Molecular Dynamics Simulations And Mechanical Properties Of POSS Nano Hybrid Materials

As a kind of new nanocomposite, polyhedral oligomeric silsesquioxane (POSS) organic-inorganic nano hybrid materials combine POSS units and polymers through physical or chemical means to improve the mechanical and thermal properties of the polymers such as elastic modulus, hardness, strength, glass transition temperature, wear resistance, etc. However, there is no widely accepted theory to explain this modification introduced by the POSS units. In this thesis, molecular mechanics, molecular dynamics and micromechanics are adopted to study the influences on the mechanical and thermal properties of several common polymers by the POSS units. And the mechanism of the reinforcement is discussed from the aspect of structure and energy.The mechanical behaviors of polystyrene (PS) and POSS hybrid polystyrene under uniaxial tensile test are researched and the mechanism of the reinforcement is studied by molecular mechanics simulations. The tensile moduli of the polymers, calculated from the stress-strain curves, indicate that the modulus and the elastic limit of PS are both remarkably increased after cooperating with POSS units. Moreover, the detailed analysis of the deformation during the stretching process reveals that with the increasing load, micro voids appear, grow, connect with each other and eventually lead to the fracture of the polymers. The POSS units behaving as strong anchors connect the polymer chains and prevent the formation of micro voids so that the copolymer may withstand much higher tension load, which results in the enhancement of the mechanical properties. With the same approach, the nano indentation simulations of the two polymers are performed. And the hardness versus indentation depth curves under indenters of different sizes are obtained. A simple equation is verified to describe the relation between the hardnesses and indentation depths. With the aid of the equation, the limit hardnesses of the polymers under different indenters are achieved, and the size effect of the indenters is discussed. The results demonstrate that the limit hardness decreases with the increasing size of the indenter, however, the effect of the reinforcement introduced by the POSS units ascends accordingly.A feasible stress and strain fluctuation formula is derived from least square method and thermodynamics to calculate the elastic constants. The formula, with simple form, good convergency and accuracy, has better universality and stability compared to the previous ones. And the stiffness matrix calculated from the new formula has better symmetry. The advantages of the formula are validated through the molecular dynamics simulations of crystalline Ar at different temperatures.The mechanical and thermal properties of polyethylene (PE) and POSS hybrid polyethylenes (POSS-PE) at different temperatures are studied by molecular dynamics simulations to explore the intrinsic cause of the influence introduced by the POSS units. Previously, three universal molecular force fields are examined via the molecular dynamics simulations of PE and POSS-PE. The results suggest that CVFF force field is appropriate for the simulation of POSS hybrid polymers. Afterwards, the volume-temperature curves of PE and POSS-PE with different POSS contents are depicted from the molecular dynamics simulations. From the break of the slope of the curves, the glass transition temperatures are obtained. Based on the analysis of every part of the potential energy, it is concluded that the torsion of the main chains and the relative movement between the chains contribute to the glass transition. The glass transition temperatures of the polymers are almost the same because of the slight effect of the POSS units on the two factors. On the other hand, the elastic constants of the polymers are calculated from the new fluctuation formula. The results show that the modulus of PE is increased when a certain proportion of POSS units are introduced. Furthermore, the detailed investigation of Hamiltonian implies that the contribution to bulk modulus of bond energy and Van der Waals energy are reinforced in the presentation of POSS units. As for shear modulus, the contribution of bond energy is enhanced, while the one of Coulomb energy is decreased after the combination of POSS units. The modification of elastic properties is due to the comprehensive effect of above causes.The effects on the polymer's moduli according to the content of POSS units are examined by means of micromechanics. And a computational method for the effective moduli of POSS hybrid materials is developed. Based on the three-phase material model, the moduli of POSS unit are calculated from neat POSS material. Afterwards, four combined methods are derived based on the two-phase sphere model. The results calculated from the combined methods are closed to those of homogenization methods. Furthermore, SCMT method is proposed to complete the homogenization methods. And a new formula is deduced from the exploration of the reversibility of the differential schemes. According to the form of the new formula, SCMT and Self-consistent methods, the general self-consistent method for two-phase composites is achieved. The general equation can be used to not only the prediction of the effective moduli of composites, but also the calculation of the moduli of POSS hybrid materials.