Atomistic Modeling of the Mechanical Properties of Carbon-based Nanomaterials


Student thesis: Doctoral Thesis

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Awarding Institution
Award date14 Aug 2020


Carbon-based nanomaterials have attracted intensive research interests due to their superlative mechanical, thermal, electrical and optical properties, and they are regarded as promising building blocks for micro-electromechanical devices and composites through “bottom-up design” strategy. To date, micro-systems with integrated nanoscale structures and multi-functions present a multidisciplinary challenge due to misunderstanding of the properties at nanoscale horizon. Owing to the experimental inaccessibility of atomic insights into the structural morphology, it becomes significant to utilized modelling techniques to unveil the working mechanism of these integrated nanoscale structures.

Generally, utilizing a bottom-up strategy, the chemical interconnecting sp3 covalent bond between carbon building blocks is an efficient way to enhance its Young's modulus and ductility. The formation of sp3 covalent bond, however, inevitably degrades its ultimate tensile strength caused by stress concentration at the junction. By performing a molecular dynamics (MD) simulation of tensile deformation for a fullerene-carbon nanotube (FCNT) structure, we propose a tunable strategy in which fullerenes with various angle energy absorption capacities are utilized as building blocks to tune their ductile behavior, while still maintaining a good ultimate tensile strength of the carbon building blocks. A higher ultimate tensile strength is revealed with the reduction of stress concentration at the junction. A brittle-to-ductile transition during the tensile deformation is detected through the structural modification. The development of ductile behavior is attributed to the improvement of energy propagation ability during the fracture initiation, in which the released energy from bonds fracture is mitigated properly, leading to the further development of mechanical properties.

A polymerized fullerite membrane based on the proposed FCNT structure is developed for water desalination. It is shown that the polymerized fullerite membrane enables outstanding water permeability with perfect salt ion rejection. Compared to the conventional reverse osmosis and nanoporous graphene, the water permeability is found to be much higher. A collective motion of hoping single-file water through a nanopore, which is tuned through desalination velocity and temperature, is identified and proved to be of great significance in enhancing water permeability. The polymerized fullerite membrane is found to suffer from bending deformation at high hydraulic pressure, leading to pore enlargement and degradation of salt rejection. An optimization scheme is provided to ensure a sustainable desalination performance. These insights shed light on polymerized fullerite as a prospective membrane for water purification and provide theoretical guidelines for achieving fast water permeation through collection motion of single-file water.

Integrating molecular and nanoscale components into polymer matrix are of great significance to achieve improvements on the systems. One-dimensional diamond nanothread (DNT) has drawn intensive research interests and become a promising candidate for nanocomposites reinforcement. The mechanical properties of DNT reinforced poly (methyl methacrylate) (PMMA) composite under tensile deformation are explored. The study shows that the Young's modulus and ultimate stress of PMMA composite are enhanced by 85% and 15% with the incorporation of DNT. Remarkably, DNT which is a hydrogenated carbon nanotube (CNT) is proved to strengthen PMMA composite more effectively than CNT for a similar structure. The outstanding strengthening of DNT is attributed to interfacial interaction and mechanical interlocking between DNT and PMMA matrix. A pull-out simulation is conducted to examine the interfacial shear strength of DNT-PMMA interface and comparison studies are made with that of CNT-PMMA interface. The results reveal that DNT has higher load transference within PMMA composite than CNT, by presenting 34% above CNT in interfacial shear strength. It is also demonstrated that DNT morphology can significantly affect the interfacial interaction and mechanical interlocking with PMMA matrix, leading to distinguished reinforcement efficiency for PMMA composite. These findings will shed light to DNT application in nanocomposites and provide an important insight into reinforcing mechanism.

Coarse-grained MD simulation is further carried out to determine the cavitation and fracture mechanisms of DNT reinforced PMMA composite under tensile deformation. It is discovered that cavities initiate at the interface between DNT and PMMA matrix, and cracks propagation is accompanied by continuous cavities nucleation in the bulk matrix as the stress increases. The DNT network is favorable to stabilize the cavities generation and block the cavities nucleation through bridging, leading to improvement of mechanical properties. Further investigations reveal that time-limited relaxation of chain segments at a higher strain rate could also postpone the cavities nucleation and improve the intensity of strain-softening. It is also disclosed that the DNT agglomerates can greatly affect the cavitation and cracks propagation. These findings will provide valuable insights into the fundamental fracture mechanism of fiber reinforced polymer composites, and contribute to theoretical guidance for engineering design of advanced polymer composites.

The influences of DNT on the glass transition temperature (Tg) of PMMA composites are further studied through MD simulation. It is demonstrated that DNT exhibits better improvement than other carbon-based nanomaterials in enhancing the Tg of PMMA composite, suggesting that DNT is a promising reinforcement for polymer nanocomposite with higher service temperature and better mechanical performances. Significantly, we find that interfacial interactions including van der Waals interaction and mechanical interlocking play an important part in glass transition of PMMA composite. The transition from glassy state to rubbery is induced through the interfacial debonding brought by the enlargement of free volume at the interface. According to the interfacial degrading mechanism, cross links between DNT reinforcement and PMMA chains are introduced to provide bidirectional hindrance for free motions of polymer chains, resulting in a 70 K enhancement of Tg of PMMA composites. These findings not only shed light to the prospective application of DNT in advanced nanocomposite, but also provide important guidance to improve the reinforcing efficiency of nanomaterials in engineering application, such as building and aerospace industry.

Moreover, we propose a novel epoxy composite reinforced with two dimensional carbon nitride (C3N) and reveal the interfacial interaction mechanism through density function theory (DFT) and MD simulations. Remarkably, C3N sheet exhibits more impressive performances than graphene (GN) sheet on improving the thermal-mechanical properties of epoxy composites. The Young’s modulus and Tg of C3N reinforced epoxy composite are 20% and 26 K larger than that of GN reinforced epoxy composite, respectively. It is noteworthy in pull-out simulations that the interfacial shear strength of C3N-epoxy interface is 22% larger than that of GN-epoxy interface. The DFT calculations indicate that C3N sheet presents better π-π stacking interaction with benzenes of epoxy chains than the GN sheet, which is attributed to stronger electron-electron adhesion between the aromatic rings of epoxy chains and honeycomb lattice of C3N. Furthermore, hydrogen bonding analysis elucidate that C3N sheet induces a higher capacity of the epoxy composites than GN sheet for hydrogen bonding at the interface, leading to better load transfer between reinforcement and epoxy matrix. These findings not only elucidate the prospective applications of C3N in polymeric systems, but also offer significant insights into the interfacial interaction mechanism for advanced design.