Interphase Region Research Articles

Effect of mesophase pitch/polyacrylonitrile mixture as the core‐layer on the interphase region structure of coaxial polyacrylonitrile/mesophase pitch composite fibers

AbstractCoaxial composite fibers with a cortex of polyacrylonitrile (PAN) and a core layer of mesophase pitch (MP) and PAN have been prepared by the wet spinning method, formed in a mixed solution of N,N‐dimethylformamide and deionized water to optimize the composition of the core‐layer mixture. The XRD curves of the PAN/MP coaxial composite fibers only showed the crystalline and amorphous diffraction peaks of PAN. With increasing MP mass fraction in the mixed PAN/MP solution, the crystallinity of the coaxial composite fiber gradually decreased from 27.95% to 26.21%, because MP intermixed between the PAN macromolecules, weakening the interaction between the PAN molecular chains. In the core layer of the coaxial composite fibers, spherical MP particles were dispersed in the network structure of PAN. Different proportions of PAN/MP mixed solution will result in different core structure. The coaxial composite fiber consisted of a core layer with a low gray value, a transition layer with increasing gray value, and a cortex with a high gray value. From the core layer to the cortex, taking Group C as an example, the C content gradually decreased from 68.64% to 61.23% and the N content gradually increased from 13.68% to 22.19%, which was consistent with the gradual decrease of the MP content and the gradual increase of the PAN content. Gray‐value analysis, micro‐region EDS, and SEM showed that a transition layer structure formed at the junction of the cortex and core layer. The size and amplitude of the transition layer were related to the viscosity of the mixed PAN/MP solution, and high MP content in the mixed PAN/MP solution was conducive to formation of a stable and good interface structure.

A novel strategy to predict the tensile strength of polymer/particle interphase based on De Gennes's self‐similar carpet theory

AbstractThe formation of polymer/particle interphase region besides affecting most of the mechanical characteristics of polymer nanocomposites also alters some of their physical properties. The physical/mechanical properties inside the interphase may change based on a specific pattern (e.g., linear, exponential, etc.). De Gennes's self‐similar carpet theory represents a unique definition for the concentration variation of the adsorbed polymer chains. Accordingly, this specific pattern was developed to predict the tensile strength of the polymer/particle interphase region based on the molecular characteristics of the matrix and polymer/particle compatibility. In addition, Pukanszky's model was developed to define the characteristics of the aggregates/agglomerates using the results of De Gennes's theory. Different high‐density polyethylene (HDPE) nanocomposite samples, containing compatibilized silica nanoparticles, were prepared and subjected to the tensile and field emission scanning electron microscopy (FE‐SEM) tests. Moreover, other data from the literature were used to further investigate the accuracy of the proposed model. It was revealed that considering the gyration radius of the polymer chains as the maximum interphase thickness and the scaling parameter as 0.49 provides accurate predictions. On the other hand, the obtained data regarding the average content and size of nanoparticle aggregates/agglomerates also were in good agreement with the actual results.

Bending and shear improvements in 3D-printed core sandwich composites through modification of resin uptake in the skin/core interphase region

Fused deposition modeling (FDM) printed polymers are rarely used as a structural material due to anisotropic and low mechanical properties compared with conventional composites. In recent years, greater need has been expressed for recycling of materials, such as recyclable FDM, at the end of service life to reduce environmental pollution and manufacture cost. However, how the amount of resin uptake in the skin and skin/core interphase affects the bending and shear performance of the sandwich composites when replacing the low strength and ductile core (conventional core) with a high strength and brittle core (FDM printed PLA (polylactic acid) core) still remains unclear. A new manufacturing routine is needed to improve the incorporation of FDM printed polymers in composite structures. In this work, FDM printed PLA was used as core material and sandwiched between two unidirectional glass fiber reinforced polymer (GFRP) skins to form a sandwich composite by compression-molding (CM) process, which provides a good manufacturing strategy for skin/core interphase modification. The significance of the CM process is proved by investigating the effect of resin uptake on bending and in-plane/out-of-plane shear performances. Current first order shear deformation (FSDT) theory lacks a direct connection between the in-plane shear stress and out-of-shear stress in the core region of sandwich composites. With the help of DIC, a connection between the in-plane shear and the out-of-plane shear strain was built and in-plane shear properties can acquire through out-of-plane shear properties, hence reducing the redundancy of sample preparation or the need for simulation. A significant improvement was found compared with the optimized resin uptake (Optimized resin uptake range: 20.43%–22.86 wt%) 3D-printed PLA core sandwich composite and lowest performance sandwich composite (Improvement: in-plane shear strength (∼34%)/modulus (∼29%), out-of-plane shear strength (∼25%)/modulus (∼31%), specific peak bending load (∼19%)). Compared with balsa core sandwich composites, the 3D-printed cores are suitable for use in composite sandwich structures in many applications with a satisfactory strength-to-weight ratio.

