Crystallization in Ordered Polydisperse Polyolefin Diblock Copolymers

Abstract

The morphologies of polydisperse ethylene-octene diblock copolymers, synthesized via a novel coordinative chain transfer polymerization process, are examined using two-dimensional synchrotron small-angle and wide-angle X-ray scattering on flow-aligned specimens. The diblock copolymers comprise one amorphous block with high 1-octene content and one semicrystalline block with relatively low 1-octene content, and each block ideally exhibits the most-probable distribution. Near-symmetric diblocks with a sufficiently large octene differential between the amorphous and semicrystalline blocks show well-ordered lamellar domain structures with long periods exceeding 100 nm. Orientation of these domain structures persists through multiple melting/recrystallization cycles, reflecting a robust structure which self-assembles in the melt. The domain spacings are nearly 3-fold larger than those in near-monodisperse polyethylene block copolymers of similar molecular weights. Although the well-ordered lamellar domain structure established in the melt is preserved in the solid state, the crystallites are isotropic in orientation. These materials display crystallization kinetics consistent with a spreading growth habit, indicating that the lamellae do not confine or template the growing crystals. The exceptionally large domain spacings and isotropic crystal growth are attributed to interblock mixing resulting from the large polydispersity; short hard blocks dissolved in the softblock-rich domains swell the domain spacing in the melt and allow hard block crystallization to proceed across the lamellar domain interfaces.

Comparison of chain structure and morphology of an olefinic blocky copolymer and a Ziegler–Natta‐based ethylene random copolymer

“…In OBCs, the hard blocks are long enough to form well‐developed lamellar crystals and space‐filling spherulites can be formed even when the crystallinity is as low as 7% . Many studies of the crystallization, morphology and mechanical properties of OBCs have been conducted . It is found that in OBCs the hard blocks are subjected to the constraint imposed by covalent linkages with non‐crystallizable soft blocks, and crystallization of the hard blocks always occurs simultaneously but in competition with microphase separation .…”

Estimating the segregation strength of microphase‐separated diblock copolymers from the interfacial width

“…Even if this analysis can be successfully carried out for a given sample, it is not easily transferrable to other samples because the nature of the paracrystalline distortions cannot be assumed constant. It is preferable to attempt to mitigate sources of paracrystallinity prior to the measurement by, for example, minimizing polydispersity during synthesis or through fractionation and annealing samples in the melt for an extended period …”

Elastomeric properties of ethylene/1-octene random and block copolymers synthesized from living coordination polymerization

“…The comonomer incorporation in the hard and soft segments is not clear. There is usually certain amount of random copolymer impurities in the commercial OBC products [18,19]. In researching OBC…”