Morphology of semicrystalline block copolymers of ethylene-(ethylene-alt-propylene)

The morphology of a series of ethylene--(ethylene-alt-propylene) diblock copolymers (E/EP), in which microphase separation is driven by crystallization, has been investigated. The E/EP diblocks were prepared by catalytic hydrogenation of narrow-distribution butadiene--isoprene diblocks. All the diblocks crystallized from single-phase melts so that the morphology and domain size were not restricted by "freezing" of a microphase-separated melt. The E/EP diblocks adopted a lamellar morphology over a full range of compositions studied (E block fractions ranging from 0.12-0.56), and also exhibited spherulitic superstructure. Thus, unlike purely amorphous systems, the thicknesses of the E and EP domains are effectively decoupled and lamellar materials can be tailored with a wide range of compositions and domain thicknesses. The scaling of the lamellar spacing with molecular weight and composition is compared with theoretical predictions, and mild deviations are found. These may be due to the inability of a single stem of an E crystallite to span the entire E domain as the latter becomes progressively thicker. Evidence suggests that samples with thick E blocks contain multiple crystallites running parallel to the domain interfaces.