Low molecular mass LCs

Before going any further, let us adopt the terminology introduced by Samulski [5]. We have already used above the abbreviation PLCs. Samulski contrasted PLCs to MLCs, and defined the latter as low molecular mass LCs— irrespective of the fact whether they can or cannot polymerize. His terminology is unequivocal and succinct. People unfamiliar with it use long and not necessarily well-defined terms, such as liquid-crystalline substances with low molecular weights —when they presumably mean MLCs. Other names such as liquid crystalline polymers (LCPs) for PLCs or LMMLCs for MLCs are also in use. The abbreviation SRPs for self-reinforcing polymers and the name in situ— composites [6] are used as weU. Moreover, PLCs are sometimes also called molecular composites. 

The differences between standard thermotropic LCs and macromolecular condis crystals are summarized in Fig. 8. The first three and the last two points make it easy to experimentally identify low molecular mass LCs. For macromolecules, however, the viscosity may be suflBciently large to lose the obvious liquid character the birefringence does not always show the well-known LC texture (55) the small ASj of LCs may be confused with partial crystallinity of the condis crystals and in polymers, some larger main-chain rigid groups are not always easily identifiable as mesogens. This leaves points four and eight for differentiation between the two mesophases. Points five and six are more difficult to establish, and solid state NMR and detailed X-ray structure-determinations may be necessary for full characterization. Furthermore, borderline structures may be possible between thermotropic LCs, amphiphilic LCs, and condis crystals. A few examples and the resolution of their structures are discussed next, to illustrate the resolution of some of these problems. 

Fig. 3 Different reaction paths for the preparation of LCEs (a) One-pot synthesis, usually via hydrosilylation (b) reaction of LC polymer with a crosslinker (c) LC polymer containing a reactive group that is activated by UV irradiation (d) mono- and bifunctional low-molecular-mass LCs that are polymerized...

Electric field is also expected as an effective external field to drive finite and fast deformation in LCEs, because, as is well known for low molecular mass LCs (LMM-LCs), an electric field is capable of inducing fast rotation of the director toward the field direction [6]. This electrically driven director rotation results in a large and fast change in optical birefringence that is called the electro-optical (EO) effect. The EO effect is a key principle of LC displays. Electrically induced deformation of LCEs is also attractive when they are used for soft actuators a fast actuation is expected, and electric field is an easily controlled external variable. However, in general, it is difficult for LCEs in the neat state to exhibit finite deformation in response to the modest electric fields accessible in laboratories. Some chiral smectic elastomers in the neat state show finite deformation stemming from electroclinic effects [7,8], but that is beyond the scope of this article we focus on deformation by director rotation. 

Apart from ES and APCI being excellent ion sources/inlet systems for polar, thermally unstable, high-molecular-mass substances eluting from an LC or a CE column, they can also be used for stand-alone solutions of substances of high to low molecular mass. In these cases, a solution of the sample substance is placed in a short length of capillary tubing and is then sprayed from there into the mass spectrometer. 

Another interesting application of LC-LC is the determination of low-molecular-mass carbonyl compounds in air. Carbonyl compounds, such as aldehydes and ketones, are now being given more and more attention, both as pollutants and as... 

A major application of LC/ESI/MS is the characterization and detection of toxins, ranging from relatively small molecules, such as mycotoxins and some marine toxins, to the large proteinaceous toxins such as ricin and botulinum toxins. The marine toxin saxitoxin and the plant toxin ricin are specifically listed in Schedule 1 of the CWC as examples of toxins. A comprehensive review of LC/MS in toxin analysis would require a major chapter in its own right. Hancock and D Agostino 1711 reviewed approaches to the mass spectrometric identification of selected low molecular mass toxins. This chapter will describe examples of LC/MS in the analysis of marine, fungal, bacterial, and plant toxins, which are of possible relevance to the CWC. 

The polysulfanes formed on reaction of DCPD with liquid sulfur have been studied by extraction of sulfur cement and analysis by LC, H-NMR, MS, and other techniques.The initial products are trisulfane and pentasulfane derived from DCPD by addition of S3 or S5 units to the norbomenyl double bond. These monomers are believed to further react with elemental sulfur to form low-molecular mass polymers (CS2 soluble), and on further heating form an insoluble material. The cyclopentenyl unsaturation of DCPD is much less reactive and is still present in the CS2-soluble products. endo-T>CP D reacts more slowly with liquid sulfur at 140 °C than eco-DCPD, while the cyclic trisulfanes of endo- and gxo-DCPD react at almost the same rate with liquid sulfur at 140°C. The stmctures of DCPD-S3, DCPD-S5, and the hkely stmcture of the low-molecular mass polymer, are shown in Figure 8. 

The haloacetic acids (HAA) are comprised of mono-, di- and trichloroacetic acid, mono-, di-, and tribromoacetic acid, and bromo-chloroacetic acid, bromo-dichloroacetic acid, and dibromo-chloroacetic acid. Toxicological studies showed that these compounds have carcinogenic properties and may have adverse reproductive consequences. HAA have no strong chromophore for sensitive UV detection electrochemical detection has been described. Analysis by GC-MS requires derivatization. Due to their relatively low molecular mass, the LC-MS analysis can be hindered by low-mass background interferences. 

An increase of the pitch with molecular mass has been observed for LC cellulose acetate in trifluoroacetic acid and for CTC in DEME (cf. also Fig. 2d). The pitch rapidly changes at low molecular masses and remains almost constant above a DP of 150 for the left-handed CTC/DEME system [54]. At very high molecular masses, the LC state was not obtained, gels formed instead. A similar leveling off was observed for the clearing temperature (Fig. 16). For the pitch P as well as for the clearing temperature T, an analytical expression was derived with fitted parameters as a function of the molecular mass [4]. This observa-... 

In this paper, one of the examples of the use of such oriented films as film matrix for information recording is examined. We used the principle of thermo-recording, which is well known for low molecular mass liquid crystals >. On the transparent film of the homeotropi-cally oriented LC polymers (Fig. la), regions of local overheating are created with the aid of a laser beam. 

The laser-recorded information can be kept for a long time if the sample is cooled below the glass transition temperature. From this viewpoint, LC polymers differ beneficially from low molecular mass liquid crystals. 

The molecules displaying LC phases are highly anisotropic. They might be seen as rigid rods or ellipsoids of revolution with I d. The basic structure of low molecular mass liquid crystals or monomers of LC polymers is given in Figure 5. 

Until the advent of SFC, there has been an analytical gap in the molecular mass range extending above the GC molecular weight range, which LC did not always satisfy. It is within this 10 -10 range that SFC excels. This is particularly true for oligomers and many polymeric raw materials. For example, polyols, used in the manufacture of many polyesters and polyurethanes, are seldom volatile enough for GC, nor do they necessarily contain a chromophore for LC, and they are excluded by SEC due to their low molecular mass [37]. 

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