Chain anisotropy

Figure 27 shows spectra of third-order susceptibility x(3) of the oriented MOPPV LB film. Owing to the orientation of MOPPV chains, anisotropy of x(3) was observed. The x(3) values of the LB film, which is enhanced by the three photon resonance, are maximized at a fundamental wavelength of 1600 nm, as observed in a MOPPV cast film. The maximum values of X ( 3) and are 3.2 x 10 10 esu... 

Equation (33) yields the differential form of the dependence of the effective chain anisotropy, b, on chain length L. 

Figure 15 shows the dependence of the chain anisotropy (with respect to the anisotropy of the Kuhn segment fiA) on its relative length x = L/a according to Eqs. (39) and (40). At x—> 0 the curves have the slope 0.5 and rapidly attain an a mptotic limit corresponding to the anisotropy of the Gaussian chain. 

Figure 1, Evolution of the main-chain anisotropy of a series of trans polynorbomenes with different spacer length -(CH2)n as a function of the temperature, a) n=4 b) n=5 c) n=6 d) n=7 e) n=8 f) n=9. R// and Ri are indicated by ( ) and ( ) respectively and are determined within 5% accuracy.

The influence of the spacer length and of the stereoregularity on the mesophase and main chain anisotropy type was studied in addition to the scattering experiments. The cis isomers from the "hexafluoro-initiator" A exhibited the change to a smectic (oblate)... 

Elzein, T. Galliano, A. Bistac, S., Chain Anisotropy in PDMS Networks Due to Friction on Gold Surfaces. J. Polym. Sci., Part B Polym. Phys. 2004, 42, 2348-2353. 

The orientation of polydomain polymers by mechanical or viscous flow fields can be achieved easiest if a macroscopic chain anisotropy that coincides with the local symmetry of the LC phase structure is induced and fixed by chemical crosslinking. Eor nematic or Sa main chain polymers which locally show a prolate (see Sect. 3) chain anisotropy, a uniaxial deformation leads to a globally prolate chain conformation. If the chain conformation of the LC polymer is locally oblate, a globally oblate chain conformation can be induced by either uniaxial compression or -equivalently - biaxial stretching of the sample (Lig. 9). 

Compared to approaches based on crossUnking of aligned LC prepolymers (Sect. 4.1.2), LSCEs with rather complex crossUnking histories are obtained. This is important to consider when discussing chain anisotropies, effects of random disorder, or pretransformational effects. 

LCEs can have different topologies depending on the attachment of the mesogenic units (see Fig. 2), which is important for the magnitude of the chain anisotropy. If they are incorporated into the main chain, a large interaction between LC phase and mechanical properties is expected. If the mesogens are attached as side groups (LC side-chain polymers), their orientation is decoupled from that of the polymer chains [26]. Thus orientational correlations will be smaller than in LC main-chain polymers [10, 27]. 

Prior to discussing LC elastomers, we will consider in Sect. 2 in some detail the conformations and chain anisotropy of their polymer counterparts because the polymer backbone generates the shape anisotropy and elastic response of the elastomer network. In this context, note that two different classes of thermotropic LC polymers exist main-chain and side-chain (comb-like), as depicted in Fig. 1). In side-chain LC polymers, the pendant mesogenic groups are linked to a linear polymer backbone... 

To summarize this section, we note that the orientational order in nematic elastomers induces a chain anisotropy, which in turn determines the macroscopic shape of the sample. The manipulation of these shape spheroids by temperature and by electrical, mechanical, and optical fields is at the origin of many of the effects... 

In this review we have discussed ordering and frustration in LC polymer networks. In the first part, we treated the dominant role of the polymer backbone anisotropy in shaping the specific properties of nematic polymers and elastomers. Using results of neutron and X-ray measurements and applying some theoretical models, we have demonstrated how orientational order induces chain anisotropy in nematic polymers, which, in turn, determines their macroscopic shape. In spite of these results there is still need for more extensive information on the anisotropic shape of LC polymers. Such results could provide clues for the application of a greater variety of polymers for crosslinking. Up to now, most elastomer systems use flexible silox-anes as the polymer backbone. 

Even though one might expect 6q to be large for chains such as BBB and PPhy, the measured overall chain anisotropy 6 is small (< 0.01), in contrast with 6 for BBL, PBO and PPTA. For the latter two chains, 6 in chlorosulfonic acid... 

When the disorder is weaker (pr 3), the weak localization contribution dominates to lower temperatures (T < K). Thus, the anisotropy in both conductivity and MC is related to the extent of misaligned chains (anisotropy on the molecular scale) and to the anisotropy in the diffusion coefficient. 

Fig. 3.3 Temperature variation of the chain anisotropy r deduced from the thermoelastic measurements. L is the length of the films in the direction of the nematic director, normalized with respect to the length Lq measured at high temperature in the weakly anisotropic state. The data for the NE obtained by photo cross-linking a NP oriented by an H-field were taken by F. Brommel (unpublished result)...

The large difference in the shear mechanical anisotropy and in the chain anisotropy between the two types of NE suggests that the nature of their elasticity is not the same. Since Gaussian elasticity is characterized by a decrease of G when the... 

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