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Relation between the crystalline structure and molecular weight of polyethylene adipate
1596
V. I. KOVAI~ENKOet al.
3. L. A. IGONIN, M. M. MIRAKHMEDOV, K. I. TURCHANINOVA and A. N. SHABADASH,
I)okl. AN SSSR 141: 1366, 1968 4. S. R. RAFIKOV, I. V. ZHURAVLEVA, R. S. AYUPOVA, S. A. PAVLOVA, P. M. VALETSKII, A. I. KALACHEV, S. V. VINOGRADOVA, V. I. STANKO and V. V. KORSHAK, Dokl. AN SSSR 207: 1133, 1972 5. L. I. ZAKHARKIN, V. I. STANKO, V. A. BRATTSEV and Yu. A. CHAPOVSKII, I)okl. AN SSSR 157: 1149, 1964 6. O. S. SAVEL'YEV, L. G. SHEVCHUK and N. A. VYSOTSKAYA, Zh. organ, khimii 7: 283, 1972 7. V. I. STANKO, V. A. BRATTSEV, N. N. OVSYANNIKOVA and T. P. KLIMOVA, Zh. obshch, khimii 44: 2482, 1974 8. P. I. SHABOLDO, V. A. PROSKURYAKOV, V. M. POTEKHIN and B. N. SUKHAREV, Zh. prikl, khimii 45: 1563, 1972
RELATION BETWEEN THE CRYSTALLINE STRUCTURE AND MOLECULAR WEIGHT OF POLYETHYLENE ADIPATE* V. I. KOVALENKO, N. P. ANOSltINA, N. A. PALIKHOV, A. A. ]~UKHUTDINOV a n d B. YA. TEITEL'BAUM A. Ye. Arbuzov Institute of Organic and Physical Chemistry, U.S.S.R. Academy of Sciences S. M. Kirov Chemico-Technological Institute, Kazan (Received 24 N o v e m b e r
1975)
Complex studies were made of the crystalline state of several polyethylene adipate samples with l~ln ~ 300-4000. It was shown that they are all characterized by the same crystalline modifications as the preparation previously obtained w i t h i n = 2000. However, the temperature limits of the range of forming an a or fl modification vary according to the value of ~ . At the same time there is no direct relation between the type of spherulitic structure and the phase condition of the polymer. For preparations with minimal values of ~ the thermodynamic stability of modifications is inverted as a result of the formation of highly damaged structures in low temperature crystallization. POLYETHYLENE adipate (PEA) is one o f crystallizing polymers capable of existence in several p o l y m o r p h o u s modifications. Two crystalline forms are k n o w n at pres e n t [1, 2]. I t was shown for P E A o f M W ~ 2 0 0 0 [3] t h a t this oligomer forms a n f o r m on crystallizing f r o m melt at a t e m p e r a t u r e higher t h a n 40 °, while below 20 °, it forms a fl form. The coexistence of b o t h forms was n o t e d for samples crystallized in t h e i n t e r m e d i a t e t e m p e r a t u r e range. Accordingly, various spherulitic s t r u c t u r e s were observed, n a m e l y "needle", radial a n d a n n u l a r types. * Vysokomol. soyed. A18: No. 6, 1392-1398, 1976.
Polyethylene adipate
1597
Up to now, however, it has not been known whether these temperature ranges are typical of the formation of polymorphous modifications of PEA, independent o f M W , j u s t a s t h e c i r c u m s t a n c e s a r e n o t k n o w n w h e n t h e r e is d e f i n i t e c o r r e l a t i o n between crystalline form and morphological formation. Further, no information is a v a i l a b l e a b o u t t h e r e l a t i o n b e t w e e n p o l y m o r p h i s m a n d M W a n d t h e c o r r e l a t i o n b e t w e e n c r y s t a l l i n e f o r m s a n d p o l y m e r m o r p h o l o g y . N e v e r t h e l e s s , a priori t h e p r o p e r t i e s d e s c r i b e d f o r P E A - 2 0 0 0 a r e s o m e t i m e s f o u n d in P E A o f o t h e r M W [4]. To explain the relation between the t y p e of polymorphous modifications a n d morphological structures a n d the length of the polymer chain, we studied* a series of P E A of different MW in the range of 300-4000, obtained and described according to a previous description [5]. I R spectroscopy, DTA, optical microscopy and thermo-optical analysis were used; X - r a y diffraction was studied in m a n y cases. Ii~ spectra a n d X - r a y photographs were recorded a t given temperatures of crystallization using thermostatic cells. Bearing in mind the high crystallizing properties of P E A , all procudures concerning the preparation of samples were carried out so as to avoid, i f possible, or minimize p r e m a t u r e crystallization. I R spectra of P E A samples in the form of thin films between K B r plates a n d brass foil films were obtained in a UR-10 spectrophotometer. Each sample was melted at 100 ° for 10 min, followed by rapid cooling to c r y s t a l lization temperature. The non-isothermal period for samples on foil was less t h a n 15 sec, whereas for samples between K B r plates it was ~ 1 min. I t is important, however, to n o t e t h a t results of b o t h methods agree completely. W h e n studying amorphous samples, spectra were recorded over the t e m p e r a t u r e range o f --150-100 °, i.e. sequentially for all phase-aggregate states of P E A . The samples were quenched directly in a VLT-2 cell produced b y R I I C (England). Transition from 100 Q (melt temperature) to -- 196 ° was completed in 2 rain. Absence of erystallinity from these vitrified samples was verified b y I R spectra. The cell was then gradually heated while scanning the spectrum from time to time until erystallinity bands appeared on the background of the spectrum of amorphous P E A and then, until they disappgarod completely. Diffraction curves were obtained using a URS-50I X - r a y diffractometer with C u K , radiation. D T A curves were recorded in a device using a PDS-021 double crossbar t y p e self re. cording instrument and 1-37 amplifier. The sample weighed 20 mg and the rate of heating was 2-5 dog/rain. The P E A melt obtained b y keeping a micro-test tube containing the sample in an oily base at 100 ° for 10 rain, was rapidly quenched b y immersing the micro-test t u b e in liquid nitrogen (to obtain amorphous samples), or transferred to a t h e r m o s t a t for crystallization, where it was k e p t at given t e m p e r a t u r e for a period of time sufficient for completing the process (ranging from 10 rain to m a n y hours, according to temperature and MW). Polarization microscopic studies were carried out using an MBI-6 universal microscope equipped with a heating stand. The intensity of depolarized light was recorded according to t e m p e r a t u r e (thermo-optical curves) b y the methods described [6]. The rate of heating was ~ 4 deg/min). The microphotographs obtained of spherulitie structures are basically similar to structures already well known [3, 7] a n d not described here. E a r l i e r i n v e s t i g a t i o n s h o w e d [3] t h a t e a c h c r y s t a l l i n e m o d i f i c a t i o n o f P E A - 2 0 0 { ) is c h a r a c t e r i z e d b y a s e t r e f l e x e s i n t h e X - r a y d i f f r a c t o g r a m s a n d c r y s t a l l i n i t y bands in IR spectra (Table). If we assume that packing of PEA chains in each * W i t h the help of T. O. K u r a s h e v a y a a n d G. G. Stepanova.
