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Deformational-strength properties and structure of polyethylene-polystyrene mixtures as a function of orientational stretching conditions
Polymer Science U.S.S,R. Vol. 30, No. 6, pp. 1341-1348, 1988 Printed in Poland
0032-3950/88 $10.00+.00 © 1989 Pergamon Press pie
DEFORMATIONAL-STRENGTH PROPERTIES AND STRUCTURE OF POLYETHYLENE-POLYSTYRENE MIXTURES AS A FUNCTION OF ORIENTATIONAL STRETCHING CONDITIONS* N. P. KRASNIKO VA, Y'E. V. KOTOVA, A. S. KECHEK'YAN, YE. K. BORISENKOVA, YE. M. ANTIPOV, S. A. KUPTSOV, Z. PEL'TSBAUER a n d V. YE. DREVAL' A. V. Topchiev Institute of Petrochemical Synthesis V. I. Lenin State Teaching Institute, Moscow Institute of Macromolecular Chemistry, Academy of Sciences of the Czechoslovakian SSR, Prague (Received 7 January 1987)
The effect of orientational stretching on the structure and deformation-strength properties of a fibrous, self-reinforced material obtained by pressing a melt of a mixture of 30 ~ by weight polyethylene (PE) with polystyrene (PS) through a capillary is studied. It is established that stretching over the temperature range Tg Ps-Traelt pl! results in an increase in the strength of the material in the solid state, the maximum strength being attained as a result of 13-fold stretching at 120°C. M u c h a t t e n t i o n has been given recently to s t u d y i n g mixtures on i n c o m p a t i b l e p o l y m e r s c a p a b l e o f self-reinforcement d u r i n g d e f o r m a t i o n [1-4]. Thus, on p r e s s i n g a mixture melt t h r o u g h n a r r o w capillaries, the p o l y m e r , which is the disperse phase, f o r m s u l t r a t h i n fibres at the e n t r a n c e to the c a p i l l a r y , o r i e n t e d a l o n g the c a p i l l a r y axis. A f t e r e x t r u s i o n the m a t e r i a l is g e n e r a l l y subjected to a d d i t i o n a l o r i e n t a t i o n a l s t r e t c h i n g to i m p r o v e its m e c h a n i c a l p r o p e r t i e s . T h e object o f the w o r k described in this a r t i c l e was to study the special s t r u c t u r a l features and d e f o r m a t i o n a l - s t r e n g t h p r o p e r t i e s o f the i n c o m p a t i b l e p o l y m e r e x t r u d a t e s thus f o r m e d , using a p o l y e t h y l e n e ( P E ) - p o l y s t y r e n e (PS) m o d e l system, a n d 1o s t u d y t h e effect o f these c h a r a c t e r i s t i c s o f the m a t e r i a l on a d d i t i o n a l o r i e n t a t i o n a l s t r e t c h i n g a n d i s o m e t r i c annealing. The mixture studied was obtained by emulsion polymerization of PS (M=4.5 x 105) with 30 wt. ~ linear PE (M=2.9 x 104). The mixture was obtained by mixing PS and PE powders in ethanol with subsequent evaporation of the ethanol. The polymer characteristics, and the method of preparing the specimens on a type KVID-2 capillary viscometer are described by Krasnikova et al. [3]. The material was pressed through the capillary at a shear stress of 2.5 x 104 Pa at 180°C, using a 3.0x 10 -3 diameter, 9.6x 10 -2 m long capillary, i.e. the specimens are formed in the region of developed fibre-like PE in a PS matrix [3]. The extrudates were subjected to orientational stretching on a type VRPS tension viscometer [5] at a constant deformation rate of 2.0 x 10 -2 sec-1 in a glycerol bath at 110-155°C. After attaining the desired deformation the specimen was instantaneously fixed in special clamps, and withdrawn quickly (after 1-2 sec) from the bath and cooled in water. * Vysokomol. soyed. A30: No. 6, 1279-1284, 1988. 1341
1342
N. P. K~.~NnCOVA et al,
The deformational-strength properties were determined on a type FPZ-10 tensile testing machine (East German) at room temperature and a stretching rate of 10 ram/rain. The structural studies involved large angle X-ray analysis by diffractometric and photo methods. The original and final structures after treatment were recorded on X-ray patterns obtained on a type IRIS-3"0 setput at room temperature (CuK, radiation). Monochromatization of the characteristic radiation was obtained by means of a curved quartz single crystal [6]. In the case of isometric plotting, the length of the oriented specimens was fixed by means of a metal frame and clamps. A special thermal attachment [7] was used for the temperature tests, which enabled the temperature to be held to an accuracy of +0.5°C. The specimen morphology was studied by photographing sections, and specimens washed free from PS by solvent were also examined under an "Opton" optical microscope and under a JSM-35 scanning electron microscope. Figure 1 shows a typical microphotograph of P E - P S mixture extrudate. The PE forms a fibre of diameter 1-10 a m (mean diameter 6.8 p m at the 5 ~ probability level). Under mechanical tests the extrudate of an individual PS undergoes brittle fracture, and the PE is stretched out, with neck formation, by 700~o (Fig. 2). The extrudate of a mixture not subjected to orientational stretching undergoes brittle fracture on deformation, amounting to a few percent, similar to an individual PS specimen, and is even somewhat lower in strength (35 MPa) than PS (45 MPa), Accordingly, the presence of PE fibres in a PS matrix does not of itself result in a change in the mechanical properties of PS. Orientational stretching of extrudate mixtures can change their deformational-
I~o. 1
Fro. 2
FIG. 1. Photomicrograph of PE fibres washed free from PS. FIG. 2. Stress calculated on the initial cross-section of a specimen as a function of the strain at 23°C for unstretched PS (1), PE (2), and P E P S mixtures (3), and also for stretched (2--7) mixtures at 110 (4), 120 (5), 126 (6), 135 (7), and 155°C (8). strength characteristics, depending on the temperature and draw ratio 2 = l/1o, where lo is the initial and l the final specimen length. On stretching the test PE above Tm,~t(128"C) the mechanical properties of the material are close to those of the unstretched extrudate of a P E P S mixture. Stretching over the T s v s - Tmolt PB range results in the formation of material capable of further deformation (120 ~ ) , which is accompanied by an increase in its strength. Khaineman [8] observed similar behaviour in an oriented polypropylenePS mixture containing polypropylene fibres.
