Structure cross-linked polyethylene

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

Smedberg, A. Hjertberg, T. Gustafsson, B. Effect of molecular structure and topology on network formation in peroxide cross-linked polyethylene. Polymer 2003, 44, 3395-3405. 

Fig. 1.2. Illustration of the molecular structure of cross-linked polyethylene in the liquid state. The spaces between the sketched net are filled with other parts of the network.

Figlire 2IJ4 Molecular structure of forms of polyethylene, (a) Low-density polyethylene has branched chains, (b) High-density polyethylene is made of long, straight chains, (c) Cross-linked polyethylene has covalent bonds between carbon chains. 

Polyethylene Properties of the different forms of polyethylene are reflected in their uses. Linear molecules of polyethylene can pack together very closely, as shown in the model of high-density polyethylene (HDPE). The branches of branched polyefhylene keep the molecules from packing fightly, as shown in the low-density polyethylene (LDPE) structure. The cross-links of cross-linked polyethylene (cPE) make it very strong. 

Figure 10.16 Structural details revealed by staining, in this case showing lamellar detail in cross-linked polyethylene (chlorosulphonation).

The gas-barrier properties of nanostructiued polymer blends based on POSS are very much influenced by the presence of aromatic moieties, polar groups such as hydroxyl groups and the degree of cross linking. The gas-barrier properties of cross-Unked polyethylene are superior to those of a similar non cross-linked polyethylene [32], Aliphatic hydrocarbon chains provide a low gas barrier. However, the non-polar aliphatic hydrocarbon moieties are connected to the POSS backbone by polar amide bonds. Amide bonds are known to increase the gas-barrier properties of a material significantly, which leads to a significantly lower transmission of oxygen or helium in polyamides compared to polyethylene. This shows that different structural elanents can exhibit different influence on the gas-barrier properties of the POSS material. 

A polymer is a macromolecule that is constructed by chemically linking together a sequent of molecular fragments. In simple synthetic polymers such as polyethylene or polystyrer all of the molecular fragments comprise the same basic unit (or monomer). Other poly me contain mixtures of monomers. Proteins, for example, are polypeptide chains in which eac unit is one of the twenty amino acids. Cross-linking between different chains gives rise to j-further variations in the constitution and structure of a polymer. All of these features me affect the overall properties of the molecule, sometimes in a dramatic way. Moreover, or... 

As shown originally by Malcolm Dole, polyethylene molecules may be cross-linked when subjected to high-energy radiation. These three-dimensional network polymers may be represented by the structure shown in Figure 1.5. 

Polyolefins, especially polyethylene, can be cross-linked into a material that is elastic when heated. The structure of polyolefins, normally entangled long chains, includes crystalline and amorphous regions. Upon heating above the crystalline melting point of the polymer the crystalline regions disappear. 

Polypropylene. A similar study on polypropylene is interesting because polypropylene has a molecular structure intermediate between polyethylene and polyisobutylene. An atactic polypropylene specimen was prepared by ether extraction and irradiated in a nitrous oxide atmosphere. The changes in gel fraction (insoluble in hot xylene) as a function of N-jO pressure are shown in Figure 6. Gel formation (cross-linking) of polypropylene is also promoted in the presence of nitrous oxide. 

Figure 2.7—Structure of polysiloxanes (silicones) and polyethylene glycols. An inventory of all the compositions of these phases that can be used for impregnation or bonding would be lengthy. The surface of the silica column can be treated with tetradimethylsiloxane in order to obtain the bound phase, which is polymerised and then cross-linked.

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