Crystallization of polyformaldehyde during flow of the molten polymer

Crystallization of polyformahlehyde (luriug fl()w of m,)lte~l polymer

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(2) P T H F of a given molecular weight is soluble in mixtures of acetone and water of a certain composition. The composition of the mixed solvent is most s t r o n g l y d e p e n d e n t on m o l e c u l a r w e i g h t o v e r t h e r a n g e of molccular weights front 300 to 5000. (3) T h e r e l a t i o n s h i p b e t w e e n t h e c o n c e n t r a t i o n of t e r m i n a l h y d r o x y l g r o u p s a n d t h e l i m i t i n g c o m p o s i t i o n of t h e a c e t o n e - w a t e r m i x t u r e for solubility is linear. This r e l a t i o n s h i p can be m a d e use of in t h e f r a c t i o n a t i o n of P T H F . Translated by E. 0. PHILLIPS REFERENCES 1. V. N. KUZNETSOV, L. V. LESNEVSKAYA, V. A. PETROVA, V. B. KOGAN and M. S.

2. 3. 4. 5. 6. 7. 8.

VILESOVA, Vysokomol. soyed. All: 213, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 1, 239, 1969) V. N. KUZNETSOV, V. B. KOGAN and M. S. VILESOVA, Vysokomol. soycd. All: 1330, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 6, 1509, 1969) K. P. MISHCHENKO and A. A. RAVDEL' (Eds.), Praktichcskic raboty po fizicheskoi khimii (Practical Work in Physical Chemistry). Goskhimizdat, 1961 F. PATAT and J. KLEIN, Makromol. Chem. 93: 230, 1966 F. PATAT and G. TRAXLER, Makromol. Chem. 33: 113, 1959 A. A. TAGER, Fiziko-khimiya polimerov (Physical Chemistry of Polymers). Goskhimizdat, 1963 S. A. GLIKMAN, Vvedenie v fiziko-khimiyu vysokopolimerov (hlt.roducti(m to the Physical Chemistry of Polymers). Izd. Saratovskogo Univ., 1959 V. B. KOGAN, V. M. FRIDMAN and V. V. KAFAROV, Ravnovesic mezhdu zhidkost'yu i parom (Liquid-Vapour Equilibrium). Vol. 1, Izd. "Nauka", 1966

CRYSTALLIZATION OF POLYFORMALDEHYDE DURING FLOW OF THE MOLTEN POLYMER* R. G. Gu~EN and V. V. KOVRIGA Plastics Research Institute

(Received 23 January 1969) I x FLOW of m e l t s of crystallizable p o l y m e r s a t t e m p e r a t u r e s close to the m e l t i n g p o i n t d e f o r m a t i o n , o r i e n t a t i o n a n d c r y s t a l l i z a t i o n occur [1, 2]. A n o m a l o u s b e h a v i o u r of a m o l t e n p o l y m e r was r e p o r t e d in reference [3] for e x a m p l e , n a m e l y increase in t h e flow of p o l y f o r m a l d e h y d e w h e n artificial crystallization c e n t r e s were added. I t is v e r y p r o b a b l e t h a t d e v i a t i o n f r o m norm~d beh a v i o u r of p o l y m e r melts is due to f o r m a t i o n of o r d e r e d aggregates, t h e existence of which h a s b e e n s h o w n e x p e r i m e n t a l l y in a n u m b e r of p o l y m e r s [4, 5]. * Vysokomol. soyed. A12: No. 12. 2639-2643, 1970.

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T h e r e are a t p r e s e n t no d a t a t h a t could p r o v i d e a reliable answer to t h e question of t h e n a t u r e of the e l e m e n t s t h a t are m o v i n g in the flow of p o l y m e r melts. D e p e n d i n g on t h e t y p e of e l e m e n t t h a t is m o v i n g in t h e m e l t as a single unit (the p o l y m e r molecule, a n a g g r e g a t e of molecules or a highly o r d e r e d e l e m e n t ) t h e m e c h a n i s m of flow will be different, a n d in t h e last analysis this m u s t affect its s t r u c t u r e a n d properties. This suggestion was p u t f o r w a r d b y t h e a u t h o r s a f t e r consideration of t h e results of a s t u d y of t h e s t r u c t u r e of p o l y f o r m a l d e h y d e ( P F A ) blocks, crystallized in t h e mould. These results indicate t h e possibility of t h e existence in such blocks of a specific t y p e of super-spherulitic s t r u c t u r e in t h e f o r m of spherulitic layers of different thickness, s e p a r a t e d b y regions of t h e p o l y m e r of less o r d e r e d s t r u c t u r e [6-8]. T h e p r e s e n t c o m m u n i c a t i o n p r e s e n t s t h e results of a s t u d y of the conditions of f o r m a t i o n of layers of s u p e r - s p h e r u l i t i c s t r u c t u r e during t h e flow a n d crystallization of a p o l y m e r m e l t in t h e mould, a n d t h e y are also considered in t h e light o f e x p e r i m e n t a l d a t a on t h e possibility of the existence of ordered e l e m e n t s in p o l y m e r melts. EXPERIMENTAL

