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J. Phys. Chem. B 2001, 105, 2966-2970
Thermoreversible Gelation of Polystyrene-b-Poly(ethylene/butylene)-b-Polystyrene Triblock Copolymers in N-Octane Jose R. Quintana, Estı´baliz Herna´ ez, 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, Spain ReceiVed: October 25, 2000
The thermoreversible gelation of three triblock copolymers polystyrene-b-poly(ethylene/butylene)-bpolystyrene, with different molar masses and a similar chemical composition, in n-octane was studied. The solvent is selective for the middle poly(ethylene/butylene) block of the copolymers. The influence of the molar mass of the copolymer on the sol-gel transition and on the mechanical properties of the gels was analyzed. The sol-gel transition temperature increased with the copolymer concentration and the copolymer molar mass. The mechanical properties of the different gels were examined through oscillatory shear and compressive stress relaxation measurements. The concentration dependence of the elastic storage modulus was established with an exponent close to that expected for systems in good solvents (2.25) that possess a structure similar to that of chemical networks. The experimental data of the three copolymers fit a sole straight line in a double-logarithmic scale. The relaxation rates observed were high for the copolymers with lower molar masses, indicating a considerable mobility in the gel over the measurement time. The relaxation rate decreased as the copolymer molar mass increased. Some of the copolymer gels examined exhibited some elasticity, allowing reversible deformation. The degree of elastic response of SEBS gels increased the higher the gelation temperature of the gel was, that is, the longer the lifetime of the gel junctions was.
Introduction It is well-known that block copolymers can form micelles in solution, and a great number of investigations have been focused on micellization of AB and ABA type block copolymers in selective solvents of their A blocks. It is now well recognized1-3 that for these copolymer systems the micelles consist of two main regions, an inner core containing the insoluble B blocks and an outer corona containing both the soluble A blocks and the solvent molecules. In most cases, the micelles have spherical shape and a narrow size distribution. The micellization process obeys the closed association mechanism, which assumes a dynamic equilibrium between copolymer chains and micelles with an association number N. The association phenomenon of BAB-type block copolymers in selective solvents of their A blocks would lead to more possible aggregate structures. Whereas an AB diblock chain in a micelle can only have the B block in the micellar core and the A block dangling in the corona, a BAB triblock chain can have both B blocks in a same micellar core with the A block forming a loop, each B block in a different micellar core with the A block forming a bridge, or a B block in a micellar core with the other B block dangling into the solution.4,5 The additional entropic penalty to the flowerlike micelle formation that arises from the loop formation of the corona blocks would cause some B blocks to extend into solution thus favoring a branched structure formation. However, the removal of poorly solvated B blocks from the core into solution also produces an interfacial free energy penalty favoring, therefore, the flowerlike micelle formation. Thus, intermediate structures between both extremes are also possible. * To whom correspondence should be addressed. Address: Avda. Basagoiti, 8, 1°C, 48990 Getxo, Vizcaya, Spain.