1343
Deformational-strength properties and structure of PE-PS mixtures
During uniaxial deformation o f mixture extrudates strengthening o f the material is observed, this being most intense at 110 and 120°C. According to X-ray diffraction data the maximum orientation of PE crystallites is attained as a result of stretching at 120°C. Since these specimens also have the maximum strength under subsequent mechanical tests (Fig. 2), the temperature T = 120°C was selected for studying the effect of 2 on the properties of the material obtained. Furthermore, as is known [9], in the PE premelting region stronger materials o f higher M M are obtained. The PE under study could be stretched only 7- to 8-fold at 120°C, probably because of the significant specimen deformation. In subsequent mechanical tests (Fig. 3) such a specimen showed high strength (ort =200 MPa) and fractured like a rigid oriented material at 20 ~ strain. When PS specimens were stretched under these conditions they retained their rigidity, but their strength was increased, and had the value ~rt= 80 MPa, which is characteristic of highly o.MPa 200 }
/
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,,,
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i
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Fla. 3 Fla. 4 FIG. 3. Values of tr-e for PE (1) and PS (2) on their own, and for PE-PS mixtures (3-6), oriented at 120°C to 2=7 (1, 2, 4), 5 (3), 9 (5), and 13 (6). FIG. 4. Values of o-, (1) and et as a function of ,Efor PE-PS mixtures. Stretching temperature 120°C. oriented PS, the strength of which does not exceed 80-90 MPa [10]. In the case of a mixture, it can be stretched to 2 = 13-14 at 120°C. In the mechanical tests a specimen of mixture fractured similarly to a PE specimen stretched to 2 = 7 . Its strength was close to that of oriented PE, and the fracture strain was somewhat less than the strain of
1344
N, P. KRASNIKOVA et at.
oriented PE, but greater than that of oriented PS. On the whole, the above indicates that material of increased strength and deformability than PS can be obtained from PE-PS mixtures. Figure 4 shows the tensile strength of compositions as calculated on the specimen cross-section at the moment of fracture, and also the strain at fracture as a function of the stretch ratio at 120°C. It is characteristic that the strength is related directly to :t up to 2 = 7-8, i.e. up to values corresponding to the PE fracture strain at 120°C. It is noted that the viscosity of PS during stretching at 120°C, as assessed from data provided by Krasnikova et al. [3] and Dreval et ai. [4], using the Williams-LandelFerry equation for low stretch ratios is equal to 3x10 ax Pa. sec, i.e. is close to the viscosity of substances at Ts. Under these conditions the PS deformation should be almost entirely reversible. When 2 > 7 the tensile strength becomes much less dependent on the stretch ratio. Moreover, the strain of compositions at 120°C becomes reversible only partially, although the contribution of the irreversible component to the total strain is much less than the contribution to high elastic strain. Comparison of the tensile strength of PE-PS compositions at 2= 13 (Fig. 4) of strength ak, as calculated from the known equation for unidirectional composites, i.e. ak = a,,~0+ a~(l -- ~0),where am and af are the tensile strengths of the material and fibre respectively, and ~ is the volume proportion of the matrix [12],
n
17
20
23
26 20 °
FIG. 5
FIG. 6
F1o. 5. Diffraction patterns of oriented 0.---!3) PE-PS mixtures after annealing for one hour at 23 (1), I17 (2), 130 (3), 133.5 (4), and 135°C (5). Fxo. 6. Photo-X-ray patterns of oriented 0.--13) PE-PS mixtures after annealing (137°C, 15 rain),
obtained at 23°C. on the assumption that the maximum tensile strength of the PE under test is attained with a specimen for which 2 = 7, and. that the strength of the PS on stretching does not exceed 80-90 MPa, shows that the experimentally determined tensile strength is about double the calculated value. Accordingly, in the compositions tested the PE and PS
Deformational-strengthproperties and structure of PE-PS mixtures
1345
behave synergistically in their mutual effect on the tensile strength of the composition. If it is assumed that additional strain and strengthening of the PE fibres occur on stretching a mixture to 2=13, and that the equation is suitable for describing the strength of such compositions, then the tensile strength of the PE fibres in the compositions is equal to 550 MPa. On the other hand the the elastic modulus of the compositions E, as assessed from the slope of the initial section of the tr-s curves is weakly dependent on 2 and lies within the range 3-4 GPa. This value is close to E for unoriented PS (2.3 GPa). The elastic modulus of an unstretched PE specimen is 0.55 GPa, and of a sevenfold stretched PE specimen 4.3 GPa. The tensile strain of the compositions is chahged, depending on 2 (Fig. 4), along a curve with a maximum in the region of 2~-5-8. In order to understand the given results, the data obtained in X-ray diffraction studies will be considered. These data show that unstretched extrudates, and also extrudates subjected to stretching at 135 and 155 (2=7) are in an isotropic state, while PS is amorphous and PE is about 70~o crystalline. Uniaxial stretching of a PE-PS extrudate at 120°C results in the formation of a typical fibrillar structure in the PE (c-axial texture). Amorphous PS also becomes oriented (presence of corresponding intensity attenuation on the X-ray pattern equator and meridian). As 2 is increased t,he degree of orientation of