Polyethylene elastic properties

Recently, Brillouin scattering has proved useful in this area for studying the frequency dependence of hypersonic (GHz zone) absorption and dispersion velocity in liquid sulphur dioxide [91] the effect of isotopes on hydrodynamic fluctuations in self-associated fluids [92] and the elastic properties of polyethylene glycol solutions in water, benzene and toluene [93]. 

Opdahl and Somorjai studied the surface deformation and surface elastic properties of stretched polyethylene in a device depicted in Fig. 3.73. It was found that the surface textures of both HDPE and LDPE changed and that the nodular structures present at the surface lengthened in the direction of the stretch and contracted perpendicular to the stretch at various elongations. This resulted in a roughening of the surface. 

The Chow equations [5] and the Halpin-Tsai equations [8,9] are also useful in modeling the effects of the crystalline fraction and of the lamellar shape (see Bicerano [23] for an example) on the moduli of semicrystalline polymers. Grubb [24] has provided a broad overview of the elastic properties of semicrystalline polymers, including both their experimental determination and their modeling. Janzen s work in modeling the Young s modulus [25-27] and yielding [27] of polyethylene is also quite instructive. 

The effective removal of crystallinity by the incorporation of p-MS provides the opportunity to obtain elastic properties in polyethylene copolymers. The copolymer containing 18.96 mol % of p-MS is actually an elastomer with a Tg of -5.7 °C. Further increase of p-MS concentration provides the copolymers with higher Tg and the copolymers become glassy at room temperature. Figure 7 shows DSC curves of poly(ethylene-co-p-methylstyrene) copolymers with (a) 18.98, (b) 32.8 and (c) 40 mole % p-MS, respectively. The Tg increases from -5.7 °C to 30.9 °C and to 38.8 C. The single Tg with sharp thermal transition in each case is indicative of homogeneous copolymer structure even at as high as 40 mol % of p-MS concentration. 

However some non-rubber compounds are also called elastomers if they exhibit a nondeforming elastic property similar to rubber at room temperature, even if the compound is relatively hard. The two main groups of non-rubber elastomers are thermoplastics, e.g., polyvinyl chloride, polypropylene and thermosets, e.g., ethylene propylene rubber, cross-linked polyethylene. These two groups are also covered by the term plastic . 

The problem with the amorphous material is that, even though it may be in a rubbery state, there are likely to be constraints on the chains due to the crystallisation process, which will give the material different properties from those of a purely amorphous rubbery polymer. The difficulty with the two averaging steps is that the states of stress and strain are not homogeneous in materials made up of components with different elastic properties. The simple assumption of uniform stress often gives results closer to experiment than does the assumption of uniform strain, but neither is physically realistic. For polyethylene, values of the average erystal modulus Ec and the average amorphous modulus are found to be about 5 x 10 Pa and 0.25 x 10 Pa, respectively. 

Let us consider the elastic properties of epoxy polymer thin films on substrates with various surface energies. The elasticity of the boundary layers and its contribution to the elasticity of the film on the substrate were assessed by the dependence of the modulus of elasticity on the thickness of the polymeric coating formed on high-energy (aluminum) and low-energy (polyethylene terephthalate) surfaces. ED-20 epoxy resin (molecular weight 420, epoxy number 21.6) was selected... 

Direct experimental verifications of the temperature dependences of the elastic moduli of perfect crystals of polyethylene in the chain-extended form, as represented in Table 4.3, present great difficulties, first because they relate to a perfect crystalline material and second because they are based on the anharmonic atomic interactions in such perfect material. Polymeric solids, even those that are highly crystalline, incorporate a variety of crystal imperfections that permit thermally assisted relaxations under stress. These dramatically attenuate the elastic properties that, at all but the lowest cryogenic temperatures, mask the temperature dependence of elastic interactions of the perfect crystal, particularly the stiffest intra-molecular interactions along the C—C backbone. In the vast majority of cases the elastic moduli of polymers reflect the soft intermolecular interactions, and the temperature dependence of these overwhelmingly dominates the intramolecular variety at all but the lowest temperatures. 

Table 14.1 Comparison of the elastic properties of Vectra B (A = 15), Spectra 1000, drawn polyethylene (A = 27) and Kevlar 49 (elastic moduli in GPa)...

Thermoplastics used to blend with NR include PS, " polyamide 6, ethylene-vinyl acetate (EVA) copolymer, poly(methyl methacrylate) (PMMA), polypropylene (PP), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) " and high-density polyethylene (HDPE). To improve the properties of TPNR, modified NR is also used. ENR is the most frequently used modified NR. TPNR blends are prepared by blending NR and thermoplastics in various proportions. The role of rubber is to improve the impact strength and ductility of the plastic. Depending on the ratio, materials with a wide range of properties are obtained. The stiffness of the rubber is increased with the incorporation of plastic into the rubber matrix. The mechanical properties of TPNR again depend on the proportions of the rubber and thermoplastic components. The elastic properties of TPNR are considerably... 

The ICI scientists used this instrument to provide information on the rate that polyethylene samples flowed through the instrument. Relatively higher flows indicated polyethylene with a relatively lower molecular weight. However, the instrument was also used by ICI scientists to measure the elastic properties of the molten polyethylene. The elasticity was expressed as a % recovery once the polyethylene sample (strand) that flowed through the die cooled to room temperature. The % recovery value was calculated from the diameter of the polymer strand (D ) and the diameter of the orifice (DJ ... 

Suiuita M, Okuma T, Miyasaka K and Ishikawa K (1982) Effect of ultrafine particles on the elastic properties of oriented low-density polyethylene compoates, J Appl Polym Sci 27 3059-3066. 

Bersted, B. H. A model relating the elastic properties of high-density polyethylene melts to the molecular weight distribution. /. App. Polym. Sd. (1976) 20, pp. 2705-2714... 

Secondly, the ultimate properties of polymers are of continuous interest. Ultimate properties are the properties of ideal, defect free, structures. So far, for polymer crystals the ultimate elastic modulus and the ultimate tensile strength have not been calculated at an appropriate level. In particular, convergence as a function of basis set size has not been demonstrated, and most calculations have been applied to a single isolated chain rather than a three-dimensional polymer crystal. Using the Car-Parrinello method, we have been able to achieve basis set convergence for the elastic modulus of a three-dimensional infinite polyethylene crystal. These results will also be fliscussed. 

As we saw in the first chapter, polymers have become important engineering materials. They are much more complex structurally than metals, and because of this they have very special mechanical properties. The extreme elasticity of a rubber band is one the formability of polyethylene is another. 

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