Carbon nanotube-polymer nanocomposites: The role of interfaces

Research aimed at producing new nanocomposites with improved properties has dramatically increased in the last decade, especially on materials tailored at a nanometric level, such as fullerenes and carbon nanotubes. The use of nanoforms as reinforcement of organic polymers has opened the possibility of developing novel ultra-strong and conductive nanocomposites. Nevertheless, the challenge of manufacturing multifunctional composite materials based on nanostructures is still open, in particular in the details of the corresponding interfacial properties, which are particularly relevant in these systems. This paper reviews the main technical activities in this field, focusing on the most important parameters that influence the behavior of their interface, discussing recent advances, as well as current and future trends in research.     

A Fluoropolymer Coating on Carbon Fibers Improves Their Adhesive Interaction with PTFE Matrix

Carbon fibers were treated in a HF glow discharge in tetrafluoroethylene and octafluorocyclobutane in order to improve their adhesion to poly(tetrafluoroethylene) matrix. As the result of the plasma treatment, a thin (20–140 nm) fluoropolymer coating was deposited onto the fiber surface. The structure of this coating was studied by means of IR spectroscopy, XPS, AFM and SEM techniques. The coating material appeared to be similar to PTFE in its chemical composition but distinguished by branched, partially crosslinked, amorphous structure and included unsaturated chemical bonds. The coating thickness of 70 nm was sufficient to effectively screen the field of molecular forces of the initial substrate, thus, decreasing the surface energy of the fibers and improving their compatibility with the PTFE matrix. The adhesive strength in the PTFE–carbon fiber systems, measured by means of the microbond test, more than doubled upon the plasma treatment (the local interfacial shear strength increased from 10.7 to 29.7 MPa, apparent IFSS from 4.3 to 7.8 MPa), and the interfacial frictional stress increased by 70%. The new composite material consisting of 20% short coated carbon fibers in the PTFE matrix showed better mechanical, thermal and tribological characteristics as compared with the composite reinforced with untreated fibers.     

Deformation and Fracture Behavior of PP/Ash Composites

In the present work, the deformation and fracture behavior of PP/ash composites with different ash content was investigated. The effect of a silane coupling agent was also analyzed. From uniaxial tensile tests, an increase in the stiffness with ash content was found as a result of the incorporation of the stiffer filler within the PP matrix. On the other hand, a decrease in tensile strength and strain at break with filler loading was observed. This result was attributed to the increased number of debonded large particles with filler content, which subsequently led to the formation of critical-size flaws. On the other side, the composites displayed higher values of fracture parameters than the matrix as a result of the development of a particle induced toughening mechanism. However, fracture properties were also found to decrease with ash content. This could be attributed to the increase in the number of critical-size flaws that induced premature failure. The incorporation of a silane coupling agent in the formulations led to composites with slightly improved tensile and fracture properties. This was probably due to improved interaction between PP and ash in the first case and a better dispersion of ash particles in the matrix and/or changes in the crystallization behavior of PP, in the latter case.     

Load transfer from fiber to polymer matrix,studied by measuring the apparent elastic modulus of carbon fiber embedded in epoxy

Load transfer from a single carbon fiber to the surrounding epoxy matrix was studied by measuring the apparent tensile modulus of the fiber while the fiber was embedded in epoxy and comparing the apparent modulus (1650 GPa) with the real modulus (230 GPa). Thus, it was found that 87% of the tensile load applied to the fiber was transferred to the epoxy.     

Effect of Fiber Surface Modification on the Interfacial and Mechanical Properties of Kenaf Fiber-Reinforced Thermoplastic and Thermosetting Polymer Composites

The surfaces of kenaf fibers were treated with three different silane coupling agents. 3-glycidoxypropyltrimethoxy silane (GPS), 3-aminopropyltriethoxy silane (APS), and 3-methacryloxypropyltrimethoxy silane (MPS). Among them, the most effective one for the property improvement was GPS when it was applied to the kenaf fiber surfaces at 0.5 wt%. Thermoplastic polypropylene (PP) and thermosetting unsaturated polyester (UPE) matrix composites with chopped kenaf fibers untreated and treated at different GPS concentrations from 0.1 wt% to 5 wt% were fabricated using compression molding technique. The present study demonstrates that the interfacial, flexural, tensile, and dynamic mechanical properties of both kenaf/PP and kenaf/UPE composites importantly depend on the GPS treatments done at different concentrations. The greatest property improvement of both thermoplastic and thermosetting polymer composites was obtained with the silane treatment at 0.5 wt% and the mechanical properties were comparable with E-glass composites prepared the same polymer matrix under the corresponding fiber length and fiber loading. The results also agreed with each other with regard to their interfacial shear strength, flexural properties, tensile properties, storage modulus, with support of fracture surfaces of the composites.     