V. I. KOVALENKOet
1598
al.
crystalline form is independent of MW, it m a y be expected that reflexes are also retained for P E A studied. We carried out investigations with all samples crystallized from melt at 41 °, in parallel with methods of Ii~ spectroscopy and radiography. It was found t h a t the positions of reflexes in diffraction curves, in fact, coincide with those observed for PEA-2000. X - R A Y A N D SPECTROSCOPIC S I G N S OF POLY1VIORPHOUS IVfODIFICATIONS
OF PEA Crystalline Reflexes form 2 0° 21 "6 24.7 20.5 21.6 24.7
Bands of erystallinity, em -1 588, 698, 735, 747, 793, 903, 907, 914, 985, 1285, 1463 700, 704, 735, 747, 835, 842, 862, 901, 914, 942, 961, 1259, 1280, 1438, 1462, 1467
However, X-ray information (Fig. 1) indicates that PEA-611, 970 and 1500 were crystallized at a given temperature, the same w a y as PEA-2000 crystallized in the a-form, since the peak 20=20-5 ° inherent for the fl-form was absent from the X-ray photograph. Bands of crystallinity of the a form were observed in I R spectra of these samples at 985, 903, 793 cm -I, respectively (Fig. 2). At the same time diffractograms of PEA-2500 and 3500 showed three reflexes, typical of the ,6-form and I R spectra of these samples had another set of erystallinity b a n d s compared with P E A of lower MW, particularly the band of the fl-form at 862 and 842 em -1. Very weak bands of an impurity of another modification were sometimes observed in the spectrum of one modification, which m a y have been due to polydispersion of P E A samples [5]. A spectroscopic study similar to that carried out with P E A of different molecnlax weights crystallized at 41 ° was also carried out with samples crystallized from melt at 11 and 27 °, at temperatures which are typical of the formation of certain forms of PEA-2000. The phase composition, results of low temperature crs~stallization and studying glass transition and melting b y methods of DTA and polarization microscopy enabled a specific phase diagram of P E A to be plotted (Fig. 3). To define more accurately boundaries of fields of crystallization, several samples were further examined, which had been crystallized at temperatures other than those mentioned. Corresponding points are also indicated in the Figure. The diagram shows that temperature limits of the formation of a given modification vary considerably according to MW. In fact., if no fl-form is produced for PEA-300 b y crystallization at any of the temperatures studied, for PEA-3500 this form is observed over an extensive range of temperature. As MW increases the upper limit of crystallization of the fl-form converges with the curve of melting PEA. With MW of over 4000 practically only the fl-modification crystallizes from melt.
Polyethylene adipate
159~
Figure 3 shows t h a t there is a broad range where both crystalline forms coexist. As shown previously [3], differences in macromolecular packing are due t o the difference in conformation of glycol fragments. A slight difference in energies: 1o(]
I00
~J
~
1o 2'q
15
'
11
b
5
laxlO-3 Crr7-1
?/g 20 °
Fro. 1
-/
-'-
Fro. 2
FIG. 1. X-ray diffraction curves of PEA samples crystallized from melt at 41° with MW= 600 (1); 970 (2); 1500 (3); 2500 (d) and 3500 (5). Fro. 2. IR spectra of the same samples as those in Fig. 1. of both forms predetermines the possibility of coexistence of crystallites over a significant range o5 MW and temperature. The course of melting P E A of different MW should be determined by concrete temperature conditions of crystal formation, which give both the internal geometry and the degree of damage. The latter also depends, of course, on the molecular weight of the sample. At the same temperatures more perfect crystals may be melted by low temperature modification and less perfect crystals by high temperature modification. This hinders the evaluation of phase composition according to results of thermal ~nalysis [8]. Figure 4 shows some DTA curves obtained for PEA samples crystallized from melt by keeping them for a considerable time at 10, 27 or 41 °. During cooling samples before DTA additional crystallization takes place frequently. This may be
V. I. KOVALENKOe t a l .
1600
seen, for example, on curves for P E A - 3 0 0 ; some e n d o t h e r m i c peaks are o b s e r v e d a t a t e m p e r a t u r e lower t h a n those a t which t h e r m o s t a t i c control was carried ,out (Fig. 4b). H o w e v e r , crystallization itself u n d e r conditions of t h e r m o s t a t i c T,° G
o
I~//o
•
•
EZ
•
C
b !