For study of the crystallization of polymers during flow of the melt the method of injection moulding was used. This permits variation of the conditions of cooling and flow of the melt in the mould within wide limits. The polymer studied was grade "B" * polyformaldehyde, with an intrinsic viscosity of 1.49, cast in the form of blade specimens with circular (diameter 5 ram) and rectangular (3 × 5 rm-n) cross-section. The temperature of the melt during casting was varied between 180 and 210 °, the mould was filled at rates from 0.3 to 4.0 cm3/sec and the degree of supercooling ~ of the melt at the surface was varied between 5 and 150°. The occurrence of super-spherulitic layers was detected by a special method [9], in which a polished section from the working part of the specimen was prepared, the plane of the section being perpendicular to the direction of casting. The section was then etched in 85% sulphuric acid for 15 rain and the etched surface was photographed in plane-polarized reflected light with crossed polarizers at magnifications from 20 to 200. As criteria of the occurrence of a super-spherulitic structure in the specimen we take two sharply differing structural patterns in etched sections in the plane perpendicular to the direction of casting, in which super-spherulitic structures are revealed (Fig. la) and not revealed by etching (Fig. lb). RESULTS AND DISCUSSION

I t was f o u n d t h a t f o r m a t i o n of a l a y e r e d super-spherulitic s t r u c t u r e is prim a r i l y d e p e n d e n t on t h e t e m p e r a t u r e of t h e melt. F o r P F A m e l t s a t 180 a n d 190 ° (pressure 800 k g / c m 2) t h e r e is a r a t e c f filling a n d a degree of supercooling resulting in f o r m a t i o n of a s u p e r - s p h e r u l i t i c s t r u c t u r e . A t 180 ° a n d a degree o f supercooling of 145 ° this occurs a t r a t e s of filling b e t w e e n 0.4 a n d 4.0 cm3/scc, a n d a t 190 ° only a t a r a t e of filling of 4.0 cm3/sec a n d in t h e supercooling inter6-05-1018-66. t The degree of supercooling is the difference between tlle temperatures of the molten polymer and the mould. * MRTU

Crystallization of polyformaldchyde during flow of molten polymer

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val from 5 to 145 °. At polyformaldehyde melt temperatures of 200 and 210 ° the layer structures are not formed at all. Study of the structure of specimens obtained at a melt temprature of 180 ° at different rates of filling of the mould showed t h a t super-spherulitic structures are formed only at high rates of filling, i.e. not less than 0.6 cm3/sec (shear rate* 5 see-l), and degrees of supercooling of 85-145 °, and less than 1 cm3/sec (shear rate l0 sec -1) at degrees of supercooling of 5-25 °. Similar results were obtained at a melt temperature of 190 °, when layer structures occur only at a rate of filling of 4 cma/sec. Increase in the degree of supercooling of the melt at the walls of the mould also favours formation of super-spherulitic structures. At a degree of supercooling of 5 ° practically no super-spherulitic structure forms through the entire volume of the specimen. Formation of a layer structure occurs when the degree of supercooling is increased. The absence of layer formation in the part of the specimen where there was practically no flow (the end of the specimen at the filling end of the mould), and also in specimens made by pressure moulding, indicates t h a t there is a relationship between the formation of super-spherulitic structures in the polymer block and the rate of flow of the melt in the cooled channel. The experimental results showing a relationship between the super-spherulitic structure and flow, shown in Fig. 2 in the form of a photograph of an etched section of a P F A specimen, obtained at a high rate of filling involving turbulent flow, are of interest. I t is seen from the photograph t h a t layers of super-spherulitie structure repeat the shape of the vortex and the flow of the melt. Two photographs of P F A super-spherulitic structure in specimens prepared at different filling rates (Fig. 3) show the effect of the rate of filling on the thickness of the super-spherulitie structures. The higher the rate the thinner is the layer. Study of the size of the spherulites in the specimen by the method described in reference [8] and comparison of this with the thickness of the layers of superspherulitic structure showed t h a t the layer can be made up of single spherulites or consist of aggregates of several spherulites. In a study of the structure of specimens cast from P F A containing 0.2% of finely divided titanium dioxide it was found t h a t introduction of artificial crystallization nuclei into P F A promotes formation of super-spherulitic structures even when the temperature of the melt is above 200 ° . I f one compares the experimental fact of formation of layered structures in polymer melts at temperatures close to the melting point with data on the existence in polymer melts of ordered aggregates, also only in the same temperature region [4, 5], it may be suggested t h a t the flow of polymer melts can occur by two different mechanisms. These are flow in a homogeneous melt (in these cases it is * The shear rate at the walls of the channel was calculated from the formula ~= 4V/zrr 3, when V is the rate of filling and r is the radius of the chanucl,

