474
J. Phys. Chem. 1992,96,414-484
Viscoelastic Micellar Solutions: Microscopy and Rheology T. M. Clausen,? P. K. Vinson,*,sJ. R. Minter,"H. T. Davis,* Y. Talmon,l and W. G. Miller*,+ Department of Chemistry, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455-0431: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455: Eastman Kodak Company, Analytical Technology Division, Kodak Park, Rochester, New York 14652; and Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel (Received: May 31, 1991)
Artifact-free electron micrographs and rheological properties of aqueous solutions of cetyltrimethylammoniumchloride (CTAC), with salicylate (from Nasal) as the binding counterion, are presented as a function of counterion concentration. At low [Sal-], CTAC solutions form globular (spherical) micelles ( 5 nm in diameter) and behave as Newtonian fluids. With increasing [Sal-], these solutions form threadlike or wormlike micelles ( 5 nm in diameter with lengths as long as several micrometers) that are viscoelastic and entangled. At moderate [Sal-], these solutions have a spectrum of relaxation times similar to semidilute polymer solutions. At high [Sal-], a single relaxation time dominates. The viscosity of the viscoelastic solutions shows shear thinning and power law behavior. As a function of [Sal-], the zero-shear viscosity (w) reaches a maximum 5 orders of magnitude greater than the initial viscosity of the solution with no salicylate. With further increase in [Sal-], the zero-shear viscosity then decrease? 3 orders of magnitude. After the formation of threadlike micelles, the rheological properties change as a function of [Sal-], whereas the electron micrographs all show similar images of entangled threadlike micelles. Some of the images have a small number of closed circular threadlike structures, indicating the existence of ringlike micelles. Samples for electron microscopy were thermally fixed before and after relaxation. In some cases, the samples imaged before relaxation revealed the threadlike micelles in an ordered state due to the shear induced during sample preparation.
of the counterion. Ulmius et aL7 and Olsson et ai." have shown Introduction that the aromatic part of the salicylate ion (Sal-) is solubilized Long threadlike micelles form in aqueous solutions of cationic surfactants above threshold surfactant and salt concentrations. The most extensively studied systems are alkyltrimethylammonium and alkylpyridinium salts. Halide anions associate only moderately (1) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869-6892. with the surfactant cations, and micellar growth is gradual. (2) Gamboa, C.; Sephlveda, L. J. Colloid Interface Sci. 1986, 113, However, with anions that associate strongly with the surfactant 566-576. cations, threadlike micellar growth occurs rapidly at low surfactant (3) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081-1086. and salt concentrations. The rheological behavior exhibited by (4) Candau, S. J.; Hirsch, E.; Zana, R.; Delsanti, M. Langmuir 1989, 5, 1225-1229. these systems is viscoelastic and analogous to that observed in ( 5 ) Imae, T. Colloid Polym. Sci. 1989, 267, 707-717. solutions of flexible polymers.'" These surfactant solutions (6) Sasaki, M.; Imae, T.; Ikeda, S.Langmuir 1989, 5> 211-215. undergo similar rheological behavior whether they are prepared (7) Ulmius, J.; Wennerstrom, H.; Johansson, L. B.-A.; Lindblom, G.; directly from surfactant salts with a strongly associating anion Gravsholt, S . J. Phys. Chem. 1979, 83, 2232-2236. or by addition of strongly associating anions to solutions prepared (8) Hoffmann, H.; Rehage, H.; Reizlein, K.; Thurn, H. Proceedings ofthe ACS Symposium on Macro- and Microemulsions; American Chemical Sofrom surfactant salts with weakly associating anions. These ciety: Washington, DC, 1985; pp 41-66. materials exhibit three regions of rheological response. First, at (9) Anet, F. A. L . J. Am. Chem. SOC.1986, 108, 7102-7103. low surfactant concentration, the solutions are Newtonian liquids (10) Manohar, C.; Rao, U.R. K.; Valaulikar, B. S.;Iyer, R. M. J . Chem. with low viscosity and nonmeasurable elastic response. Second, SOC.,Chem. Commun. 1986, 379-38 1. (11) Olsson, U.; SWerman, 0.;Gutring, P. J. Phys. Chem. 1986, 90, with increasing surfactant concentration, they behave like polymer 5223-5232. solutions in the semidilute regime, characterized by viscoelastic (12) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. behavior with a spectrum of relaxation times. Finally, with inChem. 1987, 91, 3286-3291. crease in the counterion concentration, these materials enter a (13) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354-359. (14) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1989, 5, 398-405. regime where their rheological response is similar to that of an (1 5) Makhloufi, R.; Hirsch, E.; Candau, S. J.; Binana-Limbele, W.; Zana, entangled polymer or weak gel; however, unlike polymer systems, R. J . Phys. Chem. 1989,93, 8095-8101. their relaxation after shear is dominated by a single relaxation (16) Magid, L. J. Colloids Surf 1986, 19, 129-158. time. This paper attempts to relate these regions of rheological (17) Hirata, H.; Sakaiguchi, Y. J. Colloid Interface Sci. 1988, 121, response to artifact-free electron microscope images. 300-301. (18) Hoffmann, H.; Platz, G.; Rehage, H.; Schorr, W. Adu. Colloid InSurfactant systems with strongly associating anions have been terface Sci. 1982, 17, 275-298. studied by NMR spectroscopy,'-'4 fluorescence q~enching,'~ X-ray (19) Bayer, 0.; Hoffmann, H.; Ulbricht, W.; Thurn, H. Adu. Colloid and neutron s ~ a t t e r i n g , ~ . 'static ~ . ' ~ and dynamic light scatterInterface Sci. 1986, 26, 177-203. negative-stain transmission electron m i ~ r o s c o p y , ~ ' - ~ ~ (20) Brown, W.; Johansson, K.; Almgren, M. J. Phys. Chem. 1989, 93, ing,8J 1~15~18-20 5888-5894. flow b i r e f r i n g e n ~ e , ' ~and < ~ ~a- ~variety ~ of rheological experim e n t ~ . ~ - ~ The , ~degree ~ ~ of~ counterion ~ ~ ~ ~association . ~ ~ ~ or~ ~ - ~(21)~ Sakaiguchi, Y.; Shikata, T.; Urakami, H.; Tamura, A.; Hirata, H. Colloid Polym. Sci. 1987, 265, 750-753. binding, at a given surfactant concentration, is determined by an (22) Sakaiguchi, Y.; Shikata, T.; Urakami, H.; Tamura, A.; Hirata, H. electrostatic contribution that depends on the counterion valence J . Electron Microsc. 1987, 36, 168-176. and a chemical contribution determined by the atomic components (23) Shikata, T.; Sakaiguchi, Y.; Uragami, H.; Tamura, A,; Hirata, H. *To whom correspondence should be addressed. 'Department of Chemistry, University of Minnesota. Department of Chemical Engineering and Materials Science, University of Minnesota. f Present address: The Procter and Gamble Company, Technical . . Ivorydale . Center, Cincinnati, Ohio 45217. 11 Eastman Kodak Company. Technion-Israel Institute of Technology. f
0022-3654/92/2096-474%03.00/0
