Study On The Phase-separation Behavior And Viscoelastic Relaxation For LCST-type Polymer Blends

It is well-known that most polymer materials are multiphase/multi-component polymer blends. Miscibility and phase-separation are two important issues for multi-component polymers, which directly affect their morphology and structure. Most polymer blends have lower critical solution temperature (LCST). The thorough understanding of LCST-type polymer blends is in favor of our optimizing their phase structure, domain size and ultimate mechanical properties.In this dissertation, two kinds of LCST-type polymer blends, poly (methyl methacrylate) / poly (α-methyl styrene-co-acrylonitrile) (PMMA/α-MSAN) blend and polystyrene/ poly (vinyl methyl ether) (PS/PVME) blend, were selected as model systems. Within the relatively wider annealing temperature and heating rate range, the phase-separation behaviors of these two blends were studied through time-resolved small angle light scattering (SALS), as a function of temperature and heating rate. Meanwhile, the relationship between phase-separation behavior and the viscoelasticity for PMMA/α-MSAN blend was investigated based on dynamic rheological measurements.In general, viscoelasticity is the essential characteristic for polymer systems. Their phase behaviors depend intensively on the rate (time) of promoting from the outside. The correlation between the phase behaviors and heating rates for PMMA/α-MSAN blend and PS/PVME blend was investigated by using SALS method within the relatively wide heating rates range. Although Hashimoto et. al thought that the phase-separation behavior of PS/PVME blend is linear, we found that for the abovementioned two blends, the dependence of the critical temperatures determined on heating rates deviates obviously from the linearity, even at the very low heating rates. Moreover, the cloud-point curves for PMMA/α-MSAN blends decrease gradually with the decrease of heating rates and present a trend of approaching T_gs of the blends. Based on the SALS results, the dependence of phase behaviors for PMMA/α-MSAN blends was investigated by means of dynamic rheologicalmeasurements and the similar nonlinear relationship was obtained. This indicates that owing to their nonlinear and nonequilibrium characteristic of phase-separation, the equilibrium phase-separation temperatures for these two blends could be hardly established by the linear extrapolating to zero in the plotting of cloud points versus heating rates.Based on the nonlinear phase-separation behaviors during heating process, we also studied the temperature dependence of phase behaviors for the abovementioned two blends during isothermal annealing by using SALS method. It is found that their thermal-induced phase-separation behaviors follow spinodal decomposition (SD) mechanism within the temperature region investigated. Similar to the PMMA/SAN mixtures as was investigated in our previous works, the temperature dependences of phase-separation kinetics for these two blends at either the early or late stage of SD both follow the TTS principle. The Williams-Landel-Ferry (WLF) function is applicable to describe the temperature dependence of apparent diffusion coefficient D_qpp (T) and relaxation time τ of normalized scattering intensity (I(t) -I(0)) / (I_m -I(0)) at the early stage of SD, as well as the phase behavior at the late stage of SD. Besides, the light scattering data of PS/ Poly (methyl methacrylate-stat-cyclohexyl methacrylate) (PS/PMsC) blend investigated by Edel et al. was studied in the same way and it is found that WLF-like function is also applicable to PS/PMsC blend. Hence, these present another powerful evidence for the applicability of the TTS principle and WLF-like function for describing the phase-separation behavior of binary polymer mixtures over a relatively wide temperature range.The influence of the blend film-thickness on phase-separation behavior for PMMA/α-MSAN blend was examined. It is found that with the increase of film thickness, their phase-separation temperatures decrease and phase-separation rates accelerate, and the stability of binary polymer blend decreases within the investigated thickness range of sample film. When the thickness of sample film is higher than 30μm, the volatilization of residual solvent dominates in their phase-separation and results in the intensification of the system's instability. However, when their thickness is lower than 25 μm, the geometric effect of film plays an important role. Accordingto the thermodynamic treatment as demonstrated by Hill, the relation among the film thickness, phase-separation rate and spinodal temperature can be obtained.Small amplitude oscillatory shear rheology was employed to investigate the influence of phase-separation on the linear viscoelastic behavior of PMMA/α-MSAN blend. The results reveal that when temperature approaches separation temperature, the blends exhibit some characteristic of complex thermorheological behaviors, such as a shoulder in the dynamic storage modulus (G') or the linear relaxation modulus (G(t)), the appearance of loss tangent (tanδ) peak and the additional relaxation in the relaxation spectrum (H(τ)). All of these can be attributed to the enhanced concentration fluctuations near the phase boundary. The anomalous pretransitional behavior can be quantified to yield the binodal temperature from the inflexion of variation and the spinodal temperature on the basis of the mean field theory. Furthermore, in order to predict the long-term properties of phase-separated material, we try to qualify the rheological characteristics of phase-separated blends with Palierne's emulsion model and Bousmina's emulsion model in virtue of the calculatedα/ R|-_v value. It is found that there exists some difference between the theoretical valueand experimental value for PMMA/α-MSAN (60/40) blend, because their co-continuous morphology hardly accords with the assumption of emulsion model. However, the linear viscoelastic properties of the phase-separated PMMA/α-MSAN (80/20) blends can be described well by the two emulsion models in the whole frequency ω region, which indicates that these emulsion models are effective for the blends with insular morphology to predict the variation of their viscoelastic behaviors.