Polyethylene networks

This argument was put forth In 1962 by Judge and Stein (1) to explain the behavior of polyethylene networks under a constant load. That same year Smith (4) showed that a folded-chain morphology was thermodynamically favored at lower temperatures (high crystallization) whereas at temperatures near the melting point (low crystallization) fibrillar morphology was stable a transformation of one morphology Into the other was thermodynamically... 

This may be what happens in polyethylene networks (1 ). A transformation from one morphology to another is therefore not at all required. The observed stress profile may be indicative of nothing more than a normal progression of lamellar growth whereby more and more folds are incorporated by the growing lamellae. 

Figure 9. The end-to-end distance per skeletal bond n for regular conformations of polydimethylsiloxane and polyethylene network chains (12). Maximum extensibility rm of this chain molecule occurs at rm/n = 1.34 A.

Additional support for a value of A in the neighborhood of 1.0 10-8 is furnished by the results of our calculations of the unperturbed dimensions of a number of linear polyesters (see Section III C) which consist mainly of methylene chain units and which yield similar values of A. We should also mention an as yet unpublished calculation of Hoeve [quoted in a paper by Ciferri, Hoeve and Flory (66")], according to which such a value of A is in excellent agreement with the observed temperature coefficient, as deduced from intrinsic viscosities and from stress-temperature coefficients for cross-linked polyethylene networks. 

To obtain a higher effidency of photocrossiinking and to improve technological properties of prepared films (smoothness of the surface, transparoic ), 3 to 5% of ethylene-vinyl acetate copolymer having about 12% of vinyl acetate groups ate added [90], Increase of the molecular mass of the added copolymer leads to the increase of insoluble gel which indicates that this copolymer incorporates into the polyethylene network. The decrease or increase of vinyl acetate content in the cx>polymer below/ above an optimum value always reduces the effidency of photoo-osdinldng. 

Posthuma de Boer and Pennings have investigated the stretching of polyethylene networks swollen to an equilibrium extent and followed the retroactive forces with crystallization. The product has a shish-kebab morphology identical to that normally prepared by flow-induced crystallization. 

Since the melting temperatures of networks are depressed well beyond that expected, based on the concentration of cross-linkages, it can be anticipated that the level of crystallinity will be influenced in a similar manner. This expectation is fulfilled due to the crystallization restraints placed on units adjacent to network junction points. In polyethylene networks there is a twofold decrease in the enthalpy of fusion for the networks studied.(15) This can be directly related to the decrease in the level of crystallinity. Similar results were found with poly(tetrahydrofuran) networks.(21)... 

Table 7.2. Properties of polyethylene networks formed at 17° C by the action of high-energy ionizing radiation s...

Fig. 1.15. Thermoelastic results on (amorphous) polyethylene networks and their interpretation in terms of the preferred, all-trani conformation of the chain [3, 6],...

Taylor, D.J.R., Stepto, R.F.T. and Jones, R.A. (1999) Computer simulation studies of molecular orientation and the stress-optical properties of polyethylene networks. 

Sharaf M A and Mark J E (2004) Monte Carlo simulations on the effects of nauopart-icles on chain deformations and reinforcement in amorphous polyethylene networks. Polymer 45 3943-3952. 

Figure 12 Stress-temperature curves at constant pressure and length for an amorphous polyethylene network in the unswollen state. Values of the elongation a and of the nominal stress / are calculated using the rest length and the undeformed cross-section respectively, at the highest temperature investigated (reproduced by permission of Wiley from J. E.

Cables are available in a variety of constmctions and materials, in order to meet the requirements of industry specifications and the physical environment. For indoor usage, such as for Local Area Networks (LAN), the codes require that the cables should pass very strict fire and smoke release specifications. In these cases, highly dame retardant and low smoke materials are used, based on halogenated polymers such as duorinated ethylene—propylene polymers (like PTFE or FEP) or poly(vinyl chloride) (PVC). Eor outdoor usage, where fire retardancy is not an issue, polyethylene can be used at a lower cost. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

A similar example is the formation of nonstoichiometric interpolymeric complexes between mutually complementary polyelectrolytes — polycation and polyanion [69,70], They behave like true polymer networks and are capable of swelling the interpolymeric complexes between PAAc and polyethylene piperazine swells, for instance, 16-18 times [70], Also advantageous in this case is the possibility to carry out this type of crosslinking in open systems, such as soil. 

Classify each of the following solids as ionic, network, metallic, or molecular (a) quartz, Si02 (b) limestone, CaC03 (c) dry ice, C02 (d) sucrose, C12H22011 (e) polyethylene, a... 

Synthesis of comb (regular graft) copolymers having a PDMS backbone and polyethylene oxide) teeth was reported 344). These copolymers were obtained by the reaction of poly(hydrogen,methyl)siloxane and monohydroxy-terminated polyethylene oxide) in benzene or toluene solution using triethylamine as catalyst. All the polymers obtained were reported to be liquids at room temperature. The copolymers were then thermally crosslinked at 150 °C. Conductivities of the lithium salts of the copolymers and the networks were determined. 

Yang and coworkers did the most efforts on the development of network polyester based on citric acid. " They investigated the reaction of citric acid with a series of aliphatic diols (from 3-16 carbon chains) and polyether diols such as polyethylene oxide (PEG), in which 1,8-octanediol (POC) and 1,10 decanediol (PDC) have been studied the most. 

Crosslinked polyethylene consists of molecular chains that are linked at random points to form a network, as shown schematically in Fig. 18.2 f). The crosslinks can consist of carbon-carbon bonds, which directly link adjacent chains, or short bridging species, such as siloxanes, which may link two, three, or four chains. We often refer to these materials as XLPE. 

These moment studies have been performed on polymer systems such as polyethylene (or on penetrants in polymer systems) in which the interacting spins (protons or fluorines) reside on the same or on adjacent atoms. This allows essentially no freedom of variation in the internuclear vectors upon deformation of the network. The primary informational content therefore relates to independent segmental orientation distributions. By placing single spins on alternate segments, there should be much greater sensitivity to changes in the chain extension upon bulk deformation. 

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

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