J . Colloid Interface Sci. 1987, 119, 291-293. (24) Hirata, H.; Sakaiguchi, Y . ; Akai, J. J. Colloid Interface Sci. 1989, 127, 589-591. (25) Angel, M.; Hoffmann, H.; Lobl, M.; Reizlein, K.; Thurn, H.; Wunderlich, 1. Prog. Colloid Polym. Sci. 1984, 69, 12-28. (26) Rehage, H.; Hoffmann, H.; Wunderlich, I. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1071-1075. (27) Rehage, H.; Wunderlich, I.; Ihffmann, H. Prog. Colloid Polym. Sci. 1986, 72, 51-59,
0 1992 American Chemical Society
Viscoelastic Micellar Solutions
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 415
CTAC samples were used without further purification. Sodium salicylate (Nasal >99.5%) was obtained from Riedel-de-Haen (Germany). Sodium chloride (NaCl>99.5%) was obtained from CSah ma Eb TEM‘ rheologyd E K Industries (Addison, IL) and stored in a desiccator until 0 0 I 2 required. CTAC was dissolved in water that had been purified 0.20 I 2 0.010 in a carbon filter and ion exchange cartridges followed by dis0.013 0.25 K tillation. Solutions were made by adding water to weighed 0.30 K 0.015 quantities of surfactant and salts and then stirring for several hours 0.40 I 0.020 on a magnetic stirrer rotating at about 2 Hz. Table I is a list of 0.46 2 Z 0.023 the sample compositions studied by cryo-TEM and rheological 0.50 K K 0.025 0.52 2 2 0.026 methods. Samples were kept at room temperature for 2-20 days 0.53 K 0.027 before taking measurements, and care was taken to ensure that 0.56 2 0.028 the sample composition remained constant over the time between 0.60 I 2 0.030 preparation and observation. 0.03 1 0.62 K Cryo-TEM samples were prepared in the controlled environ0.70 K 0.035 ment vitrification system, or CEVS, which is described in detail 0.038 0.75 K elsewhere.36 In the CEVS, temperature was controlled to within 0.80 Z 0.040 f O . l OC. Before introducing the sample into the CEVS, the 0.045 0.89 K environmental chamber achieved steady state at 25 OC and near 1.oo K K 0.0500 1.22 K 0.0610 water saturation (95-99% relative humidity). The humidification 1.37 K 0.0685 of the chamber was accomplished with porous sponges extending 1.61 K 0.0805 upward from liquid reservoirs. The air inside the chamber was 1.80 K 0.0900 recirculated across the sponges to reduce temperature and com2.00 K K 0.100 position gradients in the vapor. The high relative humidity within the chamber reduces evaporation of water from the sample and ‘All samples have CC-,, = 0.050 m and CNaCl = 0.100 m. bCsal/ I refers to CTAC obtained from Imae, Z from Zana, and K prevents artifacts that result from drying. C,,,,. from Kodak. First two points used Couette geometry. Thin films of sample were formed by placing a 3-pL drop of the liquid on a holey polymer support film which had been coated inside the micelle. This solubilization results in counterion binding with carbon and mounted on the surface of a standard TEM grid.37 unlike thz case of the halide ions (Br- and C1-) and promotes the The drop was blotted with filter paper so that thin (10-500 nm) formation of large threadlike micelles. The most studied systems films of the sample remained, and these spanned the 2-8-pm holes are aqueous solutions of cetyltrimethylammonium bromide in the support film. The assembly was then vitrified by rapidly (CTAB), cetylpyridinium bromide (CPyB) with sodium salicylate plunging it through a synchronous shutter at the bottom of the (Nasal), cetyltrimethylammonium salicylate (CTASal), or ceenvironmental chamber into liquid ethane a t its freezing point. tylpyridinium salicylate (CPySal). Vitrified specimens were examined at 100-120 kV in the Despite the array of techniques used to understand these sysconventional TEM mode of an analytical electron microscope tems, details of network entanglements and dynamics, micelle size (JEOL Models JEM lOOCX and 120CX and Philips CM12) with distribution, and micelle flexibility remain elusive. For these a cry0 transfer holder (Gatan Inc. Model 626 and JEOL Model reasons, we investigated a similar system: cetyltrimethylEM-SCH). The cry0 transfer holder temperature was maintained ammonium chloride (CTAC), with salicylate (from Nasal) as below -165 OC during imaging. Images were recorded on SO-163 the solubilized counterion and NaCl to increase the ionic strength. film and developed in full-strength D-19 developer (Eastman This CTAC-Nasal-NaC1 system was studied as a function of Kodak) for 12 min. Images were recorded at approximately 4 the Sal- concentration (&) by rheological methods and by cryo pm underfocus of the m i c r m p e objective lens to provide sufficient transmission electron microscopy (cryo-TEM), a direct visualiphase contrast, which is mainly responsible for gradients of optical zation technique. Cryo-TEM of vitrified hydrated specimens density in the images. Images were recorded at 20000-50000 avoids many of the artifacts that plague the staining and drying X (*5%) and photographically enlarged. TEM techniques35 previously used to study these s y s t e m ~ . ~ l - ~ ~ Steady shear and small-amplitude oscillatory, i.e. dynamic, shear Furthermore, cryo-TEM samples, up to 0.2 pm thick, allow stemeasurements were performed on a Rheometrics Fluids rheometer reographic imaging of network structure. Thus, these experiments RFR 7800. The RFR 7800 had a 10 g-cm transducer that deallow comparison of images of micelles, quenched in time and tected torque and normal force simultaneously. Oscillatory shear space, to the rheological behavior. The rheological data provide and steady shear measurements were taken over a frequency sweep a means to correlate the present results with previous work on from 0.01 to 100 rad/s. The RFR 7800 has a built-in computer other systems. which converts the torque measurements into either G’(the storage modulus) and G”(the loss modulus) in oscillatory shear experExperimental Section iments or viscosity in steady shear experiments. For each steady shear datum, the rheometer averaged results from both the Cetyltrimethylammonium chloride (CTAC) was generously clockwise and the counterclockwise directions. The instrument provided by Dr. T. Imae (Department of Chemistry, Nagoya was fitted with an environmental chamber, which allowed the test University, Nagoya, Japan), by Dr. R. Zana (Institut Charles atmosphere to be saturated with water to avoid composition Sadron, Strasbourg, France), and by Eastman Kodak Co. All changes from solvent evaporation. The humidity inside the chamber was maintained at 100% by blowing air through heated (28) Wunderlich, I.; Hoffmann, H.; Rehage, H. Rheol. Acta 1987, 26, water and circulating it inside the chamber. The air temperature 532-542. inside the chamber fluctuated between 26 and 28 OC, depending (29) Gravsholt, S . J. Colloid Interface Sci. 1976, 57, 575-577. on how recently the chamber had been opened. Parallel plate and (30) Shikata, T.; Hirata, H.; Takatori, E.; Osaki, K. J. J. Non-Newtonian Couette geometries were used in the experiments. The radius of Fluid Mech. 1988, 28, 171-182. (31) Hoffmann, H.; Lobl, H.; Rehage, H.; Wunderlich, I. Tenside Deterg. the plates was 10 mm, and they were separated by a gap of 0.5-0.8 TABLE I: Compositions of CTAC-Nasal-NaCI Samples at Room Temperature
-
1985, 22, 290-298.