both components increases and attains a maximum value at 2= 13 (the PE reflections tend to be constricted from an arc into points). The test process on the tensile testing machine (T=23°C) has an appreciable effect on the PE orientation in specimens of a P E P S mixture stretched to 2~ 5-8, which is expressed in a noticeable decrease in the transverse dimensions of the PE crystallites from 19.7 to 15.4 nm (110 reflection). The form of the X-ray patterns is thus changed, just as when 2 is increased from 5 to 13 (T=120°C), as noted above. Accordingly, in this case it is reasonable to speak of additional orientational stretching, but at room temperature. In the case of a mixture with 2= 13 the transverse dimensions of the PE crystallites before and after testing on a tensile testing machine remains equal at 18 nm. In this case the X-ray patterns of the mixture after testing on the tensile testing machine also remain almost unchanged. These also show a reflection at the angular position 20max= 19"5°, corresponding, according to Seto et al. [13], to the appearance of a monoclinic modification of part of the crystalline PE, whereas its main part (not less than 90 ~) remains in the orthorhombic form. A structural transition of this type indicates the presence in the PE during stretching of large shear stresses, which indicates a different strain mechanism at small and large ratios of preliminary specimen stretching. Temperatures curves for mixture specimens with 2 = 7 and 13 for isometric conditions showed that increase in temperature and heating over 1 hr results in typical annealing features. The last signs of orthorhombic PE reflections disappear at 135°C (Fig. 5). In certain cases it was possible to anneal specimens at 137°C without fracture (15 min). As follows from the photo-Xray diffraction patterns, obtained after cooling of the annealed specimens of mixture to room temperature (Fig. 6), annealing somewhat decreases, but does not remove anisotropy of the mixture components. The PE diffraction pattern, apart from the isotropic component, shows very clear complex texturizing.
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N. P. KRASNIKOVAet al.
This is mainly a-axial texture for which the a axis of the orthorhombic cell is oriented mainly along the specimen stretching direction, and the positions of the b and c axes show cylindrical symmetry relative to this direction. All the test specimens showed this texturizing, but this was clearest at the maximum values of 2. Gerasimo et al. [14] observed a situation indicated by the position of the PE macromolecules perpendicular to the preliminary orientation axis (layerized structure) on annealing PE under free conditions close to the melting point, and showed that the a-texture is an intermediate stage between the highly oriented (c-texture) and isotropic states. Apart from the a-texture, narrow equitorial PE reflections are observed on the photoX-patterns, which correspond to c-texture. This obviously indicates that a small proportion of the PE chains remain in the PE melt at 135-137°C in the oriented state, and on subsequent cooling, most of the PE melt is partially crystallized on these stretched chains, just as on nuclei. The result obtained confirms the conclusion arrived at by Shilov et ai. [15] that the PE chain orientation in composite material mixtures is partially retained above the mixture melting point. These results were observed by the authors with all the PE-PS mixtures studied, with 2=7-13 at points some distance from the isometric annealing frame clamps. It is noted that similar results were obtained earlier for PE-polypropylene [16] and PE-lavsan [17] mixtures. The above data show that the PS matrix has a significant effect on the crystallinity of PE in P E P S mixtures. The large number of physical" "crosslinks" between the components, which determine their adhesion, on ,the one hand ensures orientation of the PE inclusions on uniaxial stretching of the specimen, and on the other hand, holds some of the PE chains in the oriented state above T,,clt. Furthermore, if an attempt is made, using the data provided by Krasnikova et al. [3] and Dreval et al. [4], to evaluate the maximum relaxation time 0o for PS, using the Williamson-Landel-Ferry equation, it is found that 0o =t/olEo (where t/o is the initial specimen viscosity and/'o is the initial high elastic modulus) attains a high value, equal to 1.5 x 105 sec, so that during the annealing time (3.6 x 103 sec) the PS is not relaxed but remains partially oriented. On the basis of these results it is assumed that the special features of the mechanical behaviour of P E P S mixtures subjected to orientational stretching at 120°C can be associated with the presence in it of a certain proportion of highly oriented and high strength PE fibres, which increase the strength of the material, and some unoriented or only slightly oriented PE fibres. These are stretched and further oriented during the mechanical tests on the tensile testing machine, resulting in release of the local overstresses, causing brittle fracture of the composite material. This is because in specimens not subjected to orientational stretching the PE fibres are not all the same. Thus, they have different diameters (1-10/zm), can be non-uniformly distributed along the extrudate cross-section, etc. During mechanical testing and high temperature stretching of the specimens such fibres undergo different changes, since they are subjected to different local loads. In an unstretched extrudate the elastic modulus of the PE, which is equal, according to the authors' data, to 0.55 GPa, is lower by a factor of about 4 than the corresponding value for PS, so that in the mechanical tests the PS bears the main load •
