Mainchain flexibility

As sparse as the dataset describing mainchain nematic LCP blends with conventional polymers is, it is rich compared to the almost non-existent data on the blending of other types of LCPs-side chain polymers, flexible spacer polymers, smectics, etc. 

Figure 2. Polypeptide backbone with standard notation for mainchain atoms the flexible dihedral angles and and the more rigid, partially conjugated peptide bond angle a are shown. The sidechains are indicated as R. [Adapted from C. R. Cantor and P. R. Schimmel, Biophysical Chemistry (W. H. Freeman and Co., San Francisco, 1980).]...

Our interest here is to review the thermodynamic character of the flexible chain segment which joins the mesogenic cores on both sides in the so-called mainchain liquid crystals. In the LC state, the flexible segments adjust themselves to make them compatible with the environment. The flexible segment thus takes a mesophase conformation which is different from either the isotropic random-coil or the extended crystalline conformation [26,81-87]. 

For long-chain molecules there are different geometric possibilities for the orientation of molecular dipole vectors with respect to the backbone. Following the notation of Stockmayer (1967), polymers are classified as type A (with dipoles fixed parallel to the mainchain, e.g., ds-l,4-polyisoprene and polyethers), type B [with dipole moments rigidly attached perpendicular to the mainchain e.g., poly (vinyl acetate) and most synthetic polymers], or type C [with a more-or-less flexible polar sidechain e.g.,poly(n-alkyl methacrylate)s]. However, a polymer possessing only one type of dipole moment is an exceptional case. The timescale (speed) of each polarization (and subsequent relaxation) process will determine whether this process will be monitored by a particular dielectric technique. Characteristics and fundamental peculiarities of relaxations generally found in polymers are discussed hereafter. Note that cases where finite polarization is present even in the absence of an external field (e.g., the permanent polarization in ferroelectrics) are not considered. 

Polymers with Flexible Polar Sidegroups Attached to an Apolar Mainchain... 

II. COMBLIKE POLYMERS HELICAL MAINCHAIN WITH FLEXIBLE SIDECHAINS... 

A great number of flexible polymers bearing linear long alkyl sidechains have been examined and much evidence has been collected to conclude that the commonplace structure of these systems in the solid state is a biphasic arrangement of alternating layers of mainchain and sidechains [6]. A detailed X-ray diffraction study on poly(a-olefin)s carried out by Turner-Jones [7] demonstrated that the polymethylene sidechain is crystallized only if it contains seven or more... 

The conformational analyses of mainchain LCs have been reported from various laboratories. Although the results seem to vary somewhat depending on the models adopted in the treatment of experimental data, all suggest that flexible spacers prefer to take extended conformations in the nematic state. Efforts to formulate molecular theories to describe the N1 transition characteristics of the mainchain LCs in terms of the molecular parameters have also been reported [7,43,44]. In an ideal crystalline state, molecules are aligned in a perfect order, often only the most preferred conformation being permitted for the spacer [45]. The structural characteristics of chain molecules in the crystalline, liquid crystalline, and isotropic fluid states must manifest themselves in the conformational entropy of the system upon phase transitions. As the DP of the mainchain LC sample increases, however, the degree of crystallinity tends to be lower, and accordingly the CN transition becomes less sharp [11]. 

In this example, we have attempted to reveal the tme nature of nematic conformation characteristic of flexible spacers incorporated in the LC state. The nematic conformation predominates in the individual -[Ms-X-(CH2) -X] c- units constituting a given polymeric sequence. In an independent work [26], PVT studies oti the mainchain LCs carrying OE-t5q>e spacers have been carried out It is interesting that the expansivity of the nematic LC phase was found to be larger than that of the isotropic melt. According to the conventional thermodynamic theories of polymeric fluids, the expansivity is closely related to the free volume of the liquid state [46 9]. 

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