A dynamic mechanical study on unidirectional carbon fiber-reinforced polypropylene composites

Fiber-reinforced polymer composites show high specific strength and stiffness. The alignment of reinforcing fibers results in anisotropy of the material. This anisotropic behavior has been studied through dynamic mechanical analysis of unidirectional carbon fiber-reinforced polypropylene (CFRPP) composites measured in both parallel and transverse directions to fiber arrangement. Several parameters such as storage modulus (E′), loss modulus (E″), loss factor or damping factor (tan δ), and complex viscosity (MU^*) have been determined over a wide range of frequencies and at a fixed temperature. Relaxation and retardation spectra have been constructed for these composites. Modulus enhancement occurs due to stiffness imparted by the fiber and efficient stress transfer at the interface. Relaxation of the polymer matrix ceases with increase in the volume fraction of the fibers. α′-relaxation is observed for the composite having a 13% volume fraction of fibers and is ascribed to relaxation in the crystalline phase where the additional crystallinity arises out of the transcrystalline growth at the fiber–matrix interface. There exists a good correlation between theroretical curves with the experimental ones. Relaxation and retardation spectra and the dynamic parameters determined for these composites show a good correlation with the volume fraction of fibers as well as the direction of the applied load. © 1994 John Wiley & Sons, Inc.     

Dynamic mechanical analysis of the effect of water on glass bead–epoxy composites

Dynamic mechanical analysis (DMA) has been used to investigate the effect of water and glass bead surface treatment on the properties of glass bead–epoxy composites. By treating or not treating the glass beads with a silane coupling agent, we fabricated composites with ostensibly good or poor interfacial adhesion. SEM images of fracture surfaces and water uptake data confirmed this picture. We used dynamic mechanical tests to measure the material properties of dry and wet specimens. Temperature sweep tests of atmosphere-conditioned specimens indicated that the value of the loss tangent at the temperature of the α-α-relaxation peak was most sensitive to interfacial adhesion. For wet specimens, the magnitude of an additional relaxation process, denoted as the ω-relaxation, correlated strongly with water uptake and, indirectly, interfacial adhesion. Master curves constructed from frequency sweep tests also manifested differences among dry and wet specimens, but shift factor data suggested that these tests were more prone to complications due to water loss. Apparent activation energies of α- and β-relaxation processes were statistically significant indicators of interfacial adhesion in dry and wet composites, respectively. © 1996 John Wiley & Sons, Inc.     

On the dynamic mechanical behavior of poly(methyl methacrylate) reinforced with Kevlar fibers

The dynamic mechanical relaxation behavior of poly(methyl methacrylate) reinforced by continuous parallel Kevlar-49 fibers is investigated here on samples with several fiber volume fractions. The shifts in the temperature of the main relaxation of the matrix are interpreted according to free volume considerations and with the help of a thermomechanical block model. The dependence of the storage and loss moduli both on temperature and fiber content found experimentally can be reproduced by the model. It is not necessary to rely on the existence of an interphase to account for the modifications evidenced by the spectrum of the matrix, which can be explained on the basis of the two phase model.     

Interfacial,dynamic mechanical,and thermal fiber reinforced behavior of MAPE treated sisal fiber reinforced HDPE composites

The present article summarizes an experimental study on the mechanical and dynamic mechanical behavior of sisal fiber reinforced HDPE composites. Variations in mechanical strength, storage modulus (E′), loss modulus (E″), and damping parameter (tan δ) with the addition of fibers and coupling agents were investigated. It was observed that the tensile, flexural, and impact strengths increased with the increase in fiber loading up to 30%, above which there was a significant deterioration in the mechanical strength. Further, the composites treated with MAPE showed improved properties in comparison with the untreated composites. Dynamic mechanical analysis data also showed an increase in the storage modulus of the treated composites The tan δ spectra presented a strong influence of fiber content and coupling agent on the α and γ relaxation process of HDPE. The thermal behavior of the composites was evaluated from TGA/DTG thermograms. The fiber–matrix morphology in the treated composites was confirmed by SEM analysis of the tensile fractured specimens. FTIR spectra of the treated and untreated composites were also studied, to ascertain the existence of type of interfacial bonds. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3306–3315, 2006     

