Surface-Wetting Effects on Spinodal Decomposition of Poly(ethylene glycol)/poly(ethylene glycol–ran propylene glycol) Mixtures

• Main Authors: Jia-Syun Li, 李佳勳
• Other Authors: Po-Da Hong
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/dx72ch

碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 104 === In this work, the surface-wetting effects on spinodal decomposition behavior of the PEG/RAN mixtures under the confinement were investigated using turbidimetry, light scattering and microscope analyses. Firstly, the critical temperature of phase separation evaluated from the cloud point has no change with sample thickness as the same with the prediction from theoretical simulation on the surface-wetting effect. It is well known that wetting effect on the critical point of phase separation depends on the formation of three-phase system (liquid-liquid-solid phase) in general. Hence, the wetting behavior will take place in the late-stage of phase separation in which the system exists the sharp interface between two phases. From the morphological observation and the kinetics of spinodal decomposition for the near-critical composition, however, the occurrence of wetting effect was found in chronological order and this phenomenon is quite different from that reported in other literatures. In this study, the surface-wetting effect on spinodal decomposition in the confined space would be elucidated further with considering the mechanisms of structural evolution for a phase-separating system. In summary, the structural change with time during spinodal decomposition process can be divided into four stages: (1) typical hydrodynamic coarsening of spinodal decomposition; (2) wetting PEG-rich phase spreading on the quartz confined wall; (3) the nucleation-growth behavior of channels connecting to two wetting layer (there exists a critical radius, so the channels will shrink and then disappear if the channel is smaller than the critical radius); (4) the closure and shape relaxation of channels. Most importantly, the existence of equilibrium wetting layers with time was not observed in this work, whereas the phase dissolution occurs. The wetting-driven phase dissolution behavior harmonizes the inconsistency between the thermodynamic theory and kinetic experiments. Strictly speaking, the two-phase separation should be stable in the bulk (3D), while it is considered to be metastable in thin film (2D). The wetting effect will reconstruct the two-phase composition, and then this will reach a new equilibrium state. Therefore, for the surface-wetting effect in confined space, the real equilibrium state may mean that the formation of one-phase structure with surface segregation is rather important than that of two-phase phase separation. Therefore, the phase dissolution has to be expected to occur under these arguments. On the other hand, the shape relaxation rate of channels is found to correlate with the temperature and this fact might be considered due to the change in surface tension. To concern further about the effects of surface tension on the mechanism of liquid-liquid phase separation is very important in the near future.