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Probing the Melt Miscibility of a Commercial Polyethylene Blend by Small-Angle Light Scattering

Small-angle light scattering (SALS) was used to determine the binary interaction parameter χ in a molten blend of linear polyethylene (LPE, M w = 52 kg/mol, PDI = 2.9) and linear low-density polyethylenes (LLDPEs) based on homogeneous ethylene-1-butene copolymers (LLDPE-1, 18.7 mol% butane branches, M w = 58.1 kg/mol, and LLDPE-2, 5.9 mol% butene branches, M w = 70 kg/mol). Traditional cloud point measurements (cooling at 0.5–3°C/min) and melt annealing/isothermal measurements under V V (parallel-polarized) optical alignment were used to evaluate the melt miscibility and determine the binary interaction parameter χ (7.74 × 10−4 at 423 K). This value of χ is consistent with the value (7.69 × 10−4 at 423 K) determined by earlier small-angle neutron scattering (SANS) measurements obtained by Crist and Rhee on polyethylene blends with the same branching-density difference (Δ y = 0.187). In addition, it was shown that SALS can be used to determine directly the melt miscibility of undeuterated polyolefin blends over branching-density differences (Δ y > 0.06), with results that are similar to the range obtained by neutron scattering. Our results are significant because they show that the low optical contrast between coexisting phases in polyolefin blends does not limit the determination of phase boundaries by SALS as was previously assumed. The blends studied exhibit upper critical solution temperature behavior.