Laser confocal microscopical characterization of toughened epoxy resins: Correlations between structural features and mechanical properties

Laser confocal microscopy is used to analyze the morphology of an epoxy resin (DGEBA) modified with different amounts of toughening agent carboxyl terminated butadiene acrylonitrile (CTBN). The size and phase volume of the distributed spherical toughening particles is ranging from 0.7 to 1.6 µm and 5 to 40 vol %, respectively. These morphological parameters and particles/µm² reveal a nonlinear relationship with the amount of toughening agent. With increasing particle size and number the glass transition temperature and the tensile modulus are decreasing, whereas the fracture toughness increases. Particles larger than 1.3 µm and a value of particles/µm² higher than 0.15 exhibit a more significant impact on the resin properties. Linear correlations between the rubber phase volume and the glass transition temperatures as well as the mechanical properties, i.e., tensile modulus and fracture toughness are ascertained. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46094.     

Morphological and local mechanical surface characterization of ballistic fibers via AFM

As‐received morphologies, defect structures, and contact moduli of Kevlar KM2 Plus and three other ballistic fibers varying in chemistry and processing, were observed and compared using atomic force microscopy (AFM) and instrumented nanoindentation (NI) techniques. Surface features and defects were defined and measured for each fiber chemistry: p‐phenylene terephthalamides (PPTA including KM2 Plus and Twaron), co‐polymer aramid (AuTx), and ultra high molecular weight polyethylene (UHMWPE including Dyneema). Although a multitude of surface defects were observed in each fiber, the types of defects were similar from one fiber type to another. It was found that surface defects generally map to a more compliant local modulus value. Contact modulus values were compared with NI elastic modulus values to demonstrate validity for the AFM technique. Challenges and limitations of the AFM technique for cataloging defects are discussed. This study is the first which attempts to outline the various morphologies found on several fiber surfaces. These local property studies will enable future comparisons with single filament and bulk fiber properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40880.     

Structural aspects of mechanical properties of iPP‐based composites. I. Composite iPP fibers with VGCF nanofiller

Nanocomposite fibers consisting of isotactic polypropylene (iPP) as a matrix filled with vapor grown carbon nanofibers (VGCF) have been prepared and their fine crystalline structure and mechanical properties characterized. The obtained results point out that the VGCF oriented along the fiber extrusion direction induce crystallization in the surrounding iPP matrix in a special way leading to the formation of oriented iPP α‐transcrystallite layers. The VGCF content and the draw ratio (DR) affect the textural properties of the composite material and lead to the formation of an anisotropic structure. The improvements of the mechanical properties of the composite fibers in both undrawn and drawn states are attributed to the VGCF aligning effect during extrusion, which produces highly oriented iPP crystalline structure, rather than to the reinforcing effect of the nanofibers. A new detailed scheme explaining the changes in tensile strength from the structural point of view is proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41865.     

Consolidation by spark plasma sintering of polyimide and polyetheretherketone

This article presents two high‐temperature thermoplastic powders which were sintered by spark plasma sintering in order to get homogeneous mechanical properties. Dense polyimide (PI) and polyetheretherketone (PEEK) specimens were obtained at temperatures as low as 320°C for PI and 200°C for PEEK, respectively. Relative densities higher than 99% were reached for both materials. In order to characterize their properties, in situ measurements with compression and hardness tests were carried out on sintered samples. This method allowed to obtain polymeric materials with improved mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40783.     

Thermoplastic‐modified epoxy resins cured with different functionalities amine mixtures: Morphology,thermal behavior,and mechanical properties

Epoxy matrices inherent brittleness and poor crack resistance make necessary some form of toughening. In this work, to improve their fracture toughness and ductility epoxy matrices were modified by changing its architecture and by the addition of a third component. The matrices architecture were modified by stoichiometrically reacting a bifunctional epoxy resin with different functionalities amine mixtures, one of which being a monoamine that plays the role of chain extender. In the modification by the addition of a third component, poly(methyl methacrylate) (PMMA) was selected as modifier. PMMA is initially miscible with epoxy/amine systems but can phase separate during curing. The kinetics and miscibility of these systems were studied previously. At constant curing conditions, materials from completely opaque (phase separated) to transparent (miscible) can be obtained with the increase in monoamine content. In this work, the effects of the modifier content and of the monoamine : diamine ratio in stoichiometric epoxy/amine mixtures on the resultant morphologies as well as on their thermal and mechanical properties was studied.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Morphology and mechanical properties of styrene–ethylene/butylene–styrene triblock copolymer/high‐density polyethylene composites

