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Physical transformations and change in mechanical properties on thermal ageing of aliphatic polyamides
Thermal ageing of aliphatic polyamides
967
26. G. LICHTI, D. F. SANGSTER, C. C. WHANG, D. H. NAPPER and R. G. GILBERT, J. Chem. Soc. Faraday Trans. I. 78: 2129, 1982 27. V. I. LUKHOVITSKII, V. V. POLIKARPOV and V. L. KARPOV, Vysokomol. soyed. B14: 426, 1972 (Not translated in Polymer Sci. U.S.S.R.) 28. R. A. GREGG and F. R. MAYO, Disc. Faraday Soc., No. 2, 238, 1947 29. R. KHUVINK and A. STAVERMAN, Khimiya i tekhnologiya polimerov 1: 215, 1965 30. V. I. LUKHOVITSKII, V. V. POLIKARPOV and R. M. POZDEYEVA, Vysokomol. soyed. A17: 1449, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 7, 1663, 1975) 31. G. L. LEY, Ch. SCHNEIDER and D. O. HI~MMEL, J. Polymer. Sci. C., 27, 119, 1969
Polymer Science U.S.S.R. Vol. 29, No. 4, pp. 967 973, 1987 Printed in Poland
0032-3950/87 $10.00 +. 00 ii~ 1988 Pergamon Press plc
PHYSICAL TRANSFORMATIONS AND CHANGE IN MECHANICAL PROPERTIES ON THERMAL AGEING OF ALIPHATIC POLYAMIDES * N. N. PAVLOV, G. A. KUDRYAVTSEVA, I. M. ABRAMOVA, V. A. VASIL'EVA, L. A. ZEZINA and L. G. KAZARYAN "Plastmassy" Scientific Production Association (Receh~ed 8 October 1985)
In the thermal ageing of aliphatic polyamides the density and degree of crystallinity are increased, the supermolecular structure is completed, and microcracks appear in the layers neat the surface, which is accompanied by a change in tile mechanical properties of the specimens. ALIPHATIC polyamides (PA) are prospective structural thermoplastics, widely used in the manofacture of components for different surfaces, which, as a rule, are used at temperatures not exceeding 120°C. In the pressure casting of crystallized thermoplastics it is not alwa}s possible to ensure equilibrium conditions of crystallization and formation of the supermolecu[ar structures [1]. Accordingly, during thermal ageing over the temperature range not exceeding the melting point of the aliphatic PA, changes in the degree of crystallinity and the size of crystals must be expected, and also changes in the supermolecular formations, which can affect the service properties of the polymer in components. In view of this, the changes in the degree of crystallinity, the supermolecular structure, and the mechanical properties of aliphatic PA differing in the ratio of the CH2 and C O N H groups taking place on thermal ageing at 50-160°C are studied. * Vysokomol. soyed. A29: No. 4, 871-875, 1987.
N. N. PAVLOV et ai.
968
TABLE 1. PROPERTIES of P A SPECIMENS tN THE ORIGINAL STATE Value o f mrameter
Parameter
PA-6
Ratio of CH2 and C O N H groups p, g/cm 3
PA-12
5
11 1.014/1.016 3()/33 40/48 45_+3 51_+9 290 _+60
1.123/I.130
K,%
30/38
L, A Yield point under tension, M P a Fracture stress under tension, M P a Relatve breaking elongation,
55_+5 70 _+10 300 _+60
Note. N u m e r a t o r - f o r the surface layer (thickness 300 itm), d e n o m i n a t o r - f o r the central part of the specimen (at a distance of about 1500 ,urn from the surface).
Standard specimens (GOST 11262-80) in the form of two-sided blades of thickness 3 ram, obtained by pressure casting from PA-6 (OST-6-06-$9-76) a n d PA-12 (OST76-05-425-76) were studied. The characteristics of the original specimens are shown in Table 1. Apart from these specimens, a film of thickness 23 _+2 gin, obtained from PA-6 by extrusion was used for studying the polymorphous transitions. Since during pressure casting the supermolecular structure formed and the crystallinity in the cross section of the specimen differ, sections of thickness 300_+ 20 #m, obtained from the surface of the specimens in a direction parallel to the casting direction were used to study the changes taking place during thermal ageing. The degree of crystallinity K and the size of the crystallites L20o were determined by a wide angle ionization method with a model URS-50IM diffractometer (CuK~ irradiation). The density was determined by means of a radiant tube (GOST 15139-78) in a mixture of p-xylene and tetrachloromethane at 25°C. The polymorphous transitions in PA-6 were determined by X-ray diffraction analysis and IR spectroscopy, from the change in intensity of absorption close to 936 and 1075 cm-~, which is determined by the crystallinity of the PA-6 phase in the ct- and 7-modifications respectively
[2--41. Oc ~ 107 SeC -1 rl~r,e/.lln.
