Hairpin Defects in Liquid-Crystalline Polymers
- PDF / 590,432 Bytes
- 3 Pages / 576 x 777.6 pts Page_size
- 64 Downloads / 208 Views
string of length L, characterized by a bending constant e and a coupling constant a, which measures the strength of the interaction between the chain and the molecular field caused by the orientational order of the medium. Because of the rotational symmetry with respect to n, it is sufficient to consider the twodimensional behavior of the chain. In this case, the orientation of the tangent to a chain is specified by the angle 9(s) where s is the arc length along the chain. The energy of a single chain consists then of two terms:4,8
David R.M. Williams and Avi Halperin u =— Conventionally, the term "defects" in liquid-crystalline systems refers to microscopic faults in the orientational order, which are usually visible optically.1-2 These are discussed in other articles in this issue. Our use of the term defect is entirely different. The defects we shall be considering, "hairpins," occur on the scale of several angstroms and are abrupt reversals in the trajectory of a single liquid-crystalline-polymer (LCP) chain3 (Figure 1). In comparison to conventional defects, the direct observation of hairpin defects is much more difficult, yet their presence has important effects. Among the affected properties are the dimensions4 6 and the elasticity of the chains,5 the elastic behavior of the bulk nematic,7 and its dielectric response.4 Their presence should also give rise to a family of interfacial phase transitions in solutions of LCPs in nematic solvents.6 In turn, these are of interest in the design of liquid-crystalline displays. Hairpins in LCPs are superficially reminiscent of similar configurations in proteins and in homopolymers that undergo fold crystallization. These similarities are misleading because hairpins in proteins are permanent structures due to the chemical bonds. The folds in crystalline polymers are also fixed structures. In marked contrast, hairpin defects are mobile topological excitations that are created and annihilated continuously. Our emphasis in this article is on main-chain, semiflexible, nematic LCPs,3-4-8 consisting of mesogenic monomers joined by flexible spacer chains (Figure la). These polymers combine the orientational order of monomeric nematics with the flexibility and randomness inherent in polymers. The appearance of an oriented, nematic phase can be controlled either by temperature or by concentration. Here we discuss the former "thermotropic" case where the onset of orientational order oc-
MRS BULLETIN/SEPTEMBER 1995
curs upon the lowering of the temperature T. Below a characteristic temperature Tc, the polymer chains change their orientation from random to one where the chains are locally aligned along a particular direction, the director n. The simplest models of chain defects consist of a single chain in the nematic field caused by surrounding chains. The chemical sequence is smeared out and the chain is modeled as a inextensible
o Figure 1. (a) A single hairpin on a liquid-crystalline chain. The nematic mesogens are represented by the rectangles; the remainder of the chain c
Data Loading...
