SANS Studies of Liquid−Liquid Phase Separation in Heterogeneous and Metallocene-Based Linear Low-Density Polyethylenes

An ethylene-hexene copolymer, representative of many heterogeneous linear low-density polyethylenes (LLDPEs), has been shown to contain a dispersed minority phase (volume fraction similar to 10(-2).), which was manifested by departures from a Q(-2) variation of the neutron scattering cross section at low Q values. After xylene extraction, which removes the highly branched amorphous material, the dispersed phase is removed to a good approximation. In contrast, a metallocene-based LLDPE, which has a more homogeneous distribution of branch contents, does not exhibit an upturn in the cross section in the limit of low momentum transfer (Q < 10(-2) Angstrom (-1)), indicating that the LLDPE forms only a single phase in the melt. The composition variance is calculated from the comonomer composition distribution obtained by temperature-rising elution fractionation (TREF) and used to estimate proximity of the melts to the spinodal condition. These findings support previous conclusions for compositionally polydisperse LLDPEs, whereby the highly branched molecules in the distribution may phase segregate, even if the overall branch content is low. When this component is not present, as in metallocene-based LLDPEs, the system forms a single phase in the melt.