Surface properties of some styrene/vinyl pyridine diblock polymers adsorbed on rutile

Inverse gas chromatography (IGC) has been applied for the surface characterization of styrene-4-vinyl pyridine (S-4VP) diblock polymers of varying composition and of two rutile pigments. The latter were used as adsorbents for the polymers. Dispersion surface energies and acid-base interaction parameters were obtained from the IGC data. These show that the adsorbed layers form interphases where the local composition varies with the mass of adsorbed polymer and also with the acid-base interaction between rutile and the polymer. The results may be rationalized by suggesting that the more basic 4VP moiety preferentially orients to the acidic rutile surfaces, leaving surface compositions enriched in the less basic polystyrene (PS), which also has a lower surface energy. The effect was more pronounced when the strength of acid-base forces at the interface was increased. The experimental findings also permit the calculation of thicknesses for the adsorbed interphases. These were found to be in the range 30-90 nm, depending on the mass of adsorbed polymer and on its acid-base interaction with the adsorbing pigment surface.     

Surface characteristics of polymer/small molecule blends

Inverse gas chromatographic data have been obtained for polystyrene, polycarbonate, and two substituted amines used as additives in the polymers. Surface energies have been determined and evaluations made of acid/base interaction parameters and Flory–Huggins χ values for the surface bounded interphase. It was shown that acid/base considerations are implicated in the miscibility of these polymer/additive systems. Surface energy analyses showed that surface and bulk compositions in blends differed whether or not the blend components were miscible. Composition differences were the result of thermodynamic drives to minimize surface free energy. © 1994 John Wiley & Sons, Inc.     

Polycaprolactone coating with varying thicknesses for controlled corrosion of magnesium

Controlled corrosion of magnesium is critical for its clinical application to orthopedic devices. For this purpose, we coated the surface of Mg with a biodegradable polymer, polycaprolactone (PCL) and attempted to control the Mg corrosion with varied coating thicknesses in a reproducible manner. As we increased the coating thickness from 0 to 13.31 ± 0.36 μm, the volume of hydrogen gas and amount of Mg ions, the indicators of Mg corrosion, decreased by almost half from 0.57 mL/cm²/day and 0.55 mg/day to 0.20 mL/cm²/day and 0.26 mg/day, respectively. However, the elemental compositions on the surface revealed possible detachment of polymer coating and rapid water absorption at the early stage of corrosion for all coating thicknesses. Therefore, the lessons learned from this study suggest pre-treatment of the Mg surface for better polymer–metal adhesion, as well as preparation of the coating with lowered porosity as a stronger water-permeation barrier, to eventually allow precise control on Mg corrosion.     

Surface modification strategies for multicomponent polymer systems. III: Effect of diblock copolymer‐modified rutile on the morphology and mechanical properties of LLDPE/PVC blends

Rutile pigment was surface‐modified by the adsorption of various diblock copolymers and used as a component in two‐ and three‐component polymer blends involving the incompatible pair of linear, low‐density polyethylene (LLDPE) and poly(vinyl chloride) (PVC). Stress–strain analyses and electron microscopy show that the copolymer tethered to the rutile surface affects both mechanical and morphological properties of the blends. Inverse gas chromatography was used to evaluate dispersion surface energies and acid–base interaction parameters of the various solids. The mechanical and morphological characteristics of the blends can be rationalized by the concepts of acid–base and dispersion–force interaction. Of the copolymer modifiers used, the diblock based on polyisoprene and poly(4‐vinyl pyridine) (PIP‐P4VP) was best suited for use in LLDPE/PVC blends, ostensibly because of strong acid–base interaction between PVC and P4VP and mechanical interlocking between LLDPE and the PIP moiety. The properties of ternary blends were shown to be dependent on the method used for mixing the components. All mixing procedures used here resulted in time‐dependent variations of mechanical properties, suggesting that none gave rise to equilibrium morphology in the compounds. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1891–1901, 2001     