I
,
_.+
g.+
I",C °81 L
l
2'o ÷
I I
I
FIG. 1
'1 1500 Time, h~ I
EO0
FIG. 2
FIG. 1. Effect of layer depth on the rate of 'change of the degtee of crystallinity during thermal ageing at 100°C in the case o f / , 2 - P A - 6 and 3, 4 - P A - 1 2 in 1, 3 - a i r and 2, 4 - a n inert medium. FJ6. 2. Effect of ageing time in ageing PA-6 at !, 1' - 70 and 2, 2' - 100°C in 1, 2 - air and 1', 2' - an inert medium on the content of Ix- and y-crystalline modifications.
Thermal ageing of aiiphatic polyamides
969
The supermolecular structure of PA before and after various ageing transitions was studied under an electron microscope with the aid of two-step polystyrene carbon-platinum replicas, obtained from the surface of a brittle chip of the specimen. The chips were made from plates obtained by cutting the specimen into parts parallel to the casting direction. This method of preparing the specimens for electron microscope examination provided a means of revealing differences in the morphology of the individual layers of the cast specimen, and also the changes taking place in them on thermal ageing. Figure l shows the initial rate of growth K as a function of tile depth of the layer under study. It can be seen that the rate of growth is inversely to the distance From the specimen surface. On ageing of the PA under study in an inert medium an increase in K is also observed, but the rate of this process is less than when the specimen are aged in air. The rate of change of K u n d e r specific ageing conditions indicates that this process takes place more rapidly in PA-6 than in PA-12. In the latter the composition of the gas medim in which the specimens are subjected to thermal ageing has hardly any effect on the rate of change of K. Determination of the size of the crystallites From the data of X-ray diffraction analysis showed that in PA-12 the maximum increase (80~o) takes place in the surface layer (6~300 #m). The composition of the gaseous medium has a m u c h greater effect on the size of the crystallites than on the change in K. During thermal ageing the temperature K and L20o are increases, these changed depending on the temperature more than on the time. On thermal ageing at 100-160°C, not only changes in the degree of crystallinity, but also a polymorphous transition of the crystalline structure from the 7- to the ~-modification is noted in the surface layer of a PA-6 specimen.
FIG. 3. Photomicrograph of the external layer (300 #m) of PA-6 specimens before (a) and after ageing (b) at 70°C over 180 days.
Figure 2 shows the results indicating a polymorphous change in PA-6 as observed by IR spectroscopy. Accordingly, a polymorphous transition is observed in PA-6 specimens obtained by pressure casting or extrusion. The weight degree of crystallinity as calculated from the equation [5]
970
N.N. PAVLOVet al.
We-
PeP-P,,
P P~-P
is somewhat less than the degree of crystallinity determined from the data of X-ray diffraction analysis. This difference is evidently due to the presence of microdefects in the specimens. In investigating the morphology of the supermolecular structure of various layers of PA-6 and PA-12 specimens it is established that the surface layer of a PA-6 specimen has a laminar structure (Fig. 3a). The deeper layers have a coarsely spherulitic structure, which is also characteristic of PA-12 specimens and is independent of the depth. The difference in structure between the surface layer and the deeper layers of PA-6 specimens and the uniformity of supermolecular structure of the whole volume of the PA-12 specimens is eviently determined by the higher melting point of PA-6. For the same temperalure of the casting mould, the degree of supercooling of the melt is greater in the case of PA-6, which hinders the formation of the spherulitic structure in the surface layer. Thermal ageing at 70°C over 180 days is accompanied by the appearance of not Very clear individual spherulites in the surface layer of PA-6 (Fig. 3b); in the PA-12 specimens hardly any changes in spherulitic structure can be observed. Microcracks appeared in all the polymers studied, and singificantly more were observed in the surface layer than in the deeper layers. The maximum size of the microcracks in the surface layer was 2-8/tm, as agail~st 2-3 i t m in the deeper layers. Increase in the temperature of thermal ageing is accompanied by more marked changes in the surface layer structure of PA-6. Thus at an ageing structure of 160°C the supermolecular structure becomes markedly spherulitic over the whole volume of the specimen. In PA-12 specimens the spherulitic structure is retained almost without change. The dimensions of the microcracks in the surface layer vary from 5 to 8/~m, and in the case of PA-6 individual pores of diameter about 