/ / ol =2 ~3
-20
0 0
0
i
1
I
2
FIG. 3
t
3
i
4
lg
I
30 ZO
~0
qO
60
Fro. 4
:FIG. 3. Isobaric "phase diagram" of PEA according to MW: 1--=-; 2--p-form; 3--=+B. The shaded part shows formation of sphenflites of annular morphology; the broken line shows tentative boundaries of a region in which partial =--;8 transition was observed for samples obtained in low temperature crystallization: Z--melt; II--=-; III--B-modification; IV--glass. FIG. 4. ]:)TA curves of PEA crystallized from melt under conditions of thermostatic control a t 10 (a), 27 (b) and 41 ° (c); MW~4000 (•); 2500 (2); 1500 (3); 970 (4) 600 (5) and 300 (6). ~ontrol is b y no m e a n s i s o t h e r m a l (because of t h e liberation of t h e h e a t o f crystallization), corresponding t o a given t e m p e r a t u r e [9]. * I t is easy to see t h a t t h e f o r m o f D T A curves d e p e n d s b o t h on t e m p e r a t u r e a n d on MW; it reflects to some ext e n t t h e properties o f crystallization on the one h a n d a n d r e c r y s t a M z a t i o n proeesses, on the o t h e r [8]. • It should be emphasized hero that in Fig. 3 too, nominal temperatures of the super~ooled melt are plotted on the ordinate and not the actual temperatures which are difficult "to determine for melts with crystals growing in them.
Polyethylene adipate
1601
E x p e r i m e n t s with a m o r p h o u s (quenched) samples are distinguished b y specific t h e r m a l conditions, crystallization a n d melting of these samples d e p e n d i n g only on the rate of heating during DTA. Curves p l o t t e d for P E A of different M W
,, J^k__.,, \ r
I
-60
I
I
-2I)
I
I ___.I___2.
ZO
00 T,'P,
FIG. 5. DTA curves of amorphous PEA samples with MW=4000 (1); 3500 (2); 300 (3); 2500 (4); 2100 (5); 1500 (6); 970 (7); 600 (8) and 300 (9). are generally similar (Fig. 5). T h e y show: an o r d i n a r y d i s c o n t i n u i t y in the glass t r a n s i t i o n range, two e x o t h e r m i c peaks (high and low) a n d a n e n d o t h e r m i e p e a k (with low ~ W - - m u l t i p l e t ) of melting. The low e x o t h e r m i c p e a k before melting Tmelt,°C m
qO
0
FIG. 6
/40
50
~
z/O
/
30
I o
~
I i
I
I 2
I
0---0---03
I 3
I
I
¢ /~n x l O -3
Fro. 7
FIG. 6. Relation between a- (1) and fl-forms (2) and temperature for low molecular weight PEA crystallized from the hardened state in a-form (layout) (m--arbitrary units). Fie. 7. Relation between melting points of PEA samples and MW, according to results of thcrmo-optical investigations. Crystallization from melt took place at 27 (1); 41 (2)and 10° (3).
1602
V. I. KOV~E~KO
et al.
is due to thermokinetie features of low temperature crystallization with a corresponding high exothermic peak [10]. It is typical that all peaks are regularly displaced to the direction of low temperatures as MW decreases, starting from 1500. A reduction in the peak of melting points to reduction of crystallinity, which may be due to an increase in macromolecules of the relative number of end groups that should be regarded as defects in the polymer crystal. For P E A of very low MW there is a new feature: numerous peaks are observed in the range of melting. It should be noted that the form of these curves is strictly reproducible, i.e. is not due to chance. It is possible that for PEA-300 and 600, MW of which is already commensurate with MW of the monomer, we have a mixture of a small number of individual homologues, which may form individual phases and eutectics. This m a y be the cause of the complication in the curves examined. In order to study polymorphous transitions in P E A samples obtained by low temperature crystallization, a method was used of recording Ii~ spectra in the heated cell. Observations confirm that with an increase of temperature above the values specified an irreversible transition of the fl-form to the ~-form takes place, as described previously (as for samples crystallized during thermostatic control) [3, ll]. However, the behaviour of P E A with the lowest MW (300 and 600) appeared to be unexpected. Low temperature crystallization during heating hardened glass only becomes noticeable at about --45 °. Crystallinity bands appear at 985, 902, 793 cm -1 for only the ~-form. Keeping the samples for 1 hr at this temperature completed crystallization which could be seen from the discontinuation of the intensity increase of the bands mentioned (samples of all other MW crystallize in the fl-form under similar conditions). Heating PEA-300 and 600 crystallized at low temperatures to --5 ° results in the formation and a marked increase in the intensity of bands in the ]?-form, e.g. at 862 cm -1, as well as in some reduction in the intensity of bands of the ~-form. At 20 ° the ]?-form disappears and up to melting crystallinity bands of the ~-form are only retained in the spectrum. A qualitative system of the variation of crystalline phase content in these P E A samples is shown in Fig. 6. It may be assumed that around --5 ° imperfect crystals of ~-modification begin to melt and recrystallization takes place with the formation of/?-modification, but the stability range of this modification for PEA-300 and 600 is low and on increasing temperature, the newly formed crystals melt, partially changing again to the ~-form which is represented this time b y crystMlites more clearly defined than the initial ones. ~/[elting is complete at ~ 40 °. Differences in free energy due to variable numbers of defects in crystallites m a y exceed the differences due to different modifications. It is precisely because of this, as we can see, that the less clearly defined crystals of the modification with a higher temperature of thermodynamic equilibrium (a) melt at a tempera-
