Calorimetric study of nanocomposites of multiwalled carbon nanotubes and isotactic polypropylene polymer

Modulated differential scanning calorimetry (MDSC) was used to measure the complex specific heat of the crystallization and melting transitions of nanocomposites of isotactic polypropylene (iPP) and carbon nanotubes (CNT) as function of CNT weight percent and temperature scan rate. In the last few years, great attention has been paid to the preparation of iPP/CNT nanocomposites due to their unique thermal and structural properties and potential applications. As the CNT content increases from 0 to 1 wt %, heterogeneous crystal nucleation scales with the CNT surface area. Above 1 wt %, nucleation appears to saturate with the crystallization temperature, reaching ～8 K above that of the neat polymer. Heating scans reveal a complex, two‐step, melting process with a small specific heat peak, first observed ～8 K below a much larger peak for the neat iPP. For iPP/CNT samples, these two features rapidly shift to higher temperatures with increasing ?_w and then plateau at ～3 K above that in neat iPP for ?_w ≥ 1 wt %. Scan rates affect dramatically differently the neat iPP and its nanocomposites. Transition temperatures shift nonlinearly, while the total transition enthalpy diverges between cooling and heating cycles with decreasing scan rates. These results are interpreted as the CNTs acting as nucleation sites for iPP crystal formation, randomly pinning a crystal structure different than in the neat iPP and indicating complex transition dynamics. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis,characterization and properties of organoclay‐modified polyurethane/epoxy interpenetrating polymer network nanocomposites

organoclay‐modified polyurethane/epoxy interpenetrating network nanocomposites (oM‐PU/EP nanocomposites) were prepared by adding organophilic montmorillonite (oMMT) to interpenetrating polymer networks (IPNs) of polyurethane and epoxy resin (PU/EP) which had been prepared by a sequential polymerization technique. Wide‐angle X‐ray diffraction (WAXD) and transmission electronic microscopy (TEM) analysis showed that the interpenetrating process of PU and EP improved the exfoliation and dispersion degree of oMMT. The effects of the NCO/OH ratio (isocyanate index), the weight ratio of PU/EP and oMMT content on the phase structure and the mechanical properties of the oM‐PU/EP nanocomposites were studied by tensile testing and scanning electronic microscopy (SEM). Water absorption tests showed that the PU/EP interpenetrating networks and oMMT had synergistic effects on improvement in the water resistance of the oM‐PU/EP nanocomposites. Differential scanning calorimetry (DSC) analysis showed that PU was compatible with EP and that the glass transition temperature (T_g) of the oM‐PU/EP nanocomposites increased with the oMMT content up to 3 wt%, and then decreased with further increasing oMMT content. The thermal stability of these nanocomposites with various oMMT contents was studied by thermogravimetric analysis (TGA), and the mechanism of thermal stability improvement was discussed according to the experimental results. Copyright © 2005 Society of Chemical Industry     

Comprehensive high‐performance epoxy nanocomposites co‐reinforced by organo‐montmorillonite and nano‐SiO2

Comprehensive high‐performance epoxy nanocomposites were successfully prepared by co‐incorporating organo‐montmorillonite (o‐MMT) and nano‐SiO₂ into epoxy matrix. Because of the strong interaction between nanoscale particles, the MMT layers were highly exfoliated, and the exfoliated nanoscale MMT monoplatelets took an interlacing arrangement with the nano‐SiO₂ particles in the epoxy matrix, as evidenced by X‐ray diffraction measurement and transmission electron microscopy inspection. Mechanical tests and thermal analyses showed that the resulting epoxy/o‐MMT/nano‐SiO₂ nanocomposites improved substantially over pure epoxy and epoxy/o‐MMT nanocomposites in tensile modulus, tensile strength, flexural modulus, flexural strength, notch impact strength, glass transition temperature, and thermal decomposition temperature. This study suggests that co‐incorporating two properly selected nanoscale particles into polymer is one pathway to success in preparing comprehensive high‐performance polymer nanocomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Retention of Mechanical and Thermal Properties of Hydrothermal Aged Glass Fiber-Reinforced Polymer Nanocomposites

The combined effect of nano-Al₂O₃ and TiO₂ fillers on residual mechanical and thermal properties of glass fiber-reinforced polymer composites has been evaluated. The results reveal that the addition of 0.1?wt% of Al₂O₃ and 0.1?wt% of TiO₂ into the epoxy matrix reduces the water diffusivity by 12%. The residual flexural and interlaminar shear strength of the nanocomposite have been increased by 19 and 21%, respectively, as compared to those of neat epoxy glass fiber-reinforced polymer composite. In spite of reduction in water diffusivity and increase in strength, there was no improvement in glass transition temperature of the nanocomposites.     

