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Structural transitions in polyoxypropylene
STRUCTURAL TRANSITIONS IN POLYOXYPROPYLENE* V. A. K~GI~, T. I. SOGOLOVAand V. M. RUBSHTEIN L. Ya. Karpov Physical Chemistry Institute
(Received 15 Augu~ 1967) THE s t u d y o f l a r g e d e f o r m a t i o n s in c r y s t a l l i n e p o l y m e r s o v e r a wide r a n g e o f t e m p e r a t u r e s is o f g r e a t t h e o r e t i c a l a n d p r a c t i c a l interest, since p r o f o u n d s t r u c t u r a l c h a n g e s t a k e p l a c e d u r i n g t h e p r o c e s s i n g a n d use o f p o l y m e r i c m a t e r i a l s u n d e r t h e effect o f v a r i o u s m e c h a n i c a l a c t i o n s . T h i s a p p r o a c h m a k e s it possible to establish most accurately the connection between the actual supermolecular s t r u c t u r e s in t h e p o l y m e r a n d its m e c h a n i c a l p r o p e r t i e s [1-4], N e w e x p e r i m e n t a l r e s u l t s are p r e s e n t e d in t h e p r e s e n t p a p e r ; t h e s e w e r e o b t a i n e d in t h e s t u d y o f l a r g e d e f o r m a t i o n s in u n i a x i a l t e n s i o n o f p o l y o x y p r o p y l e n e ( P P 0 ) s p e c i m e n s b o t h o f t h e initial m a t e r i a l a n d also c o n t a i n i n g a r t i ficial n u c l e i o f a s t r u c t u r e - f o r m i n g a g e n t .
EXPERIMENTAL An apparatus specially constructed by us for straining sheet specimens directly on the stage of an optical MBI-6 microscope considerably broadened the experimental possibilities of the investigation, since the automatic recording of the stress-strain curve was carried ' out simultaneously with the observation of the structural changes in the deformed specimen. The apparatus consists of the straining rig which is mounted on the microscope stage, a dynamometer unit and a recorder. (A general view of the apparatus is shown in Fig. la, and the straining rig is shown in Fig. lb.) The specimen under test 1 is secured in grips one of which, 2, is connected to the lead screw, 3, of the drive, which consists of a reduction gear, 4, and an electric motor, 5. The second grip, 6, is secured to a steel plate, 7, on to which a balanced bridge consiting of four strain gauges is cemented. The force applied to the specimen as it is stretched leads to bending of the plate, causing the bridge to become unbalanced. A certain voltage appears across the measuring diagonal of the bridge; this is amplified and recorded on the EPP-09 recorder. The speed of the recorder chart is selected to be a multiple of the rate of elongation of the specimen. I n this way, the load taken by the specimen as it is stretched is plotted along the horizontal axis, and the deformation is plotted along the vertical axis. The calibration of the rig was carried out as a preliminary; that is, the value of the scale unit on the recorder was established from the bending of the plate under the action of small weights. A detachable thermal block makes it possible to test specimens under isothermal conditions over a temperature range from -- 150 to 200°C. The study of large deformations was carried out on dumbell-shaped PPO specimens which were blanked out from film (thickness 10-30 ~) (dimensions of the working section of the specimen: length, 3.5 ram; width, 1-1 ram). Films with various supermolecular structures * Vysokomol. soyed.'Al0: No. 9, 2017-2027, 1968. 2343
V . A . KAROr~ et al.
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FI(~. 1. a - - G e n e r a l view of the equipment, and b - - t h e rig for straining polymer specimens on the microscope stage. were obtained b y evaporation of the solvent from solutions of PPO in benzene at 75°C over 2 hr. FiLms having supermolecular structures With different spherulite sizes wore obtained by varying the cooling rate of the melt obtained (melting point of PPO, 73°C) from 1-2 dog C/rain to rapid cooling in nitrogen. Tests were carried out under isothermal conditions in the temperature range from - - 1 1 0 to 55°C at a constant rate of elongation of 1 ram/rain. RESULTS AND DISCUSSION
Structural changes at large deformations in uniaxial elongation of PPO over a wide temperature interval. F r o m t h e e x p e r i m e n t a l d a t a o b t a i n e d i n t h e s t u d y of the deformation
of PPO
having
a coarse spherulitic structure
(spherulite
Structural.transitions in polyoxypropylene
2345
size 250/~ and above) (Fig. 2) it may be seen that, in the temperature region from --110 to 50°C, the specimens have a high rupture strength which falls with an increase in temperature, and rupture occurs without any marked deformation; the shape and size of the spherulites thus remain practically unchanged (Fig. 3a). The development of about 150% deformation is observed from --45 to 0°C, and the strength decreases with increase in temperature. Figure 3b, shows that ~, kig/O?.~2
800I_ll0 800I_9800 ~001~70
-~5 1o
200 _ _ _ _ _ _ / 0 0
50 i
I00
200
300
i
~00 500 C,%
FIO. 2. Stress-strain curves for coarse spherulitic P P O specimens. The n u m b e r s on the curves correspond to t h e stressing t e m p e r a t u r e .
these deformations arise from the elongation of the spherulites themselves. It should be noted that, over a certain temperature range, the specimen strength changes with temperature but the elonga{ion at break remains practically constant. In the temperature region from 10 to 50°C the specimens deform by more than 300%, and the entire elongation process takes place in three stages, as may be seen from-Fig. 2, the shape of the stress-strain curves being typical of crystalline polymers. However, the rupture characteristics of the supermolecular structure have been made apparent for the first time in these experiments. The clear step-wise breakdown of the individual spherulites and of the entire specimen as a whole is shown in Fig. 3c. This type of structural breakdown which arises during tensily deformation is reminiscent externally of the way in which slip planes originate in ordinary low molecular weight crystals, bug upon more detailed examination it turns out to be a system of parallel broad consecutive bands, closely connected with the appearance and development of a large number of necks. The formation of the necks does not take place all at one time, but consecutively, and has clearly expressed discontinuous characteristics (with a rate of elongation of 1 Tn~n/min or 30%/rain, and a specimen thickness of about 20#, the formation of a new neck took place each three seconds). The boundaries separating the necks formed appear as lines in relief, which are dearly visible in polarized and reflected light (Fig. 3c, e). These bands and the lines in relief traverse practically the entire specimen (in Fig. 3c), but, in going through the different spherulites, they reproduce the outlines of the spherulite boundaries.
2346
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i~ ¸
:FIG. 3. Photomicrographs of deformed coarse spherulitic PPO specimens, a ~ / - - Breakdown of supermolecular formations in PPO specimens during elongation at 1 ram/rain in the temperature range --110 to 50°C, e--boundaries in relief between necks, under reflected visible light. FIG. 5. Photomicrographs of deformed fine spherulitic PPO specimens, a, b--Breakdown of supermolecular formations in PPO specimens with different spherulito sizes, c - e - - r u p t u r e of PPO specimens having a practically uniform structure as seen in the optical microscope, during elongation at 1 m m / m i n in the temperature range from --110 to 55°C. :FIG. 7. Photomicrographs of coarse spherulitic PPO specimens deformed at various rates at 20°C: a--300, b--30, e--3 mm/min.
Struotural transitions in polyoxypropyleno
2347
The rupture characteristics of PPO remained of the same type at temperatures above 50°C. It was observed that, under these conditions, the necks being formed break up into fibrillar formations. When this happens, the lines in relief, which ~rupf,kon/cmz 1300
,o1o
k gO0
300"1 100
-eb 40
I
o
]
eo ,oc
FIe. 4. Temperature dependence of the true strength of coarse spherulitic PPO specimens.
always arise perpendicular to the line of action of the tensile force, are retained (Fig. 3d). A further increase in deformation is observed in this temperature region (because of the formation of fibrils) and all three sections on the stress-strain curve are realized; however, they are less clearly expressed because of the flow of the material. In this way, a study of large deformations in coarse spherulitic PPO specimens over a wide temperature interval has made it possible to separate the temperature regions in which changes in the value of the deformation at break correspond to transitions from one type of breakdown of the supermolecular formations to another type of breakdown. These transitions are also clearly apparent in the form of steps on the curve giving the temperature dependence of the true strength (Fig. 4 and 3a-c).. It was essential to clarify whether the mechanism by which the supermolecular formations break down with the formation of consecutive necks was a characteristic only of coarse spherulitic structures in PPO, or whether this phenomenon is more general. For this purpose, similar experiments were carried out on PPO specimens having various supermolecular structures. The experiments showed that the characteristics of the breakdown of t h e supermolecular structures, with the formation of consecutive necks, were retained in specimens of all types, even when the structure was hardly resolvable in the optical microscope (Fig. 5a, b), but the crystallinity of the specimens had been checked by X-ray methods. With a reduction in the spherulite size, the relief lines between the necks become more linear in view of the greater homogeneity of the supermolecular formations.
