Experimental and Molecular Dynamics Simulation Study of Viscosity of Polymer Nanocomposites

Abstract

One of the important dynamic parameter characterizing the properties of polymer nanocomposite is viscosity. It is a quantity of interest on macroscopic scale also. A thorough study of viscosity in case of polymer nanocomposite has not been carried out in the existing literature. In this work we used atomic force microscope, force-distance spectroscopy to experimentally measure the viscosity of polymer and polymer nanocomposite thin films. In particular we try to tune viscosity by changing the nature of interface of polymer grafted nanoparticle and polymer melt. The interface nature in varied by changing the miscibility parameter ( f ), defined as the ratio of grafted chain length to the matrix chain length. Using coarse-grained molecular dynamic simulations, dynamics at the nanoparticle-matrix interface is explored by calculating slip length and mobility at the interface. Equilibrium molecular dynamic simulation is employed to calculate the viscosity of nanocomposite. Chapter 1 We introduce some basic models for polymer chain conformation and dynamics. The known facts about the structural and dynamics of polymer grafted nanoparticle are also described. Chapter 2 We present our experiment method and results for various nanocomposite systems for two different volume fractions of nanoparticles and for two different thicknesses. We show that introduction of nanoparticles causes reduction in viscosity of thin film with respect to the neat polymer films. Further for the low volume fraction system (0:5%) the extent of reduction decreases with increasing f -value and almost matching the neat system at the highest f . At high volume fraction (1%), for lower f we observe a reduction in viscosity and for highest f surprisingly there is an increase in viscosity of nanocomposite with respect to the neat system with a cross-over for intermediate f . We attribute the effects to possible slip at the nanoparticle-matrix interface. A rough estimation of slip length from the measured value of viscosity of nanocomposite and pure polymer is provided which strongly supports our idea of slip at the interface Chapter 3 Briefly discusses some basic aspects of molecular dynamic simulation. Chapter 4 Using MD simulation we calculate the slip-length at the grafted nanoparticle-matrix interface for various systems with different f values. A spherical core grafted with atoms same as the matrix is kept fixed at the canter of simulation box. The particle is rotated for calculating slip length. We also look at the mobility variation of matrix chains as a function of radial distance from the centre of nanoparticle. From both slip-length and mobility calculation we observe that slip length as well as mobility is higher for lower f systems as compared to higher f thus supporting our assertion of slip as the most likely cause for our experimental observations. Chapter 5 Now instead of single grafted nanoparticle we have multiple nanoparticles which are free to move in the matrix. Using Green-Kubo formalism we calculate the equilibrium viscosity for pure polymer and nanocomposite systems from MD simulations. We observe increase in viscosity for nanocomposite system as compared to the pure polymer system. We also look at various structural and dynamical changes, that occurs in the filled system with respect to neat system, that leads to such increase in viscosity.