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Interfacial interaction between host matrix and nanofillers is a determinative parameter on the mechanical and thermal properties of nanocomposites. In this paper, we first investigated interaction between carbon nanotube (CNT) and montmorillonite clay (MMT) absorbing on epoxy surface in a theoretical study based on the density functional theory (DFT) calculations. Results showed the interaction energy of -1.93 and -0.11 eV for MMT/epoxy and CNT/epoxy, respectively. Therefore, the interaction between epoxy polymer and MMT is of the chemisorptions type, while epoxy physically interacts with CNT. In addition, thermal and mechanical analyses were conducted on nanocomposites. In DSC analysis the glass transition temperature which was 70°C in neat epoxy composite showed an improvement to about 90°C in MMT nanocomposites while it was about 70°C for CNT nanocomposites. Finally, mechanical properties were investigated and MMT nanocomposite showed a change in compressive strength which increased from 52.60 Mpa to 72.07 and 92.98 Mpa in CNT and MMT nanocomposites, respectively. Also tensile strength improved to the value of 1250.69 Mpa MMT nanocomposites while it was about 890 Mpa in both CNT nanocomposite and neat epoxy composite which corresponds to the calculation result prediction.

Epoxy matrix composites are one of the most common thermosetting polymers that are widely used because of properties including solvent resistance, thermal stability, and high tensile strength and still new research continues to improve these properties [

In many cases, the results from the DFT calculations for solid-state systems agree quite satisfactorily with experimental data. Therefore, the interaction between different types of nonmaterial as fillers and the epoxy polymers can be studied using this method. In an investigation accomplished by Ghorbanzadeh et al. [_{2} functional groups increase the physisorption capability of the nanotube to approximately -0.41 and -0.38 eV.

The aim of the present work was to predict and compare the interaction between epoxy matrix and SWCNTs and clay as fillers using calculation methods and then proof the results by experimental methods. DFT calculations were used to estimate interaction between epoxy matrix and nanofillers, and the accuracy of the results was tested by mechanical and thermal properties of two different nanocomposites.

Both systems were optimized by the self-consistent charge density functional tight-binding (SCC-DFTB) calculations implemented in the DFTB+ program package [

The montmorillonite model is based on a 5.180 × 8.980 × 15.000 Å^{3} unit cell structure obtained by single crystal X-ray refinement [

Super cell of montmorillonite. (a) front view, (b) side view, and (c) top view.

The mesh cut-off was chosen to be 120 Ry for all calculations. Adding ghost atoms to the calculation of the isolated absorbent, the basis set superposition error (BSSE) was eliminated [

As a matrix for preparing a polymeric nanocomposite, D. E. R. 332 epoxy resin from Sigma Aldrich with an epoxy equivalent weight of 175 equiv.

40 g of epoxy was heated at 50°C and then optimum weight of nanofiller dispersed into the solution by mechanical stirrer in 10 min. In the next step, a stoichiometric ratio of the diamine was added to the mixture and stirred mechanically for additional 10 min and finally ultrasonicated for approximately 15 min. The samples were prepared, molded, and placed in vacuum for 15 minutes in ambient temperature, afterward cured in two cases: for about 30 min in 60°C and then 1 hour in 100°C. Neat epoxy specimens were prepared as the same method.

The optimum weight of nanofiller was obtained by determining the reaction enthalpy of the exothermic peak on dynamically cured samples involving various nanofiller concentrations (0%, 1%, 3%, 5%, 10%, and 15%). The amounts of MWCNT and MMT were selected 5% and 10%, respectively, based on the maximum value of ∆H in DSC measurements. A NETZSCH DSC 200 F3 unit was employed for calorimetric analysis. Dynamic DSC experiments were conducted in nitrogen atmosphere at temperature range of 25–300°C at constant heating rate of 20°C.

Samples were tested by a Santam testing unit. The gauge length was 50 mm. The tensile tests were conducted at 0.5 and 2 mm.

Samples were tested by a Santam testing unit. The gauge length was 12 mm. The compressions were applied at 0.25 mm.^{3} cube specimens (according to ASTM standard D695-96) and at least five specimens were tested for each system.

SEM micrographs on the fracture surface of the epoxy nanocomposite were obtained with a Tescan Scanning Electron Microscope (SEM) operated at 10 kV. In order to observe the images clearly, a thin film of gold was sputtered at 10 mA for 45 s on the surface of the probe using a Hummer 600 sputtering system.

DFT calculations were performed to determine the influence of nanotube on epoxy desorption in bisphenol A epoxy monomer and (10, 0) SWCNT systems. The diameter and length of SWCNT were 7.774 Å and 20.44 Å, respectively. The interaction energy calculated for the situation that epoxy monomer approached the sidewall of the carbon nanotube (Figures

Epoxy monomer approached the sidewall of the carbon nanotube.

The interaction energy of epoxy approaching to the TOT plane of montmorillonite was calculated (Figure

Interaction energy and the equilibrium distances between the closest atom of the epoxy and the layer of montmorillonite.

System | | |
---|---|---|

Site 1 | 2.03 | -0.7 |

Site 2 | 2.1 | -1.93 |

Different situations epoxy monomer approached the surface of MMT and the equilibrium distances between the closest atom of the epoxy and the layer of montmorillonite.

The behavior of electrons in solids depends on the distribution of energy among the electrons. This distribution determines the probability that a given energy state will be occupied, but must be multiplied by the density of states function to weight the probability by the number of states available at a given energy [

DOS diagrams of (a) montmorillonite nanoparticles and epoxy-montmorillonite nanocomposite and (b) SWCNT and Epoxy-SWCNT.