(32) Hoffmann, H.; Rehage, H.; Wunderlich, I. Rheol. Acta 1987, 26, 532-542. (33) Rehage, H.; Hoffmann, H. Faraday Discuss. Chem. SOC.1983, 76, 363-373. (34) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712-4719. (35) Vinson, P. K.; Talmon, Y. J. Colloid Interface Sci. 1989, 133, 288-289.
(36) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87-1 11. (37) Vinson, P. K. In Proceedings of the 45th Annual Meeting of the Electron Microscopy Society of America; Bailey, G. W., Ed.; San Francisco Press: San Francisco, 1987; pp 644-645.
476 The Journal of Physical Chemistry, Vol. 96, No. 1 1992
Clausen et al.
~
I
o4
I
o3
Io4
-
h
&I
1
o3
c-, 3
F
10’
1 o2 10’ -10.2
10’1 10.’
loo
10’
10‘
o (rad/s)
Figure 1. The steady shear viscosity (q) and the complex viscosity (lq*l) as a function of shear rate for C,,/CcTA, = 5 = 0.6.
10.’
loo
1 o2
IO’
w (radis)
F w e 3. The storage and loss moduli (G’and G”) as a function of shear rate for 5 = 0.52 (the continuous curves are calculated from eqs 1 and 2). 12000 10000 6000 6000
r 4000
1’ .*/ “1
o.2
10.’
‘\
IO0
IO’
J 1 o2
w (radis)
Figure 2. The storage and loss moduli (G’and G”)as a function of shear rate for [ = 0.46 (the continuous curves are calculated from eqs 1 and 2).
mm. In the Couette geometry, the bob length and radius were 55 and 24 mm, respectively, and the cup radius was 26 mm.
Results All samples listed in Table I had the same surfactant and NaCl concentration (CcTAc= 0.050 m and CNaCl= 0.100 m), differing only in the concentration of NaSal (Csal). We chose to change the embedded counterion concentration by titrating with NaSal, and as a result each sample had a different ionic strength in a narrow range. Rheological results for steady shear were obtained for all compositions listed in Table I using either Couette or parallel plate geometries, but the torque limits of the rheometer would not allow reliable oscillatory shear results below CSal/CcTAc = [ = 0.46. During sample preparation, these materials were observed between polarizers in an optical microscope. Movement of the coverslip on top of the sample produced intense birefringence in samples with [ 1 0.46. The birefringence persisted after shear was stopped for up to 10 s, which implies that orientation occurs under shear and then relaxes to a n optically isotropic state. Oscillatory shear experiments were conducted at several different strains to ensure that the materials were in the region of linear response. Strain is the amplitude of the shear oscillation and is defined as the ratio of lineal plate displacement to the gap distance. Strain ratios were investigated from 0.005 to 1.0 (0.556100% strain). From strain ratios of 0.005-0.5, there was little difference in the moduli, while from 0.5 to 1.0 the moduli decreased up to 10%. For consistency among results, experiments reported here were collected at a strain ratio of 0.2 (20% strain). Some samples were put under steady shear at 10 rad/s for up to 3 0 s with small strain oscillatory shear measurements performed immediately before and after steady shear. No significant changes in rheological behavior were observed as a result of this treatment. This indicates that any ordering during steady shear either relaxed rapidly or had no effect on the oscillatory shear response. Viscosity from steady shear measurements as well as the complex viscosity (Iv*() determined from oscillatory shear measurements are shown in Figure 1 for F = 0.60. The magnitude of 11*I is given by ( I / W ) ( G ’ ~ G”2)1/2,where w is the rate of shear oscillation. Figure 1 shows shear thinning behavior. For all 5
+
I”:
I X
1 0 -2
1
0.4
’
/
,0.8
1.2
1.6
2
5 Figure 4. The zero shear viscosity v0 and complex zero shear viscosity lqo*l
as a function of 6, plotted on linear (a) and semilog (b) scales.
which show shear thinning, the slope of the power law region is 0.89 f 0.13, which is similar to the behavior of semi-dilute polymer solutions3*and the results obtained by Shikata et al.30and Rehage et a1.34 Figures 2 and 3 are examples of the two types of results obtained from the oscillatory shear experiments. Figure 2 shows the oscillatory shear respome of the material with [ = 0.46, while Figure 3 is for = 0.52. In both figures, the curves describing G’ (the storage modulus) and G” (the loss modulus) cross as the rate increases, indicating that the materials are more elastic than viscous at high frequencies. In Figure 2, both G’and G”curves continue to slope upward with increasing rate, but in Figure 3, G’reaches a plateau value and G” reaches a maximum and then a minimum and finally increases with increasing rate. When discussing the rheological response of these materials as a function of [Sal-], we will refer to three regions: region A is where the materials behave as Newtonian fluids,l < 0.25; region B is where the materials are viscoelastic but have no plateau in G’ and a spectrum of relaxation times (cf. Figure 2), 0.50 2 2 0.25; region C is where the materials have viscoelastic response and there is a plateau in G‘and the response is dominated by a single relaxation time (cf. Figure 3), $. > 0.50. Materials in regions B and C show shear thinning in steady shear experiments (cf. Figure 1). ( 38) Ferry, J . D.Viscoelastic Properties of Polymers; Wiley: New York, 1980.
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 477
Viscoelastic Micellar Solutions TABLE II: The Plateau Modulus (eN), Relaxation Time ( T ) , Zero-Shear Viscosity (qo), and Zero-Shear Complex Viscosity (Iqo*l) as a Function of !.
