTELIS and O. V. NESTEROV, Bold. AN SSSR 148: 1323, 1963 5. H. ADKINS and Q. E. THOMPSON, J. Amer. Chem. Soe. 71: 2242, 1949 6. G. S. KOLESNIKOV, O. Ya. FEDOTOVA and E. I. KHOFBAUER, Vysokomol. soyed. AI0: 1511, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 7, 1748, 1968) 7. I. Ye. KARDASH, A. Ya. ARDASHNIKOV and A. N. PRAVEDNIKOV, Vysokomol. soyed. Al1: 1996, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2276, 1969) 8. G. T. MORGEN and R. W. THOMASON, J. Chem. Soc., 2691, 1926 9. A. BARKER and C. C. BARKER, J. Chem. See., 870, 1954 10. Ch. COURTOT and J. MOREAUS, Compt. rend. 217: 453, 1943
X-RAY DIFFRACTION STUDY OF THE REORIENTATION OF POLYETHYLENE* YA. V.
Gv.~uw, V.
I. GERASI~OV, :B. !V[. GIlVZBURG, N. SULTAI~OV a n d
D. YA. TSVAIVKIIV Heteroorganic Compounds Institute,U.S.S.R. Academy of Sciences
(Received 17 May 1971) THE reorientation of p o l y m e r samples is of m a j o r interest in t h a t it provides t h e possibility o f s t e p - b y - s t e p analysis of t h e f o r m a t i o n of oriented structures. The processes in question h a v e been a n a l y s e d b y several different m e t h o d s [1-5], including low- a n d wide-angle X - r a y s [2, 4, 5] which h a v e revealed certain features of the s t r u c t u r a l changes i n v o l v e d as well as t h e f a c t t h a t t h e reorientat i o n processes m a y differ according t o the n a t u r e of the original s t r u c t u r e a n d t h e d e f o r m a t i o n conditions. Our aim in this i n v e s t i g a t i o n was to determine t h e s t r u c t u r a l changes i n v o l v e d in t h e reorientation of annealed p o l y e t h y l e n e samples. A s t u d y was m a d e of s t r u c t u r a l t r a n s f o r m a t i o n s a p p e a r i n g as a result of def o r m a t i o n a t r o o m t e m p e r a t u r e , using samples t h a t had first of all u n d e r g o n e prolonged annealing.
EXPERIMENTAL Low-density polyethylene (LDPE) samples were used in this investigation, as was also the case in refs. [2, 4, 5]. The isotropie film samples with a thickness of 2 mm underwent 600 ~o extension at room temperature, which was followed by annealing in the stretched state at 100° for 2 hr. The annealed samples were reoriented at room temperature across the direction of elongation. When the reorientation had taken place, the samples were removed from the stretching apparatus, and were then kept for one day to allow stress * Vysokomol. soyed. A15: No. 1, 85-90, 1973.
X-ray diffraction study of reorientation of polyethylene
95
relaxation to take place. Low- and wide-angle X-ray photographs were taken of parts of the same sample with different degrees of deformation. Both the low- and wide-angle pictures were obtained simultaneously with the aid of a special low-angle c&mere [6] at room temperature, the distance between film and sample being 200 and 15 mm for low- and wide-angles respectively. DISCUSSION
OF RESULTS
Figure 1 shows the low- and wide-angle X-ray photoTwo jibrillar subgroups. graphs characterizing structural changes occurring during the reorientation of LDPE
samples.
All the regions appearring in these X-ray
photographs
are situ-
FIG. 1. Low-angle (I) and wide-angle (II) X-ray patterns obtained for different parts of an oriented sample: a-original sample; b-e-intermediate stages; f-reoriented sample. Direction of initial reorientation is vertical; the reorientation direction is horizontal.
ated along the axial line of the sample (marked by the small circles in the schematic diagram shown in Fig. 2). The X-ray patterns in Fig. l&‘, d” were obtained for the d’ and cl” regions (Fig. 2) located above and below the axial line.
YE. V. GENIN et al.