Continuum Micro-mechanics of Estimating Interfacial Shear Strength in Short Fiber Composites

A new continuum approach to micro-mechanics of short fiber composites yielded two separate methods of estimating the apparent interfacial shear strength and fiber orientation efficiency. The methods exploit the compilation of the effects of fiber length distribution and interfacial shear strength on strengthening efficiency into a function of strain. The In-Built Method derives a unique combination of apparent interfacial shear strength and fiber orientation efficiency being able to reproduce the experimental stress–strain curve of a short fiber reinforced composite with a very low residual standard deviation. The Boundary Method accomplishes rapid interfacial shear strength screening in materials selection by constructing and utilizing the proposed selection chart.     

A Fiber-Bundle Pull-out Test for Surface-Modified Glass Fibers in GF/Polyester Composite

The development of high-performance polymer composites is tightly bound with the functional surface modification of reinforcements. A new method, based on the principle of the fiber-bundle pull-out test, is proposed to analyze the interfacial properties between the long fibers in the form of a bundle and the polymer matrix. Specimen geometry and a test fixture were designed using finite element analysis. The method was verified for unsized and sized glass fibers embedded in polyester resin to demonstrate its applicability for a wide range of adhesion between fibers and the polymer matrix. The pull-out test can be used for a relative comparison of different surface modifications if the bundle geometry is unknown. The results of high reproducibility and sensitivity for interfacial properties make the method attractive.     

Micromechanical properties of poly(butylene terephthalate) nanocomposites with single- and multi-walled carbon nanotubes

Polymer nanocomposites with carbon nanotubes (CNT) are becoming important structural materials because of their superior mechanical properties and easy processability. The objective of the work is to investigate the influence of small amounts of single walled carbon nanotubes (SWCNT), as well as multi-walled carbon nanotubes (MWCNT), on the microhardness of a thermoplastic polymer such as poly(butylene terephthalate) (PBT). The nanocomposites were obtained by introducing the CNT into the reaction mixture during the synthesis of PBT. The polymers without carbon nanotubes (reference material) and with carbon nanotubes were synthesized using an in-situ polycondensation reaction process. Weight percentages ranging from 0.01 to 0.2 wt% of the single walled and from 0.01 to 0.35 wt% of the multi-walled nanotubes were dispersed in 1,4-butanediol (BD) by ultrasonication and by ultra high speed stirring. The nanocomposites were extruded followed by injection molding. The samples were characterized by electron microscopy and microindentation hardness techniques. The variations of the micromechanical properties (indentation hardness) of the nanocomposites with nanotube content and with temperature are discussed in the light of the stress transfer between the polymer matrix and nanotubes, the degree of dispersion, the nature of the tubes and other structural parameters.     

Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes

Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM–MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM–MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point, t _gel for PAM–MWNTs composite gels showed that the gel fraction exponent β obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM–MWNT gel was also measured and found to be about 2.0, which agrees with a random resistor network.     

Covalent attachment of polystyrene on multi-walled carbon nanotubes via nitroxide mediated polymerization

A new method to attach polymers on carbon nanotubes has been studied. We used nitroxide mediated polymerization (NMP) to graft polystyrene from multi-walled carbon nanotubes (MWNTs). Carboxyl groups on MWNTs were activated with thionyl chloride (SOCl₂) to give acyl chloride derivative (MWNT-COCl). NMP initiator was introduced to MWNT by esterification of 1-hydroxy-2-phenyl-2-(2′,2′, 6′, 6′-tetramethyl-1′-piperidinyloxy)ethane (HO-PE-TEMPO) with acyl chloride groups of MWNTs. The obtained MWNT-PE-TEMPO was used for initiation of bulk polymerization of styrene, yielding polystyrene-grafted MWNTs (MWNT-g-pSt). The resulting composites of MWNT-g-pSt were analyzed by TEM, SEM, FT-IR and TGA.     

