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Influence of crystallization on the structure and adsorption properties of glassy polyethylene terephthalate strained in an adsorption-active medium
1178
A . L . VOLYNSKIIet a~.
10. A. L KUZAYEV, S. D. KOLESNIKOVA and A. A. BRIKENSHTEIN, VysokomoL soyed. A17: No. 6, 1327, 1975 (Translated in Polymer Sei. U.S.S.R. 17: 6, 1524, 1975} 11. B. R. SMIRNOV, I. S. MOROZOVA, A. P. MARCHENKO, M. A. MARKEYEVICH, L. M. PUSHCHAYEVA and N. S. YENIKOLOPYAN, Dokl. AN SSSR, 254: No. 4, 891, 1980 12. C. BAMFORD, W. BARBE, A. JENKINS and P. ONION, Kinetika radikalnoi polimerizatsii vinflovykh soyedinenii (Kinetics of Radical Polymerization of Vinyl Compounds), p. 106, Foreigh Lit. Press, Moscow, 1961 13. A. Yu. SHAULOV, A. B. SHAPIRO, A. G. SKLAYROVA, A. M. WASSERMAN, A. Lo RUCHACHENKO and E. G. ROZANTSEV, Europ. Polyzn. J . 10: 1077, 1974 14. A. M. NORTH and G. A. REED, Trans. Faraday Soc. 57: 859, 1961 /
Polymer Science U.S.S.R. Vol. 23, No. 5, pp. 1178-1187,1981 Printed in Poland
0032-3950/8'1/051178-10507. 50/0 1982 PergamonPress Ltd.
INFLUENCE OF CRYSTALLIZATION ON THE STRUCTURE AND ADSORPTION PROPERTIES OF GLASSY POLYETHYLENE TEREPHTHALATE STRAINED IN AN ADSORPTION-ACTIVE MEDIUM* A. L. VOLYNSKII, V . ~ LoGI~ov and N. F. BAKEYEV M. V. Lomonosov State University, Moscow
(Received 12 February 1980) A study has been made of the influence of crystallization of glassy polyethylene terephthalate (PETP) stretched in an adsorption-active medium on its s~ructure and adsorption properties. I t was found that the annealing of strained PETP specimens in the same liquid medium results in stable highly porous materials whose structure does not depend on the drying process or on repeated humidification. I t is shown that these materials are capable of adsorbing iodine and an organic dye (Rhodamine S) from aqueous solutions. Major changes in the structure and adsorption behaviour of PETP are obtainable by varying the degree of prior drawing of the polymer in an adsorption active medium. PETP specimens prepared with crystallization of the polymer are capable of adsorbing considerable amounts of various substances from the gaseous phase as well. This makes it possible to obtain some additional data on the structure of the materials. ON PR~.vIous occasions [1, 2] we showed t h a t t h e cold d r a w i n g o f a p o l y m e r in a n a d s o r p t i o n - a c t i v e m e d i u m is a c c o m p a n i e d b y t r a n s i t i o n o f t h e p o l y m e r t o a new s t r u c t u r a l - p h y s i c a l state. T h i s ' s t a t e is c h a r a c t e r i z e d b y a new set o f p h y a i * Vysokomol. soyed. ASS: No. 5, ~1059-1065, 1981.
Structure and adsorption properties of glassy polyethylene terephthalate
1179
cal and physieochemical properties, and its main feature is t h a t the polymer has a highly developed boundary surface. This means t h a t these materials are porous adsorbents [3], whose structure m a y be varied at will. However, structural lability is a specific feature of adsorbents of this type. Certainly, drying of the polymeric porous specimens after, stretching in a n adsorption-active medium leads to major shrinkage of the latter resulting from coagulation of the highly dispersed s t r u c t u r e [1], whereas long-term retention in an adsorbate solution m a y bring about the reverse process, i.e. peptization ' o f structure previously coagulated [4]. I n the present investigation an a t t e m p t was made to prepare stable polymeric porous materials, and their properties were examined. The study object in the investigation was a commercial film of amorphous unoriented PETP of thickness 500-700/ml. Ttm adsorption of iodine and of an organic pigment (Rhodamine S} from water was investigated. The adsorption active medium in which PETP stretching was carried out was n-propyl alcohol. After crystallization via annealing in the same medium at 85° specimens that had been stretched in the adsorption active medium were placed in an adsorbate solution, and the adsorption was estimated from changes in the optical'density of the solution, using an FEK-M device• The time required to establish ~he equilibrium was determined from the data obtained from kinetic investigations bf adsorption. A solution of iodine in water was prepared, and KI was added in the ratio KI : Ii=20 : 1~ Specific adsorption values (g/g} were given by the formula v(c0-c) 1000 m '
~vhere co and c are the initial and equilibrium solution concentrations, g/l.; m is the weighed portion of adsorbent, g; V is the volume of the solution, ml. Low.angle X-ray diffraction patterns were obtained using the apparatus used in [5] in line with standard procedure; an ]~SM-2 electron microscope was used for the scanning electron microphotographs. All experiments were carried out at room temperature. Reagents used (without further puri~cation) in the investigation were all of chemically pure grade, or of analytical purity. One m a y surmise t h a t stability of the P E T P structure formed during stretching in an adsorption-active medium could result from subsequent crystallization ,of the polymer. To obtain P E T P having a stable porous structure one must, ~n the one hand, ensure crystallization of the polymer, and on the other h a n d preserve a highly developed interface. I n view of this the annealing of P E T P specimens was carried out directly in an adsorption active medium on the assumption t h a t precisely the stabilizing influence of the medium will make for preservation of t h e highly dispersed microcrack structure. Specimens strained in the adsorption~active medium were transferred to the same liquid, albeit heated to 85 °, in which the specimens were kept for 1 hr. I t was found t h a t materials obtained in this way suffer a complete loss of ability to change their dimensions during drying processes. I t m a y therefore be said t h a t the polymer .structure has acquired a sufficient level of stability as a result of previous crystallization. There are two ways in which crystallization under these conditions m a y
