- Email: [email protected]
Study of crystallization of industrial polyethylene and polyethylene mixtures
:2596
R.R.
FIKHTI~ER et a[. REFERENCES
1. A. L. SUVOROV, M. A. KOCHNEVA, I. V. YEMEL'YANOVA and L. D. DUL'TSEVA, H a s t . massy, No. 7, 65, 1977 2. A. I. SUVOROVA, N. Yu. YEZHOVA, L. D. DUL'TSEVA, A. L. SUVOROV, M. A. KOCHNEVA, N. T. NERUSH and A. A. TAGER, Plast. massy, into. 7, 65, 1977 3. A . L . SUVOROV, A. I'. SUVOROVA, L. D. DUL'TSEVA, N. Yu. YEZHOVA, M. A. KOCHNEVA and Ye. F. LOGINOVA, ~ysokomol. soyed. A2O: 2592, 1978 (Translated in Polymer Sci. U.S.S.R. 20: l l , 2909, ]978) 4. M. KOBALE and H. LOBLE, Z. Elektrochem. 65: 662, 1961 5. N. SHITO and M. SATA, J. Polymer Sci. C16: 1069, 1967 6. Ye. M. BLYAKHMAN, T. I. BORISOVA and P. M. LEVITSKAYA, Vysokomol. soyed. A12: 1544, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 7, 1756, 1970) 7. A. L. SUVOROV and M. A. KOCHNEVA, Auth. Cert. 478848; Byull. izobr., No. 8, 1975 8. V. A. KLIMOVA, Osnovnyye mikrometody analiza organicheskikh soycdinenii (Main Micro-methods for the Analysis of Organic Compounds). Khimiya, 1967 9. A. L. SUVOROV and S. S. SPASSKII, Sb. Elementoorganicheskiye soyedineniya (Heteroorganic Compounds), T r u d y I n s t i t u t a Khimii, U F AN SSSR, 13, 39, 1966 10. V. F. BABICH, Yu. M. SIVERGIN, A. A. BERLIN and A. L. RABINOVICH, Mekhanika polimerov, 3, 1966 11. Ye. M. BLYAKHMAN, T. I. BORISOVA and Ts. M. LEVITSKAYA, Vysokomol. soycd. A12: 2721, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 12, 3090, 1970) 12. V. H. BATZER, F. LOHSE and R. SCHMID, Angcw. Macromolek. Chem. 29/30: 349, 1973 13. L. I. KOMAROVA, S. N. SALAZKIN, V. V. KORSHAK, S. V. VINOGRADOVA, V. I. NIKOLAICHIK, Ye. V. ZABOROVSKAYA and I. A. BULGAKOVA, Vysokomol. soyed. BI6: 718, 1974 (Not translated in Polymer Sci. U.S.S.R.) 14. W. KAUZMAN, Revs. Mod. Phys. 43: 219, 1942
Polymer Science U.S.S.R. Vol. 21, pp. 2596-2693. © Pergamon Press Ltd. 1980. Printed in Poland
0032-3950/79/1001-2596507.50/0
STUDY OF CRYSTALLIZATION OF INDUSTRIAL POLYETHYLENE AND POLYETHYLENE MIXTURES* ]:~. ]:~. FIKHTNER, T. I. VOLKOV, S. A. SHALATSKAYA a n d M. S. TRIZ~O Lensoviet Technological Institute, Leningrad
(Received 22 September 1978) Depolarization of linear polarized light was used to examine isothermal crystallization of industrial P E distinguished by the method of synthesis. The temperature ~lependence of the half-life of crystallization was derived. A study was made of the ~tructure of low mid high density P E mixtures, according to their behaviour during * Vysokomol. soyed. A21: No. 10, 2348-2353, 1979.
Crystallization of industrial PE and PE mixtures
2597
crystallization and melting. It was shown that low density PE and linear PE in a mixture crystallize separately, forming general spherulites. Spherulite formation and increase in dimensions before collision take place during crystallization of high density PE, forming the skeleton of the spherulite. Joint extrusion of low density PE powder and high density, high molecular weight PE is not accompanied by homogenization of the alloy. No marked increase is observed in the fluidity of high density, high molecular weight PE on adding small amounts of low density PE. I ~ a previous s t u d y [ 1] concerning kinetics of isothermal crystallization of P E b y d i l a t o m e t r i c m e t h o d n a r r o w p o l y m e r fractions were used, which distinguislxes these investigations f r o m conditions of industrial p r o d u c t i o n a n d raises t h e p r o b l e m w h a t degree of f r a c t i o n a t i o n is r e g a r d e d sufficient to confirm some general conclusions. Jo'urthermore, conditions of h e a t exchange prevailing during t h e d i l a t o m e t r i e o b s e r v a t i o n of crystallization impede the investigation of rapid processes m o s t i m p o r t a n t for practical purposes. Optical methods, in which samples of low weight are used ensure i s o t h e r m a l conditions of crystallization to an increased e x t e n t , c o m p a r e d with d i l a t o m e t r y , if crystallization is carried o u t in a fairly large liquid t h e r m o s t a t . I n p r e v i o u s studies [2, 3] concerning i s o t h e r m a l crystallization of P E , optical m e t h o d s were o n l y used to e x a m i n e p o l y m e r s with b r a n c h e d molecules low d e n s i t y polye t h y l e n e ( L D P E ) a n d L u p o l e n 1800S, respectively. An i n v e s t i g a t i o n was m a d e of rapid processes of crystallization of main t y p e s of P E p r o d u c e d in this c o u n t r y (Table) and of crystallization a n d the s t r u c t u r e o f m i x t u r e s p r e p a r e d using some of these P E . CSARACWEmST~CS OF PE EXAMI~S~D Density, g/era a Polyethylene
LDPE HDPE I HDPE II HDt)E II[
with rapid erystallization 0-916 0.947 0.940 0.930
witl7 slow erystallization i
0.923 0.968 0.955 0.947
Melt flow index g/10 min 5-6 2-.9 2.,(t 0.0
K inet its of isothermal crystallization of PE were examined by depolarization of linear polarized light using a device and methods previously described [4]. Crystallization and melting of mixtures were studied by DTA using a Paulik Paulik-Erdei derivatograph (MOM), the method of depolarization of linear polarized light and an optical microscope with a heating stage. Diffraction curves of small angle polarized light scattering [5] were plotted. LDt?E and HDPE were mixed in melt using five extrusion cycles and a capillary viseometer f(~r MMI at 160° and by precipitation from solution in xylene using rapid cooling. I s o t h e r m a l curves of crystallization of the materials studied were o b t a i n e d in a wide range of t e m p e r a t u r e , where the stage of p r i m a r y crystallization is c o m p l e t e d (luring a period ranging from a few secomls to m a n y minutes, lso-
R. R. FIKH~EI~ et a/.
25.98
thermal curves for HDPE II, which occupies intermediate position among P E as regards molecular structure, are shown in Fig. 1 in standard coordinates. Isothermal curves of crystallization of other grades of PE take the same form. As shown by Fig, 1, the stage of primary crystallization is completed (particularly a t high temperatures) with comparatively low degress of crystallinity For example, at a temperature of hardening of 124 ° crystallization is practically complete with a crystallinity of less than 0.5 in relation to its value at 20 °. I'0
12
3
4
5
, ~ O.G
8
\ \ I
I
1
10
Time, rain
lho. 1. Isothermal curves of crystallization of HDPE II in standard coordinates at temperatures of 117 (1); 118.5 (2); 120 (3); 122 (4); 123 (~) and 124 ° (6). Two stages are observed during isothermal crystallization of PE, which is, apparently, a reflection of the two stage development of the crystalline phase: first, the spherulites increase in size until they collide with each other and this is followed by further crystallization of the material inside the spherulites. Analysis of experimental results, according to the Avrami-Kolmogorov equation, shows that the Avrami index varies continuously with a change in the temperature of the crystallizer. For example, for HDPE II it changes from 5.6 to 1.0 in the temperature range of 117-124 °. This, as well as results mentioned, confirms that crystallization of PE in the temperature range studied cannot be regarded, as in the case of crystallization of low molecular weight substances, as the total of processes of formation and extension of nuclei. Temperature dependences of the rate of crystallization of PE of various grades are shown in Fig. 2. Similar dependences for most polymers are extremal in nature, whereby the maximum rate of crystallization is achieved between the melting 0 point and glass temperature when T:(0"82-0"83)Tmelt [6]. For the PE grades examined the maximum rate of crystallization under isothermal conditions could not be determined. More exactly, this concept is noa-existent for this case since
Crystallization of industrial PE and PE mixtures
2599
with considerable degrees of supercooling the rate of crystallization becomes comparable with the rate of cooling the sample to the temperature of the crystallizer. According to Lindenmeyer's theory [7], slow crystallization takes place by the precipitation of molecules accumulated on the growing face of the crystallite. "Foreign" molecules and molecules of a poorly crystallizing material do not form part of the crystalline lattice. Based on the position of curves in Fig. 2 it m a y be assumed that with slow cooling of the mixture of PE samples examined it is mainly P E with linear molecules, namely HDPE I, which crystallizes first and P E with most branched molecules, LDPE, which crystallizes last. In other words, slow crystallization of the mixture is accompanied (in relation to crystallites) by fractionation first of all according to the degree of branching. Fraetionation according to molecular weight will be of lesser significance, particularly with high values of supercooling (Fig. 2). "c rain 40-
/ 23 z/
2-00~ r 9#
98
lOZ
116
/20
/2q Tcr,°C
Fie. 2. Temperature dependence of the half-period of crystallization v of LDPE (1), HDPE 13[ (2), HDPE III (3) and HDPE I (4). Results of investigating crystallization of PE and the importance of compositions from a practical point of view prompted us to investigate the mixture of industrial grades of PE. Results in the literature concerning the structure o f P E mixtures are very conflicting. Robertson and Paul [8] analysed studies by other authors and recognized that low and high density PE can be crystallized separately and interpreted results of their own investigations into mechanical properties of polyolefm mixtures in terms of compatibility and interaction of amorphous phases. Experimental results obtained by authors of another study [9] confirm the homogeneity of amorphous regions and microheterogeneity of crystalline regions in LDPE mixed with wax. The conclusion concerning the microheterogeneity of crystalline regions and the possible separate crystallization of mixture components is based on the observation of individual melting points on DTA curves. Authors of another study [10] only observed one melting peak from the mixture of radiation-polymerized PE and HDPE assuming that P E in a form which is similar to a melt is between t t D P E crystallites. This assumption, however, raises the problem of the cause of high (65% according to X-ray diffrae-
