Comb LCPs

An effective comb LCP contains three primary structural variables each of which is significant to a realization of desired thermotropic behaviour,... 

The backbones encountered are generally familiar in thermoplastics technology and tend to be flexible polymers associated with glassy, rubbery or fluid phases depending on temperature. Overwhelmingly dominant are polymethacrylates, polyacrylates and polymethylsiloxanes, in order of increasing chain flexibility (decreasing Tg). These and other comb LCP backbones are listed, with indicative references, in Table 7.1. 

A further variant on the spacer-mesogen linkage follows from the lath shape of typical mesogens. In most comb LCPs the linkage is to the end of the long axis of the mesogen (longitudinal attachment), but connection... 

With such a varied menu of comb LCP components to choose from, it is important for synthetic chemists to appreciate how decisions about molecular architecture might influence the properties of polymer products. While the spacer, the mesogen and the constitution of the backbone are fixed by preselection of reactants, the average molecular mass, the polydispersity, the degree of substitution along the backbone, and the purity are variable. Systematic syntheses with these latter variables under careful control have been performed only quite recently. We shall review some of the results once we have covered the actual constitution of the backbone. A good deal is known about the effects of variations to the spacer and mesogen components, and the major correlations are listed in Sections 7.3.2 and 7.3.3. 

In the context of comb LCPs the most important property of a polymer backbone is its flexibility. Finkelmann has pointed out that while different backbones may not cause different mesophases, they do at least bring about different transition temperatures. The major correlations are between backbone flexibility on the one hand, and Tg and mesophase persistence on the other. Tg decreases as backbone flexibility increases, while AT(= Tci — Tg) increases with increasing flexibility, although clearing temperatures themselves (the onset of I, the isotropic phase) do not necessarily increase with flexibility —that depends largely on the length of the spacer, as we shall see later. 

Four general points emerge from studies by Shibaev, on acrylic comb LCPs. Phase transition data were compared for an unfractionated specimen and a series of carefully fractionated samples of different DPs. The four major conclusions were as follows ... 

In new synthetic work, Mallon and Kantor have used coordination polymerization to prepare stereoregular hydrocarbon combs (65-90% isotactic) from monomers that are 4,4 -dialkyIbiphenyls containing a terminal double bond in the longer alkyl chain." The resulting polymers exhibit smectic B and smectic E mesophases that are uncommon in comb LCPs. "... 

In a series of comb LCPs where spacer length is the only variable, there may be two reasons for any observed differences. Firstly, we may be seeing the result of changing the effective length of the mesogen (i.e. at least a part of the spacer is involved in the meso-genic structure) which is known to affect both the nature and thermal stability of mesophases (Section 7.3.3.1). Secondly, we may be seeing... 

In small-molecule LC systems, mixtures containing several LC components tend to be more useful for commercial applications than are pure LC substances. By analogy, composite polymers may, despite expense, turn out to be the most useful comb systems. In such composites the mixing may be purely physical (e.g. blends of comb LCPs with dyestuffs, as mentioned in Section 7.6.1), or the components may be chemically bonded into copolymers. In so far as copolymerization of two monomers generally leads to properties that are modified (but not fundamentally different) with respect to either of the analogous homopolymers, we may expect that fine-tuning of comb LCP properties should be feasible by copolymerization (e.g. by utilizing the sort of structure-property correlation data we have just derived for homopolymers). 

The optical and elastic properties of these novel network LCs are quite new. Orientation via mechanical deformation, and the resultant stress-optical phenomena, can be added to the range of behaviours already associated with side chain mesogenic polymers, such as electro-optic and magneto-optic phenomena. The comb LCP design is clearly capable of delivering a wide range of novel materials. 

Polymerization can be initiated using radicals generated via the UV irradiation of appropriate initiators in the presence of the monomer. This approach to comb LCPs, first reported by Broer et a/., is attractive since it can be performed at a preselected temperature. Therefore, monomers can be polymerized in an aligned LC phase and the effects of ordered polymerization can be examined. 

Cationic polymerization of comb LCPs was first reported by Percec who used the Lewis acid boron trifluoride etherate as initiator. This approach, illustrated in Fig. 7.22, gave useful solution preparations of polyvinylethers (R = H) and polypropenylethers (R = CH3). Recently Sagane and Lenz demonstrated that the use of the cationic initiator system HI/U brought about living polymerizations of similar monomers and greatly improved the molecular mass distribution of the products. 

Percec has extended the use of cationic procedures to the synthesis of vinyl ether copolymers based on constitutional isomers, analogous to the copolysiloxanes shown in Fig. 7.16 (Section 7.4.2). Other work, by Percec and Hahn, will prove useful in the controlled synthesis of siloxane backbones (Fig. 7.23). Cationic, ring-opening polymerization of cyclic tetrameric siloxanes, in the presence of an end capper (hexamethyldisiloxane) and initiated using triflic acid, can be used to prepare homo- (Y = n = 0) or copolysiloxanes whose DP and compositions are determined by the ratio of end-capper to monomer in the feedstock. These polymers are starting materials for siloxane comb LCPs (Section 7.5.5). 

While comb LCPs exhibit display-relevant properties, they do not seem appropriate for real-time active displays of the type associated with... 

Section 7.3.1.2. Switching response times were shown to be shorter for polymers of lower DP but the variation levelled off at DP 30. Each polymer switched most quickly at a temperature near the upper end of its S phase, and switching times were around 4-15 ms—much faster than those associated with nematic comb LCPs. 

Even-order non-linear coefficients vanish if individual molecules possessing substantive P coefficients pack into a material structure that has a centre of symmetry the induced molecular dipoles cancel out in the material, which therefore has negligible The comb LCP architecture has the advantage (compared to small-molecule materials) that NLO... 

A major drawback with some GC stationary phases results from their own volatility at operating temperatures, which, over a period of time, threatens column integrity via column bleed. Small-molecule LC packings are vulnerable to this problem but the comb LCP architecture, involving both LC moieties and siloxane polymers, is an attractive solution since it incorporates chemical tethering of the mesogens to the polymer. 

In summary, useful GC stationary phases based upon comb LCPs are available. Areas of active development involve suppression of column bleed, depression of the glass-to-LC transition temperature (important for volatile analytes) and broadening of the mesophase temperature range. The latter point favours nematic systems, since that phase tends to have better longevity. In parallel with these developments, investigations are under way to apply LC stationary phases to capillary to super-... 

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