Effect of interfacial chemistry on crystallization of polypropylene/multiwall carbon nanotube nanocomposites

This study systematically investigates the polymer–carbon nanotube (CNT) interaction when the interphase is tailored. Maleic anhydride‐grafted‐polypropylene (MA‐g‐PP) or polypropylene (PP) was noncovalently coated onto acid functionalized multiwall nanotube (f‐MWNT) through solution mixing. These coated f‐MWNTs were melt microcompounded with neat PP to form PP/f‐MWNT nanocomposites. The effects of functional groups and the thin layer of solution processed polymers, namely, MA‐g‐PP or PP, at the PP/f‐MWNT interface on crystallization and on melting behavior of matrix PP were investigated. The results were compared with a pristine MWNT (p‐MWNT) incorporated system. It was shown that PP coated CNTs can serve as a strong nucleating agent for templated polymer crystal growth. Unlike other PP nanocomposites in the literature, a relatively high shift of 7°C in melting peak maximum (T_p), along with a sharp melt endotherm was achieved with the addition of 0.3 wt% f‐MWNT via PP/f‐MWNT master batch. This indicates refinement of matrix PP crystalline region due to the tailored f‐MWNT surface chemistry. With a designed self‐seeding and templated crystal growth approach, columnar crystalline interphases were found surrounding MWNT which melted at 10.5°C higher temperature than neat PP crystallized without undergoing the same heat treatment protocol. POLYM. ENG. SCI., 59:1570–1584 2019. © 2019 Society of Plastics Engineers     

Structure‐property relationship of substituted pyrrolidine functionalized CNT epoxy nanocomposite

Carbon nanotubes (CNTs) based polymer nanocomposites hold the promise of delivering exceptional mechanical properties and multifunctional characteristics. However, the realization of exceptional properties of CNT based nanocomposites is dependent on CNT dispersion and CNT‐matrix adhesion. To this end, we modified MWCNTs by Prato reaction to yield aromatic (phenyl and 2‐hydroxy‐4‐methoxyphenyl) substituted pyrrolidine functionalized CNTs (fCNT1 and fCNT2) and aliphatic (2‐ethylbutyl and n‐octyl) substituted pyrrolidine functionalized CNTs (fCNT3 and fCNT4). The functionalization of CNTs was established by Thermogravimetric analysis (TGA), Raman Spectroscopy, and XPS techniques. Optical micrographs of fCNT epoxy mixture showed smaller aggregates compared to pristine CNT epoxy mixture. A comparison of the tensile results and onset decomposition temperature of fCNT/epoxy nanocomposite showed that aliphatic substituted pyrrolidine fCNT epoxy nanocomposites have higher onset decomposition temperature and higher tensile toughness than aromatic substituted pyrrolidine fCNT epoxy nanocomposites, which is consistent with the dispersion results of fCNTs in the epoxy matrix. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42284.     

Preparation and thermal,electrical, and morphological properties of multiwalled carbon nanotubeand epoxy composites

Multiwalled carbon nanotube (MWCNT)/epoxy composites are prepared, and the characteristics and morphological properties are studied. Scanning electron microscopy microphotographs show that MWCNTs are dispersed on the nanoscale in the epoxy resin. The glass‐transition temperature (T_g) of MWCNT/epoxy composites is dramatically increased with the addition of 0.5 wt % MWCNT. The T_g increases from 167°C for neat epoxy to 189°C for 0.5 wt % CNT/epoxy. The surface resistivity and bulk resistivity are decreased when MWCNT is added to the epoxy resins. The surface resistivity of CNT/epoxy composites decreases from 4.92 × 10¹² Ω for neat epoxy to 3.03 × 10⁹ Ω for 1 wt % MWCNT/epoxy. The bulk resistivity decreases from 8.21 × 10¹⁶ Ω cm for neat epoxy to 6.72 × 10⁸ Ω cm for 1 wt % MWCNT/epoxy. The dielectric constant increases from 3.5 for neat epoxy to 5.5 for 1 wt % MWCNT/epoxy. However, the coefficient of thermal expansion is not affected when the MWCNT content is less than 0.5 wt %. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1272–1278, 2007     