R. R. Fr~u-r~E~R eta/.
2600
tion results) initial crystallinity of radiation polarized P E since, according to our results, density which is the measure of crystallinity, is the additive function of composition of the L D P E - H D P E mixture. The structure of mixtures is now being discussed within the framework of a two phase model. There are no indications as to the degree of homogeneity of P E mixtures and it is impossible to judge the level of elements of supermoleeular organization [4], where components are compatible, or whether this occurs on the grain boundaries. Meanwhile, information about the homogeneity of t h e mixture is particularly important if measurements are concerned with operational properties of compositions. I pel.un. -
II
a
60_[
#5 /
2
3
-
I
2
3
20
80
100
I20
fqO
8O
100
/20 T,°C
:Fie. 3. Intensity variation of depolarized light during heating (a) and cooling (b) samples of I ~ P E (I) and HDPE I (II) mixtures. Here and in Fig. 4 mixture compositions are as follows: 1--19 : 1; 2--5 : 1; 3--2 : l; 4--1 : 1; 5--1 : 2. Results of investigating mixtures in this study showed the following special features. Extrusion through a capillary using a device for the measurement of the melt index (MMI) of powder mixtures of L D P E and H D P E I I I did not improve homogenization of the material. Particles of bright grains containing spherulites of irregular shape on the darker background of a material also containing spherulites can be seen in the visual field of a polarization microscope. Observations of samples during heating (cooling) showed that L D P E is the continuous phase and H D P E III, the dispersed phase. The immiscibility of these grades of P E in melt is, apparently, due to the considerable difference in viscosity. No marked improvement was observed in the fluidity of the H D P E I I I melt on adding small amounts of L D P E . Microscopic studies of mixtures of L D P E and I-IDPE I showed that all the compositions studied are homogeneous materials containing spherulites. Spheru]itic dimensions, as in the case of pure components, depend on the rate of cooling ~he melt. During heating (cooling) mixtures with 19:1 and 5:1 component ratios a sudden change occurs in the intensity of Hv diffraction curves and the brightness
CrystMlization of industrial PE and PE mixtures
260k
of spherulites in the temperature interval of melting LDPE. No change is observed in the dimension of diffraction curves after prolonged (1 hr) retention of t h e material at a temperature Close to Tmelt of H D P E I. +AT
FIG. 4. Thermograms of melting LDPE (I) and HDPE (II) mixtures. Analysis of changes in Hv diffraction curves during cooling the melt of a mixture of the same composition indicates that the diffraction curve appears and its size is reduced long before the sample reaches Tcr of LDPE. The intensity of the diffraction curve undergoes considerable increase near Tcr of LDPE. Results of investigating crystallization and melting of L D P E and H D P E I mixtures b y depolarization of linear polarized light (Fig. 3) and DTA (Fig. 4) indicates tl~e existence of phase transitions near the melting points of pure components. These transitions take place most readily b y depolarization of linear polarized light using a mixture of L D P E and H D P E with a component ratio of 19:1 and b y DTA for a 5:1 mixture. Results for mixtures (19:1, 5:1 and 1 :l) obtained b y precipitation from solution do not diffe',r from those obtained for mixtures prepared from melt. This meaus that the effects observed do not in-. volve poor mixing. A combination of eX])erimental results concerning L D P E and H D P E I mixtures suggests the following. (lomponents crystallize and melt separately. Crystallization takes place with the formation of spher(~|~tes common for both components. The formation and extension of st)herulites before collision involve exclusively the crystallization of H D P E I. During crystallization of L D P E the weight of the crystalline component in s])herulitesi ncreases.