Mullikan population analysis is also accomplished and the results show 0.37 e transfer from MMT to epoxy monomer. This is in fact more than CNT-epoxy system which was 0.2 e. Mullikan population results also show stronger interaction between MMT and epoxy.

Differential Scanning Calorimetry can investigate various characteristics of thermosetting composites such as temperature and calories in curing reaction. Also DSC technique is one of the most common methods to study

Data from dynamic DSC measurements on neat epoxy and CNT/MMT epoxy composites having different loadings of nanofillers.

Mass fraction | 0% | CNT | MMT | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|

1% | 3% | 5% | 10% | 15% | 1% | 3% | 5% | 10% | 15% | ||

∆^{−1} | -470.4 | -410.7 | -420.8 | -446.6 | -375.3 | -358.1 | -417.1 | -432.0 | -443.8 | -458.2 | -437.5 |

Tg/°C | 76.17 | 72.3 | 73.28 | 73.47 | 72.02 | 69.40 | 77.16 | 79.51 | 85.34 | 89.20 | 79.60 |

Peak onset/°C | 55 | 55 | 55 | 55 | 55 | 55 | 60 | 60 | 60 | 60 | 60 |

Peak cure max/°C | 100 | 95.9 | 92.3 | 88.8 | 90.0 | 90.0 | 96.2 | 94.1 | 93.5 | 93,5 | 94 |

DSC curves of 0, 1, 3, 5, 10, and 15% Weight of MMT-epoxy nanocomposites.

DSC curves of 0, 1, 3, 5, 10, and 15% Weight of SWCNT-epoxy nanocomposites.

Obviously a better correction of the glass-transition temperature is observed in the MMT nanocomposite which reached from about 70°C in neat epoxy to about 90°C in MMT nanocomposite (detailed results are presented in Table

Tensile tests of neat epoxy, CNT, and MMT/epoxy nanocomposites specimens were conducted in two different speeds of 0.5 and 2 mm.

Tensile strength properties of block specimens: neat, MMT, and CNT epoxy composites.

Content | Neat Epoxy | CNT- Epoxy | MMT-Epoxy | |||
---|---|---|---|---|---|---|

Rate, mm.min^{−1} | 0.5 | 2 | 0.5 | 2 | 0.5 | 2 |

Bending strength, MPa | 886.28 | 670.39 | 894.05 | 697.60 | 1250.69 | 977.42 |

Bending modulus, GPa | 1.71 | 1.540 | 2.040 | 3.095 | 1.518 | 1.485 |

Elongation | 2.55 | 2.13 | 2.14 | 2.23 | 7.58 | 3.79 |

Stress, MPa | 36.63 | 27.71 | 36.95 | 28.83 | 58.37 | 40.40 |

Figures

Stress strain graphs of neat epoxy and MMT/ SWCNT-epoxy in optimum fraction, conducted at 2 mm.

Stress strain graphs of neat epoxy and MMT/SWCNT-epoxy in optimum fraction, conducted at 0.5 mm.

The results show both systems to be Fragile, so the compressive properties, which are good way to study fragile systems, were also used to measure the performance of nanocomposites.

Typical true stress-strain curves of cubic specimens loaded in static uniaxial compression were illustrated in Figure

The compressive properties of neat, MMT, and CNT epoxy composites.

Compressive property | Neat Epoxy | CNT- Epoxy | MMT-Epoxy |
---|---|---|---|

Elastic modulus, E (MPa) | 735.19 | 342.41 | 225.60 |

Compressive strength, | 52.60 | 72.07 | 92.98 |

Transition strength, | 47.9 | 67.9 | 80.4 |

Compressive stress-strain curves of neat epoxy and MMT/SWCNT-epoxy in optimum fraction, conducted at 0.25 mm.

Nanoparticle-matrix interaction plays a major role in the properties of nanocomposites. The attraction forces between particles, due to the Van der Waals and electrostatic forces, affect the particle-particle interaction and deteriorate the composite’s performances and improved mechanical properties can be achieved through improved interface between the particle and matrix [

Scanning Electron Microscopy is used to reveal micro-structural information of fractured surfaces of composites. Figures

SEM fractograph of 5 wt. % SWCNT nanocomposite.

SEM fractograph of 10 wt. % MMT nanocomposite.

In this study, the interaction energy of the bisphenol A epoxy monomer with two different nanofillers (SWCNT and montmorillonite) was assessed using DFT calculations. The calculated interaction energy for these two epoxy systems in the best situation was -0.11 eV for epoxy-SWCNT and -1.9 eV for montmorillonite epoxy nanocomposite. Calculated results showed that among two nanofillers the epoxy monomer prefers to be adsorbed on the montmorillonite and the interaction energy values obtained from the ab initio calculations are typical for the physical absorption for SWCNT-epoxy system where it was chemisorptions for montmorillonite-epoxy system. Also thermal and mechanical properties of SWCNT and MMT epoxy nanocomposites were tested in optimum loading of nanofillers. As it was predicted from calculations results, better improvement was achieved from MMT-epoxy nanocomposites rather than CNT-epoxy composites. This improvement would be caused of better interaction between montmorillonite surface and epoxy matrix. Finally, by performing thermal and mechanical analysis DFT calculation results have been proved.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

Support from University of Semnan and the Faculty of Engineering of University of Mazandaran is gratefully acknowledged.