Nanoscale characterisation of thickness and properties of interphase in polymer matrix composites

An overview is presented of the properties and effective thickness of the interphase formed between fibres and polymer matrices. Chemical and physical characterization of the interphase is discussed to portray molecular interactions comprising the interphase layers in silane-treated glass-fibre composites. The gap between physico-chemical investigation on one side and bulk material testing on the other side is bridged by implementation of novel techniques, such as nanoindentation, nanoscratch tests, and atomic force microscopy (AFM), which have been successfully used for nanoscopic characterization of the interphase in the past few years. Salient differences are identified between the major findings of these studies in terms of hardness/modulus of the interphase relative to the bulk matrix material. While there is a significant "fibre stiffening" effect that may cause misinterpretation of the interphase hardness very close to the fibre, the formation of both a softer and a harder interphase is possible, depending on the combination of reinforcement, matrix, and coupling agent applied. This is explained by different interdiffusion behaviour, chemical reactions, and molecular conformation taking place at the interphase region in different composite systems. The effective interphase thickness is found to vary from as small as a few hundred nanometers to as large as 10 µm, depending on the constituents, coupling agent, and ageing conditions.     

Electrodeposition of a polymer interphase in carbon-fiber composites

The performance of an electrodeposited interphase of poly(butadiene-co-maleic anhydride) (BMA) in carbon-fiber composites is investigated. Carbon fibers are electrocoated with BMA from an aqueous solution and the coated fibers are fabricated into composite bars for evaluation of mechanical properties. These composites show superior impact strength, but lower interlaminar shear strength, compared to composites made from commercially treated fibers. It is suggested that unsaturation in the butadiene segments of the interphase polymer leads to the formation of a crosslinked layer during electrodeposition and subsequent drying. Inadequate penetration of this interphase by bulky epoxy molecules leads to a weak interphase/matrix interface which is the locus of failure, generating the observed mechanical properties. These conclusions are supported by examination of the fracture surfaces by Scanning Electron Microscopy. Further evidence of lack of matrix penetration into the interphase comes from electron microprobe line scans for bromine performed on cross-sections of single-filament composites, the bromine being introduced into the matrix via a brominated epoxy resin. Appropriate control of the chemical structure and physical characteristics of the interphase polymer is thus indicated, for acieving simultaneous improvements in impact and interlaminar shear strengths.     

The effects of transcrystalline interphase in advanced polymer composites

Surface-induced transcrystallization in fibers has been reported in some advanced polymer composites. It is believed that transcrystalline interphase may affect stress transfer efficiency between the reinforcing fiber and the matrix. In this study, attempts were made to examine the effects of transcrystallinity on composite performance, particularly on fiber-matrix interfacial bond strength, and to investigate possible attributes of transcrystallization. Three polymer resins, poly(etherketoneketone) (PEKK), poly(etheretherketone) (PEEK), and poly(phenylenesulfide) (PPS), and four types of fiber, polyacrylonitrile (PAN)-based AU-4 (untreated AS-4) carbon, pitch-based carbon, poly (p-phenylene terephthalamide) (PPDT) aramid, and E-glass were used. It was found that PPDT aramid and pitch-based carbon fibers induce a transcrystalline interphase in all three polymers because of an epitaxial effect. Under certain conditions, transcrystallization was also observed in PAN-based carbon and E-glass fibers, which may be partially attributed to the thermal conductivity mismatch between the fiber and the matrix. Plasma treatment on fiber surface showed a negligible effect on inducing transcrystallization, whereas solution-coating of PPDT on the fiber surface showed a positive effect. The Microdebonding test, which measures the interfacial bond strength between the fiber and the matrix, consistently showed more than 40% increments for various single filament systems with transcrystalline interphase versus without. However, the effects of transcrystallinity on the interfacial bond strength appeared to decrease as the fiber content increased in composites.     