A. L. VOLX-NSXlXa a2.
1180
be carried out. One may transfer the strained (deformed) specimen to the heated adsorption-active medium, having first released the specimen from the clamps, or crystallization of the specimen may be carried out with itsdimensions fixed (in clamps). We investigated the structure and adsorption properties of P E T P specimens perpared by both methods. As a result of this it was found t h a t crystallization of P E T P specimens that have first been stretched in the liquid medium results in a polymer structure of a stable and highly porous type. Certainly, it was found by special tests that 1) a P E T P specimen crystallized through annealing in an adsorption activo medium suffers a complete loss of ability to shrink, and its adsorption properties remain constant during a prolonged period of retention in the same
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Fro. 1. Degree of adsorption of iodine x by P E T P specimens from aqueous solutions of I= (equilibrium conc. 0"8 g]l.) (a) and a~lsorption of Rhodamine S (equilibrium cone. 0.04 gfl.) (b) vs. degree of extension of the polymer in an adsorption-active medium 2: 1--noncrystallized specimens; ~2--specimens crystallized in the free state; 3--with fixed dimensions. FIG. 2. Degree of desorption of iodine (a) and Rhodamine S (b) from P E T P specimens transferred to n-propanol after adsorption from water vs. degree of prior stretching o f P E T P in an adsorption-active medium: 1--noncrystallized specimens, 2--crystallized in the free state, 3--with fixed dimensions.
Structure and adsorption properties of glassy polyethylene terephthalate
118l~
liquid; 2) the adsorption properties of materials obtained in this way do not depend on the sequence of operations: immediately after crystallization (i.e. in the moist state) the specimen is transferred to the adsorbate solution, or t h e specimen is dried, and then placed dry in the adsorbate solution. In other words, processes of coagulation and of peptization of highly dispersed structure are not characteristic of these materials. Figure 1 shows plots of the adsorption of iodine and of the pigment Rhodamine S vs. the degree of prior stretching of the polymer for crystalline P E T P specimens." For comparison, the same Figures show the data for P E T P specimens transferred to the absorbate solution immediately after stretching in a liquid medium without subsequent crystallization [6]. It is clear from Fig. 1 that t h e materials obtained via crystallization in liquid media are efficient adsorbents, which means that a well developed boundary surface is preserved during crystallization. It is apparent that as in the case of the uncrystallized specimens the adsorption properties of the materials undergo a rather marked change in the range of deforn~tion corresponding to transition from a loose structure to a compact one, as has been described in some detail in [6, 7]. Crystallization of" the polymer makes this transition still more marked (Fig. 1 ). I t shouId be n o t e d that the conditions under which the crystallization of P E T P is being carried out does not significantly influence the adsorption properties of the resulting material. Irrespective of how crystallization of the specimens is carried out (in the free state or with fixed dimensions)the adsorption behaviour was practically identical, though it has been demonstrated in earlier work [4] that annealing: of the polymer in the free state in presence of a liquid medium is accompanied by a considerable amount of shrinkage. A slight reduction in adsorption (Fig. 1): evidences coagulation of some of the highly dispersed microcrack materia~ during annealing of the polymer in an adsorption-active medium. I t was found that the extent of this coagulation does not depend on conditions under which the polymer specimen is kept (in the free state or with fixed dimensions). Fixing t h e dimensions does not prevent coagulation of highly dispersed microcrack material, but simply alters certain morphological features of the material: Moreover it follows from the results of desorption investigations that definite structural regrouping takes place in the polymer during crystallization. Figure 2 shows the data on iodine and dye desorption in n-propanol. It is apparent that as in the case examined in [6], considerable desorption takes placein the range of low deformations, but this desorption is strongly inhibited in the case of large prior deformations of P E T P in liquid media. At the same time, however, the adsorptipn behaviour of crystallized P E T P specimens also exhibit a number of characteristic differences. It appears that the deformation range in which transition from a free to an inhibited desorption of adsorbates takes, place is displaced towards lower degrees of extension: in the case of Rhodamine S, from 150 to 100%, and in the case of iodine, from 30Oto 150~/o. Another characteristic difference appears in the fact that P E T P specimens that have beerL
i182
A.L. VOLYNSKIIet ~.