The morphology and mechanical properties of a styrene–ethylene/butylene–styrene triblock copolymer (SEBS) incorporated with high‐density polyethylene (HDPE) particles were investigated. The impact strength and tensile strength of the SEBS matrix obviously increased after the incorporation of the HDPE particles. The microstructure of the SEBS/HDPE blends was observed with scanning electron microscopy and polar optical microscopy, which illustrated that the SEBS/HDPE blends were phase‐separation systems. Dynamic mechanical thermal analysis was also employed to characterize the interaction between SEBS and HDPE. The relationship between the morphology and mechanical properties of the SEBS/HDPE blends was discussed, and the toughening mechanism of rigid organic particles was employed to explain the improvement in the mechanical properties of the SEBS/HDPE blends. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Phosphor settling induced mechanical degradation of silicone/phosphor composite in light emitting diode packages

Silicone/phosphor composite is a functional material used in light emitting diode (LED) packages. In this article, effect of phosphor settling on mechanical properties and microstructure of phosphor/silicone composite is investigated experimentally and numerically. Test samples of silicones with various degrees of phosphor settling were prepared and uniaxial tensile tests were conducted. The results indicate that, for specific volume fraction of phosphor, phosphor sedimentation tends to reduce the strength and elongation of overall composite. And with increasing degree of sedimentation, the weakening effect becomes more significant. The fractographs of the test samples indicate that cracks initiate around the bottom area where phosphor particles settle. Numerical investigations, which were conducted by random unit cell model with graded particle distribution, demonstrate that strain localization and stress concentration are significant where phosphor particles concentrate. It can be concluded that, to reduce mechanical degradation, phosphor sedimentation should be minimized in silicone/phosphor composite for LED packages. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42006.     

Toughness of ABS/PBT blends: The relationship between composition,morphology, and fracture behavior

A series of acrylonitrile–butadiene–styrene (ABS) copolymer/poly(butylene terephthalate) (PBT)/acrylonitrile‐styrene‐glycidyl methacrylate (ASG) blends with various compositions were prepared and characterized in this study. When the fraction of ABS exceeds a critical value there is a rapid increase in notched impact strength of ABS/PBT blends no matter whether the compatibilizer ASG is present. By combining morphology observation and notched impact results, we found that the ductile‐brittle transition of the blends is closely related to the morphology inversion. The notched impact strength jumps from 15.9 to 33.4 kJ/m² when phase inversion of ABS occurs at its fraction of 58 wt %. Accordingly, a possible toughening mechanism involved in the blends is proposed on the basis of a careful analysis of fracture energy, crack propagation behavior and fracture surface morphology. It is believed that the continuous ABS phase plays the critical role in toughening ABS/PBT blends. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46051.     

Evaluation of the chemical resistance of high‐density polyethylene geomembranes

The chemical resistance of high‐density polyethylene geomembranes (GMs) with smooth and textured surfaces in notched and unnotched forms at different pH values and in the range of 20–80°C was examined with stress crack resistance testing. Surface microcracks in GMs were observed in scanning electron microscopy images. Smooth and textured GMs did not show significant differences in their mechanical behaviors. The yield strength decreased with the temperature, pH, and exposure time. The yield strain increased with the temperature, but there were no good correlations with pH values. The break strength also decreased with the temperature and showed no significant correlation with pH variations. The break strain did not show a good correlation with the temperature and pH variations. The stress crack resistance was independent of pH variations but significantly depended on the temperature. It was negatively correlated with the exposure time. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structure and properties of thermoplastic poly(glycerol sebacate) elastomers originating from prepolymers with different molecular weights

Thermoplastic poly(glycerol sebacate) (TMPGS) elastomers originating from three prepolymers with different molecular weights were prepared first, and then the structure and properties were studied. Specifically, by swelling tests, gel permeation chromatography, X‐ray diffraction, and differential scanning calorimetry, the crosslinking densities, sol contents and compositions, crystallization, and thermal performances of three TMPGSs were examined. Finally, the degradability in a 37°C phosphate‐buffered saline solution (pH = 7.4) was also illuminated. The three TMPGSs had similar chemical structures, but the different molecular weights of the prepolymers influenced their final compositions and properties to a great extent. Furthermore, both hydrogen bonding and plasticization action in the elastomers played important roles in balancing the overall properties of the TMPGS elastomers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1131–1137, 2007     

Effects of size and shape originated synergism of carbon nano fillers on the electrical and mechanical properties of conductive polymer composites