The ability of self-associated BAB triblock chains to bridge two insoluble regions is one main feature distinguishing them from self-associated AB diblock or ABA triblock chains. This distribution leads to a very important qualitative difference in the phase behavior of these systems. At low concentrations, solutions of AB and ABA block copolymers in selective solvents of A blocks show the existence of standard micelles,6-8 and only at high concentrations are macrolattice structures observed9-12 as a consequence of extensive entanglements of the A blocks in the corona of the close-packed micelles. In a BAB triblock system, on the other hand, well-defined micelles4,13,14 or loose and polydisperse aggregates rather than standard micelles15 can be found at low concentrations, and a gelation could occur in semidiluted solutions16-22 by bridging of the micelles in addition to entanglements at high concentrations. To improve our understanding of these systems, we have undertaken the study of the self-aggregation process of triblock copolymers in selective solvents of the middle block in both dilute14,23,24 and semidilute solutions.25,26 In a recent paper27 we have studied the self-association of three polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymers, SEBS, in noctane. The three block copolymers had similar weight percentages of polystyrene but different molar masses. n-Octane is a good solvent for poly(ethylene/butylene) block and is a precipitant for the polystyrene blocks. Whereas we found stable solutions of well-defined micelles for the two copolymers with lower molar mass, the copolymer of high molar mass was not dissolved or did not form clusters which were seen with the naked eye even at concentrations as low as 8 × 10-5 g‚mL-1. Focusing our investigations on both copolymers with lower molar mass, we found that the standard functions of micellization showed more negative values for the SEBS copolymer with large molar mass. Micelle molar mass, association number,
10.1021/jp003942z CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
Thermoreversible Gelation of Triblock Copolymers
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2967
TABLE 1: Characteristics of the Block Copolymer Samples: Mass Average Molar Mass of the Copolymer, Mw, of the Polystyrene Blocks, Mw,PS, and of the Poly(ethylene/ butylenene) Block, Mw,PEB, Polystyrene Weight Percentage and Polydispersity Index, I Mw Mw,PS Mw,PEB wt % PS I
SEBS1
SEBS2
SEBS3
60,700 2 × 9,100 42,500 30 1.09
87,300 2 × 14,000 59,400 32 1.11
260,000 2 × 39,000 182,000 30 1.18
and viscosimetric hydrodynamic radius also increased with the length of the copolymer chain. In this paper we have extended this investigation to semidiluted solutions where thermoreversible gelation has been observed for the three copolymers. We have studied the sol-gel transition and the mechanical properties of the gels. Oscillatory shear and compression measurements were carried out over the concentration and temperature ranges for which the gels had enough consistency. Experimental Section Materials and Gel Preparation. The polystyrene-b-poly(ethylene/butylene)-b-polystyrene triblock copolymer samples are commercial products kindly provided by Shell Espan˜a, S.A. The samples have been previously characterized in detail.28 They are homogeneous in chemical composition, and their mass average molar masses, polydispersities, and styrene contents are shown in Table 1. n-Octane (analytical purity grade) was used without further purification. Sample gels were prepared by dissolving the copolymer samples in n-octane at 120 °C in sealed flasks. Once the solutions were clear they were allowed to cool in order to form the gels. In this study the concentrations are expressed in wt %. The concentration ranges used were chosen in order to get a sufficient consistency of the gels and melting temperatures low enough to avoid solvent evaporation. Sol-Gel Transition. The gelation temperatures were determined by inverting a test tube containing the copolymer solutions. As the temperature was lowered, the temperature at which the solution changes from a mobile to an immobile system was considered as the gelation temperature, TGL. Oscillatory Experiments. Oscillatory shear measurements were performed in a Polymer Laboratories dynamic 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. The copolymer solutions were poured into the fluid cup at high temperature and then allowed to cool and to stabilize to the measuring temperature. The elastic storage, G′, and loss modulii, G′′, were measured as a function of frequency between 0.1 and 20 Hz at a maximum strain amplitude of 6.25 mrad. The temperature was controlled with a precision of 1 °C. Relaxation Experiments. Compression measurements were made in a Perkin-Elmer dynamic mechanical analyzer DMA7. A cup and a plate with diameters of 18 and 10 mm, respectively, were used for all measurements. The gels were formed in the cell in the same way as described above, having a height of 3 mm. The stress relaxation measurements were performed by measuring the load as a function of time, keeping the gel deformation and temperature constant. The temperature was controlled with a precision of 0.1 °C. Results and Discussion The sol-gel transition temperatures were determined by tubeinversion method. As the temperature was lowered, the tem-
Figure 1. Gelation temperature as a function of the copolymer concentration for SEBS1 (0), SEBS2 (4), and SEBS3 gels (O).
