One-comb PLCs

The values quoted above for the domain order parameter refer to the alignment of the mesogenic groups. It is known, for instance, that the order parameter of the spacer methylene units in smectic one-comb PLCs is only 0.3-0.4 whereas the mesogenic group order parameter is close to 0.9 [30]. Finkelmann et al. [31] reported the following data on the order parameter of absorbing dichroic dyes, chemically bonded to a nematic, one-comb PLC with a siloxane backbone, close to the isotropization temperature Tjt 0.48 (T/Tj = 0.90) 0.40 (T/Tj = 0.95) 0.30 (T/Tj = 0.99) 0.2 T/T. = 1.0). 

PLC elastomers are a special class of lightly crosslinked one-comb PLC which were first made by Finkelmann et al [69]. A permanently oriented PLC may be achieved by mechanical means by stretching a lightly crosslinked polymer, and the orientation is made permanent by further crosslinking in the distorted state. The crosslinks are attached to the backbone chains. The end product is crosslinked polydomain PLC which can be further aligned by drawing. 

Talrose et al. [85] reported that the response for the alignment of one-comb PLCs was faster at higher temperatures (nematics) and that the response time was proportional to the reciprocal of the square of the applied voltage, a trend which indeed was the same as for MLCs. Ringsdorf and Zentel [86] found that the response time decreased with increasing difference between the actual and glass transition temperatures. 

Figure 10.16 The order parameter of two oriented, nematic one-comb PLCs with different spacer groups as shown in the graph as a function of temperature. The samples were continuously heated and the order parameter was obtained by IR spectroscopy. (After data of Buerkle et al [128].)...

Figure 10.17 (a) The birefringence of an oriented one-comb PLC (polyacrylate obtained by photopolymerization of a surface aligned MLC) as a function of temperature. The various phases (K, crystalline S, smectic N, nematic and I, isotropic) are shown in the graph, (b) The order parameter of the same polymer (open symbol) in its nematic state as a function of reduced temperature, T/T., where Tj is the isotropization temperature. Data for the monomer (filled symbol) are shown in the same graph, ((a) and (b) from data of Broer et al [111].)... 

Figure 10,18 The temperature dependence of the order parameter for different moieties (explained in the graph) in a nematic, one-comb PLC with a polysiloxane backbone. (After data of Hempei et al. [130].)...

This chapter deals exclusively with thermotropics (pure polymer systems). This review deals with longitudinal one-comb and network PLCs and blends of longitudinal PLCs and conventional flexible chain polymers (Figure 10.2). An excellent introduction to the subject including a presentation of definitions and classifications is given by Brostow [10]. It is important to emphasize that a given PLC at a given... 

Dense PLC networks (thermosets) are often prepared from mixtures of mono- and bifunctional monomers resulting in a one-comb polymer-like structure with crosslinks. 

The acoustic absorption can also be obtained from ultrasonic measurements. However, no work has been done on longitudinal or other main chain PLCs. There is only one study [17] of the ultrasonic absorption of a comb PLC, the result of which will be discussed briefly. 

After perusing the classification in the previous section, one can ask It looks fine, but why do we need such a classification Actually, I started the development of this classification at about the same time I started to work on PLCs, simply so as not to get lost in all these structures. Analyzing the structures, I discovered another good reason molecular structures of PLCs determine their phase structures and properties. This important fact is also illustrated throughout this book hence there are separate chapters on longitudinal and on comb PLCs. Other authors have reached similar conclusions from their particular results. For instance, Ebert et aO say that In most liquid-crystalline systems it is predominantly the molecular shape which determines which kind of liquid-crystalline phase is formed. ... 

It is useful to compare the thermotropic behavior of noncovalently complexed PLCs with their analogous all-covalent PLCs. For the case of the ionically complexed comb-like PLCs, recently published series of particularly appropriate PLCs must be mentioned, one involving quatemized poly(4-vinylpyridine) [45-47], and another quatemized poly (ethylene imine) [48-50], In both series, the quatemizing moiety is a flexible spacer (based usually on methylene, but also on ethylene oxide) CO-terminated by a mesogen. The result is a pair of ionic groups at or near the polymer backbone, just as in the ionically complexed systems described above. These systems are compared schematically in Figure 3.27. 

A comment on Rule 2 also seems needed, since even some researchers working on LC materials believe that LC phase formation requires structures involving primary chemical bonds—while that Rule involves a more general concept of relations. Kato and Frechet [46] and also Bazuin and her collaborators [47,48] have found that hydrogen bonds or ionic interactions are sufficient for the formation of LC phases. Somewhat similarly, Zhao and Lei [49] have obtained ionomers which form simple one-row combs, that is class sO PLCs. Recall also the already mentioned work by Felekis and coworkers [31]. In other words, covalent bonds do not constitute a necessary condition for the formation of LC phases in polymers. 

DRS results for PLCs have been reviewed by Moscicki [92]. We shall name here two findings. First, in simple one-row combs (see again Table 41.1) there are different rates of reorientation of different polar groups in side chains. This phenomenon was observed even in MLCs [93], but is apparently stronger in PLCs. Second, also in analogy to MLCs, in PLCs we observe slow relaxational processes along the director, and noticeably faster perpendicularly to it. 

Subclass fiO, combs with one row of side chains. Hardouin and his colleagues call these side-end-fixed polymers . Usually there is a flexible tail beyond the mesogen. Sometimes there are two tails growing out of the mesogen first forked MLCs were synthesized " and later combs with forked tails. Many thermotropic eO PLCs are discussed in Chapter 7 by Simmonds, while lyotropic ones are covered in Chapter 5 by Hall and Tiddy. 

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