E. Amendola

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.

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.

Composites based on carbon fibers and liquid crystalline epoxy resins, 1 Monomer synthesis and matrix curing

The synthesis and physical characterization of a novel liquid crystalline epoxy resin, used as a matrix for carbon fiber-reinforced composites, is presented in this paper. The curing reaction was monitored by means of calorimetric and rheological measurements. Calorimetric analysis indicates that the presence of carbon fibers does not affect the reaction rate. A conventional isotropic epoxy resin in used as a model compound in the rheological analysis. According to the patent literature, two different formulations of the model compound were used, characterized by a stoichiometric ratio of epoxy and an epoxy excess, respectively, with respect to the curing agent.

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.

The elastic modulus of poly(ethylene terephthalate-co-p-oxybenzoate)

The experimental approach to improve the mechanical properties of polymers proceeds essentially along two different lines: the modification of the processing of conventional polymers in order to obtain extended chain structures with better orientation, and the design of stiff, rod-like molecules exhibiting nematic liquid crystalline behaviour in the melt, which can be easily oriented in the solid state. High orientation and excellent mechanical properties can be achieved by solid-state or solution processing of flexible polymers which do not show mesophases [1-3]. More recently, thermotropic liquid crystalline polymers have attracted attention as a consequence of their good mechanical properties and easy processability [4, 5]. In fact the orientation of the liquid crystalline domains, which leads to a superior degree of orientation of the macromolecules along the fibre axis has been related to the possibility of producing materials with excellent levels of mechanical anisotropy. Different types of thermotropic nematogenics have been investigated [6]. Among these, the class of polymers which has received the highest attention is the copolyester, produced by Tennessee Eastman Company, initially synthesized by Jackson and Kuhfuss in 1976 [7]. The polymer is a polyethylene terephthalate modified by p-hydroxybenzoic acid (PET/PHB 60). Several papers have been published on the rheological [4] and on structural characterization [8, 9] of this new material. Most effort has been concentrated on the evaluation of the mechanical properties with the purpose to relate them to the processing of the polymer [4, 5]. Acierno et al. [4] found that the higher elastic moduli were reached when the fibres were spun at a lower temperature. They inferred that an important contribution to the high modulus would arise from crystallites of the hydroxybenzoic acid sequences. For the material extruded at a higher temperature (260 ° C) they measured a relatively high tensile modulus (about 20 GPa) which increases slightly with the draw ratio. More recently Tealdi et al. [10] reported higher elastic moduli for the polymer processed at higher temperatures than those found by Acierno et al. However, no indication of the exact value of the orientation of the macromolecules is provided in either paper. Sugiyama et al. [11] calculated the function of orientation of Hermans ( f ) using wide-angle X-ray patterns for fibres processed in different conditions of temperature and drawing. They found thatfdecreases with temperature and shear rate, but is practically independent of the spinline drawdown ratio. They inferred that almost all the orientation appears to be developed in the capillary. Muramatsu and Krigbaum [5] related the orientation, measured by X-ray, with the rheological behaviour and mechanical properties of the fibres. However, they do not provide any form of the function of orientation. In this letter we refer to the mechanical properties of fibres of PET/PHB 60 spun at 260 °C. The high modulus found is related to the high level of orientation, obtained from the large values of the order parameters, calculated from the X-ray diffraction patterns. Spinning of the nematic phase was performed using a Ceast rheometer. The piston-type extruder was operated at constant velocity, with a die having a 1 mm diameter. Fibres were spun at a single temperature, 260 ° C, using a capillary with LID = 10 and varying the span draw ratio. The extrusion rate, V0, was 45 cm min and the output flow rate, Q, was 0.35cm3min ~. About 5g polymer was used for each extrusion. In order to avoid moisture, the batches were previously dried overnight in a vacuum oven at 100 ° C. The spun fibres were collected with a take-up machine placed at a distance 0.3 m from the bottom of the capillary. The Vf/Vo ratios take-up velocity, Vr, was varied to obtain between 10 and 300. The true Vr/V0 ratios were evaluated as cross-sectional area variation and determined by measuring the decrease in the diameter of the fibres, using a microscope. Wide-angle X-ray diffraction patterns were recorded with a Siemens diffractometer using a flat film camera with CuKe radiation. Because of the small dimensions of the fibres, the exposure time ranged between 8 and 24h. Samples were fractured in liquid nitrogen and the fracture surface examined using a Cambridge Stereoscan 100 scanning electron microscope. The real component of the complex modulus, E, was determined using a Dynamic-mechanical spectrometer Dynastat system. One of the most important features of the Dynastat is the rise time control circuit that allows step-functions in load or displacement to be imposed on the sample without overshoot, but with rise time of an underdumped servo. Dynamic mechanical experiments were performed at room temperature and at a frequency of 1 Hz.

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,

“In Situ” composites formed with Liquid Crystalline Polymers and Thermoplastic Matrices

The modification of polymeric materials performances are commonly achieved in the industrial practice by the addition of fillers as reinforcing agents. The effect of reinforcing fibers and fillers on the mechanical properties of polymers are, in fact, well documented. (1,2)