Multi-axial elastic–viscoplastic curves of unidirectional carbon nanotube-polymer nanocomposites considering interphase and interface debonding using finite element modeling

This paper aims to predict the elastic-viscoplastic curves of unidirectional carbon nanotube (CNT)-reinforced polymer nanocomposites under the multi-axial tensions, including bi-axial transverse-transverse, bi-axial longitudinal-transverse and tri-axial longitudinal-transverse-transverse loadings by the use of finite element (FE) modeling. The role of the interphase between the CNT and polymer matrix as well as the debonding at the CNT/interphase interface in the stress-strain curves of nanocomposites is investigated. The polymer matrix and interphase region are characterized as elastic-viscoplastic materials, while the CNT behaves as the elastic material. The present results in uni-axial loading are compared to the available numerical outcomes for indicating the accuracy of the FE modeling. The simulation of multi-axial load conditions is based on the three-dimensional representative volume element (RVE) of the unidirectional CNT-polymer nanocomposite. The effect of interface strength on the elastic-viscoplastic stress-strain curves is examined under bi-axial and tri-axial loadings.

Investigation of non-equilibrium separation time on the partitioning of cephalexin in an aqueous two-phase system composed of glucose and acetonitrile

Abstract In order to commercialize aqueous two-phase systems (ATPSs), not only the equilibrium data is essential, but also the knowledge of separation mechanisms, kinetics, settling time, and operational conditions are needed. Mixing duration and settling time are the most critical factors affecting separation and biomolecule partitioning in terms of economic aspects. This research aimed to find the desired conditions for separating cephalexin in an ATPS consisting of acetonitrile, glucose, and water. Firstly, the evolution of the interphase region was observed. Hereafter, to examine the effect of time on the experimental tie-lines and partition coefficient in non-equilibrium states, the settling time was varied from 2 min to 24 h. In addition, centrifugation was applied to help the separation at different time intervals and rotational speeds. The results of tie-lines slope and partitioning coefficients showed that the system approaches equilibrium after 5 h. However, using the centrifuge separation at 4000 rpm improved the separation time to 45 min, reaching 80 % of the actual partition coefficient. It can be concluded that with an acceptable tolerance in the partition coefficient, a remarkably diminished settling time is available for economic productivity in industrial units.

Micromechanical mapping of polycrystalline cubic boron nitride composites by means of high-speed nanoindentation: Assessment of microstructural assemblage effects

Polycrystalline cubic boron nitrides (PcBNs) are composites widely used as cutting tool materials due to their exceptional high hardness and wear resistance. Investigation of their micromechanical properties is key for optimizing PcBN’s performance through microstructural design. Within this context, high-speed nanoindentation is proposed and implemented, for three different PcBN grades, to correlate microstructure with local mechanical properties. A total of 40,000 imprints were performed in each grade. The obtained mechanical maps and data sets are statistically treated following two deconvolution approaches: 1D and 2D Gaussian fitting. The use of high-speed nanoindentation is validated not only by the reliable assessment of the intrinsic mechanical properties of cBN particles, binder and interphase region, but also by the successful mirroring of microstructural assemblage within the mechanical maps attained. Comparison of the results determined from 1D and 2D gaussian representations are in satisfactory agreement. Nevertheless, some difficulties and disparity between them arises when involving fine-grained microstructures. • Mechanical property maps replicate the microstructure assemblage of PcBN grades. • 1D and 2D representations also depict multiphase material microstructure. • Nanoindentation is validated for assessing H and E of PcBN constitutive phases. • Good agreement is generally found between 1D and 2D statistical analysis results. • Demonstration of the presence of a third composite-like/interface-related phase.