In this study, microstructural features, mechanical properties, and electrical conductivity behaviors of thermoplastic composites prepared by using of cyclic olefin copolymer (COC) as matrix and various types of carbon nano materials, expanded graphite (EG), carbon nanofiber (CNF), and multi walled carbon nanotubes (CNT) as conductive fillers were investigated. Effects of using of double and triple filler combinations on the electrical properties of composites were also quantified in detail by measuring the bulk resistance of samples under alternating current with an impedance spectrometer. The electrical percolation values of fillers were found to be 20, 10, and 5 phr for the series of composites prepared with the EG, CNF, and CNT, respectively. It was obtained that the bulk resistances of percolated samples were dramatically decreased from 10¹⁴ ohm.cm to 10³?10⁴ ohm.cm. On the other hand, it was also found that the using of double and triple filler combinations provided much lower (about 10¹ ohm.cm) bulk resistance which corresponded to higher conductivity values than the highly filled composites including of 30 and 40 phr of EG. Based on the DMA measurements and the quantifying of elastic modulus values of composites in the rubbery region, it was found that the reinforcing effects of carbon nano fillers on the elastic modulus of composites decreased in the order of CNT>CNF>EG, depending on the aspect ratio (A_f) values of fillers into the matrix. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42313.     

Enhanced oil resistance and mechanical properties of nitrile butadiene rubber/lignin composites modified by epoxy resin

This is probably the first report on developing nitrile butadiene rubber (NBR) composites with enhanced performance s via lignin bridged epoxy resin in the rubber matrix. NBR/lignin masterbatch has been prepared through latex‐compounding method, and then epoxy resin (F51) was added in the NBR/lignin compounds by the melt compounding method. Lignin‐epoxy resin networks were synthesized in situ during the curing process of rubber compounds through epoxide?hydroxyl reactions. Compared with lignin filler, lignin‐F51 networks showed an improved oil resistance ability and led to increased mechanical properties, crosslinking density, and thermal stability of the rubber composites. This method provides a new insight into the fabrication of novel interpenetrating polymer networks in rubber composites and enlarges the potential applications of lignin in high performance rubber composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42922.     

Effect of PEKK oligomers sizing on the dynamic mechanical behavior of poly(ether ketone ketone)/carbon fiber composites

Poly(ether ketone ketone) (PEKK)/unidirectional carbon fiber (CF) composites have a poor interface. Accordingly, PEKK oligomer (PEKKo) sizing with a chemical compatibility with PEKK is proposed for promoting interfacial interactions in order to enhance mechanical performances. The thermal stability until 500 °C has been shown by thermogravimetric analysis (TGA). In order to compare static and dynamic sizing methods, “lab sizing” and “pilot sizing” were carried out. Scanning electron microscopy images of freeze fractures of PEKK/unsized CF, PEKK/PEKKo lab-sized CF and PEKK/PEKKo pilot-sized CF show that the PEKKo sizing causes an improvement of fiber/PEKK interactions, regardless of the sizing method. Indeed, in both cases, there is a continuity of matter at the interface while we observe a poor wetting of CF by matrix in PEKK/unsized CF. Dynamic mechanical relaxations in shear were analyzed as a function of temperature. The increase of storage modulus upon sizing is observed for both methods but it is more important for PEKKo pilot sizing. In the same way, the mechanical energy loss increases, it reflects the optimization of stress transfer between matrix and fibers. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48818.     

Dynamic mechanical properties of very thin adhesive joints

The effective mechanical properties of a polyurethane adhesive (oligoetherdiol, ‐triol, MDI) in gold joints (bond line thickness, d_P = 35–550 µm) are studied in the linear deformation range by dynamic mechanical analysis in shear mode. These properties depend on d_P: thin ones possess a higher dynamic glass transition temperature and show a narrower glass transition than the thick ones. The storage modulus rises with decreasing d_P for the rubbery plateau. The results attest mechanical interphases in the polyurethane with increased crosslink density and reduced cooperative mobility than in bulk. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42058.     

Study on microfracture mechanism of short glass fiber reinforced polycarbonate by using acoustic emission

The influence of the difference in wettability between glass fiber (GF) and polycarbonate (PC) on the microfractures of GF reinforced PC was investigated by using an acoustic emission (AE) method. In the case of well‐coupled GF‐reinforced PC, it is suggested that in the AE amplitude region higher than about 16 mV, microfracture related to scission of polymer chains occurs at the interfacial layer between GF and PC. On the other hand, in the case of poorly‐coupled GF‐reinforced PC under stress, debonding and interfacial slippage between GF and PC occurred below the yield stress of PC, whereas interfacial fracture and GF breakage occurred above the yield stress. Debonding and interfacial slippage between GF and the PC matrix were closely related to an AE amplitude smaller than about 16 mV. The relationship between stress and AE events is expressed in this case by the Eyring model. The activation energy of interfacial slippage between GF and PC was about 74 kJ/mol, which corresponds to the energy of chain‐backbone motion of PC in the glassy state. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45664.     

