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Crazing and shear deformation in amorphous polyethylene terephthalate during elongation in air and liquid media
1530
Y~.. A. SINEWCTr a n d N. F. BAg~YEV
10. Z. CSUROS, I. RUSZNAK, G. BERTALAN, P. ANN~k a n d J. KOROSI, Makromolek. Chem. 160: 27, 1972 11. G. REINISCH and U. GOHLIKE, Faserforsch. und Textiltechn. 23: 515, 1972 12. L. N. MIZEROVSKII, A. A. KOLESNIKOV a n d Yu. S. PAIKACHEV, Trans. I K h T I , Ivanovo, No. 16, p. 50, 1973 13. L. N. M1ZEROVSKII, Yu. M. BAZAROV a n d V. M. KHARITONOV, Vysokomol. soyed. A I 6 : 2780, 1974 (Translated in P o l y m e r Sci. U.S.S.R. 16: 12, 3240, 1974) 14. D. BERTLAN, I. RUSNAK and A. PETER, Poliamidy-75 (Polyamides-75) Khrudim, p. 295, 1975 15. L. N. MIZEROVSKII, V. G. SILANTYEVA, T. S. USACHEVA, Yu. S. PAIKACHEV, N. V. SHOLICHEV and Ire. N. LYUTAYA, Khimich. volokna, No. 6, 28, 1976 16. L. N. MIZEROVSKII, Yu. S. PAIKACHEV, V. M. KHARITONOV and A. A. KLOESNIKOV, Vysokomol. soyed. A13: 1109, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1246, 1971) ]7. G. REINISCH and U. GOHLKE, Faserforsch. u n d Textilteehn. 23: 415, 1972 18. N. V. SHOLICHEV, K a n d i d a t s k a y a dissertatsiya (Candidate's dissertation), Ivanovo, Chemico-technolog. I n s t . , 1975
Polymer Science U.S.S.R. Vol. 20, pp. 1530-1536. (~) Pergamon Press Ltd. 1979. Printed in Poland.
0032-3950]78/0601-1530507.50/0
CRAZING AND SHEAR DEFORMATION IN AMORPHOUS POLYETHYLENE TEREPHTHALATE DURING ELONGATION IN AIR AND LIQUID MEDIA* Y~. A. SnVEVlCH and N. F. :BAKEYEV L. Ya. K a r p o v Scientific Research I n s t i t u t e of Physidal Chemistry
(Received 24 August 1977) A microscopic s t u d y of low t e m p e r a t u r e shear shows t h a t during elongation in surface active liquid media of thick amorphous P E T P samples deformation of external layers of the sample causes hair line cracks, whereas the deformation mechanism of internal layers m a y be quite different, it m a y be shear deformation. This has a significant effect on properties of the material obtained as a result of elongation.
AMOaPHOUS P E T P elongated in air at room temperature is deformed with neck formation. In this it differs considerably from PMMA and PS which under similar conditions break down with a slight elongation and the deformation of which is accompanied b y the formation of hair line cracks both on the surface and in the volume of the sample [1]. Numerous hair line cracks appear in P E T P during the * Vysokomol. soyed. A20: No. 6, 1358-1363, 1978.
Crazing and shear deformation in amorphous PETP
1531
elongation of the polymer in certain liquid media [2], however, these are formed on the surface of the sample and only penetrate as deformation proceeds. I f t h e hair line crack goes through the entire cross section of the sample, ultra-thi~ fibrils of the oriented polymer carry the entire load during subsequent deformation~ which join the walls of hair line cracks [3]. Subsequent elongation of this sample results in a gradual transition of the undeformed material between hair line cracks into the oriented state [4]. The elongation of thicker samples may differ considerably from the deformation of" thin polymer films. I t was found t h a t with a thickness of the sample h exceeding some threshold value of hp the limit of forced elasticity aT is the same for elongation of samples both in air and in a surface active liquid medium, although the medium causes the formation of hair line cracks both on thin and thick samples [5]. I t was found t h a t when h ~ h p the value of a~ determines the bcha~iour of internal layers of the polymer not directly affected by a liquid medium and apparently, undergoing deformation, which is different from t h a t of external layers of the sample. A study of the mutual relation of processes o f hair line crack formation and other methods of plastic deformation is very imp o r t a n t for the understanding of deformation mechanisms of structural polymer materials. This study sought to carry out a microscopic examination of the deformation of internal and external layers in elongation of P E T P samples in air and in chemically inactive liquid media. Transparent PETP (with a density of 1.335 g/cm'~), free from fille~ was used in the experiments. Samples in the form of two sided sheets with an operating surface of 6.8 X 10 mm and a thickness of about 0.6 ram, were cut out of industrial sheet material. The sample was elongated in air or in polyethylene packs with liquid media [6] at room temperature using an Instron dynamometer at a rate of 5 mm/min. After elongation in the liquid medium the samples were wetted with filter paper and dried in air in the free state. They were then frozen in liquid nitrogen and cut off along the axis of elongation. The knife and the organic glass substrate on which the sample was cut, were also previously cooled with liquid nitrogen. Relatively "smooth fractures were normally obtained which were several millimeters irL
length. It should be noted that attempts to prepare the cut by shapp bending or separatioi~ of frozen samples (with or without cut) gave less satisfactory results particularly in those cases when the samples were not very wide (e.g. in the neck after elongation). The surfaces and fractures of samples were studied using a POLAM-P-113 polarizatio~ microscope and a JSM-2 raster electron microscope was used for more detailed morphological investigations (an aluminium layer was previously deposited on the samples). The polarization microscope enabled us in many cases to show more clearly the fine hair line cracks, which are difficult to distinguish when using an electron raster microscope: passing through the sample, the light was scattered by parts of cracks situated under the sample surface, thus "shadi~g" the crack and making it more noticeable. During elongation of P E T P in air a clearly expressed neck had formed in t h e sample most often near the transition of the operating part to a wider blade. Although the neck spread through the entire cross section of the sample almost spasmodically, the pneumatic clamps of the dynamometer freed the sample a t an intermediate moment and thus disrupting elongation, made it possible t(~
1532
Y~.. A. Si~svicH and N. F. BAI~EYEV
observe neck formation. When the sample fixed in the clamps without any curvature, neck formation began perpendicular to tensile stress from the edges of the sample to the middle (Fig. 1). "Planes", in which deformation has occurred (Fig. 2) were clearly seen in polarized light (with crossed polaroids). On the surface of the fracture, parallel to the axis of elongation, this region gave a shear band t h a t can be readily distinguished (Fig. 3). Any further shear deformation produced a clearly defined macro-neck in the sample.
FIG. 1. PETP sample after elongation in air by 9"3~o. Parallel black lines show places where the shear band emerges to the upper (a) and lower (b) surface of the sample. An arrow shows the direction of elongation. Under the conditions of elongation used the deformation of the sample was localized in the neck. A study of samples in polarized light showed that luminescence is only observed in the neck region; no hair line cracks, or other clearly defined traces of plastic deformation were observed outside this region. Surfaces of longitudinal fractures of these samples outside the neck region did not in any w a y differ from fractures of undeformed P E T P samples.
FIG. 2. Same sample in polarized light using crossed polaroids. Contact with liquid media (propanol and its dilute solutions, heptane) markedly changed the pattern of deformation. A number of hair line cracks formed on the surface of the sample perpendicular to stress. Their fibrillar structure was
Crazing and shear deformation in amorphous PETP
1633
clearly seen on photographs of longitudinal fractures (Fig. 4) of samples deformed beyond the limit of forced elasticity. However, these cracks did n o t e x t e n d through the entire cross section of the sample: they ended on shear bands s i t u a t e d at an approximate angle of 40 ° to the plane of the crack (Fig. 5).
Fxo. 3. Shear band in the longitudinal fracture of ~ sample passing through the region with neck formation, as shown by Fig. 1.
A marked difference was therefore observed in the deformation of internal and external layers of thick polymer samples on contact with the liquid medium. Under our experimental conditions deformation of surface layers took place by the formation of hair line cracks, whereas in the sample volume shear deformation took place. According to a previous study [7], the micro-mechanisms of these two methods of plastic deformation have a lot in common. In both cases polymer elongation results in the formation (inside the hair line crack or shear band) of oriented fibrils 50-700 • in diameter. However, the formation of hair line cracks is controlled by tensile stresses [8, 9] and cavities are formed in hair line cracks because geometrical conditions of interaction with the non-deformed material surrounding the hair line crack only allow local Poisson contraction of the material subjected to elongation inside the hair line crack. On the other hand, in shear deformation surrounding material layers do not prevent the Poisson contraction of the polymer in the shear band. Therefore, no pores are formed in t h e shear band and polymer orientation takes place without the separation of fibrils; the existence of fibrils in the shear band is related [7] to the initial supermoleeular structure of the polymer.
1534
YE. A. SINEVICH and 1~. F. BAKEYEV
Displacement of parts of the sample along the shear plane results in the formation of phases on the sample surface (Fig. 3) and a "curvature" of hair line cracks ending on the shear bands. Photographs indicate (Fig. 4) that the fibrils joining the walls of hair line cracks form an angle with the direction of elongation. T h e sign of the gradient of fibrils in adjacent hair line cracks alternates, so that parts of the sample surface between the cracks are at variable heights in relation to the medium plane. Of the t w o shear bands usually separated from the top of the hair line crack (Fig. 5) one is more active under given conditions, which results in a displacement of walls o f the hair line crack in relation to each other.
Fro. 4
FIG. 5
FIG. 4. Near surface fracture of the sample elongated in n-propanol; x 930. FIG. 5. Shear bands in the internal layers of the sample after elongation in n-propanol. Longitudinal fracture in transmitted light with a closed aperture diaphragm of the microscope. A charge in the limit of forced elasticity, according to the thickness of t h e sample in elongation of polymers in liquid media [5] shows that the active liquid markedl]y eases deformation in the surface layers of the material, where hair line cracks are formed. This effect of the medium m a y be due to a reduction in the surface energy on the polymer-medium interface [6] resulting in the formations of a highly developed surface in the hair line cracks and plasticization of t h e polymer in the upper parts [3]. ~:hen the thickness of the sample exceeds t h e ~hre~ho]d value, at the same rates of elongation of samples lhe medium ceases t o affect the value of aT. Since in lhis ease the deformation (haracteristics of t h e scruple are deteImincd b y lhe behaviour of internal material layers it m a y b e ass~ mcd that lhe medirm has no direct influence on the formation of shear bands,
Crazing and shear deformation in amorphous PETP
1535
although the cracks formed b y the action of the medium m a y contribute to tho formation of these bands. During polymer elongation the liquid medium first of all produces hair line cracks on the surface of the sample and further extends these cracks. I t is expected, however, that the cracks formed will not in m a n y cases, penetrate the entire cross section of the sample: in the internal layers stress increases at a higher rate than at the same rate of deformation of the sample in air (when no crack formation takes place). In fact, although during elongation of the polymer in a liquid medium the macroscopic deformation of internal and external layers is the same (otherwise, the sample would separate), in the internal layer the increase of stress should correspond to the increase in tensile force acting on the sample and the reduction in the cross section of the continuous material (internal layer) related to hair line crack formation qnside the sample. If the rate of elongation is such that relaxation effects no longer prevent a marked increase in stress in ~he polymer undergoing deformation, and no irreversible breakdown takes place, the stress level in deep layers m a y reach the limit of forced elasticity of the polymer. In this case ~hear deformation takes place although the average stress in the sample still does not reach a value corresponding to the limit of forced elasticity during the elongation of the polymer at the same rate in air. Since the highest stress occurs at the top of the hair line crack, and the cracks themselves are main lines for the penetration of the medium inside the sample, shear band,s are formed precisely in the top parts of hair line cracks and not between th:~m (Fig. 5). The formation of shear bands in the internal layers is unlikely, since stress increases there at a much lower rate: the deformation of the continuot~s material between hair line cracks is reduced as a consequence of a considerable elongation of the polymer inside them. This does not prevent the formation of a ~'hear band towards the surface of the sample, on which a hair line crack has formed initiating the shear band. The shear bands formed interrupt any further increase in cracks, blunting ~heir upper parts. Since the form of hair line cracks in the polymer under given conditions of deformation is approximately the same [3], the stress required for the formation of shear bands will be achieved with about the same depth of cracks. As a result, the thickness of the surface layer through which hair line cracks have penetrated is about the same in the entire operating part of the sample elongated in liquid medium. Further deformation of internal layers has a lot in common with ~he deformation of the polymer sample in air, although in the former case the number of effective shear bands is muCh higher than in the second (with neck formation). It should be noted that the shear deformation of the material of interna| layers influences not only the morphology and mechanical properties of sample8 elor~gated in liquid media The differences in behaviour of thick samples ~nd thin films of fibres after deformation in an active liquid are also significant. It is l ~ o w a that on drying P E T P films and other polymers elongated in surface active media,
1536
YE. A. SINEVICH and N. F. B A ~ m ~ v
• he material m a y undergo strong contraction [] 0] which is due to the closure of i]~air line cracks (joining of Walls). I f these cracks do not pass through the entire ~ross-section of the sample, the shear deformation of internal layers, which is irreversible at room temperature, prevents contraction. I n fact, fragments of P E T P samples (Fig. 4) show wide open hair line cracks, although these samples were dried in the free state after elongation in liquid medium. Thus, an increase in the thickness of P E T P samples deformed in surface .active liquid media changes the efficiency of action of the media as ~ result of changing the deformation mechanism of the polymer. Elongation of thin films ,or fibres is due to formation and extension of hair line cracks; when bulky samples w i t h a large cross section are elongated, shear deformation m a y be significant, as it also influences the physical and mechanical properties of the material .obtained as a result of elongation of the polymer on contact with the liquid ,medium. The authors are grateful to A. S. Kechek'yan for the discussion of results. Translated by E. SEVERE REFERENCES
I. Y. IMAI and N. BROWN, J. Mater. Sci. 11: 425, 1976 2. N. V. PERTSOV, Ye.,A. SINEVICH and N. I. IVANOVA, Plast. massy, No. 2, 25, 1978
3. R. P. KAMBOUR, Macromolec. Rev. 7: 1, 1973 4. A. L. VOLYNSKH, V. I. GERASIMOV and N. F. BAKEYEV, VysokomoL soyed. A17: 2461, 1975 (Translated in Polymer Sci. U.S.S.R. 14: 11, 2831, 1975) ~5. Ye. A. SINEVICH,A. A. RYZHKOV and N. F. BAKEYEV, Vysokomol. soyed. BI9: 687, 1977 (Not translated in Polymer Sci. U.S.S.R.) 6. Ye. A. SINEVICH, R. P. OGORODOV and N. F. B~AKEYEV,]3old. AN SSSR, 212: 1383, 1973 7. T. E. BRADY and' G. S. Y. YEH, J. Mater. Sci. 8: 1083, 1973 8. S. S. STERNSTEIN, L. ONGCHIN and A. SILVERMAN, Appl. Polymer Syrup., No. 7, 175, 1968 9. E. J. KRAMER, J. Polymer Sci., Polymer Phys. Ed. 13: 509, 1975 40. A. L. VOLYNSKHand N. F. BAKEYEV, Vysokomol. soyed. A17: 1610, 1975 (Translated in Polymer Sci. U.S.S.R. 14: 1, 1855, 1975)
Y~.. A. SINEWCTr a n d N. F. BAg~YEV
10. Z. CSUROS, I. RUSZNAK, G. BERTALAN, P. ANN~k a n d J. KOROSI, Makromolek. Chem. 160: 27, 1972 11. G. REINISCH and U. GOHLIKE, Faserforsch. und Textiltechn. 23: 515, 1972 12. L. N. MIZEROVSKII, A. A. KOLESNIKOV a n d Yu. S. PAIKACHEV, Trans. I K h T I , Ivanovo, No. 16, p. 50, 1973 13. L. N. M1ZEROVSKII, Yu. M. BAZAROV a n d V. M. KHARITONOV, Vysokomol. soyed. A I 6 : 2780, 1974 (Translated in P o l y m e r Sci. U.S.S.R. 16: 12, 3240, 1974) 14. D. BERTLAN, I. RUSNAK and A. PETER, Poliamidy-75 (Polyamides-75) Khrudim, p. 295, 1975 15. L. N. MIZEROVSKII, V. G. SILANTYEVA, T. S. USACHEVA, Yu. S. PAIKACHEV, N. V. SHOLICHEV and Ire. N. LYUTAYA, Khimich. volokna, No. 6, 28, 1976 16. L. N. MIZEROVSKII, Yu. S. PAIKACHEV, V. M. KHARITONOV and A. A. KLOESNIKOV, Vysokomol. soyed. A13: 1109, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1246, 1971) ]7. G. REINISCH and U. GOHLKE, Faserforsch. u n d Textilteehn. 23: 415, 1972 18. N. V. SHOLICHEV, K a n d i d a t s k a y a dissertatsiya (Candidate's dissertation), Ivanovo, Chemico-technolog. I n s t . , 1975
Polymer Science U.S.S.R. Vol. 20, pp. 1530-1536. (~) Pergamon Press Ltd. 1979. Printed in Poland.
