Macromolecules 1984, 17, 342-348
34 2
Registry No. Polyethylene (homopolymer),9002-88-4; polypropylene (homopolymer), 9003-07-0.
References and Notes (1) P. J. Florv. “Statistical Mechanics of Chain Molecules”. In. terscienc4 ‘New York, 1968. (2) J. Rault, M. Sotton, C. Rabourdin, and E. Robelin, J . Phys.
(Paris),41, 1459, 1469 (1980). (3) J. Rault and E. Robelin, Polym. Bull., 2, 373 (1980);J. Rault, J. Am. Phys. SOC., 27, 258 (1982). (4) E. Robelin-Souffach6and J. Rault, Macromolecules, preceding paper in this issue.
(5) W. W. Graessley and S. F. Edwards, Polymer, 22, 1329 (1981). (6) V. R. Raju, G. Smith, G. Marin, J. R. Know, and W. W. Graessley, J . Polym. Sci., 17, 1183 (1975). (7) W. W. Graessley, Adv. Polym. Sci., 16, l(1974). (8) P.-G. de Gennes, “Scaling Concepts in Polymers Physics”,
Cornel1 University Press, Ithaca, NY, 1979. (9) J. Klein, Nature (London),271, 143 (1978);Macromolecules, 11, 852 (1978). (10) D. C. Prevorsek and B. T. de Bona. J . Macromol. Sci.. Phvs.. B19, 605 (1981). (11) G. P. Andrianova. J.Polvm. Sci.. Phvs.. 13. 95 (1975). (12) L. Wild, R. Ranghath, and D. C. Kno6eloch,Polym. Eng. Sci., 16, 12, 811 (1976).
Viscoelastic Properties of Blends of Styrene-Butadiene Diblock Copolymer and High Molecular Weight Homopolybutadiene Hiroshi Watanabe and Tadao Kotaka* Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan. Received April 15, 1983 ABSTRACT Viscoelastic properties of three well-characterized styrene-butadiene (SB) diblock copolymers blended with three high molecular weight homopolybutadienes (hB)were examined. Since hB is a nonsolvent for S blocks, micelles with precipitated S cores and dissolved B cilia are formed in the blends. The blend exhibits a shoulder or plateau often called a second plateau in the low-frequency region of the storage G‘and loss G”modu1i. When the SB content c is smaller than a certain critical value cb*, the frequency region where the second plateau appears is independent of c. On the other hand, when c is increased beyond Cb*, the second plateau rapidly extends to the lower frequency region. This critical value cb* coincides with the concentration at which the micelles begin to contact and their cilia begin to overlap with each other. These results suggest that the slow relaxation mechanism for the blends with c below cb* may be related t o the motion of the B cilia entangled with the matrix hB molecules, while that for the blends with c above cb* is related to the delayed reptation of the mutually entangled B cilia of the neighboring micelles. From this viewpoint, we are able to explain the dependence of the longest relaxation time T~ of the blends on the SB content c and on the molecular weights MbB and Mm of the B block and hB molecules, respectively, by employingthe “tube renewal” mechanism of Klein and the modified “tube” model of Doi and Edwards and of Kuzuu for star polymers. Table I Characteristics of Polymer Samples
I. Introduction I n our previous paper^,^-^ we described rheological properties of blends of styrene-butadiene (SB) diblock copolymers (with S content less t h a n 30 w t 5%) a n d a low molecular weight homopolybutadiene (with an M, of about 2 X lo3, coded as chB). T h e behavior of t h e blends was basically linear viscoelastic but was quite different from that of ordinary homopolymers in concentrated solutions a n d melt^.^^^ Namely, as t h e SB content c is increased beyond a certain critical value cb*, t h e blends begin to exhibit a slow relaxation process in t h e low-frequency reg i ~ n , l -where ~ a n ordinary homopolymer system has no such processes b u t exhibits a Newtonian f l o ~ . ~ ? ~ W e proposed t h e following mechanism for such a slow relaxation process found in the SB/chB blend^.^ Since chB itself is a 0 solvent for B blocks b u t a nonsolvent for S blocks, SB molecules tend to aggregate a n d form micelles with precipitated S cores a n d dissolved B cilia. With increasing SB content c, the B cilia of neighboring micelles begin to overlap and entangle with one another. T h e slow relaxation process is a result of retarded reptational motion of the mutually entangled B cilia, because the reptational motion of a polymer chain (in this system, a B cilium) with one end fixed a n d t h e other e n d free would be highly retarded as pointed o u t by d e Gennes.6 However, in this earlier study3 we did n o t observe t h e entanglement effect of chB molecules on the relaxation processes, presumably because the M , of chB was too small to result in effective entanglements. I n other words, only t h e entanglement network resulting from B cilia alone 0024-9297/84/2217-0342$01.50/0
