INTERFACE/INTERPHASE ENGINEERING OF POLYMERS FOR ADHESION ENHANCEMENT: PART I. REVIEW OF MICROMECHANICAL ASPECTS OF POLYMER INTERFACE REINFORCEMENT THROUGH SURFACE GRAFTED MOLECULAR BRUSHES

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material.It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples.Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Interface/interphase engineering of polymers for adhesion enhancement: Part II. Theoretical and technological aspects of surface-engineered interphase-interface systems for adhesion enhancement

Part I of this paper reviewed the theoretical principles of the macromolecular design of polymer interface/interphase systems for obtaining maximum adhesion and fracture performance of adhesively bonded assemblies. In Part II a novel, relatively simple and industry-feasible technology for surface-grafting connector molecules is demonstrated and discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theory, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

CHARACTERIZATION, PROPERTIES, AND PROCESSING OF LARC PETI-5® AS A HIGH-TEMPERATURE SIZING MATERIAL: III. ADHESION ENHANCEMENT OF CARBON/BMI COMPOSITES

A phenylethynyl-terminated imide oligomer (LaRC PETI-5®) with a number average molecular weight of 2500 g/mol has been applied onto the surfaces of PAN-based carbon fiber tows and woven carbon fabrics as a sizing material to introduce an interphase between the fiber and matrix in carbon/BMI composites. The adhesion between the fiber and matrix was enhanced by the presence of a properly processed LaRC PETI-5® interphase. The results showed that when LaRC PETI-5® was sized and processed at 150°C, the interfacial shear strength (IFSS) of unidirectional IM7/BMI composite measured by using a microindentation technique and the interlaminar shear strength (ILSS) of a carbon/BMI composite measured by short beam shear test were markedly improved by about 35% and 66%, respectively, in comparison with the unsized counterparts. The adhesion enhancement strongly depends not only on the presence or absence of LaRC PETI-5® sizing interphase but also on the temperature profile applied to the sizing before composite fabrication. Both of these factors critically influence the physical and chemical state of the sizing material. Scanning electron microscopic observations of the composite fracture surfaces support the improved interfacial property of carbon/BMI composites.     

Interface/Interphase engineering of polymers for adhesion enhancement: Part I. Review of micromechanical aspects of polymer interface reinforcement through surface grafted molecular brushes

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material.It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples.Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

ADHESION ENHANCEMENT THROUGH CONTROL OF ACID-BASE INTERACTIONS

Adhesive bond strengths have been determined for lap-shear joints of PS/LLDPE and PS/CPE, a chlorinated version of polyethylene. Joints were formed at temperatures in the range of 180–280°C. In PS/LLDPE, bond strength at lower joining temperatures is compromised by the inability of LLDPE to act as electron acceptor to the donor properties of PS. However, at T?≥?260°C, PS becomes a fluid capable of interacting through dispersion forces only, leading to enhanced diffusion across the PS/LLDPE interface and much stronger adhesive bonds. An acid–base pairing is in effect in joints of PS/CPE, resulting in strong joints made at T?≤?240°C. The probable loss of acid-base interaction between the polymers at higher T, coupled with a failure of diffusion across the interface, leads to a lowering of the joint bond strength. Control over interfacial interactions is demonstrated to be a vital factor in the development of adhesive bonds.     

ADHESION ENHANCEMENT THROUGH CONTROL OF ACID-BASE INTERACTIONS

Adhesive bond strengths have been determined for lap-shear joints of PS/LLDPE and PS/CPE, a chlorinated version of polyethylene. Joints were formed at temperatures in the range of 180-280°C. In PS/LLDPE, bond strength at lower joining temperatures is compromised by the inability of LLDPE to act as electron acceptor to the donor properties of PS. However, at T ≥ 260°C, PS becomes a fluid capable of interacting through dispersion forces only, leading to enhanced diffusion across the PS/LLDPE interface and much stronger adhesive bonds. An acid-base pairing is in effect in joints of PS/CPE, resulting in strong joints made at T ≤ 240°C. The probable loss of acid-base interaction between the polymers at higher T, coupled with a failure of diffusion across the interface, leads to a lowering of the joint bond strength. Control over interfacial interactions is demonstrated to be a vital factor in the development of adhesive bonds.     