Polyethylene adipate
1603
ture which is lower than melting points of crystals with a lower equilibrium temperature (fl) which crystallize with fewer defects. A similar condition is know~ as the inversion of thermodynamic stability and was observed for polybut-l-ene 112] and, apparently, for polyurethane prepared from di-n-butylene glycol and hexamethylenedi-isocyanate [13]. The fact that melting point depends both on the type of modification and the condition of the structure expressed in dimensions of crystalline ranges inside a given phase, was also shown recently when examining P V D P [14]. All the transitions noted in P E A take place without change in the morphological structure which is determined exclusively by conditions of initial crystallization. Observations made using a polarization microscope show that the distribution described of the temperature range of crystallization of P E A according to types of spherulitie structures [3] is maintained for all MW (only with some displacement of temperatures in the direction of reduction for polymers of lower MW (Fig. 3)) and is evidently not related directly to the phase condition of crystallites. Thus, chain packing in the crystallite is independent of other factors on the one hand emd the organization of crystallites to spherulite, on the other. In the light of the foregoing the problem concerning the phase structure of annular spherulites may be clarified. The mechanism of extension is discussed in connection with the study of PEA-2000 [15]: In spherulite layers alternate of lower and higher real temperature of formation. For P E A of MW indicated fl and ~-modifications correspond to these temperatures, however, it follows from the mechanism examined that this difference in modifications of alternating layers is b y no means necessary. Results of this investigation show that with lower MW annular spherulites are obtained which are only of ~-modification and with high MW, of fl-modification. The region of forming annular spherulites in Fig. 3 is shaded. It may be continued on the same level in the direction of higher MW. Even in PEA-9900 these spherulites formed practically within the same temperature limits [7]. Figure 3 shows that these limits and the temperature range of existence of two phases Mmost coincide for P E A of average MW, which explains the result obtained with PEA-2000 which, as is now clear, is only partially significant. We note that in the diagram shown (Fig. 3) in the region above the shaded part up to the line of melting P E A crystallizes in the form of "needle t y p e " spherulites and below the shaded part, in the form of radical spherulites. Using a polarization microscope it was possible not only to observe the morphology of crystalline formation but also to evaluate the rate of spherulitic growth. It appeared that at appropriate temperatures with a reduction of MW, this rate decreased. Thermo-optical curves enabled the point of disappearance of the last crystals to be determined, which is normally the melting point (Fig. 7). The value in question increased with MW, b u t with a MW > 2000 became almost unchanged. Melting point depended on the temperature at which the sample crystallized since this is generally typical of polymer crystals. It seems nevertheless surprising at
1604
V . I . KOVALENKOet al.
first sight t h a t the melting p o i n t of crystals o b t a i n e d at 27 ° was higher t h a n t h a t o f crystals o b t a i n e d a t 41 °. This is, however, because intensive recrystallization t o o k place during melting samples crystallized at 27 °. J u d g i n g b y t h e r m o - o p t i c a l curves characterized b y an S shape, as with those for P E A - 2 0 0 0 [6], this process t a k e s place a t the highest rate 8-12 ° before the end of melting. Thus, at the melting point crystals n o t f o r m e d primarily, b u t f o r m e d as a result of recrystallization, disappear. I n Fig. 3 the curve of melting corresponds to the highest Tme,t value obtained.
Translated by E. SEMERE REFERENCES
1. C. S. FULLER and C. J. FROSH, J. Phys. Chem. 46: 323, 1939 2. A. TURNER-JONES and C. W. BUNN, Aeta erystallogr. 15: 105, 1962 3. B. Ya. TEITEL'BAUM, N. A. PALIKHOV, L. I. MAKLAKOV, N. P. ANOSHINA, I. O. MURTAZINA and V. I. KOVALENKO, Vysokomol. soyed. A9: 1672, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 8, 1880, 1967) 4. P. SCHUSTER, Thermochim. Aeta 3: 487, 1972 5. V. S. MINKIN, A. A. MUKHUTDINOV, V. I. YASTREBOV and P. A. KIRPICHNIKOV, Vysokomol. soyed. B16: 162, 1974 (Not translated in Polymer Sei. U.S.S.R.) 6. B. ¥a. TEITEL'BAUM and N. A. PALIKHOV, Vysokomol. soyed. A1O: 1468, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 7, 1699, 1968) 7. M. TAKAYANAGI and T. YAMASHITA, J. Polymer Sei. 22: 552, 1956 8. B. Ya. TEITEL'BAUM, J. Therm. Anal. 8: 511, 1975 9. B. Ya. TEITEL'BAUM, Vysokomol. soyed. B16: 763, 1974 (Not translated in Polymer Sci. U.S.S.R.) 10. B. Ya. TEITEL'BAUM, Dokl. AN SSSR 222: 1115, 1975 11. B. ¥a. TEITEL'BAUM, V. I. KOVALENKO and N. A. PALIKHOV, Sb. Sintez i fizikokhimiya polimerov (Synthesis and Physico-Chemistry of Polymers). No. 9, Kiev, 1971 12. F. LANUSSO, Uspekhi khimii 39: 304, 1970 13. V. N. VATULEV and S. V. LAPTII, Vysokomol. soyed. BI3: 475, 1971 (Not translated in Polymer Sci. U.S.S.R.) 14. G. M. BARTENEV, A. A. REMIZOVA, I. V. KULESHOV, M. A. MARTYNOV and T. N. SARMINSKAYA, Vysokomol. soyed. A17: 2063, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2381, 1975) 15. B. Ya. TEITEL'BAUM and N. A. PALIKHOV, Vysokomol. soyed. BI2: 3, 1970 (Not translated in Polymer Sei. U.S.S.R.)