2348
V.A.K.~GxN
at a/.
Stress-strain curves are shown in Fig. 6 for specimens whose structure is shown in Fig. 5b; it m a y be seen that the general characteristics o f the temperature dependence of the strain are retained. A temperature region exists (-- 110 to ~60°C) in which the specimens rupture by brittle fracture (see Fig. 5c) and a reduction in strength is also observed with an increase in temperature. Then, in th~ temperature range from --60 to --10°C, considerable elongation of the films takes place with the formation of a "neck", the structure of which could not be seen (Fig. 5d). When the neck is formed at --10°C and above, wise breakdown is observed with the formation of consecutive necks (Fig. 5e). The new information
0", kg/c,v 2 600 :-¢fo ,-90 #00 ;-70 -80-50-#0
- fO
55 I
I
I
I
100
200
300
000
J
R
500
600
'
~,%
FIG. 6. Stress-strain curves for fine spherulitic P P 0 specimens. Tho numbers b y the curves correspond to the stressing temperatures.
which was obtained from investigations of specimens of this type (the initial specimen having structural elements not resolvable optically) in practice amounts to the fact that they exhibit temperature regions in which a marked change occurs in the properties of PPO under conditions of tensile deformation; a considerable reduction in the dimensions of the polymer's supermolecular structural elements causes the temperature at which large deformations begin to develop with the formation of consecutive necks to be 15-20°C below the corresponding temperature in specimens with a coarse spherulitic structure, and there is no sharp jump in the values o f the rupture deformation around the temperature of the second transition, i,e. the transition to breakdown with the formation of consecutive pecks. A comparison of the stress-strain curves (Figs. 2 and 6) shows that in the brittle fracture region the values of the reerystallization stress over the entire temperature and strength range investigated is considerably greater for specimens with a coarse spherulitic structure. These differences in mechanical properties are clearly connected with the special features of the supermolecular structure in the specimens investigated. Unfortunately, in the experiments with specimens having a very fine supermolecular structure, there was naturally no possibility of observing all the structural changes which had been previously observed by means of the optical microscope, However, analysis of the stress-strain relationships over a wide tern-
Structural transitions in polyoxypropylene
2349
perature range made it possible to conclude that there exist structural transitions which take place during the elongation of these PPO specimens. The structural transitions which we have observed when PPO is stretched do not depend on the dimensions of the supermolecular formations, but there is an effect of these dimensions in t h a t the structural transitions occur in different temperature and stress regions. Since the step-wise breakdown of the supermolecular structures in PPO was observed at a low rate of elongation (1 ram/rain), it was essential to clarify the effect of elongation rate on the characteristics of the structural transitions in PPO. For this purl~ose, the deformation of PPO specimens was investigated, at various rates of elongation, 3, 30 and 300 m ~ / m i n over a wide range of temperatures. From the experimental data obtained, which are shown in the Table, it m a y be seen that the values of the strength characteristics (arupt., arecr., 5) depend on the rate of elongation and on the temperature. The rise in the rupture stress values with an increase in the elongation rate at temperatures below 10°C agrees with the general rule which is true for all solid bodies. This rule does not hold above 10°C. A study of the supermolecular structures in specimens deformed at various elongation rates showed t h a t a temperature region exists in which brittle fracture occurs independent of the rate of elongation, the shape and dimensions of the spherulites being preserved. Large deformations become apparent in the temperature interval from --45 to 10°C, so that specimens, independent of the rate of elongation (within the range of elongation rates investigated by us), are deformed through the elongation of the sl~herulites themselves. Tests at 10-20°C show that a change in the elongation rate causes a substantial difference in the characteristics of the structural changes, and consequently in the mechanical properties. Figure 7 shows the types of structural change which occur in specimens tested at 20°C at various elongation rates. A high rate of elongation causes deformation of the specimens through the elongation of the spherulites themselves (it should be noted that this type of structural change occurs in tests at high rates over the entire temperature range in which large deformations appear). With a medium rate of elongation and at a temperature of 10°C, but more clearly at 20°C, a stepwise breakdown of the spherulites is observed, with the formation of elliptical concentric formations. At a low rate of elongation, the breakdown which we had already found is again observed, with the formation of necks which are formed consecutively (it should be noted that the distance between the relief lines in this case is greater than in the experiments described above, at an elongation rate of 1 ram/rain). The change in the type of structural transition is reflected in the values of deformation and rupture strength (Table). Whereas, at temperatures below 10°C an increase in the elongation rate causes an increase in strength, recrystallization stress and elongation at rupture, at temperatures from 10 to 20°C, when an increase in elongation rate gives rise to a change in the type of structural transition,
2350
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l(A~orN e t a / .
this rule a b o u t the s t r e n g t h a n d elongation a t r u p t u r e is n o t true. This a n o m a l y is u n d o u b t e d l y caused b y the different changes in the s t r u c t u r e o f the p o l y m e r during elongation a t different rates. Tests a t higher t e m p e r a t u r e s (50°C a n d above) give uniform values of the s t r e n g t h characteristics i n d e p e n d e n t o f t h e strain rate; this is c o n n e c t e d with the fact t h a t the supcrmolecular s t r u c t u r e s become fibriUar, as observed b y m e a n s o f the optical microscope. CHANGES ON
IN THE
STRENGTH
THE
TEMPERATURE
Rate of elongation
CHARACTERISTICS AND
RATE
OF
OF
PPO,
DEPENDENT
ELOI~GATION
T °, C
~rrecr,, kg/cm 2
kg/cm s
--70
650
650
450 425 450 320 290 310 280 275 300 180 140 230 115 05 170 100 9O 160 90 85 65 5O 4O
45o 425 450 320 290 360 35o 345 350 270 230 34o 235 125 260 210 320 25o 230 245 75 7o 6o
~'rupt.,
8,%
High, 300 ram/
/mh~ Medium, 30 mm/ min Low, 3 mm/min High Medium Low High Medium Low High Medium Low
--55 --45 --25
High Medium Low High Medium Low
10
High
20
Medium Low
High Medium Low
50
20o 150 140 200 150 140 240 180 150 240 260 320 300 280 320 65o 55o 5o0
Tests a t different elongation rates h a v e therefore shown t h a t step-wise breakdown with the f o r m a t i o n o f consecutive necks occurs in a definite t e m p e r a t u r e range a n d a t c o m p a r a t i v e l y low rates o f elongation, a rise in the d e f o r m a t i o n capacity a n d s t r e n g t h t h u s being observed. These e x p e r i m e n t s m a k e it possible to consider t h a t the b r e a k d o w n o f the supermolecular s t r u c t u r e with the f o r m a t i o n o f consecutive necks imparts a definite set o f mechanical properties t o the poly-
Structural transitions in polyoxypropylene
2351
met. The phenomenon which has been observed, that the supermolecular structures break down in s~ps with the formation of necks, arising in a consecutive manner and regularly positioned, requires more detailed explanation. I t should be remembered t h a t the formation of a neck proceeds as a result of structural changes which occur with time. I f the strain rate is greater than the rate of these structural changes over-stressing inevitably occurs, leading right up to rupture or to the start of a structural transition. Therefore when a neck begins to form a considerably greater stress always arises, and this stress leads to the storing of potential energy in the specimen which is deformed at first as a whole; this energy is used up in the development of the neck at a rate very much greater than the rate of movement of the grips. The practically instantaneous formation of the first neck leads to a fall in the stress in the specimen to a value at which any further structural change is impossible. But since the elongation of the specimen proceeds continuously and at a constant rate, the accumulation of elastic energy again takes place in it, and the rapid, almost instantaneous, formation of a new neck again takes place at a certain value of over-stressing. The repetition of these cycles gives a discontinuous regular breakdown. (When the rate of breakdown is so large that the growth of the neck has time to occur during the deformation of the specimen, a single neck is formed which stretches across the entire specimen.) Why does the formation of each subsequent neck begin at the place dividing the isotropic section of the specimen from that already orientated, with the formation of a transverse relief thickening ? This is explained by the fact that the transitional region from the isotropic specimen to the neck turns out to be orientated, and consequently stronger than the isotropic part of the spechnen and stronger than the part of the specimen which has already been stretched but which is thinner. Consequently, the formation of each new neck begins directly at the side of this strengthened section, within the isotropic part of the specimen. The fact t h a t these transverse lines of thickening exist until the entire isotropie part of the specimen breaks down into necks, and only become smoothed out immediately before rupture of the specimen, is an indication t h a t these regions of transverse thickening are stronger. The kinetic nature of the phenomenon is also shown by the fact that, as the rate of elongation is increased, the length of the consecutively formed necks increases. In fact, the larger value of over-stressing also implies a greater rate of formation of the neck, and this specifies that a greater amount of isotropic material passes into the neck. Moreover, it should be noted t h a t a greater stress is required to initiate the formation of a neck in crystalline polymers than is needed for its development. It is just this which also leads to the step-wise creation of the neck with a dividing boundary. Since the structural changes do not occur instantaneously under the real kinetic conditions of our experiment, but lag somewhat behind the deformation process, over-stressing peaks arise periodically in the specimen as a result of its elongation, and these lead to the repeated step-wise formation of necks.
