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|Title:||Molecular dynamics and reaction kinetics during polymerization using dielectric spectroscopy and calorimetry|
|Authors:||Wasylyshyn, Andrew Dwayne|
|Department:||Materials Science and Engineering|
|Keywords:||Materials Science and Engineering;Materials Science and Engineering|
|Abstract:||<p>The evolution of molecular dynamics during the polymerization of linear-chain and network forming liquids was studied using dielectric spectroscopy and differential scanning calorimetry. Polymerization was carried out using step-addition reactions between epoxide and amine molecules, and by catalysis of epoxide molecules with tertiary amines. The former resulted in linear-chain or network structured polymers while the latter resulted in network polymers. The step-addition polymerizations resulted in linear-chain polymers by reacting stoichiometric quantities of diepoxide and monoamine molecules, and network structure polymers by reacting stoichiometric quantities of triepoxide and monoamine molecules, or diepoxide and diamine molecules. The growth and extinction of localized (or secondary) relaxation processes during the polymerization were studied by measuring the changing dielectric properties using two techniques; fixed frequency dielectric measurements during heating of the partially polymerized samples, and isothermal dielectric measurements over the frequency range of 1 MHz to 20 GHz. The number of covalent bonds formed at any instant during the polymerization was determined by isothermal calorimetric measurements. Thus, the change in the dielectric properties during polymerization was associated with the increase in the number of covalent bonds. It was found that the localized relaxations evolve in a manner that is independent of the spontaneous increase in configurational entropy. The results also tend evidence towards the concept that these relaxations occur in regions of relatively high energy, and the collapse of such regions led to the observed changes in the dielectric properties. As well, an analogy was made between the structural relaxation of a physically metastable glass and the polymerization of a chemically metastable liquid. This has lead to the concept of a chemical fictive temperature to describe the state of the polymerizing liquid. The effects of pressure on the dielectric properties of the polymerizing liquids was studied using fixed frequency dielectric measurements. Pressure was step-increased, both at the beginning of the polymerization and at a time during the polymerization, then maintained. An increase in pressure increased the rate of polymerization, and thus the dielectric properties evolved more rapidly with time. The chemical effects of pressure during polymerization were examined using transition state theory and the concept of negative feed-back. The physical effects of pressure were investigated in terms of its effects on the equilibrium dielectric properties. It was shown that the polymerization kinetics are increased with increased pressure when the kinetics are in the mass-controlled regime, and decreased with increased pressure when the kinetics are in the diffusion-controlled regime. The transition from mass- to diffusion-controlled kinetics was investigated using the rate of change of the dielectric loss. Finally, the chemical and physical effects of step-increased pressure upon the dielectric properties of polymerizing liquids in the mass-controlled regime were mathematically simulated using current concepts and formalisms. The simulated results were qualitatively similar to those obtained experimentally, demonstrating the adequacy of the understanding.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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