Supramolecular organization of some block copolyurethanes with saccharide units in the main chain

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REFERENCES 1. P. KAMMER and J. CROW, Napolniteli dlya polimernykh kompozitsionnykh materialov (Fillers fort Polymer Composite Materials) (Eds. G. S. Kats and D. M. Milevski). p. 147, Khimiya, Moscow, 1981 2. N. S. YENIKOLOPOV and S. A. VOL'FSON, Plast. massy, No. I1, 9, 1980 3. V. A. POPOV, V. V. GUZEYEV, Yu. A. ZVEREVA, A. N. GRISHIN, T. V. PALAYEVA, A. P. SAVEL'EV and S. N. POTEPALOVA, Dokl. Akad. Nauk 275: 1109, 1984 4. V. A. POPOV, Yu. A. ZVEREVA, A. N. GRISHIN, T. V. PALAYEVA and V. A. FOMIN, Vysokomol. soyed. A24: 1654, 1982 (Translated in Polymer Sci. U.S.S.R. A24: 8, 1883, 1982) 5. Yu. A. ZVEREVA, V. A. POPOV, V. V. GUZEYEV, Ye. P. SVHAREV, G. P. GLADYSHEV and S. S. IVANICHEV, Dokl. Akad. Nauk SSSR 252: 1174, 1980 6. S. S. IVANCHEV and A. V. DMITRIYENKO, Uspekhi khimii 51:1178, 1982 7. Yu. S. LIPATOV, A. Ye. NESTEROV, T. M. GRITSENKO and R. A. VESELOVSKII, Spravochnik po khimii polimerov (Reference Book on Polymer Chemistry). p. 422, Naukova dumka, Kiev, 1971

Polymer Science U.S.S.R. Vol. 26, No. 12, pp. 2797-2803, 1984 Printed in Poland

0032-3950/84 $10.00+.00 © 1986PergamonPress Ltd.

SUPRAMOLECULAR ORGANIZATION OF SOME BLOCK COPOLYURETHANES WITH SACCHARIDE UNITS IN THE M A I N CHAIN* V. V. SHILOV, T. E. LIPATOVA, V. V. VORONA, V. N. BL1ZNYUK a n d A. I. SNEGIREV Institute of Chemistry of High Molecular Weight Compounds, Ukr.S.S.R. Academy of Sciences Institute of Organic Chemistry, Ukr.S.S.R. Academy of Sciences

(Received 15 March 1983) Structuromorphological studies have been made of block eopolyurethanes containing maltose, glucose and arabinose residues in the main chain. Polymer systems vary with the nature of the polysaccharide units in their degree of microphase separation of the components and regularity in the disposition of the corresponding supramolecular regions in the polymer volume. It is shown that the structure of the polyurethane containing maltose creats favourable conditions for its biodegradation. RECENTLY p r o m i s i n g p o l y m e r m a t e r i a l s for b i o m e d i c a l uses b a s e d o n p o l y u r e t h a n e s have been p r o d u c e d [1-3] r e p r e s e n t i n g t y p i c a l s e g m e n t e d p o l y m e r s . T h e c a p a c i t y f o r b i o d e g r a d a t i o n o f such p o l y u r e t h a n e s is due to the i n t r o d u c t i o n i n t o the r i g i d c h a i n u r e t h a n e segments o f r e a d i l y h y d r o l y s a b l e units. It a p p e a r s n a t u r a l t o link the b i o d e g r a * Vysokomol. soyed. A26: No. 12, 2496-2501, 1984.