5/~m appear irrespective of the depth of the layer. Accordingly, electron microscope studies provide a means of revealing changes in the morphology of the supermolecular structure, which is manifested on the one hand in completion of the initially formed structure (in the case of PA-6) and, on the other hand, in the appearance of microcracks, these being more numerous in the surface layers. Similar changes in supermolecular structure were also observed on heating the studied polymers at 160°C in an inert medium over seven days. Thermal ageing in the inert medium is also accompanied by the appearance of microcracks, the maximum size of these being 8 ~um. Comparison of the results of electron microscope studies of the supermolecular structure of PA-6 and PA-12 specimens before and after thermal ageing in air and in an inert medium justifies the assumption that the changes taking place are qualitatively identical, but that on ageing in air they occur more rapidly. The clearest structural changes are observed in the surface layers. Evidently these changes are facilitated by the more rapid oxidation which takes place in these layers [6]. From a comparison of the results obtained in investigating the effect of the conditions of thermal ageing on the structural transformations in aliphatic PA, it follows that
Thermal ageing of allphatlc p01yamides
971
irrespective of the method of obtaining the specimen (pressure casting or extrusion) thermal ageing is accompanied by a change in the supermolecular structure and the latter is activated by thermal oxidative reactions, and is more marked in the surface layers of the specimens obtained by pressure casting.
'Co
-lO
1.0
-18 02 I
I
I
IO
IOa FIG. 4
Time, hp
2"3
3.1
;O~/T, K -,
Fro. 5
FIG. 4. Change in relative breaking elongation of PA-6 after thermal heating in air at reduced temperatures of 1 -50, 2-70, 3-100, and 4-120°C. FIG. 5. Rate of decrease of relative breaking elongation of standard specimens of I-PA-6 and 2-PA-12 as a function of temperature. Since the transformations of the physical structure of PA considered take place more rapidly in the surface layers, which detelmine the mechanical properties of the specimen under tension [5, 6], these changes would naturally be expected during thermal ageing in air and in an inert medium. Figure 4 shows, in a general form, the effect of the temperature and duration of thermal ageing on the relative breaking elongtion of PA-6 specimens. The generalized curves were obtained by temperature-time superposition [7]. Apart from the decrease in relative breaking elongation, after ageing the yield point and the elastic modulus are increased, and the ultimate tensile stress is decreased. The change in mechanical properties after ageing in an inert medium are qualitatively similar to those on ageing in air, as shown in Table 2, which shows the ratios of the yield point ay, the rupture stress at, and the relative elongation ere before and after ageing in an inert medium. Since ageing in an inert medium is accompanied by a change in the physical structure and the appearance of microcracks, it can be assumed that one of the reasons determining the decrease in the deformational and strength properties is the micro-distribution of the specimen surface layers. In fact, removal of a layer of specimen of thickness about 200 pm from .,he specimen surface by means of a solvent (m-cresol) results in significant restoration of the mechanical properties lost during thermal ageing. For example, this treatment of PA-6 specimens after ageing enables the relative elongation to be restored from 5 ~ to almost the initial value. The results obtained indicate the important role of the micro-distribution of the surface layer. As has been established, cracking takes place irrespective of the composition of the
972
N . N . PAVLOVet a/.
gas medium in which thermal ageing takes place, and is evidently determined by the technological parameters in the manufacture of the specimen. The presence of atmospheric oxygen promotes micro-distribution. Increase in ageing temperature results in quicker development of oxidation, accompanied by the formation of a considerable number of large cracks. TABLE 2. EFFECT OF DURATION AND TEMPERATURE OF THERMAL AGEING IN AN INERT MEDIUM ON THE MECHANICAL PROPERTIES UNDER TENSlON OF CAST P A - 6 AND P A - 1 2 SPECIMENS
T,° 50 7O 100
Time, day 30 90 180 30 90 180 15 30 60
Koy
K,~
1.07/1.00 1-11/1.03 1.15/1.05 1.15/1.03
1"03/0'95 1.03/0.90 0.75/0.90 1.00/0.94 0.75/0.87 0"68/0"78 0.87/0.92
1.18/1.05 1.23/1.10 1.21/1.05 i.25/1.08 1.27/1.13
1.oo/1.83 1.00/0.67
Ker© 1.02/1.00 1.01/0.90 0.75/0.80 1.00/0.94 0-78/0.90 0.67/0.72 0.79/0.86 1.00/0.81 1.03/0.73
Note. The numerator gives the data for PA-6 and the denominator the data for PA-12.