/
Deformational-strength properties and structure of PE-PS mixtures
1 347
in such a composite material, and the material behaves similar to polystyrene on its own at low strains. With increase in stretch ratio the proportion of highly oriented PE fibres in the mixture increases, which increases the specimen tensile strength and modulus. Moreover, in all probability, on extrudates with 2 > 7 PE fibres appear in them for which the stretch ratio is higher than the limiting stretching frequency for PE on its own (2~7). This results in hardening of the material and constancy of its structure during the mechanical tests. However, with increase in stretch ratio 2.>7, the number of PE fibres capable of undergoing additional strain and orientation in mechanical tests on a tensile testing machine is decreased. In view of this, uniaxial stretching to 2.>7, when most of the PE fibres pass into the oriented state, results in an increase in brittleness of the composite material at r o o m temperature. The use in this work of high temperature orientational stretching of mixtures of incompatible polymers thus provides a means of obtaining a composite material having increased strength characteristics. Translated by N. STANDEN REFERENCES
I. D. POL, Polimernye smesi (Polymer Mixtures), vol. 2 (ed. by D. Pol and S. M. Newmen), 1981 2. M. V. TSEBRENKO, N. M. RESANOVA and G. V. VINOGRADOV, Polymer Engng. Sci. 20: 1023, 1980 3. N. P. KRASNIKOVA, V. Ye. DREVAL, Ye. V. KOTOVA, Ye. P. PLOTNIKOVA, G. V. VINOGRADOV, G. P. BEIX)V and Z. PELTSBAUER, Vysokomol. soyed. A24: 1423, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 7, 1617, 1982) 4. V. Ye. DREVAL, G. V. VINOGRADOV, Ye. P. PLOTNIKOVA, M. R. ZABUGINA, N. P. KRASNIKOVA, Ye. V. KOTOVA and Z. PELZBAUER, Rheol. Acta. 22: 102, 1983 5. V. PADUSHKEVICH, V. D. FIKHMAN and G. V. VINOGRADOV, Uspekhi reologii polimerov (Successes in the Rheology of Polymers). Moscow, 1970 6. Yu. A. ZUBOV, V. I. SELIIKHOVA, V. S. SHIRETS and A. N. OZERIN, Vysokomol. soyed. A16: 1681, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1950, 1974) 7. Ye. M. ANTIPOV, N. N. KUZMIN, Yu. N. OVCHINNIKOV and T. S. MARKOVA, Pribory i tekhnika eksperimenta, No. 2, 1958, 1975 8. P. KHAINEMAN, S. V. VLASOV, Yu. P. MIROSHNIKOV and V. N. KULEZNEV, Kolloid. zh. 48: 974, 1968 9. J. R. SHEFGEN, T. I. VEIR, J. V. BELLOU, S. L. KVOLEK, P. V. MORGAN, M. PANAR and J. TSIMMERMAN, Sverkhvysokomodulnye polimery (Superhigh Modulus Polymers). (ed. by A. Chiferr and I. L. Uord), 1983 I0. G. V. VINOGRADOV, B. V. RADUSHKEVICH, V. P. NEPASNIKOV and V. D. FIKHMAN, Rheol. Acta 17: 231, 1978 l I. J. D. FERRI, Vyazkouprugie svoistva potimerov (Viscoelastic Properties of Polymers). Moscow, 1963 12. L. NILSEN, Mekhanicheskie svoistva polimerov i polimernykh kompozitsii (Mechanical Propcrties of Polymers and Polymer Compositions). Moscow, 1978 13. T. SET, T. HARA and K. TANAKO, J. Appl. Phys. Japan 7: 31, 1968 14. V. I. GERASIMOV, Ya. V. GENIN and D. Ya. TSVANKIN, J. Polymer Sci. Polymer Phys. E d 12: 2035, 1974 15. V. V. SHILOV, Yu. P. GOMZA and Yu. S. LEPATOV, Kompozitsionnye polimernye materialy (Composite Polymer Materials). p. 28, Kiev, 1979 16. Ye. M. ANTIPOV, S. A. KUPTSOV, A. A. REMIZOVA and V. P. POPOV, Tez. dokl. XXII
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Vsesoyuz. konf. po vysokomolek, soyedineniyam (Thesis of Paper at 22-nd All-Union Conference on High Molecular Compounds), p. 156, Alma-Ata, 1985 17. S. I. BELOUSOV, Ye. M. ANTIPOV, Yu. A. MAKHNOVSKII and N. A. SHITOV, Tez. dokl. I Vsesoyuz. konf. "Smesi polimerov" (Thesis of Paper of All-Union Conference "Polymer Mixtures") Ivanovo, 1986
Polymer SciaticaU.S.S.R. Vol. 30, No. 6, pp. 1348-1357, 1988 Printed in Poland
0032-3950/88 $10.00+.00 1989 Pergamon Press plc
DETERMINATION OF THE NUMBER OF BRANCHES AND OTHER DEFECTS IN THE STRUCTURE OF DENSE POLYMER NETWORKS* A. A. ASKADSKII, V. V. KAZANTSEVA,O. A. MEL'NIK, K. A. BYCHKO, A. A. SAKHAROVA, G. L. SLONIMSKII and T. M. FRUNZE A. N. Nesmeyanov Institute of Metal-Organic Compounds, U.S.S.R, Academy of Sciences (Received 12 January 1987)
A method is proposed for determining the number of defects (branches and isolated loops) in densely crosslinked polymers, based on a comparison of experimental and calculated glass temperatures of the network, using the increment method in the calculation. T h e proposed procedure was applied to crosslinked copolymers of methyl methacrylate with carborane substituted exocyctic vinylsilane of various composition; a considerable number of defect structures was revealed.
Tt-IE PROBLEMof determining defect structures (branchings, inactive cross[inks, loops etc.) is of importance both in loosely cross[inked and in dense polymer networks. For loosely cross[inked polymers, the classical equation of viscoelasticity is applied to this end, in the form 3R T
E~ = p - - ,
Mc
(1)
where E~ is the equilibrium modulus of viscoelasticity, M~ is the mean molecular mass of an inter-crosslink segment, p is density, T is the absolute temperature and R the universal gas constant. When the Mc value calculated by means of eqn. (1) does not agree with that calculated from the stoichiometry of the reactants and the degree of conversion, the network is considered to contain defects, like branches, loops etc. In the case of dense networks eqn. (1) cannot be applied and the value of Mc and the extent of defects have to be calculated in some other way. * Vysokomol. soy'ed. 30: No. 6, 1285-1293, 1988.
