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Langmuir 1998, 14, 1586-1589
Effects of Polystyrene on the Gel Formation of Polystyrene-b-poly(ethylene/butylene)-b-polystyrene in n-Decane† Jose´ R. Quintana, Esperanza Dı´az, and Issa Katime* Grupo de Nuevos Materiales, Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Campus de Leioa, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Espan˜ a Received July 3, 1997
The effects of homopolystyrene on the gel formation in n-decane solutions of polystyrene-b-poly(ethylene/ butylene)-b-polystyrene and polystyrene were investigated. It was found that the polystyrene content in the gels influenced the sol-gel transition, mechanical properties, and swelling behavior. An increment in the percentage of polystyrene caused an increase of the sol-gel transition temperature, the elastic storage modulus, and the equilibrium swelling ratio. These effects would be related to the solubilization of polystyrene chains into the gel junctions that increases the thermal stability of the gel.
Introduction The reversible gelation of block copolymers in selective solvents has received growing attention in the nineties.1-11 A special case of these gels is those formed in semidiluted solutions of ABA triblock copolymers in a selective solvent of the B block.7-11 The thermally reversible gelation of the triblock copolymers is not yet well-established. Some authors have12 considered the existence of micelles where the middle block would form a loop so that both outer blocks stay in the micelle core. The additional entropic penalty to the micelle formation that arises from this loop causes some outer blocks to be extended into solution. If the copolymer concentration is large enough, these end blocks can form part of another micelle core or interact with another end block belonging to a different micelle in the same condition. A physical network would be formed in this way. However other authors13 suggest that the copolymer solutions show loose and polydispersed aggregates rather than spherical micelles. Thus, in semidilute solutions, a lattice would be made of end block nodes linked together by middle blocks.14 †
In memoriam of Dr. Jose´ M. Go´mez Fatou. * Address for correspondence: Dr. Issa Katime, Avda. Basagoiti, 8-1 C, 48990 Getxo, Vizcaya, Spain (1) Deng, Y.; Yu, G.-E.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 1441. (2) Nicholas, C. V.; Luo, Y.-Z.; Deng, N.-J.; Attwood, D.; Collett, J. H.; Price, C.; Booth C. Polymer 1993, 34, 138. (3) Bedells, A. D.; Arafeh, R. M.; Yang, Z.; Attwood, D.; Padget, J. C.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1993, 89, 1243. (4) Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (5) Brown, W.; Schille´n, K.; Hvidt, S. J. Phys. Chem. 1992, 96, 6038. (6) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (7) Rodrigues, K.; Mattice, W. L. Polym. Bull. 1991, 25, 239. (8) Nguyen-Misra, M.; Mattice, W. L. Macromolecules 1995, 28, 1444. (9) Sato, T.; Watanabe, H.; Osaki, K. Macromolecules 1996, 29, 6231. (10) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J.-P. Macromolecules 1996, 29, 1296. (11) Yu, J. M.; Dubois, Ph.; Teyssie´, Ph.; Je´roˆme, R.; Blacher, S.; Brouers, F.; L′Homme, G. Macromolecules 1996, 29, 5384. (12) Tuzar, Z.; Kona´k, C.; Stepa´nek, P.; Plestil, J.; Kratochvı´l, P.; Procha´zka, K. Polymer 1990, 31, 2118. (13) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J.-P. Macromolecules 1994, 27, 2956. (14) Lairez, D.; Adam, M.; Raspaud, E.; Carton, J.-P.; Bouchaud, J.-P. Macromol. Symp. 1995, 90, 203.