98
Let us consider the wide-angle has crystallites
patterns shown in Fig. 1. The original sample
of c texture type, and the texture
original direction
of stretch.
axis of the latter follows the
During the course of reorientation
situated on the equator split up into two subgroups of reflections. crystallitea
is preserved
in each of these subgroups,
a
a
b
b
the reflections The c texture of
but the texture
axes are
d
d
e
FIU. 2. Schematic drawings of the sample for the low- and wide-angle X-ray patterns, and structural models illustrating the reorientation process. The regions in whioh the photographing was done are marked in the diagram by circleswith letters oorrespondingto those in Fig. 1.
inclined at an angle to one another in the different subgroups. As the deformation degree increases, the axes diverge from one another, the angle between them increasing until it becomes 90”. In the process each of the axes turns symmetrically to an angle of 45’ to both the old and the new axes of orientation. The angle between the texture axes is reduced during further orientation, and their direction now approximates to that of the new axis of orientation (Fig. le).
X-ray diffraction study of reorientation of polyethylene
99
I n the end the two subgroups merge to form a single system having crystallites with ~ type texture and a texture axis t h a t coincides with the direction of elongation. So far we have been dealing with the X-ray patterns obtained for the different regions of the sample located along its axial line. However, if the pictures are taken instead at the points d' and d" (see Fig. 2), it then appears that, in the regions situated above and below this line, there is now only one of the two subgroups of fibrils with crystallites of c texture type, wMch means t h a t ~he reorientation process results in two separate groups of crystallites, each with its own type of c texture, in the regions above and below the axial line. Near this line the two groups of crystallites are superposed one on the other, and a structure consisting, as it were, of two cross c textures results. The wide-angle X-ray photographs obtained in this investigation for the reoriented annealed samples mainly coincide with the corresponding photographs characterizing the reorientation process occurring with unannealed samples [4]. This means t h a t the process of change in the orientation of crystallites and the reorientation of the axes of the macromolecules from the original axis of elongation to a perpendicular direction proceed in much the same manner, whether or not the originally oriented sample has undergone annealing. Changes in the long period. I t is seen t h a t the low-angle X-ray photograph of the original sample (Fig. la) has the usual striated reflection of the fibrillar type with d r = 175 A. As the process of reorientation develops, this reflection gradually contracts towards the meridian, and becomes thicker (Fig. lb, c), its shape being now roughly spherical, which is probably characteristic for layer structures [7] (Fig. 1). In the course of further stretching the intensity of this reflection is reduced, and all that remains of the latter in the completely stretched sample is a thin weak line appearing on the equator of the low-angle X-ray photograph of the reoriented sample (Fig. l f). Starting at the stage marked by :Fig. lc, additional reflections (apart from the one last-mentioned) appear in the X-ray photographs, forming the rhomboid pattern that is clearly seen in Fig. ld. These new reflections t u r n round gradually and contract towards the meridian, which coincides with the direction of reorientation, and finally form a four-spot fibrillar reflection with dr~--125 A and ~ 3 5 ° (Fig. 2). In the course of investigation of the wide-angle photographs in the previous section it was found t h a t in the process of elongation all the macromolecules split up into two groups, each having its own texture axis. Low-angle X-ray photographs of each subgroup were therefore obtained and analysed individually to elucidate the general pattern of low-angle scattering. The X-ray patterns for the regions above, below and near the axial line are shown in Fig. ld, d', d". I t should be noted t h a t similar observations were reported in [5] in regard to annealed L D P E samples t h a t had been reoriented at various angles to the direction of stretch, and in actual fact the authors were observing the behaviour of a single fibrillar subgroup. As is apparent from the X - r a y patterns in Fig. ld', d", and from the data
100
Y~.. V. G ~
et al.
