Phase Separation Dynamics and Pattern Formation in Thin Films of a Liquid Crystalline Copolyester in Its Biphasic Region

Abstract

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 Λm(t) ∝ qm(t)-1 ∝ tα, 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.