An EBM based multi-stage mechanical model to predict the time-dependent creep behavior of semi-crystalline polymer nanocomposites

In this study, a multi-stage model is proposed to predict the creep behavior of semi-crystalline polymer nanocomposites considering the impact of nanoparticle aggregation/agglomeration, polymer/particle interphase region and self/induced crystallization phenomenon. A specific equivalent box model (EBM) was designed, based on excluded volume concept, to evaluate the different roles of dispersed or aggregated/agglomerated nanoparticle domains in the system. The tensile modulus of these domains was defined using mechanical and cohesive energy-based parameters in the melt-mixing stage. In the next stage, the physical/mechanical characteristics of the polymer/particle interphase were indicated using an analytical model and mechanical properties of the system. Another EBM was developed using standard linear viscoelastic (SLV) model components to study the time-dependent creep behavior of the matrix and polymer/particle interphase region. The impact of polymer/particle compatibility on the creep behavior of the samples was also experimentally evaluated by applying different amounts of compatibilized and un-compatible silica nanoparticles to the high-density polyethylene (HDPE) matrix. Different experimental tests were used to provide the required data or verify the obtained theoretical results. Both model predictions and experimental results showed that increasing the nanoparticle content and polymer/particle compatibility increases the resistance of the semi-crystalline nanocomposite system against the time-dependent creep.

An energy‐based strategy to predict the thickness and content of randomly oriented nanolayers aggregates/agglomerates in thermoplastic polymer nanocomposites: Experimental and analytical approaches

AbstractIn this study, the formation of clay nanolayer clusters inside the high‐density polyethylene (HDPE) matrix was evaluated using a specific strategy based on energy equilibrium in the melt mixing stage. Furthermore, the formed interphase region around nanoparticle domains was precisely evaluated considering the molecular structure of the matrix and linear variation of the physical/mechanical characteristics. The obtained results were used in a developed analytical model to predict the tensile modulus of the nanocomposite system containing randomly oriented nanoparticle domains. The domains consisted of exfoliated, intercalated, or aggregated/agglomerated nanolayers placed in different excluded volumes as the components of an equivalent box model. Different comparative studies were performed using the obtained data from different tests, applied to the prepared HDPE/clay nanocomposite samples, and other data from the literature. It was found that the confrontation between the exerted dispersion energy, by the mixer, and interparticle cohesion energy defined the thickness and physical/mechanical characteristics of the final aggregates/agglomerates in a nanocomposite. Though, the size and content of aggregates/agglomerates increased with the content of initially added nanoparticles. Consequently, the random orientation of each nanoparticle domain and formation of the polymer/particle interphase were proved to have determinative impacts on the mechanical properties of the system.

Characterization of elastic modulus at glass/fiber interphase using single fiber composite tensile tests and utilizing DIC and FEM

The bonding performance and formation of the interphase region between fiber and matrix have significant effects on the strength and durability of composites. A new approach, involving combined experimental and numerical analyses, has been developed to avoid the shortcomings and scattering associated with local experimental methods. For this purpose, tensile tests are performed on specimens fabricated from a single glass fiber and epoxy resin. The elongations for specified lengths are measured using the digital image correlation (DIC) technic. The size of the fiber diameter and specimen section are also measured from SEM images. The obtained experimental displacements for micro tensile tests are used in an inverse elastic finite element solution to obtain the interphase elastic modulus. It is shown that, considering the interphase thickness of 1.0 μm that is more realistic, the interphase elastic modulus is in the range of 12 ∼ 19 GPa. However, the performed sensitivity analysis shown that considering interphase thickness ranging from zero to 2.0 μm the interphase elastic modulus varies between 9 and 46 GPa. It is shown that the proposed procedure can be used to obtain the overall mechanical properties of the matrix/fiber interphase in long fiber composites.

Finite element modeling of the effective creep compliance of carbon nanotube-polymer nanocomposites: A critical microstructure-level investigation

A micromechanical model implemented with the finite element method (FEM) using the representative volume element (RVE) approach is developed to investigate the creep compliance behavior of carbon nanotube (CNT)-polymer nanocomposites. Effects of the CNT waviness, orientation, volume fraction and aspect ratio on the creep compliance of the nanocomposite are examined. Contribution of the interphase region formed due to the interaction between CNTs and matrix to the nanocomposite creep behavior is considered. The nanocomposite creep behavior is affiliate on the waviness and orientation of CNTs. Increasing the volume fraction and length of CNT and interphase thickness improves the nanocomposite creep performance.