Effect of the core/shell latex particle interphase on the mechanical behavior of rubber-toughened poly(methyl methacrylate)

Two- and three-layer composite latex particles were used to prepare rubber-toughened poly(methyl methacrylate) (RT-PMMA). The interfacial thicknesses of the multi-layered particles were varied by using different emulsion polymerization synthesis techniques. The resulting interphases were previously characterized by ¹³C nuclear magnetic resonance techniques. The poly(divinyl benzene)/poly(butyl acrylate) (PDVB/PBA) interphase thickness was found to be in the range of 5–7 nm. It was also found that the PBA/PMMA interphase thickness could be varied from 5 to 7 nm (batch addition of MMA) to 15 to 17 nm (interphase compatibilized with PMMA macromonomer). The interphase thickness was expected to play an important role in the mechanical behavior of PMMA. The effect of the interphase of two- and three-layer particles on the tensile and fracture behavior of PMMA composites was evaluated. The fracture surfaces were examined by scanning electron microscopy. The two-layer PBA/PMMA particles with a thicker interphase (15–17 nm) exhibited higher K_IC values with the PMMA composites compared with PBA/PMMA particles with a thinner interphase (5–7 nm). The three-layer particles were found to be more effective in toughening PMMA compared with the two-layer particles. The differences in toughening behavior are speculated to arise from the morphological effects caused by a thicker interphase, which in turn results in better coverage by the PMMA shell and a more uniform distribution of the toughening particles in the PMMA matrix. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:581–593, 1997     

Prospect of nanoscale interphase evaluation to predict composite properties

For meeting the requirements of lightweight and improved mechanical properties, composites could be tailor-made for specific applications if the adhesion strength which plays a key role for improved properties can be predicted. The relationship between wettability and adhesion strength has been discussed. The microstructure of interphases and adhesion strength can be significantly altered by different surface modifications of the reinforcing fibers, since the specific properties of the interphase result from nucleation, thermal and/or intrinsic stresses, sizing used, interdiffusion, and roughness. The experimental results could not confirm a simple and direct correlation between wettability and adhesion strength for different model systems. The main objective of the work was to identify the interphases for different fiber/polymer matrix systems. By using phase imaging and nanoindentation tests based on atomic force microscopy (AFM), a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites was conducted. As model sizings for PP composites, γ-aminopropyltriethoxysilane (APS) and either polyurethane (PU) or polypropylene (PP) film former on glass fibers were investigated. The EP-matrix was combined with either unsized glass fibers or glass fibers treated with APS/PU sizing. It was found that phase imaging AFM was a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficiently small indentation force was found to be sufficient for measuring actual interphase properties within a 100-nm region close to the fiber 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 surface stiffness of sized glass fibers, micromechanical results, and the mechanical properties of real composites.     

Physical and chemical properties of polypyrrole–carbon fiber interphases formed by aqueous electrosynthesis

The properties of carbon fibers modified by aqueous electrochemical synthesis of pyrrole has been determined by using the dynamic contact angle analyzer (DCA), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Electrochemical process parameters such as the initial pyrrole concentration, electrolyte concentration, applied voltage, electrolyte type, and reaction time were systematically varied, and their impact on the polypyrrole–carbon fiber interphases surface free energy and morphology was ascertained. The surface free energies of the polypyrrole–carbon fiber interphases were obtained by using single fiber filaments. SEM analysis of the interphases revealed several distinct surface structures, including smooth, porous, granular, microspheroidal, and leafoidal morphologies. The noncoated but commercially surface oxidized carbon fibers have smooth surface morphology with occasional longitudinal striations. FTIR analysis of the polypyrrole interphases confirmed that the counterions derived from the electrolytes were incorporated into the film. The surface free energies of the electrochemically formed polypyrrole–carbon fiber interphases equivalent to 60–75 dynes/cm, was determined to be up to 40% higher than that for the surface oxidized but unsized carbon fibers equivalent to 50 dynes/cm. This improvement in the surface free energies of the polypyrrole–carbon fiber interphases suggests easy wettability by polymer matrices such as epoxy resin, γ ˜ 47 dynes/cm and, polyimide matrix, γ ˜ 45 dynes/cm. © 1996 John Wiley & Sons, Inc.     