2352
V.A.
KARGIN et al.
I t becomes clear from all t h a t has been p ut forward t h a t relaxation phenomena which occur in polymer bodies under an applied external force field, m a y also explain the breakdown of the supermolecular structures which occurs under the various conditions. The fact t h a t the formation of a regular system of necks is independent of the spherulite dimensions indicates t h a t the limiting structural element which determines the rate of the transition is not the spherulite, b u t a still finer structural unit.
Structural transitions at large uniaxial tensile strains in P P O in the presence of coarse foreign inclusions acting as artificial nuclei for structure formation, and the role of their surface in strengthening the polymer material. The second part of the work was devoted to a s t udy of problems connected with the effect of a foreign surface on the supermolecular structures of the polymer, and the effect of these structures on the properties of crystalline polymers. This was important because the introduction of artificial nuclei for structure formation in polymers capable of crystallization is a new and potentially very powerful m e t h o d of regulating
Fro. 8. Photomicrographs of PPO specimens with an artificial structure-forming agent introduced (aa~thraquinone dye): a--spherulitic band of PPO, formed at a large acieular crystal of the dye; b, c--formation of a macro-defect at a crystal lying perpendicular to the line of action of the tensile force; d and e--because of a crystal lying parallel to the line of action of the tensile force, a macro-defect developing in the specimen is eliminated. the supermolecular structure and physicomechanical properties of polymers [5-9], and P P O is a ve r y convenient material for investigating structural transitions in crystalline polymers. I n order to study these problems effectively, it was
Structural transitions in polyoxypropylene
2353
very important to obtain a supermolecular structure in the polymer on a comparatively large surface area of the foreign bodies introduced as artificial nuclei for structure-formation. Coarse particles of a fat-soluble anthraquinone clearblue dye were used for this purpose (the latter was introduced into a solution of PPO in isopropyl alcohol). The use of this structure-forming agent made it possible to obtain films in which spherulitic bands of PPO were formed at the coarse acicular crystals (approximately 1000/~ long) o£ the dye (Fig. 8a). The crystals of the artificial structure-forming agent were positioned at random in these specimens and it was noted that the processes of deformation and rupture of the PPO specimens took place differently depending on the position of these particles. Crystals positioned perpendicular to the line of action of the tensile force frequently contributed to the formation of macro-defects, and this led to the rapid rupture of the specimens (Fig. 8b, c). Coarse crystals positioned parallel to or at a small angle to the line of action of the tensile force did not prevent the development of high strains in the specimens. Moreover, if such crystals
~I
16-18# 0.3mm 5 ¢~
C 5 Fxo. 9. Schematic diagram of a PPO model specimen. occurred in the path of a macro-defect growing across the entire width of the specimen, the crystals stopped its growth (Fig. 8d, e). It was noticed for the first time in these experiments t h a t the artificial structure-forming agents create a barrier preventing the growth of a macro-defect in the specimen whilst the specimen is being strained. In connection with the essentially new effect which had been observed, the "barrier action" of coarse foreign inclusions, exploratory investigations were carried out on model PPO specimens which had an artificially produced macrodefect, a slit, and which contained a fibre as the barrier (Fig. 9). In preparing the model specimens, straight fibres lying parallel to each other were positioned in the solution of PPO in benzene. Then specimens were cut out of the film obtained, so t h a t the fibre lay along the specimen practically at its centre. The slit took up approximately 25~/o of the width of the working section of the speei-
2354
V.A.
K~GI~
et a~.
men. In order to assess the part played by the nature of the surface of the fibre used as the artificial nucleus for structure formation fibres with various chemical structures were studied, i.e. po!ypropylene, teflon and polyacrylonitrile. (The model specimens tested had a thickness of 30-50 ~ with a fibre diameter of 15-18 ~.) From observation of the specimens in polarized light, it was established t h a t the polypropylene and teflon fibres are artificial structure-forming agents, a n d a well-formed spherulitic band of PPO occurs at their surfaces, but the polyacrylonitrile fibre does not initiate the structure-formation process at its surface (Fig.
10a- }. The study of the tensile deformation of model specimens showed that if the fibres were artificial nuclei for structure formation, the slit was prevented from penetrating into the body of the specimen,' i.e. the "barrier" action effect was completely confirmed. The fact t h a t the source of the rupture was localized and actually eliminated led to its becoming possible to realize large deformations in the specimens, amounting to more than 200~.
Fro. 10. Pho~omicrographsofPPOmodelspecimenscon~aining:a--polypropylene,b--teflon, and c--acrylonitrile fibres. From the microphotographs (Fig.1 la-e) it m a y be seen that as the specimen is stretched the slit takes on the form of a very broad wedge with a rounded point, whicl~ gradually penetrates into the body of the specimen, but then the source of the rupture is eliminated and further elongation leads to deformation by the usual mechanism, as if a PPO specimen without external defects were being stretched (Fig. l ld, e). In experiments with specimens containing a polyacrylonitrile fibre, which is not an artificial nucleus for structure formation, the slit penetrated without hindrance across the entire width of the specimen as it was stretched, and the rupture characteristics did not differ in any qualitative way from that observed in stretching a slit PPO specimen not containing a fibre (see Fig. 12a-v and Fig. 13a-c). I n this way, two groups of phenomena were observed: the first occurs when a foreign body, whose surface is an artificial nucleus for structure formation, exhibits the "barrier" action, that is, the body prevents the rupture of the specimens
Structural transitions in polyoxypropylene
2355
~ii~ i~
ilil
i!
FxO. 11. Growth of a slit during the stretching of a PPO specimen containing a structureforming fibre. FIG. 12. Growth of a slit during the stretching of a PPO specimen e o n t a h ~ l g a fibre which is not a structure-forming agent. I~xG. 13. Growth of a slit during the s t ~ c h i n g of a PPO specimen not containing a fibre.
