Abstract
In this work, the viscoelastic behavior of polypropylene (PP)/multi-walled carbon nanotube (MWCNT) nanocomposites was investigated by indentation testing and phenomenological modeling. Firstly, indentation tests including two-cycle indentation were carried out on PP/MWCNT nanocomposite with three MWCNT loadings (1, 3 and 5 wt %). Next, the Maxwell–Voigt–Kelvin model coupled with two-cycle indentation tests was used to predict the shear creep compliance function and the equivalent indentation modulus. The indentation hardness and elastic modulus of the PP/MWCNT nanocomposites extracted based on the Oliver and Pharr method were compared with the equivalent indentation modulus predicted based on the Maxwell–Voigt–Kelvin mode. The experimental results indicated that the addition of nanotubes into the polypropylene has a positive effect on the micro-mechanical properties of PP/MWCNT nanocomposites. Indentation hardness and elastic modulus increased significantly with increasing MWCNT loading. The creep resistance at the micro-scale of the PP/MWCNT nanocomposites improved with the addition of MWCNTs, with creep displacement reduced by up to 20% by increasing the carbon nanotube loading from 1 to 5 wt %. The Maxwell–Voigt–Kelvin model with three and five Voigt–Kelvin units accurately predicted the shear creep function and its change with increasing MWCNT loading. However, the equivalent indentation modulus was found to be sensitive to the number of Voigt–Kelvin units: the more Voigt–Kelvin units, the better the model predicts the equivalent indentation modulus.
Highlights
Polymer/carbon nanotube (CNT) nanocomposites are currently used in many engineering applications [1,2,3,4] and a lot of research has been devoted to investigating and improving the processing–structure–property relationships, which are summarized in several reviews [5,6,7,8,9,10,11,12,13].Polymer/CNT nanocomposites are typically multiscale materials that are homogeneous in the macro-scale and heterogeneous in the micro- and nano-scales
The equivalent indentation modulus was found to be sensitive to the number of Voigt–Kelvin units: the more Voigt–Kelvin units, the better the model predicts the equivalent indentation modulus
At micro-scale level, the addition of nanotubes into the polypropylene has a positive effect on the mechanical properties of the PP/multi-walled carbon nanotubes (MWCNTs) nanocomposites
Summary
Polymer/carbon nanotube (CNT) nanocomposites are currently used in many engineering applications [1,2,3,4] and a lot of research has been devoted to investigating and improving the processing–structure–property relationships, which are summarized in several reviews [5,6,7,8,9,10,11,12,13]. While for conventional elastic-plastic materials there is a broad consensus about the design of indentation experiments and data analysis, considerable challenges remain for polymers and polymer-based nanocomposites, the key issues are generally well documented [20,23,32,33,34,35,36,37,38,39,40,41,42] This is due to the complexity of polymer matrix deformation during the indentation process. When a sharp indenter is used on viscoelastic materials to extract mechanical properties, both time-independent and time-dependent deformations occur, like time-independent elasto-plastic deformation during the loading stage, time-dependent viscoelastic or visco-plastic deformations during the initial part of the unloading curve, and time-dependent viscoelastic deformation during the holding stage. The elastic modulus, indentation hardness, and creep resistance are discussed in terms of MWCNT loading
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