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d a t i o n o f such p o l y m e r s t o the special features o f t h e i r s u p r a m o l e c u l a r structure. H o w e v e r t h e r e l e v a n t c o r r e l a t i o n s have so f a r n o t b e e n p r a c t i c a l l y c o n s i d e r e d . This is p r i maxily r e l a t e d t o the a b s e n c e o f reliable d a t a on the special aspects o f the s u p r a m o l e c u l a r o r g a n i z a t i o n o f such systems o b t a i n e d b y direct m e t h o d s o f s t r u c t u r a l analysis a n d e l e c t r o n m i c r o s c o p i c o b s e r v a t i o n s w i t h sufficiently h i g h r e s o l u t i o n . T h e p r e s e n t w o r k outlines the results o f s t u d y o f the s u p r a m o l e e u l a r structure o f b l o c k c o p o l y u r e t h a n e s with p o l y s a c c h a r i d e units in the m a i n c h a i n b y the m e t h o d s o f s m a l l a n g l e r a d i o g r a p h y a n d electron t r a n s i l l u m i n a t i n g m i c r o s c o p y . T h e results o f these studies a r e c o n s i d e r e d in r e l a t i o n to the p h y s i c o c h e m i c a l characteristics o f these systems. The test samples were synthesized on the basis of polytetramethylene glycol ( M = 1000) and hexamethylene diisocyanate. Mono- and poly-disaccharides served as chain elongators. Table 1 present the structural formulae of the residues of the sugars and sugar alcohols introduced into the polymer chain and also the values of the rupture stress ~r~,relative elongation 8, and the residual elongation (after removal of the load AI) of the systems studied (the mechanical characteristics were determined with the ZM-40 rupture roaching). The same Table indicates the temperature intervals of softening of the polymers determined with the Kefler microscope (1"2- T1)K. The X-ray investigations by the photomethod were carried out with the KRM-1 unit with point collimation of the primary beam using the radiation of a copper anode monochromatized with a nickel filter. The diffraction pattern was recorded simultaneously at large and small scatter angles using the attachment devised for these purposes [4]. The small angle diffractometric investigations were carried out with the automatic diffractometer with a Kratky type camera [5] in the radiation from the copper anode monochromatized with a nickel filter and monochromator with total internal reflexion. The diffractograms were filmed in the regime of automatic step scanning of the d e t e c t o r BDS-6 scintillation counter. The data of small angle scatter were reduced to an absolute scale with the Lupolen* standard sample [6]. Treatment of the diffract ion curves for obtaining the values of the mean square of the fluctuations of electron density Ap 2' [7], introducing the collimation correction [8], calculating the correlation functions [9] and the thickness of the interphase transitional layer E [10] was with the FFSAXS programme [11]. t For the investigation by transilluminating electron microscopy the samples were first kept in vacuo ( ~ 1 mm Hg at 313 K) for 24 hr. Then the samples were etched in the plasma of the non-electrode discharge [12] and from their surface carbon replicas shaded with chromium (angle of shading ,,=40 °) were taken, The electron microscopic films were obtained with the JEM-100 C microscope at an accelerating voltage 80 kV. In this case we used an instrumental magnification 2000 and focusing by the minimal contrast method [13]. This allows one, as is known, to reduce to a minimum the possible appearance of artefacts through the phenomenon of phase contrast. The possibility of allowing for this factor is graphically demonstrated in the recent work of Thomas and Poche [14, 15]. T h e wide angle diffraction p a t t e r n s o f the p o l y m e r s t u d i e d d i s p l a y a diffuse diffract i o n ring. T h e i n t e r p l a n a r distances c o r r e s p o n d i n g t o the d i a m e t e r o f this r i n g are given i n T a b l e 2. T h e scatter p a t t e r n o b s e r v e d a t wide angles i n d i c a t e s the a m o r p h o u s c h a r a c t e r o f the structure o f the given systems. T h e i n t r o d u c t i o n as e l o n g a t o r o f the r i g i d segments o f t h e v a r i o u s r e s i d u e s o f sugars a n d sugar a l c o h o l s led t o insignificant changes in the n e a r o r d e r l i n e s s o f the b l o c k c o p o l y u r e t h a n e s . * We are grateful to O. Kratky for making available and recalibrating the standard Lupolena sample. t We are grateful to C. Vonk for making available the FFSAXS programme.

