Rheological Properties

The rheograms for the samples prepared through the different preparations (A), (B), and (C) are shown in Figure 4.32. The measurements were carried out using an identical solution, i.e. the heating and cooling processes were conducted in the rheometer. Therefore it should not be taken into account that the even tiny shear force

(A) (B) (C)

Figure 4.31. Appearances of the samples prepared through the different preparation routs: (A) normal preparation (Section 4.1), (B) heating and cooling with agitation, (C) heating and cooling without shear. The surfactant concentration and protonation degree are 100 mM and X = 0.17.

0.01 0.1 1 10

50

100 Sample (A)

Sample (B) Sample (C)

G' [Pa]

f [Hz]

0.01 0.1 1 10

10² 10³ 10⁴ 10⁵

10⁶ Sample (A)

Sample (B) Sample (C)

|η*| [mPas]

f [Hz]

Figure 4.32. Influences of the preparations (A, ■), (B, ○), and (C, ▲) on the rheological parameters, storage modulus G’ (left) and complex viscosity |η*| (right), in 100 mM C₁₂C₈MAO/HCl (X = 0.17) aqueous solutions at 25 °C.

generated throughout the experimental procedure would influence on the aggregate structure, consequently the rheological properties. The shear moduli for all the samples are almost independent on frequency, and the complex viscosities represent the shear thinning with a slope ~ -0.97. This indicates that any solutions consist of the Lα morphologies as confirmed by birefringence in Figure 4.31. In Table 4.4, the storage modulus (G’, f = 1 Hz),

the complex viscosity (|η*|, f = 1 Hz), and the yield stress (σ0) are compared between the samples (A), (B) and (C). Samples (A) and (B) have the higher viscoelastic property than Sample (C); G’ and |η*| of Sample (C) are half or one-third of those of Sample (A) and (B). Moreover, for Sample (C), σ0 could not be measured by the cone-plate system in the same stress-controlled range as Sample (A) and (B). The yield stress also attempted to be found out by the sweep measurement of shear rate – stress (σ), whereas the significant break point, corresponding to the yield stress, could not be observed as shown in Figure 4.33. The behavior of Sample (C) thus seems to be a viscous fluid. On the basis of weaker birefringence and lower viscoelasticity, it can be plausible that Sample (C) composed of the stacked bilayer morphology or even large vesicular one. It is also of interest that Sample (B) has the higher viscoelastic properties than Sample (A). This would be due to contribution of the vesicle size, which may be explained by the similar procedure to the finely-dispersed emulsion using the phase inversion method.¹² The mechanism is essentially related to the interfacial tension. The interfacial tension at the phase inversion point reaches to the minimum,¹⁶⁸ where the emulsion turns to the microemulsion consisting of well-defined bicontinuous structure.¹⁶⁹ The shear force could tear the bicontinuous structure apart, and the dispersions would be smaller and smaller depending on the applied shear force because the interfacial tension is quite low. In turn, the mechanism can be manipulated in the present case because the clouding phenomenon originates from the similar structural modification.⁷⁷ The high

Table 4.4. Comparison of the rheological parameters, storage modulus G’ (1 Hz), complex viscosity |η*| (1 Hz), and yield stress σ0, for the L_α solutions prepared by the different routs. The surfactant concentration and protonation degree are kept at 100 mM and X = 0.17.

-Figure 4.33. Sweep curves of the stress (σ) – shear rate for the samples differently prepared through preparations (A, □), (B,

●), and (C, ▲). The surfactant concentration and protonation degree are 100 mM and X = 0.17.

0,00 0,05 0,10 0,15 0,20

0,0

yield stress value of Sample (B) implies that there exists much densely packed vesicles.

The temperature-induced structural modification, however, did not occur at higher protonation degree than X = 0.2 where the phase separation with rising temperature does not take place in the measurable temperature range.

Changes in the complex viscosity and the transmittance of Sample (A) are shown as a function of temperature in Figure 4.34. Both |η*| and the transmittance drop steeply down at around 72 °C, referring to the phase separation. The viscosity decreases by a factor of 10³, which clearly demonstrates that the viscoelastic structure arising from the densely packed multilamellar vesicles are ruined through the phase separation. Thus it is evident that the vesicle structure is ready to modify into another L_α structure by controlling shear and temperature, while the modification requires the temporarily breaking-down of the vesicle structure.

The dependence of protonation degree on the temperature-induced structure modification was studied for the series of samples with different X using rheology.

Figure 4.35 shows the plot of the storage modulus G’ against protonation degree for the samples before and after heating. The annealed samples were prepared in the measuring cell of the rheometer under no shear. As mentioned in the previous section 4.1, G’

40 45 50 55 60 65 70 75 80 85 90

0.001 0.01 0.1 1 10

Transmittance

|η*|

|η*| [mPas]

Temperature [^oC]

0 20 40 60 80 100

Transmittance [%]

Figure 4.34. Changes in the complex viscosity (|η*|, ■) and the transmittance (□) with temp-erature. The surfactant concent-ration and protonation degree are 100 mM and X = 0.2.

Figure 4.35. Plots of the storage modulus G’ versus protonation degree X for the samples before (■) and after heating (○) in C₁₂C₈MAO/HCl (X = 0.17) system at 25 °C.

The surfactant concentration is kept at 100 mM.

0,0 0,1 0,2 0,3 0,4

1 10

before heating after heating

X

G' [Pa]

increases monotonically with protonation degree in the low X range, and levels off at X

= 0.17, remaining constant up to the phase boundary. The heated samples, on the other hand, show the different change in G’; the modulus is almost independent on the charge density of the system, namely, the resulting morphology could be the same over the studied X range. Although the formed structure after heating was speculated to be the stacked L_α before, the storage moduli in fact increase by the thermal treatment at X = 0.05 and 0.1 where the large multilamellar vesicles are present (see Figure 4.16). The upwards deviation of G’ at low X would result from an experimental error; in the course of elevating temperature, more or less water is inevitable to evaporate from the sample solution although the measuring system had been covered with the plastic cover and retained under constant vapor pressure as far as possible, whereas certainly the absolute G’ values of upwards deviation at low X seem to be so much small that it would be unnecessary to account for the experimental error. At high X, rather, it is noteworthy that G’ remarkably decreases by heating, and the modulus after heating is one order smaller than that before heating. Provided the aggregate structure at X = 0.05 is identical in spite of the thermal history, the invariant G’ with protonation degree after heating elucidates that their structure can be the flat bilayer structure or such the large vesicle as seen in the cryo-TEM micrographs in Figure 4.16.