ABSTRACT:This study compared a series of experimental propylene/ethylene copolymers synthesized by a transition metal-based, postmetallocene catalyst (xP/E) with homogeneous propylene/ethylene copolymers synthesized by conventional metallocene catalysts (mP/E). The properties varied from thermoplastic to elastomeric over the broad composition range examined. Copolymers with up to 30 mol % ethylene were characterized by thermal analysis, density, atomic force microscopy, and stress-strain behavior. The xP/Es exhibited noticeably lower crystallinity than mP/Es for the same comonomer content. Correspondingly, an xP/E exhibited a lower melting point, lower glass transition temperature, lower modulus, and lower yield stress than an mP/E of the same comonomer content. The difference was magnified as the comonomer content increased. Homogeneous mP/Es exhibited space-filling spherulites with sharp boundaries and uniform lamellar texture. Increasing comonomer content served to decrease spherulite size until spherulitic entities were no longer discernable. In contrast, axialites, rather than spherulites, described the irregular morphological entities observed in xP/Es. The lamellar texture was heterogeneous in terms of lamellar density and organization. At higher comonomer content, embryonic axialites were dispersed among individual randomly arrayed lamellae. These features were characteristic of a copolymer with heterogeneous chain composition.
Miscibility of homogeneous propylene/ethylene (P/E) copolymers of relatively narrow molecular weight distribution was studied as a function of constituent comonomer content. Polymers with up to 31 mol % ethylene were blended in pairs in order to vary the comonomer content difference. Binary blends were rapidly quenched from the melt to retain the phase morphology, and the phase volume fractions were obtained from AFM images. Copolymers of molecular weight about 200 kg mol-1 were miscible if the difference in ethylene content was less than about 18 mol % and immiscible if the ethylene content difference was greater than about 20 mol %. Blends with constituent composition difference in the range of 18−20 mol % exhibited partial miscibility in the melt as indicated by a phase volume fraction that was different from the blend volume fraction. The temperature dependence of blend morphology confirmed the UCST behavior of P/E copolymer blends. The phase composition and the χ interaction parameter were extracted by using an approach that considered the molecular weight distribution. The compositional dependence of χ conformed to the copolymer equation and depended on comonomer content difference only, not on comonomer content per se.
The elastic behavior of a propylene-ethylene copolymer was investigated. An initial ''conditioning'' tensile extension up to 800% strain resulted in an elastomer with low initial modulus, strong strain hardening, and complete recovery over many cycles. Structural changes that occurred in the low crystallinity propylene-ethylene copolymer during conditioning, and that subsequently imparted elastomeric properties to the conditioned material, were investigated. Thermal analysis, wide and small angle X-ray diffraction, and atomic force microscopy measurements were performed at various strains during the conditioning process. Conditioning transformed crystalline lamellae into shish-kebab fibers by melting and recrystallization. The fibers, accounting for only 5% of the bulk, were interconnected by a matrix of entangled, amor-phous chains that constituted the remaining 95%. It was proposed that the shish-kebab fibers acted as a scaffold to anchor the amorphous rubbery network. Entanglements of the amorphous chain segments acted as network junctions and provided the elastic response. The stress-strain response of materials conditioned to 400% strain or more was described by the classical rubber theory with strain hardening. The extracted value of M c , the molecular weight between network junctions, was intermediate between the entanglement molecular weights of polypropylene and polyethylene.
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