Block copolyurethaneswith saccharideunits

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TABLE 1. STRUCTUROMECHANICAL CHARACTERISTICS OF THE POLYURETHANE SYSTEMS STUDIED

•=- .~ •.o i~' Polyurethane based on

Structural formula ~N

•

g

H~Hj Maltose

~S~

Y.

I OH

73"51171C 20

355-373

0'38

8"0

14-0 160 --

319-333

0"34

7"1

335-341

0"33

17"0

C~FIzOH

Glucose

H~HH~ H~OH H OH 14

Arabinose

OH

~11.2 43

To compare the morphological features of the polymers we used electron microscopie films (Fig. 1) the right upper corner of which gives the small angle diffraction patterns of the corresponding polyurethanes. From Fig. 1 it follows that the main structural elements of the system studied axe the highly anisodimetric supramoleeulax formations of "worm-like" form with a diameter 10 nm. Characteristic of the block copolyurethane based on arabinose is serious defectiveness in the disposition of these supramolecular formations. Portions are present with a different periodicity of their distribution and different degree of their transillumination. On passing to the glucose-based sample the defectiveness of the supramolecular structure diminishes and finally, characteristic of the block copolyurethane based on maltose is the most perfect pattern of the disposition of the microregions of heterogeneity. Table 2 presents the averaged values of the periodicity of the arrangement of the supramoleeular formations calculated from Fig. 1. It may be noted that for the block copolyurethane based on axabinose the periodicity of the side packing of the microregions is ~ 2 times greater than for the two remaining polyurethanes. From the small angle diffraction curves (Fig. 2) it follows that in the series of block copolyurethanes based on axabinose, glucose, and maltose the reflexion becomes more

V. V. SmLOV et al.

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TABLE 2. PARAMETERS OF BIPHASE STRUCTURE OF BLOCK COPOLYURETHANES

Block copolyurethane based on

Maltose Glucose Arabinose

Periodicity Values of raInterplanar of supramolee- dial distances Size of transi- Mean square distances tional zones of fluctuations ular forma(determined (from data on tions (from between rein electron from position wide angle X electron migions with difdensity of maximum radiography,), ferent electron zip ~ × 10-z croscopic of correlation nm density, nm mole.el2/cm 3 films), nm function) nm 0.43 11-0 9"6 0.44 0.56 0"59 10.7 8"0 0.32 0.29 0.41 16.3 16.6 0.37 0.06

intense. It may be noted that, as a whole, the results of small angle radiography (Fig. 2) and electron microscopy given in Table 2 are in satisfactory agreement. The electron microscopic films plus the data of X-ray scatter give information on the form o f the regions of supramolecular orderliness. Considering that the dark extended regions in these films are framed by light portions occupying a large area of the films the latter may be assigned to the matrix phase. As the latter, judging from the volumetric fractions of the components (Table 1) one must consider the phase enriched with the glycol component. The extended form of the microregions of heterogeneity is undoubtedly a consequence of the rigid chain nature of the segments formed from the fragments: diisocyanate-elongator-diisocyanate. As is known, in certain conditions the rigid chain component is capable of segregation in the anisotropic phase [16]. For segmented polyurethanes this finds expression in the purely speculative models of the segregation of the rigid segments proposed by Bonart and completely agrees with ~ e electron microscopic data obtained by us.

FIG. 1. Electron microscopic films (instrumental magnification 20,000) and small angle diffraction patterns of block copolyurethanes based on maltose (a), glucose (b) and arabinose (c). Since characteristic of the supramolecular formations of the system studied is lateral orderliness and the complete absence of order in the longitudinal direction, to analyse such structures it is necessary to use not three-dimensional but one-dimensional correlation functions representing the cosine transformation of the functions of the intensity of small angle scatter [9] (Fig. 3). It may be noted that for the polyurethane based on arabinose one weakly marked maximum of the function 7(x) is observed. For poly-

Block copolyurethanes with Saecharide units

2801

urethane based on glucose three weakly marked maxima may be observed. Polyuerthane based on maltose is characterized by the function 7(x) with four very well marked gradually decaying oscillations. Noting that the maxima of the function 7(x) reflect the degree of correlatedness o f the microregions of heterogeneity we arrive at the conclusion that the periodicity of the supramolecular structure improves in the series of polymers based on axabinose, glucose and maltose. This agrees with the results of analysis of the small angle diffractograms and electron microscopic films.