Glass transition behaviour of poly(ether ether ketone)/poly(aryl ether sulphone) blends: dynamic mechanical and dielectric relaxation studies

Blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) have been prepared in the whole composition range. The molecular dynamics and α-relaxation behaviour of these materials have been studied using dynamic mechanical and dielectric relaxation spectroscopy. From dynamic mechanical relaxation studies, two α-relaxation peaks corresponding to the segmental relaxation process of pure components in the blend was observed. Also, it was found that the temperature at which α-process of the homopolymers occurs, shows a slight change with blend composition, corresponding to a PEEK-rich and PES-rich phase. The relaxation intensities of the homopolymers in the blend compared to that in pure state were approximately proportional to their respective content in the blend. From the phase composition of the respective phases obtained using Fox equation, it has been inferred that PEEK dissolves more in PES than vice-versa. The α-relaxation of PES could not be detected from dielectric relaxation spectroscopy because of the possible influence of dc conduction and electrode polarization losses. Otherwise, the α-relaxation behaviour of PEEK-rich phase observed from dielectric relaxation studies agree with those inferred from dynamic mechanical relaxation studies. Furthermore, activation energies for molecular motions (E_a) at the α-relaxation have also been determined using an Arrhenius form of equation and it has been found that E_a for both PEEK-rich and PES-rich phase show variation with composition. Similarly, the relaxation times associated with the mobility of relaxing species in both PEEK and PES are influenced in the blends. It is likely that these observations are related to some interactions and a partial segmental mixing between the blend components, which result in changes in the local molecular environment on blending.     

Investigation of composite interphase using dynamic mechanical analysis: Artifacts and reality

The fiber-matrix interface is an important factor determining the overall mechanical properties of composites. This interface is no longer regarded as a sharp boundary, but is now considered to be an „interphase,”︁ i.e., a region surrounding the fiber where properties differ from those of the bulk matrix. Although the concept of the interphase is rapidly gaining acceptance, its in situ detection and characterization remains a largely unsolved problem. Dynamic mechanical analysis (DMA) is a technique known for its sensitivity in the detection of inhomogeneity in polymer morphology. Recent publications have claimed that the interphase in unidirectional fiber-reinforced composites is detectable using DMA. We have been evaluating a Du Pont 982 DMA on its uses in characterization of composites and their individual components. We have found that using heating rates higher than 2°C/min produces an artificial peak in the DMA loss spectrum of glass-fiberreinforced epoxy composites at temperatures above the matrix glass-transition temperature (T_g). This peak was not present in the data from the unreinforced matrix nor in data from carbon-fiber-reinforced samples. This artifact could be interpreted as evidence of an interphase. However, our investigations revealed that it is in fact due to a complex interaction of the instrument, the thermal conductivity of the sample, the heating rate, and the sample modulus above T_g. Despite this artifact, the high sensitivity of the Du Pont 982 DMA enables detection of inhomogeneities in the composite matrix that are attributable to an interphase.     

Rheology and porosity control of poly(2-hydroxyethyl methacrylate) hydrogels

Swelling and dynamic mechanical behavior in a broad frequency range of homogeneous and porous poly(2-hydroxyethyl methacrylate) gels were studied in relation to their matrix structure, gel morphology and nature of the swelling medium. Very lightly cross-linked gels showed up relatively large mechanical losses caused by slow relaxation of hydrophobic physical associates still proceeding at frequency 10^?3 Hz. The macroporous gels of fused-spheres type morphology contained many dangling aggregates not bearing the stress which caused concentration of the stresses in the load bearing paths through the matrix. This is the reason for the sudden drop of storage modulus. In the α-relaxation region, the loss factors for homogeneous and porous gels merged into a single master curve. Reswelling of gels in a poor solvent caused a decrease of the swelling degree and shifted their α-relaxation to lower frequencies; reswelling in good solvent shifted the maximum of α-relaxation to higher frequencies.     

