Disclinations and Their Interactions in Thin Films of Side-Chain Liquid Crystalline Polymers Shanju Zhang, Eugene M. Terentjev, and Athene M. Donald* Cavendish Laboratory, University of Cambridge, Madingley Road, CB3 0HE, Cambridge, UK Received August 21, 2003; Revised Manuscript Received October 24, 2003 ABSTRACT: Two-dimensional disclinations formed in thin films of a side-chain liquid crystalline polymer are investigated using transmission electron microscopy (TEM). The detailed director patterns are revealed through nanoscale stripes, which are parallel to the molecular director. One hyperbolic pattern of a negative disclination with a charge s )-1 and three patterns involving radial, circular, and spiral director fields of a positive disclination with s )+1 are observed. Positive disclination cores are found to exhibit circular dark centers while the s )-1 is shown to have a bright central core. The difference in TEM contrast is attributed to the nature of the packing within the core. From the director texture around a disclination it is possible to determine the elastic anisotropy, which is not higher than 0.1. Observing the time evolution of the disclination density, it is found that the average number of defects in the plane scales with time as N ∝ t -3/4 . It is found that both translation and rotation motions occur for a pair of (+1, -1) disclinations, leading to the formation of a certain special configuration shortly before the pair annihilation. In some cases, not the pairs but the linear clusters of three-disclination dominate the interaction. Theoretical analysis is presented to explain this new phenomenon. In addition, director inversion walls are observed to form during the late stage of disclination annihilation. It is found that inversion walls always separate a pair of (+1, +1) disclinations. Introduction Liquid crystals (LC) are characterized by a preferred orientational order, which is described by an apolar director n field. The most probable director configura- tion is determined by minimization of the total free energy with respect to the director field. In deformed liquid crystals, disclinations are rotation symmetry- breaking defects. The strength of a disclination corre- sponds to the number of rotations of the director around a path encircling the disclination. 1 Disclinations and their interactions have been theoretically investigated for many years. 2-5 Such experimental studies are limited in scope in that most of them use optical microscopy to study small molecular LCs or liquid crystalline polymers (LCP’s). 4-7 The pairwise interaction of disclinations is analogous to the electrostatic interac- tion between line charges. The interaction forces are inversely proportional to the distances. Disclinations of opposite sign attract and annihilate each other, while disclinations of the same sign repel each other. This interaction leads to the annihilation of defects and hence a decrease in their number density. The details of these processes are, however, still far from being understood due to both the complexity of potential many-body disclination interactions and the lack of suitable, high- resolution, experimental methods. Transmission electron microscopy (TEM) is a useful method for studying the microstructure at a molecular level. It has proved to be of great utility in the study of LCP’s over the past two decades. 8,9 In the case of semicrystalline LCP’s, crystalline lamellae, which are formed perpendicular to the local director, can be used to decorate molecular orientation, and the director trajectories around a disclination are, therefore, directly visualized via TEM. 10,11 This technique to explore the director orientation in semicrystalline LCP’s has been successfully applied for the analysis of disclinations and their interactions in main-chain LCP’s. 12-14 Hudson and Thomas studied the structure around cores of half- integer disclinations (charge s )( 1 / 2 ) by analysis of the elastic anisotropy. 15 They found that a rigid polymer splayed more at the near-core, while a flexible polymer bent more. Hudson and Thomas also studied disclina- tion (s )( 1 / 2 ) interactions in applied magnetic and extensional flow fields. 16 They observed that a stable four-body disclination cluster (a so-called Lehmann cluster) dominated the interaction at weak and inter- mediate field strengths. However, almost no observation of director fields of disclinations in side-chain amor- phous LCP’s systems has been reported to date. 17 Side-chain LCP’s present a unique class of materials with mesomorphic and viscoelastic behavior. 18,19 Their microstructures result from the delicate balance be- tween liquid crystalline order due to the mesogenic core and statistical disorder due to the polymer backbone entropy. In side-chain LCP’s, the orientational proper- ties are carried by the short mesogenic side chains, and the nature of the order parameter is then similar to that in small-molecule LC’s. 20 In contrast to main-chain systems, there are many theoretical and experimental indications that the values of Frank elastic constants in side-chain LCP’s are close to those in analogous small-molecule LC’s. 21,22 Therefore, the problems of molecular packing are of a different nature in main- chain and side-chain systems. Recently, we reported the observation of two-dimensional nanoscale stripes in thin films of a side-chain smectic polymer using TEM. 23 When the samples were cooled to the smectic phase, after a holding period in the nematic phase, a charac- teristic surface morphology developed. This morphology manifested itself in bright field TEM images as a series of nanostripes which aligned along the local director. This orientation was demonstrated by correlating the direction of the stripes with electron diffraction patterns. * Corresponding author. 390 Macromolecules 2004, 37, 390-396 10.1021/ma035240o CCC: $27.50 © 2004 American Chemical Society Published on Web 12/18/2003