In organic matrix composites the properties of the matrix in the vicinity of the reinforcing fiber are of interest [1]. It has been suggested that a volume of material surrounding the fiber is significantly different from the bulk matrix [2–6]. Recent work has indicated that the interphase layer may be softer than the normal matrix material [6]. For a model composite with a single fiber embedded in an epoxy/amine matrix this layer was observed to be about 500nm thick and the material had an average elastic modulus of about 1/4 of that of the normal matrix material. The objective of the present work is to observe the effect of fiber treatment on the elastic properties of the interphase.

We have developed a range of “antiplasticizing” additives for epoxy and polyimide resins which can increase the modulus of the cured polymers by > 40%, increase their tensile and shear strengths, modify their fracture behavior, and decrease their water uptake. These additives may be used alone or in conjunction with rubber modifiers. The optimum additive formulations are complex mixtures, but we describe here two relatively pure model systems. The epoxy additive is the reaction product of 4-hydroxyacetanilide (HAA) and vinylcyclohexene dioxide (VCD), while the polyimide additive is the reaction product of epoxyphenoxypropane (EPP) and HAA. These additives are mixed with the epoxy resin and curing agent (or polyamic acid solution in the case of polyimides), before curing, and the mixture is then cured in the conventional fashion. We describe here some physical properties of the modified epoxies and polyimides and their use in adhesive formulations. These materials offer the potential for constructing modulus gradients at interfaces through their use as primers, so as to allow testing of the many hypotheses concerning desirable interphase properties. Some initial experiments in this area are described.

The thermomechanical stability of organosilane surface treatments for E-glass fibers used in fiber reinforced composites was evaluated. The effect of molecular structure of 40 to 80 namometer coatings on the force transmission across the fiber/matrix interface was measured as a function of temperature and exposure to water using a fiber fragmentation test. It was found that phenyl-substituted amino silanes exhibited better thermal stability, but were less resistant to boiling water, than the commierically available γ-amino propyl silanes. A bis-trimethoxy γ-amino propyl silane showed an increase in both the hydrolytic and thermal stability when compared to the commiercial product. A good balance of thermal and hydrolytic stability was also obtained with a methylaminopropyltrimethoxy silane coating.

The strain energy released from the glass fibers upon decoupling from the poxy matrix or silane coating was found to be in the range of 145 to 186 g/m2 and varied no more than 20 percent over a temperature range of 25 to 75°C or when exposed to boiling water and then redried. It also varied very little with the silane coating used. In addition, the average shear stress attained at the fiber-matrix interface in an imbedded single fiber test at 25°C was as much as two times higher than the shear strength of the epoxy matrix and as much as five times higher at elevated temperature. These data lead one to the conclusion that the interphase failure in these composites is controlled by a plane strain fracture in the constrained region of the organic phase, near the fiber surface, rather than by the maximum shear strength in the interphase.

The study of the type and strength of the filler-polymer linkages is of great importance in understanding the reinforcement of elastomers. Silicone rubbers are weak elastomers and the addition of reinforcing fillers is essential in order to obtain useful, strong materials. The best reinforcing filler for these elastomers are fumed silicas. These fillers, like reinforcing carbon blacks, have very complex structures. Both have fractal characteristics, small particles fused together forming open aggregates that can cluster by physical forces. Silicas have sometimes more complex structures than carbon blacks, but have a better understood surface chemistry. Interactions between polydimethylsiloxanes and silica surfaces have been studied using heat of adsorption measurements of mostly low molecular weight analogs or inferring the strength of the adsorption by the shift of particular peaks in the infrared spectrum [1]. Here we will present a new technique that measures directly the strength of the adsorption of the polymer segments onto glass and between themselves. It also allows for comparison of the strength of such bonds with the strength of a polymer entanglement “link”.

Surface modification of organic reinforcement fibers by exposure to certain plasmas appears to have considerable potential as a means for improving the performance of composites. Such treatments can change fiber surface properties, leaving the core of the fiber virtually unaffected so that the mechanical properties of the fibers remain unaltered. Previous studies [1–5] have shown that plasma treatment of polymeric fibers can modify surface energetics and that the acid/base characteristics of a fiber surface can be altered by exposure to plasmas of acidic or basic gases. Although several publications [6,7] have reported that the mechanical properties of composites reinforced with plasma-treated fibers are enhanced, there has been no direct evidence to show the impact of fiber surface plasma treatment on interfacial shear strength.

Composites made with high performance polymer fibers can achieve axial properties comparable to those which use inorganic reinforcements, but with somewhat inferior interfacial properties. A high degree of chain alignment in polyaramids, such as Kevlar, produces weak interactions between adjacent polymers, resulting in poor transverse strength in the fiber, and low interfacial shear strength in composite systems. The latter controls many composite properties, such as transverse, shear and flexural strengths. Also, by reducing the tendency to form weak interfacial boundary layers, good fiber-matrix adhesion can enhance environmental stability. For these reasons, modifications to reinforcement fiber surfaces that promote fiber-matrix adhesion are frequently used to improve the performance of composites.

The effects of plasma treatments on the physical and chemical characteristics of carbon fiber surface and on the mechanical properties of carbon fiber/bismaleimide (BMI) resin composites were investigated. Scanning tunneling microscopy (STM), scanning electron microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and wettability measurements were applied for fiber surface characterization. Oxygen plasmas were found to be effective in enhancing the interfacial adhesion between carbon fiber and BMI matrix. Possible mechanisms of interfacial adhesion promotion were suggested and discussed.

Surface energies of carbon fibres at different levels at surface treatment have been determined by a wetting force technique and related to fibre-matrix adhesion in carbon fibre reinforced PEEK composite. The effect of oxidative surface treatment on the surface free energy is detailed, along with the changes in surface oxygen and nitrogen content, as determined by X-ray photoelectron spectroscopy (XPS). The work of adhesion has been calculated for the carbon fibres and thermoplastic, which correlate well with experimental determination of interfacial strength. The technique can therefore be used to predict adhesion levels in fibre reinforced composites.

Five carbon fibers from different precursors were surface treated by various chemical reagents for the elucidation of the mechanism involved in interfacial adhesion. SEM, ESCA and SAM techniques were used to characterize morphology as well as surface constituents of the treated fibers. The transverse tensile strengths of the composites made from the treated fibers and epoxy resin were also obtained. The result indicated that the interlocking mechanism by surface roughness seemed to play a major role in the fiber-epoxy adhesion.

Macroscopic properties of fiber reinforced composites are dependent on the micromechanics of the filament-matrix interphase. We present here some preliminary results of our studies aimed at understanding the behavior of the interphase when the composite is subjected to cyclic tensile loading. We have used carbon/polycarbonate mono-filament composites as a model system for studying the effects of loading direction (axial and transverse), frequency, and amplitude. The interphase was varied by etching the filament surface. Damage was characterized by fiber matrix debonding, reduction in interphase stress transfer efficiency, and changes in the locus of failure as determined by SEM fractography.

The surfaces of commercial carbon fibers are generally chemically cleaned or oxidized and then coated with an oligomeric sizing to optimize their adhesion to epoxy matrix resins. Evidence from fractography, from embedded fiber testing and from fracture energies suggests that these standard treatments are relatively ineffective for thermoplastic matrices. This evidence is reviewed and model thermoplastic composites (polyphenylene oxide/high strain carbon fibers) are used to demonstrate how differences in adhesion can lead to a two-fold change in interlaminar fracture toughness.

The potential for improved adhesion via plasma modification of fiber surfaces is discussed. Finally, a surprising case of fiber-catalyzed resin degradation is described.