High Molecular Weight Isotactic Polypropylene Research Articles

Dynamic Mechanical Thermal Analysis and Rheological Properties of Synthesized Polypropylene Reactor Blends Using Homogeneous Binary Metallocene Catalyst

ABSTRACTA homogenous binary metallocene catalytic system comprising of isospecific rac-Me2Si(2-Me-4-Ph In)2ZrCl2(I) producing high molecular weight isotactic polypropylene and oscillating (2-Ph In)2ZrCl2(II) precursor producing low isotactic elastomer polypropylene at three varying molar ratios of two types of catalysts was used to synthesize polypropylene reactor blends. Dynamic mechanical thermal analysis and rheological properties along with molecular weight of synthesized polypropylene reactor blends were studied and correlations among these properties were established. It was found that molar ratio of catalysts is a significant factor in determination of molecular weight and its distribution. The produced polymers with unimodal molecular weight distribution showed intermediate modulus during dynamic mechanical thermal experiment, while the ones with bimodal molecular weight distribution exhibited a kind of phase separation at low temperature. Depending on strength of the developed structures, determined by the presence of interfacial connectors, the modulus could be adjusted. Origins of other types of relaxations and their differences for each type of the developed products were discussed in detail. From rheological results and particularly the relaxation curves, the characteristics of the chain structure of the synthesized reactor blends could be resolved. It was revealed that one of the synthesized polymers had long chain branches unlike the rest of the samples having linear chain structure.

Correlation between chain microstructure and activation energy in the pyrolysis of a high molecular weight isotactic polypropylene

Changes in chain microstructure during the pyrolysis of a high molecular weight isotactic polypropylene (iPP) have been tracked by means of FTIR, DSC and NMR techniques. For this purpose, preparative dynamic TGA experiments have been performed up to final temperatures between 200 °C and 420 °C. The evolutions of molecular weight, tacticity and both quality and content of chain-end double bonds have been analysed and correlated with the conversion-dependent Eact build-up, estimated from analytical TGA curves.The obtained results allows correlating the earliest low Eact values with the vanishing of chemical defective structures like carbonyls, as well as with the scission of some very few regular C–C bonds. The growing trend of the Eact, found from low to advanced pyrolysis stages, is associated to the collapse of the molecular weight and the decreasing of the isotacticity degree. The evolution towards shorter and more flexible chains is thought to underlie an increasingly energy demanding scission process.

Flow-induced crystallization precursor structure in high molecular weight isotactic polypropylene (HMW-iPP)/low molecular weight linear low density polyethylene (LMW-LLDPE) binary blends

The formation of initial crystallization precursor induced by flow was investigated by synchrotron-based wide- and small-angle X-ray scattering techniques (WAXS/SAXS), using polymer blends consisting of high molecular weight isotactic polypropylene (HMW-iPP) and low molecular weight linear low density polyethylene (LMW-LLDPE) as model systems. LMW-LLDPE, the dominant component, served as the amorphous matrix. Crystallization behavior of the blends with three different HMW-iPP concentrations, i.e., 3, 6 and 9 wt%, were examined at temperatures higher than the blends' cloud points, in a time-resolved manner. It was found that the formation of a flow-induced precursor of crystallization was strongly influenced by the concentration of HMW-iPP in the blends. The blend with low HMW-iPP content (3 wt%) was not able to form a shear-induced crystalline structure. As the concentration of HMW-iPP increased, crystallization kinetics, crystallinity, and lamellar orientation were greatly enhanced.

High-energy radiation forming chain scission and branching in polypropylene

The degradation of high molecular weight isotactic polypropylene (iPP) subjected to gamma rays irradiation up to 100 kGy in inert atmosphere was analyzed. The investigation relied upon complex viscosity, elastic modulus, gel fraction, morphology of the insoluble fraction and deconvoluted molecular weight distribution (MWD) curves. At low irradiation doses, already at 5 kGy, the MWD curve is strongly shifted to the low molecular weight side showing chain scission, which is confirmed using the calculated chain scission distribution function (CSDF). At high dose levels, the appearance of a shoulder in the high molecular weight side of the MWD curve indicates the formation of chain branching. The presence of a considerable insoluble fraction at these high dose levels indicates also the formation of cross-linking, which has different morphology then the insoluble fraction present in the original iPP. The rheological results show changes in the molecular structure of irradiated samples in agreement with the gel content data. The chromatographic and rheological data has shown that gamma irradiation of iPP produces chain scission, branching and cross-linking.

On the γ-Phase of Isotactic Polypropylene

The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) (Mw = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.

Crystal structures and stereospecific propylene polymerizations with chiral hafnium metallocene catalysts

Ligand effects on stereoregulation with homogeneous catalysts have been stimulated considerable interest in Ziegler-Natta propylene polymerizations with metallocene catalysts. The major limitations to development in this area have been that the titanium catalysts are unstable at conventional polymerization temperatures, and that the zirconium analogues only produce low molecular weight oligomers in significant quantities. In this contribution, the authors describe the structures and the polymerization behavior of both rac-ethylenebis(indenyl)hafnium(IV) dichloride (rac-Et(Ind)/sub 2/HfCl/sub 2/) and rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)hafnium(IV) dichloride (rac-Et-(IndH/sub 4/)/sub 2/HfCl/sub 2/). The results are compared to those with Ti and Zr analogues. The new Hf catalysts are the first metallocenes to provide high yields of high molecular weight isotactic polypropylene.