The Role of the Residual Stresses of the Epoxy-Aluminum Interphase on the Interfacial Fracture Toughness

When an epoxy-diamine system (DGEBA-IPDA) is applied onto aluminum alloy (5754) and cured, an interphase having chemical, physical, and mechanical properties quite different from those of the bulk polymer is created between the substrate and the part of the polymer having bulk properties. To get a better understanding of the role of the interphase on the interfacial fracture toughness either a tri-layer (bulk coating/interphase/substrate) or a bi-layer model (bulk coating/substrate) were used for quantitative determination of the critical strain energy release rate (noted G_c). Indeed, as the interphase formation results from both dissolution and diffusion phenomena, we were able to control the interphase formation within coated systems by controlling the liquid-solid contact time and then to make tri- or bi-layered systems. The particularity of models used is to consider residual stress profiles developed within the entire system leading to an intrinsic parameter representing the work of adhesion between the polymer and the metallic substrate. The aim of this publication is to clearly establish the role of the interphase mechanical properties, such as Young's modulus and residual stress on the interfacial fracture toughness. Results are presented and discussed for three different aluminum surface treatments (chemical etching, degreasing and anodizing).     

The Role of the Residual Stresses of the Epoxy-Aluminum Interphase on the Interfacial Fracture Toughness

When an epoxy-diamine system (DGEBA-IPDA) is applied onto aluminum alloy (5754) and cured, an interphase having chemical, physical, and mechanical properties quite different from those of the bulk polymer is created between the substrate and the part of the polymer having bulk properties. To get a better understanding of the role of the interphase on the interfacial fracture toughness either a tri-layer (bulk coating/interphase/substrate) or a bi-layer model (bulk coating/substrate) were used for quantitative determination of the critical strain energy release rate (noted G_c). Indeed, as the interphase formation results from both dissolution and diffusion phenomena, we were able to control the interphase formation within coated systems by controlling the liquid-solid contact time and then to make tri- or bi-layered systems. The particularity of models used is to consider residual stress profiles developed within the entire system leading to an intrinsic parameter representing the work of adhesion between the polymer and the metallic substrate. The aim of this publication is to clearly establish the role of the interphase mechanical properties, such as Young's modulus and residual stress on the interfacial fracture toughness. Results are presented and discussed for three different aluminum surface treatments (chemical etching, degreasing and anodizing).     

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.     

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.     

SELF-FORCING OF A CHEMOSTAT WITH SELF-SUSTAINED OSCILLATIONS FOR PRODUCTIVITY ENHANCEMENT

The effects of self-forcing simple limit-cycle type self-sustained oscillations (SSO) on a single chemostat with the aim to enhance its productivity was investigated using four different microbial systems with increasing complexities. It was demonstrated via simulation that for a chemostat with monod-type growth and with substrate-and-product-inhibited growth and product formation, it is possible to increase its productivity several fold by self-forcing the chemostat with SSO without incurring extra costs normally associated with external periodic forcing. Despite significant difference in the models used to simulate the chemostats, all four systems show a similar pattern of behavior in their productivity-vs.-mean residence time plots.     

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.     

界面相对碳纤维增韧碳化硅复合材料性能的影响

利用减压ＣＶＩ技术制备了三维碳纤维增韧碳化硅复合材料,研究了热解碳界面相对复合材料性能的影响。结果表明：适当选择界面相厚度,可获得弯曲强度和断裂韧性较高的碳纤维增韧碳化硅复合材料。     

STRATEGIES FOR SIZE REDUCTION OF MICROREACTORS BY HEAT TRANSFER ENHANCEMENT EFFECTS

One of the likely aims of reactor miniaturization in the field of chemical production and energy generation is to increase the conversion to the desired product and the selectivity of the process through better control of heat and mass transfer. In addition to the effects related to miniaturization, a further increase of the transfer coefficients is achieved by applying microstructuring techniques. In this context, three different approaches for heat transfer enhancement in miniaturized reaction systems are presented. The ideas put forward rely on entrance flow effects, inertial flows in meandering channels, and suppression of axial heat conduction. Among these ideas the entrance flow effect, realized by an arrangement of microfins with a typical dimension of a few hundred micrometers, provides the most efficient heat transfer. It is found that a heat transfer enhancement of at least one order of magnitude can be achieved compared to unstructured channels. On this basis, a miniaturized heat-exchanger reaction system is investigated, where a kinetic model of an endothermic, heterogeneously catalyzed gas-phase reaction is used. The miniaturized heat-exchanger reactor, both with and without heat transfer enhancement, is subsequently benchmarked against conventional fixed-bed technology. It is shown that, for the reaction system under study, a substantial reduction of the required amount of catalyst can be achieved in microsystems.     

