L. Mascia

Compatibility of polyimide-silicate ceramers induced by alkoxysilane silane coupling agents

Abstract A hybrid material incorporating silica networks in polyimide films was produced using the sol-gel technique from solution mixtures of polyamic acid and partially polymerised tetraethoxysilane (TEOS) containing small amounts of a functionalised alkyl trimethoxysilane. These were converted into films by solvent evaporation, followed by imidisation and condensation reactions through temperature increases to a maximum of 350°C. Depending on the alkoxysilane solution composition and mixing parameters, different morphologies were obtained, varying from semi-interpenetrating networks of linear polyimide within highly crosslinked silica chains, to finely dispersed heterogeneous systems exhibiting either co-continuous or particulate microstructure. Gelation studies revealed that in the presence of amine catalysts, the tetraethoxysilane solutions modified with γ-glycidyloxypropyltrimethoxysilane had the shortest gelation times, indicating that the epoxy groups become involved in the formation of the network by reactions with the carboxylic acid groups of the polyamic acid and through homopolymerisation. At the maximum temperature allowable for the imidisation reactions (400°C) small amounts of residual SiOH groups were detected within the silicate network. The tensile strength of ceramers was found to go through a maximum with increasing curing time at 400°C, whereas the elongation at break decreased monotonically and then remained constant.

Influence of siloxane composition and morphology on properties of polyimide-silica hybrids

Abstract Hybrid organic-inorganic materials based on polyimide-silica systems have been produced by the sol-gel route from solution mixtures of hydrolysed tetraethoxysilane (TEOS) and an aromatic polyamic acid. Compatibilization of the two components was achieved with the addition of small amounts of γ-glycidyloxy-propyltrimethoxysilane, and the evolution of the morphology was controlled by the partial substitution of TEOS with dimethylethoxysilane. In all cases imidization and network formation for the two respective components were carried out simultaneously on cast films after evaporating the solvent through successive temperature rises in steps up to a maximum of 300°C. The results illustrate the role of the interconnected silica-rich particles within the polyimide-rich matrix in depressing the α-relaxations and reducing accordingly the thermal expansion coefficient of the film or coating by an extent substantially greater than can be expected from the usual additivity rules. Forcing the reoccurrence of a particulate morphology through desolubilization of the siloxane component and simultaneously preserving the adhesion between the two phases gives rise to substantial improvements in both tensile strength and elongation at break. For systems exhibiting a co-continuous two-phase morphology the observed changes in properties can be partially attributed to residual orientation within the polyimide phase resulting from the internal constraints imposed on its shrinkage during removal of the solvent and the imidization reactions.

Oligomeric and polymeric modifiers for toughening of epoxy resins

Abstract The enhancement of properties of cured epoxy resins by the incorporation of modifiers into standard formulations is a subject that has received a great deal of attention over the last two decades. This paper presents an overview of the principles that have been explored to overcome the main deficiencies of epoxy resins, especially their brittleness, and reviews the scientific advances that have been made in this field. Particular attention is given to improvements in properties achieved by the addition of reactive oligomeric compounds, such as: (a) carboxyl and amino terminated butadiene-acrylonitrile, low molecular weight, statistical copolymers; (b) telechelic acrylate copolymers and terpolymers; (c) amine terminated polysiloxanes; and (d) chain extended carboxyl terminated perfluoro polyethers. The use of functionalised thermally stable high molecular weight polymers, both rubbery and glassy, is also examined and their efficiency in enhancing the toughness of difunctional and multifunctional epoxy resins is considered. The polymers considered in greater detail are based on aromatic polyethers and polysiloxane elastomers, respectively.

The transport of water in a tetrafunctional epoxy resin by near-infrared Fourier transform spectroscopy

Abstract An epoxy resin formulation composed of tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) and 4,4′diamino diphenyl sulfone (DDS) was investigated by Fourier transform near infrared (FT-NIR) spectroscopy and dynamic-mechanical analysis. Both techniques have demonstrated the essentially complete cure state of the resin with the adopted curing schedule. A novel feature of this work is the possibility of monitoring the transport of water into the resin by using of an FT-NIR spectrometer as a detector in place of the currently employed gravimetric detectors. The data gathered at different temperatures have revealed a good agreement with conventional gravimetric measurements, which verifies the reliability and accuracy of the experimental approach presented herein.

Thermal-oxidative degradation of epoxy and epoxy-bismaleimide networks: Kinetics and mechanism

The thermal oxidative stability of an epoxide/diamine network (TGDDM/DDS) and of an epoxide/diamine/bismaleimide IPN system (TGDDM/DDS/BMI) have been investigated by thermogravimetric analysis (TGA), dynamic-mechanical thermal analysis (DMTA) and isothermal, time-resolved FTIR spectroscopy. Both TGA and DMTA data revealed a considerable enhancement of the stability of the material as the bismaleimide content in the system is increased. The FTIR measurements allowed us to monitor in-situ the degradation process and to obtain reliable kinetic data. These, together with the analysis of the time evolution of the infrared spectra, permitted to propose likely mechanisms to account for the experimental observations. It was found that in the IPN system the epoxide network degrades with the same mechanism occurring in the pure epoxy resin, albeit at an increased rate. No evidence has been found of alternative pathways introduced in the system by the presence of the bismaleimide component. The bismaleimide network was also found to undergo significant degradation in the IPN system, but to a considerably lesser extent than the more oxygen sensitive groups of the TGDDM/DDS network. The higher stability of the bismaleimide component represents the underlying reason for the enhanced thermal-oxidative resistance of the ternary system with respect to the binary TGDDM/DDS resin.

