Macromolecules 1996,28, 2439-2449
2439
Interdiffusion and Composition Polydispersity in Diblock Copolymers above the Ordering Transition T. Jian, S. H. Anastasiadis,*JA. N. Semenov? and G. Fytas* Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion Crete, Greece
G. Fleischer Fakultat fur Physik und Geowissenschaften, Universitat Leipzig, 0 - 0 4 103 Leipzig, Germany
A. D. Vilesov Institute of Macromolecular Compounds, Russian Academy of Sciences, 199004 St. Petersburg, Russia Received September 26, 1994; Revised Manuscript Received December 27, 1994@
ABSTRACT: Photon correlation spectroscopy and pulsed-field-gradient nuclear magnetic resonance have been utilized in order to investigate the characteristic features of the recently established (both experimentally and theoretically) diffusive “polydispersity”relaxation process for concentration fluctuations in homogeneous diblock copolymer melts and solutions. This is accomplished using semidilute solutions of a symmetric mixture of two diblock copolymers with similar molecular weights and almost mirror compositions in a common good solvent. Mixing of the two asymmetric diblocks leads to a system with almost symmetric composition and with narrow molecular weight distribution, but with large effective composition polydispersity. Above the order to disorder transition (ODT), the theoretical expressions for the amplitude and the relaxation rate of the polydispersity mode can quantitatively describe the observed diffusive relaxation. Similarly to the situation in homopolymer blends, the thermodynamic forces can significantly retard the diffusion coefficient and lead to an increase in the dynamic intensity with increasing copolymer concentration. The intervention of the ODT alters this concentration dependence a t copolymer concentrations near but below #JODT. #J
I. Introduction
revealed the presence of an additional diffusive relaxation identified7J1s21with the copolymer self-diffusion Composition fluctuations are the key quantity in the observable via the small but finite inherent composition description of the static1 and dynamic2 properties of polydispersity K~ of the system. Due t o the low and diblock copolymers. In contrast to the polymer blends, uncertain estimated values of K~ (dO-2-10-3) neither the most prominent composition fluctuations are the proposed mechanism nor the applicability of the short range with the relevant wavevector, q*, a t which theoretical expression for the amplitude and dynamics the static structure factor attains its maximum ~ a l u e ~ , ~of this process a t low wavevectors has been critically satisfying the relation q*R, = 0(1), with R, being the examined. size of the copolymer chain. In fact, for monodisperse In the present study, an investigation is presented diblock copolymer melts, the long range qq(t)are supon the characteristic features of this polydispersity mode pressed and the mean field prediction3 is a vanishingly by utilizing a symmetric mixture of two poly(styrenesmall structure factor S(q) for q , whereas in the entanglement measurements, SB-1 is probably obeying the reptation regime D,(N,@)= ~BZ'@-~.*~/(T,-,~P). The characteristic dynamics whereas SB-2 is closer t o exhibiting Rouse number of segments between entanglements in solution, behavior and the blend is somewhere in between. This Ne(@)is related to that in the bulk, Ne, by33 Ne(4) qualitatively explains the trend for the magnitude of where a is a universal constant for similar ranges of interaction densities and a = 2; thus Ne(@)= the measured diffusivities. It should be noted, however, that the crossover between the Rouse and the reptation Ne@-l. Therefore, there is a characteristic concentraregime is rather broad36and, in the melt, well-entangled tion, @e ZZ NJN, and a respective critical concentration behavior is observed only for molecular weights greater for entanglements, 4, G 2.2 @e, above which the polymer than about 5M,.36 Therefore, one cannot assume that chains are entangled. The above expressions show that even the solutions with the highest concentrations are the polymer properties get involved in the expressions well-entangled in order to obey the @-1.s3/W for the diffusivities via only the Ne values, which do not scaling, explicitly interfere in the calculations but determine the which makes quantitative comparison of the data for Rouse and the reptation regimes. The glass transition the two diblocks almost impossible. Moreover, there is temperature of the polymer, on the other hand, that a difference of a factor of about 2.5 between the diffusivities for the two diblocks even a t concentrations affects significantly the mobility in the bulk state does not appear in the expressions, and it is the solvent (5 wt %) where both systems should obey Rouse dynamics. This disparity cannot be explained by either the viscosity that controls the mobility whereas the blob size results in the explicit 4 dependence. In contrast, for previous discussion or by the slight difference between concentrated solutions or the bulk state, it is the friction the Ws of the two diblocks; this apparent dependence coefficients and, therefore, the solution or bulk viscosiof the translational diffusivity in solution on the diblock ties that control the mobility. composition is currently under i n ~ e s t i g a t i o n . ~ ~ The critical molecular weights for entanglements, M,, Another point that should be mentioned is the insenfor bulk polystyrene and polybutadiene are vastly sitivity of the translational diffusion coefficient for the different33 and are listed in Table 2 together with the ternary system to the crossing of the ODT. Depolarized values for N,, the critical number of segments for dynamic light-scattering measurements show that the reptation, calculated using the average segmental volternary solution undergoes a disorder to order transition