The existence of reversible gels of polystyrene-b-poly(ethylene/butylene)-b-polystyrene in semidiluted solutions of paraffinic oils (n-alkane mixtures) has been reported.15-17 Paraffinic oils are selective solvents of the poly(ethylene/ butylene) block. Firm gels were found at concentrations as low as 2 wt %. In previous papers15-17 we have studied the influence of copolymer and oil molar masses on the physical gels. The swelling behavior, the mechanical properties, and sol-gel transition of the reversible gels were studied. Gels swelled significantly and the copolymer concentration dependences of the swelling ratio were linear. These dependences were influenced by the copolymer and oil molar masses. The sol-gel transition temperature increased with the copolymer concentration and the copolymer and oil molar masses. The concentration dependences of the elastic storage modulus were similar and agree very well with the value expected for systems in good solvents (G′ ≈ G2.25) that have a structure close to that of covalent networks.18 The relaxation rates observed were extremely high, suggesting a considerable mobility in the gel over the measurement time. These rates decreased as the copolymer and oil molar masses increased. The capability of block copolymer micelles to solubilize compatible homopolymers with the copolymer blocks which form the micelle core has been well-established.19-22 When the homopolymer chain is shorter that the copolymer block which forms the core, the homopolymer can go into the micelle core and form part of the micelle. This paper is concerned with the possibility that homopolystyrenes of low molar mass can form part of gels (15) Quintana, J. R.; Dı´az, E.; Katime, I. Macromol. Chem. Phys. 1996, 197, 3017. (16) Quintana, J. R.; Dı´az, E.; Katime, I. Macromolecules, in press. (17) Quintana, J. R.; Dı´az, E.; Katime, I. Polymer, submitted for publication. (18) de Gennes, P. G. Scaling Concepts in polymer Science; Cornell University Press: Ithaca, New York, 1979; Chapter 5. (19) Oranly, L.; Bahadur, P.; Riess, G. Can. J. Chem. 1985, 63, 2691. (20) Price, C.; Stubbersfield, R. B. Eur. Polym. J. 1987, 23, 177. (21) Quintana, J. R.; Salazar, R. A.; Katime, I. Macromolecules 1994, 27, 665. (22) Quintana, J. R.; Salazar, R. A.; Katime, I. J. Phys. Chem. 1995, 99, 3723.
S0743-7463(97)00742-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/03/1998
Polystyrene-b-poly(ethylene/butylene)-b-polystyrene
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Table 1. Characteristics of Polymer Samples: Mass-Average Molar Mass, Mw, Polydispersity Index, I, and Styrene Weight Percentage Mw/g‚mol-1 Mw,PS/g‚mol-1 Mw,PEB/g‚mol-1 I wt % styrene
SEBS2
PS1
PS2
87 000 2 × 14 000 59 000 1.11 32
5 700
8 300
2.23 100
2.80 100
of polystyrene-b-poly(ethylene/butylene)-b-polystyrene in n-decane. Polystyrene is not soluble in n-decane and is not miscible with poly(ethylene/butylene). The influence of the concentration and molar mass of the polystyrene on the swelling, the sol-gel transition, and the mechanical properties of the gels was studied. Experimental Part Materials and Gel Preparation. The polystyrene-b-poly(ethylene/butylene)-b-polystyrene triblock copolymer sample, SEBS2, is a commercial product kindly provided by Shell Espan˜a, S.A. The sample has been previously characterized in detail.23 The polystyrene samples, PS1 and PS2, were synthesized via radical polymerization in toluene solution at 90 °C, using R,R′azobis(isobutyronitrile) as initiator. The mass-average molar mass, the polydispersity, and the styrene content of the polymer samples are summarized in Table 1. n-Decane (analytical purity grade) was used without further purification. Sample gels were prepared by dissolving the copolymer and polystyrene samples in n-decane at 120 °C in sealed flasks. Once the solutions were homogeneous, they were allowed to cool to room temperature in order to form the gels. The copolymer concentration employed was 15 wt %. In this study the polystyrene concentrations are expressed in wt % by mass of