Ian Hamerton

Recent technological developments in cyanate ester resins

Although they are relative newcomers to the composites industry, cyanate ester (CE) resins are enjoying unprecedented success in certain applications. Several major space and radome manufacturers have qualified CE resins despite the extensive database on epoxies and the inherently conservative nature of the industry. Increasing demands on the materials used in these areas have stimulated the use of CE resins over other more conventional polymers. The aim of this review is to bring to the reader’s attention the more recent developments in the processing, toughening, properties and applications of cyanate esters, an exciting, versatile and important class of resins.

Preparation and characterization of imidazole–metal complexes and evaluation of cured epoxy networks

A series of copper complexes of epoxy-imidazole adducts have been prepared and characterized by 1H nuclear magnetic resonance (NMR) spectroscopy. Differential scanning calorimetry (DSC) was employed to investigate the thermal behaviour of the curing agents and to investigate the medium-term storage stability of a one-pot composition of a commercial epoxy resin when mixed with the complexes. The cure onset temperatures of the mixtures containing copper complexes are ca. 20–50 °C higher than those of the parent epoxy-imidazole adducts and the decrease of cure onset temperatures in the early stages of storage (up to 100 h) is less. The latent nature and improved storage stability of mixtures containing the copper complex were clearly demonstrated and confirmed by the viscosity behaviour of the catalysed mixtures of the commercial epoxy resins MY720 and MY750. 1H NMR and electron paramagnetic resonance (EPR) spectroscopy were employed to monitor the thermal decomposition of the copper(II) complexes, which were found to decompose at 120–130 °C and exist in equilibrium. Glass fibre-reinforced composite samples were prepared using a commercial epoxy resin cured with the complexes and their physico-mechanical properties were evaluated.

Solving the Problem of Building Models of Crosslinked Polymers: An Example Focussing on Validation of the Properties of Crosslinked Epoxy Resins

The construction of molecular models of crosslinked polymers is an area of some difficulty and considerable interest. We report here a new method of constructing these models and validate the method by modelling three epoxy systems based on the epoxy monomers bisphenol F diglycidyl ether (BFDGE) and triglycidyl-p-amino phenol (TGAP) with the curing agent diamino diphenyl sulphone (DDS). The main emphasis of the work concerns the improvement of the techniques for the molecular simulation of these epoxies and specific attention is paid towards model construction techniques, including automated model building and prediction of glass transition temperatures (Tg). Typical models comprise some 4200–4600 atoms (ca. 120–130 monomers). In a parallel empirical study, these systems have been cast, cured and analysed by dynamic mechanical thermal analysis (DMTA) to measure Tg. Results for the three epoxy systems yield good agreement with experimental Tg ranges of 200–220°C, 270–285°C and 285–290°C with corresponding simulated ranges of 210–230°C, 250–300°C, and 250–300°C respectively.

Molecular modelling of the physical and mechanical properties of two polycyanurate network polymers

Elastic moduli and glass transition temperatures (Tgs) of two polycyanurates, based on the dicyanates of bisphenol A and an oligomeric poly(arylene ether sulfone), have been predicted from molecular simulation. The simulated mechanical and physical parameters offer reasonable agreement with the experimental values. This is one of the first preliminary reports of the prediction of properties of a network polymer.

Comparative kinetic analyses for epoxy resins cured with imidazole–metal complexes

Proton nuclear magnetic resonance (1H NMR) spectroscopy experiments were carried out for a commercial epoxy resin system in [2H6]DMSO solution and in bulk. Good agreement was obtained between the secondary rate constants found for the epoxide polyetherification reaction when monitored by Fourier-transform infrared (FTIR) or 1H NMR spectroscopy in the bulk over the range of temperatures studied. This highlights the usefulness of the high temperature bulk NMR technique in kinetic studies of polymerization.