2448 Jian et al.
in the concentration regime between 20 and 23 wt %. A flat featureless correlation function is observed a t 20 wt % (as well as at all lower concentrations), corresponding t o the disordered state, whereas a relaxation process appears in the window of the correlator for the 23 wt % solution accompanied by a significant increase in the depolarized intensity, in agreement with previous investigation^.^^,^^ The depolarized scattering again is due to the establishment of significant form anisotropy due to grains of coherently ordered lamellae in the ordered state with correlation length of the O(um). Mixing two cylinder-forming asymmetric poly(styreneb-butadiene) copolymers of molecular weights smaller than of the diblocks here in order t o get a symmetric system in the melt resulted in a mixture exhibiting lamellae ordering, as verified by small-angle X-ray scattering and transmission electron microscopy and in agreement with Monte-Carlo computer simulation^.^^ Therefore, lamellae structure is expected when our ternary system crosses the ODT. The translational diffusion data for the ternary system in Figure 7 show a smooth concentration dependence between 20 and the 25 wt % that conforms with the smooth variation of the temperature dependence of the self-diffusion and tracerdiffusion data measured by PFG-NMR and forced Rayleigh scattering, respectively, on poly(styrene-blocki ~ o p r e n e )and ~ ~ .poly(ethylenepropy1ene-block-ethyl~~ ethyleneI4O diblock copolymers in the bulk; this was attributed to concentration fluctuation effect^^,^^ that retard chain mobility even a t temperatures well above the ODT, especially for entangled diblocks.
VI. Concluding Remarks Photon correlation spectroscopy and pulsed-fieldgradient nuclear magnetic resonance measurements were used in order t o investigate the characteristic features of the newly observed diffusive relaxation process in disordered diblock copolymer systems in melts and semidilute solutions. Recent theoretical predictions attributed the process to the relaxation of the additional long-wavelength concentration fluctuations in homogeneous diblocks originating from the inherent composition polydispersity of the system. The diffusion coefficient was predicted to be controlled by the translational diffusion coefficient of the diblock copolymer chains. Utilizing a series of semidilute solutions in a common good solvent of a symmetric mixture of two poly(styrene-b-butadiene)diblock copolymers with similar molecular weights and almost mirror compositions, i.e., a system with almost symmetric composition, very small molecular weight polydispersity, but large effective composition polydispersity ( K J , it was shown that the experimental data for both the intensity and the diffusion coefficient of the observed relaxation can be quantitatively accounted for by the theoretical expressions for the amplitude and the relaxation rate of the polydispersity diffusive mode. The thermodynamic forces, expressed as X N K , ~can ~ .significantly ~~, retard the diffusion coefficient and lead to an increase in the dynamic intensity with increasing copolymer concentration, when K, is large enough. For anionically synthesized diblock copolymers, K, is usually of the O(10-2-10-3) and, thus, the diffusion coefficient of the process is the self-diffusion of the copolymer chains. The dependence of the translational diffusion coefficients of diblock copolymer chains in semidilute solutions on diblock composition, f , was discussed; the different critical molecular weights for entanglements of the two species were found t o be very important in determining
Macromolecules, Vol. 28, No. 7, 1995 the diffusion coefficients. Besides, the insensitivity of the translational diffusion coefficient to the crossing of the ODT was once more shown.
