Langmuir 2000, 16, 1447-1449
Rheological Behavior of Poly(methyl methacrylate) Dispersions Stabilized by a Diblock Copolymer. 2. Positive and Negative Electrorheological Effect† Vladimı´r Pavlı´nek and Petr Sa´ha Faculty of Technology in Zlı´n, Technical University Brno, 762 72 Zlı´n, Czech Republic Otakar Quadrat* and Jaroslav Stejskal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic Received May 11, 1999. In Final Form: September 13, 1999
Introduction The electrorheological (ER) effect is a phenomenon in which the rheological behavior of a fluid is modified by the influence of an external electric field. It has been the object of many studies and reviews.1-4 ER fluids comprise suspensions of solid particles in an electrically insulating liquid in which the polarization of particles takes places in the electric field. As a result of electrostatic interactions between induced dipoles, the organization of polarized particles into fibers or strands occurs, leading to a higher pseudoplasticity and, at higher particle concentrations, even to the yield stress appearance. The positive ER behavior, when viscosity increases after the application of the electric field, involves most of the systems described in the literature. However, a small number of cases exist when the viscosity under the influence of the electric field decreases. The causes of such a rare negative ER effect may be various. An anisotropy of the dielectric polarizability of flat particles was assumed in the suspension of magnesium hydroxide in silicone oil.5 The conductivity ratio of the solid and liquid phases was proposed by Boissy et al.6 to be decisive for the character of the electrorheological effect. Thus, if the conductivity of the liquid phase is greater than that of dispersed solids, the particles are drawn to the region of low field due to the interfacial polarization and the viscosity under electric field decreases. A viscosity decrease in the suspension of poly(tetrafluoroethylene) particles after the electric field application was considered to be a result of particle migration to the anode and the subsequent separation of the suspension into two layers.7 The negative ER effect has also been described in systems containing segmented polyurethanes8-11 as a result of the phase separation in the fluid induced by an electric field. * To whom correspondence should be addressed. E-mail:
[email protected]. † For part 1, see ref 19. (1) Block, H.; Kelly, J. P. J. Phys. D.: Appl. Phys. 1988, 21, 1661. (2) Jordan, T. C.; Shaw, M. T. IEEE Trans. Electr. Insul. 1989, 24, 849. (3) Block, H. J.; Kelly, J. P.; Qin, A.; Watson, T. Langmuir 1990, 6, 6. (4) Block, H.; Rattray, P. In Progress in Electrorheology; Havelka, K. O., Filosko, F. E., Eds.; Plenum: NewYork, 1995; p 19. (5) Trlica, J.; Quadrat, O.; Bradna, P.; Pavlı´nek, V.; Sa´ha, P. J. Rheol. 1996, 40, 943. (6) Boissy, C.; Atten, P.; Foulc, J. N. J. Intell. Mater. Syst. Struct. 1996, 7, 599. (7) Wu, C. V.; Conrad, H. J. Rheol. 1997, 41, 267. (8) Uemura, T.; Minagawa, K.; Koyama, K. Chem. Lett., Chem. Soc. Jpn. 1994, 64. (9) Gohko, N.; Uemura, T.; Minagawa, K.; Takimoto, J.; Koyama, K. Rep. Prog. Polym. Phys. Jpn. 1995, 38, 77.
