Mainchain characteristics

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. 

A.3. The Normal-Mode (n) Relaxation Process The term normalmode relaxation refers to the long-range motions of the end-to-end dipole moment vector along a polymer chain, and thus corresponds to the comparably slow motion of a whole chain (Adachi 1997). This relaxation mode is characteristic of polymers with dipoles fixed parallel to the mainchain (type A polymers). A representative class of such polymers are the polyethers (-CH2—CHR—O—) , with R H [e.g., poly(propylene glycol) (Hayakawa and Adachi 2001) poly(butylene oxide) (Casalini and Roland 2005)], for which, with the exception of a few members [e.g., poly(styrene oxide) (Hirose and Adachi 2005)], a strong normal-mode relaxation signal can be resolved... 

The basic characteristics of mainchain supramolecular polymers are presented schematically in Fig. 16. Designating the monomer core residues by Rj, monomers bearing two identical interaction/recognition groups (homoditopic), may yield either homopolymers, when R = Rj, or regularly alternating copolymers, when R( Rj. 

The characteristic features of the nematic ensemble elucidated above are put together in a simple illustration depicted in Fig. 6, which shows nematic arrangements of mainchain LCs in contrast to those of the adjacent isotropic and crystalline phases. In the nematic field, both spacer and mesogenic units at the terminals tend to align along the domain axis. Consequently, the individual mesogenic cores inevitably incline to some extent with respect to the direction of the molecular... 

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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