Uniaxially oriented elastomer

Figure 2. Scattering intensity versus azimuthal angle for a uniaxially oriented elastomer, X is 3, x is 0.2. Phantom network where , f is 3 A, f is 4 V, f is 10. Crosslink junctions fixed, X.

Ro is given by 2 cot a, where a is the angle between the dipole moment vector and the chain axis. (P2 cos0)) is the one-parameter measure of orientation normally given in the literature for uniaxially oriented samples, a is a function of the vibration in question and is known for a number of vibrations useful in studying thermoplastic elastomers (Myers and Cooper, 1994). 

For anisotropic samples, such as thermoplastic elastomers showing uniaxial orientation, the analysis of the longitudinal structure gives only a fraction of the total information concerning the nanostructure topology. By analogy with the IDF concept, the complete information should be displayed in a multidimensional function that maps the distances between all the domain surfaces. 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

A powerful technique for the study of orientation and dynamics in viscoelastic media is line shape analysis in deuteron NMR spectroscopy [1]. For example, the average orientation of chain segments in elastomer networks upon macroscopic strain can be determined by this technique [22-31]. For a non-deformed rubber, a single resonance line in the deuterium NMR spectrum is observed [26] while the spectrum splits into a well-defined doublet structure under uniaxial deformation. It was shown that the usual network constraint on the end-to-end vector determines the deuterium line shape under deformation, while the interchain (excluded volume) interactions lead to splitting [26-31]. Deuterium NMR is thus able to monitor the average segmental orientation due to the crosslinks and mean field separately [31]. 

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. 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

Smectic elastomers, due to their layered structure, exhibit distinct anisotropic mechanical properties and mechanical deformation processes that are parallel or perpendicular to the normal orientation of the smectic layer. Such elastomers are important due to their optical and ferroelectric properties. Networks with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal with respect to the normal smetic direction are mechanically deformed by uniaxial and shear deformations. Under uniaxial deformations two processes were observed [53] parallel to the direction of the mechanical field directly couples to the smectic tilt angle and perpendicular to the director while a reorientation process takes place. This process is reversible for shear deformation perpendicular and irreversible by applying the shear force parallel to the smetic direction. This is illustrated in Fig. 2.14. 

Hedden, R. C. Tachibana, H. Duncan, T. M. Cohen, C., Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 2. NMR Measurements from Uniaxially Stretched Samples. Macromolecules 2001, 34, 5540-5546. 

Different types of LC systems are found in elastomers. In nematic liquid crystals, the molecules have orientational but no positional order, their center of mass positions being randomly distributed. Most nematic elastomers are employed in uniaxial deformation. If the LC elements contain chiral groups, they are termed as cholesteric elastomers. Discotic nematic LC elastomers contain disk-shaped molecules that can be oriented in layers. Smectic LC elastomers form well-defined layers. 

However, it is well known that a mechanical deformation of a conventional, isotropic polymer network causes anisotropy. Under deformation the chain segments become oriented according to the symmetry of the external field and the state of order of the network can be characterized by an order parameter similar to that of nematic liquid crystals. Very early mechanical experiments on nematic polydomain elastomers actually demonstrate that a uniaxial deformation of a nematic elastomer converts the polydomain structure into a macroscopically xmi-formly ordered monodomain network [44]. This is shown in Fig. 2, where the opaque polydomain becomes optically transparent and converts into a monodomain... 

The concept of mechanical field induced orientation can easily be transferred to nematic elastomers with oblate chain conformation, i.e., side chain end-on elastomers with an even number of spacer atoms. In order to achieve a monodomain structure, a globally oblate chain conformation has to be established. This can be achieved by uniaxial compression or biaxial stretching of the polydomain elastomer which induces a uniform homeotropic alignment of the nematic director perpendicular to the film plane. Up to now, this orientaticMi technique has only been realized experimentally for chiral nematic elastomers [72]. 

Smectic side-chain polymers prefer locally oblate chain conformations, independent of the spacer length or attachment geometry. Analogous to oblate nematic polydomain elastomers, biaxial mechanical stretching or uniaxial compression can be used to orient Sa polydomain elastomers. This achieves a simultaneous orientaticai of the director and the smectic layer normal in a uniform homeotropic fashion [74],... 

In contrast to classical side chain elastomers, smectic-A main chain elastomers exhibit prolate chain conformations. Consequently, macroscopically oriented samples can be prepared according to the method of Kiipfer et al. utilizing a second crosslinking step under uniaxial deformation [31]. Analogous, Sa LSCEs based on side chain elastomers with side-on attached mesogenic units can be prepared [97]. 

Locally oblate lyotropic elastomers with lamellar phase structure (L -phase) can be oriented by uniaxial compression, as outlined above for thermotropic smectic-A elastomers. Fischer et al. synthesized crosslinked polysiloxane elastomers carrying non-ionic amphiphilic side-groups attached with their hydrophobic end to the polymer backbone. They were able to compress elastomer samples between Teflon half-cylinders to about half of their original thickness. The orientation of the phase structure - except for some unoriented domains - was demonstrated by means of H-NMR spectroscopy on the directly deuterated samples as well as by X-ray scattering. The preferred orientation of the director, and hence the amphiphilic side chains, was found to be parallel to the axis of compression with the amphiphilic bilayers aligned perpendicularly [98, 99]. 

Weiss et al. were able to orient a hexagonal epoxy-amine elastomer by applying uniaxial strain yielding high degrees of order. The elastomer was crosslinked in the isotropic state and successively swollen with water to give a hexagonal phase [102]. 

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