Determination of mechanical properties of nanocomposites reinforced with spherical silica nanoparticles using experiments, micromechanical model and finite elements method

In this research, experimental, numerical, and micromechanical methods are used to determine the effects of silica nanoparticle content on the elastic modulus of polymer matrix nanocomposites in tension and compression. The direct mixing method was used to prepare experimental samples containing 0.1, 0.25, and 0.5 volume fractions of silica nanoparticles. The elastic modulus of the nanocomposites is evaluated by the Einstein, Guth, Mori-Tanaka, Halpin-Tsai, Kerner, and Ji et al. micromechanical models. In addition, the elastic modulus of the nanocomposites is evaluated using the RVE method in ABAQUS software. The effects of volume fraction and diameter of silica nanoparticles, thickness, and adhesion exponent of the interphase on the elastic modulus of polymeric nanocomposite are extensively examined. Stiffer elastic modulus behavior is found in the presence of interphase region. The results indicate that the elastic modulus of nanocomposites reinforced with spherical silica nanoparticles is improved with increasing nanoparticle volume fraction, decreasing the nanoparticle diameter, increasing the interphase thickness, and decreasing the interphase adhesion exponent. The validity of the results of numerical and micromechanical models has been checked with experimental results. It can be seen that the results obtained from the experiments with 1 hour of ultrasonic waves in the tensile samples with volume fractions of 0.1, 0.25, and 0.5, compared to the pure sample, have increased by 5.4, 10.3, and 9.7 percent, respectively. Similarly, for compression samples with the same volume fractions, the results have increased by 7, 13, and 11 percent, respectively.

Effect of interphase region on the Young's modulus of polymer nanocomposites reinforced with cellulose nanocrystals

Cellulose nanocrystals (CNCs) are rod-like and highly crystalline nanofillers reinforcing polymer matrices because of their high aspect ratio, high surface area, and great Young's modulus. However, there is no simple methodology estimating the Young's modulus of polymer CNC samples. In this study, Young's modulus of polymer composites containing CNCs is predicted using a simple model. This model is validated by some experimental results of various samples. Young's modulus calculations by this model agree with measured values. Moreover, the effects of various factors such as interphase thickness, volume fraction of interphase regions, densities of polymer matrix and CNC, and CNC size on the Young's modulus of samples are examined by the current model. Thinner CNCs significantly improve the Young's modulus of polymer nanocomposite (PNC). Furthermore, thicker interphase causes a higher Young's modulus. Among the mentioned parameters, CNC radius and interphase thickness have the most important effects on the nanocomposite modulus, while the volume fraction of interphase areas causes the least influence.

Identifying of the mechanism formation of a natural nanocomposite in polymer composite materials

The object of the study is the sealing elements of the packer. The fundamental principle of the cluster model is the assumption that the sum of the relative volume fractions of two structural elements of a natural nanocomposite (loosely packed matrix and nanoclusters) is equal to one – This approach is incorrect because it is violated when the nanoclusters are immersed in a loosely packed matrix. In this regard, a new relationship is proposed that relates the relative volume fractions of the loosely packed matrix and the part of nanoclusters immersed in it – Calculation formulas are obtained for the relative fractions of the loosely packed matrix and the interphase region. The paper shows that such an assumption is unfounded due to the indispensable immersion of clusters in a loosely packed matrix, and in this regard, a new relationship is proposed linking the relative volume fractions of the loosely packed matrix and the part of nanoclusters immersed in it. For a composite with a matrix, a mixture of synthetic butadiene nitrile and hydrogenated butadiene nitrile rubber and the addition of a copper nanoparticle, it is shown that in a natural nanocomposite, which is the polymer under study in an amorphous state, the time dependences of the relative volume fractions of the regions of inter-component adhesion and the loosely packed matrix coincide quite well with each other. The application of the above relationship between the volume fractions of a loosely packed matrix and nanoclusters allows to reformulate the known equations of parallel and sequential micromechanical models, as well as the Kerner equation for a more complex micromechanical model used to describe the effect of strengthening the elastic modulus of nanocomposites

Mechanistic origins of corrosion protection of aluminum alloys by graphene/polyetherimide nanocomposite coatings

