The effects of polar dopants in polymers subjected to applied electric fields
Abstract
A model doped polymer system has been studied to understand how the local interactions between the polymer and dopant species affect the material properties of the system. This study also examines the effects of the vitrification process and glassy nature of the polymeric material. The polymer was modeled as a self-avoiding random walk on the cubic lattice, and the dopant was modeled as a rigid-rod, also fixed on a cubic lattice. These were chosen to approximate systems currently in use in the areas of electronics and photonics. The pair potentials used in the guest-host system include the hard-sphere potential (volume exclusion) and the square-well potential. These are used to describe weakly coupled systems and short range interactions, respectively. In addition, a system of dopants which interact through a dipole-dipole potential as well as a system of side-chain functionalized polymers which interact through a hard-sphere potential were also used. Monte Carlo simulations based on these models were developed to examine the model systems dynamic behavior under a variety of conditions. Simulations were performed as a function of occupied volume fraction, dopant concentration, polymer chain length, dopant length, and electric field magnitude. Polymer dynamics in the melt phase have been shown to follow Rouse dynamics. The chain length and time scale of interest are too short to observe reptation effects. The orientational distribution in the melt decays exponentially indicating the dopant mobility is largely decoupled from the effects of the polymer relaxations. In the case of the glassy system, the dopant orientational relaxation becomes intimately coupled to the polymer matrix as shown by the non-exponential behavior. Although the polymer relaxations are slowed by this coupling, the chains continue to follow Rouse dynamics. In the side-chain functionalized system, the constrained nature of the polymer-dopant configuration causes the polymer to begin to vitrify. Several precursors of the glass transition are clearly evidenced. The addition of the square-well potential and dopant dipoles greatly increases the coupling of the interspecies motion in the melt. The goal of this study is to understand how the local properties influence the overall behavior of the system.
Degree
Ph.D.
Advisors
Talbot, Purdue University.
Subject Area
Chemical engineering|Materials science|Molecules
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