Investigating the Effects of CNT Aspect Ratio and Agglomeration on Elastic Constants of Crosslinked Polymer Nanocomposite Using Multiscale Modeling Farshid Aghadavoudi, 1 Hossein Golestanian , 2 Yaghoub Tadi Beni 2 1 Faculty of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Isfahan, Iran 2 Faculty of Engineering, Shahrekord University, Shahrekord 8818634141, Iran Multiscale modeling has been developed to calculate the Young’s modulus of carbon nanotube (CNT) rein- forced epoxy-based nanocomposites. Molecular dynamics was used to construct nanocomposite mod- els consisting of crosslinked network structure of epoxy resin as the matrix material and CNT as the rein- forcement at nanoscale. Transversely, isotropic stiff- ness matrices were calculated using constant strain method on four cases with different CNT chiralities. Effective fiber method was employed to scale bridging from nanoscale to microscale. In multiscale calcula- tions, various types of micromechanical methods were investigated and Halpin–Tsai formulation was selected due to its more realistic predictions. The results showed that increasing CNT aspect ratio from 1 to 1,000 results in an increase in nanocomposite Young’s modulus by about three times for nanocomposite rein- forced with CNT(20,0). Comparing CNT(5,0) with CNT(20,0) reinforced polymer results, suggested that increasing the CNT radius resulted in a decrease in nanocomposite moduli to about one half. Multiscale results indicated that CNT agglomeration can decrease nanocomposite Young’s modulus to one-third com- pared to the dispersed CNT case. Predicted results were compared with numerical and experimental results found in the literature and good agreement was observed. POLYM. COMPOS., 00:000–000, 2017. V C 2017 Society of Plastics Engineers INTRODUCTION Recently nanostructured materials such as carbon nanotube (CNT)-reinforced nanocomposites have been studied as a new generation of materials. CNTs have mul- tipurpose characteristics such as high stiffness, strength, thermal, and electrical properties [1]. Therefore, these characteristics make CNTs excellent candidates for polymer-based nanocomposites. The ratio of surface to volume increases drastically as the size of reinforcement reduces to nanoscale. This phe- nomenon, known as nano-effect, causes some unusual mechanical behavior in nanocomposites compared to con- ventional composites [2]. Due to the nano effect in nano- structured materials, nanocomposite mechanical properties are very sensitive to filler size [3]. Nanocomposite mechanical properties are affected by different factors such as reinforcement size, distribution, orientation, and morphology [4–8]. In CNT-reinforced composites chiral- ity and aspect ratio have great influences on elastic prop- erties. Size effect in thermoset polymer nanocomposites is more observable due to crosslinked network of the matrix [9, 10]. During the crosslinking process in thermoset res- ins, a polymeric network is formed by the reaction of base resin and curing agent. Strong covalent bonding between epoxy and hardner molecules results in excellent properties for the matrix in composite materials [10]. Atomistic modeling has been used to predict physical and mechanical properties of thermoset polymeric struc- tures by many researchers. Yang and Qu used molecular dynamics to study the thermo-mechanical properties of epoxy resins formed by EPN-1180 and bisphenol A as the curing agent [10]. Tam and Lau employed molecular dynamics simulation for modeling the crosslinking of SU- 8 epoxy photo resist resin under three different force fields [11]. Wang et al. used molecular dynamics simulation to investigate the effects of composition ratios of polyacryl- amide (PAM)/poly(vinylalcohol) (PVA) blend on mechani- cal and physical properties of the polymer [12, 13]. Some researchers employed molecular dynamics for modeling the nanocomposite samples at nanoscale [14–16]. Mahboob and Islam determined mechanical properties of CNT-reinforced polyethylene composite using molecular dynamics [15]. Their results showed a decrease in nanocomposite Young’s modulus with an increase in the number of defects in the CNTs. Arash et al. used molecular dynamics to determine mechanical Correspondence to: H. Golestanian; e-mail: golestanian@eng.sku.ac.ir DOI 10.1002/pc.24557 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2017 Society of Plastics Engineers POLYMER COMPOSITES—2017