The rate of decrease of relative breaking elongation as a function of temperature is well described by the Arrhenius equations (Fig. 5), but the angular coefficient over t h e temperature range 100-120°C is changed. This important fact must be allowed for in selecting the limiting temperatures for artificial tests on PA. In general such tests are aimed at predicting the changes in a control criterion in another, generally lower range of temperatures. To explain the possible reasons for the change in the angular coefficient the inter-plane distances in the crystals of the polymers studied were investigated as a function of temperature. The tests indicated a non-linear relation over the temperature range 20-200°C. The inflection point on the plot of temperature against the inter-planar distances is within the range 100-120°C, which can be associated with the rupture of hydrogen bonds in the crystals. The position of the inflection points on the temperature axis depends on the completion of the initial structure, and as this increases is displaced in the direction of higher temperatures. A change in the angular coefficients of the rate of change of the control criterion as a function of the temperature of thermal ageing must be allowed for in selecting the maximum temperature of the artificial tests, which, in the case of aliphatic PA, should not exceed 120°C. The results obtained during thermal ageing at temperatures below 120°C can be used in forecasting the time for which the control criterion (properties) can be maintained at a given level. The authors wish to thank A. Ya. Malkin for useful discussions of the result of this work. Translated by N. STANDEN
Spontaneous graft polymerization of' aikylated 4-vinylpyridine in PE
973
REFERENCES
1. V. V. LAPSHIN, Osnovy pererabotki termoplastov metodom lit'ya pod devleniem (Fundamentals of the Processing of Thermal Plastics by Pressure Casting), Moscow, 1974 2. d. KAZUO and W. SADAO, J. Polymer Sci. Polymer Phys. Ed. 11: 1077, 1973 3. d. L. KOLNING, M. IPEKKEI and ,L B. LANDO, Macromolec. Sci. 11: 713, 1972 4. lnfrakrasnaya spektroskopiya polimerov (Infrared Spectroscopy of Polymers), (Ed. I. M. Dekhant), Moscow, 1976 51 B. VUNDERLIKH, FizJka makromolekul (Physics of Macromolecules), vol. 1, p. 439, Moscow, 1976 6. Diffuzionnaya stabilizatsiya polimerov (Diffusional Stabilization of Polymers), Vilnius, 1974 7. E. L. KALINCHEV and M. Sh. KATSEVMAN, Plast. massy, 4, 21, 1976
Polymer Science U.S.S.R. Vol. 29, No. 4, pp. 973-979, 1987 Printed in Poland
0032-3950]87 $10.00+ .00 1988 P e r g a m o n Press plc
SPONTANEOUS GRAFT POLYMERIZATION OF ALKYLATED 4-VINYLPYRIDINE IN POLYETHYLENE* B. SH. KHAKIMDZHANOV, U. N. MUSAYEV, SH. A. KURBANOV, A. V. VLASOV and B. L. TSETLtN s
V. I. Lenin State University, Tashkent (Received 8 October 1985)
It is shown that spontaneous graft polymerization of 4-vinylpyridine on polyethylene containing grafted poly-4-vinylpyridine is possible in principle by introduction of alkylating agents into the solution. The rate of polymerization is studied as a function of the nature and concentration of the alkylating agents, the monomer, the nature of the solvent, and the content of grafted poly-4-vinylpyridine, and the temperature. It is established that thg initiation and chain growth reactions involve a Zwitterion mechanism. INVESTIGATIONo f the laws of alkylation of 4-vinylpyridine (VP) by alkyl halides, as is known, led Kargin, K a b a n o v et al. to discover the Zwitterion polymerization o f quaternary salts o f VP [l, 2]. As has been shown [3, 4] polymerization takes place in accordance with a specific mechanism which ensures selection in tile polymer o f only the vinylpyridine salt molecules. Also, American werkers [5, 6] observed that the original Zwitterions in the reaction system can be formed on addition to VP o f uncharged nucleophiles, especially pyridine or vinylpyridine. It is thus o f interest to study the alkylation o f VP in the presence of grafted polylners containing grafted poly-4-vinylpyridine (PVP) in small quantities. The first tests showed that under such conditions rapid spontaneous graft polymerization o f VP is observed, and bzcause of thi:~ the process was studied in much greater detail. * Vysokomol. soyed. A29; No. 4, 876-881, 1987.