5 0 0.20 0.46 0.50 0.52 0.53 0.56 0.60 0.62 0.70 0.75 0.80 0.89 1.oo 1.22 1.37 1.61 1.80 2.00
@N,
Pa
830 1000 1050 1130 1050 1080 1170 1270 1250 1440 1350 1400 1300 1400 1510
T,
qo, Pa-s
s
0.13 0.1 1 544 2330 4920
5 .O 13 8.1 9.4 16 5.5 1.3 2.0 0.73 0.29 0.15 0.12 0.18 0.23 0.45
8270 9340 8140 5020 964 1810 472 311 163 155 215 358 755
IBO*~,Pa.s
8
t
300-
4630 11000 8840 9800
200
200 400 600 800 1000 1200
G’ (Pa) Figure 5. C o l d o l e plot (G”as a function of G’) using the data presented in Figures 2 and 3: (a) is for ,$ = 0.46 and (b) is for ,$ = 0.52. ~
~
~
-
~
~
-
~
@
~
~
G%272
1
+
w272
where w is the frequency, is the plateau modulus, and 7 is the relaxation time. In the time domain, this type of model has simple exponential decay, with G ( t ) = @,e-r/r. Maxwell elements are useful in determining the and 7 of systems dominated by a single relaxation time (region C) or in a limited portion of a material’s frequency response range if the relaxation times are well separated. The curves on Figures 2 and 3 are the result of fitting this type of model to the oscillatory shear data. Table I1 presents GoN and 7 for all of the samples that were fit with a single Maxwell element, along with the qo and (qo*l values used to prepare Figure 4. A single Maxwell element is inadequate to fit the G’and G” data shown in Figure 2. However, in Figure 3 a single element fits the G’data well, though it fails at frequencies above 1 rad/s to fit the G”data. The fit of the data seen in Figure 3 is consistent for all rheological results collected in region C where [ > 0.50. How do we justify using a single Maxwell element for our data, when the data deviates from this model at higher frequencies? Generally, each maximum in G”data as a function of frequency indicates a relaxation time. The first maximum in G”in Figure 3 corresponds to the relaxation time being fit by a simple Maxwell model. GNthen passes through a minimum with increasing frequency. After this minimum, G” increases again indicating at least a second relaxation time at a frequency beyond the limit of the rheometer (100 rad/s). We attempted to fit a second maximum using a two-element Maxwell model, but there was not
e,
e
“0
-201
e,
100 200 300 400 500 600 7 0 0 G’ (Pa)
“0
5740 1220 3190 862 395 203 175 227 321 696
Several types of information can be extracted from the rheological results. Of particular interest are the changes in the rheological properties as a function of [. Figure 4 is a plot of the steady and complex zero-shear viscosities (qo and Iqo*l) versus E. These values were extrapolated from the data by averaging the low-frequency data points once they reached a plateau value. The maximum value of zero-shear viscosity occurs a t [ = 0.6 and is 5 orders of magnitude greater than the viscosity when [ = 0. Modeling techniques must be used to quantify rheological parameters that can be obtained from oscillatory shear results. In region C, several W O ~ ~ ~ ~ S ~ have ~ ~used J a~simple J ~ Maxwell element to fit rheological data obtained from viscoelastic surfactant solutions. A simple Maxwell element describes the rheological behavior of a system as a single spring connected in series to a viscous element (dashpot). In shear experiments, this results in equations for G’and G” of the form: G’(w) =
400-
h
0
0.4
1.2
0.8
1.6
2
5 Figure 6. Plateau modulus (GoN)as a function of 5.
enough high-frequency data to accurately determine QN and 7 of the second maximum. It is sometimes difficult to determine how “good” a Maxwell model fits the data from plots of G’and G”versus w . Systems that have a spectrum of closely spaced relaxation times may appear to be fit by a single Maxwell element. C o l d o l e or Nyquist plots (plots of G”as a function of C? provide a better picture of how well the data correspond to a single relaxation time Maxwell model. A Cole-Cole plot for a perfect Maxwell element is a semicircle, whereas a plot for a system with many relaxation times in a narrow range is boxlike. Figure 5 shows C o l d o l e plots from the data presented in Figures 2 and 3. The curves on these figures represent best fits to a simple Maxwell model. The data points in Figure 5a do not fit on a semicircular curve, in agreement with our previous conclusion that these data are not well fit by a simple Maxwell model. The data points below G’ = 900 Pa in Figure 5b fall rather well on a semicircular curve indicating a better fit of the data by the simple Maxwell model. At higher [, the data fit to a single Maxwell element even better than the results shown in Figure 5b. At high G’in Figure 5b, the G”va1ues increase again, indicating a second, well-separated relaxation time as discussed above. Cole-Cole plots were prepared for all samples listed in Table I1 and indicated that single element Maxwell models gave adequate relaxation time values for data with [ 2 0.52.
470 The Journal of Physical Chemistry, Vol. 96, No. I , 1992
0
0 0
0.1
0
0.4
1.2
0.8
1.6
2
5 Figure 7. Relaxation time
(7)
as a function o f t .
The fitted parameters listed in Table I1 summarize the relationship between the oscillatory shear rheological behavior and [. Figure 6, a plot of GON vs [,shows that is dependent on [
Clausen et al. between 0.52 and 1.O but is almost independent when [ 2 1.O. Figure 7 shows the dependence of 7 on [,which is similar to the dependence of qo on [ (cf. Figure 4), both having a maximum at [ = 0.6. Figures 8-1 1 are electron micrographs of the vitrified CTACNasal-NaC1 samples with [ = 0-2.0. There was some difficulty performing cryo-TEM on samples with [ 2 0.60 because they w m highly viscous and could not be thinned easily. In the case of these highly viscous but shear-thinning samples, we compared unrelaxed specimens that were vitrified immediately after blotting to specimens that were given 20-30 s to relax before vitrification. This was done to ensure that the images observed were characteristic of an equilibrium state. When observing apparent entanglements and other features, recall that electron micrographs are two-dimensional projections of microstructures contained in three-dimensional specimens. Figure 8a,b shows spheroidal micelles (arrows) that are approximately 5 nm in diameter. There are no distinguishable differences
Figure 8. Cryo-TEM micrographs dwumenting the transition from globular to threadlike micelles: globular micelles (arrows) -5 nm in diameter (a) at = 0.0 and (b) at t = 0.2. Larger specks in (a) are frost particles; dark patches in (b) are areas of cubic ice. Threadlike micelles - 5 nm in diameter are apparent in (c) with t = 0.25 and in (d) with = 0.40. Bars = 100 nm.