0032-3950/88 $10.00+.00 © 1989 Pergamon Press pie
DEFORMATIONAL-STRENGTH PROPERTIES AND STRUCTURE OF POLYETHYLENE-POLYSTYRENE MIXTURES AS A FUNCTION OF ORIENTATIONAL STRETCHING CONDITIONS* N. P. KRASNIKO VA, Y'E. V. KOTOVA, A. S. KECHEK'YAN, YE. K. BORISENKOVA, YE. M. ANTIPOV, S. A. KUPTSOV, Z. PEL'TSBAUER a n d V. YE. DREVAL' A. V. Topchiev Institute of Petrochemical Synthesis V. I. Lenin State Teaching Institute, Moscow Institute of Macromolecular Chemistry, Academy of Sciences of the Czechoslovakian SSR, Prague (Received 7 January 1987)
The effect of orientational stretching on the structure and deformation-strength properties of a fibrous, self-reinforced material obtained by pressing a melt of a mixture of 30 ~ by weight polyethylene (PE) with polystyrene (PS) through a capillary is studied. It is established that stretching over the temperature range Tg Ps-Traelt pl! results in an increase in the strength of the material in the solid state, the maximum strength being attained as a result of 13-fold stretching at 120°C. M u c h a t t e n t i o n has been given recently to s t u d y i n g mixtures on i n c o m p a t i b l e p o l y m e r s c a p a b l e o f self-reinforcement d u r i n g d e f o r m a t i o n [1-4]. Thus, on p r e s s i n g a mixture melt t h r o u g h n a r r o w capillaries, the p o l y m e r , which is the disperse phase, f o r m s u l t r a t h i n fibres at the e n t r a n c e to the c a p i l l a r y , o r i e n t e d a l o n g the c a p i l l a r y axis. A f t e r e x t r u s i o n the m a t e r i a l is g e n e r a l l y subjected to a d d i t i o n a l o r i e n t a t i o n a l s t r e t c h i n g to i m p r o v e its m e c h a n i c a l p r o p e r t i e s . T h e object o f the w o r k described in this a r t i c l e was to study the special s t r u c t u r a l features and d e f o r m a t i o n a l - s t r e n g t h p r o p e r t i e s o f the i n c o m p a t i b l e p o l y m e r e x t r u d a t e s thus f o r m e d , using a p o l y e t h y l e n e ( P E ) - p o l y s t y r e n e (PS) m o d e l system, a n d 1o s t u d y t h e effect o f these c h a r a c t e r i s t i c s o f the m a t e r i a l on a d d i t i o n a l o r i e n t a t i o n a l s t r e t c h i n g a n d i s o m e t r i c annealing. The mixture studied was obtained by emulsion polymerization of PS (M=4.5 x 105) with 30 wt. ~ linear PE (M=2.9 x 104). The mixture was obtained by mixing PS and PE powders in ethanol with subsequent evaporation of the ethanol. The polymer characteristics, and the method of preparing the specimens on a type KVID-2 capillary viscometer are described by Krasnikova et al. [3]. The material was pressed through the capillary at a shear stress of 2.5 x 104 Pa at 180°C, using a 3.0x 10 -3 diameter, 9.6x 10 -2 m long capillary, i.e. the specimens are formed in the region of developed fibre-like PE in a PS matrix [3]. The extrudates were subjected to orientational stretching on a type VRPS tension viscometer [5] at a constant deformation rate of 2.0 x 10 -2 sec-1 in a glycerol bath at 110-155°C. After attaining the desired deformation the specimen was instantaneously fixed in special clamps, and withdrawn quickly (after 1-2 sec) from the bath and cooled in water. * Vysokomol. soyed. A30: No. 6, 1279-1284, 1988. 1341
1342
N. P. K~.~NnCOVA et al,
The deformational-strength properties were determined on a type FPZ-10 tensile testing machine (East German) at room temperature and a stretching rate of 10 ram/rain. The structural studies involved large angle X-ray analysis by diffractometric and photo methods. The original and final structures after treatment were recorded on X-ray patterns obtained on a type IRIS-3"0 setput at room temperature (CuK, radiation). Monochromatization of the characteristic radiation was obtained by means of a curved quartz single crystal [6]. In the case of isometric plotting, the length of the oriented specimens was fixed by means of a metal frame and clamps. A special thermal attachment [7] was used for the temperature tests, which enabled the temperature to be held to an accuracy of +0.5°C. The specimen morphology was studied by photographing sections, and specimens washed free from PS by solvent were also examined under an "Opton" optical microscope and under a JSM-35 scanning electron microscope. Figure 1 shows a typical microphotograph of P E - P S mixture extrudate. The PE forms a fibre of diameter 1-10 a m (mean diameter 6.8 p m at the 5 ~ probability level). Under mechanical tests the extrudate of an individual PS undergoes brittle fracture, and the PE is stretched out, with neck formation, by 700~o (Fig. 2). The extrudate of a mixture not subjected to orientational stretching undergoes brittle fracture on deformation, amounting to a few percent, similar to an individual PS specimen, and is even somewhat lower in strength (35 MPa) than PS (45 MPa), Accordingly, the presence of PE fibres in a PS matrix does not of itself result in a change in the mechanical properties of PS. Orientational stretching of extrudate mixtures can change their deformational-
I~o. 1
Fro. 2
FIG. 1. Photomicrograph of PE fibres washed free from PS. FIG. 2. Stress calculated on the initial cross-section of a specimen as a function of the strain at 23°C for unstretched PS (1), PE (2), and P E P S mixtures (3), and also for stretched (2--7) mixtures at 110 (4), 120 (5), 126 (6), 135 (7), and 155°C (8). strength characteristics, depending on the temperature and draw ratio 2 = l/1o, where lo is the initial and l the final specimen length. On stretching the test PE above Tm,~t(128"C) the mechanical properties of the material are close to those of the unstretched extrudate of a P E P S mixture. Stretching over the T s v s - Tmolt PB range results in the formation of material capable of further deformation (120 ~ ) , which is accompanied by an increase in its strength. Khaineman [8] observed similar behaviour in an oriented polypropylenePS mixture containing polypropylene fibres.
