Transport properties of graphite/epoxy composites: Thermal, permeability and dielectric characterization

In this study nanocomposites were prepared by dispersing three different grades of graphite particles, expanded graphite, commercial graphene nanoplatelets and natural graphite, in a commercial epoxy matrix. Dielectric properties, thermal conductivity and permeability to oxygen of the composites were studied and compared to those of the unfilled epoxy matrix. An increase of all properties is obtained using expanded graphite, suggesting the presence of a good dispersion of the filler in the matrix and a strong polar interactions of the filler with the matrix, attributed to the partially oxidised surfaces of the expanded graphite. All the measured transport properties were fitted with simple mathematical models obtaining good agreement between the experimental results and theoretical predictions. The model parameters were related to the aspect ratio of the filler, defined as the ratio between the in-plane average dimension and the thickness of the reinforcement. An aspect ratio between 1250 and 1550 indicates that graphite thin platelets (or graphene stacks), characterized by a thickness of the order of a few tens of nanometers, were dispersed in the epoxy matrix.     

Aging properties of styrene-butadiene rubber nanocomposites filled with carbon black and rectorite

To better understand the effect of rectorite and carbon black (CB) on the aging performance of styrene-butadiene rubber (SBR), SBR/CB, SBR/CB/rectorite and SBR/rectorite nanocomposites with the same total filler loading were prepared. The microstructure of the three SBR nanocomposites was characterized by XRD, TEM and SEM. After thermal aging, oxygen-containing molecules were found to be formed in the SBR nanocomposites, as verified by FTIR analysis. The SBR/rectorite nanocomposite showed the highest aging coefficient and the lowest change rate of tensile strength and stress at 100% strain among the three SBR nanocomposites, indicating that the introduction of nano-dispersed rectorite layers can enhance the thermal aging resistance of the nanocomposites. For the SBR/CB/rectorite nanocomposite, the addition of CB helped to improve the interfacial compatibility between the filler and matrix, resulting in the best crack resistance as the aged SBR/CB/rectorite nanocomposite always demonstrated the least cracks on the surface during either stretching or bending experiments.     

Graphite nanoplatelets/polymer nanocomposites: thermomechanical,dielectric, and functional behavior

Exfoliated graphite nanoplatelets (GNP)/epoxy resin nanocomposites were prepared and tested, varying the amount of the filler content. Systems’ morphology was investigated by means of scanning electron microscopy, while their thermal response was examined via differential scanning calorimetry (DSC). Broadband dielectric spectroscopy and dynamic mechanical thermal analysis were employed in order to characterize the produced systems. Static mechanical tests were also conducted at ambient. Reinforced systems exhibit improved performance under mechanical and electrical excitation. In particular, storage modulus increases systematically with GNP content. DSC results imply that glass transition temperature is not affected by the presence of GNP. Flexural modulus and storage modulus, as determined by static and dynamic mechanical tests, respectively, increased with filler content. Dielectric permittivity increases also systematically with GNP content. Recorded relaxation processes arise from the glass to rubber transition of the polymer matrix (α-mode), re-orientation of polar side groups of the polymer chains (β-mode), and interfacial polarization because of the accumulation of charges at the systems’ interface. Finally, the energy storing efficiency of the nanocomposites enhances with reinforcing phase in the examined frequency and temperature range. Optimum performance corresponds to the nanocomposite with maximum GNP loading.     

Polylactide/exfoliated graphite nanocomposites with enhanced thermal stability,mechanical modulus,and electrical conductivity

We have prepared a series of polylactide/exfoliated graphite (PLA/EG) nanocomposites by melt‐compounding and investigated their morphology, structures, thermal stability, mechanical, and electrical properties. For PLA/EG nanocomposites, EG was prepared by the acid treatment and following rapid thermal expansion of micron‐sized crystalline natural graphite (NG), and it was characterized to be composed of disordered graphite nanoplatelets. It was revealed that graphite nanoplatelets of PLA/EG nanocomposites were dispersed homogeneously in the PLA matrix without forming the crystalline aggregates, unlike PLA/NG composites. Thermal degradation temperatures of PLA/EG nanocomposites increased substantially with the increment of EG content up to ～3 wt %, whereas those of PLA/NG composites remained constant regardless of the NG content. For instance, thermal degradation temperature of PLA/EG nanocomposite with only 0.5 wt % EG was improved by ～10 K over PLA homopolymer. Young's moduli of PLA/EG nanocomposites increased noticeably with the increment of EG content up to ～3 wt %, compared with PLA/NG composites. The percolation threshold for electrical conduction of PLA/EG nanocomposites was found to be at 3–5 wt % EG, which is far lower graphite content than that (10–15 wt % NG) of PLA/NG composites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 850–858, 2010     

Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading

The aim of this paper is to demonstrate that the stress–strain behavior of natural rubber reinforced with short pineapple leaf fiber (PALF) can easily be manipulated by changing the cross-link density and the amount of carbon black (CB) primary filler. This gives more manageable control of mechanical properties than is possible with conventional particulate fillers alone. This type of hybrid rubber composite displays a very sharp rise in stress at very low strains, and then the stress levels off at medium strains before turning up again at the highest strains. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr) and varying carbon black contents from 0 to 30 phr. To change the cross-link density, the amount of sulfur was varied from 2 to 4 phr. Swelling ratio results indicate that composites prepared with greater amounts of sulfur and carbon black have greater cross-link densities. Consequently, this affects the stress–strain behavior of the composites. The greater the cross-link density, the less is the strain at which the stress upturn occurs. Variations in the rate of stress increase (although not the stress itself) in the very low strain region, while dependent on fillers, are not dependent on the crosslink density. The effect of changes in crosslinking is most obvious in the high strain region. Here, the rate of stress increase becomes larger with increasing cross-link density. Hence, we demonstrate that the use of PALF filler, along with the usual carbon primary filler, provides a convenient method for the manipulation of the stress–strain relationships of the reinforced rubber. Such composites can be prepared with a controllable, wide range of mechanical behavior for specific high performance engineering applications.     

Characterization of kaolinite/emulsion-polymerization styrene butadiene rubber (ESBR) nanocomposite prepared by latex blending method: Dynamic mechanic properties and mechanism

The latex blending method was chosen to prepare Kaolinite/emulsion-polymerization styrene butadiene rubber (ESBR) nanocomposite to improve the interaction between filler particles and rubber matrix chains. The influences of kaolinite particles size, filler contents, and flocculants types on dynamic mechanical properties and the relative reinforcement mechanism of the prepared composite were systematic investigated and proposed. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that the kaolinite particles were finely dispersed into the rubber matrix and arranged in parallel orientation. The prepared nanocomposites by latex blending exhibited improved crosslinking characteristic and dynamic mechanical parameters. The KAl (SO₄)₂ flocculant presented obvious modification in dynamic properties and crosslinking characteristic. Both the decrease in kaolinite particle size and the increase in kaolinite content can greatly improve the storage modulus and reinforcing effect of kaolinite/ESBR nanocomposites. The dynamic reinforcement mechanism of kaolinite can be explained by filler network including a certain thickness of rubber shell on the surface of kaolinite lamellar structure and the aggregations network between kaolinite particles The optimum way to balance the dynamic properties of rubber nanocomposites at different temperatures is to reduce the surface difference between kaolinite and rubber matrix and the degree of filler-filler networking on the basis of kaolinite with nanoscale (nanometer effect).     

Development and thermo‐mechanical behavior of nanocomposite epoxy adhesives

Two kinds of organo‐modified (OM) clays were dispersed in an epoxy resin for the preparation of nanocomposite adhesives at various filler amounts. XRD tests evidenced the formation of intercalated structures, increasing the intercalation degree with the clay hydrophilicity. The original transparency of the samples was retained up to a filler content of 3 wt%, and then decreased due to filler agglomeration. The glass transition temperature of nanocomposites filled with the more hydrophilic clay (30B) raised up to a filler content of 3 wt% and then decreased, probably because of the concurrent and contrasting effects of the physical chain blocking and reduction of the cross‐linking degree. Also elastic modulus, stress at break, and fracture toughness were sensibly improved by nanoclay addition up to filler loadings of 0.5–1 wt%. For higher concentrations the positive contribution of clay nanoplatelets was counterbalanced by the presence of agglomerated tactoids in the matrix. Mechanical tests on single‐lap composite (epoxy/glass) bonded joints evidenced an enhancement of the shear strength by about 25% for an optimal filler content of 1 wt%. Therefore, it was concluded that the addition of a proper amount of OM clay to epoxy adhesives could represent an effective way to improve the shear resistance of adhesively bonded composite structures. Copyright © 2011 John Wiley & Sons, Ltd.     

