Order parameters, aliphatic

Attempts to interpret the corresponding build-up curves according to the Lipari-Szabo approach lead to inconsistent results (for instance, order parameters greater than unity). This indicates that these remote correlations are probably not of infra-molecular origin but would rather arise from znfer-molecular dipolar interactions which could become significant when some contacts exist between neighbouring aliphatic chains. This... 

The order parameters of aliphatic hydrocarbons solubilized in a lamellar liquid crystal were determined from NMR data. The variation of order parameters along the hydrocarbon chain with varying amounts of hydrocarbon solubilized supports a model with the main part of the hydrocarbon forming a layer between the amphiphilic layers with only a small amount of it penetrating between the amphiphilic molecules. 

Deuterium NMR spectroscopy of the discotic phase of hexa-n-hexyloxy triphenylene has led to similar conclusions. Spectra of two selectively deuterated isotopic species, one in which all aromatic positions are substituted and the other in which only the a-carbon side chains are substituted, bring out the difference between the order parameters of the cores and the tails. Fig. 6.1.3 gives the quadrupole splittings of the aromatic and the a-aliphatic deuterons versus temperature in the meso-phase region. It is seen that the rigid core is highly ordered, the orientational order parameter s ranging from 0.95 to 0.90, whereas the a-aliphatic chains are in a disordered state. 

Fig. 6.1.3. Quadrupole splittings for the aromatic and a-aliphatic vj deuterons of deuterated hexa-n-hexyloxytriphenylene (THE6) as functions of temperature (r— 7 ) in the mesophase region, where 7 is the mesophase-isotropic transition point. The open circles correspond to measurements on neat THE6-ard, and THE6-adi2 separately, while the filled circles correspond to a 2 1 mixture of the two isotopic species. The scale on the upper right-hand side gives the orientational order parameter of the aromatic part. The curve at the bottom gives the ratio of the quadrupole splittings for the a-aliphatic and aromatic deuterons (Goldfarb, Luz...

In the case of LC polymers, the order parameter S [61] is usually examined with respect to mesogenic groups, although the same parameter can be used for evaluating the orientational order of the main chains and aliphatic spacers. 

An increase in the length of the spacer in a series of nematic polymers has almost no effect on the value of the order parameter, as demonstrated on the example of siloxane polymers with aliphatic spacers containing from 3 to 6 methylene groups [14]. 

Fig. 6.18. Temperature dependence of the order parameters of mesogenic groups (1) and aliphatic spacers S (2) for polymers [63] ...

Fig. 5. The temperature dependence of the micro-order parameter ZZ and trans population n of liquid crystal polyesters I-III. Full circles (left ordinate) denote S z, while open squares (right ordinate) refer to of the outer and central spacer segments. Full triangles indicate the micro-order parameter zz of the polyester, having only nine segments in the aliphatic spacer. The dashed lines denote the melting points at 7 = 433 K and T = 429 K, respectively.

A further difficulty arises during preparative electrolyses in aprotic solvents because of the bulk pH change which commonly occurs. Thus cathodic reductions often require proton abstraction from the solvent in order to yield stable products, while many anodic oxidations, mcluding those of aromatic and aliphatic hydrocarbons, give rise to a quantitative yield of proton and the consequent changes in the pH. of the electrolysis media would be expected to lead to a variation in the products with the duration of the electrolysis. Unfortunately, the pH can be a very difficult parameter to control in aprotic solvents and most work reported in the literature has been carried out in unbuffered conditions. In the case of oxidations, organic bases, e.g. pyridine, have... 

Consequentely, we have chosen solvents in order to change separately either the norm of the solubility parameter or its direction (see Fig. 4). These solvents are listed in Table 1. It can be clearly seen that the polar and hydrogen bonding interactions are zero for all of the aliphatic and cycloaliphatic alkanes. This allows one to change only the value of without changing its direction. For a second series of experiments, we compare 2,6-dimethyl-4-heptanone, dib-utylether and methyl-cyclohexane which have nearly identical lengths, but different vector directions. 

---