stretched to high degrees of preliminary extension in a liquid medium exhibit practically no desorption. This means that PETP pore sizes are reduced during •orystalhzation and become commensurate with iodine molecules, starting already as from 200-300% extensions. The number of large pores that are accessible to Rhodamine S is at t h e same time greatly reduced, starting at 100-150% extensions, as is clear from the results ofdtirect adsorption tests. The number o f pores t h a t are accessible to iodine molecules decreases only insignificantly (Fig. 2), but the pore size becomes a lot smaller. It should be noted that both iodine and Rhodamine S, in contrast to PETP specimens obtained on drying after stretching in an adsorption active medium :[8]readily pass through the surface layer of coagulated microcrack fibrils and reach the inner boundary surfaces. This is evident from the results of optical microscopy, according to which transverse sections of PETP specimens become coloured, after adsorption in solutions of iodine or Rhodamiae S, over the entire ~ross section. In view of these findings one may say that the structure of PETP specimens formed as a result of crystallization differs markedly from the Corresponding amorphous materials after removal of the liquid medium. Figure 3 shows the low angle diffraction patterns for P E T P specimens prepared during crystallization in the liquid after tlieir deformation amounting to 51, 150 and 3000/o extensions. Crystallization of the specimens was carried out in the free state (a,-~) and with fixed dimensions (d-f). It can readily be seen that with low degrees of extension (50-150~/o) crystallization does lead to the formation of a highly dispersed .structure featuring intense diffuse scattering, It is noteworthy that in contradistinction to the specimens subjected after stretching to drying in the adsorption active medium, t h e crystallized specimens give a practically symmetrical scattering patter~l. In other words, crystallization is accompanied by major structural deformatmn and by the preservation of well developed boundary surfaces. It is interesting that in the case of large (~300~)extensions in a liquid medium (Fig. 3c, f) orientation of structural fibrillar microcrack elements is largely preserved even after crystallization, irrespective of the conditions under which crystallization is conducted. It is also clear from these findings that compared with the specimens subjected after stretching to drying in a liquid medium the cryjstalline specimens must feature major dissimilarities in the :structure of the surface layer formed during coagulation of highly dispersed mierocrack material. Certainly, if the crystallized P E T P specimens were structurally similar to the dried ones [8] one would be unable to explain how adsorbate molecules could reach the inner interfaces in the microerack structure. As was ipointed out above the crystallized Sl~cimens do not cause peptization of co~g~tlated highly dispersed microcrack material, but, despite this, iodine and Rhodamine S molecules are able to get through to the surface located in internal parts of the specimen. This assumption is fully substantiated by the results of scanning electron
Structure and adsorption properties of g l a i r polyethylene terephthalate
1183
Fio. 3. Low-angle X . r a y t l i ~ i o n patterns of PETP drawn by: a, d--50%; b, e--150% and c,/--300°/o in n-propanol and crystallized in tile same medium at 85°C for 1 hr: a, b and c--specimens 'crystallized in the free condition; d, s and f--specimens crystallized with fixed dimensions; tensile axis of polymer vertical.
microscopy. Figure 4 shows the scanning electron photomicrographs for the P E T P specimen deformed with 150% extension in n-propanol and then crystallized in the latter at 85 °. It is apparent that the mierocrack structure in the crystallized specimen shows significant differences compared with the specimen dried after extension in the adsorption active medium. The microeracks which, in the initial PETP specimens [8] show quite distinct orientation relative to the drawing axis, suffer a loss of their orientation and are crooked. Microvacancies in the microcrack structure suffer a loss of asymmetry and orientation. The porotis structure of these materials resembles that of active carbon characterized by a large multitude of mierovacancies of varying shape and size with a random arrangement of one to another (Fig. 4a). Interesting results were obtained from a study of the external surfaces of crystallized PETP specimens. I t is seen from Fig. 4b, c that the structure of the surface layers of microeracks
1.184
A. L. VOLY~S~II ~ .l.
contains a large number of apertures measuring from 2 to 10/~m, whereby interna! vacancies are in communication with environm'ent~l spa~e. The origin of these apertures is suggested by some of their morphological features. I t is seen from Fig. 4c, that the apertures are not unlike craters made by explosions, e.g. from their external appearance one could suppose that they are the result of the action of a force applied to a specimen from within, as evidenced by the characteristic bulging around the rims of the apertures. The following mechanism for formation of these apertures appears feasible. When P E T P is stretched in the liquid the latter covers microvacancies of propagating mierocracks. After the P E T P specimen has been transferred to the heated adsorption active medium, processes of coagulation and crystallization of the microcraek material take place a n d are accompanied by significant thermal expansion of the liquid fillln~ up microcracks in the microcrack structure.
Fio. 4. Scanning electron photomicrographs of a PETP specimen with 100% extension in n-propanol and then annealed in n-pro~ano~ at 85° for 1 hr: a, b--viewed from the fracture surface; c--exte ~ rnal surface of specimen. Stresses developing in this case in the specimen are so great as to cause bre'akdown in individual parts of the microcrack structure, which results in the formation of apertures whereby internal microvaeancies of the latter are connected wit h surrounding space. Thus it appears from these, results that interaction between polymer and the adsorption active liquid is not limited solely to facilitating the development
Struoture and adsorption properties 0 ~
~ e n e
terephthalate
1185
of mlcrocracks having a specific structure. The adsorption active medium protects the highly dispersed polymer structure from coalescence dm~ug crystallization, and also leads to the formation of.open-porous material through effects relating to thermal expansion of the liquid (filling microcrack vacancies) during t h e heating process.
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0"8 200
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50
100
M
F1o. 6
FIO. 5. Degree of adsorption of methanol (1), n-jpropanol (2), n-hexanol (3), n-decanol (4) and water (5) by PETP specimens deformed in an adsorption-active medium to different degrees of extension 2, from saturated vapours at room temperature. FIo. 6. Amount of adsorbed normal aliphatic aloohols x vs. MW of the latter. Adsorption by crystallized PETP specimens stretched 100% in an adsorption-active medium. Since an open-porous polymeric material is formed as a result of crystallit zation of P E T P specimens deformed in liquid media one m a y surmise that this t y p e of material must equally be efficient as an adsorber from the gas phase. We accordingly used the simplest method for this t y p e of investigation. The P E T P specimens were placed under a glass c o v e r at room temperature; under the cover was an open small glass conta'ming the adsorbate. The amount o f substance ~dsorbed after establishment of the equilibrium was determined gravimetrically. Actually we found in each ease only a single point on the a d sorption isotherm, i.e. a point corresponding to the saturated vapour pressure at room temperature.* Figure 5 shows the data obtained b y the above method for a number of" aliphatic alcohols and for water. I t is seen that the P E T P specimens w h e n crystallized are able to adsorb considerable amounts of substances from t h e gas phase. For instance, the P E T P specimen stretched in an adsorption-active medium and then crystallized are capable of adsorbing more than 26 wt. % of n-propanol. The adsorption process i~in this case a reversible one, i.e. P E T P * It was found by cheek tests that this method could not be used to determine the amount of substance adsorbed by unstretched PETP specimens or by specimens stretchec[ in air with development of "necking".
