Marta Giamberini

Liquid crystalline epoxy based thermosetting polymers

Abstract In 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 substantial 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.

Curing kinetics of liquid-crystalline epoxy resins

Abstract By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.

Epoxy+liquid crystalline epoxy coreacted networks: I. Synthesis and curing kinetics

Abstract In situ copolymerization of diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) with diglycidyl ether of bisphenol F (DGEBP-F) networks using an anhydride curing agent has been investigated. DGEBP-F is a commercial epoxy while cured DGE-DHBP shows liquid crystal transitions. Curing kinetics are determined using differential scanning calorimetry (DSC). The data were fitted using an autocatalytic curing model for both pure and mixed components. Isothermal and non-isothermal methods were compared. The glass transition ( T g ) was evaluated as a function of composition using DSC. The results show that the DGE-DHBP constituent affects the curing kinetics of the epoxy resin and that the network exhibits one T g .

Rigid rod networks: Liquid crystalline epoxy resins

Abstract Liquid 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.

Epoxy + liquid crystalline epoxy coreacted networks: II. Mechanical properties

Abstract The effect of coreacting diglycidyl ether of bisphenol F (DGEBP-F) with diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) on mechanical properties is investigated. DGE-DHBP shows a liquid crystal (LC) transition upon curing. Tensile, impact and fracture toughness test results are evaluated. Dynamic mechanical analysis is conducted to determine the effect of the DGE-DHBP component. Scanning electron microscopy of fracture surfaces shows changes in failure mechanisms compared to the pure components. The results indicate that the mechanical properties of these blended samples are significantly improved at 10–20% by weight of DGE-DHBP. This is a consequence of the rigidity of the LC component which thus provides the reinforcement.

7 – Liquid Crystalline Epoxy Resins

Rigid rod glycidyl terminated monomers can be crosslinked in a liquid crystalline structure by reaction with diamines. The ordered phase forms in the initial stage of the curing reaction, during the growing of a prepolymer and is stabilized during the network formation. Temperature strongly affects the state of order of the cured thermoset. The isothermal curing of the liquid crystalline material is characterized by a double peak exotherm, whose second maximum has been related to the formation of the nematic phase. Physical characterization of the liquid crystalline epoxy resin indicates lower residual reactivity and superior fracture toughness.

ANISOTROPIC LIQUID CRYSTALLINE EPOXY THERMOSET

Abstract The processing of an anisotropic liquid crystalline thermoset formed by p-bis(2,3-epoxypropoxy)-α-methylstilbene (DOMS) and 1-methyl-2,4-diaminobenzene (DAT) with uniformly oriented mesomorphic molecules, and consequently strongly birefringent, is described. The unidirectional alignment of the molecules was achieved during the curing process by external factors such as surface-liquid crystal interactions and/or an applied electric field. The control over the molecular orientation enabled us to produce well oriented highly anisotropic and birefringent epoxy thin films. In this paper, we discuss the formation process and the properties of the anisotropic epoxy network.

Can liquid crystalline polymers find application in the field of protective coatings

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 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.

Curing Reaction Kinetics of Liquid Crystalline Resin Based on 6,6′-Bis (2,3-Epoxypropoxy)-2,2′ Binaphthyl

Abstract The curing kinetics of an epoxy stoichiometrically equivalent mixture of 6,6′-bis (2,3-epoxy-propoxy)-2,2′ binaphthyl (EPBN) and 4,4′-diaminodiphenyl sulfone (DDS) has been determined experimentally. A liquid crystalline phase forms during the curing reaction. A phenomenological model of the curing process has been developed, with parameter values extracted from the data by fitting of separate parts of the DSC curves. If the conventional reaction scheme proposed for the addition reaction of amines to epoxy groups holds true, then the overall reaction can be modeled, and the contribution of the phase transition occurring during the hardening reaction can be evaluated and modeled.

Epoxy-based liquid crystalline coatings

The use of epoxy resins fbr surface coatings accounts fbr about 50% of current output of epoxy resins, For these applications, mainly modified bisphenol A epichlorohy drin epoxies are used, Two principal types of modification are commercially used, namely combination with other resins and esterification, In the first case, bisphenol Aepichlorohydrin resins are blended with a variety of other resins containing reactive groups; on curing, linkages occur to give a crosslinked copolymer which exhibits certain characteristics of the two component resins, One example is the phenol-formaldehyde resins, which give a crosslinked network with the best chemical and heat resis tance of all epoxy coatings and are widely used for corro sion-resistant pipes and containers, In the case of esterification, the epoxy resins are reacted with fatty acids through their epoxy and hydroxyl groups, The esterification of epoxy resins may be carried out, either in the pres ence, or in the absence, of solvent, The finished products find application in the protection of equipment where less corrosive environments are encountered, or as films with good adhesion, flexibility and chemical resistance, Also they can be blended with melamine fbrrnaldehyde resins which on storing give films with superior hardness and chemical resistance,