The Effect of ZrO2 Interphase on Interfacial Frictional Stresses in SiC/ZrO2/SiCf Composites

Two types of SiC fiber tows (Hi-Nicalon? and Hi-Nicalon S?) were coated with stabilized ZrO₂ and composited using preceramic polymer impregnation pyrolysis to form SiC/SiC_f minicomposites. Properties of the fiber/matrix interface in composites were investigated using the indentation method in which a pyramidal indenter was used to push on an individual fiber and cause sliding at the interface. The interfacial frictional stresses were determined from the force–displacement relation. The composites reinforced by the ZrO₂-coated fibers have smaller interfacial frictional stresses than composites reinforced by the initial fibers and show fibers sliding relatively more easily with respect to the SiC matrix.     

PVDF-Based Nanocomposite Solid Polymer Electrolytes; the Effect of Affinity Between PVDF and Filler on Ionic Conductivity

Nanocomposite solid polymer electrolytes (NSPEs) based on poly(vinylidene fluoride) (PVDF) were prepared by dispersing two kinds of organoclay (Cloisite^® 30B, Cloisite^® 15A) consisting of silicate layers in the polymer matrix. The effect of affinity between PVDF and organoclay as the filler on ionic conductivity was investigated in relation to its content, dispersed condition of organoclay, and structural changes of nanocomposites. The characterizations of PVDF-based nanocomposites with various organoclay contents were carried out by XRD, TEM, DSC, and DMA. In order to confirm the ion conduction properties of NSPEs with LiCF₃SO₃ at room temperature, ac impedance analyzer and FT-IR spectrometer were used. As a result, a higher ionic conductivity appeared in the case of NSPE with C15A than that with C30B and the maximum conductivity was 1.04 × 10^–3 S/cm for the NSPE containing 5 wt% of C15A and 40 wt% of LiCF₃SO₃.     

Basic Spectroscopy Performance of Nd~(3 ) and Rhodamine B Doped Polymer Fibers

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Isothermal Crystallization and Melting Behavior of Composites Composed of Poly(L-lactic Acid) and Poly(glycolic Acid) Fibers

Several composites of poly (L-lactic acid) (PLLA) with poly (glycolic acid) (PGA) fibers were prepared. The isothermal crystallization kinetics and melting behavior of PLLA and all of the composites were characterized by using differential scanning calorimetry. The experimental data were processed by using the Avrami equation. The relative parameters, such as the Avrami exponent and half-time crystallization, revealed that PGA fibers had positive effects on the crystallization of PLLA, but these effects had only a minimal dependence on the PGA fiber content. Moreover, at low isothermal crystallization temperatures (85°C～110°C), recrystallization during the heating scan was observed, which could lower the melting point of the samples to a certain extent.     

Sorption and Diffusion of Supercritical Carbon Dioxide in a Biodegradable Polymer

A gravimetric method was used to study the sorption and diffusion of supercritical carbon dioxide in a temperature range from 40°C to 80°C and a pressure range from 8.0 to 18.0 MPa in a biodegradable polymer, namely poly(butylene adipate-co-terephthalate) (PBAT). The PBAT presented Fickian behavior and Fick's diffusion model was applied to determine the amount of carbon dioxide present in the samples after a predetermined exposure time as well as the diffusion coefficients. The variations of diffusion coefficients of CO₂ for the sorption under supercritical conditions and desorption at ambient conditions as well as equilibrium sorption amounts of CO₂ with variations of pressure and temperature were determined and compared.     

