Studies of the gamma form of isotactic polypropylene at atmospheric and elevated pressures

Author

Khaled Mezghani

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Paul J. Phillips

Committee Members

Joe E. Spruiell, David C. Joy, Roberto S. Benson

Abstract

Isotactic polypropylene (iPP) has three different morphological forms, α, β and γ, which are observed for different preparation procedures. Previous studies showed that the α and γ forms of iPP coexist in samples crystallized at elevated pressures up to 200 MPa (2 kbar) where only the γ-form exists. This study has the general objective of learning why and how the γ-phase occurs, to determine the thermodynamics underlying the phenomenon and to develop a fully analysis of the crystallization kinetics associated with crystallization at elevated pressures. Using a wide range of supercoolings it will be shown that the amount of each form varies with crystallization temperature at a constant pressure. It appears from experiments that the lower the supercooling the higher the amount of γ-form produced at a specific pressure. The coexistence of these two phases can be predicted from their thermodynamics parameter differences. Calculated values of the Gibbs free energy (ΔG) plotted for both a and γ-phases as a function of temperature show that as the temperature decreases from the equilibrium melting point the ΔG of the γ-form is lower than that of the α-form up to a transition temperature, where ΔG_γ = ΔG_α. This transition temperature, above which pure γ-form is predicted, is in good agreement with the experimental results for several pressures. The results of melting at high pressures show that the equilibrium melting point of iPP is linearly proportional to the pressure. Use of the modified Clapeyron equation, permits estimation of the heat of fusion (ΔH_f) of the γ-form of 145 J/g. The equilibrium melting point of the γ-form at atmospheric pressure is determined by extrapolation to be 187.6°C. These two values are cross-correlated by melting pure γ-form samples with DSC equipment. The equilibrium melting point of the γ-form at atmospheric pressure determined from the DSC and lamellar thickness studies is 187.2°. Furthermore, the average value of the ΔH_f of the γ-form, determined from the DSC, is close to 150 J/g, which is in good agreement with the value of 144.8 J/g determined from the Clapeyron equation. In addition to the γ-form prediction as a function of pressure and temperature, this study will shed some light on the effect of pressure and morphological changes on crystallization behavior of iPP. Using Lauritzen and Hoffman analysis of polymer crystallization kinetics, at atmospheric pressure high molecular weight iPP exhibits Regime II and III; whereas, at elevated pressures all Regimes are observed. It is noted from the kinetic analyses that even though the γ-content varies with crystallization temperature at a constant pressure, the growth kinetics is not affected by that phenomenon. In addition, this variation cannot be correlated to the regime changes. A very important conclusion from these analyses is that the γ-form is developed not because of kinetic reasons but because of thermodynamic factors, and specifically because of differences in the heat of fusion of both forms. On the spherulitic level, the γ-form exhibits three distinct types of spherulites classified typically on the basis of the sign of the birefringence. In addition, The complex arrangement of lamellae within a spherulite of the γ-form has been examined as a function of crystallization temperature at 2.0 kbar. The analyses of the SEM studies show that lamellae are arranged in a feather-like structure, which is a unique morphological feature of the γ-form. The birefringence results indicate that the feather-like structure is solely developed by self-epitaxial growth of the γ-lamellae. When considering the different possibilities for epitaxial growth of lamellae and the geometric factors of the triclinic and the orthorhombic crystal structures, it is possible to select the most reasonable unit cell of the γ-form. The results indicate that the triclinic structure fits the analysis better.

Recommended Citation

Mezghani, Khaled, "Studies of the gamma form of isotactic polypropylene at atmospheric and elevated pressures. " PhD diss., University of Tennessee, 1996.
https://trace.tennessee.edu/utk_graddiss/9802