V. I. KOVAI~ENKOet al.
3. L. A. IGONIN, M. M. MIRAKHMEDOV, K. I. TURCHANINOVA and A. N. SHABADASH,
I)okl. AN SSSR 141: 1366, 1968 4. S. R. RAFIKOV, I. V. ZHURAVLEVA, R. S. AYUPOVA, S. A. PAVLOVA, P. M. VALETSKII, A. I. KALACHEV, S. V. VINOGRADOVA, V. I. STANKO and V. V. KORSHAK, Dokl. AN SSSR 207: 1133, 1972 5. L. I. ZAKHARKIN, V. I. STANKO, V. A. BRATTSEV and Yu. A. CHAPOVSKII, I)okl. AN SSSR 157: 1149, 1964 6. O. S. SAVEL'YEV, L. G. SHEVCHUK and N. A. VYSOTSKAYA, Zh. organ, khimii 7: 283, 1972 7. V. I. STANKO, V. A. BRATTSEV, N. N. OVSYANNIKOVA and T. P. KLIMOVA, Zh. obshch, khimii 44: 2482, 1974 8. P. I. SHABOLDO, V. A. PROSKURYAKOV, V. M. POTEKHIN and B. N. SUKHAREV, Zh. prikl, khimii 45: 1563, 1972
RELATION BETWEEN THE CRYSTALLINE STRUCTURE AND MOLECULAR WEIGHT OF POLYETHYLENE ADIPATE* V. I. KOVALENKO, N. P. ANOSltINA, N. A. PALIKHOV, A. A. ]~UKHUTDINOV a n d B. YA. TEITEL'BAUM A. Ye. Arbuzov Institute of Organic and Physical Chemistry, U.S.S.R. Academy of Sciences S. M. Kirov Chemico-Technological Institute, Kazan (Received 24 N o v e m b e r
1975)
Complex studies were made of the crystalline state of several polyethylene adipate samples with l~ln ~ 300-4000. It was shown that they are all characterized by the same crystalline modifications as the preparation previously obtained w i t h i n = 2000. However, the temperature limits of the range of forming an a or fl modification vary according to the value of ~ . At the same time there is no direct relation between the type of spherulitic structure and the phase condition of the polymer. For preparations with minimal values of ~ the thermodynamic stability of modifications is inverted as a result of the formation of highly damaged structures in low temperature crystallization. POLYETHYLENE adipate (PEA) is one o f crystallizing polymers capable of existence in several p o l y m o r p h o u s modifications. Two crystalline forms are k n o w n at pres e n t [1, 2]. I t was shown for P E A o f M W ~ 2 0 0 0 [3] t h a t this oligomer forms a n f o r m on crystallizing f r o m melt at a t e m p e r a t u r e higher t h a n 40 °, while below 20 °, it forms a fl form. The coexistence of b o t h forms was n o t e d for samples crystallized in t h e i n t e r m e d i a t e t e m p e r a t u r e range. Accordingly, various spherulitic s t r u c t u r e s were observed, n a m e l y "needle", radial a n d a n n u l a r types. * Vysokomol. soyed. A18: No. 6, 1392-1398, 1976.
Polyethylene adipate
1597
Up to now, however, it has not been known whether these temperature ranges are typical of the formation of polymorphous modifications of PEA, independent o f M W , j u s t a s t h e c i r c u m s t a n c e s a r e n o t k n o w n w h e n t h e r e is d e f i n i t e c o r r e l a t i o n between crystalline form and morphological formation. Further, no information is a v a i l a b l e a b o u t t h e r e l a t i o n b e t w e e n p o l y m o r p h i s m a n d M W a n d t h e c o r r e l a t i o n b e t w e e n c r y s t a l l i n e f o r m s a n d p o l y m e r m o r p h o l o g y . N e v e r t h e l e s s , a priori t h e p r o p e r t i e s d e s c r i b e d f o r P E A - 2 0 0 0 a r e s o m e t i m e s f o u n d in P E A o f o t h e r M W [4]. To explain the relation between the t y p e of polymorphous modifications a n d morphological structures a n d the length of the polymer chain, we studied* a series of P E A of different MW in the range of 300-4000, obtained and described according to a previous description [5]. I R spectroscopy, DTA, optical microscopy and thermo-optical analysis were used; X - r a y diffraction was studied in m a n y cases. Ii~ spectra a n d X - r a y photographs were recorded a t given temperatures of crystallization using thermostatic cells. Bearing in mind the high crystallizing properties of P E A , all procudures concerning the preparation of samples were carried out so as to avoid, i f possible, or minimize p r e m a t u r e crystallization. I R spectra of P E A samples in the form of thin films between K B r plates a n d brass foil films were obtained in a UR-10 spectrophotometer. Each sample was melted at 100 ° for 10 min, followed by rapid cooling to c r y s t a l lization temperature. The non-isothermal period for samples on foil was less t h a n 15 sec, whereas for samples between K B r plates it was ~ 1 min. I t is important, however, to n o t e t h a t results of b o t h methods agree completely. W h e n studying amorphous samples, spectra were recorded over the t e m p e r a t u r e range o f --150-100 °, i.e. sequentially for all phase-aggregate states of P E A . The samples were quenched directly in a VLT-2 cell produced b y R I I C (England). Transition from 100 Q (melt temperature) to -- 196 ° was completed in 2 rain. Absence of erystallinity from these vitrified samples was verified b y I R spectra. The cell was then gradually heated while scanning the spectrum from time to time until erystallinity bands appeared on the background of the spectrum of amorphous P E A and then, until they disappgarod completely. Diffraction curves were obtained using a URS-50I X - r a y diffractometer with C u K , radiation. D T A curves were recorded in a device using a PDS-021 double crossbar t y p e self re. cording instrument and 1-37 amplifier. The sample weighed 20 mg and the rate of heating was 2-5 dog/rain. The P E A melt obtained b y keeping a micro-test tube containing the sample in an oily base at 100 ° for 10 rain, was rapidly quenched b y immersing the micro-test t u b e in liquid nitrogen (to obtain amorphous samples), or transferred to a t h e r m o s t a t for crystallization, where it was k e p t at given t e m p e r a t u r e for a period of time sufficient for completing the process (ranging from 10 rain to m a n y hours, according to temperature and MW). Polarization microscopic studies were carried out using an MBI-6 universal microscope equipped with a heating stand. The intensity of depolarized light was recorded according to t e m p e r a t u r e (thermo-optical curves) b y the methods described [6]. The rate of heating was ~ 4 deg/min). The microphotographs obtained of spherulitie structures are basically similar to structures already well known [3, 7] a n d not described here. E a r l i e r i n v e s t i g a t i o n s h o w e d [3] t h a t e a c h c r y s t a l l i n e m o d i f i c a t i o n o f P E A - 2 0 0 { ) is c h a r a c t e r i z e d b y a s e t r e f l e x e s i n t h e X - r a y d i f f r a c t o g r a m s a n d c r y s t a l l i n i t y bands in IR spectra (Table). If we assume that packing of PEA chains in each * W i t h the help of T. O. K u r a s h e v a y a a n d G. G. Stepanova.