perature at which the solution becomes an immobile system was considered as the gelation temperature, TGL. Gelation temperatures were determined for SEBS1, SEBS2, and SEBS3 solutions covering concentration ranges 3-39, 3-22, and 2-7 wt %, respectively. Plots of gelation temperature as a function of copolymer concentration for solutions of the three copolymers in n-octane are shown in Figure 1. All the plots were linear within the experimental error over the temperature ranges studied: -5-68 °C for SEBS1, 5-44 °C for SEBS2, and 6090 °C for SEBS3. The gelation temperature increases as the copolymer concentration increases suggesting that the higher the copolymer concentration in the solution is, the higher the temperature will be at which the minimum number of physical junctions necessary to form the gel can be obtained. This concentration dependence increases with the molar mass of the copolymer. Similar results have also been found for these copolymers in a paraffinic oil.29 Taking into account that the three copolymers have similar chemical compositions and that the polystyrene block is the block responsible for the gel junctions, the above result suggests that the chain length of this copolymer block determines the ease of gelation and consequently the thermal stability of the gels formed in a given solvent. This behavior accords with the results found for these same copolymers in diluted solutions of n-octane27 where the stability of the micelles increased as the molar mass of the copolymer increased. This behavior seems logical since the incompatibility with the solvent will increase as the molar mass of the copolymer blocks for which the solvent is precipitant increases. The upper limits of the concentration ranges studied have been chosen because of the handing difficulties found due to the high viscosity of the copolymer solutions or because the corresponding gelation temperatures were too close to the boiling point of the solvent. On the other hand, the lower limit of the concentration ranges corresponds to the critical gel concentration, CGC, defined as the lowest concentration at which a particular polymer is capable of forming a one-phase gel. The critical gel concentrations range between 2.15 and 2.48 wt % for SEBS1, 1.55 and 1.75 wt % for SEBS2, and 1.59 and 1.74 wt % for SEBS3. According to their values, one could say that the copolymer molar mass influence on the critical gel concentration hardly exists in the experimental range studied. Measurements of the real and imaginary parts, G′ and G′′, of the complex shear modulus were also made as a function of the frequency, ω, of a small deformation oscillatory shear strain. Gels of the three copolymers at different concentrations and temperatures were analyzed. Figure 2 shows the plots of log G′ and log G′′ against log ω for three gels of SEBS2 at three copolymer concentrations and at -20 °C. According to the
2968 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Figure 2. Frequency dependence of the dynamic storage and loss moduli (G′, open symbols, and G′′, filled symbols, respectively) for SEBS2 gels at -20 °C. Copolymer concentrations: 5.8 (O), 12.3 (0), and 17.5 wt % (4).
Figure 3. Dynamic storage modulus, GN, versus concentration on a logarithmic scale for SEBS1 (0), SEBS2 (4), and SEBS3 gels (O) at -20 °C.
results presented in this figure the storage and loss moduli were virtually independent of frequency over 2 orders of magnitude. This behavior is consistent with the dynamic mechanical behavior of a physical gel.30 Another noteworthy feature of Figure 2 is that G′ exceeds G′′ over the entire range of frequency examined by 1 or 2 orders of magnitude this characteristic also being indicative of a physical gel. On the other hand, as the copolymer concentration increases, the storage modulus, G′, showed higher values, meaning that the number of long-lived junctions in the gel increases. This same behavior has been observed in gels of SEBS1 and SEBS3 at a temperature of -20 °C. The block copolymer gels studied at -20 °C exhibited an elastic behavior, at least in the investigated frequency range, and therefore the constant values of G′ could accordingly be considered as the plateau modulus, GN. The variation of the plateau modulus with concentration for the three copolymers at -20 °C is plotted in Figure 3. The copolymer concentrations range from 8.7 to 33.4 wt % for SEBS1, from 3.8 to 17.6 wt % for SEBS2, and from 2.5 to 6.5 wt % for SEBS3. At these concentration intervals the gels had enough consistency, and the melting temperatures were not too high. For the three copolymer samples, the data were linear in a double-logarithmic scale and yield a scale power relation
GN ≈ c
2.22
with c in g/g. n-Octane is a selective solvent for the poly(ethylene/butylene) block; therefore, the gels should possess a fringed micellar
Quintana et al.