Poly(trimethylene terephthalate‐block‐tetramethylene oxide) elastomer/single‐walled carbon nanotubes nanocomposites: Synthesis,structure, and properties

Nanocomposites based on poly(trimethylene terephthalate)‐block‐poly(tetramethylene oxide) (PTT‐PTMO)‐segmented copolymer and COOH‐functionalized single‐walled carbon nanotubes (SWCNTs) were prepared by in situ polymerization method. The obtained nanocomposites were characterized by thermogravimetric analysis, scanning electron microscopy, differential scanning calorimetry (DSC), DMTA, wide‐angle x‐ray scattering (WAXS), small‐angle X‐ray scattering, and tensile testing. The nanocomposites with low SWCNTs loading (<0.5 wt %) shows uniform dispersion of CNT in polymer matrix. As the SWCNTs loading in the nanocomposites increase, the significant improvement of thermo‐oxidative stability was observed. It was found that the nanocomposites have slightly higher degree of crystallinity (determined by DSC and WAXS) of poly(trimethylene terephthalate) (PTT) hard phase than neat PTT‐PTMO copolymer. The melting point of PTT hard phase and glass transition temperature of poly(tetramethylene oxide)‐rich phase were not affected by the presence of CNTs in polymer matrix. The SWCNTs played a role as nucleating agent in PTT‐PTMO matrix, which led to increase in the crystallization rate. Tensile tests showed that the tensile strength of the nanocomposites with 0.05–0.3 wt % loading of SWCNTs have improved tensile strength in comparison to the neat PTT‐PTMO copolymer without reduction elongation at break. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Multifunctional liquid‐like graphene@fe3o4 hybrid nanofluid and its epoxy nanocomposites

Novel multifunctional graphene@Fe₃O₄ hybrid nanofluid with liquid‐like behavior at room temperature and low viscosity (5.5 Pa.s at 25°C) has been developed by using GO@Fe₃O₄ as a core, sulfuric acid‐terminated organosilanes as corona, and polyether amine as canopy. The obtained nanofluid has been extensively characterized by various analytical techniques. The content of graphene@Fe₃O₄ reaches up to 13.78 wt% in the hybrid nanofluid, which is a superparamagnetism material with specific magnetization of 0.39 emu/g. In addition, it shows excellent dispersion and exfoliation in solvents and polymer matrixes. It is then compounded with an epoxy resin for the development of liquid‐like graphene@Fe₃O₄ hybrid nanofluid/epoxy nanocomposites. It is found that liquid‐like graphene@Fe₃O₄ hybrid nanofluid/epoxy nanocomposites demonstrate better mechanical property than those of unmodified graphite/epoxy nanocomposites at the same fraction. For example, the multifunctional liquid‐like graphene@Fe₃O₄ hybrid nanofluid at 1.5 wt% improves the impact toughness of epoxy by 125%. This new interface modification also enhanced the T_g of neat epoxy from 98.37 to 129.46°C. POLYM. COMPOS., 37:3474–3485, 2016. © 2015 Society of Plastics Engineers     

Toughness and roughness in hybrid nanocomposites of an epoxy matrix

The present paper investigates the relationship between roughness and toughening mechanisms in hybrid epoxy nanocomposites with carbon nanotubes (CNT) and graphene nanoplatelets (GNPs). The role of adding a block copolymer (BC) to the studied systems was also investigated. The nanocomposites were prepared by means of high‐energy sonication and in situ polymerization. All nanocomposites presented higher numerical values for K_Ic than untoughened systems. The system containing 0.5 wt% of CNTs presented an increase of 35% in K_Ic compared to neat epoxy, and the hybrid nanocomposite, at the proportion of 1:1 (CNT:GNP), with 0.5 wt% total of nanoparticles and also containing 0.5 wt% of BC, had an increase of 34% compared to the neat epoxy. Systems with higher amounts of graphene showed the highest roughness values, having crack deflection/exfoliation between the GNP layers as the main toughening mechanism. On the other hand, systems with more CNTs presented a lower fracture surface roughness, and the main toughening mechanism was bridging/break‐up of the nanotubes. Hybrid systems have more types of mechanisms than simple ones. With only one type of nanoparticle, however, some of those mechanisms are not effective in increasing the toughness, only increasing the fracture surface roughness. POLYM. ENG. SCI., 59:1258–1269 2019. © 2019 Society of Plastics Engineers     

