The phase separation and the coarsening process of polymer mixtures with a thermotropic liquid crystalline polymer (LCP) as one component are investigated. The LCP used is a main-chain type copolyester X-7G, comprised of p-hydroxybenzoic acid (60 mol %) and ethylene terephthalate (40 mol %) units. The isotropic, transparent, and homogeneous test specimens of the 50/50 mixtures of X-7G and poly(ethylene terephthalate) (PET) were prepared by a rapid solution-casting with an organic solvent. The specimens were heated rapidly to the test temperature (T jump), and an isothermal phase separation process was investigated at real time and in situ under a polarized light microscope. A rapid phase separation was observed, when the temperature was higher than the melting points of PET and X-7G, allowing us to study the late stage spinodal decomposition into anisotropic and isotropic liquid phases. The following sequences of the decomposition mechanisms were found as time elapses in this stage:  (i) the self-similar growth of a percolating network of the anisotropic liquid phase rich in X-7G in the isotropic matrix phase rich in PET, (ii) disruption of the percolating network and shrinkage of the disrupted fragments into the anisotropic droplets, and (iii) diffusion and coalescence of the anisotropic droplets. The two important factors, transesterification and the liquid crystal effect which can affect the phase separation, are also discussed in the text.

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We present the experimental results concerning a phase separation dynamics and relevant pattern formation in thin film samples (ca. 10 µm in thickness) of a liquid crystalline copolyester in its biphasic region. The copolyester separates into an isotropic phase and an anisotropic phase in the biphasic region due to its composition heterogeneity, though it is homopolymer. The entire phase-separation process was in situ monitored in real space by polarized light microscopy. The structural evolution that appeared in the digital images was further analyzed using the fast Fourier transform method. The process can be described by the following steps: (1) The formation of a percolated network consisting of phase-separated isotropic and anisotropic liquids. The network growth obeys a scaling law Λ m(t) ∝ qm(t) -1 ∝ t R , where Λm(t) is the characteristic length of the domains, qm(t) is the characteristic wavenumber, and t is the time. A crossover of the exponent from 1 /3 to 1 /2 was observed. (2) The network breaks up at a critical value of Λm(t) ) 50 µm to form some anisotropic fragments with irregular shapes, followed by the further shrinking and reshaping into anisotropic drops. (3) The diffusion-coalescence of the drops to form large merged drops and the reshaping of these merged drops followed by orientational ordering within the merged drops. We studied the temperature dependence of these individual processes and discussed the mechanisms and the scaling behavior of the domain growth in the first step.