Ethylene‐octene copolymers prepared by Dow's INSITE™ constrained geometry catalyst technology present a broad range of solid‐state structures from highly crystalline, lamellar morphologies to the granular morphology of low crystallinity copolymers. As the comonomer content increases, the accompanying tensile behavior changes from necking and cold drawing typical of a semicrystalline thermoplastic to uniform drawing and high recovery characteristic of an elastomer. Although changes in morphological features and tensile properties occur gradually with increasing comonomer content, the combined body of observations from melting behavior, morphology, dynamic mechanical response, yielding, and large‐scale deformation suggest a classification scheme with four distinct categories. Materials with densities higher than 0.93 g/cc, type IV, exhibit a lamellar morphology with well‐developed spherulitic superstructure. Type III polymers with densities between 0.93 and 0.91 g/cc have thinner lamellae and smaller spherulites. Type II materials with densities between 0.91 and 0.89 g/cc have a mixed morphology of small lamellae and bundled crystals. These materials can form very small spherulites. Type I copolymers with densities less than 0.89 g/cc have no lamellae or spherulites. Fringed micellar or bundled crystals are inferred from the low degree of crystallinity, the low melting temperature, and the granular, nonlamellar morphology. © 1996 John Wiley & Sons, Inc.
The elastomeric behavior of low-crystallinity ethylene−octene copolymers prepared by Dow's INSITE constrained geometry catalyst technology is described. Deformation in uniaxial tension was examined as a function of comonomer content and molecular weight. Within the melting range of copolymers, temperature was used as an experimental variable to reveal the relationship between crystallinity and stress response. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions provided the structural basis for analysis of the elastic behavior. The rubber modulus scaled with crystallinity. Furthermore, the dimension of the fringed micellar junction obtained from the modulus correlated well with the average crystallizable sequence length of the copolymer. Because classical rubber theory could not account for the large strain dependence of the modulus, a theory which incorporates the contribution of entanglements to the network response was considered. Slip-link theory described the entire stress−strain curve. The slip-link density correlated with crystallinity; the cross-link density did not depend on crystallinity and appeared to represent a permanent network. The latter was further revealed by the effect of molecular weight on the stress−strain behavior. It is proposed that lateral attachment and detachment of crystallizable chain segments at the crystal edges provide the sliding topological constraint attributed to slip-links, and entanglements that tighten into rigid knots upon stretching function as permanent network junctions.
SYNOPSISEthylene-octene copolymers prepared by Dow's INSITETM constrained geometry catalyst technology present a broad range of solid-state structures from highly crystalline, lamellar morphologies to the granular morphology of low crystallinity copolymers. As the comonomer content increases, the accompanying tensile behavior changes from necking and cold drawing typical of a semicrystalline thermoplastic to uniform drawing and high recovery characteristic of a n elastomer. Although changes in morphological features and tensile properties occur gradually with increasing comonomer content, the combined body of observations from melting behavior, morphology, dynamic mechanical response, yielding, and largescale deformation suggest a classification scheme with four distinct categories. Materials with densities higher than 0.93 g/cc, type IV, exhibit a lamellar morphology with welldeveloped spherulitic superstructure. Type I11 polymers with densities between 0.93 and 0.91 g/cc have thinner lamellae and smaller spherulites. Type I1 materials with densities between 0.91 and 0.89 g/cc have a mixed morphology of small lamellae and bundled crystals.These materials can form very small spherulites. Type I copolymers with densities less than 0.89 g/cc have no lamellae or spherulites. Fringed micellar or bundled crystals are inferred from the low degree of crystallinity, the low melting temperature, and the granular, nonlamellar morphology. 0 1996 John Wiley & Sons, Inc.
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