Mechanical and Thermal Properties of Polymers: Insight from Atomistic Simulations
Rajesh Khara, Chemical Engineering, Texas Tech University
Monday, January 26, 2015
Biodesign Institute, Building A (BDA) 105, Tempe campus [map]
Physical properties of polymeric systems are governed by their molecular topology and the specific chemical interactions. Molecular simulations provide the unique ability to investigate the microscopic mechanisms underlying the structure-property relationships in these systems.
In the first part Khara will discuss the application of such a methodology to a model polymer nanocomposite system formed by cross-linked epoxy and carbon nanotubes (CNTs). Their simulation results show that the poor interfacial interactions between the CNTs and the polymer matrix lead to a weak, compressible interphase region between these two components. This weak interphase region has several consequences: for example, the enhanced compressibility of the interphase reduces the ability of the pristine CNTs to increase the stiffness of the matrix, thus limiting the Young’s modulus increase in the system. On the other hand, functionalization of the CNTs leads to the creation of covalent bonds between the CNTs and the matrix, and pins the CNTs to the matrix. This aspect leads to the creation of a less compressible interphase region. Consequently, the Young’s modulus of the nanocomposite containing functionalized CNTs is significantly higher than that of the neat polymer. An increase in the thermal conductivity is observed as well. The timescales sampled by atomistic simulations and those sampled by experiments differ by several orders of magnitude.
In the second part, Khara will demonstrate that the ideas of time-temperature-superposition (TTS) can be applied to bridge this large timescale gap for two aspects of interest for polymeric systems: the glass transition and viscoelasticity. The approach can be used to design high strain rate applications such as protective armor in the defense sector which are difficult to study by conventional rheometry.
Rajesh Khare has more than 20 years of experience working in the field of molecular simulations spanning both academia and industry. Following his Ph.D. (University of Delaware) and post-doctoral (University of Wisconsin-Madison) research, he worked in various scientific and managerial capacities in the R&D group at Accelrys, Inc. Subsequently, he joined the faculty of the Chemical Engineering Department at Texas Tech University in 2005 where he is currently Associate Prof. The main underlying theme of Prof. Khare’s research is the innovative use of molecular simulations for the prediction of glass transition and mechanical properties of polymeric materials, quantification of nanoscale transport phenomena and efficient production of cellulosic biofuels. An important thrust of Prof. Khare’s work is to make a quantitative connection between the results of molecular simulations and laboratory experiments.