Strain rate influence on nonlinear response of polymer matrix composites

In this article, influence of strain rate on nonlinear response of unidirectional fiber‐reinforced composites is studied. The fibers are assumed to be periodic arrays in composite structures. By studying a representative volume element, a microscopic constitutive model for characterizing macro‐mechanical response of polymer matrix composites is developed. Viscoplastic material parameters of polymer matrix are acquired by axial tension and pure shear experiment, and the proposed method is validated by experimental data. The results showed that mechanical behavior of composites, which is affected by strain rate, can be ignored in the linear stage of loading. Furthermore, with the increase in strain rate, stiffness behavior of composites tends to be stiffer at the stage of nonlinear deformation. POLYM. COMPOS., 36:800–810, 2015. © 2014 Society of Plastics Engineers     

A kinetics model for interphase formation in thermosetting matrix composites

During the cure of thermosetting polymer composites, the presence of reinforcing fibers significantly alters the resin composition in the vicinity of the fiber surface via several microscale processes, forming an interphase region with different chemical and physical properties from the bulk resin. The interphase composition is an important parameter that determines the micromechanical properties of the composite. Interphase development during processing is a result of the mass‐transport processes of adsorption, desorption, and diffusion near the fiber surface, which are accompanied by simultaneous cure reactions between the resin components. Due to complexities of the molecular‐level mechanisms near the fiber surface, few studies have been carried out on the prediction of the interphase evolution as function of the process parameters. To address this void, a kinetics model was developed in this study to describe the coupled mass‐transfer and reaction processes leading to interphase formation. The parameters of the model were determined for an aluminum fiber/diglycidyl ether of bisphenol‐A/bis(p‐aminocyclohexyl)methane resin system from available experimental data in the literature. Parametric studies are presented to show the effects of different governing mechanisms on the formation of the interphase region for a general fiber–resin system. The interphase structure obtained from the model may be used as input data for the prediction of the overall composite properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3220–3236, 2003     

Stiffness and strength of flax fiber/polymer matrix composites

Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule‐of‐mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered. POLYM. COMPOS. 27:221–229, 2006. © 2006 Society of Plastics Engineers     

Effects of short fiber tip geometry and inhomogeneous interphase on the stress distribution of rubber matrix sealing composites

The normal and interfacial shear stress distributions with flat fiber tip of short‐fiber‐reinforced rubber matrix sealing composites (SFRC) compared with the shear lag model were investigated by using the finite element method (FEM). The results indicate that stress values do not agree with those calculated by the shear lag model. The effect of different geometrical shapes of fiber tip on the stress distributions of SFRC was also investigated. The geometrical shapes of fiber tip under present investigation are flat, semi‐elliptical, hemispherical, and circular cone, respectively. The results show that the hemispherical fiber tip transfers the load with less stress concentration and is contributed to controlling the interface debonding failure more effectively than other shapes of fiber tip. Further study on the effect of the inhomogeneous interphase properties on the normal and interfacial shear stresses of hemispherical fiber tip was also conducted. The results indicate that the normal stress increases with the increase of the interphase thickness and interfacial shear stress remains unchanged, and the normal stress values of SFRC with interphase are higher than those without interphase. The interphase elastic modulus has no influence on the stress distributions along the direction to the fiber axis. The stress distributions along the radial direction in the interphase end are largely dependent on the interphase elastic modulus, and the interfacial shear stress is larger than the normal stress, which reveals that a significant part of the external load is transferred from the fiber to the matrix through shear stresses within the interphase. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41638.     

