Smectic combined polymers

Since the synthesis of the first chiral smectic C side chain LCP by Shibaev et al. [6], chemists over the last ten years have considerably extended that field. Now, the SmC mesophase can be exhibited by a variety of polymeric materials including homopolymers, copolymers and terpolymers, oligomers, combined polymers, and cross-linked polymers. 

The investigation of combined FLCPs was initiated by Zentel et al. [91-93] as a part of their approach to ferroelectric LC elastomers [94]. Figure 12 shows typical structures of combined FLCPs and cross-linkable chiral combined LC polymers. Poths et al. [67] used that approach to synthesize combined polymers with axially chiral mesogenic side groups (similar to the acrylic side-chain polymer above). The smectic C structure of polymers has been identified by optical microscopy and x-ray data, but no ferroelectric properties of the polymers have been reported yet. 

FIGURE 12 Typical structures of combined smectic C polymers (a) and cross-linkable chiral combined polymers (b). 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

Turning to the low temperature transition of the homopolymer of PHBA at 350 °C, it is generally accepted that the phase below this temperature is orthorhombic and converts to an approximate pseudohexagonal phase with a packing closely related to the orthorhombic phase (see Fig. 6) [27-29]. The fact that a number of the diffraction maxima retain the sharp definition at room temperature pattern combined with the streaking of the 006 line suggests both vertical and horizontal displacements of the chains [29]. As mentioned earlier, Yoon et al. has opted to describe the new phase as a smectic E whereas we prefer to interpret this new phase as a one dimensional plastic crystal where rotational freedom is permitted around the chain axis. This particular question is really a matter of semantics since both interpretations are correct. Perhaps the more important issue is which of these terminologies provides a more descriptive picture as to the nature of the molecular motions of the polymer above the 350 °C transition. As will be seen shortly in the case of the aromatic copolyesters, similar motions can be identified well below the crystal-nematic transition. 

Three different ways have been developed to produce nanoparticle of PE-surfs. The most simple one is the mixing of polyelectrolytes and surfactants in non-stoichiometric quantities. An example for this is the complexation of poly(ethylene imine) with dodecanoic acid (PEI-C12). It forms a solid-state complex that is water-insoluble when the number of complexable amino functions is equal to the number of carboxylic acid groups [128]. Its structure is smectic A-like. The same complex forms nanoparticles when the polymer is used in an excess of 50% [129]. The particles exhibit hydrodynamic diameters in the range of 80-150 nm, which depend on the preparation conditions, i.e., the particle formation is kinetically controlled. Each particle consists of a relatively compact core surrounded by a diffuse corona. PEI-C12 forms the core, while non-complexed PEI acts as a cationic-active dispersing agent. It was found that the nanoparticles show high zeta potentials (approximate to +40 mV) and are stable in NaCl solutions at concentrations of up to 0.3 mol l-1. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. A variation of the pH value was used to activate the dissolution of the particles. 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

The first chiral combined lc polymers prepared for this purpose showed the desired cholesteric and chiral smectic C phases only at high temperatures (8) (the melting point was always above 100°C). By using lateral substituents (see Figure 3) it is possible however to suppress the melting temperature and to obtain polymers with a glass transition temperature of about room temperature, without losing the cholesteric and chiral smectic C phases (9). 

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 ... 

Due to the interesting LC properties of combined LC polymers (i.e. broad LC phases, and the occurrence of different smectic phases and a nematic phases at different temperatures) and their intermediate nature between that of side-chain and that of main-chain polymers, a lot of research has been undertaken on these materials. Most of the research has been directed towards the preparation of cross-linkable polymers and LC elastomers [3-11] and of chiral combined LC polymers [4, 6, 7, 9, 12-16]. 

Because of the interest in chiral LC phases in general, which give rise to selective reflection of light (cholesteric phase) or fer-roelectricity (chiral smectic C phase), chiral combined LC polymers were prepared quite early on [4]. Polymers with cholesteric and chiral smectic C phases could be prepared easily. As these polymers were synthesized using to the polycondensation process shown in Scheme 1, the chiral groups had to be selected carefully in order to prevent racemization during polycondensation [4, 7, 12, 13]. 

X-ray measurements [7, 11, 13-15, 17, 19, 20] performed on combined LC polymers show that the LC phases are analogous to low molar mass liquid crystals, which also show nematic, smectic A, smectic C and higher ordered smectic phases at different temperatures (see Figure 8). 

In addition to the homopolymers also copolymers were studied, in which the comonomers were characterized by spacer groups of different length. The combinations studied were n = 2/6 n = 6/12 and n 2/12. Smectic polymers were observed by copolymerization of monomers with n = 2/6 and n = 6/12, whereas cholesteric polymers were obtained by copolymerization of monomers with n = 2/12, if the composition was approximately 1 1. Apparently large differences in spacer... 

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