Thermosets, ordered liquid crystalline

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. 

Liquid crystalline thermosets (LCT) with a rigid rod-like mesogenic group capped firom both ends by reactive fimctional groups can be used as matrices for production of advanced composites, coatings or adhesives (1, 2). hi this case ordering of the chains of the final network polymer is the only way to improve its physical and mechanical properties. 

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]. 

Liquid Crystalline Epoxy Resins can be cured in different ordered states depending on the curing agent used. With aromatic amines the thermoset is crosslinked in a nematic structure. No isotropization temperature can be detected. The reaction can be monitored by means of DSC analysis. A double peak exotherm indicates that the crosslinking reaction forms a liquid crystalline resin. Kinetic models can be applied to separate the contributions of the different reactions. 

Giambtaini M, Amendola E, Carfagna C (1997) Lightly crosslinked liquid crystalline epoxy resins the effect of rigid-rod length and applied stress on the state of order of the cured thermoset. Mataomol Chem Phys 198 3185—3196 Grebowicz JS (1996) On the formation of liquid crystalline texture in epoxy resins. Macromol... 

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