The melting behavior and structure of ethylene copolymers from metallocene catalysts

Author

Man-ho Kim

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Paul J. Phillips

Committee Members

Mark Dadmun, Joseph E. Spruiell, Edward S. Clark

Abstract

Ethylene/1-octene copolymers produced using metallocene catalysts have a homogeneous comonomer content with respect to molecular weight. Two series of molecular weight having 1-octene content ranging from zero to thirty nine branches per 1000 carbon atoms, have been studied. The influence of branch content on structure and melting behavior as well as on isothermal and non-isothermal bulk crystallization has been studied. The principal techniques used have been thermal analysis and small angle x-ray scattering.

As branch (defect) content increases in olefin copolymers, non-isothermal and isothermal crystallization rates, melting temperature, and heat of fusion decrease. Melting temperatures of homogeneous random copolymers are always lower than those of fractions of heterogeneous copolymers, having approximately the same comonomer content and molecular weight. Hence, defect distribution in copolymer systems is a more important parameter rather than defect content.

The Hoffman-Weeks plot to obtain the equilibrium melting temperatures of ethylene/1-octene random copolymers results in a non-sensible, high value or no intercept at all. The equilibrium melting temperatures of linear polyethylenes and homogeneous ethylene/1-octene random copolymers were determined as a function of molecular weight and branch content, using Thompson-Gibbs plots involving lamellar thickness data. This systematic study makes it possible to evaluate two equilibrium melting temperature depression equations for the olefin type random copolymers, the Flory equation and the Sanchez-Eby equation, as a function of defect content and molecular weight. An empirical equation determined from this study for predicting the equilibrium melting point of linear polyethylene as a function of molecular weight is more sensitive to molecular weight than the Flory-Vrij equation. The range over which the two equations can be applied depends on the defect content, after correcting the molecular weight effect on the equilibrium melting temperature. The T_m^o(n→∞) of linear polyethylene at infinite molecular weight was found to be 144.4 °C.

The equilibrium melting temperature, T_m^o(n, p_B), of ethylene/1-octene random copolymers was a function of the molecular weight and defect at low defect content (p_B% ≤ 1.0%). The T_m^o(n, p_B) was a weak function of molecular weight and a strong function of the defect content at high defect content (p_B% ≥ 1.0%). The Flory copolymer equation could predict T_m^o(n, p_B) at p_B % ≤ 1.0% when molecular weight was corrected. Sanchez-Eby's uniform inclusion model could predict T_m^o(P_b) at high defect content, 1.6% < p_B% < 2.0%. It was observed that some defects were included in crystalline phase and the excess free energy (18 ~37 kJ/mole) estimated in this study were within the theoretical range.

A mathematical and graphical method was developed to estimate the phase boundary thickness. The calculated electron density profiles showed an intermediate profile between linear-gradient and semi-sigmoidal models. Background intensity showed a systematic positive deviation as a function of defect content. Electron density fluctuation component increased and electron density difference between amorphous and crystalline phase decreased with defect contents, reflecting defect inclusion. Interfacial layer thickness depended on defect content, defect distribution, crystallization temperature and time, and the temperature at which SAXS intensity is measured. The interfacial layer thickness of the homogeneous ethylene/1-octene copolymers decreased as a function of defect content while that of the heterogeneous copolymers increases. The interfacial layer thickness of LPE and EO copolymers at low in-situ temperature were higher than those at ambient temperature. On increasing in-situ temperature, the phase boundary became thin and finally disappeared.

As a conclusion, the philosophy that defects can affect the thermodynamics properties and morphology was verified experimentally and theoretically through this study.

Recommended Citation

Kim, Man-ho, "The melting behavior and structure of ethylene copolymers from metallocene catalysts. " PhD diss., University of Tennessee, 1996.
https://trace.tennessee.edu/utk_graddiss/9777