Liquid crystalline epoxy

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. 

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Barclay, C.G. McNamee, S.G. Ober, C.K. Papathomas, K.I. Wang, D.W. Liquid crystalline epoxy thermosets mechanical and magnetic alignment. J. Polym. Sci. Polym. Chem. 1992, 30, 1845-1853. 

Epoxy + Epoxy, liquid max. 50 % liquid crystalline epoxy RT 1.0 49.0 - 73.0 02Pun... 

Pun Pxmchaipetch, P., Ambrogi, V., Giamberini, M., Brostow, W., Cafagna, C., D Souza, N. A. Epoxy + liquid crystalline epoxy coreacted networks n. Mechanical properties. Polymer 43... 

Har Harada, M., Aoyama, K., Ochi, M. Influence of the loading rate on liquid-crystalline epoxy resin systems. J. Polym. Sci. Part B - Polym. Phys. 43 (2005) 1296-1302. 

Figure 3.13 Reaction scheme for synthesis of liquid crystalline epoxy by chemical modification of DGEBA resin. Reprinted with permission from K. Sadagopan, D. Ratna and A.B. Samui, Journal of Polymer Science Part A Polymer Chemistry Edition, 2003, 41, 3375 2003, John Wiley and Sons Publishers...

Liquid crystalline epoxies have potential applications for electro optics, aconstics, and in information technology. The rubbery epoxies exhibit very high damping factor (>1) and can be used for vibration damping applications, as discussed next. 

This statement holds true especially in the case of liquid crystalline epoxy resins. In fact, the structural features and distribution of crosslinks along the molecular backbone strongly influences the liquid crystalline phase developed during the curing process and the performances and the applicability of the resulting material. 

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. 

As a final comment it has to be outlined that liquid crystalline epoxies represent a novel class of thermosetting polymers with unique properties. The relative easiness of synthesis, their production costs and eventually the possibility to taylor their properties make them potential candidates for specific applications. 

Diglycidyloxy-a-methylstilbene (1) is a liquid crystalline epoxy, while the diglycidyl ether of bisphenol A (2) is a typical non-liquid-crystalline epoxy. 

Kinetic studies have also been conducted on epoxy systems. For amine cure, it is generally assumed that the primary amine has a greater reactivity than the secondary amine. However, two studies foimd that for liquid crystalline epoxies the secondary amine is more reactive than the primary amine (62,63). There is no... 

Carfagna, C., Amendola, E., and Giamberini, M. (1997). Liquid crystalline epoxy based thermosetting polymers. Prog. Polym. Sci., 22 1607. 

Keywords Liquid crystalline thermosets, liquid crystalline epoxy resins, curing reactions, mechanical properties, thermal properties... 

Figure 11.7 Chemical structures of liquid crystalline epoxy resins with deferent mesogenic length. Reproduced from reference 27.

Figure 11.8 Chemical structures of naphthalene (a) and biphenyl (b) containing liquid crystalline epoxy resins. Reproduced from reference 28, 29.

Figures 11.11 and 11.12 explains gelation and liquid crystalline phase development in a liquid crystalline epoxy curing system.

Figure 11.21 2,2 -Bis(4-cyanatophenyl) isopropylidene (CE) and Liquid crystalline epoxy resin (LCE).

Sue et al. [350] studied the morphologies of various diglycidyl ethers of 4,4 -dihydroxy-a-methylstilbene-based liquid crystalline epoxy (LCE) formulations used as matrices for high performance composites by polishing, to 0.3jum finish, followed by etching with KMn04 in a... 

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