in ref. [5], there is a low-angle X-ray pattern, with four reflections, for each fibrillar subgroup, and these four reflections m a y be split up into two pairs of two-spot patterns. To describe the shape of these reflections we have first to examine the wide-angle structural diagram in order to determine the direction of t h e texture axis, which is the fibril axis (Fig. 3). I t can be seen from Fig. ld' that one pair of reflections is a tilting two-spot pattern [7] with dr-----175 A (Fig. 3),
F
Fie. 3. Schematic diagrams of the low- and wide-anglo X-ray patterns, and structural models corresponding to the patterns in Fig. ld, d', d". a n d this pair evidently corresponds to the low-angle reflection of the original structure, seeing that the size of the long period is the same. The other pair of reflections is in the form of a straight two-spot, but here df----125 A, i.e. the size of the long period differs. Schematic diagrams of the structures corresponding to these two two-spot patterns appear in Fig. 3. I t should be noted that both structures have a common texture axis, and moreover the direction of the macromolecular axis is the same, and the crystallites of these structures are of a planar type, as m a y be surmised from the two-spot form of the low-angle reflections [7]. However, the inclinations of the crystallite faces, as well as the sizes of the long periods differ in the structures corresponding to the two different two-spots. Previous investigations showed [7] t h a t a tilting two-spot with d-----175A corresponds approximately to bt/a-~l.0-1.5, and for t ~ 1.5 b/a:0.7-1.O, where a, b and t are the transverse and longitudinal dimensions, and the tangent of the angle of taper, respectively. For a straight two-spot bt/a~0.4-0.6 and t-~0.76, while blab--0.5-0.8, i.e. the transverse dimensions of the crystallites also differ in the two structures. As may be seen from the patterns in Fig. ld', d", the
X-ray diffraction study of reorientation of polyethylene
101
structure of the subgroups located above and below the axial line are altogether identical, apart from the direction of the texture axis. Reverting once again to the low-angle X-ray pattern obtained from the middle line (Fig. ld) this m a y be interpreted as consisting of two X-ray p a t t e r n s pertaining to different subgroups placed together near the middle line of the sample. I t will be seen that the reflection is approximately spherical, results from t h e original low-angle reflection (Fig. ld), consists of two tilting two-spots with d ~ 1 7 5 A, and is in fact a merged tilting four-spot. The other (rhomboid) intensity distribution consists of two straight two-spots with df----125 A. The stretching of the annealed sample is responsible for the difference in the size of the long periods of the two reflections. The long period of the annealed sample ( d ~ 175 A) is greater than that formed (d-~125/~) in the course of stretching at room temperature. In the end reflections corresponding to a period with d----125/~ remain in the reoriented structure, whilst those with d ~ 1 7 5 A weaken gradually, and disappear.
a
b
d
1~io. 4. Schematic diagram of the transformation of a low-angle reflection of fibrillar type into a layer one with increasing skewing of the erystallites. The letters a, b and v under the diagrams correspond to the notation in Fig. 1.
Transformation of low-angle reflections of the fibrillar type into larger ones. L e t us now consider the process whereby the original striated low-angle reflection (Fig. la) is transformed into the spherical reflection seen in Fig. ld..The transformation takes place gradually, without any alteration in the size of the long period. It is of major interest, in view of the change in crystallite shape involved in the process. In the original sample the long period is positioned along the texture axis, the direction of which coincides with that of the macromolecular axes. As the degree of deformation increases, the maeromolecular axes, and accordingly the fibrillar axes, diverge gradually from the original direction, and are finally turned through an angle of 90 ° . At the same time the centre of the reflection seen in Fig. la remains constantly in the same place, i.e. on an axis
102
YE. V. G~.~r~ et a/.
