Isothermal crystallization of metallocene-based propylene/α-olefin copolymers



Isothermal crystallization and subsequent melting behavior of two propylene/hexene-1 copolymers and two propylene/octene-1 copolymers prepared with metallocene catalyst were investigated. It is found that γ-modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (Tc) the crystallization rate constant changes more rapidly with Tc and the crystallization half-time substantially increases. Double melting peaks were also observed at high Tc, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (Tmath image) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the Tmath image decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005