Zhisheng Fu

Structure and morphology of polypropylene/poly(ethylene-co-propylene) in situ blends synthesized by spherical Ziegler–Natta catalyst

Abstract Polypropylene/poly(ethylene-co-propylene) (iPP/EPR) in situ blends of different composition were synthesized by spherical Ziegler–Natta catalyst, and were fractionated into three portions: the random copolymer (EPR), the block copolymer, and the iPP matrix. The EPR fraction was characterized by 13C NMR, and the block copolymer fraction was characterized by crystalline segregation and differential scanning calorimetry analysis. The blends showed bi-phase structure with EPR existing in the dispersed phase. Increasing EPR in the blends resulted in increase of the number and diameter of the EPR particles, but there is an upper limit for the particle number. There were only highly irregular spherulites or tiny crystallites in the isothermal crystallized blends. The morphology of the impact fracture surfaces of the blends clearly showed that they were fractured in ductile fashion. There was strong dependence of impact strength of the blends on their morphology, and the sequence distributions of the EPR and segmented copolymer fractions also markedly influenced the mechanical properties.

Temperature rising elution fractionation of PP/PE alloy and thermal behavior of the fractions

Abstract In this paper, a PP/PE alloy prepared by sequential polymerization of ethylene and propylene was fractionated with temperature rising elution fractionation and a variety of fractions were obtained. Thermal analysis was conducted to probe the microstructures of fractions. Four types of fraction with different melting behaviors were identified: random copolymer with no melting peak, multiple-block ethylene–propylene copolymer exhibiting plural melting peaks, ethylene-predominant copolymer with single melting peak and ethylene–propylene block copolymer with double melting peaks. The production of these components was also discussed in terms of polymerization process and the nature of active sites in the catalyst.

DEPENDENCE OF THE DISTRIBUTION OF ACTIVE CENTERS ON MONOMER IN SUPPORTED ZIEGLER-NATTA CATALYSTS

Distribution of active centers (ACD) of ethylene or 1-hexene homopolymerization and ethylene-1-hexene copolymerization with a MgCl2/TiCl4 type Z-N catalyst were studied by deconvolution of the polymer molecular weight distribution into multiple Flory components. Each Flory component is thought to be formed by a certain type of active center. ACD of ethylene-1-hexene copolymer with very low 1-hexene incorporation was compared with that of ethylene homopolymer to see the effect of introducing α-olefin on ethylene polymerization. On the other hand, ACD of ethylene-1- hexene copolymer with very low ethylene incorporation was compared with that of 1-hexene homopolymer. Adding small amount of 1-hexene in ethylene polymerization caused marked activation of all the Flory components of the polymer, in which the low molecular weight components are activated more than the high molecular weight components. In 1-hexene polymerization system, the activity can also be greatly enhanced by introducing small amount of ethylene, but the different Flory components (or active centers) are activated with similar extent, except a newly emerged active center producing polymer with the lowest molecular weight. The total number of active centers is markedly increased by adding small amount of ethylene in 1-hexene polymerization, but the average catalysis efficiency of the active centers decreased. The broad composition distribution of the ethylene-1-hexene copolymer can be well understood from the ACD of catalyst and its dependence on the monomer.

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

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.

PROBING THE ROLES OF DIETHYLALUMINUM CHLORIDE IN PROPYLENE POLYMERIZATION WITH MgCl2-SUPPORTED ZIEGLER-NATTA CATALYSTS *

In this article, the effect of diethylaluminum chloride (DEAC) in propylene polymerization with MgCl2-supported Ziegler-Natta catalyst was studied. Addition of DEAC in the catalyst system caused evident change in catalytic activity and polymer chain structure. The activity decrease in raising DEAC/Ti molar ratio from 0 to 2 is a result of depressed production of isotactic polypropylene chains. The number of active centers in fractions of each polymer sample was determined by quenching the polymerization with 2-thiophenecarbonyl chloride and fractionating the polymer into isotactic, mediumisotactic and atactic fractions. The number of active centers in isotactic fraction ([Ci*]/[Ti]) was lowered by increasing DEAC/Ti molar ratio to 2, but further increasing the DEAC/Ti molar ratio to 20 caused marked increase of [Ci*]/[Ti]. The number of active centers that produce atactic and medium-isotactic PP chains was less influenced by DEAC in the range of DEAC/Ti = 0–10, but increased when the DEAC/Ti molar ratio was further raised to 20. The propagation rate constant of Ci* (kpi) was evidently increased when DEAC/Ti molar ratio was raised from 0 to 5, but further increase in DEAC/Ti ratio caused gradual decrease in kpi. The complicated effect of DEAC on the polymerization kinetics, catalysis behaviors and polymer structure can be reasonably explained by adsorption of DEAC on the central metal of the active centers or on Mg atoms adjacent to the central metal.