Characterization of the interphase in carbon fiber/polymer composites using a nanoscale dynamic mechanical imaging technique

The interphase of fiber reinforced polymer composites is a narrow region around the fiber, and the mechanical performance of a composite strongly depends on the properties of the interphase. The interphase of carbon fiber reinforced polymer composites (CFRPs) is difficult to quantitatively characterize because of its nanometer dimension. To solve this problem, we present a nanomechanical imaging technique for mapping the dynamic mechanical property around the interphase region in CFRPs, and for providing nanoscale information of the interfacial dimension. The experimental results show that this method can determine the width and topography of the interphase with nanoscale lateral resolution, based on the storage modulus profile on the cross section of the composite. The average interphase thicknesses of a T300 carbon fiber/epoxy resin composite and a T700 carbon fiber/bismaleimide resin composite are 118 nm and 163 nm, respectively, and the size of interphase is uneven in width and “river-like”, which is consistent with the surface topography of the carbon fibers. Furthermore, the effect of water-aging on the interphase of the T300/epoxy composite was analyzed using the in situ imaging technique. An increase in the interphase width and interface debonding were revealed, implying a degradation in the interphase region.     

Ballistic response of polymer composites

A new microvelocity sensor unit was developed to measure the instantaneous velocity of a projectile during the penetration process. The concept of this device is based on the induced current generated in a coil resulting from the passage of a magnet. A special digital circuit was designed to yield a spatial resolution of better than 2.5 mm by elimiating the problem of signal overlap. The time delays obtained from these signals were used to determine the slowing down or energy loss of a high velocity projectile. A light gas gun was used to propel aluminum projectiles to velocities up to 320 m/s. Energy loss of these high velocity projectiles in composites reinforced with polyethylene, polyester, and graphite fabrics was investigated. Two distinct energy loss mechanisms were detected, one due to the actual fracture process and the other to the generation of frictional heat. Extensive delamination was observed in the more ductile PE and PET composites, but not in graphite. Low velocity instrumented drop-tower impact tests were also conducted, using identical specimens and similar impact geometries.     

Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase

The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (δ) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz–van der Waals work of adhesion (W _a ^LW) and the acid–base interaction parameter (I _a?b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid–base interaction (I _a?b) or by reducing the chemical incompatibility (Δδ) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.     

Fabrication and performance of mini SiC/SiC composites with an electrophoresis-deposited BN fiber/matrix interphase

The feasibility of fabricating a BN matrix/fiber interphase of SiC/SiC composites via electrophoresis deposition (EPD) was investigated based on the simplicity and non-destructiveness of the process and the excellent interfacial modification effects of BN. The BN suspension and SiC fiber surface properties were both adjusted to generate suitable conditions for the EPD process of the BN interphase. Next, the deposition dynamics and mechanism were studied under different deposition voltages and time, and the relationship between the deposition morphology of the BN interphase and mechanical properties of the fabricated mini SiC/SiC composites were also discussed. After oxidation at high temperature (600–1000 ℃), the mechanical properties of the mini SiC/SiC composites were studied to verify the oxidation resistance effect of the EPD-deposited BN interphase, whose oxidation resistance mechanism was briefly analyzed as well.     

Thermodynamic probe of intermolecular coupling in the interphase region of glass fiber reinforced polymer composites

Mechanical properties of polymer composites are enhanced by an interphase having a brush-like structure, due to the entanglement of stretched compatibilizer chains and neighboring polymer molecules. Lipatov's model provides a calorimetric method for characterizing the fiber-matrix interactions and determining the interfacial thickness, i.e., the heat capacity jump at T_g is sensitive to the combined effects of fiber glass content and intermolecular coupling in the interphase region. By a correlation between computed interfacial thickness from DSC methodology and measured tensile strength, we have a quantitative means to develop polypropylene composites having “mega coupled” tensile behavior.     

麻纤维/聚合物复合材料

综述了近年来麻纤维/聚合物复合材料的研究进展,重点阐述了环境友好型复合材料的界面处理、成型工艺及性能研究.     