1343
Deformational-strength properties and structure of PE-PS mixtures
During uniaxial deformation o f mixture extrudates strengthening o f the material is observed, this being most intense at 110 and 120°C. According to X-ray diffraction data the maximum orientation of PE crystallites is attained as a result of stretching at 120°C. Since these specimens also have the maximum strength under subsequent mechanical tests (Fig. 2), the temperature T = 120°C was selected for studying the effect of 2 on the properties of the material obtained. Furthermore, as is known [9], in the PE premelting region stronger materials o f higher M M are obtained. The PE under study could be stretched only 7- to 8-fold at 120°C, probably because of the significant specimen deformation. In subsequent mechanical tests (Fig. 3) such a specimen showed high strength (ort =200 MPa) and fractured like a rigid oriented material at 20 ~ strain. When PS specimens were stretched under these conditions they retained their rigidity, but their strength was increased, and had the value ~rt= 80 MPa, which is characteristic of highly o.MPa 200 }
/
tl
ii I I I
It
5
I0o I00 ~ / ~ x ~ i
I
25
50
,,,
~
E~%
i
L
5
I0
50
_~A
Fla. 3 Fla. 4 FIG. 3. Values of tr-e for PE (1) and PS (2) on their own, and for PE-PS mixtures (3-6), oriented at 120°C to 2=7 (1, 2, 4), 5 (3), 9 (5), and 13 (6). FIG. 4. Values of o-, (1) and et as a function of ,Efor PE-PS mixtures. Stretching temperature 120°C. oriented PS, the strength of which does not exceed 80-90 MPa [10]. In the case of a mixture, it can be stretched to 2 = 13-14 at 120°C. In the mechanical tests a specimen of mixture fractured similarly to a PE specimen stretched to 2 = 7 . Its strength was close to that of oriented PE, and the fracture strain was somewhat less than the strain of
1344
N, P. KRASNIKOVA et at.
oriented PE, but greater than that of oriented PS. On the whole, the above indicates that material of increased strength and deformability than PS can be obtained from PE-PS mixtures. Figure 4 shows the tensile strength of compositions as calculated on the specimen cross-section at the moment of fracture, and also the strain at fracture as a function of the stretch ratio at 120°C. It is characteristic that the strength is related directly to :t up to 2 = 7-8, i.e. up to values corresponding to the PE fracture strain at 120°C. It is noted that the viscosity of PS during stretching at 120°C, as assessed from data provided by Krasnikova et al. [3] and Dreval et ai. [4], using the Williams-LandelFerry equation for low stretch ratios is equal to 3x10 ax Pa. sec, i.e. is close to the viscosity of substances at Ts. Under these conditions the PS deformation should be almost entirely reversible. When 2 > 7 the tensile strength becomes much less dependent on the stretch ratio. Moreover, the strain of compositions at 120°C becomes reversible only partially, although the contribution of the irreversible component to the total strain is much less than the contribution to high elastic strain. Comparison of the tensile strength of PE-PS compositions at 2= 13 (Fig. 4) of strength ak, as calculated from the known equation for unidirectional composites, i.e. ak = a,,~0+ a~(l -- ~0),where am and af are the tensile strengths of the material and fibre respectively, and ~ is the volume proportion of the matrix [12],
n
17
20
23
26 20 °
FIG. 5
FIG. 6
F1o. 5. Diffraction patterns of oriented 0.---!3) PE-PS mixtures after annealing for one hour at 23 (1), I17 (2), 130 (3), 133.5 (4), and 135°C (5). Fxo. 6. Photo-X-ray patterns of oriented 0.--13) PE-PS mixtures after annealing (137°C, 15 rain),
obtained at 23°C. on the assumption that the maximum tensile strength of the PE under test is attained with a specimen for which 2 = 7, and. that the strength of the PS on stretching does not exceed 80-90 MPa, shows that the experimentally determined tensile strength is about double the calculated value. Accordingly, in the compositions tested the PE and PS
Deformational-strengthproperties and structure of PE-PS mixtures
1345
behave synergistically in their mutual effect on the tensile strength of the composition. If it is assumed that additional strain and strengthening of the PE fibres occur on stretching a mixture to 2=13, and that the equation is suitable for describing the strength of such compositions, then the tensile strength of the PE fibres in the compositions is equal to 550 MPa. On the other hand the the elastic modulus of the compositions E, as assessed from the slope of the initial section of the tr-s curves is weakly dependent on 2 and lies within the range 3-4 GPa. This value is close to E for unoriented PS (2.3 GPa). The elastic modulus of an unstretched PE specimen is 0.55 GPa, and of a sevenfold stretched PE specimen 4.3 GPa. The tensile strain of the compositions is chahged, depending on 2 (Fig. 4), along a curve with a maximum in the region of 2~-5-8. In order to understand the given results, the data obtained in X-ray diffraction studies will be considered. These data show that unstretched extrudates, and also extrudates subjected to stretching at 135 and 155 (2=7) are in an isotropic state, while PS is amorphous and PE is about 70~o crystalline. Uniaxial stretching of a PE-PS extrudate at 120°C results in the formation of a typical fibrillar structure in the PE (c-axial texture). Amorphous PS also becomes oriented (presence of corresponding intensity attenuation on the X-ray pattern equator and meridian). As 2 is increased t,he degree of orientation of both components increases and attains a maximum value