Liquid crystalline chemical networks

Curing of liquid crystalline cyanate ester resins in electric fields is a new trend in thermoset design and processing and can be used to control directly their mechanical and physical properties. Combining new LC materials with non-LC cyanate monomers leads to a variety of novel ordered network structures and is a convenient method for modifying and controlling their chemical and physical properties [294]. 

Polymerization of reactive monomeric liquid crystals is one method for stabilizing the liquid-crystalline thin films. Another approach is to form chemical gels of liquid crystal molecules with low molecular weight by construction of a polymer network. This method has been investigated for the stabilization of ferroelectric liquid crystal displays. Guymon et al. reported that a polymer network produced by photochemical cross-linking accumu-... 

F. 2.13 Left the chemical stmctures and a schematic representation of desorption and adsorption of 9 in the nanoporous columnar liquid crystalline polymer film. Right the X-ray diffraction of the erosslinked network at different stages of adsorption and desorption. Adapted with the permission from Ref. [66]. Cop5uight 2006 American Chemical Society and Ref [64]... 

For these transient networks formed by the interaction of an ABA triblock copolymer and a microemulsion it has been shown that their principal viscoelastic properties are not affected significantly by the chemical nature of the microemulsion, i.e., they are similar for systems with both nonionic and ionic surfactants. Also it should be noted that the phase behavior of the corresponding microemulsion is qualitatively preserved, i.e., the reversible aggregation of the nanodroplets and the phase transitions to lyotropic liquid crystalline phases remain essentially unchanged (although the concentrations at which they occur might... 

Figure 9.12 Chemical route to a macroscopically oriented liquid crystalline network (adapted from Reference [29]).

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

Fig. 2 Schematic five types of SMPs depicted as a function of their thermal behavior. Plotted is the heat flow vs temperature as measured in a differential scanning calorimetry (DSC) experiment (a) Cat. A-1, chemically crosslinked tunorphous polymer network (Tirans = 7g) (b) Cat. A-11, chemically ciossfinked semicrystaUine polymer networks (Taims = 7m)> ( ) Cat. B-1, physically crosslinked thermoplastic with Tirans = T (d) Cat. B-11, physically crosslinked thermoplastic (Tams = I m) and (e) liquid crystalline polymer (Tlrans = T -n)...

Just as for biological beings, the liquid crystalline phase structure and simultaneously the functionality of liquid crystalline elastomers are strictly limited to a defined temperature regime. Similar to low molar mass liquid crystals and LC polymers this temperature regime is determined by the chemical constitution of the polymer networks. For the synthesis and investigation of liquid crystalline elastomers the basic concepts of liquid crystals, LC polymers, and polymer networks have to be brought together. 

Conventional principles and methods concern the synthetic routes for macromo-lecular networks and the realization of the liquid crystalline state by mesogenic monomer tmits. Network chemistry has to consider the reactivity and functionality of the monomer tuiits. In most cases, this excludes ionic polymerization techniques and reduces utihzable methods to radical polymerization and polymer analog reactions for side chain networks, and to polycondensation or polyadditirai reactions for main chain elastomers. The chemistry of the crosslinking process and the chemical constitution of the crosslinker have to be adapted to the polymerization process. Applying photo-chemistry of suitable functional mmiomer units opens an additional, versatile pathway to build up the network structure. 

The effect of the chemical ermstitution of the crosslinker oti the local topology of the network is the second new feature to be considered. If the crosslinker molecule is flexible it can behave like an isotropic solvent. In that case, essentially only the phase transition and phase transformatiOTi temperatures of the LC phase are affected [90]. If, however, the chemical constitution resembles that of a mesogen of the constituent polymer backbone, the history of the crosslinking process becomes important. Under these conditions the crossUnker adopts the state of order in which the final crosslink process of the network occurs and thus determines the local topology of the crosslink [120,121]. The mechanical properties and the reorientational behavior are considerably modified for networks with the same chemical constitution but crosslinked either in the isotropic or in the liquid crystalline state [122-124]. Other important aspects of the local topology at the crosslink concern the phase transformation behavior [125] as well as the positional ordering in smectic systems [126]. 

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