1186
A. L. V O L Y S S X n
e$ a/.
specimens placed in air after adsorption suffer a practically complete loss:vf the adsorbed substance in the course of 2 days. It is noteworthy that the amount of alcohols adsorbed goes through a maxi~ m u m in the regior~ of 100-150°//o extensions of the polymer, i.e. at the point where a transition from a loose structure to a compact one was observed b y other methods [6, 7]. Let us now turn to features of adsorption by the study objects in a number of aliphatic alcohols. Figure 6 shows plots of the amount of alcohol adsorbed by crystallized P E T P specimens that had first been stretched to 100~/o extension (the area of maximal adsorption in Fig. 5) vs. MW of the adsorbates being used. I t can readily be seen that adsorption is most marked for the alcohols containing three or four methylene groups. The amount of substance adsorbed is determined for the most part by two factors. Firstly, by the intensity of intermolecular interaction of adsorbate molecules and the molecular surface layer of polymer. This increases in the series of aliphatic alcohols, whose surface activity, as is known from [9], increases with increasing length of the hydrocarbon radical, Secondly, the amount of substance adsorbed is determined by the accessibility of inner surfaces to adsorbate molecules, i.e. by the ratio of pore sizes and molecular dimensions of the adsorbate. It is obvious that the larger the molecular dimensions bf the adsorbate, the larger will be the number of small pores inaccessible to the adsorbate and the lower (other conditions being identical) the degree of adsorption. It appears that starting at C8-C ~ homologues of aliphatie alcohols hindrances due to molecular dimensions of adsorbates become a limiting factor that determiues the ~lsorption.-This means that the structure of P E T P specimens extended 100~/o in an adsorption active medium and then crystallized in the medium will contain a large number of pores that are accessible to molecules of propanol and butanol and have effective cross sections of ~25-38 [10-12] and 27-41 ~2 [10] respectively. H o w e v e r for subsequent Ce-C10 homologues the amo~mt of pores accessible in these specimens will be significantly smaller. For instance, in the case of decanol, that has the most bulky, molecules, the adsorption will be at t h e lowest level. Thus crystallization of P E T P after extension of the polymer in an adsorption active medium results in a porous highly dispersed structure that has stable adsorption properties. As was found in earlier work [8] the structure of these materials may readily be controlled, which opens the way for the preparation of porous polymeric adsorbents that are capable of adsorption both from liquid phase and from gaseous phase. Tran~latext by R. J. A. :HEI~-DBY REFERENCES
1. A. L. VOLYNS~II and N. F. BAKEYEV, Vysokomol. soyed. AIT: No. 7, 1610, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 7, 1855, 1975) 2. A. G. A~LESKFA~OV,A. L. VOLYNSKII and N. F. BAKI~,YEV,Vysokomol. soyed. AIS: No. 9, 2121, 1976 (Translated in Polymer Soi. U.S.S.R. 18: 9, 2427, 1976)
Polymerization of organobi- and organotrieyclosiloxanes
1187
3[ U:S.S.R: Pat. 611436, 1978 4. A. L. VOLYNSKII, V. I. GERASIMOV and N. F. BAI[EYEV, Vysokomol. soyed. A17: No. 11, 2461, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 11, 2831, 1975) 5. V. L GERASIMOV and D. Ya. TSVANKIN, Pribory i tekhnika eksperimcnta, No. 2, 204, 1968 ..... 6. A. L. VOLYNSKH, V. S. LOGINOV, N. A. PLATE and N. F. BAKEYEV, Vysokomol. soyed. A22: No. 12, 2727, 1980 (Translated in Polymer Sci. U.S.S.R. $2: i2, 1980) 7. A. L. VOLYNSKII, A. G. ALESKEROV, T. Ye. GROKHOVSKAYA and N. F. B ~ E Y E V , Vysokomol. soyed. A18: No. 9, 2114, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 9, 2419, 1976) 8. A. L. VOLYNSKII, V. S. LOGINOV and N. F. BAKEYEV, Vysokomol. soyed. A23: No. 6, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 6, 1981) 9. S.S. VOYUTSKII, Kurs kolloidnoi khimii~ 2-e izd. (Kolloid Chemistry Co(~rse, 2nd ed.) 126 Kblmlya, Moscow, 1976, 10. V. E. VASSERBERG, A. A. B'ALANDIN and M. P. MAKSIMOVA, Zh. fiz. khimii 35: No. 9, 858, 1961 11. J. J. van VOORHIS, R. G. GRAIG and F. E. BARTELL, J. Phys. Chem. 61: No. 11, 1513, 1957 12. C. M. HOLLABAUGH and J. J. CHESSICK, J. Phys. Chem. 65: No. 7, 109, 1961
Polymer Science U.S.S.R. Vol. 23. No. 5, pp. 1187-1197,1081 Printed in Poland
0082--39501811051187-11507.5010 1982 PergamonPress Ltd.
CALORIMETRIC INVESTIGATION OF THI~ POLYMERIZATION OF ORGANOBI- AND ORGANOTRICYCLOSILOXANES I. I. DUBOVIK, N. N. M~AaOVA and G. L. SLONIMSKII I-Ietero-organic Compounds Institute, U.S.S.R. Academy of Sciences
(Received 13 February 1980) The anionic polymerization of organobi- and organotricyclosiloxanes containing organocyclotrisiloxane fragments has been investigated by a calorimetric method. The opening of organocyclotrisiloxane fragments makes the main contribution to the enthalpy change during the catalytic polymerization. The magnitude of the enthalpy change largely depends on the nature of the diorganosiloxane unit in the organocyclotrisiloxane fragment.
I~ THE ionic polymerization of organocyclosiloxane compounds any of the siloxane bonds forming part of organocyclosiloxanes may participate in the formatiorr of a transition complex with a nucleophilic agent, and may open with the result that linear oligomeric and polymeric organosiloxanes are formed. * Vysokomol. soyed. A23: No. 5, 1066-1074, 1981.
A . L . VOLYNSKIIet a~.
