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 Å, 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.

Differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and small-angle neutron and X-ray scattering (SANS and SAXS, respectively) have been used to investigate the solid-state morphology of blends of linear (high-density) and model short-chain branched (linear-low-density) polyethylenes (HDPE/LLDPE). SANS indicates that the mixtures are homogeneous in the melt for all compositions when the ethyl branch content in the copolymer is low (i.e., <4 branches/100 backbone carbon atoms for a molecular weight of M w ∼ 105). However, due to the structural and melting point differences between HDPE and LLDPE, the components may phase segregate in the solid state. The degree of separation is therefore controlled by the crystallization kinetics. DSC, TEM, SAXS, and SANS experiments have been used to investigate the solid-state morphology as a function of component composition, the thermal history, and the rate of cooling. It is shown that the combination of scattering, microscopy, and calorimetric techniques can provide detailed insight into the compositions of the various populations of the lamellar crystals and the amorphous regions that surround them.