Micro–macro characterisation of DGEBA-based epoxies as a preliminary to polymer interphase modelling

Reactive polymer adhesives in contact to substrates are known to form so-called interphases, a notion comprising the domain within which the polymer, compared to its bulk, exhibits structural inhomogeneities and gradients in material properties. Induced by the interface between substrate and polymer the formation of such interphases is usually ascribed to processes like segregation or phase separation of polymer components, selective adsorption, steric hindrance, orientation effects or curing shrinkage. Quantitative information on mechanical interphase properties is obtainable only by considerable efforts since interphases belong to the class of buried layers, i.e. they are located between bulk polymer and substrate, which impedes a majority of experimental techniques.Within this contribution, a two-component epoxy-based model polymer (DGEBA/DETA) is examined by methods on different scales and with respect to the effects that both the resin/hardener mixing ratio and the chemical structure of the hardener exert on the mechanical bulk properties. By means of these variations the above mentioned processes disturbing the polymer network formation in the vicinity of the substrate are emulated within the bulk. Macroscopic tension tests, nanoindentation and calorimetric methods (DSC) are applied to obtain relations between structural variations and material behaviour. Inversely identifying the governing parameters of suitable constitutive laws from experimental data will later conclude the first step towards a quantitative interphase model.It is demonstrated that modifications of the resin/hardener mixing ratio and the hardener formulation lead to variations in mechanical bulk properties which are quantitatively determinable by methods on different scales and do lie in ranges similar to those of property profiles that have been observed within interphases.In future work, the local mechanical behaviour of adhesive joints under load will then be investigated by a microscale videoextensometry. The resulting data will be compared to the structure–property relations from step one to conclude on the local polymer structure within the interphase.     

SiC/SiC minicomposites with structure-graded BN interphases

BN interphases in SiC/SiC minicomposites were produced by infiltration of fibre tows from BF₃–NH₃–H₂ gaseous system. During interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By TEM characterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The minicomposites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infiltration. The specimen lifetimes at 700°C under a constant tensile loading were measured in dry and moist air. Compared to a pyrocarbon reference interphase, the BN interphases significantly improve the oxidation resistance of the SiC/SiC minicomposites.     

Mechanical properties and microstructures of Cf/SiC-ZrC composites using T700SC carbon fibers as reinforcements

C_f/SiC-ZrC composites were fabricated through mold-pressing and polymer infiltration and pyrolysis (PIP) process using T700SC carbon fibers as reinforcements. The effects of interphases on the mechanical properties and microstructures of composites were studied. Composite showed brittle fracture behavior and low bending stress of 81 ± 24 MPa when no interphase was deposited on the fiber surface. With the deposition of a PyC/SiC interphase, composite showed typical non-brittle fracture behavior and the bending stress increased to 401 ± 64 MPa and a large amount of pulled-out fibers could be observed on the fracture surface. Meanwhile, it could also be concluded from the microstructures of the composites that the existed interphases had a great hindering effect on the infiltration of ZrC into the intra-bundle zones in the slurry infiltration process. The TEM analysis results showed that the carbon fibers were almost not eroded and the brittle fracture behavior may be mainly ascribed to the strong bonding between fibers and matrix.     

