ABSTRACTS

The following is a list of the abstracts for papers which will be presented in the INTERNATIONAL SYMPOSIUM ON INTERFACES IN POLYMER COMPOSITES The listing is alphabetical by presenting author. This list is updated continually to add abstracts as they become available and make appropriate corrections. This list may be conveniently searched by using the editor provided with most popular browsers (e.g. Microsoft Explorer, Netscape, ... etc.)

Polymerization Compounding Composites: from Chemistry to End-use Applications

Polymerization compounding is a process in which the surface of a solid substrate participates to the polymerization process of the matrix of the composite. In this approach, several aspects of polymer composites can be addressed. Wetting of the surface of solid substrate is enhanced due to the fact that active sites used to initiate the polymerization process are generally distribute on nanometric scale. Interactions between the solid substrate and the polymeric matrix are enhanced since covalent links can be created during the polymerization process. Finally, the dispersion of the solid substrate in the polymeric matrix is enhance since each individual particle or fibre will be encapsulated by the polymeric matrix. These aspects generally lead to materials with enhanced overall properties. The technique can be used to obtain highly filled polymers which can not be produced by conventional techniques. It can also be used as a special treatment of the surface of solid substrates. In this case, the modified solid is incorporated in a matrix compatible with the polymer synthesized on the surface of the solid. The approach will be illustrated for two principal polymerization processes namely Ziegler-Natta polymerization of polyolefin composites and interfacial polycondensation composites. Several engineering aspects (rheology, mechanical properties, etc...) of such composites will be presented and discussed.

Wetting Dynamics of Copolymer Interfaces

Wetting dynamics of copolymers is studied by characterizing the dynamic interaction between a polymer matrix and a solid surface. It is argued that the pressure sensitive bonding of a polymer interface occurs within a critical time scale below which no wetting is established upon contact. This critical time scale is used to define an adhesion frequency for pressure sensitive adhesives. The wetting kinetics is used to model transient adhesion and take into account the growth of bonding energy with time. The mechanics of the interfacial dynamics is used to derive expressions for the adhesion frequency when inertial, viscous or elastic effects dominate the surface deformation. Transient adhesion effects are also investigated experimentally using non-axisymmetric rotating bits and commercially available copolymer adhesives. The rotating bits induce oscillatory motion on the polymer surface. Experiments were conducted with the rotating bits spinning at a wide range of speeds making it possible to obtain the dynamic response of the polymer interface as well as to measure the adhesion frequency for the tested polymers.

Tuskegee University, Tuskegee, AL 36088

The Effect of Thermal Aging on the Fracture and Fatigue Behavior of PTFE Composites

The effect of thermal aging on the fracture and fatigue behavior of particulate and Fiber filled polytetrafluroethylene (PTFE) composite materials has been investigated. The materials used were a 15% graphite particle filled PTFE and a 15% fiberglass filled PTFE. Geometrically identical unnotched and notched specimens were prepared for monotonic tensile and fatigue tensile testing. Some of the specimens were subjected to thermal cyclic aging for 56-days. Scanning electron microscopy (SEM) was used to observe changes to the interfacial characteristics between the fillers and the matrix materials, It was observed that thermal aging alone caused the formation of defects such as microcracks and porous structure in the graphite filled PTFE. Debonded fibers with increased pull-out length and less deformed matrix were associated with the thermally aged 15% fiberglass filled, PTFE. Monotonic tensile tests were conducted on aged and unaged samples. The ultimate tensi le strength was found to drop approximately 20'% and 10% for the 1.5% graphite/PTFE and the 15% fiberglass/PTFE samples respectively due to the thermal aging. Torsion-tension fatigue crack propagation tests were performed on both thermal aged and un-aged materials at a frequency of 3 Hz at room temperature. The fatigue fracture and failure behavior of these two materials displayed even greater sensitivity than the static behavior to the change in interfacial conditions caused by the thermal aging. For unaged materials, the fatigue lifetime of the 15%, fiberglass filled PTFE reached 705,000 cycles. This is three times that of the 15% graphite particle filled PTFE (245,000 cycles). After thermal aging, the fatigue lifetime of the 15% fiberglass filled PTFE decreased to 110,000 cycles, which is less than 15% of that of the unaged material. The fatigue lifetime of the 15% graphite particle filled PTFE material also decreased drastically to 26,000 cycles. Fatigue fracture surface morphology in the stable fatigue crack propagation region shows low energy consuming features associated with fatigue crack growth for the thermally aged specimens. This reveals that the weakened filler/PTFE. interfacial bonding observed in both materials prior to mechanical testing was responsible for the weakened mechanical properties and fatigue resistance.