967
26. G. LICHTI, D. F. SANGSTER, C. C. WHANG, D. H. NAPPER and R. G. GILBERT, J. Chem. Soc. Faraday Trans. I. 78: 2129, 1982 27. V. I. LUKHOVITSKII, V. V. POLIKARPOV and V. L. KARPOV, Vysokomol. soyed. B14: 426, 1972 (Not translated in Polymer Sci. U.S.S.R.) 28. R. A. GREGG and F. R. MAYO, Disc. Faraday Soc., No. 2, 238, 1947 29. R. KHUVINK and A. STAVERMAN, Khimiya i tekhnologiya polimerov 1: 215, 1965 30. V. I. LUKHOVITSKII, V. V. POLIKARPOV and R. M. POZDEYEVA, Vysokomol. soyed. A17: 1449, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 7, 1663, 1975) 31. G. L. LEY, Ch. SCHNEIDER and D. O. HI~MMEL, J. Polymer. Sci. C., 27, 119, 1969
Polymer Science U.S.S.R. Vol. 29, No. 4, pp. 967 973, 1987 Printed in Poland
0032-3950/87 $10.00 +. 00 ii~ 1988 Pergamon Press plc
PHYSICAL TRANSFORMATIONS AND CHANGE IN MECHANICAL PROPERTIES ON THERMAL AGEING OF ALIPHATIC POLYAMIDES * N. N. PAVLOV, G. A. KUDRYAVTSEVA, I. M. ABRAMOVA, V. A. VASIL'EVA, L. A. ZEZINA and L. G. KAZARYAN "Plastmassy" Scientific Production Association (Receh~ed 8 October 1985)
In the thermal ageing of aliphatic polyamides the density and degree of crystallinity are increased, the supermolecular structure is completed, and microcracks appear in the layers neat the surface, which is accompanied by a change in tile mechanical properties of the specimens. ALIPHATIC polyamides (PA) are prospective structural thermoplastics, widely used in the manofacture of components for different surfaces, which, as a rule, are used at temperatures not exceeding 120°C. In the pressure casting of crystallized thermoplastics it is not alwa}s possible to ensure equilibrium conditions of crystallization and formation of the supermolecu[ar structures [1]. Accordingly, during thermal ageing over the temperature range not exceeding the melting point of the aliphatic PA, changes in the degree of crystallinity and the size of crystals must be expected, and also changes in the supermolecular formations, which can affect the service properties of the polymer in components. In view of this, the changes in the degree of crystallinity, the supermolecular structure, and the mechanical properties of aliphatic PA differing in the ratio of the CH2 and C O N H groups taking place on thermal ageing at 50-160°C are studied. * Vysokomol. soyed. A29: No. 4, 871-875, 1987.
N. N. PAVLOV et ai.
968
TABLE 1. PROPERTIES of P A SPECIMENS tN THE ORIGINAL STATE Value o f mrameter
Parameter
PA-6
Ratio of CH2 and C O N H groups p, g/cm 3
PA-12
5
11 1.014/1.016 3()/33 40/48 45_+3 51_+9 290 _+60
1.123/I.130
K,%
30/38
L, A Yield point under tension, M P a Fracture stress under tension, M P a Relatve breaking elongation,
55_+5 70 _+10 300 _+60
Note. N u m e r a t o r - f o r the surface layer (thickness 300 itm), d e n o m i n a t o r - f o r the central part of the specimen (at a distance of about 1500 ,urn from the surface).
Standard specimens (GOST 11262-80) in the form of two-sided blades of thickness 3 ram, obtained by pressure casting from PA-6 (OST-6-06-$9-76) a n d PA-12 (OST76-05-425-76) were studied. The characteristics of the original specimens are shown in Table 1. Apart from these specimens, a film of thickness 23 _+2 gin, obtained from PA-6 by extrusion was used for studying the polymorphous transitions. Since during pressure casting the supermolecular structure formed and the crystallinity in the cross section of the specimen differ, sections of thickness 300_+ 20 #m, obtained from the surface of the specimens in a direction parallel to the casting direction were used to study the changes taking place during thermal ageing. The degree of crystallinity K and the size of the crystallites L20o were determined by a wide angle ionization method with a model URS-50IM diffractometer (CuK~ irradiation). The density was determined by means of a radiant tube (GOST 15139-78) in a mixture of p-xylene and tetrachloromethane at 25°C. The polymorphous transitions in PA-6 were determined by X-ray diffraction analysis and IR spectroscopy, from the change in intensity of absorption close to 936 and 1075 cm-~, which is determined by the crystallinity of the PA-6 phase in the ct- and 7-modifications respectively
[2--41. Oc ~ 107 SeC -1 rl~r,e/.lln.