0032-3950]78/0601-1530507.50/0
CRAZING AND SHEAR DEFORMATION IN AMORPHOUS POLYETHYLENE TEREPHTHALATE DURING ELONGATION IN AIR AND LIQUID MEDIA* Y~. A. SnVEVlCH and N. F. :BAKEYEV L. Ya. K a r p o v Scientific Research I n s t i t u t e of Physidal Chemistry
(Received 24 August 1977) A microscopic s t u d y of low t e m p e r a t u r e shear shows t h a t during elongation in surface active liquid media of thick amorphous P E T P samples deformation of external layers of the sample causes hair line cracks, whereas the deformation mechanism of internal layers m a y be quite different, it m a y be shear deformation. This has a significant effect on properties of the material obtained as a result of elongation.
AMOaPHOUS P E T P elongated in air at room temperature is deformed with neck formation. In this it differs considerably from PMMA and PS which under similar conditions break down with a slight elongation and the deformation of which is accompanied b y the formation of hair line cracks both on the surface and in the volume of the sample [1]. Numerous hair line cracks appear in P E T P during the * Vysokomol. soyed. A20: No. 6, 1358-1363, 1978.
Crazing and shear deformation in amorphous PETP
1531
elongation of the polymer in certain liquid media [2], however, these are formed on the surface of the sample and only penetrate as deformation proceeds. I f t h e hair line crack goes through the entire cross section of the sample, ultra-thi~ fibrils of the oriented polymer carry the entire load during subsequent deformation~ which join the walls of hair line cracks [3]. Subsequent elongation of this sample results in a gradual transition of the undeformed material between hair line cracks into the oriented state [4]. The elongation of thicker samples may differ considerably from the deformation of" thin polymer films. I t was found t h a t with a thickness of the sample h exceeding some threshold value of hp the limit of forced elasticity aT is the same for elongation of samples both in air and in a surface active liquid medium, although the medium causes the formation of hair line cracks both on thin and thick samples [5]. I t was found t h a t when h ~ h p the value of a~ determines the bcha~iour of internal layers of the polymer not directly affected by a liquid medium and apparently, undergoing deformation, which is different from t h a t of external layers of the sample. A study of the mutual relation of processes o f hair line crack formation and other methods of plastic deformation is very imp o r t a n t for the understanding of deformation mechanisms of structural polymer materials. This study sought to carry out a microscopic examination of the deformation of internal and external layers in elongation of P E T P samples in air and in chemically inactive liquid media. Transparent PETP (with a density of 1.335 g/cm'~), free from fille~ was used in the experiments. Samples in the form of two sided sheets with an operating surface of 6.8 X 10 mm and a thickness of about 0.6 ram, were cut out of industrial sheet material. The sample was elongated in air or in polyethylene packs with liquid media [6] at room temperature using an Instron dynamometer at a rate of 5 mm/min. After elongation in the liquid medium the samples were wetted with filter paper and dried in air in the free state. They were then frozen in liquid nitrogen and cut off along the axis of elongation. The knife and the organic glass substrate on which the sample was cut, were also previously cooled with liquid nitrogen. Relatively "smooth fractures were normally obtained which were several millimeters irL
length. It should be noted that attempts to prepare the cut by shapp bending or separatioi~ of frozen samples (with or without cut) gave less satisfactory results particularly in those cases when the samples were not very wide (e.g. in the neck after elongation). The surfaces and fractures of samples were studied using a POLAM-P-113 polarizatio~ microscope and a JSM-2 raster electron microscope was used for more detailed morphological investigations (an aluminium layer was previously deposited on the samples). The polarization microscope enabled us in many cases to show more clearly the fine hair line cracks, which are difficult to distinguish when using an electron raster microscope: passing through the sample, the light was scattered by parts of cracks situated under the sample surface, thus "shadi~g" the crack and making it more noticeable. During elongation of P E T P in air a clearly expressed neck had formed in t h e sample most often near the transition of the operating part to a wider blade. Although the neck spread through the entire cross section of the sample almost spasmodically, the pneumatic clamps of the dynamometer freed the sample a t an intermediate moment and thus disrupting elongation, made it possible t(~
1532
Y~.. A. Si~svicH and N. F. BAI~EYEV
observe neck formation. When the sample fixed in the clamps without any curvature, neck formation began perpendicular to tensile stress from the edges of the sample to the middle (Fig. 1). "Planes", in which deformation has occurred (Fig. 2) were clearly seen in polarized light (with crossed polaroids). On the surface of the fracture, parallel to the axis of elongation, this region gave a shear band t h a t can be readily distinguished (Fig. 3). Any further shear deformation produced a clearly defined macro-neck in the sample.