-2602
R . R . FIKHTNER et al. g
Comparing the behaviour of L D P E mixed with H D P E I with properties of other systems some similarities m a y be noted. First--this is the similarity of this mixture to the solution of H D P E I in LDPE; the latter behaves like a solvent reducing the melting point and temperature of crystallization of t I D P E I. In fact, L D P E differs markedly from H D P E I both in its ability to crystallize (Fig. 2) a n d in molecular structure. It m a y be regarded as a copolymer of ethylene with 4 mole~o ~-hexene [8], whereas H D P E I is a homopolymer of ethylene. The similarity of the mixture of H D P E I and L D P E to the solution of the former in the ilatter suggests that the spherulite of the mixture is formed of widely spaced shafts o f H D P E I, the spaces between them being filled with crystalline LDPE. P E .crystallizes from solution in paraffin precisely in the form of spherulites with -widely spaced shafts. The other similarity is between a polyethylene mixture and pure PE. The mixture crystallizes in the form of spherulites, the dimension of which during remelting depends on the rate of cooling of the melt. With a low content of L D P E in the mixture methods affected b y crystallinity do n o t distinguish the mixture from pure H D P E I in its behaviour during crystallization and melting. I t m a y hence be concluded that two stage crystallization of the mixture reflects features of primary and secondary stages of isothermal crystallization of individual PE. Crystallization of unfractionated industrial grade P E , studied b y depolarization of linear polarized light reveals the same properties as crystallization of fractionated samples studied dilatometrically. On the other hand, a comparative ~study of industrial P E samples obtained b y different procedures and distinguished as products of polymerization of various monomers, shows individual features, related to the molecular structure of the polymer. A study of polyethylene mixtures functioning as models of industrial products contributed to a better under~standing of crystallization (and melting) of polymers. Translated by E. SEVERE REFERENCES
1. L. MANDELKERN, Kristallizatsiya polimerov (Crystallization of Polymers). Khimiya, 1966 2. N. N. SIROTA, G. S. GORNISTOVA, V. E. PAVLOVSKAYA and A. G. SIROTA, Dokl. AN BSSR 19: 406, 1975 3. T. PAKULA and M. KRYSZEWSKI, Europ. Polymer J. 12: 47, 1976 4. R. R. FIKHTNER, Kandidatskaya dissertatsiya (Doctor's Degree). Leningrad, LTI im. Lensovieta, 1974 5. T. I. VOLKOV and V. G. BARANOV, (book) Novoye v metodakh issledovaniya polimerov (Progress in Methods of Investigating Polymers). Mir, 1968 ~6. Yu. K. GODOVSKII, Vysokomol. soyed. All: 2129, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 10, 2423, 1969) 7. R. N. LINDENMEYER, J. Polymer Sci. 1320: 145, 1967 ::8. R. E. ROBERTSON and D. R. PAUL, J. App1 Polymer Sei. 17: 2579, 1973
Deformation of densely crosslinked polymers
2603
9. A. G. SIROTA, M. D. PUKSHANSKII, Ye. L. VINOGRADOV, A. Ya. GOL'DMAN, P. A, IMCHENKO, V. G, NIKOL'SKII, N. A. MIRONOV and T. I. TROFIMOVA, VysokomoL soyed. B17: 695, 1975 (Not translated in Polymer Sci. U.S.S.R.) 10. K. YAMAGUCHI, T. YAGI, S. MACHI and M. TAKEHISA, J. Appl. Polymer Sci. 19: 1959, 1975 l l . H. D. KEITH, E. J. PADDEN and R. G. VADIMSKY, J. Polymer Sci. 4, A-2: 267, 1966
Polymer Science U.S.S.R. Vol. 21, pp. 2603-2609. C Pergamon Press Ltd. 1980. Printed in Poland
FEATURES
0032-3950170/1001-2603507.50/@
OF DEFORMATION OF DENSELY CROSSLINKED POLYMERS* V. I. KARTSOV~CIKand B. A. ROZmV~ERG
Branch of the I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 25 September 1978) A study was made of the mechanical behaviour of densely erosslinked epoxy polymers u n d e r conditions of uniaxial elongation at constant rates of deformation. Physical a n d mechanical properties of polymers were investigated after annealing previously deformed samples. I t was shown that differences in the behaviour of densely crosslinked and linear glassy polymers are due to the considerable effect of chemical bond rupture on forced elastic deformation of densely erosslinked polymers.
IT was previously concluded [1] that forced elastic deformation of densely crosslinked polymers (~ 1021 erosslinks/cm3) is ensured not only by the conformation rearrangement of chains between units by the action of stress, but also by intensive decomposition of chemical bonds of the network. In order to develop these theories, results are deseribect irt this study of investigating deformation of densely crosslinked epoxy polymers. An investigation was made under conditions of uniaxial elongation of epoxy polymers based on diglycidie ester of hydroquinone, resorcinol and 2,6-diam[nopyridine (polymer I), a n d diglyeidic ester of resorcinol and 2,6-diaminopyridine (polymer II). Components for both polymers were taken in equifunctional ratios. Conditions of hardening ensuring complete conversion (N96%) for polymer I were as follows: 100° - 1 hr, 150°--6 hr; for polymer II: 90 ° - 5 hr, 150 ° - 3 hr. Samples in the form of dumb-bell test pieces with a length of the cylindrical operating part of 25 m m and diameter 5 m m were cast in evacuated m e t a l moulds equipped with fiuoroplastic sectional inserts. Solidified samples were annealed a t temperatures higher t h a n the glass temperature for 1 hr, followed by a reduction of tern* VysokomM. soyed. A21: No. 10, 2354-2359, 1979.