Effect of the interphase properties on the fracture energy and fatigue behavior of thermoset resins containing spherical fillers

Interphase region in polymer based nanocomposites is a very thin layer that is created between the reinforcing phase and the matrix surface due to reaction forces between the nanoparticles and the matrix. The ability to determine the behavior of the interphase region can facilitate the understanding and prediction of the fracture toughness and fatigue behavior through multiscale modeling. In the present study, by using the fully analytical multiscale hierarchical modeling method, fracture toughness and also fatigue behavior of thermoset resins containing spherical fillers with consideration the influences of the main damage mechanisms and interphase properties (thickness and elastic modulus of the interphase region) were investigated. The novelty of this investigation is that it enables the application of a range of properties to the interphase zone and describes a technique for multiscale modeling based on this interphase zone. The present multiscale approach quantifies the dissipation energy due to main damage mechanisms at the nanoscale and accounts for the emergence of an interphase region as functionally graded (FG) properties surrounding nanofillers. Modeling of FG interphase power-varying properties, the derivation of governing equations, and the evaluation of the findings, all are parts of the achievements of this research. In addition, multiscale analyses have been carried out on fracture energy and fatigue behavior in various fiber volume fractions with and without interphase properties. It was found that the fracture toughness and fatigue behavior are significantly dependent on the interphase elastic properties and thickness. Furthermore, the critical debonding stress and the fracture energy were assessed with various interfacial fracture energy, elastic modulus, and thickness of interphase. Finally, the accuracy of the utilized multiscale approach with consideration of interphase properties was verified by comparing the modeling results with experimental tests on thermoset resins containing spherical fillers.     

MICROBIALLY FORMED CATALYSTS FOR ENHANCEMENT OF DIRECT COAL LIQUEFACTION

The present study examines the potential to biologically precipitate active catalytic compounds on the surface of coal particles in a water slurry. These compounds could be active forms of iron, an element abundant in coal, or active forms of molybdenum added as a soluble salt. The acidophilic microorganisms Thiobacillus ferrooxida/ns and Acidianus brierlyi were added to a 5% coal slurry at a pH of 3·0 or less. Experiments were carried out for 21 days in shake flasks/automated fermentor at constant temperature and pH. Deposition of iron and molybdenum compounds on the particle surface was confirmed by several spectroscopic methods. After bio-treatment, the coal was removed by filtration and dried. A sample was then liquified in a bomb at 385°C in tetralin and hydrogen for 15 minutes. In some runs the sulfiding agent dimethyl disulfide (DMDS) was added to the mixture. Separation of the products was by soxhlet extraction. Total conversion, gas + oil + asphaltenes + preasphaltenes, of the pretreated coal was compared with the conversion of raw, untreated coal. Statistically significant increases in conversion were achieved for various pre-treatment methods. The maximum conversion achieved was with coal pre-treatment in the presence of Acidianus brierleyi, and with the addition of 200 mg/1 of ammonium molybdate to the slurry. Under these conditions, liquefaction of 2 separate runs, in the presence of DMDS, produced conversion of 78 and 81%. This compared with 48% conversion for the raw coal under the same liquefaction conditions.     

PARTICLE ADHESION AT THE NANOSCALE

This article attempts to connect macroscopic observations of particle adhesion with the known interatomic forces which bind particulate interfaces together by studying contact between a plane surface and a sphere of smaller and smaller diameter. The fracture of a contact between a plane and a macroscopic sphere depends on the nonuniform stress distribution across the contact spot, causing atomic attraction at the edges of the contact region. Interface atoms some distance inside the contact region do not contribute to the adhesion. In fact, these inner atoms are in compression and are pushing the particles apart rather than causing adhesion. When a smaller sphere adheres to a plane at the nanoscale, this nonuniform stress distribution cannot be possible and the stress across the contact must be more even. To prove this hypothesis, molecular dynamics (MD) simulations have been carried out to study the fracture behaviour of subnano sodium chloride crystals. The MD models show clean fracture across the contact junction, in agreement with the macroscopic fracture studies. The models included explicit interatomic potentials to calculate the adhesion forces and contact stress distributions during particle pulloff as sodium chloride particles were altered in size. The results show that there is stress concentration at the contact edge for the smallest particles with 16 atoms (4?×?4) in contact.     

PARTICLE ADHESION AT THE NANOSCALE

This article attempts to connect macroscopic observations of particle adhesion with the known interatomic forces which bind particulate interfaces together by studying contact between a plane surface and a sphere of smaller and smaller diameter. The fracture of a contact between a plane and a macroscopic sphere depends on the nonuniform stress distribution across the contact spot, causing atomic attraction at the edges of the contact region. Interface atoms some distance inside the contact region do not contribute to the adhesion. In fact, these inner atoms are in compression and are pushing the particles apart rather than causing adhesion. When a smaller sphere adheres to a plane at the nanoscale, this nonuniform stress distribution cannot be possible and the stress across the contact must be more even. To prove this hypothesis, molecular dynamics (MD) simulations have been carried out to study the fracture behaviour of subnano sodium chloride crystals. The MD models show clean fracture across the contact junction, in agreement with the macroscopic fracture studies. The models included explicit interatomic potentials to calculate the adhesion forces and contact stress distributions during particle pulloff as sodium chloride particles were altered in size. The results show that there is stress concentration at the contact edge for the smallest particles with 16 atoms (4 × 4) in contact.