Crystallization Behavior of Poly(Lactic Acid) Filled with Modified Carbon Black

A novel method was employed to modify the surface of carbon black (CB) by an organic small molecule in a Haake Rheomix mixer. The modified carbon black (MCB) was dispersed uniformly in poly(lactic acid; PLA). The crystallization behaviors of PLA, PLA/CB and PLA/MCB composites were investigated by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and polarizing optical microscopy. It is found that the addition of CB or MCB can influence the crystallization behavior of PLA. PLA/MCB has a faster crystallization rate and higher crystallization peak temperature than PLA/CB. For non-isothermal studies, Jeziorny and Mo equations were employed. The Mo equation can well describe the non-isothermal crystallization of the three samples. For PLA/CB and PLA/MCB composites containing 3wt% fillers, the nucleating activity for CB is about 0.32, and about 0.16 for MCB. All these results show that MCB is an effective nucleating agent. PLA/MCB has a higher nucleation rate than PLA/CB because of the finer dispersed particles size and improved interaction between MCB and PLA.     

Confined Crystallization in Polymer Blends: DSC Studies of the Behavior and Kinetics of Isothermal Crystallization of Polypropylene in Poly(cis-butadiene) Rubber Blends

Thermal properties of polypropylene with poly(cis-butadiene) rubber (iPP/PcBR) blends have been measured by differential scanning calorimetry (DSC), and the melting point T_m, crystallization temperature T_c, enthalpy Δ H (melting enthalpies and crystalline enthalpies), and equilibrium melting point T⁰ _m have been measured and calculated. The variation of T_m, T_c, Δ H and T⁰ _m with composition in the blends was discussed, showing that an interaction between phases is present in iPP/PcBR blends. The degree of supercooling characterizing the interaction between two phases in the blends and the crystallizability of the blends which bears a relationship to the composition of the blends was discussed. The kinetics of isothermal crystallization of the crystalline phase in iPP/PcBR blends was studied in terms of the Avrami equation, and the Avrami exponent n and velocity constant K were obtained. The Avrami exponent n is between 3 and 2, meaning that iPP has a thermal nucleation with two dimensional growths. The variation of the Avrami exponent n, velocity constant K, and crystallization rate G bear a relation to the composition of the blends, n increases with increasing content ofPcBR. K also increased with increasing content of PcBR. All of the K for the blends are greater than for pure iPP. The crystallization rate G (t_1/2) depends on the compositions in the blends; all G of the blends are greater than for iPP.     

Dynamic Scaling and Fractal Behavior of Phase Dispersion in Multicomponent Polymer Blends

Dynamic scaling and fractal behavior of phase dispersion in a binary polymer mixture of polypropylene with poly(cis-butadiene) rubber during processing in a batch mixer was studied by means of a back small-angle laser scattering (BSALS) online system. In the late stage of phase dispersion, a special fractal behavior was found; different overlapped line groups existed simultaneously and the temporal distribution was stochastic. We defined the special behavior as quasi-self-similarity and calculated the fractal dimension, D _p, by using the power spectrum method. The results show that the evolution of different fractal dimensions with time is consistent with average chord length of the dispersed phase.     

Sorption of Organic Solvents in Poly(Dimethyl Siloxane) Membrane. II. Effect of Sorption on the Polymer Crystals upon Cooling

Polymer crystalline properties in poly(dimethyl siloxane) (PDMS) film after solvation by various solvents was determined using low temperature differential scanning calorimetry (DSC). At various solvent uptake levels, the crystalline thermal properties of the solvated polymer were modified to different extents as revealed by the shifts in crystalline melting point (T_m) and its enthalpy (ΔH_m). Water uptake in PDMS was very limited (<0.01 g/g) and T_m did not significantly change during the sorption process. For toluene and cyclohexane penetrants, T_m moved toward a much lower temperature depending on the sorption levels. At low solvent uptakes, the T_m values decreased linearly with solvent uptakes due to formation of a miscible phase. Beyond a threshold, the T_m remained stable and an additional penetrant fusion peak appeared, implying the onset of a microphase separation phenomenon. The ΔH_m values for the swollen membranes were decreased, with the exception of the water penetrant. This indicates that a lower percentage of polymer chains were involved in the crystalline domain for swollen PDMS.     

一种校正XRF光谱基体吸收效应的新方法

根据校正XRF光谱基体吸收效应的基本理论,提出了一种用基本参数法校正基体吸收效应的新方法,该法既不需要标准,又对样品和衬底靶无特别要求,仅用一些基本的物理参数(ω,J,τ)和荧光强度的测量值便可算得样品中待分析元素的含量,方便简单。用岩石标准SY-2,SY-3和MRG-1检验该法,其结果与文献报道的值相符。