Search

There remain practical problems to predicting structures and properties of materials from first principles, though the foundation, quantum mechanics, has been established for many years. The goals of this research are to develop methods and tools that are accurate and practical, and apply them to important problems. Two aspects of the methodology are focused. 1. The development of accurate force fields based on ab initio quantum mechanical calculations on prototype systems. Procedures were developed on polyvinyl chloride (PVC) and successfully applied on other types of polymers. They are very important to studying of amorphous polymers materials, for which current methods have not been useful in predicting important properties (e.g. moduli and glass temperature). 2. The development of Massive Parallel Simulation (MPSim) Software. MPSim is suitable for large systems (millions of atoms). It has the ability of including environmental variables (temperature, pressure, tension, and shear) and extracting physical properties (moduli and glass temperatures). The theories and algorithms implemented are summarized in the Appendix. These methods and tools are applied to the accurate simulation of structures and properties of amorphous polymer materials and nano-materials. Molecular dynamics (MD) simulation on polyethylene (chapter 6) was used to develop a general strategy for predicting glass transition temperatures which is expected to be very important in polymer industry. In chapter 7, these strategies were successfully applied to three important fluoro polymers. Single-walled carbon nanotubes (SWNT), recently discovered but not very well characterized, is an interesting new class of materials. Using an accurate force field, structures and mechanical properties of these systems are studied. Chapter 2 shows that the dominating factor for deciding stable structures and mechanical properties is the tube size, not chirality. The behavior of (10, 10) nano-tube under bending are

Single- and multi-walled carbon nanotubes have being increasingly used as nanoreinforcements for polymer-based nanocomposites. Since the nanotubes are resilient and have an extremely high elastic modulus, only adding a few percentages of nanotubes into the composites during the manufacturing process could enhance the composites' mechanical strength and fracture toughness. However, many arguments have been arisen regarding which types of nanotube provide the better strengthening performance in composite structures. An experimental observation showed that the inner tubes of multi-walled nanotubes were pulled out after tensile testing. Similar observations were also made regarding multi-walled nanotube /polymer composites. The inner layers of nanotubes were pulled out after an outmost layer of some of the nanotubes was broken. There is no comprehensive study of the straining behaviour inside single- and multi-walled nanotubes to date. Without fully understanding this, it is hard to anticipate the strength and effectiveness of using nanotubes for nanotube/polymer composites. In this letter, the results extracted from molecular dynamic simulations of single- and multi-walled nanotubes subjected to uniaxial straining to study the mechanical properties of these nanotubes is reported.

In this paper, we present extensive molecular mechanics and molecular dynamics studies on the energy, structure, mechanical and vibrational properties of single-wall carbon nanotubes. In our study we employed an accurate interaction potential derived from quantum mechanics. We explored the stability domains of circular and collapsed cross section structures of armchair (n,n), zigzag(n,0) , and chiral (2n,n) isolated single-walled carbon nanotubes (SWNTs) up to a circular cross section radius of 170 Å. We have found three different stability regions based on circular cross section radius. Below 10 Å radius only the circular cross section tubules are stable. Between 10 and 30 Å both circular and collapsed forms are possible, however, the circular cross section SWNTs are energetically favorable. Beyond 30 Å (crossover radius) the collapsed form becomes favorable for all three types of SWNTs. We report the behavior of the SWNTs with radii close to the crossover radius ((45, 45), (80, 0), (70, 35)) under uniaxial compressive and tensile loads. Using classical thin-plane approximation and variation of strain energy as a function of curvature, we calculated the bending modulus of the SWNTs. The calculated bending moduli are [kappa][sub](n,n)=963.44 GPa, [kappa][sub](n,0)=911.64 GPa, and [kappa][sub](2n,n)=935.48 GPa. We also calculated the interlayer spacing between the opposite sides of the tubes and found d[sub](n,n)= 3.38 angstroms, d[sub](2n,n)= 3.39 angstroms, and d[sub](n,0)= 3.41 angstroms. Using an enthalpy optimization method, we have determined the crystal structure and Young's modulus of (10,10) armchair, (17,0) zigzag and (12, 6) chiral forms (which have similar diameter as (10,10)). They all pack in a triangular pattern in two dimensions. Calculated lattice parameters are a[sub](10,10)= 16.78 angstroms, a[sub](17,0)= 16.52 angstroms and a[sub](12,6)= 16.52 angstroms. Using the second derivatives of potential we calculated Young's modulus along the tube axis an

Recent experiments by Lee et al. [Nature (London) 363, 255 (1997)] show that doping carbon single-wall nanotube (SWNT) ropes with K, Rb, or Br2 leads to metallic conductivity, but the structure and properties are not known. We used molecular dynamics to predict structures and properties which should help motivate and interpret experiments on SWNT/K. We find the optimum stoichiometry to be KC16 if the K cannot penetrate the tubes and K1C10 ( K5exoK3endoC80, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for KnC80 from n = 0–10 (optimum is n = 5). The Young's modulus per tube along the tube axis varies from 640 to 525 GPa for n = 0 to 5.