Cybotactic groups

Compound 43 (F pre 10) exhibited a nematic phase over a range of 120 °C, and its FE switching has been demonstrated by combining repolarization current measurements, electro-optical characterizations, XRD, and computer simulations. It was concluded that the response is due to field-induced reorganization of polar cybotactic groups within the nematic phase. 

Figure 14. (a) Cybotactic groups in the nematic phase (b) diffraction pattern from a nematic phase with SmC-type cybotactic clusters. 

In some compounds (usually the lower members of a homologous series), the transition to the Nd phase takes place directly from the crystal, while in others (the higher members of the series) it takes place via a columnar phase (Table 2) [31, 32]. The trend is somewhat analogous to the behaviour of the smectic A-nematic transition for a homologous series, and as will be discussed in Sec. 3 this trend can be explained by an extension of the McMillan model of the A-N transition to systems of discotic molecules. There is also X-ray evidence of skew cybotactic groups in the Np phase prior to the transition to the D, phase, very much like the situation seen near a nematic-smectic C transition [34]. 

W = -COO-, X = -N=CH- and A = C6H4) [6] is that of a usual nematic phase of elongated molecules with evidence of tilted Sc cybotactic groups the apparent length of the molecule is 51.3 A and the tilted angle reaches 60° near the N-Sc transition. The found angles are those of the crystalline phase about 50-60°. 

Secondary nematic structure was found by Blumstein and co-workers during the x-ray investigation of polyester prepared from 4,4 -hydroxy(2,2 -methyl) azoxybenzene and dodecanedioic acid. The diffraction pattern of the nematic shows the development of enhanced order characteristic of smectic phase (Fig. 16) the first of the meridional arcs splits up into four sharp spots. This phenomenon is incompatible with the classical definition of the nematic phase and suggests an additional order of macromolecules within cybotactic groups for which a structural model is proposed. 

That is why various supramolecular heterogeneities in oligomer liquids such as aggregates, associates or cybotactic groups are not microfhases . The term micro can be used here, but the term phase in inapplicable on the basis of Gibbs approximation. 

As noted above, the term cybotactic grouping (xiPcoxoi - ark) was introduced in the 1920-ies [79], however, it was finally approved much later, thanks to the fundamental summaries of Ya.Frenkel [80]. Now this term is widely used in the descriptions of the structure of both traditional low molecular weight organic and metallic liquids [87], Papers on polymers often use another term, cluster (bimch, swarm). This term is close to cybotactic grouping , however, the probability of spatial orientation of the primary particles of this structural element has not been taken into accoimt here. For this reason, another term, nematic cluster has been introduced, which, as well as the word combination anisotropic associate , is sometimes used to denote the cybotactic areas of the liquid. These terms do reflect the physical meaning of the concept, however, they are too cumbersome. 

Later on, numerous indirect confirmations of the hypothesis about the existence of regions with spatial molecular arrangement in oligomer liquids, were obtained. In addition to the analysis of early works [72, 146, 191], allowing us to formulate and prove the hypothesis about anisotropy of molecules inside the aggregates of liquid oligoester acrylates, we will study only one experimental result, which was not analyzed in the above mentioned papers. Associative cybotactic grouping model was used to explain this result. 

The initial growth of Wo in thickened systems in comparison with Wo for clear MB, results from the increase of number and lifetime of cybotactic groupings, which occms when high-viseous components are added. This corresponds to the idea described in works [146,188,200]. 

For the mixed systems (Figure 2.33), the Wo velocity decrease cannot be directly proportional to the dilutant concentration. Furthermore, at equimolar ratio of reactive and nonreactive molecules in the solution, the supramolecular structures do not influence the polymerization processes. This fact occurs due to the pair-wise interaction of two methyl acrylate groups and one-methyl acrylate and isobutyrate group inside the cybotactic groupings. Hence, the laws of classical kinetics must operate in this range of concentrations. As we can see from the experiment (Figure 2.32, curve 3), at cpi 0.5 the velocity does not depend on cpi. 

The fact that the experiment for the system with high viscosity of the nonreactive component shows an extremum on the curve, lFb=f(cpi) is explained by competition between the positive influence of the second component (thickener) on polymerization (the number of cybotactic groupings with the lifetime comparable to the elementary acts time increases) and its negative influence (the addition of nonactive molecules to the structures with kinetically preferred order results in the decrease of the elementary acts number). It is natural that at small dosages of the thickener the first factor prevails over the second one, while at large dosages of the thickener the second factor is predominant. 

Therefore, the spontaneous formation of the equilibrium supramolecular system structure, which provides its kinetic curing stability, and does not depend on the prehistory of the sample, occurs after the completion of swelling, within the exposure period 0contact with the liquid phase. During this period, local molecule concentration redistribution and spontaneous supramolecular structure elements formation (cybotactic groupings and associates) occur in the macroscopic sample volume. The characteristics of these elements (lifetime, correlation radius, strength of physical bonds) differ from the similar indices or coefficients in the initial oligomer and in the polymer-oligomer film, which is in contact with liquid. 

---