Flow History - Morphology - Property Interactions in Poly(vinylidene Fluoride)
Khomami, Bamin
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https://hdl.handle.net/2142/69784
Description
Title
Flow History - Morphology - Property Interactions in Poly(vinylidene Fluoride)
Author(s)
Khomami, Bamin
Issue Date
1987
Doctoral Committee Chair(s)
McHugh, A.J.
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Engineering, Materials Science
Abstract
A numerical and experimental analysis of the melt flow history of poly(vinylidene fluoride), PvF$\sb2$, in an extensional flow, film-processing geometry as well as morphological characterization and property measurements of the melt extruded films are presented.
Numerical computations were carried out for both Newtonian and powerlaw flow in an impinging channels die using both finite difference and element techniques. Computations demonstrated that a strong extensional flow exists in the region from the stream impingement point to a distance about 0.75 D downstream where D is the channel height at the impingement point. Measurements of the stress fields using the technique of flow birefringence showed that in consequence of the changing flow kinematics from the stream impingement region to the downstream converging channels region, both the isochromatics and isoclinic patterns exhibit a pronounced axial positional dependence. Excellent qualitative and reasonably good quantitative agreement was found between measured and calculated stress fields.
Row nucleated structures were observed at two-dimensional flow rates higher than 0.01 cm$\sp2$/sec and the resultant extruded films exhibited either $\alpha$-phase of (PvF$\sb2$) or a mixture of $\alpha$ and $\beta$ phases of (PvF$\sb2$) as shown by DSC, x-ray diffraction, FTIR and hot stage microscopy, depending on the processing history. Films with draw ratios less than 5 exhibited only the $\alpha$-polymorph while above the critical draw ratio a mixture of both polymorphs existed. The extent of the conversion of the $\alpha$-phase to $\beta$ is shown to be a very strong function of the draw temperature. Furthermore, the tensile property of the extruded films is an increasing function of draw ratio.
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