Compatible blends of polypropylene with an amorphous polyamide

Compatible polymer blends of polypropylene (PP) with an amorphous polyamide (aPA) were obtained through reactive compatibilization by adding 20% maleic anhydride‐modified copolymer (PP‐g‐MA) to the blends. The blends were made up of a pure PP phase and an aPA‐rich phase where very small amounts of PP were detected. The dispersed phase particle size decreased considerably indicating that compatibilization occurred. Young's modulus of the compatibilized blends increased with respect to that of the uncompatibilized ones. The compatibilized blends were highly ductile, and the impact strength also improved, proving that compatibilization occurred under a broad range of experimental conditions. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013     

Structure‐mechanical properties relationship of poly(ethylene terephthalate) fibers

The correlation between the fiber structure and mechanical properties of two different poly(ethylene terephthalate) fiber types, that is, wool and cotton types produced by three producers, was studied. Fiber structure was determined using different analytical methods. Significant differences in the suprastructure of both types of conventional textile fibers were observed, although some slight variations in the structure existed between those fibers of the same type provided by different producers. A better‐developed crystalline structure composed of bigger, more perfect, and more axially oriented crystallites was characterized for the cotton types of PET fibers. Crystallinity is higher, long periods are longer, and amorphous domains inside the long period cover bigger parts in this fiber type in comparison with the wool types of fibers. In addition, amorphous and average molecular orientation is higher. The better mechanical properties of cotton PET fiber types, as demonstrated by a higher breaking tenacity and modulus accompanied by a lower breaking elongation, are due to the observed structural characteristics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3383–3389, 2003     

Synthesis and characterization of nanosilica/waterborne polyurethane end‐capped by alkoxysilane via a sol‐gel process

A novel method was used to synthesis nanosilica/waterborne polyurethane (WPU) hybrids by in situ hydrolysis and condensation of tetraethyl orthosilicate (TEOS) and/or 3‐aminopropyltriethoxylsilane bonding at the end of the WPU molecular chain. The hybrid was characterized by scanning electron microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy, Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS). The results showed that the nanosilica/WPU hybrids with well‐dispersed nanosilica particles were synthesized, in which the particles had typical diameters of about 50 nm. In addition, XPS and FTIR analyses demonstrated that chemical interaction occurred between WPU and silica. The effects of TEOS on surface wettability, water resistance, mechanical strength, and thermal properties of the hybrid were also evaluated by contact angle measurements, water absorption tests, mechanical tests, and differential scanning calorimetry, respectively. An increase in advancing contact angles, water resistance, and tensile strength, as well as decrease in elongation at break and glass transition temperature, were obtained with the addition of TEOS. Water absorption decreased from 17.3 to 5.5%. The tensile strength increased to a maximum of 29.7 MPa, an increase of about 34%. Elongations at break of the hybrids decreased 191%. These results were attributed to the effects of the nanosilica and the chemical interaction between WPU and silica. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of microcapsule shell material on the mechanical behavior of epoxy composites for self‐healing applications

In this article, we have studied the effect of microcapsule shell material on the mechanical behavior of self‐healing epoxy composites. Liquid epoxy healant was encapsulated in melamine‐formaldehyde (MF) and urea‐formaldehyde (UF), using emulsion polymerization technique to prepare microcapsules of different shell walls. The core content of the microcapsules, as determined by solvent extraction technique was found to be 65 ± 4%, irrespective of the shell wall of microcapsule. Morphological investigations reveal a rough texture of the spherical microcapsules, which was attributed to the presence of protruding polymer nanoparticles on the surface. Epoxy composites containing UF and MF microcapsules (3–15% w/w) were prepared by room temperature curing and their mechanical behaviour was studied under both quasi‐static and dynamic loadings. The tensile strength, modulus, and impact resistance of the matrix was found to decrease with increasing amount of microcapsule in the formulation, irrespective of the shell wall material used for encapsulation. Interestingly, substantial improvement in the fracture toughness of the base resin was observed. Morphological investigations on the cracked surface revealed features like crack pinning, crack bowing, microcracking and crack path deflection, which were used to explain the toughened nature of microcapsule containing epoxy composites. Our studies clearly indicate that the microcapsule shell wall material does not play any significant role in defining the mechanical properties of the composites. In addition, presence of secondary amine functionalities in UF and MF shell wall do not interfere with the reaction of epoxy with triethylene tetramine hardener during the curing process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40572.