Experimental,analytical, and numerical investigation of interphasial stress and strain fields in MWCNT polymer composites

In this investigation, the effect of polymer matrix‐MWCNT interphase on the stress and strain fields developed at the close vicinity of MWCNT was studied. The recently developed concept of the hybrid interphase (Papanicolaou et al., 2002) was applied. According to this concept, the interphase thickness depends on the property considered at the time. The parameter of imperfect bonding between the primary constituent materials is also introduced by means of the degree of adhesion. Experimental findings combined with analytical and numerical results gave a better understanding of the structural and mechanical performance of epoxy resin‐carbon nanotubes composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Dynamic properties of short fiber–EPDM matrix composites as a function of strain amplitude

The dynamic properties of composite materials consisting of an ethylene–propylene rubber matrix (EPDM) and short polyester polyethylene-terephthalate (PET) fiber vary with dynamic stress amplitude applied to the material. These variations support the statement that fiber treatment with 1,4-carboxy-sulphonyl-diazide, which acts as a bridge between the fiber and the matrix and hence enhances the strength of the interface enabling it to resist greater strain applied to the composite and, as a consequence, yielding greater retention values of the storage modulus, measured longitudinally to preferential fiber orientation, E′_L. By means of transversal measurements of the storage modulus, E′_T, of these materials it is possible to determine a parameter b, which eventually indicates the degree of matrix–fiber bonding and which is consistently higher for materials filled with surface-treated fiber. This enhanced phase adhesion is further confirmed by higher equivalent interfacial thickness values, ΔR, which, in addition, vary less with increasing dynamic strain amplitude. Finally dissipated energy variation or mechanical energy loss, E_loss, is studied as a function of fiber content and strain amplitude. Experimental findings show E_loss to increase with fiber content and strain amplitude, when measured at constant strain amplitude ?₀, and to yield higher values for treated fiber samples. © 1994 John Wiley & Sons, Inc.     

先进树脂基复合材料扩大应用的关键

先进树脂基复合材料已经成为重要航空结构材料之一。本文主要介绍为扩大先进树脂基复合材料在飞机结构上的应用,近期国内外在进一步降低复合材料成本、提高复合材料应用效能方面的部分研究工作。     

The meaning of the critical length concept in composites: Study of matrix viscosity and strain rate on the average fiber fragmentation length in short-fiber polymer composites

Computer modeling is used in order to provide a theoretical understanding of the concept of critical length in composites, and of the factors influencing the critical aspect ratio. The effects of interphase and matrix properties have been investigated. We have identified the interphase parameters which minimize the critical length. Contrary to the assumption that the critical aspect ratio is related to interfacial shear strength and fiber strength only, we find a significant dependence on matrix viscosity and strain rate, for fixed interphase properties. We therefore conclude that the fragmentation test, which relates the measured critical aspect ratio to a value for the interfacial shear strength, has to be reinterpreted in terms of more parameters than those simply present in the Kelly-Tyson formalism. Moreover, the significance of the concept of critical length for tailoring mechanical properties of composites needs to be reassessed.     

Delamination growth during fatigue of advanced polymer matrix composites

Abstract

Advanced polymer matrix composites such as carbon fibre reinforced polymers (CFRP), offer many advantages over more traditional materials such as metals. Usually, CFRP have greater strength/weight and stiffness/weight ratios than traditional engineering materials, which makes them ideal for use in many weight sensitive applications, especially in the aerospace sector. To maximise the use of these materials there is a need to gain a better understanding of how CFRP, and more generally FRPs, behave under fatigue load conditions. This work investigates the fatigue response and damage mechanisms found in a CFRP. Previous work has highlighted that fatigue with a compressive element is more damaging than pure tensile fatigue and that delamination is the dominant damage mechanism in both cases. However, in the tensile fatigue tests the primary delamination was on a different interface from the primary delamination found in the compression fatigue tests. The cause of this trend to delaminate along a particular interface has been investigated using mixed mode bend tests. These tests have been used to investigate the response of the interface to both static and fatigue loads. Initial tests have been carried out on the 0°/45° interface. Delamination growth was monitored at three levels of mode mixity, ratios of MI/MII of 1:1, 1:3 and 3:1. PRC/1848