Polymethylene -type spacers

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Fig. 3 Effect of the linking group on the odd-even character of the polymethylene-type spacer [ 14]. The latent entropy, A5ni/R, oscillates with the number of methylene units ( ) of the spacer. The three upper curves represent the dimer, and the two lower curves are for the monomer LCs. Filled symbols indicate the carbonate-type dimer, a,o)-bis[(4,4 -cyanobiphenyl) oxycarbonyloxy]alkane (CBC-n filled circles) and the monomer LC, 4 -n-alkoxycarbonyloxy-4-cyanobiphenyl (/lOCCB filled squares). The open symbols indicate the ether-type dimer, a,co-bis[(4,4 -cyanobiphenyl)oxy] alkane (CBA-n open circles), the ester-type dimer, a,o)-bis[(4,4 -cyanobiphenyl)carbonyloxy] alkane (CB-n open triangles), and the ether-type monomer LC, 4 - -alkoxy-4-cyanobiphenyl (nOCB open squares)...

The distinct odd-even alternation in ASjsn is a character peculiar to the polymethylene (PM)-type spacers. A comparison between the two different types of spacer, BCBOn (designated as CBA-n in this paper) [25] and MBBE-x with the oxyethylene (OE)-type spacer [26, 27] is shown in Fig. 5, where the latent entropy ASjsfi/R is plotted against the number of crmstituent atoms ( ) of the spacer. For MBBE-x with the OE spacer, irrespective of the parity of n, the A5ni versus n plot tends to decrease monotonicaUy with some tiny bumps. The odd-even trend characteristic of the tetrahedrally bonded chain system is rapidly smeared out by the... 

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

To conclude this discussion, it is quite obvious that the flexible spacer plays a very important role in determining not only the transition temperature but also the type of mesophase. In almost all cases, an increase in flexible spacer length leads to a decrease in transition temperatures. For the polymethylene spacers, this decrease is associated with an even-odd effect, with lower transition temperatures for the odd-numbered members of the series. For some mesogenic types, this effect can also be accompanied by a change from nematic to smectic mesophases. 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

Field-dependent CIDNP experiments in low magnetic fields reveal that CIDNP is of the comparatively rare S-T+i-type. The curves exhibit the bell-shaped behaviour t /pical for intersystem crossing at a level intersection, which has long been known for biradicals with a polymethylene chain. However, the rigidity of the dyads removes a complication present in flexible biradicals, namely, that the CIDNP experiment weighs certain conformations and distances between the radical termini more heavily than others.Hence, in the dyad experiments the value Bq at which the extremum of the field-dependence occurs corresponds directly to the condition y/lBo=2 /. An exponential dependence of / on the spacer length n was found with a damping factor of about 0.75A, which is comparable to that in other biradical systems. 

The use of chromophores covalently bonded to the polymer chain (NLO polymer) is a natural evolution of guest-host systems. In this case the polymer is obtained from a monomer (or a co-monomer) which is a chromophore too. There are clearly different choices for the way the chromophore is oriented with respect to the polymer chain. One possibility is to put the chromophore unit as a side pendant of the polymer chain (side-chain NLO polymer). This, in turn, can be accomplished by connecting the chromophore to the chain through suitable spacer groups, for instance conforma-tionally flexible polymethylenic units (polymer type I) in these polymers, no atoms of the chromophore unit, between the donor and acceptor groups, formally belong to the polymer chain. 

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