V. I. KOVALENKOet
1598
al.
crystalline form is independent of MW, it m a y be expected that reflexes are also retained for P E A studied. We carried out investigations with all samples crystallized from melt at 41 °, in parallel with methods of Ii~ spectroscopy and radiography. It was found t h a t the positions of reflexes in diffraction curves, in fact, coincide with those observed for PEA-2000. X - R A Y A N D SPECTROSCOPIC S I G N S OF POLY1VIORPHOUS IVfODIFICATIONS
OF PEA Crystalline Reflexes form 2 0° 21 "6 24.7 20.5 21.6 24.7
Bands of erystallinity, em -1 588, 698, 735, 747, 793, 903, 907, 914, 985, 1285, 1463 700, 704, 735, 747, 835, 842, 862, 901, 914, 942, 961, 1259, 1280, 1438, 1462, 1467
However, X-ray information (Fig. 1) indicates that PEA-611, 970 and 1500 were crystallized at a given temperature, the same w a y as PEA-2000 crystallized in the a-form, since the peak 20=20-5 ° inherent for the fl-form was absent from the X-ray photograph. Bands of crystallinity of the a form were observed in I R spectra of these samples at 985, 903, 793 cm -I, respectively (Fig. 2). At the same time diffractograms of PEA-2500 and 3500 showed three reflexes, typical of the ,6-form and I R spectra of these samples had another set of erystallinity b a n d s compared with P E A of lower MW, particularly the band of the fl-form at 862 and 842 em -1. Very weak bands of an impurity of another modification were sometimes observed in the spectrum of one modification, which m a y have been due to polydispersion of P E A samples [5]. A spectroscopic study similar to that carried out with P E A of different molecnlax weights crystallized at 41 ° was also carried out with samples crystallized from melt at 11 and 27 °, at temperatures which are typical of the formation of certain forms of PEA-2000. The phase composition, results of low temperature crs~stallization and studying glass transition and melting b y methods of DTA and polarization microscopy enabled a specific phase diagram of P E A to be plotted (Fig. 3). To define more accurately boundaries of fields of crystallization, several samples were further examined, which had been crystallized at temperatures other than those mentioned. Corresponding points are also indicated in the Figure. The diagram shows that temperature limits of the formation of a given modification vary considerably according to MW. In fact., if no fl-form is produced for PEA-300 b y crystallization at any of the temperatures studied, for PEA-3500 this form is observed over an extensive range of temperature. As MW increases the upper limit of crystallization of the fl-form converges with the curve of melting PEA. With MW of over 4000 practically only the fl-modification crystallizes from melt.
Polyethylene adipate
159~
Figure 3 shows t h a t there is a broad range where both crystalline forms coexist. As shown previously [3], differences in macromolecular packing are due t o the difference in conformation of glycol fragments. A slight difference in energies: 1o(]
I00
~J
~
1o 2'q
15
'
11
b
5
laxlO-3 Crr7-1
?/g 20 °
Fro. 1
-/
-'-
Fro. 2
FIG. 1. X-ray diffraction curves of PEA samples crystallized from melt at 41° with MW= 600 (1); 970 (2); 1500 (3); 2500 (d) and 3500 (5). Fro. 2. IR spectra of the same samples as those in Fig. 1. of both forms predetermines the possibility of coexistence of crystallites over a significant range o5 MW and temperature. The course of melting P E A of different MW should be determined by concrete temperature conditions of crystal formation, which give both the internal geometry and the degree of damage. The latter also depends, of course, on the molecular weight of the sample. At the same temperatures more perfect crystals may be melted by low temperature modification and less perfect crystals by high temperature modification. This hinders the evaluation of phase composition according to results of thermal ~nalysis [8]. Figure 4 shows some DTA curves obtained for PEA samples crystallized from melt by keeping them for a considerable time at 10, 27 or 41 °. During cooling samples before DTA additional crystallization takes place frequently. This may be
V. I. KOVALENKOe t a l .
1600
seen, for example, on curves for P E A - 3 0 0 ; some e n d o t h e r m i c peaks are o b s e r v e d a t a t e m p e r a t u r e lower t h a n those a t which t h e r m o s t a t i c control was carried ,out (Fig. 4b). H o w e v e r , crystallization itself u n d e r conditions of t h e r m o s t a t i c T,° G
o
I~//o
•
•
EZ
•
C
b !
