Properties of Zone-Annealed Miscible Polymer Blends

We examine the morphology and properties of zone-annealed (ZA) poly(ethylene oxide) (PEO) and its miscible blends with poly(methyl methacrylate) (PMMA) over a range of compositions where PEO is semicrystalline. Small angle X-ray scattering, and transmission electron microscopy show that the ZA results in layered structures with alternating crystal-amorphous layers oriented parallel to the ZA direction (i.e., the crystal-amorphous interphases are normal to the ZA direction). Instead of concentrating impurities at the end of the sample as has been found for metal and semiconductor materials, the noncrystalline polymer (PMMA) is concentrated into the amorphous regions between the lamellae. Differential scanning calorimetry shows that the ZA samples have higher crystallinity, as expected. The mechanical properties of both neat PEO and its blends, processed by quenching and ZA, were examined using tensile testing. The most striking result is that ZA leads to increased blend toughness (despite toughness decreasing in the quenched samples), with these differences particularly manifesting for PMMA weight fractions larger than 0.2. There is no directionality associated with this property, i.e., we get the same toughness parallel vs perpendicular to the ZA direction. Similarly, ZA had no (directional) effect on the blends’ Young’s modulus, even though the modulus itself increased with the addition of the glassy PMMA. In contrast, the modulus of neat PEO is significantly increased in the direction parallel to the ZA relative to the perpendicular direction and the quenched sample. We propose that the anisotropic properties that arise from ZA of neat PEO results from the contrast in mechanical properties of the crystalline lamella and rubbery amorphous region, while in the blends, both the crystal and the amorphous phases are hard solids with little contrast-this effect being caused by the interlamellar PMMA being a glassy polymer at room temperature-evidently yielding no directionality to properties.