Waxy Maize Starch Nanocrystals as Filler in Natural Rubber

Starch nanocrystals obtained from acid hydrolysis of waxy maize starch granules consist in crystalline nanoplatelets about 6–8 nm thick with a length of 20–40 nm and a width of 15–30 nm. New nanocomposite materials, i.e. natural rubber (NR) filled with waxy maize starch nanocrystals were processed by casting. Dynamic mechanical analysis has shown that starch nanocrystals were a good reinforcing agent for NR at temperatures higher than the glass transition temperature of NR. Tensile tests have shown that until a weight fraction of 20 wt%, this new filler presented the advantage to reinforce natural rubber without decreasing significantly the strain at break of the material. These properties may be due to both the morphological nature of starch nanocrystals, and the formation of a percolating starch nanocrystals network within the NR matrix, resulting from hydrogen bonding forces between starch aggregates.     

Highly enhanced electrical and mechanical properties of methyl methacrylate modified natural rubber filled with multiwalled carbon nanotubes

Developing conductive networks in a polymer matrix with a low percolation threshold and excellent mechanical properties is desired for soft electronics applications. In this work, natural rubber (NR) functionalized with poly(methyl methacrylate) (PMMA) was prepared for strong interfacial interactions with multiwalled carbon nanotubes (MWCNT), resulting in excellent performance of the natural rubber nanocomposites. The MWCNT and methyl methacrylate functional groups gave good filler dispersion, conductivity and tensile properties. The filler network in the matrix was studied with microscopy and from its non-linear viscoelasticity. The Maier-Göritze approach revealed that MWCNT network formation was favored in the NR functionalized with PMMA, with reduced electrical and mechanical percolation thresholds. The obvious improvement in physical performance of MWCNT/methyl methacrylate functionalized natural rubber nanocomposites was caused by interfacial interactions and reduced filler agglomeration in the NR matrix. The modification of NR with poly(methyl methacrylate) and MWCNT filler was demonstrated as an effective pathway to enhance the mechanical and electrical properties of natural rubber nanocomposites.     

On the degradation and stability of high abrasion furnace black (HAF)/acrylonitrile butadiene rubber (NBR) and high abrasion furnace black (HAF)/graphite/acrylonitrile butadiene rubber (NBR) under cyclic stress-strain

Acrylonitrile butadiene rubber (NBR) compounds filled with 40 phr of high abrasion furnace black (HAF) and HAF (20 phr)/graphite (20 phr) were experimentally investigated. The stress-strain curves of the composites were studied, which are described by applying Ogden's model. The effect of cyclic fatigue and hysteresis was also examined. The dissipation energy that indicates the vibration damping capacity for all samples was determined. A continuum damage model is used to investigate the fatigue damage behavior for elastomers. Experiments on the cyclic fatigue of a carbon-filled NBR rubber and carbon/graphite filled NBR rubber were conducted to determine the relation between the number of cyclic fatigue and the strain amplitude. The results indicate that the theoretical formula for the number of cyclic fatigue as a function of the strain amplitude, derived from the damage model, can describe experimental data for the prepared samples very well.     

