NDLI: Extensional-flow-induced crystallization of isotactic polypropylene

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Extensional-flow-induced crystallization of isotactic polypropylene

Content Provider	Springer Nature Link
Author	Bischoff White, Erica E. Henning Winter, H. Rothstein, Jonathan P.
Copyright Year	2011
Abstract	A filament stretching extensional rheometer with a custom-built oven was used to investigate the effect of uniaxial flow on the crystallization of polypropylene. Prior to stretching, samples were heated to a temperature well above the melt temperature to erase their thermal and mechanical histories and the Janeschitz-Kriegl protocol was applied. The samples were stretched at extension rates in the range of $0.01\,\mbox{s}^{-1}\le \dot{{\varepsilon }}\le 0.75\,{\rm s}^{-1}$ to a final strain of ε = 3.0. After stretching, the samples were allowed to crystallize isothermally. Differential scanning calorimetry was applied to the crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate is required for an increase in percent crystallization to occur and that there is an extension rate for which percent crystallization is maximized. No increase in crystallization was observed for extension rates below a critical extension rate corresponding to a Weissenberg number of approximately Wi = 1. Below this Weissenberg number, the flow is not strong enough to align the contour path of the polymer chains within the melt and as a result there is no change in the final percent crystallization from the quiescent state. Beyond this critical extension rate, the percent crystallization was observed to increase to a maximum, which was 18% greater than the quiescent case, before decaying again at higher extension rates. The increase in crystallinity is likely due to flow-induced orientation and alignment of contour path of the polymer chains in the flow direction. Polarized light microscopy verified an increase in number of spherulites and a decrease in spherulite size with increasing extension rate. In addition, small angle X-ray scattering showed a 7% decrease in inter-lamellar spacing at the transition to flow-induced crystallization. Although an increase in strain resulted in a slight increase in percent crystallization, no significant trends were observed. Crystallization kinetics were examined as a function of extension rate by observing the time required for molten samples to crystallize under uniaxial flow. The crystallization time was defined as the time at which a sudden increase in the transient force measurement was observed. The crystallization time was found to decrease as one over the extension rate, even for extension rates where no increase in percent crystallization was observed. As a result, the onset of extensional-flow-induced crystallization was found to occur at a constant value of strain equal to ε c = 5.8.
Starting Page	303
Ending Page	314
Page Count	12
File Format	PDF
ISSN	00354511
Journal	Rheologica Acta
Volume Number	51
Issue Number	4
e-ISSN	14351528
Language	English
Publisher	Springer-Verlag
Publisher Date	2011-10-28
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Extensional flow Polymer melt Crystallization Flow-induced orientation Uniaxial extension Soft and Granular Matter, Complex Fluids and Microfluidics Food Science Mechanical Engineering Polymer Sciences Characterization and Evaluation of Materials
Content Type	Text
Resource Type	Article
Subject	Mechanics of Materials Condensed Matter Physics Materials Science

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