R.R.
FIKHTI~ER et a[. REFERENCES
1. A. L. SUVOROV, M. A. KOCHNEVA, I. V. YEMEL'YANOVA and L. D. DUL'TSEVA, H a s t . massy, No. 7, 65, 1977 2. A. I. SUVOROVA, N. Yu. YEZHOVA, L. D. DUL'TSEVA, A. L. SUVOROV, M. A. KOCHNEVA, N. T. NERUSH and A. A. TAGER, Plast. massy, into. 7, 65, 1977 3. A . L . SUVOROV, A. I'. SUVOROVA, L. D. DUL'TSEVA, N. Yu. YEZHOVA, M. A. KOCHNEVA and Ye. F. LOGINOVA, ~ysokomol. soyed. A2O: 2592, 1978 (Translated in Polymer Sci. U.S.S.R. 20: l l , 2909, ]978) 4. M. KOBALE and H. LOBLE, Z. Elektrochem. 65: 662, 1961 5. N. SHITO and M. SATA, J. Polymer Sci. C16: 1069, 1967 6. Ye. M. BLYAKHMAN, T. I. BORISOVA and P. M. LEVITSKAYA, Vysokomol. soyed. A12: 1544, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 7, 1756, 1970) 7. A. L. SUVOROV and M. A. KOCHNEVA, Auth. Cert. 478848; Byull. izobr., No. 8, 1975 8. V. A. KLIMOVA, Osnovnyye mikrometody analiza organicheskikh soycdinenii (Main Micro-methods for the Analysis of Organic Compounds). Khimiya, 1967 9. A. L. SUVOROV and S. S. SPASSKII, Sb. Elementoorganicheskiye soyedineniya (Heteroorganic Compounds), T r u d y I n s t i t u t a Khimii, U F AN SSSR, 13, 39, 1966 10. V. F. BABICH, Yu. M. SIVERGIN, A. A. BERLIN and A. L. RABINOVICH, Mekhanika polimerov, 3, 1966 11. Ye. M. BLYAKHMAN, T. I. BORISOVA and Ts. M. LEVITSKAYA, Vysokomol. soycd. A12: 2721, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 12, 3090, 1970) 12. V. H. BATZER, F. LOHSE and R. SCHMID, Angcw. Macromolek. Chem. 29/30: 349, 1973 13. L. I. KOMAROVA, S. N. SALAZKIN, V. V. KORSHAK, S. V. VINOGRADOVA, V. I. NIKOLAICHIK, Ye. V. ZABOROVSKAYA and I. A. BULGAKOVA, Vysokomol. soyed. BI6: 718, 1974 (Not translated in Polymer Sci. U.S.S.R.) 14. W. KAUZMAN, Revs. Mod. Phys. 43: 219, 1942
Polymer Science U.S.S.R. Vol. 21, pp. 2596-2693. © Pergamon Press Ltd. 1980. Printed in Poland
0032-3950/79/1001-2596507.50/0
STUDY OF CRYSTALLIZATION OF INDUSTRIAL POLYETHYLENE AND POLYETHYLENE MIXTURES* ]:~. ]:~. FIKHTNER, T. I. VOLKOV, S. A. SHALATSKAYA a n d M. S. TRIZ~O Lensoviet Technological Institute, Leningrad
(Received 22 September 1978) Depolarization of linear polarized light was used to examine isothermal crystallization of industrial P E distinguished by the method of synthesis. The temperature ~lependence of the half-life of crystallization was derived. A study was made of the ~tructure of low mid high density P E mixtures, according to their behaviour during * Vysokomol. soyed. A21: No. 10, 2348-2353, 1979.