I
,
_.+
g.+
I",C °81 L
l
2'o ÷
I I
I
FIG. 1
'1 1500 Time, h~ I
EO0
FIG. 2
FIG. 1. Effect of layer depth on the rate of 'change of the degtee of crystallinity during thermal ageing at 100°C in the case o f / , 2 - P A - 6 and 3, 4 - P A - 1 2 in 1, 3 - a i r and 2, 4 - a n inert medium. FJ6. 2. Effect of ageing time in ageing PA-6 at !, 1' - 70 and 2, 2' - 100°C in 1, 2 - air and 1', 2' - an inert medium on the content of Ix- and y-crystalline modifications.
Thermal ageing of aiiphatic polyamides
969
The supermolecular structure of PA before and after various ageing transitions was studied under an electron microscope with the aid of two-step polystyrene carbon-platinum replicas, obtained from the surface of a brittle chip of the specimen. The chips were made from plates obtained by cutting the specimen into parts parallel to the casting direction. This method of preparing the specimens for electron microscope examination provided a means of revealing differences in the morphology of the individual layers of the cast specimen, and also the changes taking place in them on thermal ageing. Figure l shows the initial rate of growth K as a function of tile depth of the layer under study. It can be seen that the rate of growth is inversely to the distance From the specimen surface. On ageing of the PA under study in an inert medium an increase in K is also observed, but the rate of this process is less than when the specimen are aged in air. The rate of change of K u n d e r specific ageing conditions indicates that this process takes place more rapidly in PA-6 than in PA-12. In the latter the composition of the gas medim in which the specimens are subjected to thermal ageing has hardly any effect on the rate of change of K. Determination of the size of the crystallites From the data of X-ray diffraction analysis showed that in PA-12 the maximum increase (80~o) takes place in the surface layer (6~300 #m). The composition of the gaseous medium has a m u c h greater effect on the size of the crystallites than on the change in K. During thermal ageing the temperature K and L20o are increases, these changed depending on the temperature more than on the time. On thermal ageing at 100-160°C, not only changes in the degree of crystallinity, but also a polymorphous transition of the crystalline structure from the 7- to the ~-modification is noted in the surface layer of a PA-6 specimen.
FIG. 3. Photomicrograph of the external layer (300 #m) of PA-6 specimens before (a) and after ageing (b) at 70°C over 180 days.
Figure 2 shows the results indicating a polymorphous change in PA-6 as observed by IR spectroscopy. Accordingly, a polymorphous transition is observed in PA-6 specimens obtained by pressure casting or extrusion. The weight degree of crystallinity as calculated from the equation [5]
970
N.N. PAVLOVet al.
We-
PeP-P,,
P P~-P
is somewhat less than the degree of crystallinity determined from the data of X-ray diffraction analysis. This difference is evidently due to the presence of microdefects in the specimens. In investigating the morphology of the supermolecular structure of various layers of PA-6 and PA-12 specimens it is established that the surface layer of a PA-6 specimen has a laminar structure (Fig. 3a). The deeper layers have a coarsely spherulitic structure, which is also characteristic of PA-12 specimens and is independent of the depth. The difference in structure between the surface layer and the deeper layers of PA-6 specimens and the uniformity of supermolecular structure of the whole volume of the PA-12 specimens is eviently determined by the higher melting point of PA-6. For the same temperalure of the casting mould, the degree of supercooling of the melt is greater in the case of PA-6, which hinders the formation of the spherulitic structure in the surface layer. Thermal ageing at 70°C over 180 days is accompanied by the appearance of not Very clear individual spherulites in the surface layer of PA-6 (Fig. 3b); in the PA-12 specimens hardly any changes in spherulitic structure can be observed. Microcracks appeared in all the polymers studied, and singificantly more were observed in the surface layer than in the deeper layers. The maximum size of the microcracks in the surface layer was 2-8/tm, as agail~st 2-3 i t m in the deeper layers. Increase in the temperature of thermal ageing is accompanied by more marked changes in the surface layer structure of PA-6. Thus at an ageing structure of 160°C the supermolecular structure becomes markedly spherulitic over the whole volume of the specimen. In PA-12 specimens the spherulitic structure is retained almost without change. The dimensions of the microcracks in the surface layer vary from 5 to 8/~m, and in the case of PA-6 individual pores of diameter about 