/ / ol =2 ~3
-20
0 0
0
i
1
I
2
FIG. 3
t
3
i
4
lg
I
30 ZO
~0
qO
60
Fro. 4
:FIG. 3. Isobaric "phase diagram" of PEA according to MW: 1--=-; 2--p-form; 3--=+B. The shaded part shows formation of sphenflites of annular morphology; the broken line shows tentative boundaries of a region in which partial =--;8 transition was observed for samples obtained in low temperature crystallization: Z--melt; II--=-; III--B-modification; IV--glass. FIG. 4. ]:)TA curves of PEA crystallized from melt under conditions of thermostatic control a t 10 (a), 27 (b) and 41 ° (c); MW~4000 (•); 2500 (2); 1500 (3); 970 (4) 600 (5) and 300 (6). ~ontrol is b y no m e a n s i s o t h e r m a l (because of t h e liberation of t h e h e a t o f crystallization), corresponding t o a given t e m p e r a t u r e [9]. * I t is easy to see t h a t t h e f o r m o f D T A curves d e p e n d s b o t h on t e m p e r a t u r e a n d on MW; it reflects to some ext e n t t h e properties o f crystallization on the one h a n d a n d r e c r y s t a M z a t i o n proeesses, on the o t h e r [8]. • It should be emphasized hero that in Fig. 3 too, nominal temperatures of the super~ooled melt are plotted on the ordinate and not the actual temperatures which are difficult "to determine for melts with crystals growing in them.
Polyethylene adipate
1601
E x p e r i m e n t s with a m o r p h o u s (quenched) samples are distinguished b y specific t h e r m a l conditions, crystallization a n d melting of these samples d e p e n d i n g only on the rate of heating during DTA. Curves p l o t t e d for P E A of different M W
,, J^k__.,, \ r
I
-60
I
I
-2I)
I
I ___.I___2.
ZO
00 T,'P,
FIG. 5. DTA curves of amorphous PEA samples with MW=4000 (1); 3500 (2); 300 (3); 2500 (4); 2100 (5); 1500 (6); 970 (7); 600 (8) and 300 (9). are generally similar (Fig. 5). T h e y show: an o r d i n a r y d i s c o n t i n u i t y in the glass t r a n s i t i o n range, two e x o t h e r m i c peaks (high and low) a n d a n e n d o t h e r m i e p e a k (with low ~ W - - m u l t i p l e t ) of melting. The low e x o t h e r m i c p e a k before melting Tmelt,°C m
qO
0
FIG. 6
/40
50
~
z/O
/
30
I o
~
I i
I
I 2
I
0---0---03
I 3
I
I
¢ /~n x l O -3
Fro. 7
FIG. 6. Relation between a- (1) and fl-forms (2) and temperature for low molecular weight PEA crystallized from the hardened state in a-form (layout) (m--arbitrary units). Fie. 7. Relation between melting points of PEA samples and MW, according to results of thcrmo-optical investigations. Crystallization from melt took place at 27 (1); 41 (2)and 10° (3).
1602
V. I. KOV~E~KO
et al.
is due to thermokinetie features of low temperature crystallization with a corresponding high exothermic peak [10]. It is typical that all peaks are regularly displaced to the direction of low temperatures as MW decreases, starting from 1500. A reduction in the peak of melting points to reduction of crystallinity, which may be due to an increase in macromolecules of the relative number of end groups that should be regarded as defects in the polymer crystal. For P E A of very low MW there is a new feature: numerous peaks are observed in the range of melting. It should be noted that the form of these curves is strictly reproducible, i.e. is not due to chance. It is possible that for PEA-300 and 600, MW of which is already commensurate with MW of the monomer, we have a mixture of a small number of individual homologues, which may form individual phases and eutectics. This m a y be the cause of the complication in the curves examined. In order to study polymorphous transitions in P E A samples obtained by low temperature crystallization, a method was used of recording Ii~ spectra in the heated cell. Observations confirm that with an increase of temperature above the values specified an irreversible transition of the fl-form to the ~-form takes place, as described previously (as for samples crystallized during thermostatic control) [3, ll]. However, the behaviour of P E A with the lowest MW (300 and 600) appeared to be unexpected. Low temperature crystallization during heating hardened glass only becomes noticeable at about --45 °. Crystallinity bands appear at 985, 902, 793 cm -1 for only the ~-form. Keeping the samples for 1 hr at this temperature completed crystallization which could be seen from the discontinuation of the intensity increase of the bands mentioned (samples of all other MW crystallize in the fl-form under similar conditions). Heating PEA-300 and 600 crystallized at low temperatures to --5 ° results in the formation and a marked increase in the intensity of bands in the ]?-form, e.g. at 862 cm -1, as well as in some reduction in the intensity of bands of the ~-form. At 20 ° the ]?-form disappears and up to melting crystallinity bands of the ~-form are only retained in the spectrum. A qualitative system of the variation of crystalline phase content in these P E A samples is shown in Fig. 6. It may be assumed that around --5 ° imperfect crystals of ~-modification begin to melt and recrystallization takes place with the formation of/?-modification, but the stability range of this modification for PEA-300 and 600 is low and on increasing temperature, the newly formed crystals melt, partially changing again to the ~-form which is represented this time b y crystMlites more clearly defined than the initial ones. ~/[elting is complete at ~ 40 °. Differences in free energy due to variable numbers of defects in crystallites m a y exceed the differences due to different modifications. It is precisely because of this, as we can see, that the less clearly defined crystals of the modification with a higher temperature of thermodynamic equilibrium (a) melt at a tempera-