copolymer and range between 1 and 7 wt %. Sol-Gel Transition. Two methods were used to determine the sol-gel transition. The gelation temperatures were determined by inverting a test tube containing the polymer solution. On lowering the temperature, the temperature at which the solution changes from a mobile to an immobile system was considered as the gelation temperature, TGL. The melting temperatures were determined by measuring the elastic storage modulus G′ and the loss modulus G′′, as a function of temperature at a frequency of 1 Hz. The temperature at which G′ ) G′′ was considered as the melting temperature, Tm. However, it should be noted that the melting temperature so defined is frequencydependent. A heating rate of 0.5 °C min-1 was admitted to be slow enough to keep the gels in an equilibrium state and, therefore, not dependent on gel history.15 Rheometry. Rheological properties were determined using a Polymer Laboratories dynamical mechanical thermal analysis system. The mechanical mode used was the torsion one with a fluid cup and a torsion plate whose diameters were 44 and 38 mm, respectively. Gel bits were introduced into the cup prior to increasing the chamber temperature to 100-120 °C in order to melt the gel. Once a homogeneous solution was obtained the sample was quickly cooled to 25 °C. The elastic storage modulus G′, and the loss modulus G′′ were measured as a function of temperature at a frequency of 1 Hz and a heating rate of 0.5 °C min-1. The temperature was controlled with a precision of 0.1 °C. Gel Swelling. The gel samples were immersed in an excess of n-decane to achieve an equilibrium swelling. The swelling kinetics were followed by measuring the sample weight until equilibrium was reached (or nearly reached). The equilibrium swelling ratio, G∞, is defined as the ratio of the final weight to the initial weight of the sample.
Results and Discussion Sol-Gel Transition. The sol-gel transition temperatures were determined by the tube-inversion method and (23) Villacampa, M.; Quintana, J. R.; Salazar, R. A.; Katime, I. Macromolecules 1995, 28, 1025.
Figure 1. Temperature dependence of G′(O) and G′′ (O) for a SEBS2/PS1/n-decane gel at 1 Hz: SEBS2 concentration, 15 wt %; PS1 concentration, 2 wt % by mass of copolymer; heating rate, 0.5 °C min-1.
Figure 2. Sol-gel transition temperature as a function of the polystyrene percentage by mass of copolymer for SEBS2/PS1/ n-decane (O) and SEBS2/PS2/n-decane gels (0). SEBS2 concentration: 15 wt %. Gel formation temperatures determined by the tube-immersion method (filled symbols), and melting temperature was determined by oscillatory shear measurements (unfilled symbols).
oscillatory shear measurements. A plot of log G′ and log G′′ as a function of temperature for a SEBS2/PS1/n-decane gel with a PS1 percentage by mass copolymer of 2 wt % is shown in Figure 1. A sharp drop of G′ is observed when the gel melting takes place. The temperature at which G′ ) G′′ was considered as the melting temperature, Tm, since it marks the transition from a solid-like state to a viscoelastic liquid-like state. It should be noted that the melting temperature so defined is frequency-dependent. This criterion has been chosen due to the uncertainty found in SEBS2/paraffinic oil gels to determine the temperature at which G′ and G′′ follow the known scaling relation.24,25
G′(ω) ≈ G′′(ω) ≈ ωn The influence of the polystyrene concentration on the sol-gel transition is shown in Figure 2, where the gel melting and gelation temperatures are plotted as a function of polystyrene percentage (by mass of copolymer) for two polystyrene samples of different molar masses. (24) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367. (25) Chambon, F.; Winter, H. H. J. Rheol. 1987, 31, 683.
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Figure 3. Elastic storage modulus, G′ as a function of polystyrene percentage by mass of copolymer on a semilogarithmic scale for SEBS2/PS1/n-decane (O) and SEBS2/PS2/ndecane gels (0) at 25 °C: SEBS2 concentration, 15 wt %; frequency, 1 Hz.