2356
V. A. KAmax~ e~ ed.
at a macro-defect, and in this way makes it possible to achieve laxge deformations; and t h e second occurs when a foreign body, which is not a nucleus for structure formation, does not substantially prevent the spread of the macro-defect in the specimen. Analysis of the experimental data showed that rupture of the fibre inevitably occurs during the elongation of model specimens (Fig. l l e and Fig. 12c). This is explained by the fact that orientated fibres are not capable of large deformations. I f the fibre is not a structure-forming agent, its adhesion to the polymer is comparatively small and such fibres break as the specimen is stretched and tear away from the polymer. Therefore, such a fibre itself is not a barrier preventing the propagation of the slit. On the other hand, in the initial experiments with PPO specimens containing coarse long crystals of a dye, the crystals being nuclei for structure formation, it was noted that crystals positioned in the direction in which the tensile force acted broke up into small pieces and did not separate from the polymer, i.e. the adhesion of an artificial structure-forming agent to the polymer was fairly high. The effect of the barrier action was thus observed both for crystals and also for fibres. Evidently, it m a y be accurately stated that the foreign body itself is not the barrier which substanially prevents the propagation of the slit, with the relative fibre and film dimensions studied. What then explains the barrier action effect which occurred in experiments when the foreign body introduced into PFO was an artificial nucleus for structure formation? We shall discuss this phenomenon in somewhat more detail. A considerably greater number of centres of crystallization (as compared with the remaining part of the specimen) occur on the surface of the artificial nucleus for structure formation, and this leads to the adhesion of the unusual "spherulites" in the band closely adjacent to the surface of the structure-forming agent. The area of spherulites, closely aggregated in the band and having centres positioned on the surface of the structure-forming agent (the crystal or fibre), is most highly ordered close to the surface of the structure-forming agent, and is less well ordered at its periphery. During stretching the slit passes through the outer, not well ordered, boundary of the spherulitie band and reaches its centre, which is the artificial structure-forming agent. However the foreign body itself, as has been shown above, is not a hindrance to the further propagation of the slit, since at this time the artificial nucleus for structure formation is not intact. Further penetration of the slit turns out to be impossible because it encounters the surface of the ordered region of the spherulitic band, which is the internal edge of the unruptured portion of the specimen. One could have suggested that the termination of growth of the slit is caused by the presence of the foreign body (the fibre or crystal) in the film, or also by the increased strength of the spherulitic band. In order to check this suggestion, PPO specimens were prepared without a fibre, b u t containing a band having a spherulitic structure obtained by a special method. It turned out that such bands did n o t inhibit the growth of the slit. In this way, the presence of a structural
Structural transitions in polyoxypropylene
2357
non-uniformity in t h e form of a band of spherulites is insufficient to stop the initiated rupture. It follows from this that the spherulitic bands obtained because of the presence of a structure-forming agent differ from bands of spherulites formed without such an agent, either in their internal structure or in some special feature of the surface. Since the spherulitic band has two essentially different surfaces (an external surface and an internal surface adjacent to the fibre), it is necessary to consider this in some detail. As we have already shown, the growth of the slit is not halted when it encounters the external, less well ordered, surface of the spherulitic band. I t easily passes through it and through the internaI well ordered surface, and exits at the internal ~, kg/cm~ /
20 y o
i 60
3
I
I
I
120
180
2#0
~,%
FIG. 14. Stress-strain curves for the following specimens: / - - t h e initial PPO; 2 - - a s 1, b u t containing a polypropylene fibre; 3 - - a s 1, b u t containing a teflon fibre; 4---as 1, b u t containing a pplyacrylonitrile fibre.
surface of the spherulitic band. The termination of further growth of the slit m a y be explained by the exit of its tip into the cavity occupied by the structureforming agent (by the fibre or by the crystal). However, the growth of the slit is not halted in the case of specimens with fibres incapable of causing structure formation in the specimen, but which do form cavities around themselves in the specimen. This points to the essential role played by the structure-forming activity of the fibre or any other structure-forming agent. Therefore, the spherulitic band should have not only increased strength, but also a special structure in its interior and on the surface adjacent to the fibre, the structure-forming agent. The non-uniform structure of the spherulitic band itself is apparent from the fact that it is easily ruptured when the tip of the slit tears through it from its outer edge, whereas, when the tip of the slit acts on its interior surface (the surface of the cavity in which the fibre fragraents are located), the band successfully resists rupture and the growth of the slit is terminated. The "structural barrier" effect is graphically illustrated by the stress-strain curves (Fig. 14): the capacity for deformation of specimens with polypropylene or teflon fibres, which are artificial nuclei for structure formation, is more than twice as great as the capacity for deformation of specimens without a fibre~ or containing a polyacrylonitrile fibre. It is strange that the recrystallization stress turned out to be highest for specimens with a polyacrylonitrile fibre, which is
2358
V.A.
~A~Gn~ e~ a/.
not a structure-formlng agent, but which has the highest strength of the fibres selected by us for the model specimens. It follows from all that has been put forward above that we have observed a structural barrier effect, which consists in the localization and prevention of further growth of the macro-defect at which rupture of specimens takes place during their elongation. Naturally, the barrier action effect should depend on the ratio of the specimens deformation rate to the rate of development of the macro-defect, and also on the ratio between the film thickness and the dimensions of the particles of the structure-forming agent introduced. CONCLUSIONS
(1) With polyoxypropylene as an example, it has been shown that the substantial change in the mechanical properties of crystalline polymers, depending on temperature, is caused by the different characteristics of the way in which the supermolecular structures break down, the breakdown taking place on different levels depending on the temperature and rate of elongation. (2) It has been observed that over a certain range of temperatures and rates of elongation the rupture of the supermolecular structures proceeds as a result of the consecutive step-wise creation of necks formed in a regular fashion. This type of breakdown of supermolecular structures specifies a set of polymer properties which is not observed in other types of structural transition and creates additional possibilities for regulating the mechanical properties of polymeric materials. (3) Experhnents at low rates of elongation have made it possible to develop more profound concepts about the mechanism by which necks 'form, since the existence of two processes taking l~lace at different rates has been graphically demonstrated: the first process is the elongation of the specimen at a given rate by movement of the grips, and the second is the process of structural transition, the rate of which is determined by the magnitude of the over-stress created in the specimen and by the structure of the given polymeric body. (4) A new method of regulating the structure of polymers has been developed by the method of introducing fairly coarse structure-forming agents of different dimensions and shapes, the effect of these agents not being apparent over the entire volume of the body, but in the individual places most liable to failure. The polymeric body thus acquires a resistance to rupture by active mechanical effects. Trans/a~ed by G. F. MODLEN REFERENCES 1. V. A. KARGIN and T. I. SOGOLOVA, Zh. fiz. k]~irnll 27: 7, 1208, 1214, 1325, 1953 2. T. I. SOGOLOVA, Dissertation, 1963
3. V. A. KARGIN, T. I. SOGOLOVAand L. L NADAREISHVILI, Vysokomo]. soyed. 6: 1407, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 8, 1554, 1964)
Copolymerization of p-isopropenylphenylacetate and butadiene
2359
4. V. A. KARGIN, T. I. SOGOLOVA and V. M. RUBSHTEIN, Delft. Akad. Nauk SSSR 175: 1087, 1967 5. V. A. KARGIN, T. I. SOGOLOVA and T. K. SHAPOSHNIKOVA, Dokl. Akad. Nauk SSSR 156: 1156, 1964 6. V. A. KARGIN, T. I. SOGOLOVA and N. Ya. RAPOPORT, Dokl. Akad. Nauk SSSR 163: 1194, 1965 7. T. I. SOGOLOVA, Mekhanika polimerov, No. l, 5, 1965 8. V. A. KARGIN, T. I. SOGOLOVA and V. M. RUBSHTEIN, Vysokomol. soyed. 8: 645, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 4, 707, 1966) 9. T. I. SOGOLOVA, Mekhanika polimerov, 643, 1966
RELATIVE REACTIVITIES IN THE COPOLYMERIZATION OF p-ISOPROPENYLPHENYLACETATE AND BUTADIENE* L. M. KOGA17, A. I. YEZRIYELEV, A. B. PEIZl~ER a n d A. V. LEBEDEV S. V. Lebedev All-Union Scientific Research Institute for Synthetic Rubber
(Received 25 August 1967) THE a i m o f t h e p r e s e n t w o r k w a s to clarify t h e effect of t h e s t r u c t u r a l f e a t u r e s o f p - i s o p r o p o n y l p h e n y l a c e t a t e ( I P P h A ) on its r e a c t i v i t y in c o p o l y m e r i z a t i o n w i t h b u t a d i e n e (BD). T h e r e is a reference in t h e p a t e n t l i t e r a t u r e to a m e t h o d of o b t a i n i n g I P P h A f r o m p - i s o p r o p o n y l p h e n o l ( I P P h ) b y t h e S c h o t t e n - B a u m a n n r e a c t i o n [1]; howe v e r no p h y s i c a l c o n s t a n t s a n d no evidence of t h e s t r u c t u r e o f t h e c o m p o u n d o b t a i n e d were given, a n d t h e r e a c t i v i t y o f t h e p r o d u c t , including its r e a c t i v i t y in r a d i c a l reactions, w a s n o t s t u d i e d a t all. I t s e e m e d v e r y i m p o r t a n t t o us to fill t h e l a s t - n a m e d gap, since I P P h A is a s t r u c t u r a l a n a l o g u e of a - m e t h y l s t y r e n e and, as one should e x p e c t , also h a s a fairly r e a c t i v e e s t e r group, whose presence in t h e p o l y m e r w o u l d m a k e it possible t o carry out a number of interesting polymer-analogue transformations. The synthesis of I P P h A was caried out by the usual method [1]. Because of the presence of the conjugated double bond in the IPPhA molecule's isopropenyl group, exaltation of the molecular reaction EM=1.282 (Rexalt=51.980; Bad d =50"698) is observed, the specific exaltation EZ=0.73 agreeing well with the data in the literature which were established for the first members of the homologous series
~-~=cH--
[2]
R
* Vysokomol. soyed. A10: No. 9, 2028-2033, 1968.