0'6' I, iip/rnin 0"4 !500 1 I I

O'2 ,,

. 1

!~00

//~ - ' - "

500

0

-0.2

"~.

l

200

, oi°~

o

p, A

2 20,deg Fro. 2

FIG. 3

FtG. 2. Small angle diffraction patterns of block copolyurethanes based on maltose (1), glucose (2) and arabinose (3) reduced to point collimation of the primary beam. FIG. 3. One dimensional correlation functions of block copolymers based on maltose (1), glucose (2") and arabinose (3). Table 2 presents the values of the radial distances characteristic of the positions of the maxima of the function 7(x). The radial distances corresponding to the first maxima reflect the distances between the adjacent microregions. Table 1 shows that these distances are close to the values of the large periods found from the position of the maxima of the ftmctions of intensity and evaluated from the electron microscopic films. Table 2 also presents the value of the thicknesses of the interphase transitional layer between the regions of rigid and flexible chain components. The values obtained are similar for block copolyuretlmnes of the series studied and have tb_e same order of magnitude as for typical block copolyurethanes in accord with the regularity growing

280"2

V . V . SHILOV et al.

in the given series of the rnicrolattice of the systems studied. As is known, [7] the values ztp 2' may be related to the intensity of the processes of microphase separation occurring in a heterogeneous system. In view of the amorphous structure of the polymers studied such processes may find a reasonable explanation only on the basis of the notions of •separation of blocks of a different chemical nature. The degree of phase separation on the basis of the small angle data may be evaluated on the following premises. The experimentally found values of the mean square of the fluctuations in electron density on complete separation of the blocks of a different chemical nature may be taken as equal to the calculated values of the mean square of the fluctuation in electron density ,jp2 [131 Ap

= 0(1 -

where p~ and ,02 are the electron densities of the flexible and rigid blocks; ~0is the volumetric fraction of rigid blocks. Table 1 shows that for the system studied the values of tp differ insignificantly. It may also be expected that slight differences are characteristic of the electron densities of the rigid blocks, while the P2 values of the flexible chain blocks match. In such a case on complete microphase separation for th°e systems studied we ought to observe roughly equal values of Ap 2'. Consequently, the values of Ap 2' given in Table 2 may be considered proportional to the degree of microphase separation of the flexible and rigid chain blocks. The lowest degree of segregation of the components is characteristic of the arabinose-based and the highest of the maltose-based polymer. Comparing the data on the special aspect of the microphase structure of the block copolyurethanes with the physicomechanical characteristics of these systems (Table 1) it may be noted that the block copolyurethane based on maltose, for which the capacity for segregation of the components is marked to the highest degree and which has the most perfect macrolattice, has the best mechanical properties and a higher softening point. It is necessary to see how the structural and morphological features typical of the polymers studied may be reflected in their capacity for specific biodegradation. In the electron microscopic films (Fig. l) it is possible to observe more or less regular alternation of the microregions corresponding to the light or dark portions. Together with greater regularity in the arrangement of these microregions in Fig. la (block copolyurethane based on maltose) one may note a larger fraction of light regions. As the fraction of the rigid chain component in a given polymer characterized by a comparatively high degree of segregation of the components does not exceed 4 0 ~ (Table 1) the light microregions in Fig. la, must be assigned to the flexible chain and the dark to the rigid chain regions. Evidently this is quite natural since the stronger rigid chain component is less subject to etching in the plasma of the non-electrode discharge and as a result of shading with chromium gives less transparent portions on the carbon replica. On passing from a polymer based on maltose to one based on glucose and then to the arabinose-based polymer, the fraction of the light microregion decreases. At first sight, this contradicts the growing fraction of flexible chain component in the given

