Effect of Viscoelastic Effect on Phase Separation Kinetics of Polymer Mixtures

Effects of Viscoelasticity on Phase Separation Kinetics of Polymer Mixtures ZHANG Jia-Ning, JIANG Xiu-Li, ZHANG Hong-Dong, YANG Yu-Liang (Department of Polymer Science, Fudan University, Shanghai 200433, China) When the components of viscoelasticity are the same, the phase separation morphology is similar to that of simple binary fluids. In the non-critical composition, a few of the phases are in the disperse phase. When the critical composition is in the bicontinuous phase, the scattering function shows a single peak. However, the growth index of the phase region is much smaller than that of the simple binary fluid mixture. On the other hand, when the viscoelastic contrasts between the two components of the polymer mixture are present, the reverse rotation occurs in the non-critical composition, and the growth index in the phase region becomes large. When the critical composition is formed, a permanent sponge-like structure appears. The regional growth index fell instead. At the same time, due to the appearance of the network structure, the scattering function has double peaks and shoulder peaks, indicating that there are two kinds of feature sizes. At this time, the growth index will not be meaningful if the whole circle average scattering function is used, and the two peaks should be processed separately or the interface profile should be adopted. Line statistics method to calculate growth index.

Phase separation is a phenomenon commonly found in nature, including polymer solutions, polymer mixtures and block copolymers, surfactants, colloids, and emulsions. The phase separation behavior of these high-molecular complex fluids is closely related to material properties. , which has attracted much attention. In recent years, people have already had very different morphological evolution processes when the symmetric viscoelastic model and the viscoelastic contrast model are composed of non-critical components. If the component with a small total number of integrals (' = 0.35) is A component, then in the symmetry viscoelastic model, the A component-rich phase will form a dispersed phase in the initial phase of phase separation, and the domain size will follow the evaporation condensation mechanism. The mechanisms of evaporation-condensation mechanics and coalescence continue to increase, ie, they basically follow the conventional phase separation process, but are significantly slower than the phase separation process of simple fluids. On the other hand, for the viscoelastic contrast model, the A-rich phase (polymer-rich phase) forms a continuous phase at the beginning of phase separation, and the B-rich phase (solvent-rich phase) forms a dispersion group, followed by a solvent-rich phase. The (solvent-rich phase) grows, compresses the polymer-rich phase around it to further concentrate, and withstands a certain amount of viscoelastic stress. When the polymer-rich phase network is initially compressed by the solvent-rich phase, the polymer network experiences a maximum stress. Afterwards, the part of the polymer network that has been broken down begins to shrink due to the effect of surface tension, and the polymer-rich phase changes from the initial continuous phase to the final dispersed phase, achieving a reversed phase. It is worth mentioning that the morphological evolution results obtained by this simulation are in phase with the phase diagram of the binary polymer mixed system used in this paper. The phase separation of the polymer solution made by Tanaka and the polymer mixture with viscoelastic contrast Fig.1 Phasediagram of the binary pofymer The experiment is very consistent. The mechanism of continuous solvent growth and the fact that the polymer phase is not condensed due to the concentration of the polymer can be explained from the point of view of the contrast between the bulk modulus of the two: from 35,X=2.7; the initial concentration fluctuates, causing the concentrations in some regions to rise, becoming High bulk modulus (Mb (2) critical composition '= 0.5, X = 2.7 = 5), where the solvent's diffusion motion is hindered by viscoelastic stress; and the area where polymer concentration decreases, its modulus Also falling, the solvent movement in this area is hindered by the smaller viscoelastic stress (Mb = 0). In this way, when interdiffusing, the solvent of the solvent-rich phase tends to enter into the polymer-enriched phase and is hindered by the bulk modulus; it is difficult to enter; however, the solvent of the polymer-enriched phase can easily enter the solvent-rich phase. Collectively, it's hard to come back once you go out. The more dense the result, the thinner the rarer, because the solvent entering the solvent-enriched phase also occupies a certain volume, so only the polymer-rich phase around the pressure, it is between this dense phase and the dilute phase. The asymmetry of the dynamics (including the bulk modulus contrast and the disparity in the ability of the diffusion motion) led to the occurrence of a reversed phenomenon. The change process of the secondary peak corresponds to the process of retreating and thickening of the network skeleton after the polymer-rich phase forms a network structure. At the later stage of phase separation, the original secondary peak has become the major new peak because only the polymer-rich phase has characteristic dimensions in the later stage. It can be seen that the reverse process is accompanied by a process in which the two feature sizes alternate dominantly. At this point, it is no longer meaningful to study the long index using the full-range average scattering function. Therefore, only two peaks can be separated to obtain different feature sizes. Or use the interface contour statistics method to find the long index of its phase area.

Different long indices are defined: in the case of two dimensions, the long index of the mechanism of evaporation condensation = 3, the mechanism of droplet coalescence, and the hydrodynamic flow mechanism of a = ff. with a binary simple fluid mixture in The long index of the critical composition phase region is smaller than that of the symmetric viscoelastic model of the binary polymer mixture, ie a=0.23. This is consistent with the observation that the smaller long index is often observed in the phase separation experiment of the polymer mixture. It also confirmed that the viscoelasticity of polymers proposed by Doi and Onuki et al. will inhibit concentration fluctuations and make the phase separation rate slow down. For the case of viscoelastic contrast, due to the reversed phase, the polymer-rich phase network undergoes hydrodynamic flow when the network is retracted. There is a faster coarsening mechanism than the former, so the phase area is long. The velocity is much faster than that of symmetric viscoelasticity, a = 0.48. This is also consistent with the trend that the large bulk modulus contrasts we discovered in the previous study will lead to a large long-term index. At the later stage of phase separation, the thermodynamic driving force for phase separation will gradually be depleted, and the viscoelastic stress that resists the diffusion of solvent to the polymer-rich phase will also be weakened. As a result, this sponge-like structure may be maintained for a long time. Aubert's experimental results also observed a long-term network structure. Recalling the process of phase separation of viscoelastic contrast, we can explain its special morphological evolution from the perspective of the apparent phase diagram: Due to the existence of viscoelastic stress, the apparent phase diagram is shifted. The initial state point is shifted from the area on the phase diagram that forms the polymer dispersed phase to the area where the solvent dispersed phase is formed. In this way, during the initial phase separation, the solvent dispersed phase is formed first. Subsequently, the fracture of the network caused the viscoelastic stress to decrease, and the phase diagram also began to recover to the thermodynamic phase diagram. In the course of recovery, the state point is also returned to the polymer dispersed phase area accordingly, and a reversed turn occurs. In the initial phase of viscoelastic contrast phase separation, a longer freeze period (see the first picture in part (b)) is also caused by the excessive viscoelastic stress causing the state point to shift even into the long nucleation zone outside the spinodal line. The viewpoint and his experimental results also support the above conclusions (see Fig. 1). Tanaka also gave the qualitative expression of the apparent phase diagram (Eq. 228 in Fig. 9) and the quantitative basis for the degree of phase shift (/OGb). (0) For a more detailed discussion of this section please