(Received 15 Augu~ 1967) THE s t u d y o f l a r g e d e f o r m a t i o n s in c r y s t a l l i n e p o l y m e r s o v e r a wide r a n g e o f t e m p e r a t u r e s is o f g r e a t t h e o r e t i c a l a n d p r a c t i c a l interest, since p r o f o u n d s t r u c t u r a l c h a n g e s t a k e p l a c e d u r i n g t h e p r o c e s s i n g a n d use o f p o l y m e r i c m a t e r i a l s u n d e r t h e effect o f v a r i o u s m e c h a n i c a l a c t i o n s . T h i s a p p r o a c h m a k e s it possible to establish most accurately the connection between the actual supermolecular s t r u c t u r e s in t h e p o l y m e r a n d its m e c h a n i c a l p r o p e r t i e s [1-4], N e w e x p e r i m e n t a l r e s u l t s are p r e s e n t e d in t h e p r e s e n t p a p e r ; t h e s e w e r e o b t a i n e d in t h e s t u d y o f l a r g e d e f o r m a t i o n s in u n i a x i a l t e n s i o n o f p o l y o x y p r o p y l e n e ( P P 0 ) s p e c i m e n s b o t h o f t h e initial m a t e r i a l a n d also c o n t a i n i n g a r t i ficial n u c l e i o f a s t r u c t u r e - f o r m i n g a g e n t .
EXPERIMENTAL An apparatus specially constructed by us for straining sheet specimens directly on the stage of an optical MBI-6 microscope considerably broadened the experimental possibilities of the investigation, since the automatic recording of the stress-strain curve was carried ' out simultaneously with the observation of the structural changes in the deformed specimen. The apparatus consists of the straining rig which is mounted on the microscope stage, a dynamometer unit and a recorder. (A general view of the apparatus is shown in Fig. la, and the straining rig is shown in Fig. lb.) The specimen under test 1 is secured in grips one of which, 2, is connected to the lead screw, 3, of the drive, which consists of a reduction gear, 4, and an electric motor, 5. The second grip, 6, is secured to a steel plate, 7, on to which a balanced bridge consiting of four strain gauges is cemented. The force applied to the specimen as it is stretched leads to bending of the plate, causing the bridge to become unbalanced. A certain voltage appears across the measuring diagonal of the bridge; this is amplified and recorded on the EPP-09 recorder. The speed of the recorder chart is selected to be a multiple of the rate of elongation of the specimen. I n this way, the load taken by the specimen as it is stretched is plotted along the horizontal axis, and the deformation is plotted along the vertical axis. The calibration of the rig was carried out as a preliminary; that is, the value of the scale unit on the recorder was established from the bending of the plate under the action of small weights. A detachable thermal block makes it possible to test specimens under isothermal conditions over a temperature range from -- 150 to 200°C. The study of large deformations was carried out on dumbell-shaped PPO specimens which were blanked out from film (thickness 10-30 ~) (dimensions of the working section of the specimen: length, 3.5 ram; width, 1-1 ram). Films with various supermolecular structures * Vysokomol. soyed.'Al0: No. 9, 2017-2027, 1968. 2343
V . A . KAROr~ et al.
2344
FI(~. 1. a - - G e n e r a l view of the equipment, and b - - t h e rig for straining polymer specimens on the microscope stage. were obtained b y evaporation of the solvent from solutions of PPO in benzene at 75°C over 2 hr. FiLms having supermolecular structures With different spherulite sizes wore obtained by varying the cooling rate of the melt obtained (melting point of PPO, 73°C) from 1-2 dog C/rain to rapid cooling in nitrogen. Tests were carried out under isothermal conditions in the temperature range from - - 1 1 0 to 55°C at a constant rate of elongation of 1 ram/rain. RESULTS AND DISCUSSION
Structural changes at large deformations in uniaxial elongation of PPO over a wide temperature interval. F r o m t h e e x p e r i m e n t a l d a t a o b t a i n e d i n t h e s t u d y of the deformation
of PPO
having
a coarse spherulitic structure
(spherulite
Structural.transitions in polyoxypropylene
2345
size 250/~ and above) (Fig. 2) it may be seen that, in the temperature region from --110 to 50°C, the specimens have a high rupture strength which falls with an increase in temperature, and rupture occurs without any marked deformation; the shape and size of the spherulites thus remain practically unchanged (Fig. 3a). The development of about 150% deformation is observed from --45 to 0°C, and the strength decreases with increase in temperature. Figure 3b, shows that ~, kig/O?.~2
800I_ll0 800I_9800 ~001~70
-~5 1o
200 _ _ _ _ _ _ / 0 0
50 i
I00
200
300
i
~00 500 C,%
FIO. 2. Stress-strain curves for coarse spherulitic P P O specimens. The n u m b e r s on the curves correspond to t h e stressing t e m p e r a t u r e .
these deformations arise from the elongation of the spherulites themselves. It should be noted that, over a certain temperature range, the specimen strength changes with temperature but the elonga{ion at break remains practically constant. In the temperature region from 10 to 50°C the specimens deform by more than 300%, and the entire elongation process takes place in three stages, as may be seen from-Fig. 2, the shape of the stress-strain curves being typical of crystalline polymers. However, the rupture characteristics of the supermolecular structure have been made apparent for the first time in these experiments. The clear step-wise breakdown of the individual spherulites and of the entire specimen as a whole is shown in Fig. 3c. This type of structural breakdown which arises during tensily deformation is reminiscent externally of the way in which slip planes originate in ordinary low molecular weight crystals, bug upon more detailed examination it turns out to be a system of parallel broad consecutive bands, closely connected with the appearance and development of a large number of necks. The formation of the necks does not take place all at one time, but consecutively, and has clearly expressed discontinuous characteristics (with a rate of elongation of 1 Tn~n/min or 30%/rain, and a specimen thickness of about 20#, the formation of a new neck took place each three seconds). The boundaries separating the necks formed appear as lines in relief, which are dearly visible in polarized and reflected light (Fig. 3c, e). These bands and the lines in relief traverse practically the entire specimen (in Fig. 3c), but, in going through the different spherulites, they reproduce the outlines of the spherulite boundaries.
2346
V . A . KARGIN e t a / .
i~ ¸
:FIG. 3. Photomicrographs of deformed coarse spherulitic PPO specimens, a ~ / - - Breakdown of supermolecular formations in PPO specimens during elongation at 1 ram/rain in the temperature range --110 to 50°C, e--boundaries in relief between necks, under reflected visible light. FIG. 5. Photomicrographs of deformed fine spherulitic PPO specimens, a, b--Breakdown of supermolecular formations in PPO specimens with different spherulito sizes, c - e - - r u p t u r e of PPO specimens having a practically uniform structure as seen in the optical microscope, during elongation at 1 m m / m i n in the temperature range from --110 to 55°C. :FIG. 7. Photomicrographs of coarse spherulitic PPO specimens deformed at various rates at 20°C: a--300, b--30, e--3 mm/min.
Struotural transitions in polyoxypropyleno
2347
The rupture characteristics of PPO remained of the same type at temperatures above 50°C. It was observed that, under these conditions, the necks being formed break up into fibrillar formations. When this happens, the lines in relief, which ~rupf,kon/cmz 1300
,o1o
k gO0
300"1 100
-eb 40
I
o
]
eo ,oc
FIe. 4. Temperature dependence of the true strength of coarse spherulitic PPO specimens.
always arise perpendicular to the line of action of the tensile force, are retained (Fig. 3d). A further increase in deformation is observed in this temperature region (because of the formation of fibrils) and all three sections on the stress-strain curve are realized; however, they are less clearly expressed because of the flow of the material. In this way, a study of large deformations in coarse spherulitic PPO specimens over a wide temperature interval has made it possible to separate the temperature regions in which changes in the value of the deformation at break correspond to transitions from one type of breakdown of the supermolecular formations to another type of breakdown. These transitions are also clearly apparent in the form of steps on the curve giving the temperature dependence of the true strength (Fig. 4 and 3a-c).. It was essential to clarify whether the mechanism by which the supermolecular formations break down with the formation of consecutive necks was a characteristic only of coarse spherulitic structures in PPO, or whether this phenomenon is more general. For this purpose, similar experiments were carried out on PPO specimens having various supermolecular structures. The experiments showed that the characteristics of the breakdown of t h e supermolecular structures, with the formation of consecutive necks, were retained in specimens of all types, even when the structure was hardly resolvable in the optical microscope (Fig. 5a, b), but the crystallinity of the specimens had been checked by X-ray methods. With a reduction in the spherulite size, the relief lines between the necks become more linear in view of the greater homogeneity of the supermolecular formations.