Block copolyurethanes with saccharide units

280~

polymer series. However, in interpreting this effect it must be borne in mind that in block copolyurethanes based on glucose and arabinose the degree of microphase separation is very low and therefore the degree of "erosion" of the flexible chain blocks must also be comparatively low. Evidently, we have a similar situation for the biodegradation processes. The most subject to specific enzymatic hydrolysis are the saccharide elongators of the rigid blocks. As follows from Fig. la, the corresponding rigid chain domains forming a macrolattice create in the polymer volume "channels" exposed to the m a x i m u m degree to biodegradation. At the same time the low degree of the phase separation in block copolyurethanes based on glucose and arabinose must h a m p e r the process of biodegradation as a result of the blocking by the glycol segments of the saccharide prone to enzymes destruction. Thus, structuromorphologieal studies of block copolyurethanes with saecharide units introduced into the main chain suggest that, depending on the nature of the latter, a variable degree of microphase separation and variable regularity in the arrangement of the corresponding supramolecular regions in the polymer volume are possible. W i t h this are associated the mechanical characteristics of the polymer and their capacity for softening with rise in temperature. The most acceptable characteristics are shown by a polymer with a high degree of segregation of the c o m p o n e n t s - b l o c k copolyurethane based on maltose. F r o m the qualitative analysis made it may be assumed that in presence of good physicochemical characteristics the structure of such a polymer will ocoer favourable conditions for its biodegradation. Translated by A. CROZY REFERENCES

1. T. E. LIPATOVA, G. A. PKHAKADZE, D. V. VASIL'CHENKO and Yu. S. LIPATOV, Dokl. Akad. Nauk SSSR 251: 368, 1980 2. M. M. LYNN, V. T. STANNET and R. D. GILBERT, J. Polymer Sci. Polymer Chem. Ed. 18: 1967, 1980 3. K. KAZUHIYO and S. HIROOSHI, Macromolecules 13: 234, 1980 4. V. VORONA and T. M. GRITSENKO, Pribor. i tekhn, eksperim. No. 1, 183, 1983 5. Ch. KRATKY, O. KRATKY and E. WRENTSCHUR, Acta phys. austriaca 41: 105, 1975 6. O. KRATKY, I. PILZ and P. SCHMITS, J. Colloid Interface Sci. 21: 24, 1966 7. A. GUINIER and G. FORNET, Small Angle Scattering of X-rays. p. 354, Wiley, N.Y., 1955 8. C. G. VONK, J. Appl. Cryst. 4: 340, 1971 9. P. DEBYE and A. M. BUECHE, J. Appl. Phys. 20: 518, 1949 10. J. T. KOBERSTEIN, B. MORRA and R. S. STEIN, J. Appl. Cryst. 13: 34, 1980 11. C. G. VONK, J. Appl. Cryst. 8: 340, 1975 12. Ye. V. LEBEDEV, Yu. S. LIPATDV and L. I. BEZRUK, Novye metody issledovaniya polimerov (New Methods for Polymer Investigations) (Ed. Yu. S. Lipatov) p. 3, Naukova dumka, Kiev, 1975 13. A. U. AGAR, Teklmika elektronnoi mikroskopii (Technique of Electron Microscopy) p. 30, Mir, Moscow, 1965 14. E. I. POCHE and E. L. THOMAS, Polymer 22: 333, 1981 15. D. L. HANDLIN, W. I. MACKNIGHT and E. L. THOMAS, Macromolecules 14: 795, 1981 16. T. HASHIMOTO, M. SHIDAYAMA, M. FUJIMURA and H. KAWAI, Memoirs of the Faculty. of Engineering, Kyoto University 43: 184, 1981