FIG. 1. PETP sample after elongation in air by 9"3~o. Parallel black lines show places where the shear band emerges to the upper (a) and lower (b) surface of the sample. An arrow shows the direction of elongation. Under the conditions of elongation used the deformation of the sample was localized in the neck. A study of samples in polarized light showed that luminescence is only observed in the neck region; no hair line cracks, or other clearly defined traces of plastic deformation were observed outside this region. Surfaces of longitudinal fractures of these samples outside the neck region did not in any w a y differ from fractures of undeformed P E T P samples.
FIG. 2. Same sample in polarized light using crossed polaroids. Contact with liquid media (propanol and its dilute solutions, heptane) markedly changed the pattern of deformation. A number of hair line cracks formed on the surface of the sample perpendicular to stress. Their fibrillar structure was
Crazing and shear deformation in amorphous PETP
1633
clearly seen on photographs of longitudinal fractures (Fig. 4) of samples deformed beyond the limit of forced elasticity. However, these cracks did n o t e x t e n d through the entire cross section of the sample: they ended on shear bands s i t u a t e d at an approximate angle of 40 ° to the plane of the crack (Fig. 5).
Fxo. 3. Shear band in the longitudinal fracture of ~ sample passing through the region with neck formation, as shown by Fig. 1.
A marked difference was therefore observed in the deformation of internal and external layers of thick polymer samples on contact with the liquid medium. Under our experimental conditions deformation of surface layers took place by the formation of hair line cracks, whereas in the sample volume shear deformation took place. According to a previous study [7], the micro-mechanisms of these two methods of plastic deformation have a lot in common. In both cases polymer elongation results in the formation (inside the hair line crack or shear band) of oriented fibrils 50-700 • in diameter. However, the formation of hair line cracks is controlled by tensile stresses [8, 9] and cavities are formed in hair line cracks because geometrical conditions of interaction with the non-deformed material surrounding the hair line crack only allow local Poisson contraction of the material subjected to elongation inside the hair line crack. On the other hand, in shear deformation surrounding material layers do not prevent the Poisson contraction of the polymer in the shear band. Therefore, no pores are formed in t h e shear band and polymer orientation takes place without the separation of fibrils; the existence of fibrils in the shear band is related [7] to the initial supermoleeular structure of the polymer.
1534
YE. A. SINEVICH and 1~. F. BAKEYEV
Displacement of parts of the sample along the shear plane results in the formation of phases on the sample surface (Fig. 3) and a "curvature" of hair line cracks ending on the shear bands. Photographs indicate (Fig. 4) that the fibrils joining the walls of hair line cracks form an angle with the direction of elongation. T h e sign of the gradient of fibrils in adjacent hair line cracks alternates, so that parts of the sample surface between the cracks are at variable heights in relation to the medium plane. Of the t w o shear bands usually separated from the top of the hair line crack (Fig. 5) one is more active under given conditions, which results in a displacement of walls o f the hair line crack in relation to each other.
