Side-chain LCPs

Both thermotropie and lyotropic liquid crystal polymers exhibit eharacteristic micro-strueture features [9,10]. Anisometrieal monomers such as rods or disks are conneeted to ehains in an appropriate manner. These anisometrieal monomers are considered the mesogens and may be part of main ehain LCP, side chain LCP, or of both types together (Fig. 6). Flexible spacers of nonmesogenic character are located between the mesogens. A sufficient flexibility is a prerequisite for the liquid crystal formation with an increase in temperature or solvent concentration. 

Table 1.25 Effect of the Flexible Tail on the Structure of a Side-Chain LCP...

In this chapter particular emphasis is placed on recent progress in interpreting the behavior of aromatic polyesters which have been pursued commercially. The large body of literature on LCPs with spacers and the side chain LCPs are not discussed except peripherally. A more detailed discussion of these topics is available [19]. The major themes presented in this chapter are summarized as follows ... 

Figure 2.21 Side-chain LCPs organized in different mesophases.

Side-chain LCPs can be prepared by attaching a mesogen pendant to a flexible polymer backbone (18). These materials often have optical properties similar to low molar mass LCs and have generated interest for such applications as non-linear optics, filters and optical storage devices. 

In the following two sections, a number of recent developments in the syntheses of main-chain and side-chain LCP s are discussed. Other synthetic aspects are also found in many of the chapters concerned with physical characteristics of LCP s. 

This paper presents summaries of unique new static and dynamic theories for backbone liquid crystalline polymers (LCPs), side-chain LCPs, and combined LCPs [including the first super-strong (SS) LCPs] in multiple smectic-A (SA) LC phases, the nematic (N) phase, and the isotropic (I) liquid phase. These theories are used to predict and explain new results ... 

In this paper, an earlier theory (12-13) for binary mixtures of backbone LCPs (and/or nonpolymeric molecules) in the nematic (N) LC phase and the isotropic (I) liquid phase has been extended to treat binary mixtures in multiple smectic-A (SA) LC phases, the N LC phase, and the I liquid phase. Either component 1 (Cl) and/or component 2 (C2) in the mixture can be a backbone LCP, a nonpolymeric LC molecule, a polymeric non-LC molecule, or a nonpolymeric non-LC molecule. Cl can also be a side-chain LCP or a combined LCP (including a SS LCP). The new theory of this paper is the mixture analogue of an earlier theory for pure (single-component) systems derived and presented in detail in References 3 and 7-10 (see also References 14-23). When only one component is present, the new mixture theory of this paper reduces to this earlier single-component theory. Therefore, only a short summary of the new mixture theory is presented here. 

Ei ft 0, (In t ) — (In O3) - 0, and E2 — Ej - 0. For the particular Cl side-chain LCPs and Cl combined LCPs (including SS LCPs) studied (3-13) with the earlier single-component analogue of this mixture theory, it has been found that the N LC phase and the I liquid phase for these polymers involve the packing of plate-like sections of backbones and side chains thus, (ln 0- ) 0, (ln f ) 0,... 

A. For side-chain LCPs and combined LCPs (3-10). vq33/3... 

Next to side-chain LCPs and main-chain LCPs, supramolecular networks were obtained by complex-ation of bipyridines with polyacrylates containing pendant benzoic acid groups. In a related approach, Kato and Frechet have studied supramolecular networks based on low-molecular-weight components, in which a trifunctional benzoic acid derivative was combined with a difunctional pyridine derivative.62 The hierarchy of the LC-phase that was formed turned out to be dependent on the flexibility of the trifunctional compound used. 

Most LCPs of commercial interest are thermotropic main-chain nematic materials. The other main structural type of interest are the side chain LCPs which are generally more complex to synthesise and have not yet found significant commercial applications. The author s Mesogenic Index model, which is useful for predicting whether a given polymer has a mesophase, has been applied only to main-chain LCPs to date. 

The fairly good quality of the fits validates both Leadbetter s assumptions and the Maier-Saupe distribution function. However, the values of S obtained and even the quality of the fits obviously depend on the odd or even number of (CH2) groups in the flexible spacer. This odd-even effect is widespread and well known in the field of main-chain LCPs and will be discussed later in this article. The nematic order parameter of main-chain LCPs may reach values as high as 0.85 which demonstrates the very high orientation of the nematic phase of these polymers. Such a large orientation is undoubtedly responsible for the good mechanical properties of this type of materials. The treatment described above therefore provides a very easy way of characterizing the orientational order of a nematic phase. It has also been tested for thermotropic side-chain LCPs and found to be satisfactory as well [15]. Unfortunately, it has not been used yet in the case of lyotropic LCPs except for some aqueous suspensions of mineral ribbons (Sect. 5) which are not quite typical of this family of materials. 

Side-chain LCPs are, in a way, more subtle than their main-chain counterparts because, thanks to the flexible spacers, the polymer main chain (called backbone) still retains many degrees of freedom. In fact, there is a true balance between the natural disorder of the backbones and the liquid-crystalline order of the rod-like moieties. For instance, in the SmA phase, it was recently demonstrated by SANS that the backbone performs a 2-dimensional random walk between the smectic layers [22]. Therefore, these materials have poor mechanical properties but display a rich variety of mesophases, even at room temperature. This makes them more suitable for applications in displays and electro-optic devices. 

In the special case of side-chain LCPs, it was found that the smectic order extends over about 1000 A [24], This value increases with temperature which was interpreted by the annealing of defects. The correlation length also increases with increasing spacer length which means that the smectic order develops more easily when the coupling between the mesogenic cores and the backbone gets weaker. 

From a different point of view, the most striking difference between the SmA phases of side-chain LCPs and those of LMMLCs is that the former usually show many orders of smectic reflections whereas the latter usually only show one [27, 28]. This observation will help us to answer the following basic question how are the backbones of the side-chain LCPs affected by the smectic field (Fig. 9) Do they keep a more or less random 3-dimensional conformation or are they strongly confined between adjacent sublayers of mesogenic cores ... 

This method can be extended to neutron diffraction by considering two side-chain LCPs labelled PMA(H,D)OC4H9 of formula... 

We have just seen that the backbones are very often confined in the SmA phase of side-chain LCPs. Therefore, they have lost many degrees of freedom and entropy. The backbones usually react to this confinement by inducing different kinds of fluctuations and disorder. This is obviously seen on overexposed X-... 

Let us now turn to another type of fluctuations which very often affect the SmA phase of side-chain LCPs [la,b, 30]. Figure 12 also shows some diffuse spots (d) located in the same reciprocal planes perpendicular to the director as the smectic reflections. Because of the uniaxial symmetry of the SmA phase, these diffuse spots actually represent the intersection of diffuse tori with the Ewald sphere. These diffuse tori arise from a transverse... 

The X-ray scattering patterns of side-chain LCPs sometimes show some diffuse scattering which cannot be explained in terms of fluctuations but has to be interpreted in terms of localized defects [la,b, 33]. For instance, the X-ray scattering patterns of the polyacrylates of formula... 

ESR Measurements. The side chain LCPs 1 and 2 and the corresponding low molecular weight analogue 5 were studied by dynamic ESR techniques [35]. The weight fraction of the CSL spin probe employed was < 10 . Sample cells were constructed of two quartz plates coated with tin dioxide to make them conducting. The thickness of the cell was 250 pm. 

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