Figure 4. Frequency dependence of the dynamic storage modules for a SEBS1 solution (c ) 8.7 wt %) at various temperatures: -20 (0), -10 (4), 0 (O) and 10 °C (3).
structure characterized by knots formed by aggregated polystyrene blocks and linked by well-solvated poly(ethylene/ butylene) blocks when the connections are direct. The mesh size will be determined by the poly(ethylene/butylene) block length and the strength and lifetime of the junctions by the stability of the polystyrene micelle cores. Considering that one is dealing with gels as flexible as those prepared chemically, the result found agrees well with the scaling predicted value (2.25).31 Taking into account that the data of the three copolymer fit a unique straight line so well, one can say that the copolymer molar mass has no influence in the dynamic storage modulus. In consequence, one can assume that the gels of the three copolymer samples have a similar number of cross-links for the same concentration. Oscillatory shear measurements were also carried out as a function of temperature. Figure 4, shows the evolution of the frequency dependence of the dynamic storage modulus with the temperature for an SEBS1 solution with a concentration of 8.7 wt %. The gelation temperature of this solution is around 9 °C. As the temperature decreases the frequency dependence of G′ decreases, reaching a constant value at lower frequencies, and the GN values increase. This result means that as the temperature becomes lower than the gelation temperature, the lifetime of the gel junctions increases, and therefore, the number of longlived cross-links in the gel increases. The same behavior has been found when this kind of experiment has been carried out on SEBS2 gels. The stress relaxation responses to a compressive deformation were determined for SEBS2 and SEBS3 gels at different concentrations and temperatures. Figure 5 shows the relaxation responses for several gels of SEBS2 and SEBS3 at -20 °C. At an early stage, the relation between stress, σ, and time, t, can be considered as linear in a double logarithmic plot. The stress relaxation rates m ) -d(log σ)/d(log t) were relatively independent of the copolymer concentration in the experimental range studied and for deformations between λ ) 0.9 and λ ) 0.7. At relaxation times t < 2000 s the following average values were determined at -20 °C.
SEBS1 m ) 0.27 ( 0.07 SEBS2 m ) 0.21 ( 0.02 SEBS3 m ) 0.03 ( 0.02 The relaxation rates would be related to the lifetime and weakness of the physical cross-links. A higher m value suggests the existence of weak junctions or with a short lifetime compared
Thermoreversible Gelation of Triblock Copolymers
Figure 5. Compressive stress, σ, versus time on a logarithmic scale for SEBS2 and SEBS3 gels at -20 °C. SEBS2 concentrations: 8.5 (4), 12.3 (]), and 17.5 wt % (O). SEBS3 concentrations: 4.9 (3), and 6.8 wt % (0). Deformation λ ) 0.7.
Figure 6. Stress relaxation rate as a function of temperature for SEBS2 and SEBS3 gels. Deformation λ ) 0.7. SEBS2 concentrations: 8.5 (0), 11.2 (4), and 17.5 wt % (O). SEBS3 concentrations: 4.9 (]) and 6.8 wt % (3).
to the measurement time. Values similar to those found have been reported for physical gels with an absence of crystalline order in the physical junctions32,33 (m ) 0.08-0.2). The physical gels where junctions are crystalline show even lower relaxation rates34,35 (m ) 0.01-0.03) and quite similar to those reported for chemical gels with permanent cross-links.36 The large relaxation rates found for SEBS1 and SEBS2 gels suggest a high mobility in the network over the time of measurement and could be explained according to a threedimensional network stabilized by micelles of the glassy polystyrene end blocks in which the junctions break and reform. This fact would be confirmed by the dynamic equilibrium free chain/micelle that the block copolymers show in selective solvent solutions.3 The SEBS gels would be in a dynamic equilibrium state where some junctions are broken and new ones are formed continuously. The higher m values found for SEBS1 and SEBS2 would be then a consequence of the shorter life of their junctions compared to those of the SEBS3 gels. The lifetime of the junctions will be related to the thermal stability of the polystyrene block domains. Then the differences found in the stress relaxation rates are in accord with those found in the gelation temperatures (Figure 1). Longer PS blocks will increase its incompatibility with the solvent and cause stronger and more lasting gel junctions. Figure 6 shows the evolution of the stress relaxation rates with temperature for SEBS2 and SEBS3 gels at several copolymer concentrations. At temperatures significantly lower
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2969
Figure 7. Compressive stress relaxation curve for a SEBS3 gel (c ) 4.9 wt %) with a initial deformation λ1 ) 0.7 and a second deformation λ2 ) 0.8. Temperature ) -10 °C.
