Chain Structure, Aggregation State Structure, and Tensile Behavior of Segmented Ethylene–Propylene Copolymers Produced by an Oscillating Unbridged Metallocene Catalyst

Abstract

Segmented ethylene-propylene copolymers (SEPs) with different propylene contents were prepared by an unbridged metallocene bis(2,4,6-trimethylindenyl)zirconium dichloride [(2,4,6-Me3Ind)2ZrCl2] catalyst. Due to oscillation of the unbridged ligands in the catalyst, the SEPs are composed of segments with low propylene contents, alternated by the segments with high propylene contents. Such a chain structure was verified by (13)C NMR and successive self-nucleation and annealing (SSA). As the propylene/ethylene feed ratio during copolymerization increases, the comonomer contents in both segments are increased, leading to noncrystallizability of the high propylene segments and smaller crystallinity of the low propylene segments. Consequently, SEPs may be used as thermoplastic elastomers (TPEs). The aggregation state structures at nano- and micro-scales were characterized with small angle X-ray scattering, transmission electron microscopy and polarized optical microscopy, and compared with those of ethylene-octene multiblocky copolymers (OBCs) with similar crystallinity. It is found that SEPs form thinner lamellar crystals with a lower melting temperature due to shorter length and higher comonomer content of the low propylene segments. Moreover, the short length of the high propylene segments in SEPs results in an evidently thinner amorphous layer among the lamellar crystals, thus lots of amorphous phases are excluded out of the interlamellae. Accordingly, ill-developed spherulites or even bundle crystals are formed in SEPs, as compared with the well-developed spherulites in OBCs. SEPs exhibit the tensile property of typical TPEs with diffused yielding and large strain at break.

A Generalized Avrami Equation for Crystallization Kinetics of Polymers with Concomitant Double Crystallization Processes

“…28−34 Another example is that, when the polymer chain structure (such as comonomer and tacticity) is not uniform, the polymers with different chain structures may crystallize at the same temperature in a similar time scale, forming crystals with different lamellar thicknesses, melting temperatures, and fusion enthalpies. 35,36 Moreover, stepwise crystallization behavior was observed in the ultrathin films of poly(ethylene terephthalate) (PET), resulting from the different glass transition relaxations in the bulk and surface layers. 37 The overall kinetics for single crystallization process in polymers, which is controlled by nucleation and growth, can be well described by the Avrami equation: where n is the Avrami exponent, k is the overall crystallization rate constant, x c is the relative crystallinity, and t is crystallization time.…”

“…Double or multiple crystallization processes frequently occur in ceramics, alloys, colloidal particles, and polymers. − Polymers provide excellent examples to study the concomitant crystallization behavior because of the existence of different nucleation mechanisms, polymorphism, inhomogeneous spatial environment, and nonuniform chain structure. − For instance, fractional crystallization upon cooling from melt can be observed when the crystalline polymers are dispersed in domains with huge differences in size and thus are nucleated via different mechanisms. − Polymorphism means that more than one crystalline form is formed in the solid state of the same substance, which can happen in many polymers, including polypeptides, isotactic polypropylene (iPP), polyesters, syndiotactic polystyrene, and poly­(vinylidene fluoride) (PVDF). − Another example is that, when the polymer chain structure (such as comonomer and tacticity) is not uniform, the polymers with different chain structures may crystallize at the same temperature in a similar time scale, forming crystals with different lamellar thicknesses, melting temperatures, and fusion enthalpies. , Moreover, stepwise crystallization behavior was observed in the ultrathin films of poly­(ethylene terephthalate) (PET), resulting from the different glass transition relaxations in the bulk and surface layers …”

Effect of interface and confinement size on the crystallization behavior of PLLA confined in coaxial electrospun fibers

“…During the past decades, polymer crystallization under nano‐spatial confinement has attracted a great attention, which is important to guide the exploration of nanomaterials and their applications . The crystallization of many semicrystalline polymers, such as polyethylene oxide (PEO), poly(ɛ‐caprolactone) (PCL), poly(vinylidene fluoride) (PVDF), poly( L ‐lactic acid) (PLLA), polyethylene (PE) and etc., in various confinement environments, including anodic alumina oxide (AAO) template, ultrathin polymer layers fabricated by layer‐multiplying coextrusion process, ultrathin films, microphase‐separated block copolymers, coaxial electrospinning fibers, etc ., has been widely studied. The crystallization behavior of semicrystalline polymers under nano‐spatial confinement may be fundamentally different from that in bulk.…”

Synthesis and Crystallization Behavior of Equisequential ADMET Polyethylene Containing Arylene Ether Defects: Remarkable Effects of Substitution Position and Arylene Size

“…The defects in polymer chains include chain ends, comonomer units, coupling units, and stereo- and regio-defects. When the polymer chain is crystalline, these defects can be either included into or excluded from the polymer crystal, depending on the type and size of the defects. − …”