Interface Shear Strength of Dental Composites by the Microbond Test

(Abstract not yet available)

Theoretical Phase Diagrams of Polymer/Clay Nanocomposites: The Role of Grafted Organic Modifiers

We combine a density functional theory (DFT) with a self-consistent field model (SCF) to calculate the phase behavior of thin, oblate colloidal particles that are coated with surfactants and dispersed in a polymer melt. These coated particles represent organically modified clay sheets. By integrating the two methods, we can investigate the effect of the surfactants' characteristics (grafting density, length and the polymer-surfactant interaction energy) on the polymer-clay phase diagram. Depending on the values of these critical parameters and the clay volume fraction, the system can be in an isotropic or nematic phase (which corresponds to an exfoliated composite). The system can also form a smectic, crystal, columnar, or "house-of-cards" plastic solid, as well as a two-phase (immiscible) mixture. Using this model, we isolate conditions that lead to the stabilization of the homogeneous, exfoliated phases (the isotropic and nematic regions) and to the narrowing of the immiscible two-phase regions.

1) Rubber Technology Center, Indian Institute of Technology, Kharagpur-721 302, INDIA

2)School of Materials Science and Engineering, University of New South Wales, Sydney 2052, AUSTRALIA

Atomic Force Microscopy Studies of Short Melamine Fiber Reinforced EPDM Rubber

Atomic Force Microscopy (AFM) was used to investigate the morphology and interfacial properties of unaged and aged Ethylene Propylene Diene (EPDM) rubber-melamine fiber composites. Interfacial adhesion between the fiber and the matrix was weak in the absence of a dry bonding system consisting of hexamethylene tetramine, resorcinol and hydrated silica (HRH). AFM images revealed the formation of an interface between the fiber and the matrix with the addition of the bonding agents. Ageing of the composites improved the adhesion between the fiber and the matrix, which was evident from the topographic images of the aged composites. It was found that two-dimensional and three-dimensional topographic images from AFM could be used to determine fiber geometry, fiber diameter and fiber-matrix adhesion in short fiber-rubber composites.

1) Dept. of Materials Science and Technology, University of Limerick, Ireland.

Surface Treatment of Polymer Matrix Composites for Adhesion

In this study various methods of surface treatment for polymer composites are examined. Polymer composites can be sub-divided according to their matrix into thermosets and thermoplastics. Methods of surface treatment for thermosets include abrasion, grit blasting, tear ply and peel ply techniques. These treatments increase surface roughness, increase surface area, thereby producing intimate contact between the substrates and hence a strong and durable joint. However, surface treatment of thermoplastics involves altering surface chemistry, since altering surface roughness is not usually enough to form successful adhesive joints. Various techniques, such as plasma treatment, corona discharge, oxidising flame treatments and laser treatments are employed to enhance the surface properties of thermoplastic composites. In the case of plasma treatment the substrate surface is exposed to an excited gas, which consists of atoms, ions and free radicals. This results in an increase in the number of polar groups on the surface, an increase in surface tension, better wetting and ultimately an increase in bond strength. Surface tension and surface roughness results obtained for the surface treatment of carbon fibre reinforced polyetheretherketone (APC2) indicate that argon plasma treatment gave the greatest increase in hydrophylicity, surface tension and surface roughness. This was echoed in the bond strength results. In general it can be seen that some method of surface treatment is necessary to improve adhesion to polymer matrix composites, whether it is surface roughening for thermoset composites or more complex chemical treatments for thermoplastics.

Characterisation of the Interaction Regions in Polymer Matrix Composites by Mixed Numerical-Experimental Methods

In composite systems the interaction level between the basic components, fibers - matrix and lamina, is an essential element for the thermomechanical global behaviour. In most of the theoretical models perfect adhesion is used in order to describe the interactions. It is possible to introduce imperfect interface conditions as proposed by J. Achenbach, [1] , later developed by Z. Hashin, [2] , [3]. It is also possible to introduce a transition region of interaction between the the basic components of a composite as proposed by many authors, see A. Cardon, [4] , [5].

Whatever the model used for the description of the interaction between phases a direct experimental measure of the characteristics of the interface, interphase or interaction region is very difficult except in some very specific academic conditions.

When one of the phases has a time dependent behaviour local creep and/or relaxation effects occur between the viscoelastic and the elastic phase. Those stress transfers are depending on the interaction level and are most important for perfect interface conditions. The moduli introduced by imperfect interface conditions are a measure of the stress transfer capacity between the phases.

By introducing some a priori unknown parameters in the model of the interaction region it is possible to compute some global characteristic of the composite and by comparison of the computed result with an experimental measure of the same global characteristic it becomes possible, after some iterations, to obtain a good measure of the interaction parameters.

We will discuss the two possible models to describe the interaction between phases and the different problems related to the application of such a mixed numerical-experimental methods on graphite-epoxy composites.

[1] Achenbach J. and Zhu H. (1989), "Effect of the interfacial zone on mechanical behaviour and failure of fiber reinforced composites", Journal of Mechanics and Physics of Solids, vol. 37, pp. 381-393.

[2] Hashin Z. (1991), "Composite Materials with interphase: Thermoelastic and inelastic effects", Inelastic Deformation of Composite Materials (Ed. G. Dvorak) Springer, pp. 3-34.

[3] Hashin Z. (1991) Composite Materials with viscoelastic interphase: creep and relaxation" Mechanics of Materials 11, pp. 135-148.

[4] Cardon A. H. (1991), "Global and Internal Time dependent Behaviour of Polymer Matrix Composites", Inelastic Deformation of Composite Materials (Ed. G. Dvorak) Springer, pp. 489 - 499.

[5] Cardon A. H. (1993), "From Micro- to Macroproperties of Polymer Based Composite Structures by integration of the Charcteristics of the interphase regions ", Composite Structures, 24 , pp. 213 - 217.

Composites Based on Carbon Fibers and Liquid Crystalline Epoxy

Resins

Thermosets are basically very brittle and this feature is generally related to their chemical structure. In fact network crosslinking strongly locks the chain segments reducing the freedom of the chains that cannot dissipate energy involved through a chain conformational motion. The solution of this problem is very complex because it involves the chemical composition of the monomers and even more the curing process that can generate large tensile stresses. These tensions can be due to different crosslinking densities within the resin, which, in turn, can be generated by non-homogeneous distribution of the curing agent or by an incomplete crosslinking.

Many scientific papers have been published in order to solve these problems. One solution has been proposed by mixing the monomers with functionalized rubbers. In this case, even if the toughness of the thermoset is improved, the Tg and the tensile modulus are reduced. As an alternative approach, the inclusion of thermoplastic polymers can reduce the brittleness in the thermoset, without considerably sacrificing other properties.

Chemistry of epoxy resins was initially dominated by Bisphenol A and since the end of World War Two, many new monomers have been subsequently synthesized due to the increasing demand of advanced composites.

A novel approach consists in the tailoring of new molecular structure in the thermoset by introducing rigid groups in the molecular skeleton and forcing the formation of a liquid crystalline phase during the crosslinking reactions. The experimental results have shown that liquid crystalline epoxy resins exhibit superior fracture toughness if compared to conventional epoxies due to the presence of two phases constituted by domains of liquid crystalline resin embedded in region of isotropic resin.

Also when carbon fiber composites are prepared with the new liquid crystalline resin superior mechanical properties have been experimentally found.

In this paper physical properties of composites prepared with conventional and novel liquid crystalline matrices are presented and related to the liquid crystalline structure developing during the curing process.

Research Council, 75 de Mortagne Blvd., Boucherville (Quebec) CANADA J4B 6Y4

Fiber/Matrix Interaction in Glass Fiber/Polypropylene Composites: Effect of Matrix Composition and Morphology

Polypropylene (PP) is extensively used in the composite industry since it is an inexpensive material, with relatively good performance in terms of strength and easily available in a wide range of molecular weights. This paper presents results of our research done in order to understand the mechanisms responsible for the creation of optimum interfacial properties in glass fiber/polypropylene composites (GF/PP). Effect of the modification of the PP matrix composition, glass fiber treatment and processing conditions on the morphology, interfacial strength and fracture toughness of the composite have been investigated. The increase of the reactivity of the PP matrix by the addition of functionalized PP like maleic anhydride grafted PP and acrylic acid grafted PP was found to be essential for the creation of strong fiber matrix interaction in the composite; the best results were obtained for maleic anhydride grafted PP of low viscosity. The crystalline morphology of the PP matrix was also found to be a critical parameter governing the interfacial strength and the mechanical behavior of the GF/PP; highly crystalline GF/PP obtained by process like vacuum molding showed reduced fracture toughness. A modification of the PP composition is proposed to minimize this problem.

1) Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899

2) Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005

Elucidation of Damage in a Glass Reinforced Composite Using Optical Coherence Tomography

Study of the initiation and propagation of damage in a polymer matrix composite is critical to understanding ultimate failure. Many studies of composite failure have been hampered by the difficulty in quantifying damage non-destructively. Optical Coherence Tomography (OCT') is a highly sensitive (> 100dB) non-destructive technique that can probe damage and reinforcement architecture with 10 gm to 20 p,m spatial resolution. Typical depth of penetration is 2-5 mm.