Light-weighting vehicular components through adoption of light-metal structural alloys holds promise for reducing the fuel consumption of internal combustion engine vehicles and increasing the range of battery electric vehicles. However, the alloyed microstructure and surface precipitates of aluminum alloys render these materials susceptible to corrosion under modest excursions from neutral pH. Traditional chromium-based anodic passivation layers are subject to increasingly stringent environmental regulations, whereas options for sacrificial cathodic films are sparse for electropositive metals. While hybrid nanocomposite coatings have shown initial promise, mechanistic underpinnings remain poorly understood. Here, a fully imidized polyetherimide (PEI) resin is utilized as the continuous phase with inclusion of unfunctionalized exfoliated graphite (UFG). A comprehensive investigation of the mechanisms of corrosion protection reveals key fundamental design principles underpinning corrosion inhibition. First, strong interfacial adhesion, which for PEI is facilitated by binding of imide carbonyl moieties to Lewis acidic sites on Al surfaces. Second, the miscibility of ion-impervious nanoscopic UFG fillers and stabilization of a substantial interphase region at UFG/PEI boundaries that result in minimizing the free volume at the filler/polymer interface. Finally, extended tortuosity of ion diffusion pathways imbued by the below-percolation-threshold 2D fillers. These three design principles help govern and modulate ion transport from electrolyte/coating interfaces to the coating/metal interface and are crucial for the extended preservation of barrier properties. The results suggest an approach to systematically activate multiple modes of corrosion inhibition through rational design of hybrid nanocomposite coatings across hard-to-abate sectors where light metal alloys are likely to play an increasingly prominent role.

Influence of Curing Agents Molecular Structures on Interfacial Characteristics of Graphene/Epoxy Nanocomposites: A Molecular Dynamics Framework

AbstractThis paper presents a comprehensive molecular dynamics study on the effects of the stoichiometric ratio of epoxy:hardener, hardener's linear and cyclic structure, and number of aromatic rings on the interfacial characteristics of graphene/epoxy nanocomposite. The van der Waals gap and polymer peak density as a function of the type of the hardener is calculated by analyzing the local mass density profile. Additionally, steered molecular dynamics are used to conduct normal pull‐out of graphene to study the effect of the mentioned features of hardeners on the interfacial mechanical properties of nanocomposites, including traction force, separation distance, and distribution quality of reacted epoxide rings in the epoxy. Influence of the hardeners on the damage mechanism and its initiation point are also studied by analyzing the evolution of local mass density profile during the normal pull‐out simulation. It is seen that stoichiometric ratio and geometrical structure of the hardeners affect the interfacial strength. It is also revealed that the hardener type can change the epoxy damage initiation point. The damage occurs in the interphase region for a higher stoichiometric ratio or cyclic structure of hardener. In comparison, for hardener's lower stoichiometric ratio and non‐cyclic structure, failure begins in the epoxy near graphene layers.

Optimization of interfacial mixing for thermal transport along Si/Ge heterostructures: A molecular dynamics study

Interfaces can play critical roles in the dissipation of heat, generated during the electronic nanodevices operation. Therefore, engineering of the interfacial thermal resistance is a key factor in the thermal management design of nanoelectronics. In this research, the engineering of the interfacial thermal resistance of silicon/germanium heterostructure is explored, by varying the content of silicon or germanium atoms at the interphase (mixing region). For this goal, we conducted systematic non-equilibrium molecular dynamics simulations. For a specific interphase length, it is noticeably predicted that the interfacial thermal resistance versus the percentage of germanium atoms shows an optimal minimum value. By substituting germanium atoms with silicon atoms, but with preserving their atomic mass, it is interestingly shown that the interfacial thermal resistance increases significantly, although the optimal value for the interfacial thermal resistance is still observable. By altering the length of the interphase region and the samples’ temperature (300 and 600 K), the optimal interfacial thermal conductance in terms of the percentage of germanium atoms in the mixing region was also detectable. The findings of this work can be useful in the thermal management engineering of nanodevices, especially in nanoelectronics.