Acknowledgment. We would like to thank Dr. E. Melenevskaya of the Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia, for synthesizing and providing the diblock copolymers used in this work. A.N.S. acknowledges the hospitality and financial support of the Foundation for Research and Technology-Hellas. S.H.A. would like t o acknowledge that part of this research was sponsored by NATO’s Scientific Affairs Division in the framework of the Science for Stability Programme and by the Greek General Secretariat of Research and Technology. G. Fytas acknowledges the financial support of the Alexander von Humboldt Foundation (Grant No. FOKOOP USS 1685). G. Fleischer acknowledges financial support from the Deutsche Forschungsgemeinschaft (SFB 294). References and Notes (1) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. Bates, F. S. Science 1991, 251, 898.
(2) Fytas, G.; Anastasiadis, S. H. In Disorder Effects on Relaxation Processes; Richert, R., Blumen, A,, Eds.; Spinger Verlag: Berlin, 1994; p 697. (3) Leibler, L. Macromolecules 1980, 13, 1602. (4) Fredrickson, G. H.; Helfand, E. J.Chem. Phys. 1987,87,697. ( 5 ) Akcasu, A. Z.; Benmouna, M.; Benoit, H. Polymer 1986,27, 1935. Akcasu, A. Z.; Tombakoglu, M. Macromolecules 1990, 23, 607. Akcasu, A. Z. Macromolecules 1991, 24, 2109. (6) Borsali, R.; Vilgis, T. A. J . Chem. Phys. 1990, 93, 3610. (7) Anastasiadis, S. H.; Fytas, G.; Vogt, S.; Fischer, E. W. Phys. Rev. Lett. 1993, 70, 2415. (8)Vogt, S.; Anastasiadis, S. H.; Fytas, G.; Fischer, E. W. Macromolecules 1994, 27, 4335. (9) Borsali, R.; Benoit, H.; Legrand, J.-F.; Duval, M.; Picot, C.; Benmouna, M.; Farago, B. Macromolecules 1989, 22, 4119. Duval, M.; Picot, C.; Benoit, H.; Borsali, R.; Benmouna, M.; Lartigue, C. Macromolecules 1991,24, 3185. (10) Duval, M.; Haida, H.; Lingelser, J. P.; Gallot, Y. Macromolecules 1991, 24, 6867. (11) Jian, T.; Anastasiadis, S. H.; Semenov, A. N.; Fytas, G.; Adachi, K.; Kotaka, T. Macromolecules 1994,27, 4762. (12) Borsali, R.; Fischer, E. W.; Benmouna, M. Phys. Reu.A 1991, 43, 5732. (13) Benmouna, M.; Benoit, H.; Borsali, R.; Duval, M. Macromolecules 1987, 20, 2620. (14) Haida, H.; Lingelser, J . P.; Gallot, Y.; Duval, M. Die Makromol. Chem. 1991,192,2701. (15) Konak, C.; Podesva, J. Macromolecules 1991, 24, 6502. Konak, C.; Vlcek, P.; Bansil, R. Macromolecules 1993, 26, 3717. (16) Anastasiadis, S. H.; Fytas, G.; Vogt, S.; Gerharz, B.; Fischer, E. W. Europhys. Lett. 1993,22, 619. (17) Vogt, S.; Jian, T.; Anastasiadis, S. H.; Fytas, G.; Fischer, E. W. Macromolecules 1993,26, 3357. (18) Balsara, N. P.; Stepanek, P.; Lodge, T. P.; Tirrell, M. Macromolecules 1991,24, 6227. (19) Tsunashima, Y.; Kawamata, Y. Macromolecules 1994, 27, 1799. (20) Pan, C.; Maurer, W.; Liu, Z.; Lodge, T. P.; Stepanek, P.; von Meerwall, E. D.; Watanabe, H. Macromolecules, submitted for publication. (21) (a) Fytas, G.; Anastasiadis, S. H.; Semenov, A. N. Makromol. Chem., Macromol. Symp. 1994, 79, 117. (b) Semenov, A. N.; Fytas, G.; Anastasiadis, S. H. Polym. Prepr. 