at 2= 13 (the PE reflections tend to be constricted from an arc into points). The test process on the tensile testing machine (T=23°C) has an appreciable effect on the PE orientation in specimens of a P E P S mixture stretched to 2~ 5-8, which is expressed in a noticeable decrease in the transverse dimensions of the PE crystallites from 19.7 to 15.4 nm (110 reflection). The form of the X-ray patterns is thus changed, just as when 2 is increased from 5 to 13 (T=120°C), as noted above. Accordingly, in this case it is reasonable to speak of additional orientational stretching, but at room temperature. In the case of a mixture with 2= 13 the transverse dimensions of the PE crystallites before and after testing on a tensile testing machine remains equal at 18 nm. In this case the X-ray patterns of the mixture after testing on the tensile testing machine also remain almost unchanged. These also show a reflection at the angular position 20max= 19"5°, corresponding, according to Seto et al. [13], to the appearance of a monoclinic modification of part of the crystalline PE, whereas its main part (not less than 90 ~) remains in the orthorhombic form. A structural transition of this type indicates the presence in the PE during stretching of large shear stresses, which indicates a different strain mechanism at small and large ratios of preliminary specimen stretching. Temperatures curves for mixture specimens with 2 = 7 and 13 for isometric conditions showed that increase in temperature and heating over 1 hr results in typical annealing features. The last signs of orthorhombic PE reflections disappear at 135°C (Fig. 5). In certain cases it was possible to anneal specimens at 137°C without fracture (15 min). As follows from the photo-Xray diffraction patterns, obtained after cooling of the annealed specimens of mixture to room temperature (Fig. 6), annealing somewhat decreases, but does not remove anisotropy of the mixture components. The PE diffraction pattern, apart from the isotropic component, shows very clear complex texturizing.
1346
N. P. KRASNIKOVAet al.
This is mainly a-axial texture for which the a axis of the orthorhombic cell is oriented mainly along the specimen stretching direction, and the positions of the b and c axes show cylindrical symmetry relative to this direction. All the test specimens showed this texturizing, but this was clearest at the maximum values of 2. Gerasimo et al. [14] observed a situation indicated by the position of the PE macromolecules perpendicular to the preliminary orientation axis (layerized structure) on annealing PE under free conditions close to the melting point, and showed that the a-texture is an intermediate stage between the highly oriented (c-texture) and isotropic states. Apart from the a-texture, narrow equitorial PE reflections are observed on the photoX-patterns, which correspond to c-texture. This obviously indicates that a small proportion of the PE chains remain in the PE melt at 135-137°C in the oriented state, and on subsequent cooling, most of the PE melt is partially crystallized on these stretched chains, just as on nuclei. The result obtained confirms the conclusion arrived at by Shilov et ai. [15] that the PE chain orientation in composite material mixtures is partially retained above the mixture melting point. These results were observed by the authors with all the PE-PS mixtures studied, with 2=7-13 at points some distance from the isometric annealing frame clamps. It is noted that similar results were obtained earlier for PE-polypropylene [16] and PE-lavsan [17] mixtures. The above data show that the PS matrix has a significant effect on the crystallinity of PE in P E P S mixtures. The large number of physical" "crosslinks" between the components, which determine their adhesion, on ,the one hand ensures orientation of the PE inclusions on uniaxial stretching of the specimen, and on the other hand, holds some of the PE chains in the oriented state above T,,clt. Furthermore, if an attempt is made, using the data provided by Krasnikova et al. [3] and Dreval et al. [4], to evaluate the maximum relaxation time 0o for PS, using the Williamson-Landel-Ferry equation, it is found that 0o =t/olEo (where t/o is the initial specimen viscosity and/'o is the initial high elastic modulus) attains a high value, equal to 1.5 x 105 sec, so that during the annealing time (3.6 x 103 sec) the PS is not relaxed but remains partially oriented. On the basis of these results it is assumed that the special features of the mechanical behaviour of P E P S mixtures subjected to orientational stretching at 120°C can be associated with the presence in it of a certain proportion of highly oriented and high strength PE fibres, which increase the strength of the material, and some unoriented or only slightly oriented PE fibres. These are stretched and further oriented during the mechanical tests on the tensile testing machine, resulting in release of the local overstresses, causing brittle fracture of the composite material. This is because in specimens not subjected to orientational stretching the PE fibres are not all the same. Thus, they have different diameters (1-10/zm), can be non-uniformly distributed along the extrudate cross-section, etc. During mechanical testing and high temperature stretching of the specimens such fibres undergo different changes, since they are subjected to different local loads. In an unstretched extrudate the elastic modulus of the PE, which is equal, according to the authors' data, to 0.55 GPa, is lower by a factor of about 4 than the corresponding value for PS, so that in the mechanical tests the PS bears the main load •
