Periodic Reporting for period 1 - NANOMEC (Mechanical Properties of Polymer Nanocomposites via Multi-scale Modeling: Towards Non-classical Properties)
Reporting period: 2021-04-01 to 2023-03-31
Nowadays, the use of polymer-based nanostructured materials (PNMs) is constantly increasing and expanding over every field of industry and technology. The major challenge in the PNMs research lies in enabling the precise control of their microstructure, and their heterogeneous behaviour, from the molecular level to the nano- and micrometric scale, meeting the high-performance requirements of advanced applications. Furthermore, the challenges are related to the design and the prediction of the mechanical properties, due to the inherently enormous range of spatiotemporal scales, which characterize macromolecules (from nm up to μm length scales, and from a few fs up to sec timescales) and the complexities that manifest themselves at interphases. For these reasons, the modelling of the PNMs behaviour cannot be fully captured by any single simulation technique. At large length scales , computational and analytical techniques, which are based on continuum approaches, have played a great role in our understanding of these processes and they have revealed many important generic phenomena. Nevertheless, these predictions are often obscured by the simplicity of the model, the approximations needed to make them mathematically tractable and that they ignore many fundamental atomistic aspects. At smaller length scales, particle-based computer simulation techniques e.g. atomistic molecular dynamics (MD), are robust techniques to elucidate PNM behaviours but with a limited capacity to predict large length scale cooperative phenomena. Understanding the mechanical behaviour of interphases at the molecular level, with evaluation of their independent structures and properties, must precede the modelling of PNMs. In NANOMEC we aim to integrate detailed microscopic molecular dynamic (MD) simulations and continuum modelling (homogenization) approaches in a systematic multiscale computational framework. The proposed hierarchical computational framework involves a nano/micro/macro coupling approach for predicting the mechanical properties of PNMs.
During the period of the fellowship, the fellow has developed a multi-scale computational methodology, which consists of detailed microscopic simulations and continuum modelling (homogenization) approaches for predicting the spatial distribution of the mechanical properties in polymer nanocomposites (PNCs). The homogenization methodology is based on a systematic nano-meso-macro coupling between detailed atomistic molecular dynamics (MD) simulations and the variational approach based on the Hill-Mandel lemma. The results from MD are used by homogenization theory to develop an effective continuum PNC model via a three-phase micromechanical approach, which takes into account the interphase region.
Moreover, the fellow has developed a methodology starting from detailed atomistic MD simulations of the interphase in order to construct a strain gradient continuum theory of initially the polymer/NP interphase and subsequently of the entire PNC, at the level of the repetitive unit cell. To reach this objective, he obtains the spatial distribution of the mechanical properties of the PNC, including the local hyperstress and gradient field distributions through atomistic MD simulations. From these, we identify a strain gradient constitutive law for the polymer/NP interphase region which then informs the development of a strain gradient homogenized theory for PNCs based on a local strain gradient interphase model.
As graphene is a potential candidate to notably enhance the overall mechanical performance of PNCs, the fellow investigated the mechanical properties of graphene via a hierarchical multiscale approach, combining atomistic simulations and continuum mechanics. Furthermore, he studied the origin of the mechanism of the mechanical reinforcement and the importance of the adsorbed (train) segments at the low-loading regime and of the bridges at the high-loading one. He also found that the mechanical moduli of the matrix region increase non-linearly with volume fraction of nanofiller, unlike the mechanical properties of the interphase region which remain constant. The origin of the mechanical reinforcement in PNCs is attributed to the heterogeneous chain conformations at the vicinity of the NPs, involving a two-fold mechanism. In the low-loading regime, the reinforcement comes mainly from a thin, single-molecule, 2D-like layer of adsorbed (train) polymer segments on the NP, whereas in the high-loading regime, the reinforcement is dominated by the coupling between train and bridge conformations, which involve segments connecting neighboring nanoparticles.
To date, as an output from the fellowship, the fellow has successfully published 4 papers in high impact and prestigious journals as well as three papers under review. The results have been also presented orally, some by the fellow, in prestigious international conferences. In addition, the fellow has invited presentations at the EUROMECH conference in 2022 Nancy-France; Architected materials; recent development and scientific challenges.
Moreover, the fellow has developed a methodology starting from detailed atomistic MD simulations of the interphase in order to construct a strain gradient continuum theory of initially the polymer/NP interphase and subsequently of the entire PNC, at the level of the repetitive unit cell. To reach this objective, he obtains the spatial distribution of the mechanical properties of the PNC, including the local hyperstress and gradient field distributions through atomistic MD simulations. From these, we identify a strain gradient constitutive law for the polymer/NP interphase region which then informs the development of a strain gradient homogenized theory for PNCs based on a local strain gradient interphase model.
As graphene is a potential candidate to notably enhance the overall mechanical performance of PNCs, the fellow investigated the mechanical properties of graphene via a hierarchical multiscale approach, combining atomistic simulations and continuum mechanics. Furthermore, he studied the origin of the mechanism of the mechanical reinforcement and the importance of the adsorbed (train) segments at the low-loading regime and of the bridges at the high-loading one. He also found that the mechanical moduli of the matrix region increase non-linearly with volume fraction of nanofiller, unlike the mechanical properties of the interphase region which remain constant. The origin of the mechanical reinforcement in PNCs is attributed to the heterogeneous chain conformations at the vicinity of the NPs, involving a two-fold mechanism. In the low-loading regime, the reinforcement comes mainly from a thin, single-molecule, 2D-like layer of adsorbed (train) polymer segments on the NP, whereas in the high-loading regime, the reinforcement is dominated by the coupling between train and bridge conformations, which involve segments connecting neighboring nanoparticles.
To date, as an output from the fellowship, the fellow has successfully published 4 papers in high impact and prestigious journals as well as three papers under review. The results have been also presented orally, some by the fellow, in prestigious international conferences. In addition, the fellow has invited presentations at the EUROMECH conference in 2022 Nancy-France; Architected materials; recent development and scientific challenges.
A main challenge in the computational studies of multi-component nanostructured materials involves the prediction of their mechanical properties directly from their moleculular structure, which are inherently heterogeneous. This is particularly important for multi-component systems, such as polymer nanocomposites (PNCs). For this reason, research on the theoretical and computational modeling of mechanical properties of polymer-based nanostructured systems, and especially of PNCs with nanoscale reinforcement fillers, has intensified during the last few decades. Due to the density and conformational heterogeneities in PNCs, a main challenging issue in the theoretical and computational studies of their mechanical behavior is to accurately predict the spatial distribution of stress/strain fields, and, consequently, the distribution of their mechanical properties. In addition, if such data are obtained at the local-atomic level, they can be further used to derive accurate micromechanical models that consider explicitly the polymer/nanofiller interphase. To address such challenges and requirements, we present the method termed “NANOMEC”, which gives us a direct estimation of the local mechanical properties at the atomic level of any, arbitrarily chosen, subdomain within heterogeneous polymer-based nanostructured model systems. As an application, the methodology is used for determining the distribution of mechanical properties in atomistic model PNCs, focusing on the polymer/nanofiller interfacial regions. The new methodology called “NANOMEC” that refers to a computational methodology that involves the direct estimation of the local stress and strain distributions of atomistic model PNCs under an external applied (global) strain field. The local atom-based stresses are calculated via the standard Virial formalism. To compute local strain fields at the atomic level, we propose a new approach which takes into account the local environment around a specific reference atom to compute the deformation gradient. Then, the distribution of the local strain in the atomistic PNCs is presented in 2D slice and 3D cases. The resource of Python codes are available in https://github.com/SimEA-ERA/NANOMEC.git(opens in new window)