Crystallization of industrial PE and PE mixtures
2597
crystallization and melting. It was shown that low density PE and linear PE in a mixture crystallize separately, forming general spherulites. Spherulite formation and increase in dimensions before collision take place during crystallization of high density PE, forming the skeleton of the spherulite. Joint extrusion of low density PE powder and high density, high molecular weight PE is not accompanied by homogenization of the alloy. No marked increase is observed in the fluidity of high density, high molecular weight PE on adding small amounts of low density PE. I ~ a previous s t u d y [ 1] concerning kinetics of isothermal crystallization of P E b y d i l a t o m e t r i c m e t h o d n a r r o w p o l y m e r fractions were used, which distinguislxes these investigations f r o m conditions of industrial p r o d u c t i o n a n d raises t h e p r o b l e m w h a t degree of f r a c t i o n a t i o n is r e g a r d e d sufficient to confirm some general conclusions. Jo'urthermore, conditions of h e a t exchange prevailing during t h e d i l a t o m e t r i e o b s e r v a t i o n of crystallization impede the investigation of rapid processes m o s t i m p o r t a n t for practical purposes. Optical methods, in which samples of low weight are used ensure i s o t h e r m a l conditions of crystallization to an increased e x t e n t , c o m p a r e d with d i l a t o m e t r y , if crystallization is carried o u t in a fairly large liquid t h e r m o s t a t . I n p r e v i o u s studies [2, 3] concerning i s o t h e r m a l crystallization of P E , optical m e t h o d s were o n l y used to e x a m i n e p o l y m e r s with b r a n c h e d molecules low d e n s i t y polye t h y l e n e ( L D P E ) a n d L u p o l e n 1800S, respectively. An i n v e s t i g a t i o n was m a d e of rapid processes of crystallization of main t y p e s of P E p r o d u c e d in this c o u n t r y (Table) and of crystallization a n d the s t r u c t u r e o f m i x t u r e s p r e p a r e d using some of these P E . CSARACWEmST~CS OF PE EXAMI~S~D Density, g/era a Polyethylene
LDPE HDPE I HDPE II HDt)E II[
with rapid erystallization 0-916 0.947 0.940 0.930
witl7 slow erystallization i
0.923 0.968 0.955 0.947
Melt flow index g/10 min 5-6 2-.9 2.,(t 0.0
K inet its of isothermal crystallization of PE were examined by depolarization of linear polarized light using a device and methods previously described [4]. Crystallization and melting of mixtures were studied by DTA using a Paulik Paulik-Erdei derivatograph (MOM), the method of depolarization of linear polarized light and an optical microscope with a heating stage. Diffraction curves of small angle polarized light scattering [5] were plotted. LDt?E and HDPE were mixed in melt using five extrusion cycles and a capillary viseometer f(~r MMI at 160° and by precipitation from solution in xylene using rapid cooling. I s o t h e r m a l curves of crystallization of the materials studied were o b t a i n e d in a wide range of t e m p e r a t u r e , where the stage of p r i m a r y crystallization is c o m p l e t e d (luring a period ranging from a few secomls to m a n y minutes, lso-
R. R. FIKH~EI~ et a/.
25.98
thermal curves for HDPE II, which occupies intermediate position among P E as regards molecular structure, are shown in Fig. 1 in standard coordinates. Isothermal curves of crystallization of other grades of PE take the same form. As shown by Fig, 1, the stage of primary crystallization is completed (particularly a t high temperatures) with comparatively low degress of crystallinity For example, at a temperature of hardening of 124 ° crystallization is practically complete with a crystallinity of less than 0.5 in relation to its value at 20 °. I'0
12
3
4
5
, ~ O.G
8
\ \ I
I
1
10
Time, rain
lho. 1. Isothermal curves of crystallization of HDPE II in standard coordinates at temperatures of 117 (1); 118.5 (2); 120 (3); 122 (4); 123 (~) and 124 ° (6). Two stages are observed during isothermal crystallization of PE, which is, apparently, a reflection of the two stage development of the crystalline phase: first, the spherulites increase in size until they collide with each other and this is followed by further crystallization of the material inside the spherulites. Analysis of experimental results, according to the Avrami-Kolmogorov equation, shows that the Avrami index varies continuously with a change in the temperature of the crystallizer. For example, for HDPE II it changes from 5.6 to 1.0 in the temperature range of 117-124 °. This, as well as results mentioned, confirms that crystallization of PE in the temperature range studied cannot be regarded, as in the case of crystallization of low molecular weight substances, as the total of processes of formation and extension of nuclei. Temperature dependences of the rate of crystallization of PE of various grades are shown in Fig. 2. Similar dependences for most polymers are extremal in nature, whereby the maximum rate of crystallization is achieved between the melting 0 point and glass temperature when T:(0"82-0"83)Tmelt [6]. For the PE grades examined the maximum rate of crystallization under isothermal conditions could not be determined. More exactly, this concept is noa-existent for this case since
Crystallization of industrial PE and PE mixtures
2599
with considerable degrees of supercooling the rate of crystallization becomes comparable with the rate of cooling the sample to the temperature of the crystallizer. According to Lindenmeyer's theory [7], slow crystallization takes place by the precipitation of molecules accumulated on the growing face of the crystallite. "Foreign" molecules and molecules of a poorly crystallizing material do not form part of the crystalline lattice. Based on the position of curves in Fig. 2 it m a y be assumed that with slow cooling of the mixture of PE samples examined it is mainly P E with linear molecules, namely HDPE I, which crystallizes first and P E with most branched molecules, LDPE, which crystallizes last. In other words, slow crystallization of the mixture is accompanied (in relation to crystallites) by fractionation first of all according to the degree of branching. Fraetionation according to molecular weight will be of lesser significance, particularly with high values of supercooling (Fig. 2). "c rain 40-
/ 23 z/
2-00~ r 9#
98
lOZ
116
/20
/2q Tcr,°C
Fie. 2. Temperature dependence of the half-period of crystallization v of LDPE (1), HDPE 13[ (2), HDPE III (3) and HDPE I (4). Results of investigating crystallization of PE and the importance of compositions from a practical point of view prompted us to investigate the mixture of industrial grades of PE. Results in the literature concerning the structure o f P E mixtures are very conflicting. Robertson and Paul [8] analysed studies by other authors and recognized that low and high density PE can be crystallized separately and interpreted results of their own investigations into mechanical properties of polyolefm mixtures in terms of compatibility and interaction of amorphous phases. Experimental results obtained by authors of another study [9] confirm the homogeneity of amorphous regions and microheterogeneity of crystalline regions in LDPE mixed with wax. The conclusion concerning the microheterogeneity of crystalline regions and the possible separate crystallization of mixture components is based on the observation of individual melting points on DTA curves. Authors of another study [10] only observed one melting peak from the mixture of radiation-polymerized PE and HDPE assuming that P E in a form which is similar to a melt is between t t D P E crystallites. This assumption, however, raises the problem of the cause of high (65% according to X-ray diffrae-