Alpha Relaxation Study of Poly(Vinyl Chloride Co-Vinylacetate-Co-2-Hydroxypropyl Acrylate)

By using thermally stimulated depolarization current (TSDC) technique, and coupling it with the thermal sampling (TS) method, the relaxation behavior of Poly(vinyl chloride-co-vinylacetate-co-2-hydroxypropyl acrylate) (PVVH) has been investigated in the vicinity of its glass transition temperature, T_g. The global TSDC spectra of amorphous PVVH with varying the poling field and poling temperature revealed the presence of two TSDC peaks. The first peak at about 347 K and is ascribed to the glass transition temperature and can be referred as α-relaxation. The second one is obtained in the temperature range 353–383 K and is attributed to the space charge relaxation and can be referred as ρ-relaxation. Fine structure of α-relaxation peak was obtained using the thermal sampling method. All the molecular parameters, such as activation energy (E_a) and preexponential factor (τ₀), have been estimated. In addition, the compensation law was found to be valid; and the compensation parameters such as compensation temperature (T_c) and compensation time (τ_c) have been determined.     

The interpretation of dynamic mechanical behavior in ultra high modulus polymers

Recent studies of the dynamic mechanical behavior of ultra high modulus polyethylene? are discussed in the context of our present understanding of the structure of these materials. In particular, the Takayanagi model is shown to achieve a new status in the light of direct measurements of crystal continuity from wide angle X-ray diffraction data. It is further shown that the Takayanagi model in one formulation is compatible with the Cox model for a short fiber reinforced composite. The fiber composite model offers a simple physical understanding of the fall in modulus due to the α-relaxation in terms of shear lag. This reduces the effectiveness of the continuous crystal fraction postulated in the Takayanagi model. The γ-relaxation is considered to be associated primarily with an amorphous relaxation, consistent with the conclusions of previous workers for materials of lower draw ratio.     

Effect of a Boron Nitride Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites

Typically, the debonding and sliding interface enabling fiber pullout for SiC-fiber-reinforced SiC-matrix composites with BN-based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress-rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner-rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress-state at the BN/matrix interface of melt-infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.     

Influence of structural parameters on the dynamic mechanical properties of polyacetals

The dynamic mechanical behaviour of various polyacetals was determined by means of torsion pendulum measurements. The products used had different structural parameters such as molecular weight distribution or comonomer nature and content, respectively; thus it was possible to investigate the influence of these parameters on the temperature dependence of the storage modulus G′, which is a measure for the rigidity of the material, and the loss tangent tan δ from -100 to 160°C. The storage moduli G′ are not influenced by different molecular weight distributions, but correlate with the crystallinity in the samples, which is, beside others, dependent on the concentration of the incorporated comonomer components. A decreasing crystallinity - i.e. an increasing comonomer content - is combined with a loss in rigidity. The distance between the peak location of the α-relaxation in the loss tangent curves and the melting points (differential scanning calorimetry) of the investigated acetal resins is nearly constant; thus the molecular motions causing the β-process take place in the crystalline regions. The α-process (around ?7°C) arises predominantly from molecular motions of the comonomer units, whereas the intensity of this relaxation can be correlated with the mobility of these units. The peak of the γ-relaxation is located around ?67°C and does not hardly shift despite of different structural parameters in the sample, whereas its intensity decreases with increasing both comonomer contents and contents of low-molecular-weight components. For unimodally distributed polyacetals with different comonomer contents, the intensity of the β-relaxation increases with increasing comonomer contents; simultaneously the intensities of the α- and γ-relaxatìon decrease.     

The effect of the interface/interphase on fiber composite properties

The interfacial region between fibers and matrix in fiber composites governs the transfer of forces between the relatively weak and compliant matrix and the reinforcing fibers. An effective interphase can ensure that the mechanical properties of the composite reflect the high strength and modulus of the fibers. Although composites can be made with the expected strengths and moduli, it is not entirely clear why this is achieved: Tests with critical composites, i.e., those containing very short aligned fibers, do not show the expected stress-strain behavior. This paper examines the effect of an interphase having a shear modulus that is less than that of the matrix. It is found that to explain the Young's moduli of the short fiber composites, the interphase must have a very low modulus indeed; i.e., a few kPa at most. In addition, the strength results can be accounted for only if we assume that the short lengths of fiber used in the experiments had higher strengths than anticipated. Although agreement between experiments and theory is thus not very good, the small amount of experimental evidence available indicates a need for further systematic experiments on critical (i.e. short aligned fiber) composites before firm conclusions are drawn.     

Effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures

Interphase between the fibers and matrix plays a key role on the properties of fiber reinforced composites. In this work, the effect of interphase on mechanical properties and microstructures of 3D C_f/SiBCN composites at elevated temperatures was investigated. When PyC interphase is used, flexural strength and elastic modulus of the C_f/SiBCN composites decrease seriously at 1600°C (92 ± 15 MPa, 12 ± 2 GPa), compared with the properties at room temperature (371 ± 31 MPa, 31 ± 2 GPa). While, the flexural strength and elastic modulus of C_f/SiBCN composites with PyC/SiC multilayered interphase at 1600°C are as high as 330 ± 7 MPa and 30 ± 2 GPa, respectively, which are 97% and 73% of the values at room temperature (341 ± 20 MPa, 41 ± 2 GPa). To clarify the effect mechanism of the interphase on mechanical properties of the C_f/SiBCN composites at elevated temperature, interfacial bonding strength (IFBS) and microstructures of the composites were investigated in detail. It reveals that the PyC/SiC multilayered interphase can retard the SiBCN matrix degradation at elevated temperature, leading to the high strength retention of the composites at 1600°C.     

Dynamic properties of thermoplastic butadiene–styrene (SBS) and oxidized short carbon fiber composite materials

In composites consisting of a thermoplastic butadiene–styrene (SBS) elastomer matrix reinforced with oxidized short carbon fiber, scanning electron microscopy (SEM) reveals the existence of matrix–fiber interactions, which are not detected when employing commercial carbon fiber. Interpretation of the dynamic properties and other parameters, such as equivalent interfacial thickness, and glass transition temperature, measured in terms of maximum damping temperature, as well as the apparent activation energy of the relaxation process, helps to explain the existence of such interactions. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1819–1826, 1998     

Dielectric and dynamic mechanical measurements of poly(4-methyl pentene-1)

The α-relaxation process of poly(4-methyl pentene-1) was studied by dielectric and dynamic mechanical means. The complex dielectric constant was determined at nine discrete frequencies from 100 to 10,000 Hz and over a temperature range of ?50–90°C. The complex dynamic mechanical Young's modulus was determined over the audiofrequency range of 10–22,000 Hz and a temperature range of 21–76°C, from which a master curve was constructed. The relaxation process was studied by comparing the activation energies and width of the dispersion curves. The results of a logarithmic frequency vs. reciprocal temperature plot of the loss peak maxima show that both the dielectric and mechanical curves are roughly linear but have different slopes. From the slopes the activation energies were determined. For the dielectric data an activation energy of 39 kcal/mol was obtained, whereas for the mechanical data a value of 106 kcal/mol was found. The width of the dispersion curves was determined by using a Cole–Cole empirical fit. The width of the dielectric dispersion curve is narrower by as much as a factor of 3 than the mechanical dispersion curve. It is concluded that the energy to cause the large scale molecular motion involved in the α-relaxation is lower when excited by an alternating electric field than by an alternating stress field. Also the number of repeat units involved is smaller in the dielectric case than in the mechanical case.     

New composites based on short melamine fiber reinforced epdm rubber

Melamine fibre is a new category of advanced synthetic fiber having superior heat and flame resistance with decomposition temperature above 350°C. It suitability as a reinforcing fiber for ethylene propylene diene terpolymer, abbreviated as EPDM rubber, where ‘M’ stands for polymethylene chain, was investigated. It has been observed that tensile strength and stress at 100% strain of EPDM‐melamine fiber composites increase with the addition of a three‐component dry bonding system, comprising hexamethylene tetramine (hexa), resorcinol, and hydrate silica, abbreviated HRH system. Moreover, the fiber‐filled composites anisotropy in stress‐strain properties due to preferential of the short fibers along the milling direction (longitudinal), which is substantiated by the results of swelling and fractography studies. Aging causes an increase in the modulus, tensile strength and hardness of the composites. The fractographs show an increase in interfacial adhesion between the fibers and the matrix during aging, which is further confirmed by the reduction in tan δ peak height of the aged composites during dynamic mechanical studies. Atomic Force Microscopy (AFM) studies reveal the formation of an interphase with the addition of bonding agents and a better fiber‐matrix adhesion due to aging. AFM images also confirm the role of dry bonding systems in improving the fiber‐matrix adhesion of the aged vulcanizates. The composite modulus has been theoretically calculated using the well‐known Halpin‐Tsai equation. It is found that in the transverse direction, observed modulus values are greater than the calculated values, while in the longitudinal direction, the experimental modulus values are found to be lower than the calculated values for both unaged and aged composites owing to some degree of anisotropy in fiber orientation.     