OCT is a confocal reflection technique that is enhanced by interferometric rejection of out-of-plane image scattering. Briefly, OCT uses a low coherence source such as a superluminescent diode laser with a fiber optic based Michelson interferometer. In this configuration, the composite is the fixed arm of the interferometer and the fiber optic acts as the confocai aperture. Reflections from heterogeneities within the sample are mapped as a function of thickness (z axis) for any one position. Volume information is generated by translating the sample on a motorized stage through x and y axes. Quantitative information about the location and size of a feature within the composite is obtained.

In this work, a region of impact damage in an epoxy/E-glass composite was imaged. Several damage mechanisms were found and were identified to be kink banding, fiberlmatrix de-bonding, longitudinal cracking, delamination and matrix deformation. All of these mechanisms were consistent with damage expected from bending a composite with a moderately tough matrix and poor fiber-matrix bonding. Results from OCT were compared to an established quantitative NDE technique: x-.ray computed tomography, The advantages and limitations of each technique are discussed.

1) Center for Composite Materials, University of Delaware, Newark, DE 19716-3144

2) Army Research Laboratory, Aberdeen, Maryland

Fiber/Matrix Interphase Characterization using the Dynamic Interphase Loading Apparatus

The Dynamic Interphase Loading Apparatus (DILA), originally developed to determine the high strain, rate properties of the interphase, has been extended to characterize the hygrothermal and fatigue loading of the interphase. The apparatus is a specialized version of a microindentation or fiber push-out test, which allows one to test at a wide range of loading rates and environmental exposure. The test sample configuration, a thin slice of composite material cut perpendicular to the fibers, is specifically designed to minimize the amount of the interphase in tension as well as provide for uniform shear stress within the interphase along the length of the fiber. It was shown through finite element analysis that the fiber and matrix properties significantly affect the optimum sample thickness as well as the ideal support hole diameter through which to push the fiber. It was shown that the response of the interphase of a glass/vinyl ester composite system varied significantly under different loading profiles and temperature exposure. The DILA can easily be used to characterize the effect on the durability of different sizing materials on the microscale.

Application of the Short Beam Shear Test For Monitoring Environmental Aging Effects On Interfaces in Fiber Reinforced Polymer (FRP) Composites

As FRP composite use expands into infrastructure applications, the necessity to understand the durability of these materials under a variety of exterior exposures is needed. Simple testing protocols need to be adapted from current standards or developed to evaluate this class of materials. Interlaminar shear strength (ILSS) measurements based on the short beam shear test (ASTM D 2344) are used to determine matrix-fiber interface compatibility. This qualitative test can be used as a mechanical property indicator to assess retained "apparent" ILSS after environmental exposure. This test is extensively used in industry as a screening tool to evaluate new resin-fabric compositions, and compatibility of new fiber sizings with matrix resins. The FRP composites examined in this study included 1) pultruded E-glass-vinyl ester, 2) pultruded E-glass vinyl ester/ polyester blend, 3) pultruded E-glass phenolic, 4) VARTM E-glass vinyl ester, 5) hand lay-up compaction molded E-glass isophthalic/terephthalic polyester blend and 6) hand lay-up compaction molded E-glass phenolic. The environmental aging tests included water immersion, salt water immersion, alkali immersion (CaCO₃ pH 9.5), diesel fuel immersion, dry heat resistance (60 degrees C), freeze-thaw resistance, natural weathering, and artificial weathering (accelerated UV exposure). ILLS degradation rates were determined by making measurements at prescribed times during the aging exposures. The effect of environmental exposures on the ILSS of composites 1,4, and 5 indicated no degradation of the interface, while composites 3 and 2 showed minor reduction 15 to 22% and 0 to 15% in ILSS, respectively as the result of environmental exposure. Composite 6 experienced unacceptable levels of ILSS reduction (5 to 40%) as a result of the environmental aging exposures. The results of this study indicate that the short beam shear test is well suited to evaluating the effect of environmental aging exposures on FRP composite interface properties.

Flexible Interfaces-- Creation of a New Type of Composite

There are several levels of interfaces such as interface around single fiber, interface inside of fiber bindle, interface around fiber bundle and interl amina. Chemical modification on fiber affects properties of whole composites. In this presentation one of attempt for interface inside of fiber bundle is demonstrated; that is Flexible Interfaces. The concept of Flexible Inter faces is that flexible resin is applied to fiber bundles before impregnation of rigid normal matrix, so that inside of fiber bundles is occupied by flexible resin. When fiber fracture occurs fracture energy can be absorbed in this flexible matrix and consequently tensile strength increases. Also damping and other dynamic properties can be improved by this concept. Normally it is difficult to create high strength and high damping materials, however according to this flexible interface concept both high properties can be obtained.

Development of Copolymer- Adhesion Layers for Space Applications

Kapton is the polymeric material used to cover space vehicles because it has a high thermal conductivity. However, Kapton as well as other polymers are susceptible to erosion by oxygen atoms which are present in low-earth orbit (LEO) at a concentration of 90% 300 to 700 km above earth's surface. The spacecraft move at a velocity of 8 km/s so the collisions with thermal 0 atoms is equivalent to collisions of 5 eV 0 atoms with a stationary surface The 0 atom flux is approximately 10¹⁵ atoms/cm²-s in the flight direction.

During the last 15 years, we have developed an atomic oxygen (AO) source which produces a flux of 10¹⁵ atoms/cm²-s and an average energy of - 5 eV. this source produces a negligible amount of 0 ions and electrons. The A.0 is produced by electron stimulated desorption of adsorbed 0 atoms at a Ag membrane surface. The availability of this source allows for simulation of the LEO environment in earth laboratories and testing of polymeric materials in order to develop coatings which are resistant to AO erosion.

A,0 erosion studies have been performed on Kapton, and in-situ analysis of the AO exposed surfaces have been performed using X-ray photoelectron spectroscopy (XPS)_ Contrary to results reported in previous studies, the 0 content of the near-surface region is decreased by oxidation of CO groups in the Kapton to form C0₂ which desorbs The surface become quite rough as the erosion progresses.

In an effort to find AO-erosion resistant surfaces, polyoligomeric silsesquioxane siloxane (POSS) copolymers have been tested for AO erosion. With exposure time these surfaces form a Si0₂ layer which protects the underlying polymer from further erosion. Efforts are in process to graft this POSS-containing polymer onto Kapton.

Rate Dependent Properties of Epoxy-silane Interpenetrating Networks

Experimental study and measurement of the properties of the interphase have traditionally been very difficult, due to the small nano to micro volume scales of material present in an actual composite as well as masking by the fiber reinforcement. Therefore, a model epoxy-silane interpenetrating network (IPN) was examined to elucidate the thermal and mechanical properties of the interphase. This synthesis is based on the aqueous sol-gel reaction of 3-glycidoxypropyltrimethoxysilane in the presence of a nonionic surfactant and epoxy film former. The aqueous phase of the sol-gel reaction was evaporated from the condensed sizing and the insoluble fractions of film former and surfactant were removed through Soxhlet extraction with acetone, leaving a large quantity of insoluble siloxane powder. The insoluble fraction of model sizing was incorporated into an epoxy matrix (diglycidyl ether of bisphenol A epoxy resin cured with bis (p-aminocyclohexyl) methane) in large volume fractions (50 %) to form a "composite" absent of glass fibers. Viscoelastic micro-mechanics combining rules were used to derive the rate dependent mechanical properties of the model interphase material using experimental data obtained from dynamic mechanical analysis. The ultimate goal of this research is to relate the chemical and physical properties of the interphase to the high strain rate properties of the composite.

Morphology and Failure in Nanocomposites

Nanocomposites are heterogeneous systems where the significant volume fraction of polymer is affected and strongly interacted by the very small nano-sized filler particles. The structure of nanofillers suggests the polymer matrix dispersion on the molecular level is a three dimensional net of nanofillers which produce the special constrained matrix structure and explain the specific morphology of nanocomposites.

For cur studies we have selected the nanofillers (< 100nm) kaolin and calcium carbonate(CacO₃) that were introduced in the copolymer poly(acrylate)(PA) matrix and poly(vinyl acetate)(PVAc) matrix respectively, by simple mixing in aqueous dispersions and extracting water to produce the thin films. Throughout the presentation we demonstrate how various microscopic(SEM), X-ray scattering (WAXD), thermal(DSC, DMA), mechanical tensile, contact angle and gas permeation tools were employed to interrogate the structures and properties of these, heterogeneous materials. The goal is to investigate the relations between the composite morphology and mechanisms of failure as a result of filler and matrix rnicrostructures and the adhesion phenomena at. the interface. The PA/kaolin and PVAc/CacO₃ nanocomposites all failed cohesively in the matrix. When the work of adhesion, W_a increases between the filler and the polymer .matrix, the tensile strength of the composite materials under investigation increases too. This points out W_a as an important parameter for optimization, besides the interfacial free energy and the coefficient of wetting.

The nature of reinforcing which assures the inorganic phase was determined through the interaction coefficients, calculated from the models which shown the best fitting to the experiments, The results show that the improved mechanical properties are the results of the strong interfacial interactions between matrix and nanofillers.

Improvement of the Mechanical Potential of Natural Fibre Reinforced Thermosets

Better mechanical properties of fibre reinforced plastics depend on different material properties: the mechanical behaviour of the fibres, the non-woven fabrics, the matrix and the adhesion between fibre and matrix. These items are especially important for natural fibre reinforced plastics which show particular characteristics when compared to synthetic fibre reinforced plastics. Four different natural fibres are compared in this work. The matrix resin which is used with these natural fibres is an unsaturated polyester resin. The used coupling agents are from the group of silane methacrylsilane (MS) and carbamatsilane (CS), and from the group of titanium derivates butyltitanate.

There are two ways to improve the adhesion of the fibre to the matrix: The coupling agent can be coated on the fibre ("fibre finishing process") or be mixed into the matrix just like an additive ("resin additive process"). The coupling agent butyltitanate improves the mechanical properties when added to the fibre finishing as well as the resin additive process. The addition of titanium derivatives to the silane coupling agent and the use of carbamat silane increases the mechanical properties of the natural fibre reinforced thermosets.

Using these coupling agents the mechanical properties of the composites can be increased in the resin additive process as well as in the fibre finishing process. A comparison of coupling agents using the fibre finishing procedure is done. It shows that butyltitanate and methacryl silane activated with titanum acetyl acetone are most suitable to improve the material properties of all natural fibres. In order to characterize the mechanisms of the different coupling agents, especially the mechanism of butyltitanate, different microscopic (REM, TEM, AFM) and spectroscopic (EDX, FTIR-spectroscopy) methods are applied.

Interface Adhesion of Polymer Composites: Mixed Mode Energy Release Rate and Essential Work of Fracture

The measurement of interfacial energy release rate with a mixed-mode loading device of a bi-material consisting of glass and polyethylene provides a significant increase, whenever the magnitude of the bond tangential shear load of the asymptotic elastic mixed-mode state is increased in either direction. Between these extremes the interfacial toughness curve exhibits a pronounced minimum, which is believed to represent the so-called intrinsic adhesion. This method can only be applied as long as non-linearities due to crack-wall contact and plastic flow are contained within a zone small enough compared to the extension of the near-tip opening-dominated fields. However, bulk polymers show very often large scale inelastic behaviour in front of a crack.

The essential work of fracture method (EWF) to determine the fracture toughness of bulk polymers, blends and filled polymers is extended to determine the essential interface work of fracture (EIWF) of polymer-polymer bi-materials. It was found that the EWF-concept is appropriate to determine the interface toughness between two different polymers, where at least one of them shows large-scale inelastic (viscoelastic, viscoplastic or plastic) behaviour.

The specific dissipation energies and the volume of the zones are different. For the specific work of fracture we get in this case:

w_f = w₁₂ + (B₁w¹_d + B₂w²_d)

with w₁₂ as the essential interface work of fracture,B₁, B₂, the shape factors for the dissipation zones in material 1 and 2, and w¹_d and w²_d the corresponding dissipation energies.

1) Polymers Division, National Institute of Standards and Technology

100 Bureau Drive, Stop 8543, Gaithersburg, MD 20899

2) Chemical Engineering Department, North Carolina State University, Raleigh, NC 27695

3) Institute of Materials Science and Chemical Engineering Department

University of Connecticut, Storrs, CT 06269

Studying The Buried Glass/Resin Interfacial Region By Immobilizing

A Fluorescent Probe On The Glass Surface

Adhesion of polymers to a solid surface is an important issue in many technical applications including fiber and particulate reinforced composites, nano-composites, electronic devices, biomaterials, and general adhesive applications. The properties of the polymer near the solid surface will govern the adhesive strength, location of bond failure, and durability of the polymer l substrate bond. The interaction between the polymer and the substrate, the substrate roughness, and the presence of a sizing layer or coupling agent on the surface can alter the interfacial structure. In addition the interfacial region can be affected by the preferential adsorption of monomers, catalyst, or low molecular weight species to the substrate surface. Because of these factors, the polymer structure near the surface can be very different than the bulk polymer. While many techniques are available to study polymer surfaces and thin films, very few are available to study the buried polymer/ substrate interfacial region.

In this work, we are developing a technique to study the buried glass/resin interfacial region by covalently grafting a fluorescent probe with coupling agent layers on glass surfaces. The emission from the grafted dye was sensitive to both the chemistry and structure of the adsorbed silane layer. A dimethyl-amino-vitro-stilbene fluorescent dye was tethered to a triethoxy-silane coupling agent, generating a fluorescently labeled silane coupling agent (FLSCA). Silane coupling agent layers on glass were doped with small levels of the FLSCA molecule. When the coated glass was immersed in epoxy resin, a blue shift and increase in fluorescence intensity could be followed during resin cure. By comparing the fluorescence of grafted FLSCA with the dye dissolved in bulk resin, both chemical and mobility differences were detected in the interfacial region relative to the bulk resin. When dissolved in bulk resin, a red shift in emission was observed from the dye at the glass transition. A similar red shift was observed from the grafted dye, suggesting that the technique can probe the glass transition of the buried interfacial region. The apparent interfacial transition could be lower or higher than the bulk glass transition, depending on the initial structure of the silane coupling agent layer. The technique can potentially be combined with glass fiber optics to make a practical sensor for composite cure monitoring.

Flow Micro-calorimetry and Ftir Studies on the Adsorption of Saturated and Unsaturated Carboxylic Acids onto Metal Hydroxide Flame Retardant Fillers

Despite the fact that carboxylic acids (fatty acids) are by far the most widely used surface treatment systems for inorganic fillers, there is not always an equally strong understanding of how they interact with filler surfaces. A complex interplay exists between the method of treatment, molecular structure of the carboxylic acid, and the chemistry of the filler surface. During the ongoing unravelling of this interplay we studied the adsorption of isostearic acid and stearic acid and a range of C18 unsaturated fatty acids, together with acrylic acid, onto magnesium hydroxide and aluminium hydroxide. The thermal behaviour of adsorption/desorption together with salt solubility and solvent effects were investigated using flow micro-calorimetry (FMC); whilst the mode of adsorption in the dry state was studied using diffuse reflectance Fourier transform infrared spectroscopy on filler samples isolated form the FMC cell.

The most important findings thus far are: The charge of the metal ion in the hydroxide influences the solubility of acid salts arising from adsorption. The adsorption solvent also significantly effects the solubility of any salt, and the heat of adsorption. Increasing the number of double bonds in C18 unsaturated fatty acids (i.e., working from oleic through linoleic to linolenic acids), forces the orientation of adsorption to become progressively flatter in nature. Sequential adsorptions of acrylic acid and isostearic acid (and vice versa) have also been carried out and show that acrylic acid can be adsorbed through a layer of pre-adsorbed isostearic acid, but not the other way around.

Prospect of Nanoscale Interphase Evaluation to Predict Composites Properties

The local microstructure can be altered significantly by various fiber surface modifications, causing property differences between the interphase region and the bulk matrix. Since the specific properties of the interphase resulted from nucleation, thermal stresses, sizings used, interdiffusion, and roughness, the interphase influences the stress transfer between fiber and matrix, and thus, mechanical properties of composites.

It is a main topic of our work to clarify the effect of interphases on the mechanical properties. By using phase imaging and nanoindentation tests based on atomic force microscopy, a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin and glass fiber reinforced polypropylene matrix composites was conducted. It was found that phase imaging AFM is a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficient small indentation force was found to be sufficient for measuring actual interphase properties within 100 nm region close to the fibre surface. Subsequently, it also indicated a different gradient in the modulus across the interphase region due to different sizings.

The possibilities of controlling bond strength between fiber surface and polymer matrix are discussed in terms of elastic moduli of the interphases compared with micromechanical results (static and dynamic loading of model composites) and the mechanical properties of real composites.

32^nd and Chestnut Streets, Philadelphia, PA 19104-2875;

Bioactive Fiber/Polymer Interfacial Interactions Lead to Enhanced Bone Bonding

(Abstract not yet available)

Recent Developments on Interfacial Adhesion in Natural Fiber Reinforced Thermoplastic Composites: an Overview

The lofty goals set by US government for the creation of bio-based economy are challenging industry, government and agriculture. Auto makers now see strong promise in natural fiber composites. Car makers are looking increasingly at natural fiber reinforced thermoplastics to cut weight and cost in interior and engine components. Thermoplastics will surpass thermosets in the 21^st century because of recycling possibility and convenience of servicing of composite products. North American demand for natural fibers in plastic composites is forecast to grow 15 to 50% annually. Preliminary estimates place the North American market for natural fiber composites at 400 million lb as evidenced from a recent study by Kline & company, New Jersey. Besides auto-parts other emerging significant markets of bio-composites include building products, furniture and industrial/consumer applications.

Advantages of natural fibers over traditional reinforcing materials such as glass, carbon are: low cost, low density, high toughness, acceptable specific properties, ease of separation, carbon dioxide sequesterization and biodegradability. The main drawback of natural fibers is their hydrophilic nature that lowers their compatibility with hydrophobic polymer matrices. This leads not only to week interfacial adhesion between the fiber and the matrix but also results composites having high water absorption characteristics that reduce their utility in many applications. Most early work on the development of composite materials considered fiber-matrix adhesion to be a necessary condition to ensure good composite properties.