R. R. Fr~u-r~E~R eta/.
2600
tion results) initial crystallinity of radiation polarized P E since, according to our results, density which is the measure of crystallinity, is the additive function of composition of the L D P E - H D P E mixture. The structure of mixtures is now being discussed within the framework of a two phase model. There are no indications as to the degree of homogeneity of P E mixtures and it is impossible to judge the level of elements of supermoleeular organization [4], where components are compatible, or whether this occurs on the grain boundaries. Meanwhile, information about the homogeneity of t h e mixture is particularly important if measurements are concerned with operational properties of compositions. I pel.un. -
II
a
60_[
#5 /
2
3
-
I
2
3
20
80
100
I20
fqO
8O
100
/20 T,°C
:Fie. 3. Intensity variation of depolarized light during heating (a) and cooling (b) samples of I ~ P E (I) and HDPE I (II) mixtures. Here and in Fig. 4 mixture compositions are as follows: 1--19 : 1; 2--5 : 1; 3--2 : l; 4--1 : 1; 5--1 : 2. Results of investigating mixtures in this study showed the following special features. Extrusion through a capillary using a device for the measurement of the melt index (MMI) of powder mixtures of L D P E and H D P E I I I did not improve homogenization of the material. Particles of bright grains containing spherulites of irregular shape on the darker background of a material also containing spherulites can be seen in the visual field of a polarization microscope. Observations of samples during heating (cooling) showed that L D P E is the continuous phase and H D P E III, the dispersed phase. The immiscibility of these grades of P E in melt is, apparently, due to the considerable difference in viscosity. No marked improvement was observed in the fluidity of the H D P E I I I melt on adding small amounts of L D P E . Microscopic studies of mixtures of L D P E and I-IDPE I showed that all the compositions studied are homogeneous materials containing spherulites. Spheru]itic dimensions, as in the case of pure components, depend on the rate of cooling ~he melt. During heating (cooling) mixtures with 19:1 and 5:1 component ratios a sudden change occurs in the intensity of Hv diffraction curves and the brightness
CrystMlization of industrial PE and PE mixtures
260k
of spherulites in the temperature interval of melting LDPE. No change is observed in the dimension of diffraction curves after prolonged (1 hr) retention of t h e material at a temperature Close to Tmelt of H D P E I. +AT
FIG. 4. Thermograms of melting LDPE (I) and HDPE (II) mixtures. Analysis of changes in Hv diffraction curves during cooling the melt of a mixture of the same composition indicates that the diffraction curve appears and its size is reduced long before the sample reaches Tcr of LDPE. The intensity of the diffraction curve undergoes considerable increase near Tcr of LDPE. Results of investigating crystallization and melting of L D P E and H D P E I mixtures b y depolarization of linear polarized light (Fig. 3) and DTA (Fig. 4) indicates tl~e existence of phase transitions near the melting points of pure components. These transitions take place most readily b y depolarization of linear polarized light using a mixture of L D P E and H D P E with a component ratio of 19:1 and b y DTA for a 5:1 mixture. Results for mixtures (19:1, 5:1 and 1 :l) obtained b y precipitation from solution do not diffe',r from those obtained for mixtures prepared from melt. This meaus that the effects observed do not in-. volve poor mixing. A combination of eX])erimental results concerning L D P E and H D P E I mixtures suggests the following. (lomponents crystallize and melt separately. Crystallization takes place with the formation of spher(~|~tes common for both components. The formation and extension of st)herulites before collision involve exclusively the crystallization of H D P E I. During crystallization of L D P E the weight of the crystalline component in s])herulitesi ncreases.