2348
V.A.K.~GxN
at a/.
Stress-strain curves are shown in Fig. 6 for specimens whose structure is shown in Fig. 5b; it m a y be seen that the general characteristics o f the temperature dependence of the strain are retained. A temperature region exists (-- 110 to ~60°C) in which the specimens rupture by brittle fracture (see Fig. 5c) and a reduction in strength is also observed with an increase in temperature. Then, in th~ temperature range from --60 to --10°C, considerable elongation of the films takes place with the formation of a "neck", the structure of which could not be seen (Fig. 5d). When the neck is formed at --10°C and above, wise breakdown is observed with the formation of consecutive necks (Fig. 5e). The new information
0", kg/c,v 2 600 :-¢fo ,-90 #00 ;-70 -80-50-#0
- fO
55 I
I
I
I
100
200
300
000
J
R
500
600
'
~,%
FIG. 6. Stress-strain curves for fine spherulitic P P 0 specimens. Tho numbers b y the curves correspond to the stressing temperatures.
which was obtained from investigations of specimens of this type (the initial specimen having structural elements not resolvable optically) in practice amounts to the fact that they exhibit temperature regions in which a marked change occurs in the properties of PPO under conditions of tensile deformation; a considerable reduction in the dimensions of the polymer's supermolecular structural elements causes the temperature at which large deformations begin to develop with the formation of consecutive necks to be 15-20°C below the corresponding temperature in specimens with a coarse spherulitic structure, and there is no sharp jump in the values o f the rupture deformation around the temperature of the second transition, i,e. the transition to breakdown with the formation of consecutive pecks. A comparison of the stress-strain curves (Figs. 2 and 6) shows that in the brittle fracture region the values of the reerystallization stress over the entire temperature and strength range investigated is considerably greater for specimens with a coarse spherulitic structure. These differences in mechanical properties are clearly connected with the special features of the supermolecular structure in the specimens investigated. Unfortunately, in the experiments with specimens having a very fine supermolecular structure, there was naturally no possibility of observing all the structural changes which had been previously observed by means of the optical microscope, However, analysis of the stress-strain relationships over a wide tern-
Structural transitions in polyoxypropylene
2349
perature range made it possible to conclude that there exist structural transitions which take place during the elongation of these PPO specimens. The structural transitions which we have observed when PPO is stretched do not depend on the dimensions of the supermolecular formations, but there is an effect of these dimensions in t h a t the structural transitions occur in different temperature and stress regions. Since the step-wise breakdown of the supermolecular structures in PPO was observed at a low rate of elongation (1 ram/rain), it was essential to clarify the effect of elongation rate on the characteristics of the structural transitions in PPO. For this purl~ose, the deformation of PPO specimens was investigated, at various rates of elongation, 3, 30 and 300 m ~ / m i n over a wide range of temperatures. From the experimental data obtained, which are shown in the Table, it m a y be seen that the values of the strength characteristics (arupt., arecr., 5) depend on the rate of elongation and on the temperature. The rise in the rupture stress values with an increase in the elongation rate at temperatures below 10°C agrees with the general rule which is true for all solid bodies. This rule does not hold above 10°C. A study of the supermolecular structures in specimens deformed at various elongation rates showed t h a t a temperature region exists in which brittle fracture occurs independent of the rate of elongation, the shape and dimensions of the spherulites being preserved. Large deformations become apparent in the temperature interval from --45 to 10°C, so that specimens, independent of the rate of elongation (within the range of elongation rates investigated by us), are deformed through the elongation of the sl~herulites themselves. Tests at 10-20°C show that a change in the elongation rate causes a substantial difference in the characteristics of the structural changes, and consequently in the mechanical properties. Figure 7 shows the types of structural change which occur in specimens tested at 20°C at various elongation rates. A high rate of elongation causes deformation of the specimens through the elongation of the spherulites themselves (it should be noted that this type of structural change occurs in tests at high rates over the entire temperature range in which large deformations appear). With a medium rate of elongation and at a temperature of 10°C, but more clearly at 20°C, a stepwise breakdown of the spherulites is observed, with the formation of elliptical concentric formations. At a low rate of elongation, the breakdown which we had already found is again observed, with the formation of necks which are formed consecutively (it should be noted that the distance between the relief lines in this case is greater than in the experiments described above, at an elongation rate of 1 ram/rain). The change in the type of structural transition is reflected in the values of deformation and rupture strength (Table). Whereas, at temperatures below 10°C an increase in the elongation rate causes an increase in strength, recrystallization stress and elongation at rupture, at temperatures from 10 to 20°C, when an increase in elongation rate gives rise to a change in the type of structural transition,
2350
V.A.
l(A~orN e t a / .
this rule a b o u t the s t r e n g t h a n d elongation a t r u p t u r e is n o t true. This a n o m a l y is u n d o u b t e d l y caused b y the different changes in the s t r u c t u r e o f the p o l y m e r during elongation a t different rates. Tests a t higher t e m p e r a t u r e s (50°C a n d above) give uniform values of the s t r e n g t h characteristics i n d e p e n d e n t o f t h e strain rate; this is c o n n e c t e d with the fact t h a t the supcrmolecular s t r u c t u r e s become fibriUar, as observed b y m e a n s o f the optical microscope. CHANGES ON
IN THE
STRENGTH
THE
TEMPERATURE
Rate of elongation
CHARACTERISTICS AND
RATE
OF
OF
PPO,
DEPENDENT
ELOI~GATION
T °, C
~rrecr,, kg/cm 2
kg/cm s
--70
650
650
450 425 450 320 290 310 280 275 300 180 140 230 115 05 170 100 9O 160 90 85 65 5O 4O
45o 425 450 320 290 360 35o 345 350 270 230 34o 235 125 260 210 320 25o 230 245 75 7o 6o
~'rupt.,
8,%
High, 300 ram/
/mh~ Medium, 30 mm/ min Low, 3 mm/min High Medium Low High Medium Low High Medium Low
--55 --45 --25
High Medium Low High Medium Low
10
High
20
Medium Low
High Medium Low
50
20o 150 140 200 150 140 240 180 150 240 260 320 300 280 320 65o 55o 5o0
Tests a t different elongation rates h a v e therefore shown t h a t step-wise breakdown with the f o r m a t i o n o f consecutive necks occurs in a definite t e m p e r a t u r e range a n d a t c o m p a r a t i v e l y low rates o f elongation, a rise in the d e f o r m a t i o n capacity a n d s t r e n g t h t h u s being observed. These e x p e r i m e n t s m a k e it possible to consider t h a t the b r e a k d o w n o f the supermolecular s t r u c t u r e with the f o r m a t i o n o f consecutive necks imparts a definite set o f mechanical properties t o the poly-
Structural transitions in polyoxypropylene
2351
met. The phenomenon which has been observed, that the supermolecular structures break down in s~ps with the formation of necks, arising in a consecutive manner and regularly positioned, requires more detailed explanation. I t should be remembered t h a t the formation of a neck proceeds as a result of structural changes which occur with time. I f the strain rate is greater than the rate of these structural changes over-stressing inevitably occurs, leading right up to rupture or to the start of a structural transition. Therefore when a neck begins to form a considerably greater stress always arises, and this stress leads to the storing of potential energy in the specimen which is deformed at first as a whole; this energy is used up in the development of the neck at a rate very much greater than the rate of movement of the grips. The practically instantaneous formation of the first neck leads to a fall in the stress in the specimen to a value at which any further structural change is impossible. But since the elongation of the specimen proceeds continuously and at a constant rate, the accumulation of elastic energy again takes place in it, and the rapid, almost instantaneous, formation of a new neck again takes place at a certain value of over-stressing. The repetition of these cycles gives a discontinuous regular breakdown. (When the rate of breakdown is so large that the growth of the neck has time to occur during the deformation of the specimen, a single neck is formed which stretches across the entire specimen.) Why does the formation of each subsequent neck begin at the place dividing the isotropic section of the specimen from that already orientated, with the formation of a transverse relief thickening ? This is explained by the fact that the transitional region from the isotropic specimen to the neck turns out to be orientated, and consequently stronger than the isotropic part of the spechnen and stronger than the part of the specimen which has already been stretched but which is thinner. Consequently, the formation of each new neck begins directly at the side of this strengthened section, within the isotropic part of the specimen. The fact t h a t these transverse lines of thickening exist until the entire isotropie part of the specimen breaks down into necks, and only become smoothed out immediately before rupture of the specimen, is an indication t h a t these regions of transverse thickening are stronger. The kinetic nature of the phenomenon is also shown by the fact that, as the rate of elongation is increased, the length of the consecutively formed necks increases. In fact, the larger value of over-stressing also implies a greater rate of formation of the neck, and this specifies that a greater amount of isotropic material passes into the neck. Moreover, it should be noted t h a t a greater stress is required to initiate the formation of a neck in crystalline polymers than is needed for its development. It is just this which also leads to the step-wise creation of the neck with a dividing boundary. Since the structural changes do not occur instantaneously under the real kinetic conditions of our experiment, but lag somewhat behind the deformation process, over-stressing peaks arise periodically in the specimen as a result of its elongation, and these lead to the repeated step-wise formation of necks.
