4.2 Thermal Treatment
4.3.3 Rheological Properties for Non-Sheared Solution
One recalls the preparation of the non-shear solution; the test solution was prepared inside of the double-gap system in the rheometer. Every chemical was weighted exactly at X = 0.17 and the solution had left for three days before the
1.462μm
Figure 4.41. FF-TEM image of the C12C8MAO/HCOOCH3 solution (X = 0.17) prepared by means of the chemical reaction. The surfactant concentration is 100 mM.
0.01 0.1 1 10
0.1 1 10
G''C G'C
G'' G'
G', G'' [Pa]
frequency [Hz]
101 102 103 104 105
|η*|
|η*|C
|η*| [mPas]
Figure 4.42. Dynamic rheological measurements for two solutions prepared by the normal preparation route (hollow symbols) and using the chemical reaction (filled sym-bols) at 25 °C. The subscrip-tion
“C” indicates the chemical reac-tion sample. The surfactant conce-ntration and protonation degree are fixed at 100 mM and X = 0.17.
measurements. Figure 4.42 shows the rheogram in the linear regime as a function of frequency. Apparently the rheogram is similar to that of the sheared sample in Figure 4.8, whereas the moduli, G’ and G’’, and the complex viscosity |η*| decreases by a factor of 10 by removing pre-shear from the system. The viscoelasticity of the sample subject to the chemical reaction is surprisingly further lower than that of the heated, non-sheared sample (Figure 4.43).
The steady shear measurement in Figure 4.44 demonstrates that the applied shear induces to phase transitions from the bilayer structure (Lαh) to the unilamellar vesicle
0.01 0.1 1 10
0.1 1
G', G'' [Pa]
frequency [Hz]
G'
G''
|η*|
|η*|C
G'C
G''C 10
1
102 103 104
|η*| [mPas]
Figure 4.43. Dynamic rheolo-gical measurements for two solutions prepared by thermal treatment without shear (hollow symbols) and using the chemical reaction (filled symbols) at 25
°C. The subscription “C”
indicates the chemical reaction sample. The surfactant concent-ration and protonation degree are fixed at 100 mM and X = 0.17.
0.1 1 10 100 1000
0.01 0.1
Lαh+Lαl
ULV (Lαl,ULV) MLV
(Lαl,MLV)
Lαh
η [mPas]
shear rate [1/s]
Figure 4.44. Steady shear measurement for the solution produced by using the chemical reaction at 25 °C. The surfactant concentration and protonation degree are fixed at 100 mM and X = 0.17. Each notation indicates as follows: Lαh -stacked lamellae, Lαl,MLV - multilamellar vesicle, Lαl,ULV - unilamellar vesicle.
(ULV, Lαl,ULV) via the multilamellar vesicle (MLV, Lαl,MLV) with increasing shear rate.183 Their characteristic slopes of shear thinning manifest the phase transition points.
Furthermore one can observe a plateau range between Lαh and Lαl,MLV, which would be correspondent to the coexistence regime of their structures or transformation regime. It is found thus that the transition from Lαh and Lαl,MLV is of first-order.
Figure 4.45 represents the irreversible morphology transition in the non-linear regime. In this figure, one can divide into four regimes where different power laws hold:
(1) the shear thinning with the slope = -0.866, (2) the viscosity plateau, (3) the shear thinning with the slope = -0.689, and (4) the shear thinning with the slope = -0.464. The first shear thinning in the first shear cycle can be principally due to the formation of ordered bilayer structures being aligned in the flow direction.185,186 The initial shear thinning is quite pronounced, which could be understood that the layers orient normal to the flow direction.187 When shear rate exceeds a critical shear rate ~ 0.5 1/s, the plateau is observed. This could be explained by the fact that at this point some of the bilayers start to close themselves. However, the formation of vesicles would cause the shear thickening as referred to Ref. [183]. Again the shear thinning occurs at the shear rate = 3.4 1/s where most of the bilayers form the closed structure and behave in the same way as the Lαl phase. For the first cycle, the measurement stopped at an intermediate shear rate and the shear rate was decreased down to the starting point. Upon reduction of the shear rate, no reversibility for the viscosity values is observed and the viscosity is about one order of magnitude larger than that of the non-sheared phase, suggesting that the vesicles formed once are quasi-stable. Then, when the shear rate increases again (second
0.1 1 10 100 1000
0.01 0.1 1
Lαh y ~ x-0.464
y ~ x-0.689
y ~ x-0.866
η [mPas]
shear rate [1/s]
Lαh+Lαl
MLV (Lαl,MLV)
ULV (Lαl,ULV)
Figure 4.45. Shear history effect on the lamellar morphologies in C12C8MAO/HCOOCH3
system at 25 °C. The surfactant concentration and protonation degree are fixed at 100 mM and X = 0.17. The square (■,□) and circle symbols (●,○) represent respectively the first- and second-cycle of measurements. The solid and hollow symbols are in the shear up-stream and down-stream. Each notation indicates as follows: Lαh - stacked lamellae, Lαl,MLV - multilamellar vesicle, Lαl,ULV - unilamellar vesicle.
cycle), the viscosity decreases in the same way as the viscosity change with decreasing shear rate in the first cycle. The shear thinning in this regime would results from the deformation or rupture of vesicles. Further high shear rate causes another orientation corresponding to the unilamellar vesicle (ULV) as with the transformation observed in Figure 4.14. The transition point could be the breaking point of the straight lines with the different power laws at the shear rate ~ 50 1/s. The morphological transition however takes place in different manner: the transition from MLV to ULV seems to be of second-order. Since such a weak first-order transition may disappear under strongly shearing,88 particularly in the non-linear regime, the steady shear measurement was repeated more carefully. Figure 4.46 shows the η − γ& and σ − γ& curves in the system same as above, where the time durations at each steady shear are prolonged up to 30 minutes. This figure clearly demonstrates that the MLV-ULV transition is of first-order while the coexistence region is narrowly limited. After the maximum shear rate in the second cycle (Figure 4.45), the viscosity profile was measured again with the decreased shear rate. The transition point from ULV to MLV is observed at a critical shear rate, so that the transition is reversible and it can be mentioned that the multilamellar vesicles are stable morphology at rest. However the breaking points between up- and down-streams of shear rate are inconsistent. From Figure 4.46, the MLV-ULV transition is of very weak first-order, hence, the inner energy of vesicles supplied by shearing could not be dissipated instantaneously even at less than the corresponding shear rate. As a consequence, the inconsistence between up- and down-streams of shear flow would appear.
1 10 100 1000
1
0.01 0.1 1
η
σ slope = 0.23
slope = 0.54
σ [Pa]
shear rate [1/s]
η [Pas]
Figure 4.46. Changes in the shear stress σ (■) and the apparent viscosity η (○) with shear rate in C12C8MAO/HCOOCH3 system at 25 °C. Each shear rate was retained for 30 minutes. The surfactant concentration and protonation degree are fixed at 100 mM and X = 0.17.
Chapter 5 CXDMAO SYSTEM EFFECT OF COUNTER-ION ON AGGREGATE STRUCTURE
5.1 AGGREGATE STRUCTURE AND ZERO-SHEAR VISCOSITY