Crystallization of isotactic polypropylene at atmospheric pressure and elevated pressure

Author

Richard A. Campbell

Date of Award

8-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Paul J. Phillips

Committee Members

Roberto S. Benson, Edward S. Clark, Joe E. Spruiell, Alexander Van Hook

Abstract

The crystallization of iPP was studied, at atmospheric and elevated pressures. The objective was to study the effect of pressure on the regime transitions in order to gam further insight into the crystallization mechanism. However, the elevated pressure kinetics exhibited behavior that has not been previously observed in polymers. At pressures between atmospheric and 200 MPa, where two crystal forms, α and γ, are known to exist, the product of the fold and lateral surface free energies σσ_e increased to a value nearly three times that at atmospheric pressure. But, on further increasing the pressure to 200 MPa, where essentially all the material is in the γ form, the kinetics exhibited behavior similar to that at atmospheric and the value of σσ_e was comparable to atmospheric values. With increasing in pressure, the regime behavior changes from Regime II-Regime III at atmospheric to Regime I-Regime II at intermediate pressures (35 MPa < Pc <150 MPa) , and back to Regime IIRegime III behavior at higher pressures. Crystallization behavior of competing crystal forms has not been previously reported. This behavior could only be interpreted on the basis of the proposed orthorhombic unit cell for the γ form, and in the difference in the arrangement of chain stems between the α and the γ forms A 'poisoning' mechanism is proposed where a stem deposited on the growth face of a given form could poison the face if deposited in a manner consistent with the alternative form. This poisoning would mostly affect the rate of surface nucleation i since a chain deposited essentially in a crystallographic manner could rapidly complete the substrate, but a chain deposited noncrystallographically would require, either, removal from the growth face, or, a significant time for realignment during which time the growth face is essentially inactive, or 'poisoned'. The morphology and thermal behavior of the elevated pressure crystallized materials were consistent with the proposed orthorhombic crystal structure. The lamellar thickness decreased with increasing pressure (or increasing γ content) and this decrease is expected from the differences in arrangement of chain stems in the lamellae for the α and γ forms. The elevated temperature WAXD and SAXS supported the orthorhombic crystal structure also. WAXD indicated a conversion from the γ to α form on increasing the sample temperature to a value below the atmospheric pressure DSC peak melting temperature. The γ material would convert to a at a temperature where only a small amount of the material was shown to melt. The elevated temperature SAXS indicated that the lamellar thickness of the elevated pressure crystallized material increased with increasing temperature. The observed increase in lamellar thickness for the pressure crystallized material was larger than that for the atmospheric pressure crystallized material. At a SAXS scan temperature of 145°C, the lamellar thickness of the γ form approached that of the α form.

Recommended Citation

Campbell, Richard A., "Crystallization of isotactic polypropylene at atmospheric pressure and elevated pressure. " PhD diss., University of Tennessee, 1991.
https://trace.tennessee.edu/utk_graddiss/11070