5/~m appear irrespective of the depth of the layer. Accordingly, electron microscope studies provide a means of revealing changes in the morphology of the supermolecular structure, which is manifested on the one hand in completion of the initially formed structure (in the case of PA-6) and, on the other hand, in the appearance of microcracks, these being more numerous in the surface layers. Similar changes in supermolecular structure were also observed on heating the studied polymers at 160°C in an inert medium over seven days. Thermal ageing in the inert medium is also accompanied by the appearance of microcracks, the maximum size of these being 8 ~um. Comparison of the results of electron microscope studies of the supermolecular structure of PA-6 and PA-12 specimens before and after thermal ageing in air and in an inert medium justifies the assumption that the changes taking place are qualitatively identical, but that on ageing in air they occur more rapidly. The clearest structural changes are observed in the surface layers. Evidently these changes are facilitated by the more rapid oxidation which takes place in these layers [6]. From a comparison of the results obtained in investigating the effect of the conditions of thermal ageing on the structural transformations in aliphatic PA, it follows that
Thermal ageing of allphatlc p01yamides
971
irrespective of the method of obtaining the specimen (pressure casting or extrusion) thermal ageing is accompanied by a change in the supermolecular structure and the latter is activated by thermal oxidative reactions, and is more marked in the surface layers of the specimens obtained by pressure casting.
'Co
-lO
1.0
-18 02 I
I
I
IO
IOa FIG. 4
Time, hp
2"3
3.1
;O~/T, K -,
Fro. 5
FIG. 4. Change in relative breaking elongation of PA-6 after thermal heating in air at reduced temperatures of 1 -50, 2-70, 3-100, and 4-120°C. FIG. 5. Rate of decrease of relative breaking elongation of standard specimens of I-PA-6 and 2-PA-12 as a function of temperature. Since the transformations of the physical structure of PA considered take place more rapidly in the surface layers, which detelmine the mechanical properties of the specimen under tension [5, 6], these changes would naturally be expected during thermal ageing in air and in an inert medium. Figure 4 shows, in a general form, the effect of the temperature and duration of thermal ageing on the relative breaking elongtion of PA-6 specimens. The generalized curves were obtained by temperature-time superposition [7]. Apart from the decrease in relative breaking elongation, after ageing the yield point and the elastic modulus are increased, and the ultimate tensile stress is decreased. The change in mechanical properties after ageing in an inert medium are qualitatively similar to those on ageing in air, as shown in Table 2, which shows the ratios of the yield point ay, the rupture stress at, and the relative elongation ere before and after ageing in an inert medium. Since ageing in an inert medium is accompanied by a change in the physical structure and the appearance of microcracks, it can be assumed that one of the reasons determining the decrease in the deformational and strength properties is the micro-distribution of the specimen surface layers. In fact, removal of a layer of specimen of thickness about 200 pm from .,he specimen surface by means of a solvent (m-cresol) results in significant restoration of the mechanical properties lost during thermal ageing. For example, this treatment of PA-6 specimens after ageing enables the relative elongation to be restored from 5 ~ to almost the initial value. The results obtained indicate the important role of the micro-distribution of the surface layer. As has been established, cracking takes place irrespective of the composition of the
972
N . N . PAVLOVet a/.
gas medium in which thermal ageing takes place, and is evidently determined by the technological parameters in the manufacture of the specimen. The presence of atmospheric oxygen promotes micro-distribution. Increase in ageing temperature results in quicker development of oxidation, accompanied by the formation of a considerable number of large cracks. TABLE 2. EFFECT OF DURATION AND TEMPERATURE OF THERMAL AGEING IN AN INERT MEDIUM ON THE MECHANICAL PROPERTIES UNDER TENSlON OF CAST P A - 6 AND P A - 1 2 SPECIMENS
T,° 50 7O 100
Time, day 30 90 180 30 90 180 15 30 60
Koy
K,~
1.07/1.00 1-11/1.03 1.15/1.05 1.15/1.03
1"03/0'95 1.03/0.90 0.75/0.90 1.00/0.94 0.75/0.87 0"68/0"78 0.87/0.92
1.18/1.05 1.23/1.10 1.21/1.05 i.25/1.08 1.27/1.13
1.oo/1.83 1.00/0.67
Ker© 1.02/1.00 1.01/0.90 0.75/0.80 1.00/0.94 0-78/0.90 0.67/0.72 0.79/0.86 1.00/0.81 1.03/0.73
Note. The numerator gives the data for PA-6 and the denominator the data for PA-12.
