Structural Investigation at Nanometric Length Scale of Ethylene/1-Octene Multiblock Copolymers from Chain-Shuttling Technology

The chain-shuttling technology has emerged as a powerful tool for the efficient production of ethylene/1- octene multiblock copolymers (EOBCs) characterized by a statistical distribution in block length (BL) and numbers of blocks per chain (BN) (blockiness) and unique properties of elastomers with high melting temperature and low density. The crystallization properties and the morphology at nanometric length scales of some commercial grades of EOBC samples are analyzed. The samples are characterized by alternating soft and hard blocks and similar molecular characteristics, including the octene concentration in the hard and soft blocks, the fraction of hard blocks, and the melting temperature of ∼120 °C. Differences occur for the molecular mass of the hard (MH ≈ 2 or 3 kg/mol) and soft (MS, from 3 to 5 times MH) block and BN (in the range of 2−17). Small-angle X-ray scattering (SAXS) measurements, coupled with differential scanning calorimetry thermal analysis of isothermally crystallized samples, indicate that the hard blocks crystallize in separated domains forming chain-folded lamellae organized in stacks with little or no inclusion of soft segments in the interlamellar amorphous regions. The lamellar thickness, which develops at identical undercooling, varies from sample to sample and depends on the differences in the statistical distribution of block lengths, which generates a variety of environments for the crystallization. Two relevant length scales are identified for the description of the hierarchical structural organization of EOBCs in the solid state, the lamellar length scale, and the scale of domain spacing. The organization at lamellar length scale involves the stacking of the chain-folded lamellae at average separation distance L (long spacing), to form fringed lathlike entities having low thickness, wherein the lamellar entities lay down with the chain axes oriented parallel rather than perpendicular to the basal planes. The organization of the laths at higher length scale is also layered and involves the relative arrangement of the fringed laths in the compliant matrix populated by soft blocks bridging the laths, to form an interwoven elastomeric network characterized by an average layer periodicity D that is 4 to 5 times higher than the long spacing L. However, the contribution to the SAXS intensity at large length scale deriving from the presence of long hard blocks crystallizing as isolated lamellae passing through the soft matrix may not be excluded. The thickness of the laths is small (∼10 nm), as the lateral growth of the crystalline lamellae is hampered by the attached soft segment. This small size, and the parallel chain axis orientation to the basal plane of the laths, complies well with the need to accommodate the covalently bonded noncrystallizable soft blocks emerging as fringes from the laths at low cost of free energy, reducing to a minimum the loss of conformational entropy