Plasma-treatment of polyetheretherketone (PEEK) for adhesive bonding

Polyetheretherketone (PEEK) has been treated with oxygen-, air-, argon- and ammonia-plasmas, which greatly improve adhesion to an epoxide film adhesive. Treated surfaces can be stored under laboratory conditions for up to 90 days without significant loss of the improved adhesion properties. Contact angle measurements show that the surface energy of PEEK is much increased by plasma-treatment. X-ray photoelectron spectroscopy shows that the plasmas increase the amounts of oxygen and in some cases the amounts of nitrogen, and that new surface groups include -OH and -CO-. Wiping treated surfaces with acetone can reverse the effects of plasma-treatment.

Curing and morphology of epoxy resin-silica hybrids

Hybrids of epoxy resin and silica cured respectively with methyl nadic anhydride (MNA) and 4,4′-diaminodiphenyl sulfone (DDS) were prepared. Control of the morphology was achieved through functionalisation of a diglycidyl ether resin respectively with monofunctional and difunctional secondary amine trialkoxysilanes prior to being mixed with a solution of tetraethoxysilane (TEOS) and hardener. Scanning and transmission electron microscopy examinations (SEM and TEM) were carried out to study the morphology of the samples. The results have shown that the preparation conditions and nature of solvent play a vital role in the compatibilisation of the final hybrid. The addition of a hydrolysed TEOS solution into the epoxy resin to produce the corresponding hybrid was found to interfere with the cross-linking reactions with the hardener, inevitably resulting in a reduction in the T g of the epoxy resin component. This was attributed to side reactions of MNA with the ethanol released from hydrolysis and condensation of the TEOS, producing monofunctional and difunctional esters which act as plasticisers and to decreased functionality of the epoxy resin from the reaction with the HCl used for the hydrolysis of TEOS.

Carbon fibre composites based on polyimide/silica ceramers: aspects of structure-properties relationship

Abstract Polyimide/silica ceramers, based on the products of the hydrolysis of tetraethoxysilane (TEOS) and a commercial poly(amic acid) solution, were used to fabricate unidirectional carbon fibre composites, which were subsequently evaluated with respect to thermal and mechanical properties. There is evidence to suggest that the silica component of these ceramers is present as dispersed discrete particles at low silica concentration (i.e. ∼7 wt%) and as fine interconnected domains trapped within the polyimide matrix at higher silica content (i.e. ∼14 wt%). The dimensions of the silica domains were in the region of 7–20 nm. Carbon fibre composites produced from ceramer solutions (CF/ceramers) were found to exhibit lower thermal expansion and a greater retention of flexural and interlaminar shear properties at elevated temperature than the corresponding polyimide-matrix composites (CF/polyimide). The properties of CF/ceramers were generally better for systems containing the higher amount of silica and were improved further by lowering the pH value of the precursor ceramer solution. This is believed to have resulted from the enhanced fluidity of the ceramer gel within the pre-impregnated fibres, giving rise to a higher packing density of the fibres and a more homogeneous distribution of fibres. CF/ceramers were also found to exhibit a better thermal oxidative stability at 350°C than the corresponding CF/polyimide, although a substantial amount of porosity developed in the case of ceramers with the higher silica content.

Corona-discharge treatment of polyetheretherketone (PEEK) for adhesive bonding

Poly(etheretherketone) (PEEK) has been treated by corona-discharge in air with added oxygen, argon, ammonia or sulfur dioxide. The observed effect is to almost double the strength of lap joints and increase surface polarity. Treated surfaces have been stored for 90 days without significant loss of joint strength. The improvements brought about by corona-treatments are not removed by wiping the surface with solvents.

Polyimide–silica hybrids derived from an isoimide oligomer precursor

Organic–inorganic hybrids of a cross-linked polyimide and silica were prepared via the sol–gel route using an acetylene-terminated oligomeric isoimide and tetraethoxysilane (TEOS) precursors. Two coupling agents, γ-mercaptopropyltrimethoxysilane (MPTMS), and triethoxysilane were employed to compatibilise the organic and inorganic components of the precursor solution mixture. Homogeneous co-continuous phase morphologies could be produced with the use of MPTMS and their properties were evaluated by rheological and analytical techniques. Dynamic mechanical tests were carried out on unidirectional carbon fibre composites based on a polyimide–silica hybrid matrix. The results indicate that the thermooxidative stability of the hybrids at high temperatures is improved considerably over that of the base polyimide. However, the compatibilisation reactions of the coupling agent with the organic precursor, prior to the isomerisation–curing stages, causes a reduction in cross-linking density and a concomitant reduction in the glass transition temperature.