Solution History Effects and Crystal Phase Transformation in Polybutene-1 and Domain Morphology in Polyurethanes
Chau, Kin Wah
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Permalink
https://hdl.handle.net/2142/71807
Description
Title
Solution History Effects and Crystal Phase Transformation in Polybutene-1 and Domain Morphology in Polyurethanes
Author(s)
Chau, Kin Wah
Issue Date
1983
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
Isotactic polybutene-1 (PB-1) can crystallize in four different crystal forms in which the molecules assume three different helical conformations. When PB-1 crystallizes from a dilute solution the crystal form which precipitates is determined by the thermal history imposed on the solution and the crystal structure of the initial PB-1 solid. It is proposed that on dissolution, the helix conformation originally present in the solid may be temporarily retained. With time, it transforms into the helix which is thermodynamically stable at the given solution temperature. On crystallization, such helices form the crystal nuclei and which generate the corresponding crystal form. Measurements of the solution's intrinsic viscosity show that the hydrodynamic volume of a PB-1 molecule in solution varies considerably with the solution's thermal history, lending support to the idea of retention of helices in solution. The tetragonal (--->) twinned hexagonal crystal phase transformation significantly alters the physical and mechanical properties of melt-crystallized PB-1. A study of the transformation mechanism at the molecular level was made. Results from an Avrami analysis imply that nucleation of the twinned hexagonal phase crystals is instantaneous in time and growth of the twinned hexagonal phase constitutes the rate-determining step. Transmission electron microscopy (TEM) reveals multiple nucleation within each tetragonal fibril in the melt-crystallized sample or in each solution-grown tetragonal single crystal. Nucleation is thought to be initiated by stress exerted between irregularly stacked crystal lamellae. Although interlamellar taut tie molecules are believed to accelerate transformation, they are not essential for the process. Many linear polyurethanes composed of alternating hard and soft segments have been shown to be composed of two phases. The hard and soft segments tend to aggregate individually to form hard- and soft-segment domains which are believed to be responsible for the unusual physical and mechanical properties exhibited by polyurethanes. The morphology of such domains in two polyurethanes was characterized by direct TEM observation and other techniques. A correlation was found to exist between the extent of domain formation and the physical and mechanical properties.
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