10. A. L KUZAYEV, S. D. KOLESNIKOVA and A. A. BRIKENSHTEIN, VysokomoL soyed. A17: No. 6, 1327, 1975 (Translated in Polymer Sei. U.S.S.R. 17: 6, 1524, 1975} 11. B. R. SMIRNOV, I. S. MOROZOVA, A. P. MARCHENKO, M. A. MARKEYEVICH, L. M. PUSHCHAYEVA and N. S. YENIKOLOPYAN, Dokl. AN SSSR, 254: No. 4, 891, 1980 12. C. BAMFORD, W. BARBE, A. JENKINS and P. ONION, Kinetika radikalnoi polimerizatsii vinflovykh soyedinenii (Kinetics of Radical Polymerization of Vinyl Compounds), p. 106, Foreigh Lit. Press, Moscow, 1961 13. A. Yu. SHAULOV, A. B. SHAPIRO, A. G. SKLAYROVA, A. M. WASSERMAN, A. Lo RUCHACHENKO and E. G. ROZANTSEV, Europ. Polyzn. J . 10: 1077, 1974 14. A. M. NORTH and G. A. REED, Trans. Faraday Soc. 57: 859, 1961 /
Polymer Science U.S.S.R. Vol. 23, No. 5, pp. 1178-1187,1981 Printed in Poland
0032-3950/8'1/051178-10507. 50/0 1982 PergamonPress Ltd.
INFLUENCE OF CRYSTALLIZATION ON THE STRUCTURE AND ADSORPTION PROPERTIES OF GLASSY POLYETHYLENE TEREPHTHALATE STRAINED IN AN ADSORPTION-ACTIVE MEDIUM* A. L. VOLYNSKII, V . ~ LoGI~ov and N. F. BAKEYEV M. V. Lomonosov State University, Moscow
(Received 12 February 1980) A study has been made of the influence of crystallization of glassy polyethylene terephthalate (PETP) stretched in an adsorption-active medium on its s~ructure and adsorption properties. I t was found that the annealing of strained PETP specimens in the same liquid medium results in stable highly porous materials whose structure does not depend on the drying process or on repeated humidification. I t is shown that these materials are capable of adsorbing iodine and an organic dye (Rhodamine S) from aqueous solutions. Major changes in the structure and adsorption behaviour of PETP are obtainable by varying the degree of prior drawing of the polymer in an adsorption active medium. PETP specimens prepared with crystallization of the polymer are capable of adsorbing considerable amounts of various substances from the gaseous phase as well. This makes it possible to obtain some additional data on the structure of the materials. ON PR~.vIous occasions [1, 2] we showed t h a t t h e cold d r a w i n g o f a p o l y m e r in a n a d s o r p t i o n - a c t i v e m e d i u m is a c c o m p a n i e d b y t r a n s i t i o n o f t h e p o l y m e r t o a new s t r u c t u r a l - p h y s i c a l state. T h i s ' s t a t e is c h a r a c t e r i z e d b y a new set o f p h y a i * Vysokomol. soyed. ASS: No. 5, ~1059-1065, 1981.
Structure and adsorption properties of glassy polyethylene terephthalate
1179
cal and physieochemical properties, and its main feature is t h a t the polymer has a highly developed boundary surface. This means t h a t these materials are porous adsorbents [3], whose structure m a y be varied at will. However, structural lability is a specific feature of adsorbents of this type. Certainly, drying of the polymeric porous specimens after, stretching in a n adsorption-active medium leads to major shrinkage of the latter resulting from coagulation of the highly dispersed s t r u c t u r e [1], whereas long-term retention in an adsorbate solution m a y bring about the reverse process, i.e. peptization ' o f structure previously coagulated [4]. I n the present investigation an a t t e m p t was made to prepare stable polymeric porous materials, and their properties were examined. The study object in the investigation was a commercial film of amorphous unoriented PETP of thickness 500-700/ml. Ttm adsorption of iodine and of an organic pigment (Rhodamine S} from water was investigated. The adsorption active medium in which PETP stretching was carried out was n-propyl alcohol. After crystallization via annealing in the same medium at 85° specimens that had been stretched in the adsorption active medium were placed in an adsorbate solution, and the adsorption was estimated from changes in the optical'density of the solution, using an FEK-M device• The time required to establish ~he equilibrium was determined from the data obtained from kinetic investigations bf adsorption. A solution of iodine in water was prepared, and KI was added in the ratio KI : Ii=20 : 1~ Specific adsorption values (g/g} were given by the formula v(c0-c) 1000 m '
~vhere co and c are the initial and equilibrium solution concentrations, g/l.; m is the weighed portion of adsorbent, g; V is the volume of the solution, ml. Low.angle X-ray diffraction patterns were obtained using the apparatus used in [5] in line with standard procedure; an ]~SM-2 electron microscope was used for the scanning electron microphotographs. All experiments were carried out at room temperature. Reagents used (without further puri~cation) in the investigation were all of chemically pure grade, or of analytical purity. One m a y surmise t h a t stability of the P E T P structure formed during stretching in an adsorption-active medium could result from subsequent crystallization ,of the polymer. To obtain P E T P having a stable porous structure one must, ~n the one hand, ensure crystallization of the polymer, and on the other h a n d preserve a highly developed interface. I n view of this the annealing of P E T P specimens was carried out directly in an adsorption active medium on the assumption t h a t precisely the stabilizing influence of the medium will make for preservation of t h e highly dispersed microcrack structure. Specimens strained in the adsorption~active medium were transferred to the same liquid, albeit heated to 85 °, in which the specimens were kept for 1 hr. I t was found t h a t materials obtained in this way suffer a complete loss of ability to change their dimensions during drying processes. I t m a y therefore be said t h a t the polymer .structure has acquired a sufficient level of stability as a result of previous crystallization. There are two ways in which crystallization under these conditions m a y
