Semicrystalline polyolefins

The visco-elastic interactions of connected micro-elements of semicrystalline polyolefins are approximated by the Maxwell model, which formally consists of the spring with modulus E and the viscous dashpot characterized by the viscosity rf connected in series,... 

Although the catalyst - homogeneous or heterogeneous - mainly determines the oligomerization or polymerization process and hence the polymer properties, the technical process also plays an important role. The oligomers or polymers may or may not precipitate during the synthesis, depending on several factors. Hydrocarbons with six or more carbon atoms (n-hexane, n-heptane, decalin, toluene, xylene, and others) are solvents for polyolefins, but semicrystalline polyolefins are not soluble at room temperature. Only amorphous polyolefins can be dissolved. 

Polybutylene is a semicrystalline polyolefin thermoplasitic based on poly-1-butene and includes homopolymers and a series of copolymers... 

As suggested in the introduction the development of TREF was in response to the need to know something more about polyolefins, in addition to their molecular weight distributions, which might relate to those properties dependent on crystallinity. The hope and expectation is that through crystallizability separation it will prove possible to provide a more complete picture and understanding of the relationship between synthesis conditions and structure and between structure and properties for semicrystalline polyolefins. The way in which TREF has been used in the evaluation of the various polyolefin systems is reviewed in this section. 

Semicrystalline polyolefin blends and method of their preparation were described. The blends were reported to show enhanced inter-spherulitic and interlamellar strength. The first polymer should have higher crystallinity and crystallization temperature than the second. Thus, 50-99.9 wt% PP was blended with ethylene-a-olefin copolymers, either a stereo block polypropylene or an ethylene-propylene copolymer, EPR... 

Semicrystalline polyolefin blends were prepared by mixing two different random copolymers of propene with 4-lOC alpha-olefin at a ratio from 1 3-1 1. The first copolymer contained 1-10 wt% of C4 io alpha-olefin (1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-pentene), whereas the second 15 0 wt% of the same comonomer. The mixing was carried out in reactors, polymerizing the monomers in the presence of stereospecific catalysts supports on active magnesium dihalides, in at least two sequential stages. The resulting R-TPOs showed limited... 

In semicrystalline polyolefins, failure is also often by embrittlement, but for different reasons. The main mechanism is chain scission, with lowering of molecular weight and of Tg. Failure occurs at very low levels of oxidation the molecular weight typically falls by less than a factor of 2 and there is less than one O2 molecule per 100 carbon atoms. The reason for this sensitivity lies in the complex morphology of many polymers. 

Microporous polymer membranes are the most commonly used separators in lithium-ion batteries. Majority of the microporous polymer membrane separators are based on semicrystalline polyolefin materials, such as polyethylene (PE), polypropylene (PP), PE-PP blends and high-density polyethylene (HDPE)-ultrahigh molecular polyethylene (UHMWPE). 

These are a group of techniques based on the different crystallization behaviour of semicrystalline polyolefins in dilute solution to get information about CCD. The crystallization behaviour of polyolefins is determined by the molecular structure including the type of monomer, the copolymer composition and the molecular size. The crystallization behaviour as will be discussed in this section is dominated by the type of monomer and the copolymer composition. 

Recently a fascinating new development in crystallization based techniques for CCD analysis of semicrystalline polyolefins has been published. This new technique is called CEF. As demonstrated by Monrabal and Heirro [75] the use of only TREE or CRYSTAF for polyolefin analysis might lead to equivocal results and the use of both techniques for unequivocal results was recommended. CEF is an important step forward in this direction and it combines the separation power of TREE and CRYSTAF in one single fast run. 

In the case of semicrystalline polyolefin blown films, haze is affected by light scattering and reflection/refraction due to both bulk and surface roughness effects. References... 

Solubility (S) of oxygen in semicrystalline polyolefins depends on the content of amorphous phase and can be expressed by the following equation ... 

It is well known that the permeability of some semicrystalline polymers is reduced by increases in orientation of the amorphous phase, which greatly hinder the diffusion of the penetrant [479]. Similarly, increases in crystallinity are known to decrease the permeability of most semicrystalline polyolefins by reducing the volume available to the penetrant and by limiting the mobility of the amorphous segments nearby [1710]. 

Polyethylene melts at temperatures that are relatively low in comparison with those of other commercial semicrystalline polyolefins and much lower than... 

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