Viscoelastic Micellar Solutions
The Journal of Physical Chemistry, Vol. 96, No. I , 1992 479
--rT
-el.---.---
figure 9. Cryo-TEM micrograph of different areas (a-c) in a sample at = 0.46. Apparent entanglements (X), possible micelle terminals (Y), and an apparent micelle ring (Z) are seen. The dark spots ( A ) show micelles oriented parallel to the electron beam. Region W is void of micelles. The micelle denoted B exceeds 2.3 pm in length. C depicts a short helical coil. Bars = 100 nm.
between images of spheroidal micelles at = 0 and those at 0.20. The dark patches observed in Figure 8b are small crystallites of cubic ice that formed from vitreous ice upon irradiation by the electron beam. These crystallites are not artifacts associated with hexagonal ice formation due to slow cooling. Figure 8 demonstrates that long threadlike micelles are formed between 5 = 0.20 and 0.25. Figures 8c-11 show long threadlike micelles, approximately 5 nm in diameter, with lengths that are hard to determine from the images but at least in one instance exceed 2 pm. Figure 9a is a low-magnification image of the threadlike micelles showing several examples of entanglements (X). Entanglements can comprise one, two, or several micelles. Other features seen in this micrograph are possibly ends or terminals of micelles (Y)and a possible ringlike structure (Z). Some of the dark spots (A) in Figures 9-1 1 m u r where the threadlike micelles bend and are oriented parallel to the electron beam. Figure 9a illustrates some of the problems encountered in cryo-TEM. Surface tension makes the sample film, which spans a hole in a support film, take on a biconcave shape. This often results in a region void of micelles near the center of hole-spanning
films, as shown (W)in Figure 9a. The sample thickness near the center of a hole can range from micellar dimensions to hundreds of nanometers depending upon the size of the hole in the support film and the amount of sample that remains after blotting. Thickness is greatest near the edge of a hole. In this region, the images are difficult to interpret because of overlapping projections from many micelles. Figure 9a also illustrates the problems encountered in the de termination of micelle length, flexibility, and polydispersity. Because the threadlike micelles overlap, it is not possible to identify where they begin and end. Furthermore, without recording images at many tilt angles, it is often not possible to identify the corresponding incoming and outgoing portions of a micelle in a region of overlapping projections. These two factors prevent an accurate measure of micelle length and polydispersity from the micrographs. By measuring the distance between adjacent locations of overlapping projections along a micelle, a lower limit of the micellar length can be determined. The length of the micelle denoted (B) in Figure 9a is >2.3 pm (only a portion of the micelle is shown). Figure 9c shows a. micelle that has formed a short helical coil (C)
480 The Journal of Physical Chemistry, Vol. 96, No. I , 1992
Clausen et al.
Figure IO. Cryo-TEM micrographs of -5 nm diameter threadlike micelles. Images of (a) unrelaxed sample at ( = 0.5 and (b) relaxed sample. Images (c) and (d) are a stereo-pair of a sample with ( = 0.52. Note change in appearance (arrows) upon a 5 O tilt from (c) to (d). Bars = 100 nm.
along its length. Figures 9b and IOa,b are micrographs of regions that appear largely unaffected by sample preparation. The micelles appear fully entangled. A comparison of parts a and b of Figure 9 illustrate the dinidties associated with flexibility measurements. Persistence length, a measure of the flexibility, is a characteristic length of the micelle. On a length scale greater than the persistence length, a micelle can be thought of as a flexible structure, whereas at lesser lengths the micelle appears to be linear. These micrographs are from the same specimen, yet the micelles appear visually different. The micelles in Figure 9a were straightened during the formation of the thin film-probably much like what happens under shear in the rheometer. Thus, it is not possible to determine the persistence length from Figure 9a. In Figure 9b, the micelles have not been straightened, but the projection overlap and two-dimensionality still do not allow determination of persistence length from this single image. Images must be recorded at numerous tilt angles to reconstruct the three-dimensionality from the many projections and to determine quan-
tities such as persistence length, total length, and polydispersity. Parts c and d of Figure 10 are a stereopair of micrographs taken at different tilt angles to emphasize the three-dimensional character of the micelles. Figure IOc, taken at Oo tilt, demonstrates a feature (arrow) that could be an entanglement or simply an overlap in the projections of two micelle portions. Furthermore, it is not possible to correlate a micelle portion coming into this feature with its corresponding outgoing portion. Figure lod, taken at 5 O tilt, shows this feature (arrow) not to be an entanglement and allows the corresponding incoming and outgoing portions to be identified. Thickness gradients in the liquid specimen film may cause microstructures to segregate by size or to order. Size segregation ~~*~~ is commonly observed in images of vesicular s t r ~ c t u r e s , and order is sometimes seen in systems of threadlike micelles.4' As (39) Talmon, Y. Colloids Surf. 1986, 19, 237-248. (40) Vinson, P. K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J . Colloid Interface Sci. 1991, 142, 74-91.
The Journal of Physical Chemistry, Vol. 96, No. I , 1992 481
Viscoelastic Micellar Solutions W
I
T
"
=
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T
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. ' I
... ,.
.
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Figure 11. Cryo-TEM micrographs of -5 nm diameter threadlike micelles. Image (a) is of an unrelaxed sample with [ = 1.0; note c l o d circular threadlike, i.e. ringlike, micclles (X) and small spheroidal micelles (Y). Image (b) is of an relaxed sample with [ = 1.0. Images (c) and (d) are both of an unrelaxed sample with ( = 2.0. Note the order in (c). Bars = 100 nm.
the sample thins down to less than micellar dimensions, flow may carry micelles away from the central region. Alternatively, micelles may be absent from the central portion of the biconcave sample film of micellar solution as a result of a shift in equilibrium from micellar to monomeric that is driven by the film thickness dependence of the disjoining potential.40 Although threadlike micelles sometimes become aligned with one another within the thin liquid film (cf. Figure 9a), many images (cf. Figure IOa,b) show no signs of partitioning, alignment, or segregation. Alignment of threadlike micelles may occur when the sample is not allowed sufficient time to relax. In all samples, the time between blotting and vitrification was at most 30 s. The relaxation times determined rheologically at low strain are on the order of 1-10 s (cf. Table 11), about the same as the blotting times. Hence some of the images presented may represent a nonequilibrium (41) Magid, L. J.; Gee, J.; Talmon, Y.Lungmuir 1990, 6, 1609-1613.
state. The sample imaged in Figure lOa,b with = 0.50 was first prepared by vitrification immediately after blotting (Figure loa) and then by letting the sample relax for 20 s after blotting before vitrification (Figure lob). We found little difference between the relaxed and unrelaxed samples; both are similar to the images presented in Figure 9. Specimens shown in Figure 11 with 4: = 1.0 and 2.0 were prepared in the same manner as in Figure 10a,b. For 4: = 1.0, the unrelaxed (Figure 1 1a) and relaxed (Figure 1 1b) samples show similar entangled threadlike micelles. However, Figure 1la shows a rather thin area that lets us observe more details of the individual micelles. Note ring shaped micelles (X) and spheroidal micelles (Y). Figure 1 l b is much thicker (except in the lower central portion of the micrograph), which makes it difficult to observe the details of individual micelles. Parts c and d of Figure 1 1 are images from two different 'unrelaxed" specimens with = 2.0. Figure 1 IC shows the threadlike micelles in an ordered state, while Figure 1 Id shows no ordering and looks like Figures 9 and 10.
