Van Son Vo

Clay nanoparticles (CNP) are abundantly available low-cost natural resources with numer- ous positive attributes such as large surface area, impermeability to gas, superior mechanical and thermal properties so that they have attracted over the last three decades significant attention, notably for the reinforcement of polymer-based materials. However, CNP suffer from incompati- bility, hence weak interfacial interactions and poor dispersion with/in most of organic polymeric materials because of their intrinsic hydrophilicity and strong interlayer interactions. Thus, one of the key challenges in developing clay-based polymer nanocomposites (PCNs) with advanced me- chanical properties relies on the control at the molecular level of the interface properties of clay nanoplatelets-filled polymer resins.

Taking into account the criteria for sustainable development, civil engineering and green econ- omy, we have developed, in the first part of this thesis, reactive and pre-exfoliated clay nanofillers that may be further incorporated in a diverse set of biopolymer matrices and giving rise to strong energy interactions with the said matrices for improved mechanical behavior. To ensure a closer fit of these specifications we have implemented green approaches for the preparation of these generic nanofillers, namely photopolymerisation was used as a low energy consumption and fast method for the surface functionalization of native clays, solvent-free protocols were applied to prepare polymer nanocomposites, while biopolymers (starch, cellulose) or bio-based precursors (epoxidized vegetal oils) served as dispersion media.

Our major results from the first part can be summarized as follows:

  • Morphology and reactivity of clay nanofillers are easily controlled though adjusting the pho- topolymerization time and selecting adequate vinyl monomer.
  • The newly preparation methods allow preparation of samples beyond the gram-scale.
  • Reactive and surface chemistry of pre-exfoliated clay nanofillers can be tuned to providecompatibility with both conventional preformed biopolymers and bio-based epoxy resins.
  • The mechanical properties of the resulting polymer nanocomposites are improved as com- pared to the neat polymeric matrices owing to the strong interface interaction between fillers and dispersion matrices.Due to the complexity of chemical components at the nanoscale and the uncertainties in controlling the dispersion of nanofillers, experimental characterization techniques may not be suf- ficient for understanding most important factors that improve the expected behavior of developed PCNs. Therefore, the second part of this thesis focused on developing some models and numerical frameworks for investigating the multiscale mechanical behavior of the PCNs.

    At the nanoscale, the main interest is the arrangement of constitutive components of PCNs, particularly in the interphase zone. By using the research code DL POLY, molecular dynamics (MD) simulations were performed, providing insights into the molecular interaction in the inter- phase zone for nanocomposite structure with different morphological configuration such as exfoli- ation or intercalation. The thermodynamic properties of the confined polymer in each structure

are also investigated in order to clarify the influence of silicate layer on the properties of nanocom- posites. In addition, these molecular models allow us to numerically estimate the effective elastic properties of the PCN models at different temperatures.

At the microscale, a 3D finite element model has been developed to compute the macroscopic elastic properties of the PCNs according to their microstructures and the mechanical properties of each of the constitutive components. Microstructural parameters of clay or clay stacks such as elasticity modulus, aspect ratio, basal spacing, and its volume fraction have been taken into account in the model. A parametric study on effects of these parameters on the macroscopic effective elastic properties of the PCNs has been carefully investigated. The important role of interphase on the mechanical properties enhancement of PCNs has also been demonstrated through our numerical models. The comparison with the experimental result or analytical prediction extracted from the literature has been performed.

Keywords: clay nanoparticles, reactive clay nanofillers, polymer/clay nanocomposites, in situ polymerization , structure-property relationships, finite element method, molecular dynamics simulation.

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