Effect of complexation with copper (II) on cured neat resin properties of a commercial epoxy resin using modified imidazole curing agents

A commercial epoxy prepolymer (MY750) was cured with novel modified imidazole curing agents under both isothermal and dynamic scanning conditions. The incorporation of an imidazole–copper(II) chloride complex curing agent to the epoxy prepolymer effected full cure after an isothermal cure schedule and post-cure treatment. The thermal stability of polymers arising from the isothermal cure schedule were generally higher than those for the corresponding dynamic cure. For samples cured via the dynamic curing process, a lower heating rate resulted in superior thermal stability. The same findings were obtained for samples cured via the isothermal curing process where the lower initial cure temperatures were optimal. The results of the present study show that the addition of metal atoms to the polymer systems does not have an adverse effect on either the water absorption or the dielectric properties of the final product. The cured resin displayed comparable thermal stability and absorbed the same amount of water at saturation (and a marginally lower amount at lower curing agent loadings) to a similar sample cured with an unmodified imidazole adduct.

Molecular modelling of interactions at the composite interfaces between electrolytically surface-treated carbon fibre and epoxy resin

Two carbon fibre models, based on microscopic and XPS evidence of electrochemical surface treatment, have been proposed. A diagonal graphitic plane, comprising 52 six-membered rings (in a 4×13 configuration) and of 150 carbon atoms, was built as the principal, non-surface-treated carbon fibre model. Three layers of graphitic planes, each comprising 117 six-membered rings (in a 9×13 configuration) and of 300 carbon atoms, formed the multi-layer graphitic model. The nature and level of surface treatment was represented by the introduction of hydroxy (OH) and carboxy (COOH) groups: each time, a C–C bond was broken along the edge of the plane, and a pair of OH and COOH groups was added to the graphitic plane. Six pairs of OH and COOH were introduced gradually. The functional groups were distributed evenly along the edge of each graphitic plane. Their non-covalent bonding interaction with various amine-cured epoxy polymer models was simulated using the Cerius 2 BLENDS method. Δ mix G was used to indicate the interaction, and hence the interfacial adhesion. The results show a trend, in relation to the level of surface treatment, in agreement with experimental data of composite interlaminar shear strength (ILSS).

Molecular modelling of interactions at the composite interface between surface-treated carbon fibre and polymer matrices: the influence of surface functional groups

To simulate the results of surface treatments, commonly encountered functional groups were introduced onto the surface of the carbon fibre model. The carbon fibre model used in this study is based on the single layer diagonal graphitic plane, comprising 52 six-membered rings, in a 4×13 configuration, and of 150 carbon atoms. Surface treatment was represented by the introduction of functional groups (–R): each time, a C–C bond was broken along the edge of the plane, and a pair of –R groups was added to the graphitic plane. The total number of functional groups (n) was six. The effect of these functional groups on the non-covalent bonding interactions at the composite interface between carbon fibre and epoxy resin was investigated using a previously established BLENDS method. The compatibility of the resin and fibre in this model, indicated by the interaction parameter χ(T ), was dependent upon two factors: steric bulk and electrostatic interactions. The halogen substituents show a decrease in χ(T ), as one descends the group. Maximum interaction tends to be a function of steric bulk and polarizability in this group. The alkyl (CnH2n+1) and phenyl substituents also show a decreasing trend inχ(T ) with increasing size, although the interaction parameter with methyl is anomalously low in all cases.

Blocked Shape Memory Effect in Negative Poisson's Ratio Polymer Metamaterials

We describe a new class of negative Poissons ratio (NPR) open cell PU-PE foams produced by blocking the shape memory effect in the polymer. Contrary to classical NPR open cell thermoset and thermoplastic foams that return to their auxetic phase after reheating (and therefore limit their use in technological applications), this new class of cellular solids has a permanent negative Poissons ratio behavior, generated through multiple shape memory (mSM) treatments that lead to a fixity of the topology of the cell foam. The mSM-NPR foams have Poissons ratio values similar to the auxetic foams prior their return to the conventional phase, but compressive stress-strain curves similar to the ones of conventional foams. The results show that by manipulating the shape memory effect in polymer microstructures it is possible to obtain new classes of materials with unusual deformation mechanisms.

Kinetic and simulation studies of linear epoxy systems

The kinetics and mechanism of the reaction between Bisphenol-A phenylglycidylether (BADGE) and 1,2-dianilinoethane (DAE) has been studied by a radiochemical method using tritium as the label and also by gel permeation chromatography (GPC). In parallel with these studies molecular simulation has been used to build models of the linear polymers formed, and their physical and mechanical properties have been calculated. Cast resin samples of the polymer have been produced for experimental determination of the physical and mechanical properties and the results of these determinations have been compared with the calculated values. These were found to be in reasonable agreement.