Molecular Dynamics Study on the Effect of Interfacial Cellullose Polymers in Strengthening the Stress Transfer Between Alumina Nanoparticles and Epoxy

AbstractCellulose polymers are widely used to fabricate green composites, implemented as fiber, matrix, and adhesive material between them. In this study, cellulose polymers are used as interphase material between spherical nanoparticles of hydroxylated alumina (Al2O3) and epoxy. Molecular dynamics simulations using the large‐scale atomic/molecular massively parallel simulator (LAMMPS) are utilized to investigate the effect of the cellulose content on the stiffness and tensile strength of epoxy/alumina nanocomposites. Polymer consistent forcefield (PCFF) and its supplement provided by MedeA software (PCFF+) define the interactions between the atoms. The simulation results indicate that incorporating cellulose into epoxy would increase the stiffness and strength properties considering that the cellulose polymers do not experience any agglomeration in the hosting matrix and have good interfacial adhesion with epoxy and alumina. This interfacial adhesion is needed since inserting cellulose polymers between alumina nanoparticles and epoxy would increase the porosity in the interphase region, limiting the stress transfer at the interphase and reducing the tensile strength of epoxy‐alumina nanocomposites.

Interphase characterization of glass/epoxy composite using peridynamic method and micro tensile test

ABSTRACT In this paper, a novel procedure is developed to characterize the interphase region using the nonlocal peridynamic method and micro tensile tests of glass/epoxy specimens with a single fiber of glass. For this purpose, micro tensile tests are performed by an accurate tensile test device and high-quality camera imaging for data sampling. The obtained displacements are analysed by the image processing method and used as target values in the peridynamic analyses. Multiscale mass points from macro to micro scale are generated to model the interphase region in single fiber specimens using the state-based peridynamic method. With the coupling of peridynamic and multivariate optimization codes, the program is automatically executed by the prediction of thickness and elastic modulus of interphase to achieve the desired displacements similar to micro tensile test results. In this innovative method, unlike nanoindentation tests, where the results are limited to a specific section or point, the interphase characterization gives an overall elastic modulus value along the examined length of the sample. The obtained results from the presented procedure show that the interphase elastic modulus is between 20.8 and 28.86 GPa and the interphase thickness is between 1.149 and 1.986 microns. The obtained results are in the range of nanoindentation tests results presented in the literature, and the difference could be due to the manufacturing conditions, epoxy properties, and quality of the fiber silane coatings.

Interphase elastic modulus characterization in glass/epoxy composite using combined peridynamics and experimental method

This paper introduces a novel combined technique to characterize the elastic modulus gradient for glass/epoxy interphase. The developed procedure is applicable to all fiber-reinforced polymer (FRP) composites. Interphase in FRP composites plays a vital role in the mechanical performance of the material. Scientists almost characterized this region using nanoscale test methods such as atomic force microscopy (AFM) and scratch tests. These local methods characterize the elastic properties at a specific region near the fiber and, usually, are utilized for all regions around the fiber. This paper proposes a combined peridynamic and single-fiber micro tensile test method to characterize the overall elastic modulus gradient at the interphase region. The advantages of peridynamic as a non-local method are used to model the multiscale regions of interphase, fiber, and matrix. Single-fiber micro tensile tests are performed by the authors, and variations of displacements and strains are measured by the digital image correlation method. The obtained displacements are used as target values in the developed peridynamic code, and by performing peridynamic analyses, the elastic modulus variation along the interphase is automatically extracted. Furthermore, the variations of stresses and strains are investigated at the specimens’ cross-section using different hypotheses for the interphase region.

Effect of the Interface/Interphase on the Water Ingress Properties of Joints with PBT-GF30 and Aluminum Substrates Using Silicone Adhesive

The aim of this work is to analyze the difference between silicone/composite and silicone/metal interphases, both in terms of water diffusion behavior and failure of the aged joints. For that, silicone joints with two different suhbstrates were prepared. The substrates were polybutylene terephthalate with 30% of short glass fiber (PBT-GF30) and 6082-T6 aluminum. It is assumed that the water uptake of the joints is equal to the water uptake of the substrate, adhesive, and interphase. Therefore, knowing the first three, the last could be isolated. To study the water diffusion behavior of the complete joint, rectangular joints were prepared, immersed in water and their water uptake was measured. The water immersion was conducted at 70 °C. It was concluded that the aluminum/silicone joints absorbed more water through the interphase region than the PBT-GF30/silicone joints, since the difference between the expected water uptake and the experimentally measured mass gain is significantly higher, causing adhesive failure of the joint. The same was not observed in the PBT-GF30/silicone, with a more stable interphase, that does not absorb measurable quantities of water and always exhibits cohesive failure.