Effect of interfacial chemistry on crystallization of polypropylene/multiwall carbon nanotube nanocomposites

This study systematically investigates the polymer–carbon nanotube (CNT) interaction when the interphase is tailored. Maleic anhydride‐grafted‐polypropylene (MA‐g‐PP) or polypropylene (PP) was noncovalently coated onto acid functionalized multiwall nanotube (f‐MWNT) through solution mixing. These coated f‐MWNTs were melt microcompounded with neat PP to form PP/f‐MWNT nanocomposites. The effects of functional groups and the thin layer of solution processed polymers, namely, MA‐g‐PP or PP, at the PP/f‐MWNT interface on crystallization and on melting behavior of matrix PP were investigated. The results were compared with a pristine MWNT (p‐MWNT) incorporated system. It was shown that PP coated CNTs can serve as a strong nucleating agent for templated polymer crystal growth. Unlike other PP nanocomposites in the literature, a relatively high shift of 7°C in melting peak maximum (T_p), along with a sharp melt endotherm was achieved with the addition of 0.3 wt% f‐MWNT via PP/f‐MWNT master batch. This indicates refinement of matrix PP crystalline region due to the tailored f‐MWNT surface chemistry. With a designed self‐seeding and templated crystal growth approach, columnar crystalline interphases were found surrounding MWNT which melted at 10.5°C higher temperature than neat PP crystallized without undergoing the same heat treatment protocol. POLYM. ENG. SCI., 59:1570–1584 2019. © 2019 Society of Plastics Engineers     

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

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

Compositional variation within the epoxy/adherend interphase

At a polymer-solid interface, an inorganic adherend may influence the structure and/or chemistry of the polymer in the near-interface region. This region between the adherend and the homogeneous polymer is referred to as the interphase. While there is general agreement that interphases exist, extensive debate revolves around the characteristic length scale of the interphase and its composition and structure. The present study examines the size and composition of the epoxy-aluminum interphase using spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Aliphatic bis(p-aminocyclohexyl)methane (PACM20) curing agent and aromatic diglycidyl ether of bisphenol-A (DGEBA) epoxy resin were selected as a model system. Spatially resolved π - π ^*, carbon, and thickness profiles were measured using EELS in the epoxy region immediately adjacent to the interface between the bulk epoxy and the nanoporous oxide on the aluminum adherend surface. These profiles systematically show deviations in the spectral intensities characteristic of carbon and aromaticity over distances extending 90±15 nm from the oxide surface. Simulations of such data indicate that these fluctuations cannot be accounted for by variations in specimen thickness but rather result from changes in epoxy composition near the oxide surface. The results show that this epoxy- aluminum interphase is enriched in curing agent, as indicated by a gradual compositional change from 25 ± 5 vol% PACM20 in the bulk epoxy to 80 ± 15 vol% PACM20 at the epoxy/oxide interface. This chemical segregation may have important implications on the properties and performance of epoxy-aluminum adhesive joints.     

Bioactive composites with designed interphases based on hyperbranched macromers

A series of designed interphases was produced by grafting chemically modified poly (aryl ether sulfones) (PSF) onto bioactive glass (BAG) particles. Macromolecular architecture, polymer morphology, composition and crosslink density of these PSF hybrid interphases were studied with respect to influence on mechanical properties. The hybrid PSF interphases improved mechanical strength by 20% over conventional silane treatments and increased the overall energy to failure by nearly 100%. The structure of the interphase was modeled with a three‐phase viscoelastic model. The results demonstrated the ability to engineer an interphase having hyperstructures containing mobile species and inorganic functionalities that improve adhesion and favor energy release during fracture processes. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1153–1166, 2006     

Adhesion of rubber compounds to nano-copper-coated iron plates as studied by AES depth profiles

Three nano-copper-coated iron plates with different thicknesses of the copper coating from 29 to 56 nm were prepared by sputtering and their adhesion properties to rubber compounds were examined. At the optimum cure of rubber, the nano-copper-coated iron plate with thick copper coating (56 nm) showed the best adhesion to rubber compound, whereas the thin copper coating (29 nm) showed the best adhesion to rubber compound at overcure. This may be explained as follows: at optimum cure, diffusion of iron to the copper surface in the adhesion interphase is dominant, whereas copper sulfide formation in the adhesion interphase is dominant at overcure. This fact was confirmed by Auger electron spectroscopy (AES) depth profiles of adhesion interphases using Ar⁺ ion sputtering.     