1994,35 (11,618. (22) Jian, T.; Fytas, G.; Anastasiadis, S. H.; Vilesov, A. D. Polym. Mater. Sci. Eng. 1994, 71, 767. (23) Fleischer, G.; Fujara, F.; Stiihn, B. Macromolecules 1993,26, 2340. (24) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford Science Publishers: Oxford, U.K., 1986. (25) Schaefer, D. W.; Han, C. C. In Dynamic Light Scattering; Pecora, R., Ed.; Plenum Press: New York, 1985. (26) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornel1 University Press: Ithaca, NY, 1979. (27) Nesterov, V. V.; Kurenbin, D. I.; Krasikov, V. D.; Beleukii, B. G. Talanta 1987, 34, 161.
Macromolecules, Vol. 28, No. 7, 1995 Karger, J.; Pfeifer, H.; Heink, W. Adu. Magn. Reson. 1988, 12, 1. Fredrickson, G. H.; Leibler, L. Macromolecules 1989,22,1238. Burger, C.; Ruland, W.; Semenov, A. N. Macromolecules 1990, 23, 3339. Jian, T.; Anastasiadis, S. H.; Fytas, G.;Adachi, K.; Kotaka, T. Macromolecules 1993,26, 4706. Meier, G.; Fytas, G.; Momper, B.; Fleischer, G. Macromolecules 1993,26,5310and references therein. Jordan, E. A.; Ball, R. C.; Donald, A. M.; Fetters, L. J.;Jones, R. A. L.; Klein, J. Macromolecules 1988,21, 235. Composto, R. J.; Kramer, E. J.; White, D. M. Macromolecules 1988, 21, 2580. Kanetakis, J.; Fytas, G. Macromolecules 1989,22,3452. Sillescu, H. Makromol. Chem., Rapid Commun. 1984, 5, 519; 1987, 8, 393. Graessley, W. W.; Edwards, S. F. Polymer 1981,22, 1329. Tsenoglou, C. J . Polym. Sei., Polym. Phys. Ed. 1988,26,2329. Tsenoglou, C. In New Trends in the Physics and Chemistry ofpolymers; Lee, L.-H., Ed.; Plenum Press: New York, 1989.
Diblock Copolymers above the Ordering Transition 2449 (35) Composto, R. J.; Mayer, J. W.; Kramer, E. J.; White, D. M. Polymer 1990, 31, 2320. Composto, R. J.; Kramer, E. J.; White, D. M. Macromolecules 1992,25, 4167. (36) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1970. (37) Anastasiadis, S. H.; Fleischer, G.; Adachi, K.; Kotaka, T. Manuscript in preparation. (38) Vilesov, A. D.; Floudas, G.; Pakula, T.; Melenevskaya, E. Yu.; Birshtein, T. M.; Lyatskaya, Yu. V. Makromol. Chem. 1996, in press. (39) Ehlich, D.; Takenaka, M.; Hashimoto, T. Macromolecules 1993,26, 492. (40) Dalvi, M. C.; Eastman, C. E.; Lodge, T. P. Phys. Rev. Lett. 1993, 71, 2591. Dalvi, M. C.; Lodge, T. P. Macromolecules 1994,27, 3487. (41) Bates, F. S.; Rosedale, J . H.; Fredrickson, G. H. J . Chem. Phys. 1990, 92, 6255.
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