Long branches

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

Polymers with long branches do not fit these equations and different relations exist with polymers of different degrees of long branching. In many cases the equation... 

G. Eleury, G. Schlatter, and R. Muller, Non linear rheology for long branching characterization, comparison of two methodologies Fourier transform rheology and relaxation, Rheol. Acta, 44, 174—187, 2004. 

Pseudo Kinetic Rate Constant Method for Copolymers with Long Branches... 

Dipartimento di Scienze Chimiche, Alimentari, Farmaceutiche e Farmacologiche, Universitd del Piemonte Orientale, 28100 Novara, Italy Department of Pathology and Laboratory Medicine, Monmouth Medical Center, Long Branch, NJ 07740, USA... 

Department of Pathology and Laboratory Medicine, Monmouth Medical Center, Long Branch, NJ 07740, USA... 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

It has not yet been possible to obtain samples of amylopectin which do not show some slight evidence of uptake of iodine by linear material in the early stages of an accurate potentiometric titration. Although this effect is presumably due to contaminating amylose, the presence of some long branches in the amylopectin cannot be excluded. Anderson and Greenwood190 have shown that in 0.01 M iodide solution, for concentrations of total free iodine less than 1 X 10-6 M, the amount of iodine bound by... 

The reaction temperature is above the critical temperature of ethylene so that the ethylene is in gas phase. High pressures are needed for propagation reaction. Only about 6-25 per cent of ethylene is polymerised. Rest of monomer is recycled. Extensive chain transfer reactions takes place during polymerisation to yield a branched chain polyethylene. In addition to long branches, it also contains a large number of short branches of upto 5 carbon atoms produced by intra-molecular chain transfer reactions. A typical molecule of Low density polyethylene contains a short branch for about every 50 carbon atoms and one or two long branches per molecule. 

Finally, it is worth noting the effect of these errors on the resulting phylogenetic analysis. If a complete outlier is included, it will tend to join the rest of the sequences on a long branch, as it will be only randomly related to each of them. These can be identified relatively easily when the final analysis is carried out. Sequences with frameshifts are harder... 

Antigen-presenting cells are also called dendritic cells because of their long branched processes on dendrites. They were discovered as early as 1868 by Paul Langerhans, after whom they are sometimes named. It was 100 years later diat it was appreciated diat diey had a role in the immune system. 

It can also be expected that any calculations of long chain branching frequency (12,13) will be severely compromised by errors resulting from supermolecular structures. This is because the frequency of long branches resulting from chain transfer to... 

We found that results from one spectrum to another were within an order of one to two branches per thousand carbons as far as consistency was concerned. Spectra, which were obtained on other systems under less than equilibrium conditions, show that the total methyl content, surprisingly, stays very constant. You find that the low end is enhanced and the long branches are saturated slightly however, the total remains the same. 

Make sketches or diagrams showing (a) a linear polymer, (b) a polymer with pendant groups, (c) a polymer with short branches, (d) a polymer with long branches, and cross-linked polymers with (e) low and (f) high cross-linked density. 

LDPE has a combination of short-to-long branches, with long branches occurring at a rate of about 10 short branches to every long branch. 

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