Influence of rubber content on mechanical,thermal, and morphological behavior of natural rubber toughened poly(lactic acid)–multiwalled carbon nanotube nanocomposites

The effects of natural rubber (NR) on the mechanical, thermal, and morphological properties of multiwalled carbon nanotube (CNT) reinforced poly(lactic acid) (PLA) nanocomposites prepared by melt blending were investigated. A PLA/NR blend and PLA/CNT nanocomposites were also produced for comparison. The tensile strength and Young's modulus of PLA/CNT nanocomposites improved significantly, whereas the impact strength decreased compared to neat PLA. The incorporation of NR into PLA/CNT significantly improved the impact strength and elongation at break of the nanocomposites, which showed approximately 200% and 850% increases at 20 wt % NR, respectively. However, the tensile strength and Young's modulus of PLA/NR/CNT nanocomposites decreased compared to PLA/CNT nanocomposites. The morphology analysis showed the homogeneous dispersion of NR particles in PLA/NR/CNT nanocomposites, while CNTs preferentially reside in the NR phase rather than the PLA matrix. In addition, the incorporation of NR into PLA/CNT lowered the thermal stability and glass‐transition temperature of the nanocomposites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44344.     

Creep performance of CNT reinforced glass fiber/epoxy composites: Roles of temperature and stress

Carbon nanotubes (CNTs) have been emerged as a potential nanofiller to reinforce polymeric materials to improve their mechanical properties, like strength and modulus. However, time-dependent deformation of such materials under a constant load and elevated temperature is a matter of concern for long-term durability of these materials. The present article primarily demonstrates the effects of creep temperature and stress on the reinforcement efficiency of CNT in a glass fiber/epoxy (GE) composite. Two types of materials were investigated in this study—GE which was used as a control material, as well as CNT embedded GE composite. To elucidate the impact of CNT on the long-term durability of GE composite, creep tests have been performed at different temperatures (50, 80, and 110 °C) under bending loading. As applied stress has also significant contribution toward the elevated creep deformation of materials, creep tests have also been carried out under different stresses (5, 10, and 40 MPa). The strength of the CNT-GE composite exhibited 8.7 and 18.3% higher than that of control GE composite under tensile and bending load, respectively. Results suggest CNT reinforcement to be beneficial for low temperature applications, both in terms of creep strain and strain rate. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47674.     

Facile preparation of modified carbon nanotube‐reinforced PBT nanocomposites with enhanced thermal,flame retardancy,and mechanical properties

In this study, the carbon nanotube was modified by inorganic acids to introduce the carboxylic acid groups on the surface. The modified carbon nanotube (m‐CNT)‐reinforced poly(1,4‐butylene terephthalate) (PBT) nanocomposites were prepared through melt blending method. Morphological observations revealed that the m‐CNT particles were homogeneously dispersed in PBT matrix. Differential scanning calorimeter (DSC) analysis showed that a very small quantity of m‐CNT can significantly increase the crystallization temperature of PBT. The improvement of thermal stability and tensile strength/modulus of the nanocomposites strongly depended on the uniform dispersion of m‐CNT and the interactions between m‐CNT and PBT through hydrogen‐bonding formation. In the cone calorimeter testing, the PHRR decreased from 1189 kW/m² for neat PBT to 737 kW/m² for PBT/0.9m‐CNT containing 0.9 wt% m‐CNT with a reduction of 38%. The remarkable enhancement of flame retardancy properties was attributed to the condensed‐phase effect acted by the m‐CNT. POLYM. COMPOS., 37:1812–1820, 2016. © 2014 Society of Plastics Engineers     

