Liquid Crystalline Thermoset Epoxy Resins

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

The name liquid crystals suggests that it is a state of a matter in between the liquid and crystal that mixes the properties of both liquid and sohd states and intermediate between the two in many of its property. In other words, liquid crystals are partially ordered, anisotropic fluids, thermodynamically located between the three 

Vikas Mittal (ed.) High Performance Polymers and Engineering Plastics, (387-422) 

To quantify just how much order is present in a material, an order parameter (S) is defined. Traditionally, the order parameter is given as follows  

9 is the angle between the director and the long axis of each molecule. In an isotropic liquid, the average of the cosine terms is zero, and therefore the order parameter is equal to zero. For a perfect crystal, the order parameter evaluates to one. Typical values for order parameter of a liquid crystal range between 0.3 and 0.9, with exact value a function of temperature, as a result of kinetic molecular motion [1-10]. 

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Balamumgan R, Kannan P (2009) Synthesis and properties of a liquid crystalline thermoset epoxy resin containing 1,3,4-oxadiazole groups. High Perform Polym 21 251-264... 

Castell P, Sena A, Galia M (2003a) New liquid-crystal line thermosets from liquid-crystalline bisazomethynic epoxy resins with naphthylene disrupters in the central core. J Polym Sci Polym Chem 41 1536-1544... 

The claimed compound of the invention is produced by the reaction of epichloro-hydrin and bisphenol A and was well known in the art and was also known to be useful in the preparation of thermosetting epoxy resins. The claim preamble As a manufacture..., is equivalent to saying As a product of manufacture..., which in turn means that these two claims are compound claims. Although the compound was well known in the literature, it was reported in the context where it made up to 70% to 90% of a complex liquid mixture. According to the applicant s patent application specification, no method had been described that would allow the production of pure 2,2-bis-[(4)-2,3-epoxypropoxyphenyl]propane directly from a reaction mixture. Prior attempts to recover the product of the reaction resulted in, at best, a viscous liquid that adversely affected the usefulness of the product. In contrast to the prior art, applicant s methods resulted in material of sufficient purity that the compound was isolated as a free-flowing, crystalline powder. This material could be easily handled and allegedly could be used to prepare epoxy resins equal or superior to the liquid material prepared previously. 

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 Based on Epoxy Resins... 

Balamumgan R, Kannan P (2010) 1,3,4-Oxadiazole epoxy resin-based liquid crystalline thermosets and their cure kinetics. J Mater Sci 45 1321-1327... 

Castell P, Sena A, Galia M (2004) Liquid-crystalline thermosets from liquid-crystalline epoxy resins containing bisazomethinebiphenylene the central core copolymerization with a nonmesomorphic epoxy resin. J Polym Sci Polym Chem 42 3631-3643 Chtm S, Hsu SH, Wu MC, Su WF (2011) Kinetics studies on the accelerated curing of liquid crystalline epoxy resin/multiwalled carbon nanotube nanocomposites. J Polym Sci Polym Phys 49 301-309... 

In practice, it is necessary to avoid using amorphous resins in structural applications above, or even near to, the T. (This restriction applies to all thermosets, because they are invariably amorphous, but it does not apply to crystalline thermoplastics, otherwise polypropylene chairs, with a of around -15°C, could only be used safely in Alaska and in Siberian winters). A margin of safety of 20°C is required for reasonable durability, and a much larger one in humid conditions. Traces of water, or any other solvent, lower the Tg to below its usual value, so the maximum safe working temperature of a load-bearing amorphous resin is lower in humid conditions than in a dry environment. There is a widely quoted rule of thumb that each 1% moisture absorbed by the matrix lowers the Tg by 20°C. This comes from epoxy resin data in wet atmospheres and different figures apply to other resins and liquids. 

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... 

Harada M, Ando J, Hattori S, Sakurai S, Sakamoto N, Yamasaki T, Masunaga H, Ochi M (2013) In-situ analysis of the structural formation process of liquid-crystalline epoxy thermosets by simultaneous SAXSAVAXS measurements using synchrotron radiation. Polym J 45 43-49 Hirano Y, Akiba M, Yokota A, Nakamura H, Naitoh S (1999) Epoxy resin, resin composition, and resin-encapsulated semiconductor device. U.S. Patent 5,939,473 Hsu SH, Chen RS, Chang YL, Chen MH, Cheng KC, Su WF (2012a) Biphenyl liquid crystalline epoxy resin as a low-shrinkage resin-based dental restorative nanocomposite. Acta Biomater 8 4151 161... 

Mormann W, Broche M, Schwarz P (1997) Mesogenic azomethine based diepoxides— monomers for the synthesis of liquid crystal thermoset networks. Macromol Chem Phys 198 3615-3626 Mossety-Leszczak B, Wlodarska M (2011) Liquid crystallinity in polymers—liquid crystalline epoxy resins. In Iwan A, Schab-Balcerzak E (eds) Liquid crystalline organic compounds and polymers as materials of the XXI century from synthesis to applications. Transworld Research Network, Kerala... 

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