Magnetic and Viscoelastic Study on Molten State of Low Molecular Weight Isotactic Polypropylene

2000 ◽  
Vol 33 (22) ◽  
pp. 8421-8425 ◽  
Author(s):  
Takahiko Kawai ◽  
Tsunehisa Kimura
Keyword(s):  
Molecular Weight ◽  
Isotactic Polypropylene ◽  
Low Molecular Weight ◽  
Molten State

A Real-Time Study of Homogeneous Nucleation, Growth, and Phase Transformations in Nanodroplets of Low Molecular Weight Isotactic Polypropylene Using AFM

2007 ◽  
Vol 40 (20) ◽  
pp. 7223-7230 ◽  
Author(s):  
Lekshmi Kailas ◽  
Cvetelin Vasilev ◽  
Jean-Nicolas Audinot ◽  
Henri-Noël Migeon ◽  
Jamie K. Hobbs
Keyword(s):  
Molecular Weight ◽  
Phase Transformations ◽  
Real Time ◽  
Isotactic Polypropylene ◽  
Homogeneous Nucleation ◽  
Low Molecular Weight ◽  
Time Study ◽  
Nucleation Growth

Scattering Studies of Cyrstallization in Highly Oriented Polymers

1986 ◽  
Vol 79 ◽  
Author(s):  
J. M. Schultz
Keyword(s):  
Molecular Weight ◽  
Glass Formation ◽  
Molecular Orientation ◽  
Local Strain ◽  
Temperature Fields ◽  
Low Molecular Weight ◽  
Molten State ◽  
Molecular Configuration ◽  
Oriented Polymers ◽  
Coiled State

The solidification of low molecular weight materials from their melts (in the absence of temperature fields) generally leads to a product with no preferred orientation, independent of the extent or rate of deformation of the melt. The solidification of polymers from a highly deformed melt or solution nearly always leads to a product with preferred molecular orientation. Further, the rate of solidification can be significantly increased by melt deformation. The change in rate, relative to that in a quiescent melt or solution, can be several orders of magnitude [1]. These differences, relative to small-molecule systems, arise from the degree to which orientation and local strain can be maintained in the melt. Due to the long-range connectivity within a polymer molecule, it is possible to impart large extensions to these molecules in the molten state, and significant time is often required for the molecules to relax back to their undistorted, coiled state. If crystallization or glass formation occurs before the chain can relax, an extended molecular configuration can be retained. Thus in the solidification of oriented polymers there exists a competition between the “freezing” of the molecular orientation and the relaxation (re-coiling) of the molecules.

A Rigid C1-Bridged Ansa-Zirconocene-Derived Catalyst System Suited for Stereoselective Low Molecular Weight Polypropylene Formation

1995 ◽  
Vol 50 (4) ◽  
pp. 469-475 ◽  
Author(s):  
Gerhard Erker ◽  
Christian Psiorz ◽  
Roland Fröhlich
Keyword(s):  
Molecular Weight ◽  
Fractional Crystallization ◽  
Isotactic Polypropylene ◽  
Catalyst System ◽  
Low Molecular Weight ◽  
X Ray Diffraction ◽  
X Ray ◽  
Tert Butyl ◽  
Catalyst Systems

2,5-Hexanedione was converted into the bisfulvene 2, then treated with two molar equivalents of methyllithium to yield the [4-cyclopentadienylidene-4,7,7-trimethyl-4,5,6,7-tetrahydroindenyl] dilithio compound 4. Hydrolysis, followed by treatment with acetone/pyrrolidine, gave the corresponding fulvene system 5. Reaction of 5 with methyllithium followed by treatment with ZrCl4 furnished the ring-annulated C1-bridged ansa-metallocene 8, bearing a tert-butyl substituent at the Cp ring, as a 1:1 mixture of two diastereoisomers. Treatment of the fulvene 5 with LiAlH4 followed by ZrCl4 yielded the respective isopropyl-substituted ansa-metallocene diastereomers 9a and 9b. Complex 9b was separated by fractional crystallization and characterized by X-ray diffraction. Complexes 8 and 9 provide active homogeneous Ziegler-type catalyst systems upon activation with excess methylalumoxane producing low molecular weight isotactic polypropylene with high catalyst activities.

A high-performance oil-free backing pump for electron microscopes

1978 ◽  
Vol 36 (1) ◽  
pp. 110-111
Author(s):  
G.K.W. Balkau ◽  
E. Bez ◽  
J.L. Farrant
Keyword(s):  
Molecular Weight ◽  
Electron Microscope ◽  
Hot Spots ◽  
High Performance ◽  
Carbonaceous Material ◽  
Contamination Rate ◽  
Low Molecular Weight ◽  
Principal Source ◽  
Rotary Pumps ◽  
Electron Microscopes

The earliest account of the contamination of electron microscope specimens by the deposition of carbonaceous material during electron irradiation was published in 1947 by Watson who was then working in Canada. It was soon established that this carbonaceous material is formed from organic vapours, and it is now recognized that the principal source is the oil-sealed rotary pumps which provide the backing vacuum. It has been shown that the organic vapours consist of low molecular weight fragments of oil molecules which have been degraded at hot spots produced by friction between the vanes and the surfaces on which they slide. As satisfactory oil-free pumps are unavailable, it is standard electron microscope practice to reduce the partial pressure of organic vapours in the microscope in the vicinity of the specimen by using liquid-nitrogen cooled anti-contamination devices. Traps of this type are sufficient to reduce the contamination rate to about 0.1 Å per min, which is tolerable for many investigations.

Low-molecular-weight heparin in the prevention and treatment of thromboembolic disorders

1998 ◽  
Vol 1 (5) ◽  
pp. 166-174 ◽  
Author(s):  
Evelyn R Hermes De Santis ◽  
Betsy S Laumeister ◽  
Vidhu Bansal ◽  
Vandana Kataria ◽  
Preeti Loomba ◽  
...  
Keyword(s):  
Molecular Weight ◽  
Low Molecular Weight Heparin ◽  
Low Molecular Weight ◽  
Prevention And Treatment
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