Development of Coarse-grained Models in Multi-scale Studies on Materials

多尺度材料研究中的粗粒化模型構建

Student thesis: Doctoral Thesis

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Award date28 Jul 2017

Abstract

At nano- and macro- scale, materials modeling can be achieved by atomistic and continuum approaches separately. At the transition scale in between (from tens of nanometers to several micrometers), mesoscopic modeling technique constitutes as a bridge that smoothly transfers information from the nanoscale to the macroscale. The materials study which involves smoothly connected models spanning multiple length scales is called “multi-scale study”. The target of multi-scale study is to investigate multi-level architectures, which extensively exist in many biological as well as synthetic materials. The outcome of multi-scale study should be contributory to comprehensive understandings of structure-property relationships in materials, spanning the length scales. In the scheme of multi-scale studies, coarse-grained modeling technique plays an important role at the mesoscale. The concept of coarse-graining is to represent a group of fine particles by one interactive site in molecular dynamics. Atoms (containing nucleus and electrons), molecules (a group of atoms) and higher-order assemblies (such as fiber) are all subjected to the coarse-grained concept (atom is finer, though) and can be used as building blocks to construct computational models of material structures. With reduced degree of freedoms, coarse-grained models gain orders of magnitude increase in computational efficiency, which are useful for up-scaling material simulations. In this thesis, coarse-grained models are developed and multi-scale material studies are conducted. The motivation is that coarse-grained models can contribute to solving mesoscale problems. The objective is to illustrate material morphology at the mesoscopic scale. The target materials include chitin and cement. The former material, chitin, is akin to cellulose and is the second most abundant natural polymer in the world. Natural chitin-based materials are featured with extraordinary mechanical properties such as high strength-to-weight ratio, where a typical example is the exoskeleton of crustaceans like crab shells and lobster cuticles. Lobster cuticles are hierarchically structured materials made of chitin and protein. The interface between chitin and protein plays an important role in determining mechanical properties of those natural chitin materials. In applied engineering, chitin can be extracted from natural sources to regenerate textile products, which are of great use in the biology-related fields due to good biocompatibility. For the regenerated chitin products, the mesoscopic structure is closely related to mechanical performance and can be studied following the multi-scale material modeling scheme. In the thesis, a molecule-based, 4-to-1 mapping scheme is employed to construct a coarse-grained model of α-chitin and this model is used to study chitin-protein interface. Meanwhile, as chitin often exists in the form of fibrils, a dissipative particle dynamics (DPD) model of chitin fibrils is also developed and used to study mesoscale (around 100 nm) assembly of chitin fibrils. The latter material, cement, is a major component in concrete. Cement binds aggregates together in concrete and contributes to mechanical properties of concrete. In cement, calcium silicate hydrate (C-S-H) is the major hydration product contributing to the binding force of cement. At the nanoscale, C-S-H is featured with layered structure which can be studied by atomistic simulations. At the mesoscopic scale, C-S-H is found to be composed of disk-like building blocks which can be modeled by coarse-grained particle systems. In this thesis, atomistic and mesoscopic models are employed to investigate material structures as well as loading-bearing mechanics. An atomistic model of double-layer C-S-H is constructed and employed to calculate adhesion energy between the C-S-H layers. Based on the results from atomistic simulations, a coarse-grained model with the concept of disk-like building blocks is developed. Simulations of packing behavior of disk-like objects are performed. C-S-H is supposed to be composed of disk-like building blocks around 5-nm diameter, so, this coarse-grained model reflects the mesoscale (around 100 nm) structure of C-S-H and it is used to study effect of aspect ratio of the disk-like objects. Overall, this thesis reports three kinds of coarse-grained modeling techniques including 4-to-1 mapping (the MARTINI force field), DPD (dissipative particle dynamics) model and aspherical-object modeling. This work demonstrates coarse-graining techniques and successfully employs the techniques in sub-micron modeling of crystalline as well as amorphous materials.