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.     

The interphasial regions in interlayer fiber composites

The interaction between the fiber and matrix in a fiber-reinforced material plays an important role in determining the mechanical behavior of the composite. An efficient technique to simultaneously improve fiber-matrix interfacial shear strength and impact behavior of the composite is to deposit a flexible interlayer onto the fiber. This results in the creation of three bulk phases, the fiber, matrix, and the interlayer and two interphasial regions. A phenomenological model that defines the variation of the fiber-interlayer interphase and that of the interlayer-matrix interphase has been developed. In the model, the elastic moduli of these regions vary continuously, so as to bridge the two bulk phases on either side of the interphase. The interaction between the bulk phases is also taken into consideration. The model has the potential for the use of dynamic mechanical analysis to obtain, relatively, adhesion/interaction parameters of different fiber-interlayer-matrix systems. These parameters can be used to determine the optimum interlayer thickness for improved toughness and good stress transfer efficiency.     

The silane/sizing composite interphase

Silane coupling agents are but one of many ingredients in commercial sizings that are applied to glass fibers. The chemistry of a silane coupling agent alone allows it to react with both the glass fiber surface and the epoxy matrix to increase the fiber/matrix adhesion. However, the action of a commercial sizing system containing a silane coupling agent along with other components is not well understood. Research has been conducted in which the physical properties have been measured of blends of epoxy-compatible silane/sizing made with bulk matrix at concentrations representing likely compositions found at the fiber-matrix interphase as a result of processing and fabrication. It has been shown that the silane/sizing interaction with the epoxy matrix produces a material with vastly different properties than those of the bulk matrix. In this particular system, the model interphase has a lower T_g higher modulus, and greater tensile strength, but lower toughness. The results from the present study show that a themical interaction theory of adhesion alone is not sufficient to explain the role of silane coupling agents in glass fiber-epoxy matrix adhesion. Consideration must be given to the interphase and its mechanical properties.     

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACT

Effect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.     

The reinforcement mechanism of fiber-glass reinforced plastics under wet conditions: A review

Reinforcement mechanisms of fiber-glass reinforced plastics (FRP) under wet conditions are reviewed with emphasis on molecular structures of glass/matrix interfaces. Included are studies on glass surface, the glass/coupling agent interface, silane coupling agents on glass surfaces as well as in solution, the coupling agent/matrix interface, extending to the interphase of particulate-filled composites, and matrix resin. For a better understanding of wet strength of FRP, the structures under dry conditions are extensively, reviewed. The chemical bonding theory still dominates other reinforcement theories. The importance of other factors such as orientation of silane coupling agents and the restriction of matrix polymer conformations are also considered. Based on recent development in spectroscopy, molecular level research of the glass/matrix interfaces has been initiated in the past decade, yet only a few spectroscopic investigations on the function of water have appeared. It is concluded that the correlation between spectroscopic investigations and mechanical properties of a FRP is indispensable.     

Characterization of interfacial interactions in high density polyethylene filled with glass spheres using dynamic-mechanical analysis

The dynamic-mechanical properties of high density polyethylene filled with 20% by volume of untreated glass spheres or glass spheres treated with a silane-based coupling agent were studied as a function of temperature and imposed tensile deformation. The coupling agent used is capable of providing covalent bonding between the polymeric matrix and the glass spheres. It is assumed that an interphase region is formed in the matrix around each filler particle with properties depending on the surface treatment of the filler, but different to that of the bulk matrix. It is shown how the mechanical loss factor can be used to characterize the properties of the interphase region and the degree of adhesion between the two phases, as affected by the surface treatment. We suggest that these kinds of measurements can be valuable when determining the effectiveness of various surface treatments of filler particles from a mechanical point of view.     

THE EFFECT OF INTERPHASE CURING ON INTERPHASE PROPERTIES AND FORMATION

Fiber-optic evanescent wave FTIR spectroscopy was combined with phase imaging AFM to examine two thermosetting polymer matrix composite systems. The epoxy/NMA system data from the fiber-optic, evanescent wave FTIR analysis showed incomplete curing (∼75% complete) in the region near the fiber, but essentially complete (∼95% complete) curing in the bulk. Conversely, the unsaturated polyester system exhibited essentially complete curing (∼95% complete) both near the fiber and in the bulk material. For the same samples, phase imaging AFM indicated that the epoxy/NMA system had an ∼2.5 micron thick interphase, while the unsaturated polyester system showed no interphase between the fiber and the matrix. Therefore, the presence of the interphase in the epoxy/NMA system can be attributed to the incomplete curing next to the fiber. In addition, the systems chosen allowed the reactivity of adsorbed γ-APS coupling agent to be assessed simultaneously with polymer curing. For the epoxy/NMA system, the amine band decreased about 54% during curing. For the polyester system, the amine band decreased 43% during curing.     