A. L. VOLX-NSXlXa a2.
1180
be carried out. One may transfer the strained (deformed) specimen to the heated adsorption-active medium, having first released the specimen from the clamps, or crystallization of the specimen may be carried out with itsdimensions fixed (in clamps). We investigated the structure and adsorption properties of P E T P specimens perpared by both methods. As a result of this it was found t h a t crystallization of P E T P specimens that have first been stretched in the liquid medium results in a polymer structure of a stable and highly porous type. Certainly, it was found by special tests that 1) a P E T P specimen crystallized through annealing in an adsorption activo medium suffers a complete loss of ability to shrink, and its adsorption properties remain constant during a prolonged period of retention in the same
D,%
CL
I
8o
x,,1o ,g/g qO
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,
200 200
qO0
0.8~
qO0
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o
200 Fro. 1
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Fro. 1. Degree of adsorption of iodine x by P E T P specimens from aqueous solutions of I= (equilibrium conc. 0"8 g]l.) (a) and a~lsorption of Rhodamine S (equilibrium cone. 0.04 gfl.) (b) vs. degree of extension of the polymer in an adsorption-active medium 2: 1--noncrystallized specimens; ~2--specimens crystallized in the free state; 3--with fixed dimensions. FIG. 2. Degree of desorption of iodine (a) and Rhodamine S (b) from P E T P specimens transferred to n-propanol after adsorption from water vs. degree of prior stretching o f P E T P in an adsorption-active medium: 1--noncrystallized specimens, 2--crystallized in the free state, 3--with fixed dimensions.
Structure and adsorption properties of glassy polyethylene terephthalate
118l~
liquid; 2) the adsorption properties of materials obtained in this way do not depend on the sequence of operations: immediately after crystallization (i.e. in the moist state) the specimen is transferred to the adsorbate solution, or t h e specimen is dried, and then placed dry in the adsorbate solution. In other words, processes of coagulation and of peptization of highly dispersed structure are not characteristic of these materials. Figure 1 shows plots of the adsorption of iodine and of the pigment Rhodamine S vs. the degree of prior stretching of the polymer for crystalline P E T P specimens." For comparison, the same Figures show the data for P E T P specimens transferred to the absorbate solution immediately after stretching in a liquid medium without subsequent crystallization [6]. It is clear from Fig. 1 that t h e materials obtained via crystallization in liquid media are efficient adsorbents, which means that a well developed boundary surface is preserved during crystallization. It is apparent that as in the case of the uncrystallized specimens the adsorption properties of the materials undergo a rather marked change in the range of deforn~tion corresponding to transition from a loose structure to a compact one, as has been described in some detail in [6, 7]. Crystallization of" the polymer makes this transition still more marked (Fig. 1 ). I t shouId be n o t e d that the conditions under which the crystallization of P E T P is being carried out does not significantly influence the adsorption properties of the resulting material. Irrespective of how crystallization of the specimens is carried out (in the free state or with fixed dimensions)the adsorption behaviour was practically identical, though it has been demonstrated in earlier work [4] that annealing: of the polymer in the free state in presence of a liquid medium is accompanied by a considerable amount of shrinkage. A slight reduction in adsorption (Fig. 1): evidences coagulation of some of the highly dispersed microcrack materia~ during annealing of the polymer in an adsorption-active medium. I t was found that the extent of this coagulation does not depend on conditions under which the polymer specimen is kept (in the free state or with fixed dimensions). Fixing t h e dimensions does not prevent coagulation of highly dispersed microcrack material, but simply alters certain morphological features of the material: Moreover it follows from the results of desorption investigations that definite structural regrouping takes place in the polymer during crystallization. Figure 2 shows the data on iodine and dye desorption in n-propanol. It is apparent that as in the case examined in [6], considerable desorption takes placein the range of low deformations, but this desorption is strongly inhibited in the case of large prior deformations of P E T P in liquid media. At the same time, however, the adsorptipn behaviour of crystallized P E T P specimens also exhibit a number of characteristic differences. It appears that the deformation range in which transition from a free to an inhibited desorption of adsorbates takes, place is displaced towards lower degrees of extension: in the case of Rhodamine S, from 150 to 100%, and in the case of iodine, from 30Oto 150~/o. Another characteristic difference appears in the fact that P E T P specimens that have beerL
i182
A.L. VOLYNSKIIet ~.
stretched to high degrees of preliminary extension in a liquid medium exhibit practically no desorption. This means that PETP pore sizes are reduced during •orystalhzation and become commensurate with iodine molecules, starting already as from 200-300% extensions. The number of large pores that are accessible to Rhodamine S is at t h e same time greatly reduced, starting at 100-150% extensions, as is clear from the results ofdtirect adsorption tests. The number o f pores t h a t are accessible to iodine molecules decreases only insignificantly (Fig. 2), but the pore size becomes a lot smaller. It should be noted that both iodine and Rhodamine S, in contrast to PETP specimens obtained on drying after stretching in an adsorption active medium :[8]readily pass through the surface layer of coagulated microcrack fibrils and reach the inner boundary surfaces. This is evident from the results of optical microscopy, according to which transverse sections of PETP specimens become coloured, after adsorption in solutions of iodine or Rhodamiae S, over the entire ~ross section. In view of these findings one may say that the structure of PETP specimens formed as a result of crystallization differs markedly from the Corresponding amorphous materials after removal of the liquid medium. Figure 3 shows the low angle diffraction patterns for P E T P specimens prepared during crystallization in the liquid after tlieir deformation amounting to 51, 150 and 3000/o extensions. Crystallization of the specimens was carried out in the free state (a,-~) and with fixed dimensions (d-f). It can readily be seen that with low degrees of extension (50-150~/o) crystallization does lead to the formation of a highly dispersed .structure featuring intense diffuse scattering, It is noteworthy that in contradistinction to the specimens subjected after stretching to drying in the adsorption active medium, t h e crystallized specimens give a practically symmetrical scattering patter~l. In other words, crystallization is accompanied by major structural deformatmn and by the preservation of well developed boundary surfaces. It is interesting that in the case of large (~300~)extensions in a liquid medium (Fig. 3c, f) orientation of structural fibrillar microcrack elements is largely preserved even after crystallization, irrespective of the conditions under which crystallization is conducted. It is also clear from these findings that compared with the specimens subjected after stretching to drying in a liquid medium the cryjstalline specimens must feature major dissimilarities in the :structure of the surface layer formed during coagulation of highly dispersed mierocrack material. Certainly, if the crystallized P E T P specimens were structurally similar to the dried ones [8] one would be unable to explain how adsorbate molecules could reach the inner interfaces in the microerack structure. As was ipointed out above the crystallized Sl~cimens do not cause peptization of co~g~tlated highly dispersed microcrack material, but, despite this, iodine and Rhodamine S molecules are able to get through to the surface located in internal parts of the specimen. This assumption is fully substantiated by the results of scanning electron
Structure and adsorption properties of g l a i r polyethylene terephthalate
1183
Fio. 3. Low-angle X . r a y t l i ~ i o n patterns of PETP drawn by: a, d--50%; b, e--150% and c,/--300°/o in n-propanol and crystallized in tile same medium at 85°C for 1 hr: a, b and c--specimens 'crystallized in the free condition; d, s and f--specimens crystallized with fixed dimensions; tensile axis of polymer vertical.