1447
We have found that in poly(methyl methacrylate) (PMMA) dispersions stabilized by polystyrene-block-poly(ethylene-co-propylene) in decane, both positive and negative ER effects occur, depending on particle concentration and the content of the diblock copolymer stabilizer. The demonstration of this behavior is the objective of this study. Experimental Section Dispersions. A series of dispersions of poly(methyl methacrylate) particles in decane (purum; Fluka, Switzerland) with increasing volume fraction of particles φP were prepared12-14 by the dispersion polymerization of methyl methacrylate (MMA; purum, Lachema, Czech Republic) (volume fraction φM), initiated by 10-3 g cm-3 azobisisobutyronitrile (AIBN; purum, BDH, England) and stabilized by the steric stabilizer (volume fraction φK) polystyrene-block-poly(ethylene-co-propylene) diblock copolymer (Kraton G 1701, Shell; 42 wt % styrene units, Mw ) 1.1 × 105 g mol-1, Mw/Mn ≈ 1.2). Before the monomer was added, the stock (3 wt %) solution of the stabilizer was heated for 10 min at 100 °C. During this operation, the metastable structures formed by block copolymer molecules surviving in decane after the dissolution converted to stable well-defined micelles.13 The mixtures of a monomer (MMA), a block copolymer, an initiator (AIBN), and decane were sealed into glass ampules and were allowed to polymerize at 60 °C for 70 h. Such a process guarantees monomer conversions >98%, while all the stabilizer was incorporated into the dispersion particles.12 While the monomer concentration increased from zero, the volume fraction of stabilizer was decreased according to the relation φK ) 0.022 (1 - φM) (Table 1). The macroscopic precipitation of the polymer during the polymerization was observed at φM > 0.36. Dynamic Light Scattering. The hydrodynamic radius of the particles was measured using an Autosizer Lo-C (Malvern, U.K.). Viscometry. The measurement of the apparent viscosity was carried out in the range of shear rates 15.5-1259 s-1 using a coaxial cylinder rotational viscometer Rheotest (type RV 2, Pru¨fgera¨te-Werk Medingen, Dresden, Germany) modified for ER experiments. The rotating inner cylinder, 39.2 mm in diameter, and the outer cylinder separated by the gap 0.4 mm were connected to a dc power supply, U ) 0-1.0 kV (high-voltage source NB 411, Tesla, Czech Republic), which corresponded to the electric field strength E ) 0-2.5 kV mm-1. The sample temperature during experiments was maintained within the interval 24 ( 0.1 °C.
Results and Discussion Polystyrene-block-poly(ethylene-co-propylene) forms micelles in alkanes. The insoluble polystyrene blocks constitute the micellar core, and the polyolefinic blocks afford the shell. The interaction of shell chains in alkanes, reported in many papers,13,15-18 results in the high viscosity of micellar solutions.13 When methyl methacrylate is added to the micelles and subsequently polymerized, the dispersion particles are obtained. As the content of the monomer increases, the volume fraction of the particles grows and the particle size increases (Table 1). The number of stabilizer chains per unit particle mass decreases at the same time. (10) Gohko, N.; Uemura, T.; Minagawa, K.; Takimoto, J.; Koyama, K. Abstracts, 36th IUPAC International Symposium on Macromolecules, Seoul, Korea, Aug. 4-9, 1996; p 1026. (11) Uemura, T.; Minagawa, K.; Koyama, K. Polym. Prepr. 1994, 35 (2), 360. (12) Stejskal, J.; Kratochvı´l, P.; Koubı´k, P.; Tuzar, Z.; Helmstedt, M.; Jenkins, A. D. Polymer 1990, 31, 1816. (13) Stejskal, J.; Hlavata´, D.; Sikora, A.; Konˇa´k, C ˇ .; Plesˇtil, J.; Kratochvı´l, P. Polymer 1992, 33, 3675. (14) Stejskal, J.; Kratochvı´l, P. Makromol. Chem., Macromol. Symp. 1992, 58, 221.
10.1021/la990574l CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
1448
Langmuir, Vol. 16, No. 3, 2000
Notes
Table 1. Volume Fraction of the Monomer (Methyl Methacrylate) φM, and of the Steric Stabilizer (Diblock Copolymer) φK in the Reaction Mixture Producing PMMA Dispersions with the Particle Size RP and a Low Shear (γ ) 15.5 s-1) Viscosity in the Absence of Electric Field, ηL,E)0, and at Electric Field Strength of 2.5 kV mm-1, ηL,E)2.5. φM (vol %)
φK (vol %)
RP (nm)
ηL,E)0 (mPa s)
ηL,E)2.5 (mPa s)
τ0a (Pa)
ERb
0 5 8 10 15 20 23 27 30 33 36
2.20 2.09 2.03 1.98 1.87 1.76 1.70 1.61 1.54 1.48 1.43
42c 60 66 75 136 210 318 500 617 745 1129
74 196 420 526 274 98 74 70 78 94 99
74.5 215 196 196 184 102 110 149 180 228 259
0 0.1 4.2 5.9 3.3 0 0 0 0 0 0
0 0 0 + + + + +
a τ is the yield stress. b The observed electrorheological effect, 0 ER, is either negative, absent, or positive. The total volume fraction of dispersion particles was taken as φP = φK + φM. c Taken from ref 13.
Figure 1. Dependence of the low-shear viscosity (shear rate ) 15.5 s-1) in the absence of an electric field (b) and at the electric field strength 2.5 kV mm-1 (O) on the volume fraction of particles φP.