A review of recent progress in the studies of molecular and microstructure of coupling agents and their functions in composites,coatings and adhesive joints

Recent progress in the studies of molecular and microstructure of interfaces and interphases in composites, coatings, and adhesive joints is reviewed. Remarkable progress has been made in elucidating the structure of silane coupling agents and their function with respect to dry and wet strengths of multiphase systems. Aminosilanes attracted major effort in the past. It is now understood that the structure of partially cured hydrolyzate is complicated. When adsorbed from a natural pH solution and dried in air at room temperature, approximately half of the amine groups form amine bicarbonate salt with the CO₂ in air. The rest of the amine groups are either intra- and intermolecularly hydrogen bonded to neighboring silanol groups or free from hydrogen bonding. There exists chemical bonding at the glass/silane or metal/silane interfaces. The surface characteristics, including acidity, topology and homogeneity, influence the structure of the coupling agent. The coupling agent interphase shows a gradient in various properties. Silanes tend to be ordered in the interphase and the degree of organization depends largely on the organofunctionality. The orientation and organization of the silane affects the reinforcement mechanism. There are chemisorbed and physisorbed silanes in the interphase. The coupling agent/matrix interface is a diffuse boundary where intermixing takes place due to penetration of the matrix resin into the chemisorbed silane layers and the migration of the physisorbed silane molecules into the matrix phase. With proper selection of the organofunctionality and the curing conditions, silanes can chemically react with the matrix to form copolymers. The existence of the matrix interphase is now well accepted and the effect of the interphase on the mechanical properties has been studied. It has been recognized that modification of the matrix interphase, such as a coating applied on the fiber using a similar resin as the matrix, has a complex effect on the mechanical performance. It is noteworthy that attempts to synthesize new coupling agents and to utilize the existing coupling agents more effectively still continue. Based on the molecular understanding, new concepts in the reinforcement mechanism have appeared which recognize the importance of interpenetrating networks, the structure of silane in the treating solution, and the microheterogeneity of the glass surfaces. The knowledge obtained through the studies of composites can be applied to organic coatings and adhesive joints provided that the geometrical factors are taken into consideration.     

Physico-Chemical Characterisation of the Influence of Moisture on the Fibre/Matrix Interaction in Epoxy/Anhydride Composites

The effect of moisture on the fibre/matrix interaction in epoxy/anhydride composites is investigated and explained through the characterisation of the matrix cure in the interphase. The fibre/matrix interaction is inferred from ILSS measurements on the composite, which are compared with a recently-introduced DSC interaction parameter. The matrix cure in the bulk as well as in the interphase is characterised through FT-IR microspectroscopy. Complementary information is gained by measuring the overall T_g value. Moisture is shown to lower the fibre/matrix interaction if the prepreg (= composite precursor) is stored at room temperature or lower prior to final cure. This is due to the reduced crosslink density of the matrix. For PE prepreg stored in ordinary atmospheric conditions, moisture from the surroundings lowers the fibre/matrix interaction sharply. If the prepreg is stored in a dry environment, nearly no effect of the storage is detected on the fibre matrix interaction in PE (polyethylene) fibre composites, while for PVAL (polyvinylalcohol) fibre composites a strong decrease is still found, caused by water adsorbed at the more hydrophilic fibre surface.     

The effect of interphase curing on interphase properties and formation

Fiber-optic evanescent wave FTIR spectroscopy was combined with phase imaging AFM to examine two thermosetting polymer matrix composite systems. The epoxy/NMA system data from the fiber-optic, evanescent wave FTIR analysis showed incomplete curing (～75% complete) in the region near the fiber, but essentially complete (～95% complete) curing in the bulk. Conversely, the unsaturated polyester system exhibited essentially complete curing (～95% complete) both near the fiber and in the bulk material. For the same samples, phase imaging AFM indicated that the epoxy/NMA system had an ～2.5 micron thick interphase, while the unsaturated polyester system showed no interphase between the fiber and the matrix. Therefore, the presence of the interphase in the epoxy/NMA system can be attributed to the incomplete curing next to the fiber. In addition, the systems chosen allowed the reactivity of adsorbed γ-APS coupling agent to be assessed simultaneously with polymer curing. For the epoxy/NMA system, the amine band decreased about 54% during curing. For the polyester system, the amine band decreased 43% during curing.     