Crystallization Behavior of the Blends of Isotactic Polypropylene and Ethylene-Propylene Blocky Copolymers

Two ethylene-propylene copolymer fractions (EP90 and EP120) were separated from a polypropylene in-reactor alloy by extraction with n-octane at different temperatures. 13C-NMR shows that these two fractions have a blocky structure and WAXD reveals that both ethylene and propylene sequences in these two fractions are crystallizable. However, EP90 has higher propylene content and the average length of propylene sequences is longer. These two fractions were blended with isotactic polypropylene (PP) at various proportions, respectively, and crystallization behavior and morphology of the blends were investigated. It is found that both EP90 and EP120 are partially compatible with PP. The phase-separated domains have a nucleation effect on crystallization of PP, leading to increase in crystallization temperature and crystallinity of PP in the blends. EP90 and EP120 also affect the relative content of β crystals in an irregular way. The number of EP90-rich domains in PP/EP90 blends is larger than that of EP120-rich domains in PP/EP120 blends, but the size of EP90-rich domains is smaller, indicating that EP90 has better compatibility with PP than EP120. Spherulites are formed in all the blends. The data were analyzed with Hoffman-Lauritzen theory of crystallization regime and the free energy of the folding surface (σe) was derived. Addition of EP90 and EP120 has little effect on the transition temperature from regime II to regime III. The value of σe for the PP/EP90 blends is similar to that of neat PP, but σe of the PP/EP120 blends is a little higher than that of neat PP.

Effect of Microstructure of EPR on Crystallization and Morphology of PP/EPR Blends

Two ethylene-propylene random copolymer (EPR) fractions (sol-EPR: soluble part and insol-EPR: insoluble part in n-octane) were blended with polypropylene (PP). It was found hat sol-EPR has a random sequence distribution and is nearly amorphous, whereas insol-EPR contains long ethylene and propylene sequences and is partially crystalline. The crystallization and melting behaviors, linear spherulitic growth rate, crystal structure, and morphology were investigated with differential scanning calorimetry, polarized optical microscopy, wide angle X-ray diffraction, and scanning electron microscopy. It was observed that the PP/insol-EPR blends have a smaller domain size of EPR and a rougher fracture surface. The better compatibility between insol-EPR and PP leads to lower melting temperature (T m) of PP/insol-EPR blends than the neat PP. For the blends cooled in air from the melt, both α and β crystals were observed. At low weight fraction (0–10%), EPR enhances the relative content of β crystals. When the weight fraction of EPR exceeds 10%, sol-EPR and insol-EPR decrease the relative content of β crystals in the blends to different extents. This difference can be correlated to the fact that sol-EPR and insol-EPR reduce the linear spherulitic growth rate (G) of PP to different extents.

Regulating the Structure of Ethylene-Propylene Copolymer for Polyolefin In-reactor Alloy with Improved Properties

Abstract Ethylene-propylene copolymers were synthesized with TiCl 4 /MgCl 2 /diester type high-yield supported catalysts. Composition distribution and chain structure of the copolymer were studied by fractionating the product into n -octane soluble and insoluble parts and characterizing the fractions by 13 C NMR and thermal analysis. The n -octane insoluble part is a kind of segmented copolymer. It is proposed that segmented copolymer fractions found in PP/EP in-reactor alloy are formed during the copolymerization stage. Differences in catalyst, cocatalyst and addition of hydrogen were found to strongly influence the composition distribution and chain structure of the copolymer.

Structure and Rheological Properties of the Products of Solid-State Graft Polymerization of Styrene in Annealed Polypropylene Reactor Granules

Solid-state grafting polymerization of styrene (St) in polypropylene (PP) reactor granules was conducted with different St load using tertiary-butyl perbenzoate as initiator. The graft copolymers showed higher weight average molecular weight than the original PP when the St load is high enough. A typical graft copolymer was fractionated into six fractions according to crystallinity, and the fractions showed similar St content. Comparing to the original PP, complex viscosity of the grafting products was significantly increased and the tan α value decreased at low frequency. Increasing the grafting degree exerts a positive effect on these changes. These phenomena prove the existence of long chain branches in the grafting products, and St played important roles in the formation of long chain branches.

Characterization of a Poly(propylene-g-styrene) Graft Copolymer by Temperature Rising Elution Fractionation

Abstract A polystyrene-grafted polypropylene sample was prepared by solid-state grafting polymerization in the presence of 2,2,6,6-tetramethyl-piperidinyloxy. Temperature rising elution fractionation of the grafted PP was performed and the fractions were characterized by 1H- and 13C-NMR, FT-IR, GPC, DSC, and WAXD. Results show that the grafted PP fraction eluted at 114°C accounts for 42% of the graft copolymer and has the highest grafting degree, high melting temperature and crystallinity, and high molecular weight. All the phenomena suggest that the grafted polymer contains a major component that has low branch density and long branch chain and a minor component with high branch density and short branches.