Figure 8. Compressive stress relaxation curve for two SEBS2 gels (c ) 12.3 wt % (O) and c ) 17.5 wt % (0)) with an initial deformation λ1 ) 0.7 and a second deformation λ2 ) 0.8. Temperature ) -20 °C.
than the gelation temperature the relaxation rate is practically independent of temperature, but as the temperature increases, a point is reached from which the relaxation rate increases abruptly with the temperature. This result suggests that near the gelation temperature, an increment in the temperature decreases the stability of the micelles, and the lifetime of the gel junctions decreases. The same phenomenon has been observed for both copolymers with the difference that, as the SEBS2 gels have lower gelation temperatures, the m increment starts at lower temperature. To check if the studied SEBS gels behave according to the definition of physical gel proposed by Guenet,37 a mechanical test, devised by Daniel et al.,38 has been applied to SEBS2 and SEBS3 gels for evaluating the degree of elastic response of such systems. The idea is that a gel should exhibit some elasticity, allowing reversible deformation. The test consisted in applying a compressive deformation λ1 ) 0.7 and allowing the gel to relax for 60-70 min, and then the deformation was quickly changed to λ2 ) 0.8 by raising the piston of the compression apparatus. This caused a sudden drop in stress to zero. According to the authors,38 if the gel still possesses elasticity, then it will recover its initial height partially, whence the reappearance of the stress. If the gel is not at all elastic, then the stress should remain null. The compressive experiments carried out on SEBS3 and SEBS2 gels are shown in Figures 7 and 8, respectively. Clearly, the SEBS3 gels show a reversible deformation, implying a degree of elastic response of such gels. However, SEBS2 gels show a different behavior. Whereas a gel with a copolymer concentration of 17.5 wt % presents a elastic response, the gels
2970 J. Phys. Chem. B, Vol. 105, No. 15, 2001 with concentrations equal or lower than 12.3 wt % undergo a permanent and irreversible deformation. Considering that the kind of the gel structure does not depend on concentration, the proposed criterion for recognizing thermoreversible gels is not a general one. The above results suggest that the degree of elastic response of SEBS gels increases the higher the gelation temperature of the gel is or, in other words, the longer the lifetime of the gel junctions is. Conclusions Polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymers show thermally reversible gelation in semidiluted solutions of n-octane. The gelation temperature increases with the copolymer concentration and molar mass. The copolymers studied present a critical gel concentration, defined as the lowest concentration at which the copolymer is capable of forming a one-phase gel. This concentration depends slightly on the copolymer molar mass, being higher the lower the molar mass is. The three copolymers studied show the same concentration dependence of the elastic storage modulus with an exponent close to that expected for systems in good solvents that possess a structure similar to that of chemical networks (2.25). The relaxation rates found for the copolymer samples with lower molar masses are high, suggesting a considerable mobility in the gels over the measurement time, and this could be explained according to a classical network in which the junctions break and reform. The relaxation rate increases as the copolymer molar mass decreases, suggesting the existence of junctions with a shorter lifetime for those gels formed by copolymers with lower molar mass. Gels formed by copolymers of high molar mass and with a high copolymer concentration show a reversible deformation, implying a degree of elastic response. References and Notes (1) Brown, R. A.; Masters, A. J.; Price, C.; Yuan, X. F. In ComprehensiVe Polymer Science; Allen, G., Bevington, J. C., Eds.; Pergamon Press: Oxford, 1989; Vol. 2, Chapter 6. (2) Tuzar, Z.; Kratochvı´l, P. In Surface and Colloid Science; Matigevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15(1). (3) Quintana, J. R.; Villacampa, M.; Katime, I. In The Polymeric Materials Encylopedia. Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press: Florida, 1996; Vol. 1, p 815.
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