Glass fiber 'sizings' and their role in fiber-matrix adhesion

Silane coupling agents are but one of the many ingredients in commercial sizings that are applied to glass fibers. The action of epoxy-compatible silane coupling agents alone is to increase the fiber-matrix adhesion; however, the action of a silane coupling agent-containing sizing system is not well understood. Research has been conducted in order to determine to what degree an epoxy-compatible glass fiber sizing alters the adhesion between fiber and matrix, as well as to what degree it changes the mechanical properties of the resulting composite. By using blends of epoxy-compatible sizing with bulk matrix, it has been possible to model the properties of the fiber-matrix interphase formed when the sizing interacts with the matrix during composite processing and fabrication. It has been shown in this case that the sizing's interaction with the matrix produces a material with a higher modulus, a greater tensile strength, but a lower toughness. The level of fiber-matrix adhesion increases along with a change in failure mode of the composite caused by the presence of the lower toughness interphase. The results from this study show that a chemical interaction theory of adhesion is not sufficient to explain the effect of fiber-matrix adhesion on composite properties. An interphase-based theory in which the mechanical properties of the interphase are considered along with the chemical interactions between the fiber surface and the sizing offers the best approach for developing these relationships.     

Characterizing elastic properties of carbon nanotube‐based composites by using an equivalent fiber

Effective elastic properties for carbon nanotube (CNT)‐reinforced composites are obtained through a variety of micromechanics techniques. An embedded CNT in a polymer matrix and its surrounding interphase is replaced with an equivalent fiber for predicting the mechanical properties of the CNT/polymer composite. Formulas to extract the effective material constants from solutions for the representative volume element under three loading cases are derived based on the elasticity theory. The effects of an interphase layer between the nanotubes and the polymer matrix as result of effective interphase layer are also investigated. Furthermore, this research is aimed at characterizing the elastic properties of CNTs‐reinforced composites using Eshelby–Mori–Tanaka approach based on an equivalent fiber. The variations of mechanical properties with tube radius, interphase thickness, and degree of aggregation are investigated. It is shown that the presence of aggregates has stronger impact than the interphase thickness on the effective modulus of the composite. This is because aggregates have significantly lower modulus than individual CNTs. POLYM. COMPOS., 2013 © 2013 Society of Plastics Engineers     

Evolution of the interfacial stress transfer ability between a glass fibre and a polypropylene matrix during polymer crystallization

A rigorous evaluation of the influence of a transcrystalline interphase on the adhesion between a fibre and a polymer matrix is difficult because it is generally not possible to consider this parameter without other factors. Indeed, in the case of a reinforcing material allowing spontaneous transcrystallization of a thermoplastic matrix, the inhibition of this phenomenon is, for instance, possible by modifying the surface topography or the physico-chemical nature of the fibre by appropriate coatings. In the same way, the fibres which do not intrinsically favour transcrystalline growth can behave as active substrates by applying a shear stress onto the fibre-matrix interface. In this case, however, processing of the sample for the classical micromechanical tests remains extremely difficult. In order to understand better the participation of a transcrystalline interphase in the interfacial adhesion in a polypropylene–glass fibre system, an experimental protocol has been developed which allows us to evaluate the evolution of the interfacial shear stress during the matrix crystallization, the latter being spherulitic or cylindritic. The results show that a transcrystalline interphase in a polypropylene–glass fibre composite does not significantly alter the adhesion between the two materials after total crystallization of the matrix. Nevertheless, it is shown that an important hooping of the fibre occurs during the development of a transcrystalline superstructure.     

玻璃纤维增强聚丙烯界面处理研究进展

本文综述了提高玻璃纤维增强聚丙烯复合材料界面粘结强度和改善界面层结构的各种有效方法,包括玻璃纤维的偶联剂涂覆、浸润剂浸润、表面接枝等表面处理方法以及在聚丙烯基体中添加功能化聚丙烯对基体进行共混改性等,对玻璃纤维与聚丙烯的粘的结机理进行了讨论,并论述了玻纤／聚丙烯界面横晶对界面粘结强度的影响。     

长玻纤增强聚丙烯复合材料的研究

研究了长玻璃纤维/PP复合材料的力学性能和纤维在基体中的分布状况。结果表明：长玻纤在基体中呈现三维空间交叉分布,这种结构大幅度提高了复合材料的拉伸强度、冲击强度、硬度及软化点温度；玻纤的长度和用量对三维交叉结构的形成有很大影响；偶联剂KH-550处理玻纤表面后,材料的力学性能提高不大。     

Study of interphase in glass fiber–reinforced poly(butylene terephthalate) composites

