Polymer, density

The more recently developed so-called linear low-density polyethylenes are virtually free of long chain branches but do contain short side chains as a result of copolymerising ethylene with a smaller amount of a higher alkene such as oct-1-ene. Such branching interferes with the ability of the polymer to crystallise as with the older low-density polymers and like them have low densities. The word linear in this case is used to imply the absence of long chain branches. 

With the availability of the higher density polymers the value of the melt flow index as a measure of molecular weight diminishes. For example, it has been found that with two polymers of the same weight average molecular weight (4.2 X 10 ), the branched polymer (density = 0.92 g/cm ) had only 1/50 the viscosity of the more or less unbranched polymer (density = 0.96 g/cm ). This is due to long chain branches as explained above. 

Figure 10.6. Effect of temperature on the tensile stress-strain curve for polyethylene. (Low-density polymer -0.92g/cm . MFI = 2.) Rate of extension 190% per minute ...

Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction and with a melting point of about 100°C, there are no solvents at room temperature. Low-density polymers will dissolve in benzene at about 60°C but the more crystalline high-density polymers only dissolve at temperatures some 20-30°C higher. Materials of similar solubility parameter and low molecular weight will, however, cause swelling, the more so in low-density polymers Table 10.5). 

Calendering processes, of great importance in the production of sheet materials from PVC compounds, are little used with polyethylene because of the difficulty in obtaining a smooth sheet. Commercial products have, however, been made by calendering low-density polymer containing a small amount of a peroxide such as benzoyl peroxide to give a stiff but crinkly sheet (Crinothene) which was suitable for lampshades and other decorative applications. 

Application of Statistical Experimental Design to Development of Low-Density Polymer Foams... 

We are applying the principles of statistical experimental design to the development of low-density polymer foams for use as direct-drive high-gain targets for Laser Inertial Confinement Fusion (ICF). Both polystyrene (PS) and resorcinol-formaldehyde (RF) foams are being developed. 

Initial width of fluid channel Final width of fluid channel Height of fluid channel Metal plate thickness Metal thickness at the edges Channel length Volumetric polymer flowrate Polymer inlet temperature Temperature of Dowtherm Polymer density Polymer heat capacity Polymer thermal conductivity Metal thermal conductivity... 

Tsujii, Y, Ohno, K., Yamamoto, S., Goto, A. and Fukuda, T. (2006) Structure and properties of high-density polymer brushes prepared by surface-initiated living radical polymerization. Adv. Polym. Sci., 197, 1-45. 

Low-density polyethylene (polythene) is a relatively cheap, tough, flexible plastic. It has a low softening point and is not suitable for use above about 60°C. The higher density polymer (950 kg/m3) is stiffer, and can be used at higher temperatures. Polypropylene is a stronger material than the polyethylenes and can be used at temperatures up to 120°C. The chemical resistance of the polyolefines is similar to that of PVC. 

Melting point alone cannot uniquely identify an OBC. For example, blends of high and low density polyolefins also exhibit an elevated melting point at equivalent density. Sample 3 in Fig. 17 (small circle) is a 70 30 physical blend of 0.86 and 0.94 g cm-3 ethylene-octene copolymers, and the melting point is similar to the OBCs. Physical blends of polymers of such disparate densities are not phase-continuous, however, and segregate into domains of the high and low density polymers. Figure 18 reveals differences in appearance of pressed plaques of the polymer samples... 

In addition to the polymer grafting density, polymer molecular weight is another important molecular parameter that profoundly influences the properties of surface-anchored polymers. As discussed earlier, the thickness of... 

Patent Number US 5844009 A 19981201 CROSSLINKED LOW-DENSITY POLYMER FOAM... 

It was discovered by Ziegler in Germany and Natta in Italy in the 1950s that metal alkyls were very efficient catalysts to promote ethylene polymerization at low pressures and low temperatures, where free-radical polymerization is very slow. They further found that the polymer they produced had fewer side chairrs because there were fewer growth mistakes caused by chain transfer and radical recombination. Therefore, this polymer was more crystalline and had a higher density than polymer prepared by free-radical processes. Thus were discovered linear and high-density polymers. 

Structure and Properties of High-Density Polymer Brushes... 

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