Ph.D Thesis


Ph.D StudentTamir Erez
SubjectViscoelastic Properties Prediction of Fluoropolymers
through Multiscale Modeling
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Yachin Cohen
ASSOCIATE PROF. Simcha Srebnik
DR. Arieh Sidess
Full Thesis textFull thesis text - English Version


Abstract

The time-dependent mechanical behavior of polymers subjected to continuous stress is an engineering issue of high importance to industrial applications. Since this behavior is governed by molecular characteristics (i.e., atomic structure and molecular weight of the polymer chains) it is extremely important to develop prediction capabilities of the time-dependent response of polymers, by linking the molecular structure to macroscopic properties.

This research is focused on developing a multiscale bottom-up modeling procedure for fluoroelastomers, to predict viscoelastic properties such as stress relaxation.

Thermodynamic, structural and mechanical properties of atomistic models were calculated applying five different force fields found in literature using molecular dynamics simulations. Since these force fields were originally developed for small fluorocarbon systems, their transferability to atomistic amorphous systems of relatively long chains of Viton A was assessed. Differences in thermodynamic and mechanical behaviors observed between the various systems were found to be correlated with structural measures. OPLS (Optimized Parameter for Liquid Simulation) based force fields show preference towards extended spatial structures, which are more mobile under uniaxial deformation due to the relatively soft intermolecular interactions. Among the force fields tested, the PCFF (Polymer Consistent Force Field) force field predicted most satisfactory structural features and thermodynamic behavior around the glass transition temperature (Tg) when compared with experimental observations for Viton A. Thus PCFF was used to calculate the stress relaxation of the atomistic Viton A system using two approaches: (1) Stress-strain curves calculated at various temperatures and strain rates, assuming the data represents linear behavior and time and strain effects on stress are separable, and (2) stress autocorrelation function sampled at equilibrium at various temperatures by using Green-Kubo relation. Data obtained by the two approaches was superimposed onto a master curves at reference temperature of T0 = 25 °C by applying the time-temperature superposition principle. The results were compared to the experimental stress relaxation, derived from dynamic mechanical properties measured by DMA (Dynamic Mechanical Analysis) apparatus. Shifted factors used for the superposition were also compared to the experimental ones. Excellent correlation with experiments was observed for times corresponding to β-relaxation at the onset of the glass transition. However, at shorter times the calculated values were found to be somewhat higher than the experimental. Shift factors obtained from simulations correlate well with Arrhenius equation, while the experimental ones correlate with WLF (Williams, Landel, and Ferry) theroy, suggesting the occourance of sub-Tg β-relaxations at the simulation conditions, while at the experimental conditions they are observed at much lower temperatures.

Finally, coarse-grained model of Viton A was developed using structural matching procedure known as iterative Boltzmann inversion. In the coarse-grained model each underlying atomistic chain was divided into 30 beads containing 3-4 backbone carbons. The elimination of degrees of freedom resulted in relatively soft potentials, systems with fast dynamics. Relaxation modulus values obtained from non-equilibrium simulations were lower than the experimental; however higher values obtained at equilibrium simulations where the behavior observed to match WLF theory with good agreement with the experimental behavior.