-2602
R . R . FIKHTNER et al. g
Comparing the behaviour of L D P E mixed with H D P E I with properties of other systems some similarities m a y be noted. First--this is the similarity of this mixture to the solution of H D P E I in LDPE; the latter behaves like a solvent reducing the melting point and temperature of crystallization of t I D P E I. In fact, L D P E differs markedly from H D P E I both in its ability to crystallize (Fig. 2) a n d in molecular structure. It m a y be regarded as a copolymer of ethylene with 4 mole~o ~-hexene [8], whereas H D P E I is a homopolymer of ethylene. The similarity of the mixture of H D P E I and L D P E to the solution of the former in the ilatter suggests that the spherulite of the mixture is formed of widely spaced shafts o f H D P E I, the spaces between them being filled with crystalline LDPE. P E .crystallizes from solution in paraffin precisely in the form of spherulites with -widely spaced shafts. The other similarity is between a polyethylene mixture and pure PE. The mixture crystallizes in the form of spherulites, the dimension of which during remelting depends on the rate of cooling of the melt. With a low content of L D P E in the mixture methods affected b y crystallinity do n o t distinguish the mixture from pure H D P E I in its behaviour during crystallization and melting. I t m a y hence be concluded that two stage crystallization of the mixture reflects features of primary and secondary stages of isothermal crystallization of individual PE. Crystallization of unfractionated industrial grade P E , studied b y depolarization of linear polarized light reveals the same properties as crystallization of fractionated samples studied dilatometrically. On the other hand, a comparative ~study of industrial P E samples obtained b y different procedures and distinguished as products of polymerization of various monomers, shows individual features, related to the molecular structure of the polymer. A study of polyethylene mixtures functioning as models of industrial products contributed to a better under~standing of crystallization (and melting) of polymers. Translated by E. SEVERE REFERENCES
1. L. MANDELKERN, Kristallizatsiya polimerov (Crystallization of Polymers). Khimiya, 1966 2. N. N. SIROTA, G. S. GORNISTOVA, V. E. PAVLOVSKAYA and A. G. SIROTA, Dokl. AN BSSR 19: 406, 1975 3. T. PAKULA and M. KRYSZEWSKI, Europ. Polymer J. 12: 47, 1976 4. R. R. FIKHTNER, Kandidatskaya dissertatsiya (Doctor's Degree). Leningrad, LTI im. Lensovieta, 1974 5. T. I. VOLKOV and V. G. BARANOV, (book) Novoye v metodakh issledovaniya polimerov (Progress in Methods of Investigating Polymers). Mir, 1968 ~6. Yu. K. GODOVSKII, Vysokomol. soyed. All: 2129, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 10, 2423, 1969) 7. R. N. LINDENMEYER, J. Polymer Sci. 1320: 145, 1967 ::8. R. E. ROBERTSON and D. R. PAUL, J. App1 Polymer Sei. 17: 2579, 1973
Deformation of densely crosslinked polymers
2603
9. A. G. SIROTA, M. D. PUKSHANSKII, Ye. L. VINOGRADOV, A. Ya. GOL'DMAN, P. A, IMCHENKO, V. G, NIKOL'SKII, N. A. MIRONOV and T. I. TROFIMOVA, VysokomoL soyed. B17: 695, 1975 (Not translated in Polymer Sci. U.S.S.R.) 10. K. YAMAGUCHI, T. YAGI, S. MACHI and M. TAKEHISA, J. Appl. Polymer Sci. 19: 1959, 1975 l l . H. D. KEITH, E. J. PADDEN and R. G. VADIMSKY, J. Polymer Sci. 4, A-2: 267, 1966
Polymer Science U.S.S.R. Vol. 21, pp. 2603-2609. C Pergamon Press Ltd. 1980. Printed in Poland
FEATURES
0032-3950170/1001-2603507.50/@
OF DEFORMATION OF DENSELY CROSSLINKED POLYMERS* V. I. KARTSOV~CIKand B. A. ROZmV~ERG
Branch of the I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 25 September 1978) A study was made of the mechanical behaviour of densely erosslinked epoxy polymers u n d e r conditions of uniaxial elongation at constant rates of deformation. Physical a n d mechanical properties of polymers were investigated after annealing previously deformed samples. I t was shown that differences in the behaviour of densely crosslinked and linear glassy polymers are due to the considerable effect of chemical bond rupture on forced elastic deformation of densely erosslinked polymers.
IT was previously concluded [1] that forced elastic deformation of densely crosslinked polymers (~ 1021 erosslinks/cm3) is ensured not only by the conformation rearrangement of chains between units by the action of stress, but also by intensive decomposition of chemical bonds of the network. In order to develop these theories, results are deseribect irt this study of investigating deformation of densely crosslinked epoxy polymers. An investigation was made under conditions of uniaxial elongation of epoxy polymers based on diglycidie ester of hydroquinone, resorcinol and 2,6-diam[nopyridine (polymer I), a n d diglyeidic ester of resorcinol and 2,6-diaminopyridine (polymer II). Components for both polymers were taken in equifunctional ratios. Conditions of hardening ensuring complete conversion (N96%) for polymer I were as follows: 100° - 1 hr, 150°--6 hr; for polymer II: 90 ° - 5 hr, 150 ° - 3 hr. Samples in the form of dumb-bell test pieces with a length of the cylindrical operating part of 25 m m and diameter 5 m m were cast in evacuated m e t a l moulds equipped with fiuoroplastic sectional inserts. Solidified samples were annealed a t temperatures higher t h a n the glass temperature for 1 hr, followed by a reduction of tern* VysokomM. soyed. A21: No. 10, 2354-2359, 1979.