Abstract
Processing parameters (e.g., exfoliation extent and volume fraction) of clay particles in polymeric resins play a crucial role in the mechanical properties of polymer nanoclay composites (PNCs). This paper is aimed to investigate the effects of clay aspect ratio and volume fraction on the global mechanical properties (e.g., effective stiffness, yield strength, and ultimate tensile strength) of PNCs. During the process, computational micromechanics models are adopted to simulate the nonlinear elastoplastic behavior of the PNCs of varying clay particle volume fractions and aspect ratios subjected to uniaxial tension. A representative volume element (RVE) of the PNCs is employed for the finite-element-method (FEM) based computational simulations. The polymeric matrix is treated as an idealized elastoplastic solid with isotropic hardening behavior, and the clay particles are treated as stiff elastic platelets distributed evenly in the stack and stagger configurations in the matrix. Seven volume fractions (Vf = 0.5%, 1%, 2%, 5%, 7.5%, 10%, and 15%) and seven aspect ratios (the ratio of platelet length over thickness ρ = 1, 2, 5, 10, 20, 50 and 100) of the reinforcing clay particles are utilized. Numerical experiments show that the effective modulus of the PNCs at small strains increases with the increase of either the clay volume fraction or the platelet aspect ratio largely following those predicted by classic micromechanics models. However, at the low particle aspect ratios (e.g., ρ = 1, 2, 5 and 10), the ultimate tensile strength of the clay composite is nearly independent of the clay volume fraction up to 5% in the present study, i.e., the polymeric matrix governs the PNC strength; at the large particle aspect ratios (e.g., ρ = 20 and 50), the ultimate tensile strength is significantly enhanced with growing clay volume fraction higher than 5% and reaches ~150% of that of the polymeric matrix at ρ = 50 and Vf = 10%. A comparative study is conducted for stack and stagger models for the prediction of the mechanical properties of PNCs. It shows that the stack model predicts slightly larger values of the effective stiffness and tensile strength than the stagger model. The numerical study shows that a large platelet aspect ratio through full exfoliation of the clay particles in matrix is crucial to achieving the preferable mechanical properties of PNCs as evidenced in experiments. The present results can be utilized to quantitatively explain the mechanical properties of clay particle-reinforced composites and PNCs within the framework of classic micromechanics, and provide guidelines for computer-aided nanocomposites design for processing property-tailorable PNCs.
Highlights
Nanocomposites made of polymeric matrices reinforced with intercalated or exfoliated clay nanoparticles have become a focus of research in polymer composites in the last two decades after the seminal research initiated successfully by the Toyota research group in 1980s [1,2]
Nylon-6 nanocomposites prepared through intercalative ring-opening polymerization of ε-caprolacetam modified montmorillonite to form fully exfoliated nano platelets can double the tensile modulus of the virgin nylon matrix at the filler weight content at 4.1% [3]; such giant modulus improvement cannot be achieved when the clay particles are dispersed in polymeric matrix to form intercalated polymer microcomposites [4]
In the present computational micromechanics scaling study, FEM-based nonlinear analysis was conducted to cover a wide range of clay-particle aspect ratios (ρ = 1, 2, 5, 10, 20, 50 and 100) and volume fractions (Vf = 0.5%, 1%, 2%, 5%, 7.5%, and 10%) for both the stack and stagger models
Summary
Nanocomposites made of polymeric matrices reinforced with intercalated or exfoliated clay nanoparticles have become a focus of research in polymer composites in the last two decades after the seminal research initiated successfully by the Toyota research group in 1980s [1,2]. With the assumption that the nanofillers are evenly distributed in the matrix, the effective mechanical properties, mainly the effective moduli, of the resulting nanocomposites can be approximately modeled by adopting Eshelby’s equivalent model, self-consistent models of finite-length fillers, Mori-Tanaka type models, bound models, Halpin-Tsai model and its extensions, and modified shear-lag models, among others [15] By comparison with these classic micromechanics models plus detailed finite element analysis (FEA), Tuchker and Liang concluded that the simple shear-lag model is capable of giving a good estimate of the longitudinal modulus (E11) of short-fiber reinforced composites when the fiber aspect ratio is greater than 10 [15]. Conclusions of the present study and relevant applications were addressed in the last section of the paper
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