microscopy. Figure 4 shows the scanning electron photomicrographs for the P E T P specimen deformed with 150% extension in n-propanol and then crystallized in the latter at 85 °. It is apparent that the mierocrack structure in the crystallized specimen shows significant differences compared with the specimen dried after extension in the adsorption active medium. The microeracks which, in the initial PETP specimens [8] show quite distinct orientation relative to the drawing axis, suffer a loss of their orientation and are crooked. Microvacancies in the microcrack structure suffer a loss of asymmetry and orientation. The porotis structure of these materials resembles that of active carbon characterized by a large multitude of mierovacancies of varying shape and size with a random arrangement of one to another (Fig. 4a). Interesting results were obtained from a study of the external surfaces of crystallized PETP specimens. I t is seen from Fig. 4b, c that the structure of the surface layers of microeracks
1.184
A. L. VOLY~S~II ~ .l.
contains a large number of apertures measuring from 2 to 10/~m, whereby interna! vacancies are in communication with environm'ent~l spa~e. The origin of these apertures is suggested by some of their morphological features. I t is seen from Fig. 4c, that the apertures are not unlike craters made by explosions, e.g. from their external appearance one could suppose that they are the result of the action of a force applied to a specimen from within, as evidenced by the characteristic bulging around the rims of the apertures. The following mechanism for formation of these apertures appears feasible. When P E T P is stretched in the liquid the latter covers microvacancies of propagating mierocracks. After the P E T P specimen has been transferred to the heated adsorption active medium, processes of coagulation and crystallization of the microcraek material take place a n d are accompanied by significant thermal expansion of the liquid fillln~ up microcracks in the microcrack structure.
Fio. 4. Scanning electron photomicrographs of a PETP specimen with 100% extension in n-propanol and then annealed in n-pro~ano~ at 85° for 1 hr: a, b--viewed from the fracture surface; c--exte ~ rnal surface of specimen. Stresses developing in this case in the specimen are so great as to cause bre'akdown in individual parts of the microcrack structure, which results in the formation of apertures whereby internal microvaeancies of the latter are connected wit h surrounding space. Thus it appears from these, results that interaction between polymer and the adsorption active liquid is not limited solely to facilitating the development
Struoture and adsorption properties 0 ~
~ e n e
terephthalate
1185
of mlcrocracks having a specific structure. The adsorption active medium protects the highly dispersed polymer structure from coalescence dm~ug crystallization, and also leads to the formation of.open-porous material through effects relating to thermal expansion of the liquid (filling microcrack vacancies) during t h e heating process.
x,g/g x, lO,g/g
0"2
_
2.O
I'6
0"1
0"8 200
F~o. 5
I./00 ~,%
L
I
I
50
100
M
F1o. 6
FIO. 5. Degree of adsorption of methanol (1), n-jpropanol (2), n-hexanol (3), n-decanol (4) and water (5) by PETP specimens deformed in an adsorption-active medium to different degrees of extension 2, from saturated vapours at room temperature. FIo. 6. Amount of adsorbed normal aliphatic aloohols x vs. MW of the latter. Adsorption by crystallized PETP specimens stretched 100% in an adsorption-active medium. Since an open-porous polymeric material is formed as a result of crystallit zation of P E T P specimens deformed in liquid media one m a y surmise that this t y p e of material must equally be efficient as an adsorber from the gas phase. We accordingly used the simplest method for this t y p e of investigation. The P E T P specimens were placed under a glass c o v e r at room temperature; under the cover was an open small glass conta'ming the adsorbate. The amount o f substance ~dsorbed after establishment of the equilibrium was determined gravimetrically. Actually we found in each ease only a single point on the a d sorption isotherm, i.e. a point corresponding to the saturated vapour pressure at room temperature.* Figure 5 shows the data obtained b y the above method for a number of" aliphatic alcohols and for water. I t is seen that the P E T P specimens w h e n crystallized are able to adsorb considerable amounts of substances from t h e gas phase. For instance, the P E T P specimen stretched in an adsorption-active medium and then crystallized are capable of adsorbing more than 26 wt. % of n-propanol. The adsorption process i~in this case a reversible one, i.e. P E T P * It was found by cheek tests that this method could not be used to determine the amount of substance adsorbed by unstretched PETP specimens or by specimens stretchec[ in air with development of "necking".
