Doctoral Dissertations

Kenneth Monar

Date of Award

8-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Paul J. Phillips

Committee Members

Roberto Benson

Abstract

Three key methodologies in polymer physics structural and phase analysis: Thermal Analysis, Waxd, and Saxs, are bought to bear on the problem of crystal structure and its development in a new class of Metallocene, Random Ethylene Octene copolymers and especially at high comonomer content. The course of the work is understood through specific problems and their solution. First, the problem of the parallel Tm-Tc behavior exhibited in the Hoffman-Weeks plots of isothermally crystallized samples of ethylene copolymers and the inability to abstract equilibrium melting points has been found to be the result of annealing, but not thickening, at short times at the isothermal crystallization temperature. The annealed part of the melting endotherm is not associated with the melting of the orthorhombic crystal structure and suggest a major new understanding in the so called ‘multiple melting peaks’ in ethylene copolymers. This work indicates that there is only one melt peak. Second, this work sets out to fully characterize the physical and thermophysical properties of the crystalline state(s) to address the claims of multiple phases by other authors. In this regard, none are found but a smoothly changing physical properties landscape of increasing disorder and crystal disruption. Third, Waxd and Saxs analysis are refined to describe the phase morphology where the infinite stack model will be inadequate. Along these lines, several new theoretical and experimental developments have been made. First, a new method of treating the amorphous scattering in reciprocal space has proved crucial in the results. Second, the separation of size and distortion from single Bragg reflections. A new model of distortion is proposed based on ‘a thermally activated slip/kink’ process driven by interfacial content is found to be consistent with the results with respect to unit cell distortion and the mode of lattice expansion. The combined phase analysis suggests that the additional melting peak in ethylene copolymers is the result of interfacial material and increases in proportion with increasing comonomer to eventually represent a hexagonal ordering a part of the interfacial content. Temperature dependent Waxd and reveals a smooth expansion of the unit cell to near coincidence with the hexagonal phase at the melting point and this is the first such study to identify this state (as well as a composite orthorhombic crystal structure) in unmodified polyethylenes. Both the thermophysical constants and microcrystal structure offer no support for the fringed micelle model as an additional (or sole) crystal phase in the high comonomer limit. At the highest branch contents, for samples crystallizing at low temperatures, very low surface are documented and parallel the changes in distortion of the orthorhombic lattice. Saxs analysis confirms the crystal thicknesses determined by the Waxd, but necessitate a new ‘isolated lamella.

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