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.     

Surface-Enhanced Raman Scattering As an In-Situ Probe of Polyimide/Silver Interphases

The molecular structure of interphases formed by chemically curing the polyamic acid of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) against meta-aminothiophenol (m-ATP)-primed silver substrates was determined using surface-enhanced Raman scattering (SERS) and reflection-absorption infrared spectroscopy (RAIR). It was found that m-ATP was adsorbed dissociatively onto silver substrates through the sulfur atoms. When polyamic acid was deposited onto silver substrates pretreated with m-ATP, acid groups of the polyamic acid combined with amino groups of m-ATP to form ammonium carboxylate salts near the interphase. SERS and RAIR results indicated that the structure of the interphase was significantly different from that of the bulk polymer. Chemical curing of the polyamic acids located in the interphase was suppressed because of the formation of ammonium carboxylate salts. However, the bulk of the polyamic acid films was highly cured to form polyimide. It was also found that more isoimide groups were formed when thin polyamic acid films were chemically cured in acetic anhydride/pyridine solutions than in acetic anhydride/triethylamine solutions.     

The influence of the interphase on composite properties: Poly(ethylene-co-acrylic acid) and poly(methyl vinyl ether-co-maleic anhydride) electrodeposited on graphite fibers

The electrodeposition of saturated copolymers onto carbon fibers is investigated, focusing particular attention on improvement of shear and impact properties of the corresponding composites. Carbon fibers are electrocoated with poly(ethylene-co-acrylic acid) and poly(methyl vinyl ether-co-maleic anhydride) from aqueous media, and fabricated into epoxy composites. The results of interlaminar shear strength (ILSS) tests, initially employed to assess fibermatrix adhesion, are vitiated by the occurrence of mixed-mode failure. Interfacial shear strength (IFSS) is hence evaluated by stressing single-fiber composite specimens to obtain ultimate aspect ratios of the fiber fragments. The data are combined with fiber strengths by a recently developed statistical theory (1) to yield a distribution for IFSS. Both copolymer interphases improve fiber-matrix bonding to an extent greater even than that obtained with commercial fiber surface treatment. Good fiber-matrix adhesion is further apparent from SEM studies of fractured ILSS test specimens. A key to this improved adhesion is the interpenetration of matrix resin and interphase polymer, revealed by electron microprobe analysis (2). Notched Izod impact strength is also increased over uncoated-fiber composites. These copolymer interphases behave as deformable interlayers, absorbing impact energy and blunting the growing crack tip. Further energy is absorbed in deflecting the crack through a more tortuous path. Simultaneous improvements in impact and shear strengths are thus obtained, which may be further enhanced by optimizing the electrodeposition parameters and the coating thickness. The influence of the interphase on composite properties is better understood from this study, paving the way for refinement in interphase design.     

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

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

Flow Instability in LLDPE Processing and Its Control by Fluoropolymer Additives

Flow instabilities during the capillary extrusion of an octene-LLDPE have been measured by signals from an elongational rheometer used to wind up the extruding polymer filaments. The presence of fluoropolymers at concentrations above 400 ppm suppressed or eliminated the instability signal, but only after several minutes of extrusion. The time required to suppress instability was used as an indicator of additive effectiveness. Fluoropolymers were found to increase in effectiveness with increasing degree of polarity, as measured by acid/base interaction indexes and by non-dispersion surface energies. The relative apparent melt viscosities of host and additive polymer also were involved in effectiveness ratings. It is suggested that fluoropolymer additives suppress sporadic adhesive failure of the matrix polymer by forming an interphase between the extruder (die) wall and the flowing bulk polymer.