1186
A. L. V O L Y S S X n
e$ a/.
specimens placed in air after adsorption suffer a practically complete loss:vf the adsorbed substance in the course of 2 days. It is noteworthy that the amount of alcohols adsorbed goes through a maxi~ m u m in the regior~ of 100-150°//o extensions of the polymer, i.e. at the point where a transition from a loose structure to a compact one was observed b y other methods [6, 7]. Let us now turn to features of adsorption by the study objects in a number of aliphatic alcohols. Figure 6 shows plots of the amount of alcohol adsorbed by crystallized P E T P specimens that had first been stretched to 100~/o extension (the area of maximal adsorption in Fig. 5) vs. MW of the adsorbates being used. I t can readily be seen that adsorption is most marked for the alcohols containing three or four methylene groups. The amount of substance adsorbed is determined for the most part by two factors. Firstly, by the intensity of intermolecular interaction of adsorbate molecules and the molecular surface layer of polymer. This increases in the series of aliphatic alcohols, whose surface activity, as is known from [9], increases with increasing length of the hydrocarbon radical, Secondly, the amount of substance adsorbed is determined by the accessibility of inner surfaces to adsorbate molecules, i.e. by the ratio of pore sizes and molecular dimensions of the adsorbate. It is obvious that the larger the molecular dimensions bf the adsorbate, the larger will be the number of small pores inaccessible to the adsorbate and the lower (other conditions being identical) the degree of adsorption. It appears that starting at C8-C ~ homologues of aliphatie alcohols hindrances due to molecular dimensions of adsorbates become a limiting factor that determiues the ~lsorption.-This means that the structure of P E T P specimens extended 100~/o in an adsorption active medium and then crystallized in the medium will contain a large number of pores that are accessible to molecules of propanol and butanol and have effective cross sections of ~25-38 [10-12] and 27-41 ~2 [10] respectively. H o w e v e r for subsequent Ce-C10 homologues the amo~mt of pores accessible in these specimens will be significantly smaller. For instance, in the case of decanol, that has the most bulky, molecules, the adsorption will be at t h e lowest level. Thus crystallization of P E T P after extension of the polymer in an adsorption active medium results in a porous highly dispersed structure that has stable adsorption properties. As was found in earlier work [8] the structure of these materials may readily be controlled, which opens the way for the preparation of porous polymeric adsorbents that are capable of adsorption both from liquid phase and from gaseous phase. Tran~latext by R. J. A. :HEI~-DBY REFERENCES
1. A. L. VOLYNS~II and N. F. BAKEYEV, Vysokomol. soyed. AIT: No. 7, 1610, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 7, 1855, 1975) 2. A. G. A~LESKFA~OV,A. L. VOLYNSKII and N. F. BAKI~,YEV,Vysokomol. soyed. AIS: No. 9, 2121, 1976 (Translated in Polymer Soi. U.S.S.R. 18: 9, 2427, 1976)
Polymerization of organobi- and organotrieyclosiloxanes
1187
3[ U:S.S.R: Pat. 611436, 1978 4. A. L. VOLYNSKII, V. I. GERASIMOV and N. F. BAI[EYEV, Vysokomol. soyed. A17: No. 11, 2461, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 11, 2831, 1975) 5. V. L GERASIMOV and D. Ya. TSVANKIN, Pribory i tekhnika eksperimcnta, No. 2, 204, 1968 ..... 6. A. L. VOLYNSKH, V. S. LOGINOV, N. A. PLATE and N. F. BAKEYEV, Vysokomol. soyed. A22: No. 12, 2727, 1980 (Translated in Polymer Sci. U.S.S.R. $2: i2, 1980) 7. A. L. VOLYNSKII, A. G. ALESKEROV, T. Ye. GROKHOVSKAYA and N. F. B ~ E Y E V , Vysokomol. soyed. A18: No. 9, 2114, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 9, 2419, 1976) 8. A. L. VOLYNSKII, V. S. LOGINOV and N. F. BAKEYEV, Vysokomol. soyed. A23: No. 6, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 6, 1981) 9. S.S. VOYUTSKII, Kurs kolloidnoi khimii~ 2-e izd. (Kolloid Chemistry Co(~rse, 2nd ed.) 126 Kblmlya, Moscow, 1976, 10. V. E. VASSERBERG, A. A. B'ALANDIN and M. P. MAKSIMOVA, Zh. fiz. khimii 35: No. 9, 858, 1961 11. J. J. van VOORHIS, R. G. GRAIG and F. E. BARTELL, J. Phys. Chem. 61: No. 11, 1513, 1957 12. C. M. HOLLABAUGH and J. J. CHESSICK, J. Phys. Chem. 65: No. 7, 109, 1961
Polymer Science U.S.S.R. Vol. 23. No. 5, pp. 1187-1197,1081 Printed in Poland
0082--39501811051187-11507.5010 1982 PergamonPress Ltd.
CALORIMETRIC INVESTIGATION OF THI~ POLYMERIZATION OF ORGANOBI- AND ORGANOTRICYCLOSILOXANES I. I. DUBOVIK, N. N. M~AaOVA and G. L. SLONIMSKII I-Ietero-organic Compounds Institute, U.S.S.R. Academy of Sciences
(Received 13 February 1980) The anionic polymerization of organobi- and organotricyclosiloxanes containing organocyclotrisiloxane fragments has been investigated by a calorimetric method. The opening of organocyclotrisiloxane fragments makes the main contribution to the enthalpy change during the catalytic polymerization. The magnitude of the enthalpy change largely depends on the nature of the diorganosiloxane unit in the organocyclotrisiloxane fragment.
I~ THE ionic polymerization of organocyclosiloxane compounds any of the siloxane bonds forming part of organocyclosiloxanes may participate in the formatiorr of a transition complex with a nucleophilic agent, and may open with the result that linear oligomeric and polymeric organosiloxanes are formed. * Vysokomol. soyed. A23: No. 5, 1066-1074, 1981.