482 The Journal of Physical Chemistry, Vol. 96, No. I, 1992
Clausen et al.
with different but analogous surfactants and halide counterions have rheological results that correspond remarkably. This implies that the micrographs of our materials should give reasonable approximations for the structures present in these other systems. Parts a and b of Figure 8, corresponding to 5 = 0 and 0.20, show globular micelles in solution. These samples are in region Discussion A, they are not viscoelastic, and they have Newtonian response to steady shear with viscosities similar to those expected for dilute Electron microscopy provides images of the structures in masolutions of suspended spheres. Samples with [ between 0 and terials and may be used to determine their size and shape. 0.40 were not examined in oscillatory shear experiments because Rheological measurements are used to probe the properties of their moduli were too small to detect with our instrument. With networks and can help to understand the nature of the entanan increase in [Sal-] to 5 = 0.25, Figure 8c shows that the globular glements in the network as well as the average distance between micelles have become long threadlike micelles with diameters entanglements or the mesh size. Previous work has not been able approximately 5 nm and some having lengths in the micrometer to present artifact-free micrographs of these materials. We use range. As t increases from 0.25 to 0.50, there is little difference our rheological results to correlate our electron micrographs to among the images. All show what appear to be networks of the systems studied by other workers. We also hope to provide entangled threadlike micelles. However, as t increases from 0.20 some insight, from our micrographs, into the theories proposed to 0.46, the zero-shear viscosity jumps 5 orders of magnitude to understand these systems. (Figure 4), the materials become viscoelastic, 2nd they have a The effects of changing counterion concentration on the spectrum of relaxation times like polymers in the semidilute rerheological properties are well understood for these systems; g i ~ n This . ~ ~region B behavior is the direct consequence of the however, the mechanism through which these changes occur is change from spherical to threadlike micelles. As increases not. Several models have been proposed to explain the origin of throughout this region, 7, v0,and increase, implying that the the viscoelastic behavior of these systems. lengths of the threadlike micelles are increasing. On the basis of linear dichroism and ‘H N M R studies, Ulmius Comparisons of Figures 8 and 9 with Figures 10 and 1la,b show et al.’ proposed that “rod-shaped” micelles orient during prolonged little difference in the apparent structures: all imply a fully steady shear to form a periodic colloidal structure that extends entangled network. Nevertheless, the oscillatory shear results over macroscopic distances. We see evidence in Figure 1 IC for change to region C behavior, where the response is dominated by shear induced ordered structures. a single relaxation time. The changes in the system as a result On the basis of IH N M R studies, Manohar et a1.l0 and Rao of increasing [ are not manifested in the micrographs and may et a1.12suggested that extended micellar structures consisted of only affect the dynamics of the network, not the morphology. spheroidal micelles linked together like a string of beads. They Some liquid crystalline polymers have rheological behavior similar proposed that linkages between micelles were provided by electo these systems. Poly(p-phenylenebenzobisthiazole)(PBT)42 in trostatic attractions between salicylate ions and the cationic 97% H$O4/3% H20 is a polymer with a rigid backbone that has surfactant head-groups of adjacent micelles. The micellar chain a rheological response almost exactly the same as shown in Figures model does not explain the viscoelastic behavior of CTAB solutions 3 and 5b, with only a single dominant relaxation time. with electrically neutral salicylic acid.I4 Using N M R relaxation Figure 1IC, an unrelaxed or nonequilibrium state, shows an data, Anet’ found that the motion of the salicylate ions in the ordered phase. A similar ordered phase was observed in cryeTEM region where the material behaves as a Newtonian liquid is very micrographs of hexadecyltrimethylammonium dichlorobenzoate different from their motion when the solutions are viscoelastic, threadlike micelles by Magid et aL41 Threadlike micelles order which is inconsistent with the micellar chain model. In the visduring steady shear flow, which may be seen in optical birefrincoelastic region, our micrographs show evidence of threadlike gence e ~ p e r i m e n t s ~ ~at- shear ~ ~ - * rates ~ as low as 0.001 Hz. The micelles, with no structures resembling strings of beads. flow that occurs during sample preparation for electron microscopy Shikata et a1.3,13114*30 made small strain oscillatory shear and also would tend to order these threadlike micelles. The drop in steady shear measurements on various concentrations of aquwus CTAB over a range of [Sal-]. Shikata and c o - w o r k e r ~ ~also ~ - ~ ~ the steady shear viscosity and relaxation time after = 0.6 may be due in part to this tendency to order under shear. In Figure studied these systems by transmission electron microscopy, but 1IC, the transition from an ordered phase to as discadered phase they used a staining and drying technique that certainly led to is well resolved. Since both phases are present, it may be that artifacts in the micrograph^.^^ Hoffmann and co-workthis “caught” between order e r ~did both~ small strain ~ oscillatory ~ ~shear and ~ steady~ ~ micrograph ~ ~ ~the transition ~ ~ ~ and disorder ~ ~ during relaxation, particularly k u s e the order is present in the shear rheological experiments with CPyB and CPyC in the thinnest section of the micrograph. presence of Sal-, as well as CTASal and CPySal. Note that It is curious that our long threadlike micelles do not form an Hoffmann and co-workers for the most part prepared their samples ordered equilibrium phase. Although some spherical micelles are with constant ionic strength, whereas Shikata and co-workers observed in Figures 9-1 1, it is reasonable to assume that almost prepared their samples as we did by titrating with Sal- varying all CTAC above the CMC is contained in threadlike micelles. the ionic strength in a narrow range. In both cases, the rheological Since the CTAC concentration is not changing in these samples, properties were controlled by the embedded Sal- counterion and the total length of all threadlike micelles and their volume fraction essentially identical. The results of these rheological studies are is constant in all samples where threadlike micelles are present. similar to ours. With increasing counterion concentration, A first approximation of the transition to an ordered phase would Hoffmann and co-workers saw rheological behavior corresponding happen when the aspect ratio of the threadlike micelles reached to regions A, B, and C; Shikata and co-workers only saw regions the point where excluded volume would force them to order. B and C because they did not perform steady shear experiments F 1 0 r y ~worked ~ out this condition for nonassociative rodlike on low [Sal-] samples. Rehage and H ~ f f m a n nobserved ~~ a5 u* = (8/x)(1 - 2/x), where u* is the critical volume polymers: orders of magnitude jump in vo as a function of E. In Figure 4, fraction for the transition and x is the aspect ratio. For our we see similar behavior. In Rehage and H ~ f f m a n n ’ work, s ~ ~ the CTAC-Nasal-NaC1 micelles, this corresponds to a length of maximum in qo(E) corresponds to the maximum in the relaxation threadlike micelle >1200 nm. Several micelles in our micrographs time r , which we see in Figures 4 and 7. Both Hoffmann and are this length or greater. Furthermore, in association colloid co-workers and Shikata and co-workers re rt some dependence systems as is the case in this study, rod growth would be enhanced on E at low E, but with increasing .$‘,G’$x”es independent of as in our Figure 6. Both Shikata and co-workers and Hoffmann and co-workers observe a maximum in 7 as in Figure 7. In all (42) Miller, W. G.; Youngquist, M.; Chakrabarti, S.; Zhao, H.; Russo, P. cases, this maximum either in 7 or in vo occurs before the counPoIym. Prepr. (Am. Chem. SOC.,Diu.PoIym. Chem.) 1986, 27, 233. (43) Flory, P. Proc. R. SOC.London, A 1956, 234, 73-89. terion to surfactant ratio reaches 1.0. All of these experiments The zone of transition between the ordered and disordered states is clearly seen in Figure 1IC. The existence of this ordered state is consistent with our birefringence observations and shows that the threadlike micelles may be ordered before they relax to an equilibrium state.