Both temperatures are found to be very similar, although two different experimental methods were used for the copolymer gels containing the PS2 sample. These temperatures are more different in the case of the PS1 polystyrene. However, in both cases a linear dependence of the polystyrene percentage by mass of copolymer on the sol-gel transition temperature can be considered. A higher polystyrene content of the gels implies a higher sol-gel transition temperature. This fact suggests that the homopolystyrene, which will probably be located at the junctions formed by the polystyrene blocks, improves the thermal stability of these junctions, leading to more stable copolymer gels. This behavior has also been observed in poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) gels in water containing homopoly(propylene oxide).26 The higher influence that the PS1 sample has on the sol-gel transition is remarkable. This larger influence would be due to the lower molar mass that PS1 has. It is well-known20,21,27 that the capability that the block copolymer micelles have to solubilize homopolymer by placing the homopolymer in the micelle core increases as the homopolymer molar mass decreases. The micelles cannot solubilize homopolymer with a higher molar mass than that of the copolymer block which forms the micelle core. We have found that the polymer blend gels were not totally clear. This means that a number of the polystyrene chains are not solubilized into the PS block junctions. It should be noted that the PS samples are polydispersed; therefore, a certain number of homopolystyrene chains will have larger molar mass than that of the polystyrene blocks. A larger number of PS1 chains will be solubilized in the gel junctions. The homopolystyrene effect on the elastic storage modulus, G′, was also studied. To this purpose the elastic storage modulus was measured at 25 °C and 1 Hz for several SEBS2 gels containing different percentages of samples PS1 and PS2. Figure 3 shows a plot of log G′ against the polystyrene percentage by mass of copolymer. The variation of log G′ with the PS percentage increases linearly as the PS content is raised. No difference has been found between the gels containing PS1 or PS2. This fact could be explained taking into account that the number of gel joints should not be (26) Malmsten, M.; Lindman, B. Macromolecules 1993, 26, 1282. (27) Skoulios, A.; Helffer, P.; Gallot, Y.; Selb, J. Macromol. Chem. 1971, 148, 305.
Quintana et al.
Figure 4. Swelling ratio P/P0 as a function of time for SEBS2/ PS1/n-decane gels at 25 °C. SEBS2 preparation concentrations: 15 wt %. PS1 preparation concentrations: 1 (O), 3 (0) and 5 (4) wt % by mass of copolymer.
Figure 5. Equilibrium swelling ratio, λ∞, against the PS1 percentage by mass of copolymer, for SEBS2/PS1/n-decane gels at 25 °C.
affected by the number of PS chains solubilized. The light increment of log G′ with the homopolystyrene content could be due to the introduction of a less solvated component into the gel. Finally the homopolystyrene influence on the swelling behavior of SEBS2 gels has been analyzed. When SEBS2/ PS1 gels were immersed into an excess of n-decane, they swelled up to reach an equilibrium swelling. The swelling ratio λ is defined as P/Po, where Po is the sample weight after preparation but just before the immersion in a solvent excess and P is the sample weight after a certain time from the immersion. The swelling equilibrium ratio λ∞ is then expressed as
λ∞ ) (P/Po)tf∞ The swelling behavior was similar for all the polymer blend gels studied. As an example, the evolution of the swelling ratio as a function of time is plotted in Figure 4 for several SEBS2/PS1/n-decane gels with different PS1 percentages. A quasi-equilibrium was obtained in every gel after days. The swelling equilibrium ratio as a function of PS1 percentage by mass of SEBS2 is plotted in Figure 5. Though the experimental error can be considerable due to the lack of consistency of gels after being immersed for a long time in an excess of n-decane, the PS1 percentage dependence of the swelling equilibrium ratio can be
Polystyrene-b-poly(ethylene/butylene)-b-polystyrene
considered to be linear within the preparation concentration range studied. An increment of the PS1 content leads to a higher λ∞ value. This behavior agrees with the conclusion we came to after studying the solvent influence on the physical gelation of SEBS copolymers in paraffinic oils.17 The swelling capability of a copolymer gel would be related with the number or strength of the gel junctions.
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We have seen above how an increment in the homopolystyrene content of the gel causes a larger thermal stability of the gel. Acknowledgment. The authors thank the Vicerrectorado de Investigacio´n de la Universidad del Paı´s Vasco, CYTED, and CICYT for their financial support. LA970742E