/
Deformational-strength properties and structure of PE-PS mixtures
1 347
in such a composite material, and the material behaves similar to polystyrene on its own at low strains. With increase in stretch ratio the proportion of highly oriented PE fibres in the mixture increases, which increases the specimen tensile strength and modulus. Moreover, in all probability, on extrudates with 2 > 7 PE fibres appear in them for which the stretch ratio is higher than the limiting stretching frequency for PE on its own (2~7). This results in hardening of the material and constancy of its structure during the mechanical tests. However, with increase in stretch ratio 2.>7, the number of PE fibres capable of undergoing additional strain and orientation in mechanical tests on a tensile testing machine is decreased. In view of this, uniaxial stretching to 2.>7, when most of the PE fibres pass into the oriented state, results in an increase in brittleness of the composite material at r o o m temperature. The use in this work of high temperature orientational stretching of mixtures of incompatible polymers thus provides a means of obtaining a composite material having increased strength characteristics. Translated by N. STANDEN REFERENCES
I. D. POL, Polimernye smesi (Polymer Mixtures), vol. 2 (ed. by D. Pol and S. M. Newmen), 1981 2. M. V. TSEBRENKO, N. M. RESANOVA and G. V. VINOGRADOV, Polymer Engng. Sci. 20: 1023, 1980 3. N. P. KRASNIKOVA, V. Ye. DREVAL, Ye. V. KOTOVA, Ye. P. PLOTNIKOVA, G. V. VINOGRADOV, G. P. BEIX)V and Z. PELTSBAUER, Vysokomol. soyed. A24: 1423, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 7, 1617, 1982) 4. V. Ye. DREVAL, G. V. VINOGRADOV, Ye. P. PLOTNIKOVA, M. R. ZABUGINA, N. P. KRASNIKOVA, Ye. V. KOTOVA and Z. PELZBAUER, Rheol. Acta. 22: 102, 1983 5. V. PADUSHKEVICH, V. D. FIKHMAN and G. V. VINOGRADOV, Uspekhi reologii polimerov (Successes in the Rheology of Polymers). Moscow, 1970 6. Yu. A. ZUBOV, V. I. SELIIKHOVA, V. S. SHIRETS and A. N. OZERIN, Vysokomol. soyed. A16: 1681, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1950, 1974) 7. Ye. M. ANTIPOV, N. N. KUZMIN, Yu. N. OVCHINNIKOV and T. S. MARKOVA, Pribory i tekhnika eksperimenta, No. 2, 1958, 1975 8. P. KHAINEMAN, S. V. VLASOV, Yu. P. MIROSHNIKOV and V. N. KULEZNEV, Kolloid. zh. 48: 974, 1968 9. J. R. SHEFGEN, T. I. VEIR, J. V. BELLOU, S. L. KVOLEK, P. V. MORGAN, M. PANAR and J. TSIMMERMAN, Sverkhvysokomodulnye polimery (Superhigh Modulus Polymers). (ed. by A. Chiferr and I. L. Uord), 1983 I0. G. V. VINOGRADOV, B. V. RADUSHKEVICH, V. P. NEPASNIKOV and V. D. FIKHMAN, Rheol. Acta 17: 231, 1978 l I. J. D. FERRI, Vyazkouprugie svoistva potimerov (Viscoelastic Properties of Polymers). Moscow, 1963 12. L. NILSEN, Mekhanicheskie svoistva polimerov i polimernykh kompozitsii (Mechanical Propcrties of Polymers and Polymer Compositions). Moscow, 1978 13. T. SET, T. HARA and K. TANAKO, J. Appl. Phys. Japan 7: 31, 1968 14. V. I. GERASIMOV, Ya. V. GENIN and D. Ya. TSVANKIN, J. Polymer Sci. Polymer Phys. E d 12: 2035, 1974 15. V. V. SHILOV, Yu. P. GOMZA and Yu. S. LEPATOV, Kompozitsionnye polimernye materialy (Composite Polymer Materials). p. 28, Kiev, 1979 16. Ye. M. ANTIPOV, S. A. KUPTSOV, A. A. REMIZOVA and V. P. POPOV, Tez. dokl. XXII
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A. A. ASKADSKIIet al. •
Vsesoyuz. konf. po vysokomolek, soyedineniyam (Thesis of Paper at 22-nd All-Union Conference on High Molecular Compounds), p. 156, Alma-Ata, 1985 17. S. I. BELOUSOV, Ye. M. ANTIPOV, Yu. A. MAKHNOVSKII and N. A. SHITOV, Tez. dokl. I Vsesoyuz. konf. "Smesi polimerov" (Thesis of Paper of All-Union Conference "Polymer Mixtures") Ivanovo, 1986
Polymer SciaticaU.S.S.R. Vol. 30, No. 6, pp. 1348-1357, 1988 Printed in Poland
0032-3950/88 $10.00+.00 1989 Pergamon Press plc
DETERMINATION OF THE NUMBER OF BRANCHES AND OTHER DEFECTS IN THE STRUCTURE OF DENSE POLYMER NETWORKS* A. A. ASKADSKII, V. V. KAZANTSEVA,O. A. MEL'NIK, K. A. BYCHKO, A. A. SAKHAROVA, G. L. SLONIMSKII and T. M. FRUNZE A. N. Nesmeyanov Institute of Metal-Organic Compounds, U.S.S.R, Academy of Sciences (Received 12 January 1987)
A method is proposed for determining the number of defects (branches and isolated loops) in densely crosslinked polymers, based on a comparison of experimental and calculated glass temperatures of the network, using the increment method in the calculation. T h e proposed procedure was applied to crosslinked copolymers of methyl methacrylate with carborane substituted exocyctic vinylsilane of various composition; a considerable number of defect structures was revealed.
Tt-IE PROBLEMof determining defect structures (branchings, inactive cross[inks, loops etc.) is of importance both in loosely cross[inked and in dense polymer networks. For loosely cross[inked polymers, the classical equation of viscoelasticity is applied to this end, in the form 3R T
E~ = p - - ,
Mc
(1)
where E~ is the equilibrium modulus of viscoelasticity, M~ is the mean molecular mass of an inter-crosslink segment, p is density, T is the absolute temperature and R the universal gas constant. When the Mc value calculated by means of eqn. (1) does not agree with that calculated from the stoichiometry of the reactants and the degree of conversion, the network is considered to contain defects, like branches, loops etc. In the case of dense networks eqn. (1) cannot be applied and the value of Mc and the extent of defects have to be calculated in some other way. * Vysokomol. soy'ed. 30: No. 6, 1285-1293, 1988.