Binary blends of linear ethylene copolymers over a wide crystallinity range: Rheology, crystallization, melting and structure properties
Résumé
This study deals with the physical properties of melt-compounded blends of three linear ethylene copolymers covering a large crystallinity range, namely 77% – 46% – 16% for the high density – linear low density – ultra low density copolymers, respectively. The melt behavior assessed from the zero-shear viscosity (ηo) reveals immiscibility of the three binary systems over the whole composition range. However, the change from positive to negative deviation of ηo with respect to the log-additivity mixing law as a function of composition suggests a structural transition from partial miscibility at the interface of the phase-separated domains to incompatibility. Crystallization and melting behaviors of the blends corroborate the occurrence of phase separation in the three systems. For most blends, the temperature shift of the crystallization (Tc) and melting (Tm) peaks as compared to the ones of the pure copolymers yet indicates partial miscibility in the crystalline and/or in the amorphous regions. It is pointed out that miscibility in the amorphous phase resulting from partial miscibility in the melt may, on its own, entail Tm depression of the crystals via surface free energy effect without necessarily implying cocrystallization and crystal thickness reduction. In several cases, the presence of intermediate endotherm and exotherm between the two main peaks of the melting and crystallization traces, respectively, discloses hybrid crystals assigned to a composition gradient at the interface of the phase-separated domains. A marked positive deviation of the upper Tc from the linear mixing rule is observed for the three systems. A nucleating effect from the interface of the phase-separated domains is suggested to promote early crystallization in the upper Tc phase. The SAXS data reveal electron density fluctuations at a much larger scale than that of the semi-crystalline structure demonstrating the occurrence of micro-phase separation in the melt prior to crystallization. Solubility of low Tm chain species in the amorphous layers of the high Tm phase is also evidenced. AFM and DMTA support micro-phase separation in the three systems and provide complementary information on the crystalline habits in the phase-separated domains of the blends.