Micromechanical models of young's modulus of NR/organoclay nanocomposites

Small strain Young's moduli of natural rubber (NR)/organoclay nanocomposites were estimated using the Guth–Gold, Halpin–Tsai (HT), and Krieger–Dougherty (KD) models, and compared with experimental measurements of NR vulcanizates containing organo‐montmorillonite (OM) or organo‐sepiolite (OS). To account for the effect on modulus of the NR matrix of the vulcanization‐active modifier in the organoclay, a matrix modulus correction (MMC) term was derived from the vulcanization parameters of the nanocomposites. The KD model gave a better empirical fit with the experimental data than the Guth–Gold model, with both giving good agreement with particle shape factors estimated from transmission electron microscope (TEM) images. The HT model gave the best fit with experiment for both types of nanocomposite, and use of the MMC term meant that the empirical shape factor was sufficiently close to that estimated from TEM images that the model could potentially be used to accurately predict the Young's moduli of NR/OM and NR/OS nanocomposites. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1621–1627, 2011     

Rheological behavior of fumed silica filled polyethylene oxide

Influence of molecular weight of polymer matrix on nanocomposites rheology is not yet well understood. Herein dynamic rheological responses of fumed silica (FS)/polyethylene oxide (PEO) nanocomposites are investigated as a function of viscosity‐averaged molecular weight (M_η) of PEO, volume fraction (?) and surface characteristics (hydrophilic or hydrophobic) of FS. In the nanocomposites, FS does not influence the glass transition and crystallinity of PEO in the mobile PEO phase while the interfacial interactions tend to immobilize a small fraction of PEO chains that could not undergo glass transition. In spite of the common observation that the reinforcement decreases with increasing M_η of PEO and improving hydrophobicity of FS, linear rheological responses are well reproduced by the two‐phase model, revealing the crucial contribution of the non‐Newtonian matrix undergoing microscopic strain amplified by the filler. Furthermore, nonlinear rheological responses of the nanocomposites are collapsed into master curves plotted against local strain of the matrix. Analyzing the nonlinear rheology by Fourier transform and stress waveform methods reveal the dominating contribution of the matrix and the role of strain amplification played by the filler. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 397–405     

Biodegradable Nanocomposites Based on the Polyester Poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) and Layered Silicate or Expanded Graphite

Novel nanocomposites based on the biodegradable polymer poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBHx) and layered fillers, specifically layered silicate (clay25A) and expanded graphite (EG), were prepared by melt intercalation. The dispersion of the fillers in the PHBHx was characterized by wide‐angle X‐ray diffraction (WAXD) and transmission electron microscopy (TEM). The effects of the fillers on the polymer structure, thermal stability and mechanical properties of the nanocomposites were also studied, by differential scanning calorimetry, thermogravimetric analysis, and strain‐stress measurements in elongation, respectively. The WAXD and TEM results showed that the clay25A and EG was exfoliated into well‐dispersed sheets in the polymer matrix, especially when the filler concentration were relatively low. This gave rise to considerable improvements in Young's modulus, and resulted in increases in the thermal degradation. It should be possible to convert the EG dispersions obtained thus far to ones yielding filler‐filler networks that show electrical conductivity.     

Payne effect in silica‐filled styrene–butadiene rubber: Influence of surface treatment

The nonlinear effect at small strains (Payne effect) has been investigated in the case of silica‐filled styrene‐butadiene rubber. The originality of this study lies in the careful preparation of samples in order to fix all parameters except one, that is, the modification of the silica surface by grafting silane (introduced at different concentrations) via reactive mixing. The organosilane can be either a coupling or a covering surface treatment with an octyl alkyl chain. A careful morphological investigation has been performed prior to mechanical characterization and silica dispersion was found to be the same whatever the type and the amount of silane. The increasing amount of covering agents was found to reduce the amplitude of the Payne effect. A similar decrease is observed for low coupling agent concentration. At higher concentrations, the evolution turns through an increase due to the contribution of the covalent bonds between the matrix and the silica acting as additional crosslinking. The discussion of the initial modulus was done in the frame of both the filler–filler and filler–polymer models. It is unfortunately not possible to distinguish both scenarios, because filler–filler and filler–matrix interactions are modified in the same manner by the grafting covering agent. On the other hand, the reversible decrease of the modulus versus strain (Payne effect) is interpreted in terms of debonding of the polymeric chains from the filler surface. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 286–298, 2007     

Electrical and mechanical behavior of filled rubber. III. Dynamic loading and the rate of recovery