Fabrication and performance of mini SiC/SiC composites with an electrophoresis-deposited BN fiber/matrix interphase

The feasibility of fabricating a BN matrix/fiber interphase of SiC/SiC composites via electrophoresis deposition (EPD) was investigated based on the simplicity and non-destructiveness of the process and the excellent interfacial modification effects of BN. The BN suspension and SiC fiber surface properties were both adjusted to generate suitable conditions for the EPD process of the BN interphase. Next, the deposition dynamics and mechanism were studied under different deposition voltages and time, and the relationship between the deposition morphology of the BN interphase and mechanical properties of the fabricated mini SiC/SiC composites were also discussed. After oxidation at high temperature (600–1000 ℃), the mechanical properties of the mini SiC/SiC composites were studied to verify the oxidation resistance effect of the EPD-deposited BN interphase, whose oxidation resistance mechanism was briefly analyzed as well.     

Relationship between interfacial phenomena and viscoelastic behavior of laminated materials

A literature review shows that the main arguments used to describe viscoelastic behavior of polymer composites are the existence of an interphase and/or physico-chemical matrix-reinforcement interactions. The purpose of this investigation was to study the influence of both of these parameters on the viscoelastic behavior of a sandwich structure. Using a theoretical approach of the mechanical coupling between phases in laminate composites, the interphase influence is shown to be negligible. In order to understand the influence of an interphase on viscoelastic features of laminates, some metal/polymer/metal laminates were processed under various conditions to obtain different degrees of metal/polymer adhesion. Dynamic mechanical spectroscopy tests reveal that both the amplitude of the main loss factor peak and the low temperature apparent modulus increase with the adhesion. Finite elements calculations show that discontinuities of displacements at the metal/polymer interface explain the loss peak changes. The continuity of displacements is ensured only from a threshold value of the peel energy.     

Effect of absorbed water on dynamic modulus for short fiber–CR composites

Short fiber–elastomer composites with 10 vol % fiber, nylon 6–CR and PET–CR composites, absorbed water either in the moisture atmosphere or in water. The effect of absorbed water on the viscoelastic properties for these composites was investigated. The temperature dependence of tan σ for the nylon–CR composite showed that the α-dispersion peak of nylon shifted to lower temperatures with increasing absorbed water content and that after displacement of the α-dispersion peak the additional small hump appeared at about 90°C. For the PET composite, the α-dispersion peak of PET shifted slightly to lower temperatures and the small shoulder at 90°C diminished with increasing absorbed water. The additional dispersion probably was caused by the interface between fiber and CR matrix and was independent of fiber orientation. The results suggested that nylon fiber absorbed a larger amount of water than CR matrix, while the water absorption for PET fiber was considerably less than for nylon fiber. The absorbed water in nylon fiber bonded stronger than that in CR matrix and was only slightly diminished by heat treatment under 100°C.     

Study of interphase in glass fiber–reinforced poly(butylene terephthalate) composites

It is well known that application of a coupling agent to a glass fiber surface will improve fiber/matrix adhesion in composites. However, on commercial glass fibers the coupling agent forms only a small fraction of the coating, the larger part being a mixture of processing aids whose contribution to composite properties is not well defined. The interfacial region of the composite will therefore be affected by the coating composition but also by the chemical reactions involved in the vicinity of the fiber and inside the surrounding matrix. The main feature of this study consists in dividing the interface region into two separate regions: the fiber/sizing interphase and the sizing/matrix interphase. A wide range of techniques was used, including mechanical and thermomechanical tests, infrared spectroscopy, gel permeation chromatography, carboxyl end group titrations, extraction rate measurements, and viscosity analysis. Experiments were performed on poly(butylene terephthalate) composites and results indicate that the adhesion improvement is due to the presence of a short chain coupling agent and of a polyfunctional additive, which may react both with the coupling agent and the matrix. According to the nature of this additive, it may be possible to soften the interphase and then to increase the composite impact strength.