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 l ow-density polyethylenes (LLDPEs), has been shown to contain a dispersed mi nority phase (volume fraction similar to 10(-2).), which was manifested by departures from a Q(-2) variation of the neutron scattering cross section a t 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 distr ibution of branch contents, does not exhibit an upturn in the cross section in the limit of low momentum transfer (Q < 10(-2) Angstrom (-1)), indicati ng that the LLDPE forms only a single phase in the melt. The composition va riance is calculated from the comonomer composition distribution obtained b y temperature-rising elution fractionation (TREF) and used to estimate prox imity of the melts to the spinodal condition. These findings support previo us conclusions for compositionally polydisperse LLDPEs, whereby the highly branched molecules in the distribution may phase segregate, even if the ove rall branch content is low. When this component is not present, as in metal locene-based LLDPEs, the system forms a single phase in the melt.