Fro. 4
FIG. 5
FIG. 4. Near surface fracture of the sample elongated in n-propanol; x 930. FIG. 5. Shear bands in the internal layers of the sample after elongation in n-propanol. Longitudinal fracture in transmitted light with a closed aperture diaphragm of the microscope. A charge in the limit of forced elasticity, according to the thickness of t h e sample in elongation of polymers in liquid media [5] shows that the active liquid markedl]y eases deformation in the surface layers of the material, where hair line cracks are formed. This effect of the medium m a y be due to a reduction in the surface energy on the polymer-medium interface [6] resulting in the formations of a highly developed surface in the hair line cracks and plasticization of t h e polymer in the upper parts [3]. ~:hen the thickness of the sample exceeds t h e ~hre~ho]d value, at the same rates of elongation of samples lhe medium ceases t o affect the value of aT. Since in lhis ease the deformation (haracteristics of t h e scruple are deteImincd b y lhe behaviour of internal material layers it m a y b e ass~ mcd that lhe medirm has no direct influence on the formation of shear bands,
Crazing and shear deformation in amorphous PETP
1535
although the cracks formed b y the action of the medium m a y contribute to tho formation of these bands. During polymer elongation the liquid medium first of all produces hair line cracks on the surface of the sample and further extends these cracks. I t is expected, however, that the cracks formed will not in m a n y cases, penetrate the entire cross section of the sample: in the internal layers stress increases at a higher rate than at the same rate of deformation of the sample in air (when no crack formation takes place). In fact, although during elongation of the polymer in a liquid medium the macroscopic deformation of internal and external layers is the same (otherwise, the sample would separate), in the internal layer the increase of stress should correspond to the increase in tensile force acting on the sample and the reduction in the cross section of the continuous material (internal layer) related to hair line crack formation qnside the sample. If the rate of elongation is such that relaxation effects no longer prevent a marked increase in stress in ~he polymer undergoing deformation, and no irreversible breakdown takes place, the stress level in deep layers m a y reach the limit of forced elasticity of the polymer. In this case ~hear deformation takes place although the average stress in the sample still does not reach a value corresponding to the limit of forced elasticity during the elongation of the polymer at the same rate in air. Since the highest stress occurs at the top of the hair line crack, and the cracks themselves are main lines for the penetration of the medium inside the sample, shear band,s are formed precisely in the top parts of hair line cracks and not between th:~m (Fig. 5). The formation of shear bands in the internal layers is unlikely, since stress increases there at a much lower rate: the deformation of the continuot~s material between hair line cracks is reduced as a consequence of a considerable elongation of the polymer inside them. This does not prevent the formation of a ~'hear band towards the surface of the sample, on which a hair line crack has formed initiating the shear band. The shear bands formed interrupt any further increase in cracks, blunting ~heir upper parts. Since the form of hair line cracks in the polymer under given conditions of deformation is approximately the same [3], the stress required for the formation of shear bands will be achieved with about the same depth of cracks. As a result, the thickness of the surface layer through which hair line cracks have penetrated is about the same in the entire operating part of the sample elongated in liquid medium. Further deformation of internal layers has a lot in common with ~he deformation of the polymer sample in air, although in the former case the number of effective shear bands is muCh higher than in the second (with neck formation). It should be noted that the shear deformation of the material of interna| layers influences not only the morphology and mechanical properties of sample8 elor~gated in liquid media The differences in behaviour of thick samples ~nd thin films of fibres after deformation in an active liquid are also significant. It is l ~ o w a that on drying P E T P films and other polymers elongated in surface active media,
1536
YE. A. SINEVICH and N. F. B A ~ m ~ v
• he material m a y undergo strong contraction [] 0] which is due to the closure of i]~air line cracks (joining of Walls). I f these cracks do not pass through the entire ~ross-section of the sample, the shear deformation of internal layers, which is irreversible at room temperature, prevents contraction. I n fact, fragments of P E T P samples (Fig. 4) show wide open hair line cracks, although these samples were dried in the free state after elongation in liquid medium. Thus, an increase in the thickness of P E T P samples deformed in surface .active liquid media changes the efficiency of action of the media as ~ result of changing the deformation mechanism of the polymer. Elongation of thin films ,or fibres is due to formation and extension of hair line cracks; when bulky samples w i t h a large cross section are elongated, shear deformation m a y be significant, as it also influences the physical and mechanical properties of the material .obtained as a result of elongation of the polymer on contact with the liquid ,medium. The authors are grateful to A. S. Kechek'yan for the discussion of results. Translated by E. SEVERE REFERENCES
I. Y. IMAI and N. BROWN, J. Mater. Sci. 11: 425, 1976 2. N. V. PERTSOV, Ye.,A. SINEVICH and N. I. IVANOVA, Plast. massy, No. 2, 25, 1978
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