corresponding to the original direction of the macromolecular axes. This means that the position of a crystallite face, the normal to which is directed towards the centre of a reflection, will likewise remain unaltered. Owing to the constant rotation of the fibrillar axis, there will be a corresponding constant increase in the angle between the macromolecular axes and the normal to the face of a crystallite. Therefore, as the process of contraction continues, a crystallite will be transformed from a rectangular parallelepiped into an oblique-angled parallelepiped, and the degree of bevelling, characterized b y the angle fp between the normal the face of the erystallite and the axis of the fibril will be still further increased. The process involved is like the shear deformation of a crystallite if the direction of shear coincides with that of the macromolecules. As a result of this "shearing" a crystallite is gradually transformed into a plate where the axes of macromolecules pass approximately along the plane of the plate (Fig. 4). I t was found in ref. [7] that the value of bt/a is a general parameter influencing the shape of a low-angle reflection. With increased bevelling the value of ~ rises, and t = t a n ~, hence the magnitude of bt/a will likewise be increased. This increase governs the change in the shape of a reflection from the striated to spherical type. Consequently the gradual change in the shape of the reflection with d = 1 7 5 4 during the reorientation process marks a smooth transition during which a erystallite is transformed from a parallelepiped into a plate. The process of transition from a reflection of fibrillar t y p e to a layer reflection (analysed above) differs from the case considered in ref. [7] involving the shrinkage of oriented polyethylene sampies. In the process of shrinking the fibrillar structure is transformed into a layer one owing to a change in the size of the crystallites, while in the case of reorientation only a change in crystallite shape is involved. It was pointed out above that during the reorientation process the other pair of reflections with d-----125 A has the appearance of a straight two-spot, and therefore characterizes a fibrillar structure with crystallites in the form of parallelepipeds. One may assume that during the deformation process the plates m a y split up into separate blocks forming a second system of fibrils with d = 1 2 5 4. In view of the X-ray data obtained b y investigating the reorientation of oriented L D P E samples the structural changes occurring in the polymer could be described as follows. The reorientation process results in the formation of two regions on either side of the axial line. Each of these regions consists of fibrillar subgroups, the crystallites of the latter having a planar type c texture, and these subgroups turn in the direction of elongation. Moreover there is another region close to the axial line of the sample, and in this region there are fibrillar subgroups superposed on one another. The direction of bevelling of crystallites in the subgroups shows that in the regions on either side of the axial line the effect of what would appear to be shear deformations is conductive to more rapid slipping of the regions located closer to the "neck". The reorientation of the P E samples, whether or not t h e y have been annealed, proceeds in accordance with this mechanism. I t is in respect to changes in the size of the
X-ray diffraction study of reorientation of polyothyleno
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long period t h a t the behaviour of the annealed and unannealed P E samples differs. I n the case of the annealed samples the long period of the original films is determined by the annealing temperature, and during the process of reorientation, by the temperature of the medium, i.e. room temperature; in the case of unannealed P E samples the initial stretching and reorientation take place at the same temperature, and so there is no change in the size of the long periods in this case. CONCLUSIONS
(1) I t has been shown t h a t during the reorientation of low-density polyethylene (LDPE) films two regions with dissimilarly oriented texture axes are formed on either side of the axial line; near this line the two regions in question are superposed on one another. The stretching process is accompanied by gradual turning of the fibrils and the macromolccules. (2) The transition from low-angle reflections of fibrillar type to layer ones takes place, in the case under consideration, through rectangular crystaUites undergoing shear deformation, and changing into elongated plates. (3) The difference in the behaviour of the annealed and unannealed L D P E samples is due to the fact t h a t in the case of the former the size of the long period changes, whilst for the latter it remains constant. REFERENCES 1. H. HENDUS, Kolloid-Z. 165: 32, 1959 2. V. S. KUKSENKO, S. NIZAMIDINOV and A. I. SLUTSKER, Vysokomol. soyed. Ag: 2352, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 11, 2659, 1967) 3. A. KELLER and J. G. RIDER, ft. Mater. Sei. 1: 389, 1966 4. V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. BI2: 653, 1970 (Not translated in Polymer Sei. U.S.S.R.) 5. N. SULTANOV, B.M. GINZBURG and S. Ya. FRANKEL', Vysokomol. soyed. 13: 2691, 1971 (Not translated in Polymer Sci. U.S.S.R.) 6. V. I. GERASIMOV, D. Ya. TSVANKIN, Pribory i tekhnika eksperimenta, No. 2, 204, 1968 7. V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. A12: 599, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 11, 2944, 1970)