2352
V.A.
KARGIN et al.
I t becomes clear from all t h a t has been p ut forward t h a t relaxation phenomena which occur in polymer bodies under an applied external force field, m a y also explain the breakdown of the supermolecular structures which occurs under the various conditions. The fact t h a t the formation of a regular system of necks is independent of the spherulite dimensions indicates t h a t the limiting structural element which determines the rate of the transition is not the spherulite, b u t a still finer structural unit.
Structural transitions at large uniaxial tensile strains in P P O in the presence of coarse foreign inclusions acting as artificial nuclei for structure formation, and the role of their surface in strengthening the polymer material. The second part of the work was devoted to a s t udy of problems connected with the effect of a foreign surface on the supermolecular structures of the polymer, and the effect of these structures on the properties of crystalline polymers. This was important because the introduction of artificial nuclei for structure formation in polymers capable of crystallization is a new and potentially very powerful m e t h o d of regulating
Fro. 8. Photomicrographs of PPO specimens with an artificial structure-forming agent introduced (aa~thraquinone dye): a--spherulitic band of PPO, formed at a large acieular crystal of the dye; b, c--formation of a macro-defect at a crystal lying perpendicular to the line of action of the tensile force; d and e--because of a crystal lying parallel to the line of action of the tensile force, a macro-defect developing in the specimen is eliminated. the supermolecular structure and physicomechanical properties of polymers [5-9], and P P O is a ve r y convenient material for investigating structural transitions in crystalline polymers. I n order to study these problems effectively, it was
Structural transitions in polyoxypropylene
2353
very important to obtain a supermolecular structure in the polymer on a comparatively large surface area of the foreign bodies introduced as artificial nuclei for structure-formation. Coarse particles of a fat-soluble anthraquinone clearblue dye were used for this purpose (the latter was introduced into a solution of PPO in isopropyl alcohol). The use of this structure-forming agent made it possible to obtain films in which spherulitic bands of PPO were formed at the coarse acicular crystals (approximately 1000/~ long) o£ the dye (Fig. 8a). The crystals of the artificial structure-forming agent were positioned at random in these specimens and it was noted that the processes of deformation and rupture of the PPO specimens took place differently depending on the position of these particles. Crystals positioned perpendicular to the line of action of the tensile force frequently contributed to the formation of macro-defects, and this led to the rapid rupture of the specimens (Fig. 8b, c). Coarse crystals positioned parallel to or at a small angle to the line of action of the tensile force did not prevent the development of high strains in the specimens. Moreover, if such crystals
~I
16-18# 0.3mm 5 ¢~
C 5 Fxo. 9. Schematic diagram of a PPO model specimen. occurred in the path of a macro-defect growing across the entire width of the specimen, the crystals stopped its growth (Fig. 8d, e). It was noticed for the first time in these experiments t h a t the artificial structure-forming agents create a barrier preventing the growth of a macro-defect in the specimen whilst the specimen is being strained. In connection with the essentially new effect which had been observed, the "barrier action" of coarse foreign inclusions, exploratory investigations were carried out on model PPO specimens which had an artificially produced macrodefect, a slit, and which contained a fibre as the barrier (Fig. 9). In preparing the model specimens, straight fibres lying parallel to each other were positioned in the solution of PPO in benzene. Then specimens were cut out of the film obtained, so t h a t the fibre lay along the specimen practically at its centre. The slit took up approximately 25~/o of the width of the working section of the speei-
2354
V.A.
K~GI~
et a~.
men. In order to assess the part played by the nature of the surface of the fibre used as the artificial nucleus for structure formation fibres with various chemical structures were studied, i.e. po!ypropylene, teflon and polyacrylonitrile. (The model specimens tested had a thickness of 30-50 ~ with a fibre diameter of 15-18 ~.) From observation of the specimens in polarized light, it was established t h a t the polypropylene and teflon fibres are artificial structure-forming agents, a n d a well-formed spherulitic band of PPO occurs at their surfaces, but the polyacrylonitrile fibre does not initiate the structure-formation process at its surface (Fig.
10a- }. The study of the tensile deformation of model specimens showed that if the fibres were artificial nuclei for structure formation, the slit was prevented from penetrating into the body of the specimen,' i.e. the "barrier" action effect was completely confirmed. The fact t h a t the source of the rupture was localized and actually eliminated led to its becoming possible to realize large deformations in the specimens, amounting to more than 200~.
Fro. 10. Pho~omicrographsofPPOmodelspecimenscon~aining:a--polypropylene,b--teflon, and c--acrylonitrile fibres. From the microphotographs (Fig.1 la-e) it m a y be seen that as the specimen is stretched the slit takes on the form of a very broad wedge with a rounded point, whicl~ gradually penetrates into the body of the specimen, but then the source of the rupture is eliminated and further elongation leads to deformation by the usual mechanism, as if a PPO specimen without external defects were being stretched (Fig. l ld, e). In experiments with specimens containing a polyacrylonitrile fibre, which is not an artificial nucleus for structure formation, the slit penetrated without hindrance across the entire width of the specimen as it was stretched, and the rupture characteristics did not differ in any qualitative way from that observed in stretching a slit PPO specimen not containing a fibre (see Fig. 12a-v and Fig. 13a-c). I n this way, two groups of phenomena were observed: the first occurs when a foreign body, whose surface is an artificial nucleus for structure formation, exhibits the "barrier" action, that is, the body prevents the rupture of the specimens
Structural transitions in polyoxypropylene
2355
~ii~ i~
ilil
i!
FxO. 11. Growth of a slit during the stretching of a PPO specimen containing a structureforming fibre. FIG. 12. Growth of a slit during the stretching of a PPO specimen e o n t a h ~ l g a fibre which is not a structure-forming agent. I~xG. 13. Growth of a slit during the s t ~ c h i n g of a PPO specimen not containing a fibre.