The various chemical surface modifications of natural fibers like dewaxing, alkali treatment, acetylation, cyanoethylation, bleaching, isocyanate treatment, vinyl grafting and treatment with coupling agent etc. are logical ways to improve fiber-matrix adhesion of the resulting bio-composites. The main attraction of bio-composites is the low cost of the natural fiber. Every efforts should be given to find economically viable low cost surface treatment of natural fiber to find bio-composites of commercial interest. The recent importance of well-known coupling agent (maleated polypropylene, MAPP) on performance of natural fiber-polypropylene bio-composites will be highlighted. The low cost alkali treatment and ammonia fiber explosion treatment have brilliant future to improve fiber-matrix adhesion in natural fiber composites. The treatment of natural fibers in an environmentally benign manner (through unique processing technique like heating the natural fibers in water at an optimum temperature in a pressure vessel with subsequent drying followed by heating again in the dry state) without the addition of chemicals produces reactive bio-mass and improves considerably the fiber-matrix adhesion in the resulting bio-composites. Hybrid bio-composites containing two different natural fibers like a bast fiber and a leaf fiber with optimum surface treatments can lead bio-composites of much commercial success.

Thermodynamic Characterization of Separation Phenomena at Silica/Polymer Interfaces

(Abstract not yet available)

Tethering of Polymer Chains to Filler/Fiber Surfaces to Strengthen Interfaces in Polymer Composites

The presence of polymer chains permanently tethered to the surface of a fiber or reinforcing filler can enhance the performance of a polymer composite by preventing debonding at the interface. However, the preparation of these durable interfaces is done by means of a tethering process that is dramatically influenced by the changing nature of the surface as more and more chains are tethered. By means of a unique method based on size exclusion chromatography, well-defined polymers with chain-end functional groups are monitored as they react with the surface of the reinforcement to become permanently tethered. After a rapid initial rate of tethering, the process slows down significantly. The number of chains tethered during the initial period is predictable, and is related to the molecular weight of the polymer. Our current research is focused on the conditions under which the slow part of the process can be exploited for preparation of more complex and versatile tethered layers.

Interfacial Characteristics of Wood/Polyolefin Composites

(Abstract not yet available)

Thin Composite Layers from Octamethylcyclotetrasiloxane-plasma Coated Paper

(Abstract not yet available)

The Role of Interfacial Interactions on the Mechanical and Viscoelastic Properties of Cellulose Fibre Reinforced Polymer Composites

(Abstract not yet available)

!) Under the Bridge Consulting, P.O. Box 1220, Corvallis, OR 97339-1220

2) Hewlett Packard Company, 1000 NE Circle Boulevard,Corvallis, OR 97330

Functionally Gradient Composite Interfaces

Materials are selected for their individual bulk properties. Unfortunately their surface properties are dissimilar, making it difficult to form a permanent adhesive interface. This necessitates the tailoring of the surfaces propertiesof the adherents and the adhesive. Functionalizing the surfaces of the components of a composite has received a great deal of research efforts. Surface treatment techniques include such as plasma, ion beam, corona, flame, and infrared treatments as well as coupling agents and compatibilizers. It is conceivable to create layers of functionalities, using one or a combination of surface treatment techniques. The onset of research in monolayers are proving this out, where several layers build an adhesive layer that began with two non-adhering surfaces and ends with a permanently bonded interface. It is conceivable that the interface then becomes stronger than the individual materials bulk properties This paper will discuss methods of enhancing the composite interface through functional gradient material techniques.

1) International Joint Lab between Institute of Photographic Chemistry and Max Planck Institute of Colloid and Interface Sciences, Chinese Academy of

Science, De Wai, Bei Sha Tan, Beijing 100101, CHINA

2) Max Planck Institute of Colloid and Interface Sciences, D-12489, Berlin, GERMANY

3) Institute of Technical Ecology, Donetsk, UKRAINE

Adsorption Kinetics of Phospholipids with Proteins at Different Aqueous Solution/fluid Interfaces

In the present work, the drop volume method was used to study the dynamics of the competitive adsorption of the zwitterionic phospholipids (DPPC, DPPE, DMPC and DMPE) mixed with proteins (b-Lactoglobulin, b-Casein, and Human Serum Albumin, respectively) at the chloroform/water interface. In order to investigate the main factors influencing the equilibrium interfacial tension of the mixed system (drops of lipid in chloroform formed in an aqueous protein solution environment), proteins of different conformation and concentration, and phospholipids of different structure have been investigated. It is observed that, with constant external protein concentration, the equilibrium interfacial tension g decreases with the increase of internal lipid concentration. When the phospholipid concentration is close to the CAC, both the conformation and concentration of the protein do not influence the equilibrium interfacial tension of the mixed systems remarkably. With the same internal phase containing phospholipid in oil solvent and different external phases containing the protein in water, the g ~ C isotherms show similar tendencies. Moreover, the structure of the phospholipid determines the equilibrium interfacial tension, where the lipid head group is much more significant rather than the chain length. The experimental results show that in DMPC - protein systems, the equilibrium interface tension g decreases with the phospholipid concentration more rapidly than in DMPE - protein systems.