The associative interactions between the particles dominate the rheological behavior of the dispersion at low PMMA loading, and the viscosity of the system increases (Figure 1). At the viscosity maximum (φP ≈ 0.12), the interactions were so strong that the dispersion became gel-like, and the yield stress τ0 was needed to produce a flow (Table 1). The reduced surface concentration of stabilizer chains on large particles produced at the high content of PMMA resembles the behavior of non-interacting spheres, and the viscosity decreases. Despite the increasing particle concentration, the apparent viscosity fell down to a broad minimum and then, obviously as a result of the growing particle concentration, increased again. The gradual conversion of the system of interacting spheres into non-interacting ones during the increasing loading of the particles with PMMA has been described in the previous paper in this series.19 The apparent viscosity of dispersions measured at low shear rate (15.5 s-1) is affected by the application of an electric field (Figures 1 and 2), and the flow properties of dispersions changed in a different way according to particle (15) Price, C.; Hudd, A. L.; Wright, B. Polymer 1982, 23, 170. (16) Price, C. Pure. Appl. Chem. 1983, 55, 1563. (17) Higgins, J. S.; Blake, S.; Tomlins, P. E.; Ross-Murphy, S. B.; Staples, E.; Penfold, J.; Dawkins, J. V. Polymer 1988, 29, 1968. (18) Tsunashima, Y.; Hirata, M.; Kawamata, Y. Macromolecules 1990, 23, 1089. (19) Pavlı´nek, V.; Sa´ha, P.; Stejskal, J.; Quadrat, O. J. Rheol., in press.
Figure 2. Dependence of the apparent viscosity η on the shear rate γ for various electric field strengths (kV mm-1): (O) 0, (]) 0.5; (3) 1.0; (4) 1.5; (0) 2.0; (open triangle pointing to the left) 2.5. Volume fraction of particles φP: (a) 0.022; (b) 0.12; (c) 0.25; (d) 0.36.
concentration φP. In the solution of block copolymer micelles (i.e., in the absence of any PMMA, φK ) φP ) 0.022) and in the dispersion at a low particle concentration (φP ) 0.071), no change due to the electric field was found (Figure 2a). At higher φP, a negative ER effect appeared (Figure 2b), reaching the highest intensity at the particle concentration corresponding to the maximum viscosity in the absence of an electric field (Figure 1). At the same time, when the viscosity decreased due to the applied electric field, the yield stress virtually diminished. After passing the viscosity maximum, the negative ER effect diminished, and as soon as ηL,E)0 reached a minimum, it completely disappeared (Figure 2c). A further increase in φP caused a rise in the dispersion viscosity and the appearance of the positive ER effect (Figure 2d). The reasons for observation of positive and negative ER behavior are different. In the former case, the apparent viscosity continuously increased with the field strength. In the latter, a viscosity lowering occurred only after overcoming a certain critical threshold field strength needed for the disintegration of the particle network structure (Figure 3). Apparently, at moderate particle concentrations, a physical network of the organized particles is produced, due to specific conditions for interactions of stabilizer macromolecules. In the presence of an electric field, the network structure is destroyed by electrostatic forces acting on polarized particles, reorganizing them into chains parallel to an electric field, whose resistance against the flow is lower than that of the original
Notes
Langmuir, Vol. 16, No. 3, 2000 1449
Conclusions
Figure 3. Dependence of the apparent viscosity ηL on the electric field strength E at the shear stress γ ) 15.5 s-1. Volume fraction of particles φP: ([) 0.12; (]) 0.364.
network structure. Consequently, the negative ER effect appears. At higher PMMA loading, due to a progressive decrease in specific particle interactions, the viscosity ηL,E)0 increases as a result of increasing filling of the system by polymer solids. Under these conditions, the viscosity of the organized particle-chain structure formed after electric field application is higher than that of the original dispersion and the positive ER effect occurs.
The poly(methyl methacrylate) particles stabilized in decane by polystyrene-block-poly(ethylene-co-propylene) copolymer can exhibit both negative and positive electrorheological effects. When the loading of dispersion particles by PMMA is low, the particles are interacting through the association of the stabilizer chains in the shells. The reorganization of the particles after the application of the electric field reduces also the extent of interactions, and the viscosity of the dispersions decreased (negative electrorheological effect). At high PMMA concentration in the particles, the relative proportion of the stabilizer is low, and the interactions promoted by the stabilizer are restricted. The organization of virtually noninteracting particles in the electric field, manifested by the increase of the apparent viscosity, is found (positive electrorheological effect). Acknowledgment. The authors wish to thank to the Grant Agency of the Czech Republic (104/97/0759) and the Academy of Sciences of the Czech Republic (KSK 2050602) for financial support. LA990574L
