Thousand‐Fold Acceleration of Phase Decomposition in Polymer/Liquid Crystal Blends

Abstract

Micro-phase-separated blends of liquid crystals (LC) and polymers are widely used for display applications, gas-flow sensors, optical gratings and memories. [1][2][3][4][5] The phase separation process is slow, both in the sense of actual time (of minutes to hours) and-as it is controlled by diffusion-in the sense of the power law evolution of the mean size of the domains. In addition to the traditional adjustment of the natural parameters of the process (temperature, composition), conceptually new advances attempt to affect the phase separation with an external electric field (EF). The use of an EF brings into focus the role of ionic constituents. Tsori et al. [6] showed theoretically an induction of the hexagonal-to-bcc phase transition in diblock copolymers upon application of a direct-current EF above a critical magnitude, and that free ions can reduce the value of the critical field by an order of magnitude, from 70 V mm À1 to 6 V mm À1. Tsori and Leibler followed up [7] with a result that ions can also influence the phase separation process in binary mixtures through a mechanism based on 1) a contrast of conductivity, due to a difference in chemical potential of ions, in each of the two phases and 2) on motion of the interface as a result of the electrophoretic force acting on the ions. We have recently shown [8] that a low-frequency oscillating EF (OEF) can induce a dynamic charge separation (dyCHASE) even over macroscopic distances (mm) in a LC meniscus.Herein we demonstrate experimentally that OEFs can be used to 1) accelerate the rate of phase separation by up to three orders of magnitude, and 2) change the character of the phase separation process from a power to an exponential evolution of the mean size of the domains.We studied the phase-separation process in the binary mixture of 5CB/PS (4-cyano-4'-n-pentyl-biphenyl/polystyrene) and 8CB/PS (4-cyano-4'-n-octyl-biphenyl/polystyrene). [9] In order to track the phase-separation process, we first annealed the samples at T > 60 8C (5CB) and T > 65 8C (8CB). Next, we mounted the samples in the light-scattering apparatus [9] and performed a quench of the temperature to the I + I or N+ I region in the phase diagram.[9] We monitored the evolution of the size L(t) of the domains by recording the scattering intensity SA C H T U N G T R E N N U N G (q,t).Generically, the plots of the scattering intensity SA C H T U N G T R E N N U N G (q,t) show [9] a single clear maximum at a wavevector q max (t), which gives the reciprocal of L(t). We found that in the absence of EF applied to the sample, following a quench into the I + I or N+ I region, L(t) evolves algebraically: L(t)~t Àa with a = À0.3 AE 0.04 for all investigated concentrations of 5CB/PS and 8CB/PS. [9] In our previous work, we obtained similar results for 8CB with PS, irrespective of polymer mass. [10][11][12] We then performed experiments [9] with OEF of intensity up to 3.3 V mm À1 (3.3 MV m À 1 ). We found that the application of a low-frequency (f~10 Hz) OEF significantly accelerates the phase-separati...

Ionic polarization of liquid-liquid interfaces; dynamic control of the rate of electro-coalescence

“…Our results are consistent with the dynamic charge separation mechanism (dyCHASE) that we identified earlier for freely suspended liquid crystalline film 17 and a polymer/liquid crystal blend. 18 Frequency of oscillating EF can be used to control the magnitude of electrostatic interactions in a wide class of softmatter systems.…”

Pitch Gradation by Ion-Dragging Effect in Polymer-Stabilized Cholesteric Liquid Crystal Reflector Device

“…By shining UV light to initiate the polymerization for the reactive LC monomer, a cured network was able to be achieved within the cell to stabilize the LC orientation directors or freeze the ordered structures (Figure 2aii). Normally, applying an external electric field on such an already-made CLC cell can adjust the pitch length in the PSCLC cell, and it is well-known that impurity ions trapped by an ester group in a polymer chain are prone to be driven to the corresponding electrodes and thus induce a translational motion of the polymer network [22,[29][30][31]. Consequently, pitch length modulation ( Figure 2aiii) with a highly recoverable feature ( Figure 2aiv) was able to be obtained.…”