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FIG. 1. Photographs of the structure of P F A in a cast block, revealed by etching: a - - a l t e r nating structure of less ordered and highly ordered layers; b--structure of uniform degree of order. FIG. 2. Photograph of the layered super-spherulitic structure in a specimen prepared at rate of flow of the melt in the channel of the mould gre~ter than 4 cmS/sec (the vortex currents in the melt can be seen, × 20). lvio. 3. Layered structure of P F A in specimens prepared at different rates of flow in the mould: a - - 4 cm3/sec; b--0.6 cms/sec.

Crystallization of p o l y f o r m a h l e h y d e during flow of m(dt(,n p o l y m e r

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difficult to distinguish polymeric aggregates moving as single units) and flow in a pseudo-heterogeneous melt of elements relatively to one another. The difference in the conditions of flow can be suitably illustrated by a diagram of the change in shear rate and the shear rate gradient in flow of a melt in a channel of circular cross-section (Fig. 4). In the case of a homogeneous melt the variation in shear rate and the shear rate gradient along the radius of the channel can be represented in the form of smooth curves (Fig. 4a).

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FiG. 4. Profile and rate g r a d i e n t d i a g r a m of flow of a m e l t in a channel of circular crosssection, and f o r m a t i o n of layered super-spherulitic structures: a - - f o r a m e l t w i t h o u t ordered strueturM elements; b - - f o r a m e l t containing ordered structural elements, denoted by (lark circles.

In the second ease the melt is inhomogeneous, it contains ordered aggregates of polymer molecules, the density of which is somewhat higher than t h a t of the unordered polymer. In flow in a channel of such a structurally inhomogeneous pseudo-heterogeneous melt there will also be a shear rate gradient along the radius of the channel but the variation in shear rate cannot be smooth. Shear detbrmation in such a melt must be localized at the boundaries between the ordered aggregates, because deformation of the polymer within an aggregate requires greater force t h a n in the unordered polymer. By localization of shear deformation is meant the development (concentration) of shear deformation in isolated regions along the direction perpendicular to the direction of flow. A diagram of the variation in rate and the shear rate gradient in an inhomogeneous melt is sho~m in Fig. 4b. The question of whether or not localization of shear deformation occurs between ordered aggregates of polymer molecules in a melt will depend on the competition between the shear rate in the chalmel and the strength of the aggregate. The effect of shear rate is confirmed by a number of experimental results where

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formation of a layered structure occurs only at high rates filling of the channel and increase in the rate of filling leads to decrease in the thickness of the layer, i.e. to more frequent localization of shear. This is also indicated by the fact that in the centre of the specimen, where the shear rate gradient is less, thicker layers are formed. On the basis of the above general approach to the problem of the crystallization of polymers during flow of melts we can distinguish five basic processes in the crystallization of a polymer in a block specimen, occurring during the process of injection moulding. The first two occur when the temperature of the melt is much higher than the melting point of the polymer and it contains no ordered structural elements. 1. The degree of supercooling of the melt in the surface layers is small. In these circumstances the crystallization process does not differ essentially from that of crystallization in an immobile molten polymer, because even the effect of orientation of polymer molecules during flow mainly disappears because of the long time available for relaxation in the mould at a high temperature. In practice flow occurs first and cooling of the melt occurs later. 2. The degree of supercooling in the surface layers is substantial. Crystallization in the specimen involves formation of a so-called "envelope", formed by instantaneous solidification of the molten polymer on the cold walls of the mould. Crystallization proceeds at a high rate in the envelope itself and it differs considerably from the process of crystallization in the space within the envelope. Orientation has a considerable effect on the crystallization process, especially in the layer close to the walls of the mould. At the same time a difference in principle from crystallization of an immobile melt should not be expected, because there is a time interval between flow and crystallization of the main bulk of the polymer. Flow of the polymer into the mould until the latter is filled occurs first followed by crystallization of the now stationary polymer. The following three processes occur when the temperature of the melt is close to the melting point of the polymer and it contains ordered structural elements. 3. The degree of supercooling in the surface layers of the specimen is close to zero and the mould is filled without turbulent flow. Here again crystallization and flow are separated in time, i.e. crystallization occurs in the stationary melt after the mould has been filled. It must be borne in mind, however, that during the time of flow the ordered elements are arranged on the surfaces at equal rates of flow and thus fix the subsequent order of arrangement of the spherulites throughout the specimen. The difference in the nature of the boundaries between the spherulites cannot be due to growth of spherulites from the stationary melt, since in all the cases taken the structure revealed by etching corresponds to the photograph in Fig. lb. 4. The degree of supercooling of the surface layers is high and the mould is filled without turbulence. Flow and crystallization are simultaneous. The localization of shear deformation that occurs at the boundaries between supermolecular