Polyethylene adipate
1603
ture which is lower than melting points of crystals with a lower equilibrium temperature (fl) which crystallize with fewer defects. A similar condition is know~ as the inversion of thermodynamic stability and was observed for polybut-l-ene 112] and, apparently, for polyurethane prepared from di-n-butylene glycol and hexamethylenedi-isocyanate [13]. The fact that melting point depends both on the type of modification and the condition of the structure expressed in dimensions of crystalline ranges inside a given phase, was also shown recently when examining P V D P [14]. All the transitions noted in P E A take place without change in the morphological structure which is determined exclusively by conditions of initial crystallization. Observations made using a polarization microscope show that the distribution described of the temperature range of crystallization of P E A according to types of spherulitie structures [3] is maintained for all MW (only with some displacement of temperatures in the direction of reduction for polymers of lower MW (Fig. 3)) and is evidently not related directly to the phase condition of crystallites. Thus, chain packing in the crystallite is independent of other factors on the one hand emd the organization of crystallites to spherulite, on the other. In the light of the foregoing the problem concerning the phase structure of annular spherulites may be clarified. The mechanism of extension is discussed in connection with the study of PEA-2000 [15]: In spherulite layers alternate of lower and higher real temperature of formation. For P E A of MW indicated fl and ~-modifications correspond to these temperatures, however, it follows from the mechanism examined that this difference in modifications of alternating layers is b y no means necessary. Results of this investigation show that with lower MW annular spherulites are obtained which are only of ~-modification and with high MW, of fl-modification. The region of forming annular spherulites in Fig. 3 is shaded. It may be continued on the same level in the direction of higher MW. Even in PEA-9900 these spherulites formed practically within the same temperature limits [7]. Figure 3 shows that these limits and the temperature range of existence of two phases Mmost coincide for P E A of average MW, which explains the result obtained with PEA-2000 which, as is now clear, is only partially significant. We note that in the diagram shown (Fig. 3) in the region above the shaded part up to the line of melting P E A crystallizes in the form of "needle t y p e " spherulites and below the shaded part, in the form of radical spherulites. Using a polarization microscope it was possible not only to observe the morphology of crystalline formation but also to evaluate the rate of spherulitic growth. It appeared that at appropriate temperatures with a reduction of MW, this rate decreased. Thermo-optical curves enabled the point of disappearance of the last crystals to be determined, which is normally the melting point (Fig. 7). The value in question increased with MW, b u t with a MW > 2000 became almost unchanged. Melting point depended on the temperature at which the sample crystallized since this is generally typical of polymer crystals. It seems nevertheless surprising at
1604
V . I . KOVALENKOet al.
first sight t h a t the melting p o i n t of crystals o b t a i n e d at 27 ° was higher t h a n t h a t o f crystals o b t a i n e d a t 41 °. This is, however, because intensive recrystallization t o o k place during melting samples crystallized at 27 °. J u d g i n g b y t h e r m o - o p t i c a l curves characterized b y an S shape, as with those for P E A - 2 0 0 0 [6], this process t a k e s place a t the highest rate 8-12 ° before the end of melting. Thus, at the melting point crystals n o t f o r m e d primarily, b u t f o r m e d as a result of recrystallization, disappear. I n Fig. 3 the curve of melting corresponds to the highest Tme,t value obtained.
Translated by E. SEMERE REFERENCES
1. C. S. FULLER and C. J. FROSH, J. Phys. Chem. 46: 323, 1939 2. A. TURNER-JONES and C. W. BUNN, Aeta erystallogr. 15: 105, 1962 3. B. Ya. TEITEL'BAUM, N. A. PALIKHOV, L. I. MAKLAKOV, N. P. ANOSHINA, I. O. MURTAZINA and V. I. KOVALENKO, Vysokomol. soyed. A9: 1672, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 8, 1880, 1967) 4. P. SCHUSTER, Thermochim. Aeta 3: 487, 1972 5. V. S. MINKIN, A. A. MUKHUTDINOV, V. I. YASTREBOV and P. A. KIRPICHNIKOV, Vysokomol. soyed. B16: 162, 1974 (Not translated in Polymer Sei. U.S.S.R.) 6. B. ¥a. TEITEL'BAUM and N. A. PALIKHOV, Vysokomol. soyed. A1O: 1468, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 7, 1699, 1968) 7. M. TAKAYANAGI and T. YAMASHITA, J. Polymer Sei. 22: 552, 1956 8. B. Ya. TEITEL'BAUM, J. Therm. Anal. 8: 511, 1975 9. B. Ya. TEITEL'BAUM, Vysokomol. soyed. B16: 763, 1974 (Not translated in Polymer Sci. U.S.S.R.) 10. B. Ya. TEITEL'BAUM, Dokl. AN SSSR 222: 1115, 1975 11. B. ¥a. TEITEL'BAUM, V. I. KOVALENKO and N. A. PALIKHOV, Sb. Sintez i fizikokhimiya polimerov (Synthesis and Physico-Chemistry of Polymers). No. 9, Kiev, 1971 12. F. LANUSSO, Uspekhi khimii 39: 304, 1970 13. V. N. VATULEV and S. V. LAPTII, Vysokomol. soyed. BI3: 475, 1971 (Not translated in Polymer Sci. U.S.S.R.) 14. G. M. BARTENEV, A. A. REMIZOVA, I. V. KULESHOV, M. A. MARTYNOV and T. N. SARMINSKAYA, Vysokomol. soyed. A17: 2063, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2381, 1975) 15. B. Ya. TEITEL'BAUM and N. A. PALIKHOV, Vysokomol. soyed. BI2: 3, 1970 (Not translated in Polymer Sei. U.S.S.R.)