สำนักราชบัณฑิตยสภา

103 Jaruwan Krongsin, Roongkan Nuisin, Supaporn Noppakundilograt, Suda Kiatkamjornwong วารสารราชบัณฑิตยสถาน ปีที่ ๓๗ ฉบับที่ ๔ ต.ค.-ธ.ค. ๒๕๕๕ Stability of leave-on conditioner Storagemodulus (G ' ), loss modulus (G " ) and its ratio can be used as an indicator for evaluating the stability of chitosan/menthol microcapsules. Dynamic viscoelastic moduli G ' is always higher than G " (Pa) as shown in Figure 3a which were monitored as a function of temperature at a time sweep test. When increasing the temperature, the G ' and G " decreased with the minimal value at 40 º C . In contrast, at the low temperature of 4 º C , the maximum G ' and G " were obtained. Likewise, the leave-on con- ditioner with incorporation of chitosan/menthol microcapsules can resist shear stress because of their dynamic elastic properties as well as structure deformation. A typical increase in the dynamic storage modulus G ' with time is shown in Figure 3a in all the three cycles. The trend of the curves in each cycle is similar during temperature profile. The parameters that quantify the rheological behavior depending on the leave-on conditioner formulation are useful to tailor made of a stable fluid. 18 The stability of leave-on conditioner was contributed by the cetearyl alcohol and dipalmitoylethyl hydroxyethylmo - nium methosulfate as a thickener pair, which remained unchanged with the addition of the emulsion of chitosan/menthol microcapsules. The loss tangent (tan δ), which is defined to a ratio of G ' /G " , as a function of temperature was obtained in Figure 3b. The curves of tan δ beyond room temperature (25 º C ) revealed the overlapped patterns (lines crossover), and upon increasing temperatures higher than room temperature, the tan δ decreased, i.e., G ' was higher than G " and more elastic system was obtained besides thermal effect on mobility of molecular chains. In this system, it decreased most at 40 º C . Additionally, evaporation of menthol may cause deformation of the microcapsules. Some parts of the polymeric microcapsule wall can force other components out to move or migrate upon apply- ing forces. When the storage modulus G ' increases, the tan δ decreases as governed by the ratio of G ' /G ". It can be noted that, the G ' began to decrease rapidly near the temperature of 43 º C , the melting point of menthol. The relationship of dynamic viscosity and temperature as shown in Figure 4 gave a unique curve for each cycle. Increasing the temperatures decreased the dynamic viscosity of leave-on conditioner and vice versa. It should be noted that high temperature might accelerate evaporation of the encapsulated menthol in the microcapsules. Deformation of the microcapsule walls occurred and they could be densely combined to become aggregates or agglomerates. The leave-on conditioner having the thixotropic property in the second and third cycles of the test indicated that the viscosity increased with increasing numbers of sweeping cycle as a function of temperature 19 which implied that evapora- tion of menthol could possibly take place or some kind of a stronger interaction might exist between the conditioner base and the component in the chitosan/menthol emulsion.