1,721,004 research outputs found
Liquid Crystal Epoxy Resins
No full textLiquid Crystalline Epoxy Resins (LCER) have been extensively studied in recent years, due to their challenging structures and properties. This class of materials is able to develop an ordered structure upon curing, which is then stabilized by crosslinks. From this structure, remarkable properties, such as higher fracture toughness, optical behaviour and adhesion result
Hybrid organic-inorganic UV-cured films containing liquid-crystalline units
No full textHybrid organic-inorganic films have been prepared through a dual-cure process, involving photopolymerization (UV-curing) of a methacrylic resin and hydrolysis/condensation of alkoxysilane groups, in the presence of three different liquid crystalline oligomers bearing acrylic reactive groups and synthesized on purpose. These UV-curable mixtures have been coated on glass substrates, exposed to UV radiation in inert (nitrogen) atmosphere, then thermally treated in order to promote the occurrence of the sol-gel reaction and finally peeled off from the substrate. The morphology of the obtained films has been investigated by means of scanning electron microscopy, environmental scanning electron microscopy and atomic force microscopy measurements. Significant silica enrichment on the "gas side" of the films (i.e. the side exposed to the UV radiation) has been found, as well as the occurrence of the formation of interesting silica conical shapes on the same side with respect to the glass counterpart. Because of the constraints exerted by the inorganic domains on the mobility of the polymer chains, the Tg values of the obtained networks have been found to increase in the presence of the silica phases, which also might have contributed to the occurrence of segregation phenomena of the liquid crystalline phase
Epoxy-based liquid crystalline coatings
No full textLiquid crystalline epoxy resins display properties which make them suitable for coating application. In fact, they exhibit superior fracture resistance, low coefficient of thermal expansion, good bonier properties and good heat and solvent stability. Moreover, the onset during the curing reaction of the liquid crystalline phase, creates on increase in the reaction rate which could lower the reaction times or temperature, whilst increasing the conversion obtained on curing. In this paper, an account is presented of the properties exhibited by these materials
Liquid crystalline epoxy based thermosetting polymers
No full textIn recent years many examples of liquid crystalline thermosetting polymers have been reported in the literature. The main reason for the development of this new class of materials comes from both technological and theoretical implications. Among the different anisotropic thermosets, epoxy resins represent a family of polymers whose properties make them primary candidates in different fields ranging from high performance matrices in advanced composites to polymers for optical applications. The chemistry of curing process of amine hardened conventional epoxy resins is well known from the literature; however some substancial differences arise during liquid crystalline epoxy monomers crosslinking. The level of order of the cured resin can be strongly affected by the nature of the hardener, as well as the physical properties of the cured material. This review will present the results of synthesis and physico-chemical characterization of liquid crystalline epoxy resins in relation to their applications in some specific fields
Rigid rod networks: Liquid crystalline epoxy resins
No full textLiquid crystalline polymers are well known for their unique mechanical and rheological properties. In recent years, some interest has been devoted to the study of a new class of liquid crystalline thermoset based on epoxy resins. Liquid crystalline epoxy resins can be obtained either by curing glycidyl terminated prepolymers over a range of temperature in which the mesophase is stable, or by reacting epoxy functionalized rigid monomers with a suitable curing agent. In our work this second approach has been followed. An unusual behaviour has been found for the form of the exotherm during the isothermal curing. Fracture toughness, Kq, was found to decrease with increased curing temperature. This experimental evidence has been correlated with the reduction of the extent of liquid crystalline character with temperature
Role of Curing Agent on the Nature of the Mesophase and the Properties of Mesogenic Epoxy Resins
No full textReaction of poly-functional epoxy with suitable curing agent is a well established practice to produce thermosetting materials with chemical and physical properties strongly depending upon the chemical nature of the reacting molecules as well as on the processing conditions. Among the curing agent most extensively used and investigated are the primary and secondary amines, both aromatic and aliphatic, carboxylic acids and anhydrides. The suitability of epoxy group to react via ring opening mechanism catalyzed by a wide variety of experimental conditions is the primary reason for the wide applications based on epoxy resins. On the other hand the draw back of the ready reactivity is the difficulty to control the crosslinking density and other structural parameters affecting the properties of the materials. 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
Can liquid crystalline polymers find application in the field of protective coatings?
No full textGlycidyl terminated rigid-rod monomers can be cured in a liquid crystalline state. The resulting thermoset offers high potential as protective coating thanks to its outstanding properties. In particular a superior fracture toughness and a reduced internal stress are two typical parameters offered by this new class of compounds. Transport properties are not strongly affected by the state of order of the cross-linked resin, in the sense that gas permeabilities are of the same order of magnitude of conventional epoxy resins. Glycidyl terminated rigid-rod monomers can be cured in a liquid crystalline state. The resulting thermoset offers high potential as protective coating thanks to its outstanding properties. In particular a superior fracture toughness and a reduced internal stress are two typical parameters offered by this new class of compound. Transport properties are not strongly affected by the state of order of the cross-linked resin, in the sense that gas permeabilities are of the same order of magnitude of conventional epoxy resins
Liquid-crystalline epoxy resins: curing of blends of mesogenic and non-mesogenic epoxy monomers
No full textNovel approach to the tailoring of polymers for advanced composites and optical applications, involving the synthesis of liquid crystalline epoxy resins
No full textThe authors describe the features of liquid crystalline epoxy resins having both high and low crosslinking densities. These materials seem to be very promising as matrices in advanced composites and in optical and electro-optical applications because of their peculiar properties. The importance of factors like molecular rigidity, nature of the curing agent, and curing temperature for the development of such properties are analyzed. The authors showed that amines produce thermosets having high T gs and high fracture toughness. In particular, the enhancement of this last parameter is very promising if a new generation of matrices is to be sought for manufacturing composites. In the case of acids as curing agents, thermosets having lower T gs and more ordered mesophases can be obtained. In this case, LC elastomers can be prepared that can find applications in the optical industry as waveguides or electro-optical devices
Liquid crystalline epoxy thermosets
No full textRigid rod epoxy compounds can be cured in liquid crystalline structure. The so obtained networks exhibit better mechanical properties with respect to the isotropic ones. The mesogenic character of the epoxy compounds appears more crucial than the molecular geometry of the curing agent in developing liquid crystallinity. The curing temperature plays an important role in affecting the state of order of the thermosets
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