Polyethylene ultrahigh molecular weight linear

Polyethylene produets exhibit excellent wear resistance, especially when made from ultrahigh molecular weight linear resins, in which form it surpasses all but the most speeialized of polymers in their neat state (some fllled polymers can exhibit superior wear resistance, but this is more a function of the flfler than of the polymer). The eoefficient of friction of high density polyethylene is very low, on a par with all but the most slippery of polymers. It rises as a fimetion of molecular weight and increased branching levels. 

A) is much smaller than the lamellar length of ordinary PE crystals ( 100A), thus the PE chains are prevented from folding. In this manner, extended-chain crystalline nanofibers of linear polyethylene with an ultrahigh molecular weight (6,200,000) and a diameter of 30 to 50 nm are formed. 

The findings described above form a basis for our current work that examines the influence of the interphase on the structure and ultimately on the properties of linear polymers. In this chapter we will first show that, depending on the crystallization conditions, the amount of loops or entanglements in the interphase, and thus the deformation behavior of polymers, can be varied. We will initially consider the example of ultrahigh molecular weight polyethylene (UHMW-PE) and the role of entanglements upon its drawability. 

As illustrated above, selection of cocatalyst is often predicated on cost. In some cases, however, use of an alternative cocatalyst may transcend the cost factor. This could be because the alternative cocatalyst provides enhanced polymer properties or improved process performance. For example, use of TMAL as cocatalyst in place of TEAL in a gas phase process has been shown to provide linear low density polyethylene with lower hexane extractables and superior film tear strength (14). Ultrahigh molecular weight polyethylene and polyethylene with broader molecular weight distribution can be produced using "isopre-nylaluminum" as cocatalyst (15-17). 

Linear low-density polyethylene (LLDPEK Ultrahigh molecular weight polyethylene (HMWPE). 

Polyolefins constitute the largest volume class of polymeric materials. Polyethylene, polypropylene, and ethylene-propylene rubber are major products in this family, with many subset variations with each material. Polyethylene variants include high density polyethylene (HDPE), low density polyethylene (LDPE), ultrahigh molecular weight polyethylene (UHMWPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE) and various ethylene copolymers (including comonomers of vinyl acetate, ethyl and methyl acrylate, acryhc acid and methacryhc acid and their metal salts (ionomers)). Polypropylene has fewer variations, of which low amounts of ethylene are included while maintaining crystaUinity. More recently, ethylene-styrene copolymers have been introduced. 

A maximum of draw ratio as a function of deformation temperature is also observed for many samples [117,122]. Initially, increased temperature facilitates the relief of excessive strain on individual chain segments by molecular slippage, but above a critical temperature the disentanglement rate overtakes the deformation rate. The temperature at which this occurs in linear samples increases as a function of molecular weight [123]. In ultrahigh molecular weight polyethylene samples, the maximum draw ratio is not obtained until the deformation temperature exceeds the melting temperature of the sample. 

Capaccio, G. and Ward, I.M. (1973) Properties of ultra-high modulus linear polyethylenes. Nature Phys. Sci., 243, 143 (1974) Preparation of Ultrahigh Modulus Linear Polyethylenes - Effect of Molecular-weight and Molecular-weight Distribution on Drawing Behavior and Mechanical-properties. Polymer, 15,223. 

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