Synthesis and characterization of (Fe3O4/MWCNTs)/ epoxy nanocomposites

A sonochemical technique is used for in situ coating of iron oxide (Fe₃O₄) nanoparticles on outer surface of MWCNTs. These Fe₃O₄/MWCNTs were characterized using a high‐resolution transmission electron microscope (HRTEM), X‐ray diffraction, and thermogravimetric analysis. The as‐prepared Fe₃O₄/MWCNTs composite nanoparticles were further used as reinforcing fillers in epoxy‐based resin (Epon‐828). The nanocomposites of epoxy were prepared by infusion of (0.5 and 1.0 wt %) pristine MWCNTs and Fe₃O₄/MWCNTs composite nanoparticles. For comparison purposes, the neat epoxy resin was also prepared in the same procedure as the nanocomposites, only without nanoparticles. The thermal, mechanical, and morphological tests were carried out for neat and nanocomposites. The compression test results show that the highest improvements in compressive modulus (38%) and strength (8%) were observed for 0.5 wt % loading of Fe₃O₄/MWCNTs. HRTEM results show the uniform dispersion of Fe₃O₄/MWCNTs nanoparticles in epoxy when compared with the dispersion of MWCNTs. These Fe₃O₄/MWCNTs nanoparticles‐infused epoxy nanocomposite shows an increase in glass transition (T_g) temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of EP‐POSS mixture and the properties of EP‐POSS/epoxy,SiO2/epoxy,and SiO2/EP‐POSS/epoxy nanocomposite

The hybrid material of EP‐POSS mixture was synthesized by the hydrolysis and condensation of (γ‐glycidoxypropyl) trimethoxysilane. A series of binary systems of EP‐POSS/epoxy blends, epoxy resin modified by silica nanoparticles (SiO₂/epoxy), and ternary system of SiO₂/EP‐POSS/epoxy nanocomposite were prepared. The dispersion of SiO₂ in the matrices was evidenced by transmission electron micrograph, and the mechanical properties, that is, flexural strength, flexural modulus, and impact strength were examined for EP‐POSS/epoxy blends, SiO₂/epoxy, and SiO₂/EP‐POSS/epoxy, respectively. The fractured surface of the impact samples was observed by scanning electron micrograph. Thermogravimetry analysis were applied to investigate the different thermal stabilities of the binary system and ternary system by introducing EP‐POSS and SiO₂ to epoxy resin. The results showed that the impact strength, flexural strength, and modulus of the SiO₂/EP‐POSS/epoxy system increased around by 57.9, 14.1, and 44.0% compared with the pure epoxy resin, T_i, T_max and the residues of the ternary system were 387°C, 426°C, and 25.2%, increased remarkably by 20°C, 11°C and 101.6% in contrast to the pure epoxy resin, which was also higher than the binary systems. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 810‐819, 2013     

Effect of processing techniques on the performance of Epoxy/MWCNT nanocomposites

The aim of this study is to investigate the optimum technique to disperse Multi‐Walled Carbon Nanotubes (MWCNTs) in SC‐1 epoxy uniformly and to evaluate the effect of processing technique on the performance of SC‐1 epoxy. To achieve better dispersion, MWCNT was mixed with SC‐1 resin directly or premixed with a solvent and then mixed with SC‐1 resin after evaporating the solvent using sonication, thinky mixing and three‐roll mill methods either in isolation or combination. Flexural tests were performed to evaluate mechanical performances and results exhibit up to 27.13, 13.51, and 21.99% improvement on flexural strength, flexural modulus, and maximum strain, respectively, over neat epoxy with only 0.2 wt % loading. Dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) indicated improvement in storage modulus, T_g, inflection temperature, and residue content, respectively over neat SC‐1 epoxy. Thermal and mechanical properties at higher loading conditions were seen to either reduce or not significantly improve. This was attributed to high viscosity of nanocomposites as determined by rheological analysis which prevents good dispersion of MWNCTs into epoxy system at 0.4 wt % loading. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Analysis of the dielectric spectroscopy of an epoxy‐ZnO nanocomposite using the universal relaxation law

There is currently no unified theory that explains the role of nanofillers in polymeric insulation. Consequently, different methods such as dielectric spectroscopy analysis are used to understand this role. The existence of a strong interaction between nanoparticles and a polymer is widely accepted. However, the interaction between dipoles has been taken into consideration in Jonscher's Universal Relaxation Law (URL) but not in the Debye theory. Hence, the URL was used in this article as a method to analyze the spectra of epoxy‐ZnO nanocomposites and to try to determine the nanoeffect mechanism. At 160°C above T_g (116°C), the typical phenomenon of low‐frequency dispersion occurs in the epoxy‐ZnO nanocomposites, and with filler loading increased up to 5 wt %, the value of n (calculated from the ε_r″ spectrum of the nanocomposites) increases both at high and low frequencies. This result indicates that higher loadings correspond to lower dielectric losses changing per radian. Moreover, the decrease in quasi‐direction conduction indicates that the gross dielectric losses of the nanocomposites are also less than pure epoxy resin (EP) at low frequencies. This finding may be attributed to the strong bonding between the nanofillers and the EP, which limits the movement of some macromolecular chains. The URL provides an analytical method for determining the mechanism of the effect of nanofillers on polymers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

