Jim D. Earls

Fracture behaviour of liquid crystal epoxy resin systems based on the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene and sulphanilamide: Part I Effects of curing variations

The fracture behaviours of the pour-cast, unoriented diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene/sulphanilamide liquid crystalline epoxies (LCE) cured at various temperature steps are investigated. It is found that, depending on how the LCE is cured, the liquid crystalline (LC) domain size varies dramatically. These, in turn, affect how the LCEs fracture. The operative toughening mechanisms in the toughest LCE are studied in detail and found to include the formation of numerous segmented, unlinked microcracks in front of the main crack. When the crack opens up, the matrix material between the segmented microcracks acts as a bridge between the opening crack planes. Furthermore, crack bifurcation appears to take place when the segmented cracks are eventually linked with the main crack. This entire fracture process accounts for the high fracture toughness (GIC=580 J m-2) of this particular LCE with respect to conventional epoxies (GIC=180 J m-2). The relationship between the LCE morphology and the corresponding fracture mechanisms is discussed.

Effects of liquid crystalline structure formation on the curing kinetics of an epoxy resin

The curing kinetics of a system containing 4,4′-diglycidyloxy-α-methylstilbene (DOMS) and different functionality amines, N-ethylaniline (NEA), aniline, benzenesulfonamide (BSA), and sulfanilamide (SAA), have been studied by differential scanning calorimetry (DSC) under isothermal conditions. The phase transformations during curing of the systems have been monitored by a crosspolarized optical microscope equipped with a hot-stage and photo detector. It has been found that the growth of a nematic liquid crystal structure does not cause a discrepancy from the autocatalytic model for the reactions between aniline and epoxy. There is no liquid crystalline structure formed for the systems containing NEA or BSA, which follow the autocatalytic kinetic models within the temperature range of 120–150°C. For the curing reactions between DOMS and SAA, there is a big deviation from the autocatalytic model when the liquid crystals transfer from a nematic structure to a smectic structure. Unlike the usual decrease of reaction rate resulting from diffusion in a heterogeneous reaction, the reaction rate is enhanced. A modified kinetic model has been constructed for this reaction system by introducing a pseudoconcentration term caused from the liquid crystalline structure formation.

Fracture behaviour of liquid crystal epoxy resin systems based on diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene: Part II Effect due to blending with TACTIX 556 epoxy resin and phenolic monomers

The mechanical behaviours of unoriented, poured resin castings based on formulated blends containing the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene monomer are studied. It is found that the mechanical and fracture behaviours of these liquid crystalline epoxy (LCE) blends vary significantly. In general, the LCE blends possess much higher fracture toughness and fatigue crack resistance than conventional epoxy resins. At low temperatures (−40°C), the KIC values of the LCE blends are slightly higher than those measured at room temperature. The common fracture mechanisms observed in the ductile LCE blends are crack segmentation, crack branching, crack bridging and crack blunting. The fracture surfaces of the tougher LCE blends only exhibit limited ductile drawing (furrow pattern) at the slow crack growth region; no signs of shear lips on the edges of the starter crack region are observed. The optical microscopy and transmission electron microscopy work suggests that orientation and/or transformation toughening may be the source for such high fracture toughness of the LCE blends. The possible cause(s) of the unusual fracture behaviour of the LCEs is discussed. Approaches for making high performance LCE blends are also addressed.

The use of tetraalkylphosphonium-tetrafluoroborate-tetrafluoroboric acid in the curing of a liquid crystalline epoxy resin

Tetraalkylphosphonium–Tetrafluoroborate–Tetrafluoroboric Acid was used as a catalyst in the curing of a liquid crystalline epoxy. Under some conditions the Tetraalkylphosphonium–Tetrafluoroborate–Tetrafluoroboric Acid actually retarded the reaction. An extensive experimental and kinetic analysis is presented anda mechanism for the reaction retardation is proposed.

High modulus liquid crystalline thermosetting resins through novel processing techniques

The authors have demonstrated the effects of magnetic field alignment for the epoxy resin system, the diglycidyl ether of dihydroxy-{alpha}-methylstilbene cured with sulfanilamide. With increasing magnetic field, the tensile modulus of the thermoset can be enhanced by a factor of three, without compromise to the transverse value. Measurements of the coefficients of thermal expansion and the order parameter as determined by x-ray diffraction elucidate the high degree of alignment obtained at field strengths of 15--18 Tesla. The interrelationship between the reaction kinetics of curing with the molecular alignment process is central to successful processing of this class of thermosets. Preliminary relationships between the field strength, time in the magnetic field, reaction rate, and extent of reaction have been developed. Process control and sequencing allow a wide adjustment in the properties of the produced samples. These studies demonstrate the increased value of multi-mode processing in the development of tailored property, lightweight materials.