2356
V. A. KAmax~ e~ ed.
at a macro-defect, and in this way makes it possible to achieve laxge deformations; and t h e second occurs when a foreign body, which is not a nucleus for structure formation, does not substantially prevent the spread of the macro-defect in the specimen. Analysis of the experimental data showed that rupture of the fibre inevitably occurs during the elongation of model specimens (Fig. l l e and Fig. 12c). This is explained by the fact that orientated fibres are not capable of large deformations. I f the fibre is not a structure-forming agent, its adhesion to the polymer is comparatively small and such fibres break as the specimen is stretched and tear away from the polymer. Therefore, such a fibre itself is not a barrier preventing the propagation of the slit. On the other hand, in the initial experiments with PPO specimens containing coarse long crystals of a dye, the crystals being nuclei for structure formation, it was noted that crystals positioned in the direction in which the tensile force acted broke up into small pieces and did not separate from the polymer, i.e. the adhesion of an artificial structure-forming agent to the polymer was fairly high. The effect of the barrier action was thus observed both for crystals and also for fibres. Evidently, it m a y be accurately stated that the foreign body itself is not the barrier which substanially prevents the propagation of the slit, with the relative fibre and film dimensions studied. What then explains the barrier action effect which occurred in experiments when the foreign body introduced into PFO was an artificial nucleus for structure formation? We shall discuss this phenomenon in somewhat more detail. A considerably greater number of centres of crystallization (as compared with the remaining part of the specimen) occur on the surface of the artificial nucleus for structure formation, and this leads to the adhesion of the unusual "spherulites" in the band closely adjacent to the surface of the structure-forming agent. The area of spherulites, closely aggregated in the band and having centres positioned on the surface of the structure-forming agent (the crystal or fibre), is most highly ordered close to the surface of the structure-forming agent, and is less well ordered at its periphery. During stretching the slit passes through the outer, not well ordered, boundary of the spherulitie band and reaches its centre, which is the artificial structure-forming agent. However the foreign body itself, as has been shown above, is not a hindrance to the further propagation of the slit, since at this time the artificial nucleus for structure formation is not intact. Further penetration of the slit turns out to be impossible because it encounters the surface of the ordered region of the spherulitic band, which is the internal edge of the unruptured portion of the specimen. One could have suggested that the termination of growth of the slit is caused by the presence of the foreign body (the fibre or crystal) in the film, or also by the increased strength of the spherulitic band. In order to check this suggestion, PPO specimens were prepared without a fibre, b u t containing a band having a spherulitic structure obtained by a special method. It turned out that such bands did n o t inhibit the growth of the slit. In this way, the presence of a structural
Structural transitions in polyoxypropylene
2357
non-uniformity in t h e form of a band of spherulites is insufficient to stop the initiated rupture. It follows from this that the spherulitic bands obtained because of the presence of a structure-forming agent differ from bands of spherulites formed without such an agent, either in their internal structure or in some special feature of the surface. Since the spherulitic band has two essentially different surfaces (an external surface and an internal surface adjacent to the fibre), it is necessary to consider this in some detail. As we have already shown, the growth of the slit is not halted when it encounters the external, less well ordered, surface of the spherulitic band. I t easily passes through it and through the internaI well ordered surface, and exits at the internal ~, kg/cm~ /
20 y o
i 60
3
I
I
I
120
180
2#0
~,%
FIG. 14. Stress-strain curves for the following specimens: / - - t h e initial PPO; 2 - - a s 1, b u t containing a polypropylene fibre; 3 - - a s 1, b u t containing a teflon fibre; 4---as 1, b u t containing a pplyacrylonitrile fibre.
surface of the spherulitic band. The termination of further growth of the slit m a y be explained by the exit of its tip into the cavity occupied by the structureforming agent (by the fibre or by the crystal). However, the growth of the slit is not halted in the case of specimens with fibres incapable of causing structure formation in the specimen, but which do form cavities around themselves in the specimen. This points to the essential role played by the structure-forming activity of the fibre or any other structure-forming agent. Therefore, the spherulitic band should have not only increased strength, but also a special structure in its interior and on the surface adjacent to the fibre, the structure-forming agent. The non-uniform structure of the spherulitic band itself is apparent from the fact that it is easily ruptured when the tip of the slit tears through it from its outer edge, whereas, when the tip of the slit acts on its interior surface (the surface of the cavity in which the fibre fragraents are located), the band successfully resists rupture and the growth of the slit is terminated. The "structural barrier" effect is graphically illustrated by the stress-strain curves (Fig. 14): the capacity for deformation of specimens with polypropylene or teflon fibres, which are artificial nuclei for structure formation, is more than twice as great as the capacity for deformation of specimens without a fibre~ or containing a polyacrylonitrile fibre. It is strange that the recrystallization stress turned out to be highest for specimens with a polyacrylonitrile fibre, which is
2358
V.A.
~A~Gn~ e~ a/.
not a structure-formlng agent, but which has the highest strength of the fibres selected by us for the model specimens. It follows from all that has been put forward above that we have observed a structural barrier effect, which consists in the localization and prevention of further growth of the macro-defect at which rupture of specimens takes place during their elongation. Naturally, the barrier action effect should depend on the ratio of the specimens deformation rate to the rate of development of the macro-defect, and also on the ratio between the film thickness and the dimensions of the particles of the structure-forming agent introduced. CONCLUSIONS
(1) With polyoxypropylene as an example, it has been shown that the substantial change in the mechanical properties of crystalline polymers, depending on temperature, is caused by the different characteristics of the way in which the supermolecular structures break down, the breakdown taking place on different levels depending on the temperature and rate of elongation. (2) It has been observed that over a certain range of temperatures and rates of elongation the rupture of the supermolecular structures proceeds as a result of the consecutive step-wise creation of necks formed in a regular fashion. This type of breakdown of supermolecular structures specifies a set of polymer properties which is not observed in other types of structural transition and creates additional possibilities for regulating the mechanical properties of polymeric materials. (3) Experhnents at low rates of elongation have made it possible to develop more profound concepts about the mechanism by which necks 'form, since the existence of two processes taking l~lace at different rates has been graphically demonstrated: the first process is the elongation of the specimen at a given rate by movement of the grips, and the second is the process of structural transition, the rate of which is determined by the magnitude of the over-stress created in the specimen and by the structure of the given polymeric body. (4) A new method of regulating the structure of polymers has been developed by the method of introducing fairly coarse structure-forming agents of different dimensions and shapes, the effect of these agents not being apparent over the entire volume of the body, but in the individual places most liable to failure. The polymeric body thus acquires a resistance to rupture by active mechanical effects. Trans/a~ed by G. F. MODLEN REFERENCES 1. V. A. KARGIN and T. I. SOGOLOVA, Zh. fiz. k]~irnll 27: 7, 1208, 1214, 1325, 1953 2. T. I. SOGOLOVA, Dissertation, 1963
3. V. A. KARGIN, T. I. SOGOLOVAand L. L NADAREISHVILI, Vysokomo]. soyed. 6: 1407, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 8, 1554, 1964)
Copolymerization of p-isopropenylphenylacetate and butadiene
2359
4. V. A. KARGIN, T. I. SOGOLOVA and V. M. RUBSHTEIN, Delft. Akad. Nauk SSSR 175: 1087, 1967 5. V. A. KARGIN, T. I. SOGOLOVA and T. K. SHAPOSHNIKOVA, Dokl. Akad. Nauk SSSR 156: 1156, 1964 6. V. A. KARGIN, T. I. SOGOLOVA and N. Ya. RAPOPORT, Dokl. Akad. Nauk SSSR 163: 1194, 1965 7. T. I. SOGOLOVA, Mekhanika polimerov, No. l, 5, 1965 8. V. A. KARGIN, T. I. SOGOLOVA and V. M. RUBSHTEIN, Vysokomol. soyed. 8: 645, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 4, 707, 1966) 9. T. I. SOGOLOVA, Mekhanika polimerov, 643, 1966
RELATIVE REACTIVITIES IN THE COPOLYMERIZATION OF p-ISOPROPENYLPHENYLACETATE AND BUTADIENE* L. M. KOGA17, A. I. YEZRIYELEV, A. B. PEIZl~ER a n d A. V. LEBEDEV S. V. Lebedev All-Union Scientific Research Institute for Synthetic Rubber
(Received 25 August 1967) THE a i m o f t h e p r e s e n t w o r k w a s to clarify t h e effect of t h e s t r u c t u r a l f e a t u r e s o f p - i s o p r o p o n y l p h e n y l a c e t a t e ( I P P h A ) on its r e a c t i v i t y in c o p o l y m e r i z a t i o n w i t h b u t a d i e n e (BD). T h e r e is a reference in t h e p a t e n t l i t e r a t u r e to a m e t h o d of o b t a i n i n g I P P h A f r o m p - i s o p r o p o n y l p h e n o l ( I P P h ) b y t h e S c h o t t e n - B a u m a n n r e a c t i o n [1]; howe v e r no p h y s i c a l c o n s t a n t s a n d no evidence of t h e s t r u c t u r e o f t h e c o m p o u n d o b t a i n e d were given, a n d t h e r e a c t i v i t y o f t h e p r o d u c t , including its r e a c t i v i t y in r a d i c a l reactions, w a s n o t s t u d i e d a t all. I t s e e m e d v e r y i m p o r t a n t t o us to fill t h e l a s t - n a m e d gap, since I P P h A is a s t r u c t u r a l a n a l o g u e of a - m e t h y l s t y r e n e and, as one should e x p e c t , also h a s a fairly r e a c t i v e e s t e r group, whose presence in t h e p o l y m e r w o u l d m a k e it possible t o carry out a number of interesting polymer-analogue transformations. The synthesis of I P P h A was caried out by the usual method [1]. Because of the presence of the conjugated double bond in the IPPhA molecule's isopropenyl group, exaltation of the molecular reaction EM=1.282 (Rexalt=51.980; Bad d =50"698) is observed, the specific exaltation EZ=0.73 agreeing well with the data in the literature which were established for the first members of the homologous series
~-~=cH--
[2]
R
* Vysokomol. soyed. A10: No. 9, 2028-2033, 1968.