The effect of fluorosilicone modifiers on the carbon nanotube networks in epoxy matrix

Structure and properties of polymer compositions based on carbon nanotubes (CNTs) filled epoxy matrix containing fluorosilicone copolymers as additives is discussed. Electrical conductivity and dielectric (microwave) permittivity of the composites can be varied by approximately one order of magnitude without changing the CNT concentration, by careful selection of the additive type and concentration. The mutual solubility of the modifiers and epoxy is a key factor determining both rheological properties of the uncured compositions and electrical properties of cured CNT‐nanocomposites. CNT‐nanocomposites modified with amino‐functional (i.e., epoxy crosslinkable) copolymers demonstrate improved electrical conductivity values at increased additive concentration, connected with the formation of specific segregated microstructure. Fluorosilicone additives added in a specific amount also allow for a decrease of the viscosity of uncured epoxy CNT‐nanocomposites, improving their processability. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46539.     

Properties of a reactive‐graphitic‐carbon‐nanofibers‐reinforced epoxy

Our previous studies showed that herringbone graphitic GNFs surface‐derivatized with reactive linker molecules bearing pendant primary amino functional groups capable of binding covalently to epoxy resins. Of special importance, herringbone GNFs derivatized with 3,4′‐oxydianiline (GNF‐ODA) were found to react with neat butyl glycidyl ether to form mono‐, di‐, tri‐, and tetra‐glycidyl oligomers covalently coupled to the ODA pendant amino group. The resulting reactive GNF‐ODA (butyl glycidyl)_n nanofibers, r‐GNF‐ODA, are especially well suited for reactive, covalent incorporation into epoxy resins during thermal curing. Based on these studies, nanocomposites reinforced by the r‐GNF‐ODA nanofibers at nanofiber loadings of 0.15–1.3 wt% were prepared. Flexural property of cured r‐GNF‐ODA/epoxy nanocomposites were measured through three‐point‐bending tests. Thermal properties, including glass transition temperature (T_g) and coefficient of thermal expansion (CTE) for the nanocomposites, were investigated using thermal mechanical analysis. The nanocomposites containing 0.3 wt% of the nanofibers gives the highest mechanical properties. At this 0.3‐wt% fiber loading, the flexural strength, modulus and breaking strain of the particular nanocomposite are increased by about 26, 20, and 30%, respectively, compared to that of pure epoxy matrix. Moreover, the T_g value is the highest for this nanocomposite, 14°C higher than that of pure epoxy. The almost constant change in CTEs before and after T_g, and very close to the change of pure epoxy, is in agreement with our previous study results on a chemical bond existing between the r‐GNF‐ODA nanofibers and epoxy resin in the resulting nanocomposites. POLYM. COMPOS., 28:605–611, 2007. © 2007 Society of Plastics Engineers     

Relationships between stoichiometry,microstructure, and properties for amine‐cured epoxies

Changes in microstructure and mechanical properties are investigated as a function of epoxy–amine stoichiometry. The epoxy–amine system studied exhibits a two‐phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material. The size distribution of the microgel regions tends to increase with increasing amine content. Concurrently, the connectivity of the softer phase increases dramatically. This two‐phase structure is inherently fractal, exhibiting a single glass transition temperature, T_g. The T_g and elevated‐temperature properties of the epoxy are directly correlated with crosslink density and the percentage of microgel phase observed in microstructure studies. The fracture toughness at room temperature increases with increasing amine content, most likely due to the increased presence of the soft phase, which absorbs more energy during crack growth. Changes in modulus values at 30°C with stoichiometry are explained by considering the effective aspect ratio of the polymer structure in the determination of sample rigidity. Relationships between microgel sizes and the sizes of interphase regions that form in composite and adhesive systems are also discussed in terms of interphase properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 699–712, 1999