It is well known that application of a coupling agent to a glass fiber surface will improve fiber/matrix adhesion in composites. However, on commercial glass fibers the coupling agent forms only a small fraction of the coating, the larger part being a mixture of processing aids whose contribution to composite properties is not well defined. The interfacial region of the composite will therefore be affected by the coating composition but also by the chemical reactions involved in the vicinity of the fiber and inside the surrounding matrix. The main feature of this study consists in dividing the interface region into two separate regions: the fiber/sizing interphase and the sizing/matrix interphase. A wide range of techniques was used, including mechanical and thermomechanical tests, infrared spectroscopy, gel permeation chromatography, carboxyl end group titrations, extraction rate measurements, and viscosity analysis. Experiments were performed on poly(butylene terephthalate) composites and results indicate that the adhesion improvement is due to the presence of a short chain coupling agent and of a polyfunctional additive, which may react both with the coupling agent and the matrix. According to the nature of this additive, it may be possible to soften the interphase and then to increase the composite impact strength.     

Improved interfacial shear strength and durability of single carbon fiber reinforced isotactic polypropylene composites using water-dispersible graft copolymer as a coupling agent

A synthesized water-dispersible graft copolymer (WDGP) was evaluated as a coupling agent in carbon fiber/isotactic polypropylene composites. Measurements of the interfacial shear strength (IFSS) were made using single fiber composite (SFC) specimens. A graft copolymer of isotactic polypropylene backbone grafted with polyacrylamide (PAAm) was synthesized for testing as a coupling agent. Optimal conditions for the coupling agent treatment and SFC specimens preparation were established. Improvement in the IFSS was 18% at room temperature, whereas improvement was >60% under wet conditions, after 1 h in boiling water. This improvement is due to chemical bonding in the interphase between functional groups in carbon fiber and PAAm in WDGP coupling agent, and interdiffusion in the interphase between IPP in WDGP coupling agent and matrix IPP. The hydrophobic nature of the interfacial region and crosslinking between amide groups in grafted PAAm chains based on inter- and intramolecular interactions may be the reason for the high improvement of the IFSS under wet conditions. In addition, the spherulite size and transcrystallinity seem to correlate with the IFSS of SFC specimens.     

Functional interphases with multi-walled carbon nanotubes in glass fibre/epoxy composites

The interphase between reinforcing fibre and matrix is a controlling element in composite performance. We deposited multi-walled carbon nanotubes (MWCNTs) onto electrically insulating glass fibre surfaces leading to the formation of semiconductive MWCNT-glass fibres and in turn multifunctional fibre/polymer interphases. The deposition process of MWCNTs onto glass fibre surfaces involved both electrophoretic deposition (EPD) and conventional dip coating methods. The EPD coating method produces a more homogeneous and continuous nanotube distribution on the glass fibre surface compared with the dip coating. According to fragmentation test results, the interphase with a small number of heterogeneous MWCNTs in the EPD fibre/epoxy composites, mimicking a biological bone structure, can remarkably improve the interfacial shear strength. We found that the semiconductive interphase results in a high sensitivity of the electrical resistance to the tensile strain of single glass fibre model composites. This material provides a possible in situ mechanical load sensor and early warning of fibre composite damage.     

玻璃纤维与聚丙烯界面性能的研究

利用小跨度变曲的方法研究了玻璃纤维增强PP复合材料的层间剪切强度(ILSS)。进行了偶联剂的评选,考察了偶联剂和接核物PP-g-MAH用量对层间剪切强度的影响。试验表明,玻璃纤维经偶联剂处理后,提高了与PP的层间剪切强度,PP-g-MAH在玻璃纤维与PP之间起到了较好的增容作用,研究了接技物PP—g—MAH的增容效果。     

Chemical modification and adhesion in carbon fiber/epoxy micro‐composites; coupling and surface coverage

Four compounds were used to improve adhesion between carbon fibers and an epoxy matrix. Triglycidyl isocyanurate (TGIC) and 3‐glycidoxy‐propyl‐triethoxysilane (EPS) contained reactive epoxy groups, while N‐(3‐trimethoxysilane‐propyl)ethylene diamine (AMS) a primary and a secondary amino group. The fourth coupling agent was 4,4' diphenylmethane‐diisocianate (MDI). The interaction of the fiber and the coupling agents was studied by dissolution experiments. Chemical reactions taking place on the surface of the fiber were followed by FTIR spectroscopy. Interfacial shear stress determined by fragmentation was used for the characterization of matrix/fiber adhesion. Beside coupling to the surface, EPS, AMS and MDI formed a polymer layer on the surface, but TGIC also entered into secondary reactions during the treatment. Both the type and the amount of the coupling agent affects strongly interfacial adhesion, which is determined by the thickness and properties of the formed coupling agent layer. The combination of dissolution experiments with the fragmentation test yields valuable information about the processes taking place on the surface of the fiber, facilitate the selection of the best coupling agent, as well as the development of surface treatment technology.