The rate of decrease of relative breaking elongation as a function of temperature is well described by the Arrhenius equations (Fig. 5), but the angular coefficient over t h e temperature range 100-120°C is changed. This important fact must be allowed for in selecting the limiting temperatures for artificial tests on PA. In general such tests are aimed at predicting the changes in a control criterion in another, generally lower range of temperatures. To explain the possible reasons for the change in the angular coefficient the inter-plane distances in the crystals of the polymers studied were investigated as a function of temperature. The tests indicated a non-linear relation over the temperature range 20-200°C. The inflection point on the plot of temperature against the inter-planar distances is within the range 100-120°C, which can be associated with the rupture of hydrogen bonds in the crystals. The position of the inflection points on the temperature axis depends on the completion of the initial structure, and as this increases is displaced in the direction of higher temperatures. A change in the angular coefficients of the rate of change of the control criterion as a function of the temperature of thermal ageing must be allowed for in selecting the maximum temperature of the artificial tests, which, in the case of aliphatic PA, should not exceed 120°C. The results obtained during thermal ageing at temperatures below 120°C can be used in forecasting the time for which the control criterion (properties) can be maintained at a given level. The authors wish to thank A. Ya. Malkin for useful discussions of the result of this work. Translated by N. STANDEN
Spontaneous graft polymerization of' aikylated 4-vinylpyridine in PE
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REFERENCES
1. V. V. LAPSHIN, Osnovy pererabotki termoplastov metodom lit'ya pod devleniem (Fundamentals of the Processing of Thermal Plastics by Pressure Casting), Moscow, 1974 2. d. KAZUO and W. SADAO, J. Polymer Sci. Polymer Phys. Ed. 11: 1077, 1973 3. d. L. KOLNING, M. IPEKKEI and ,L B. LANDO, Macromolec. Sci. 11: 713, 1972 4. lnfrakrasnaya spektroskopiya polimerov (Infrared Spectroscopy of Polymers), (Ed. I. M. Dekhant), Moscow, 1976 51 B. VUNDERLIKH, FizJka makromolekul (Physics of Macromolecules), vol. 1, p. 439, Moscow, 1976 6. Diffuzionnaya stabilizatsiya polimerov (Diffusional Stabilization of Polymers), Vilnius, 1974 7. E. L. KALINCHEV and M. Sh. KATSEVMAN, Plast. massy, 4, 21, 1976
Polymer Science U.S.S.R. Vol. 29, No. 4, pp. 973-979, 1987 Printed in Poland
0032-3950]87 $10.00+ .00 1988 P e r g a m o n Press plc
SPONTANEOUS GRAFT POLYMERIZATION OF ALKYLATED 4-VINYLPYRIDINE IN POLYETHYLENE* B. SH. KHAKIMDZHANOV, U. N. MUSAYEV, SH. A. KURBANOV, A. V. VLASOV and B. L. TSETLtN s
V. I. Lenin State University, Tashkent (Received 8 October 1985)
It is shown that spontaneous graft polymerization of 4-vinylpyridine on polyethylene containing grafted poly-4-vinylpyridine is possible in principle by introduction of alkylating agents into the solution. The rate of polymerization is studied as a function of the nature and concentration of the alkylating agents, the monomer, the nature of the solvent, and the content of grafted poly-4-vinylpyridine, and the temperature. It is established that thg initiation and chain growth reactions involve a Zwitterion mechanism. INVESTIGATIONo f the laws of alkylation of 4-vinylpyridine (VP) by alkyl halides, as is known, led Kargin, K a b a n o v et al. to discover the Zwitterion polymerization o f quaternary salts o f VP [l, 2]. As has been shown [3, 4] polymerization takes place in accordance with a specific mechanism which ensures selection in tile polymer o f only the vinylpyridine salt molecules. Also, American werkers [5, 6] observed that the original Zwitterions in the reaction system can be formed on addition to VP o f uncharged nucleophiles, especially pyridine or vinylpyridine. It is thus o f interest to study the alkylation o f VP in the presence of grafted polylners containing grafted poly-4-vinylpyridine (PVP) in small quantities. The first tests showed that under such conditions rapid spontaneous graft polymerization o f VP is observed, and bzcause of thi:~ the process was studied in much greater detail. * Vysokomol. soyed. A29; No. 4, 876-881, 1987.