a
-
~
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 483
Viscoelastic Micellar Solutions by the phase t r a n ~ i t i o n . ~The ~ increasing values of tobetween f = 0.25 and 0.60 indicate that the threadlike micelles are getter longer. The decreasing values of topast f = 0.60 would indicate a trend toward an ordered state. However, a t f = 1.O when the sample was allowed to relax in Figure 11b, there is no equilibrium ordered state apparent. Parts a, c, and d of Figure 11 are all of samples with f > 0.6 that were not allowed to relax completely before fixing them, but only Figure l l c shows evidence of a shear-induced ordered state. Therefore in the [Sal-] range studied, these samples do not form an equilibrium ordered state, but they do order when sheared. In polymer systems, @ON is a measure of the average chain length between entanglements in a network, Le., the mesh size. As the , increases. In our systems, is almost mesh size decreases, @ independent of f , indicating the mesh size remains constant. Between f = 0.25 and 0.56 there is little change in the micrographs of the threadlike micelle network, in agreement with a constant mesh size. Over the same range of f , the relaxation time T goes through a maximum. In entangled polymer networks, the primary mode of polymer motion is thought to be reptation. Reptation is a diffusive process where motion parallel to the contour of the polymer chain is more probable than motion perpendicular to the ~ h a i n . 4 ~A terminal relaxation time T~ (analogous to a relaxation time determined rheologically), dependent on the length of the chain, may be calculated using this type of theory. Experiments on polymer systems show that T~ N3.3, where N is the number of units in the ~ h a i n . 4Because ~ T , depends on the number of units in the chain, this diffusional process leads to a spectrum of relaxation times for polydisperse polymers. This type of behavior does not describe our systems in region C as demonstrated by Figures 3 and 6. Entanglement networks of threadlike surfactant micelles differ from polymer networks in that the chains can break and recombine, as well as grow by monomer addition. In polymer networks, the polydispersity of the system is fixed (barring chemical degradation) after the chemical synthesis of the polymer chains, but in micellar networks, the polydispersity is a function of tempera t ~ r e . ’ ~ >Breakage ~ * ~ ’ and recombination of chains in surfactant networks would affect the dynamics of these systems in different ways that depend on the time scale of the process. Hence, micellar networks would have two types of relaxation processes: one associated with diffusion of long polymer-like chains, and another associated with breaking and joiniiig of chains. Cates48-50explored these two relaxation processes using a simple model of a surfactant chain. H e modeled the diffusion of the chains with a reptation model and the breaking and joining process with a kinetic model involving only the chain ends. If diffusion dominated the relaxation of the chains, Cates calculated that the material would have a nonexponential stress decay-very different from that outlined in eqs 1 and 2 and seen in Figure 3. However, if the relaxation is dominated by the kinetics of breaking and joining, then the material’s relaxation would be described by eqs 1 and 2. Cates’s model implies that there would be many ends of chains present, since relaxation occurs as a result of breaks in the chains. Although we see a few ends in our micrographs, it appears that they are a relatively rare phenomenon. C a t e does ~~~ predict that micelle rings would be present, as seen in Figures 9a and 1la. Cates predicts that the number of rings would be significant until the volume fraction of threads reached the point where they overlap or entangle. Figure 8 documents the transition from globular to threadlike micelles. As soon as threadlike micelles appear, they are entangled; no region is seen where there are significant numbers of rings. It should be borne in mind that the models Cates used do not take into account the effect of stresses
-
(44) Flory, P. Macromolecules 1978, 1 1 , 1126-1 133. (45) de Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: London, 1979. (46) Mukerjee, P. J . Phys. Chem. 1972, 76, 565-570. (47) Porte, G.J . Phys. Chem. 1983,87, 3541-3550. (48) Cates, M. E. Mncromolecules 1987, 20, 2289-2296. (49) Cates, M. E. J . Phys. (Paris) 1988, 49, 1593-1600. (50) Cates, M. E. J . Phys. Chem. 1990, 94, 371-375.
in flow that may disturb the kinetics of chain making and breaking.5’ Shikata et al.I3J4 also proposed a model where the relaxation of the chains is dominated by a kinetic process. They built on the work of LodgeSZJ3and YamamotoS4who had independently proposed a quasi-network model, where transient junctions that break and reform control the relaxation of networks of long chains. Shikata et al. calculated that the relaxation process would be exponential as in eqs 1 and 2. From ‘H N M R results,I3 Shikata et al. found that the relaxation time was dependent on the amount of free Sal- (unbound to the threadlike micelles) and suggested that the free Sal- mediates the relaxation process. Shikata et al. envisioned a transient junction forming as two micelles come in to intimate contact. Since they see few micelle ends in their negative-stain transmission electron micrographs, they proposed that relaxation occurs as the micelles merge and pass through each other. Preparation of samples for TEM by staining and drying results in a two-dimensional compaction of structures from which all three-dimensional information is lost and can introduce microstructural changes as a result of pH and ionic strength changes inherent in the process. Therefore the TEM results reported by Shikata et al. are suspect3sbecause they are susceptible to artifacts from the staining and drying techniques used. Neither of the theories proposed by Hoffmann and co-workers or Shikata and co-workers attempt to determine why there is a maximum in the relaxation time and zero-shear viscosity (cf. Figures 4 and 7) as a function of f ; instead each attempts to explain why the rheological results are dominated by a single relaxation time. These theories also do not take into account the stiffness of the chains when considering their relaxation. Olsson et al.” suggested that the effects of ionic strength on micelle length and stiffness dominate the behavior of these systems. They postulated that increasing the [Sal-] lengthens the chains and increases their flexibility as a result of the lower surface charge density. This increase in length and flexibility stops when the counterion binding exceeds 100% at f > 1.O and the surface charge changes sign. They further state that continued addition of Salincreases the surface charge density, which stiffens the micelle and promotes a change in morphology from threadlike to spherical. Our images show that at least up to [ = 2.0 the materials form a network of long entangled threadlike micelles. Olsson et al.Il also presented evidence that Sal- binds by embedding its aromatic part in the micelles. This would have the effect of controlling the headgroup area of the CTAC surfactant. The presence of NaCl in our samples limits long-range electrostatic interactions. Therefore, short-range interactions along the chain, controlled by Sal-, probably cause the observed f dependent effects by affecting micellar stiffness, length, and kinetics of scission/recombination. Conclusions
The rheological behavior presented is controlled by [Sal-]. For f between 0 and 0.20, the material behaves as a Newtonian fluid,
corresponding to a solution of spherical micelles. As [Sal-] increases beyond 0.20, the material forms long threadlike or wormlike micelles, as observed by cryo-TEM, that remain visually constant independent of [Sal-]. Dramatic changes in the rheological behavior are accompanied by little change in the static images of the micellar structures. This suggests that dynamic changes in the network structure, unobservable in these static images, are controlling the rheological behavior. It is reasonable to assume for f = 0.25-2.0 the network structuresare similar under shear and include ordering, but the kinetics of breaking and reforming of micelles change with 5. This supports the hypothesis of Shikata et al. and Cates that the relaxation behavior is dominated by an entanglement scission process whose rate is controlled (51) Wang, S.-Q.; Gelbart, W. M.; Ben-Shaul, A. J . Phys. Chem. 1990,
94, 2219-2221.