M,/
code SB3 SB4 SB5 hB1 hB2 hB3
lO-’M,
134 192 294 27.6 60.7 177
M, 1.07
1.08 1.10 1.05 1.05 1.06
wt % ofPS
24.0 17.0 11.0 0 0 0
1 0 . ~ ~ 41 ,0 - 3 ~ , of S of B block block
32 32 32 0 0 0
102 160 262 27.6 60.7 177
affected the slow relaxation process in the SB/chB blends. To clarify the effect of entanglements between matrix homopolybutadiene and B cilia of the micelles, we examined viscoelastic properties of blends of SB diblock copolymers a n d high molecular weight homopolybutadiene with M , at least a few times greater t h a n t h e average molecular weight Mebulkbetween entanglement points in bulk h o m ~ p o l y b u t a d i e n e . T ~ h~ e~ mechanisms of slow relaxation processes in such blends will be discussed in terms of t h e tube mode17-10 for B cilia entangled with matrix homopolybutadiene a n d with B cilia of neighboring micelles.
11. Experimental Section 1. Materials. Three SB diblock copolymers, SB3, SB4, and SB5, employed in this study were prepared by anionic polymerization using sec-butyllithium as the initiator and benzene as the so1vent.l’ All of these SB samples were prepared from the same batch of precursor living polystyrene (PS) and, therefore, have S blocks of the same characteristics. Three homopoly-
8 1984 American Chemical Society
Properties of Styrene-Butadiene Blends 343
Macromolecules, Vol. 17, No. 3, 1984 butadiene samples, hB1, hB2, and hB3, were also synthesized by anionic polymerization using the same initiator and solvent as the SB samples. Table I shows the characteristics of these SB and hB samples determined by a gel permeation chromatograph equipped with a triple-detector system consisting of a refractometer, an UV-absorption monitor, and a low-angle laser-light scattering photometer (Toyo Soda LM., System HLC 801AW8-LS8).11 It has been known that hB is miscible with B blocks of SB molecules only when the molecular weight M m of the hB is lower than that of the B blocks Mbe.'2''3 On the other hand, when the molecular weights Mb8and hfbe of the blocks are too small, the microphase separation between the S and B blocks does not take place.14 We prepared six blends: SB3/hB1, SB4/hB1 and -hB2,and SB5/hB1, -hB2, and -hB3. These blends satisfy the above two criterions. In fact, we confirmed the presence of spherical S microdomains randomly dispersed in the B matrix phase in SB/chB blends, which are similar to the blends examined here.'J6 To prepare a blend, we dissolved prescribed amounts of the SB, hB, and an antioxidant, butylhydroxytoluene (BHT; about 1 wt % of the blend), in a large amount of methylene chloride, which was subsequently evaporated to obtain a blend of desired composition. 2. Rheological Measurements. Dynamic measurements were made with a conventional rheometer (an Autoviscometer L-111, Iwamoto Seisakusho, Kyoto) equipped with a cone-and-plate assembly. The radius of the cone was 15.0 mm, and the angle between the cone and plate was 3.68". The measurements were carried out at several temperatures below 160 OC. To suppress cross-linkingand oxidative degradation of B segments, we carried out the measurements at temperatures between 50 and 160 OC under a stream of nitrogen. To eliminate the prehistory of the microphase-separated structure during the blend preparation, the blends were annealed at about 80 OC for 20 min before measurements were made. In thisway, we could obtain a reproducible (and presumably close to an equilibrium) domain structure.16 The angular frequency u/(rad 8-l) was examined between 0.01 and 5. Under these conditions all the blends exhibited linear viscoelastic behavior as long as the amplitude of strain was kept below 0.3. Especially at the highest temperature examined, they exhibited almost Newtonian behavior. The data were analyzed by using the Markovitz equation16to determine the storage G'and loss G ''moduli. The timetemperature superposition principle6 was applied to obtain the master curves. In general, multiphase systems such as the block copolymer systems do not obey the time-temperature superposition principle. However, the superposition worked well for the present blends, presumably because of the low total styrene content (
