Phase Behavior of Blends of Linear and Branched Polyethylenes on Micron Length Scales via Ultra-Small-Angle Neutron Scattering

Abstract

Small-angle neutron scattering (SANS) experiments have indicated that mixtures of linear (high density) and long chain branched (low density) polyethylenes (HDPE/LDPE) form a one-phase mixture in the melt. However, the maximum spatial resolution of pinhole SANS cameras is ∼10 3 A, and it has been suggested that such experiments do not provide unambiguous evidence for a homogeneous melt. Thus, the SANS data might also be interpreted as arising from a biphasic melt with a very large particle size (∼3 μm), because most of the scattering from the different phases would not be resolved. We have addressed this hypothesis by means of ultra-small-angle neutron scattering (USANS) experiments, using a newly developed Bonse-Hart USANS facility, which can resolve particle dimensions up to 30 μm. The experiments confirm that HDPE/LDPE blends are homogeneous in the melt on length scales probed by pinhole SANS and also by USANS. We have also studied blends of linear and short-chain branched polyethylenes, which phase separate when the branch content is sufficiently high. It is shown that USANS can directly resolve both the size of the dispersed phase (∼4 μm) and the forward cross section [dΣ/dΩ(0) ∼ 10 8 cm -1 ], which is 6 orders of magnitude higher than for homogeneous blends.