LC Elastomer

Crosslinked LC elastomers (Figure 19d) are very promising for piezoelectric and ferroelectric applications, and also as non-linear optic materials. The first synthetic step to such materials is the preparation of usual side chain or combined LC copolymers doped with a small part of side chains containing a polymerizable >C=C< double bond at the end (Figure 23 shows a particular example of a crosslinkable LC polymer64). The copolymer can be further photocrosslinked, giving an elastic polymer film which reveals... 

The ability to align LC elastomers with mechanical forces means that applications where LC alignment is necessary may be accessible through... 

This opens the possibility to tailor block copolymers with a wide variety of LC phases and phase transition temperatures. A interesting possibility is the preparation of thermoplastic LC elastomers of the ABA-type with amorphous A-blocks having a high Tg and an elastomeric LC B-block with low Tg. An uniform director orientation can be achieved in these systems by stress as shown recently for chemically crosslinked elastomers (12). Various applications of these systems in which optical uniaxiality and transparency are induced by strain can be envisaged. 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

Figure 1. Schematic representation of different types of lc elastomers.

First X-ray measurements show that the helical superstructure of the cholesteric and chiral smectic C phase can be untwisted by stretching the elastomer (5). High strains of 300% are necessary for this purpose (compared to 20% for the achiral elastomers). Nevertheless these results show that the chiral lc elastomers have the potential to act as mechano-optical couplers (cholesteric phase) or as piezo-elements (chiral smectic C phase) (5), because the mechanically induced change of the helical superstructure has to change the optical transmission or reflection properties or the spontaneous polarization. Both effects however have not yet been measured directly. 

Finkelmann H, Wermter H (2000) LC-elastomers as artificial muscles. ACS Abstr 219 189... 

Due to the interesting LC properties of combined LC polymers (i.e. broad LC phases, and the occurrence of different smectic phases and a nematic phases at different temperatures) and their intermediate nature between that of side-chain and that of main-chain polymers, a lot of research has been undertaken on these materials. Most of the research has been directed towards the preparation of cross-linkable polymers and LC elastomers [3-11] and of chiral combined LC polymers [4, 6, 7, 9, 12-16]. 

In order to combine LC properties and rubber elasticity a variety of LC elastomers were prepared [8]. In these systems a reor-... 

Figure 6. Schematic representation of LC elastomers prepared from chiral combined main-chain/side-chain copolymers, d] main-chain mesogens Idl side-chain mesogens chiral groups [7],...

Cross-linked LC elastomers from combined LC polymers were mostly obtained by means of the hydrosililation reaction shown in Figure 7 [3-7, 10]. Later, a thermal or photochemical polymerization of acrylates was used [9, 11]. As can be seen from Figure 7, this cross-linking process, which... 

All the LC elastomers prepared from combined LC elastomers were soft materi-... 

Scheme 4. Cross-linking of LC elastomers by radical polymerization of pendant acrylate groups.

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

Figure 15. Logarithmic plot of the piezosignal for an LC elastomer with the phase sequence g 1 SmC 81 I [19]. The cross-linking was done according to Scheme 4.

The techniques used to obtain the untwisted SmC phase structure in low molar mass LCs and SCLCPs are limited to thin layers. In contrast to this, LC elastomers can be macroscopically uniformly oriented by mechanical deformations [230], and this orientation process is not limited to thin samples or suitable dielectric anisotropy of the material. Furthermore, for LC elastomers the oriented structure can be chemically locked in by crosslinking, resulting in the so-called liquid single crystal elastomers [231],... 

Quite recently, Finkelmann and co-work-ers [232, 233] showed that an appropriate mechanical deformation of an SmC elastomer 50 yields a permanent macroscopically uniform orientation. This process also unwinds the helicoidal superstructure, and accordingly, frequency doubling is observed where the intensity of the SHG is directly related to the perfection of the uniform smectic layer orientation. The 22 < 23= 34 coefficients for a highly oriented sample were reported to be 0.1 pm/V and 0.15 pm/V, respectively. Taking into account that only 50% of the mesogenic units in the LC elastomer are active groups, these values are of the same order as those reported for low molar mass LCs containing similar chromophores [222]. 

Conventional low molar mass LCs as well as linear LC-polymers can be macroscopically ordered by external electric or magnetic fields, which is widely applied in optoelectronics in the case of low molar mass LCs. For LC-elastomers it is very important to know whether a macroscopic mechanical deformation of the polymer network influences the liquid crystalline side groups and whether a mechanical stress or strain produces similar effects as observed for conventional LCs by external fields. 

Mechanical behavior of uniaxial elongated and compressed LC-elastomers... 

In Fig. 3 i/e will demonstrate the thermoelastic behavior of the LC-elastomer with m = 3 (x=0) at constant load (mean relative deformation X=0.78). Above T in the isotropic state cr and the modulus arespectively increase linearly with increasing temperature corresponding to the behavior of common elastomers. 

Temperature dependence of CT of the LC-elastomer i/ith the spacer length m=3 (x=60) (see Table 1) in the isotropic state (T>T ) NR=natural rubber optical negative, uniaxial... 

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