Following the earlier articles in this series, the changes in the electrical resistivity and mechanical behavior as a result of static and dynamic deformation have been studied. Cyclic shear and tensile loading were used to follow the changes in stress and resistivity with strain, including the recovery with time from the effects of a large strain as monitored by the small‐strain behavior. The recovery of resistivity from a prestrain was not complete even after 7 days at room temperature or at 50 °C, but swelling with a solvent and subsequent drying produced rapid recovery. It appears from the detailed results that there are two strain regions. Below about 10% the resistance and the modulus are strongly dependent on the filler–filler structure, which can break down and reform fairly readily, but the changes at higher strains are probably influenced by changes in the elastomer matrix and also by slippage at the filler–rubber interface. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1649–1661, 2005     

Natural rubber nanocomposites with SiC nanoparticles and carbon nanotubes

Single-walled carbon nanotubes (SWNTs) and SiC nanoparticles were dispersed in natural rubber (NR) polymer solution and subsequently evaporated the solvent to prepare NR nanocomposites. Using this technique, nanoparticles can be better dispersed in the NR matrix. The influence of nano-fillers on the mechanical properties of the resulting nanocomposites was quantified.Mechanical test results show an increase in the initial modulus with nanoscale reinforcements for up to 50% strain compared to pure NR. The modulus and strength of natural rubber with 1.5% SiC nanoparticles appear to be superior to those of SWNTs with the same filler content. In addition to mechanical testing, these nanocomposites were studied using the SEM and Raman spectroscopy techniques in order to understand the morphology of the resulting system and the load transfer mechanism, respectively. The Raman spectrum of the SWNT/NR system is characterized by a strong band at 1595 cm⁻¹ (G mode—C-C stretching) and other two bands at 1300 cm⁻¹ (D mode-disorder induced) and 2590 cm⁻¹ (D* band). A shift of the 2590 cm⁻¹ Raman band to the lower wavenumber was observed after subjecting SWNT/NR sample to cyclic stress testing. Ageing SWNT/NR specimen in distilled water for 30 days also provided a similar result. The Raman shift in aged samples indicates internal stress transfer from the natural rubber matrix to the SWNTs implying the existence of bonding at the interface.     

The Effect of Filler Type on the Mechanical Properties of Composite Materials Based on Ultra-High-Molecular-Weight Polyethylene

The effect of the filler type (graphite nanoplates, organomodified montmorillonite, molybdenum disulphide, or shungite) on the deformation and strength properties of composite materials based on UHMWPE synthesized by in situ polymerization with a filler content of up to 0.06 vol fraction was studied. The significant effect of the filler type on the dependence of the tensile strength and the tensile elongation of the studied composites on the filler content was established. A significant difference between the tensile stress–strain curves for composites with different fillers was observed.     

Polymer–filler interactions in sol–gel derived polymer/silica hybrid nanocomposites

The effect of polymer–filler interaction on solvent swelling and dynamic mechanical properties of the sol–gel derived acrylic rubber (ACM)/silica, epoxidized natural rubber (ENR)/silica, and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites has been described for the first time. Tetraethoxysilane (TEOS) at three different concentrations (10, 30, and 50 wt %) was used as the precursor for in situ silica generation. Equilibrium swelling of the hybrid nanocomposites in respective solvents at ambient condition showed highest volume fraction of the polymer in the swollen gel in PVA/silica system and least in ACM/silica, with ENR/silica recording an intermediate value. The Kraus constant (C) also followed a similar trend. In dynamic mechanical analysis, the storage modulus dropped at higher strain (>1%), which indicated disengagement of polymer segments from the filler surfaces. This drop was maximum in ACM/silica, intermediate in ENR/silica, and minimum in PVA/silica, both at 50 and 70 °C. The drop in modulus with theoretical volume fraction of silica (ϕ) was interpreted with the help of a Power law model ΔE′ = a₁ϕ, where a₁ was a constant and b₁ was primarily a filler attachment parameter. Strain dependence of loss modulus was observed in ACM/silica hybrid nanocomposites, while ENR/silica and PVA/silica nanocomposites showed almost strain‐independent behavior. The storage modulus showed sharp increase with increasing frequency in ACM/silica system, while that was lower in both ENR/silica (at higher frequency) and PVA/silica systems (in the entire frequency spectrum). The increase in modulus with ϕ also followed similar model ΔE′ = a₂ϕ proposed in the strain sweep mode. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2399–2412, 2005