Cryst,all ization of polyformaldehyde during flow of molten polymer

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f o r m a t i o n s hinders flow in the melt at the boundaries b y p u r e l y mechanical action. This she~r d e f o r m a t i o n during flow can cause f o r m a t i o n of a l t e r n a t i n g ordered a n d disordered layers, which is w h a t the etching m e t h o d reveals (Fig. la), w h e n b r e a k d o w n of the less o r d e r e d regions in the specimen occurs predomin~ntly. I t is seen from Fig. 4b, which illustrates the proposed scheme of f o r m a t i o n of b~yers in the melt, t h a t m o v e m e n t of the ordered s t r u c t u r a l elements in the direction of the x axis occurs at an equal rate, and g r o w t h of spherulites in this direction will proceed u n h i n d e r e d until their boundaries touch. The picture is different w h e n we consider g r o w t h of the spherulites in the direction of the y axis. H e r e shear d e f o r m a t i o n s are localized b e t w e e n the ordered elements a n d these h i n d e r g r o w t h of the spherulites. 5. The degree of supercooling is high a n d the flow of the melt during filling of the mould is t u r b u l e n t . Crystallization a n d flow again occur simultaneously. Shear d e f o r m a t i o n is localized at the boundaries b e t w e e n ordered elements in the m(,lt, b u t the distribution of the shear d e f o r m a t i o n and the f o r m a t i o n of l a y e r e d s t r u c t u r e s will be controlled b y the flow a n d v o r t e x p a t t e r n s (Fig. 2). CONCLUSIONS

The crystallization of p o l y m e r s from the molten state is d e p e n d e n t to a considerable e x t e n t on the conditions of flow. Crystallization occurring during flow of m o l t e n p o l y f o r m a l d e h y d e (PFA) results in f o r m a t i o n of a specific t y p e of s t r u c t u r e of alternating less ordered and highly ordered regions. The f o r m a t i o n of a l a y e r e d s t r u c t u r e can be caused b y localization of shear d e f o r m a t i o n at the boundaries b e t w e e n ordered aggregates of p o l y m e r molecules present in a p s e u d o - h e t e r o g e n e o u s melt. T h e e x p e r i m e n t a l results of this work make possible control of the s t r u c t u r e of P F A in the p r o d u c t i o n of articles b y injection moulding. Translated by E. O. PHILLIPS REFERENCES

I. G. SCHUUR, Kolloid-Z. 208: 123, 1966 2. B. MAXWELL, J. Polymer Sci. C9: 43, 1965 3. M. S. AKUTIN, B. V. ANDRIANOV, M. B. KOTItELEV m~d Y. A. KARGIN, Vysokomol. soyed. 8: 2053, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 12, 2266, 1966) 4. V. A. KARGIN, T. I. SOGOLOVA and N. Ya. ItAPOPORT, Dokl. Akad. Nauk SSSR 15,6: 1406, 1964 5. H. G. ZACHMANN, For~schr. Hochpolym.-Forsch. 3: 481, 1964 6. V.V. KOVRIGA and R. G. GUMEN, Dokl. Akad. Nauk SSSR 176: 1314, 1967 7. V. V. KOVRIGA and It. G. GUMEN, Mekhanika polimerov, 205, 1968 8. V. V. KOVRIGA and It. G. GUMEN, Mekhgnika polimerov, 394, 1968 9. V. V. KOVRIGA, It. G. GUMEN and E. A. SAAKYAN, Plast. massy, No. 3, 60, 1967