(52) Lodge, A. S. Trans. Faraday SOC.1956, 52, 120-130. (53) Lodge, A. S. Elastic Liquids; Academic: London, 1964. (54) Yamamoto, M. J . J . Phys. Soc. Jpn. 1956, 1 1 , 413-421.
J. Phys. Chem. 1992, 96, 484-491
a4
by the free Sal-. Microscopy supports the formation of entanglements and, to a l i i t e d extent, their scission and nonequilibrium ordering as a result of shear. Acknowledgment. We are grateful to Prof. L. E. Scriven for helpful discussions during the preparation of this paper. We would
like to thank the following organizations a t the University of Minnesota for their support: the NSF Center for Interfacial Engineering, the Graduate School, and the Academic Computing Center. Registry No. CTAC, 112-02-7; Nasal, 54-21-7; NaCI, 7647-14-5.
X-ray Structure Analyses and Vibrational Spectral Measurements of Model Compounds of Liquid-Crystalline Arylate Polymers. 3. Structural Phase Transitions in a Series of Model Compounds Jian-an Hou, Kohji Tashiro,* Masamichi Kobayashi, Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan
and Toshihide Inoue The Research Association of Polymer Basic Technology, Toranomon, Kanda, Tokyo 101, Japan (Received: May 28, 1991)
The phase transitional behavior has been investigated by the differential scanning calorimetry, X-ray diffraction, and infrared/Raman spectroscopic methods for a series of model compounds of liquid-crystalline arylate polymers: H5C20COPhOCOPh,COOPhCOOC2H5 (n = 1,2, and 3). The crystal phases of nl, n2 (8 form), and n3 have crystallographically isomorphous structures and were found to show relatively similar phase transitional behavior between crystal, smectic, nematic, and isotropic phases. Interspacing between the adjacent molecular layers was found to change drastically at the transition points and correspond well to the remarkable changes in infrared and Raman spectra, suggesting a close relation between the changes in the conformation and packing mode of the molecules. The a crystal phase of the n2 was found to transform to the 8 phase in a solid-to-solid transition at ca. 100 "C.Thus generated 8 phase transforms to the smectic C phase in the same fashion as that observed for the sample starting from the pure fi phase. The spatial correlation between the (Y and 8 phases was discussed on the basis of X-ray diffraction data.
Introduction To clarify the mechanical properties of liquid-crystalline polymers from the molecular theoretical viewpoint, detailed and exact structural information must be accumulated in association with the crystal-to-liquid crystalline phase transition. Unfortunately, however, the liquid-crystalline polymers show in general only a few and diffuse X-ray reflections, making it difficult to carry out a detailed structural study. As one approach to solve this problem, we have utilized model compounds for the liquidcrystalline arylate polymers, i.e., the compounds with the chemical structures H 5 c , 0 C O ~ O C O ~ C o o ~ c o c € 2 H 5 1
where n = 1,2, and 3 (abbreviated as nl, n2, and n3, respectively). In previous paper~,l-~ we reported the crystal structures of these model compounds on the basis of X-ray diffraction, infrared, and Raman spectra. Brief descriptions of the results obtained so far are as follows. (1) At room temperature the n2 compound exhibits two crystal modifications, a and (3, whose molecular conformation and packing fashion in the unit cell are remarkably different from each other. Especially, the torsional angle around the benzenebenzene linkage of the biphenyl group is very different: 48" for the a and 0" for the 8 form. (1) Tashiro, K.; Hou,J.; Kobayashi, M.; Inoue, T. J . Am. Chem. SOC. 1990, 112, 8273.
(2) Tashiro, K.; Hou,J.; Kobayashi, M.; Inoue, T. Polym. Prepr. Jpn. 1990,39,749. (3) Hou, J.; Tashiro, K.; Kobayashi, M.; Inoue, T. Polym. Prepr. Jpn. 1990, 39, 2355.
(2) Such a difference in the molecular conformation is reflected clearly on the Raman spectra. In particular, the bands at 420 and 320 cm-I were found to be characteristic of the twist structure (and motion) of the biphenyl group. (3) The structural features of the nl, n2-8, and n3 crystal phases are essentially similar, Le., they are crystallographically isomorphous. The parameters b, c, and 0 of the monoclinic unit cell are almost common,but the a-axis length or the interlayer spacing increases by the length of one benzene ring on going from the nl to n2-8 and n3. The DSC thermograms measured in these studies have shown the quite complicated features of the phase transitions for the nl, n2, and n3 compounds. It may be important to clarify the relationship between the structural isomorphism and the phase transitional behavior among these model compounds, because such an information will give us useful guiding principles for the structural study of liquid-crystalline polyarylates. In this paper, the phase transition behavior of these compounds was investigated in detail by means of X-ray diffraction, infrared and Raman spectroscopies, DSC, and optical microscopic observation. Experimental Section Single crystals of the n l and n2 compounds were prepared from toluene solution by slow evaporation of solvent at room temperature. A single crystal of the n3 compound has not yet been prepared successfully. The a and 8 forms of the n2 crystal were subjected to DSC and infrared/Raman measurements after an X-ray check of the crystal form. X-ray diffraction powder patterns were obtained by a Rigaku RAD-ROC diffractometer with a graphite-monochromatized Cu Ka radiation (A = 1.5418 A). The temperature dependence of the X-ray pattern was measured for the sample set in a homemade furnace (temperature fluctuations were within *0.5 "C). The
0022-3654/92/2096-484%03.00/0 0 1992 American Chemical Society
