Exciting new developments in polyolefin synthesis give rise to olefinic block copolymers with properties typical of thermoplastic elastomers. The blocky copolymers synthesized by chain shuttling technology consist of crystallizable ethylene-octene blocks with low comonomer content and high melting temperature (hard blocks), alternating with amorphous ethylene-octene blocks with high comonomer content and low glass transition temperature (soft blocks). This paper describes the materials science of these unique polymers as characterized by thermal analysis, X-ray diffraction, microscopy, and tensile deformation. The crystallizable nature of the hard block and the crystalline morphologies are consistent with an average hard block length that is well in excess of 200 carbon atoms. The crystallizable blocks are long enough to form well-organized lamellar crystals with the orthorhombic unit cell and high melting temperature. The lamellae are organized into space-filling spherulites in all compositions even in copolymers with only 18 wt % hard block. The morphology is consistent with crystallization from a miscible melt. Crystallization of the hard blocks forces segregation of the noncrystallizable soft blocks into the interlamellar regions. Good separation of hard and soft blocks in the solid state is confirmed by distinct and separate β-and R-relaxations in all the blocky copolymers. Compared to statistical ethylene-octene copolymers, the blocky architecture imparts a substantially higher crystallization temperature, a higher melting temperature and a better organized crystalline morphology, while maintaining a lower glass transition temperature. The differences between blocky and statistical copolymers become progressively more apparent as the total comonomer content increases.

This work compared the elastomeric properties of two low-crystallinity ethylene-octene copolymers. One was a block copolymer with lamellar crystals and the other was a random copolymer with fringed micellar crystals. The comparison of the stress-strain behavior at 23 C revealed that the initial elastic modulus and the yield stress depended only on the crystallinity of the copolymer. When the temperature was raised above 23 C, melting of the fringed micellar crystals of the random copolymer caused a rapid decrease in the modulus. Some decrease in the modulus of the block copolymer over the same temperature range was attributed to the crystalline arelaxation. Both polymers exhibited strain-hardening, ultimate fracture at high strains, and high recovery after fracture. However, in the block copolymer, the onset of strain-hardening and the ultimate fracture occurred at higher strains. The block copolymer also showed higher recovery from high strains. The initial stretching resulted in a permanent change in the stress-strain curve. It was suggested that following the onset of crystal slippage at the yield, the crystals underwent permanent structural changes through the course of the strainhardening region. The transformation of the fringed micellar crystals occurred at lower strains than the transformation of the lamellar crystals. The extent of the structural transformation was described by the crosslink density and the strain-hardening coefficient extracted from elasticity theory.

When two polymers are brought into intimate contact, the interface is not perfectly sharp. Instead, localized molecular mixing produces an interphase with thickness on the order of 10 nm. Forced assembly by layer multiplying coextrusion makes it possible to create films that are entirely interphase. The new interphase materials can be characterized using conventional tools of polymer analysis. In this study we vary the ratio of poly(methyl methacrylate) (PMMA) to polycarbonate (PC) in the coextrusion process in order to obtain very thin PMMA nanolayers sandwiched between thicker (∼50 nm) PC layers. We use oxygen permeability to probe the nature of the interphase as the PMMA layer thickness is reduced below the interphase thickness to the limit of layer stability. If the PMMA nanolayers are made thinner than the 12 nm interphase dimension, they more closely resemble a thin interphase region sandwiched between thicker PC layers than they do discrete PMMA layers. The interphase region becomes progressively thinner with decreasing PMMA layer thickness until the limit in melt stability of the PMMA layer is reached. Efforts to obtain PMMA nanolayers thinner than 5 nm resulted in layer instability and breakup. Oxygen permeability suggested that the nanolayer fragments had a very high aspect ratio.

New challenges and opportunities for polyolefin blends arise from the recent introduction of olefin block copolymers (OBCs). In this study, the effect of chain blockiness on the miscibility and phase behavior of ethylene-octene (EO) copolymer blends was studied. Binary blends of two statistical copolymers (EO/EO blends) that differed in comonomer content were compared with blends of an EO with a blocky EO copolymer (EO/OBC blends). The blends were rapidly quenched to retain the phase morphology in the melt and the phase volumes were obtained by atomic force microscopy (AFM). Two EOs of molecular weight about 100 kg/mol were miscible if the difference in octene content was less than about 10 mol % and immiscible if the octene content difference was greater than about 13 mol %. The blocky nature of the OBCs reduced the miscibility and broadened the partial miscibility window of the EO/OBC blends compared with the EO/EO blends. The EO/OBC blends were miscible if the octene content difference was less than 7 mol % and immiscible above 13 mol % octene content difference. It was also found that the phase behavior of EO/OBC blends strongly depended on blend composition even for constituent polymers of about the same molecular weight. Significantly more demixing was observed in an OBC-rich blend (EO/OBC 30/70 v/v) than in an OBC-poor blend (EO/OBC 70/30 v/v). An interpretation based on extractable fractions of the OBC described the major features of the EO/OBC (30/70 v/v) blends. V V C 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys