Polyethylene Homopolymer Research Articles

The drawing behavior of linear polyethylene. I. Rate of drawing as a function of polymer molecular weight and initial thermal treatment

AbstractThe drawing behavior of a series of linear polyethylene homopolymers with weight‐average molecular weight (M̄w) ranging from 67,800 to ∼3,500,000 and variable distribution (M̄w/M̄n = 5.1−20.9) has been studied. Sheets were prepared by two distinct routes: either by quenching the molten polymer into cold water or by slow cooling below the crystallization temperature (∼120°C) followed by quenching into cold water.When the samples (2 cm long) were drawn in air at 75°C using a crosshead speed of 10 cm/min it was found that for low M̄w polymers the initial thermal treatment has a dramatic effect on the rate at which the local deformation proceeds in the necked region. At high M̄w such effects are negligible. An important result was that comparatively high draw ratios (λ > 17) and correspondingly high Young's moduli could be obtained for a polymer with M̄w as high as 312,000. It is shown how some of the structural features of the initial materials (mainly studied by optical microscopy, small‐angle x‐ray scattering and low‐frequency laser Raman spectroscopy) can be interpreted in terms of the molecular weight and molecular weight distribution of the polymers. Although crystallization and morphology can be important at low M̄w, it suggested that the concept of a molecular network which embraces both crystalline and noncrystalline material is more helpful in understanding the drawing behavior over the whole range of molecular weights.

Static coefficients of friction for polyethylenes

AbstractThe static coefficients of friction of solid polyolefins against metal surfaces has been determined as functions of pressure, temperature, and metal surface finish. The polymers studied were a low debsity (branched) polyethylene, a high density copolymer, and a high density (linear) polyethylene homocopolymer, and a high density (linear) polyethylene homopolymer. Four metal surfaces were used, with surface roughnesses ranging from 1 1/2 to 18 microinches. The pressure and temperature ranges were 0 to 355 psig and 72 to 225°F, respectively. Data were obtained for the polymers in two forms: granules and flat smooth dise, except for the high density homopolymer which was tested in the disc from only. In general, the results indicated that, for all granules, the coefficient was essentially independent of pressure, but decreased with increasing temperature until the melting point was reached; it varied with metal surface roughness which was greatest for the low density polymer. For the smooth surfaced disc samples, the coefficient also decreased with increasing pressure with high values at low pressure indicating strong adhesion between the polymer and metal surfaces.

Effects of morphology and molecular structure on tensile impact behavior of linear polyethylene

AbstractAn instrumented tensile impact test which allows a load time curve to be obtained coincident with the energy to rupture values has been applied to the examination of a series of linear polyethylene homopolymers and copolymers of varying molecular weight and thermal history. The effect of test specimen geometry was also investigated. A morphological examination of these resins indicated a strong dependence of spherulite size on molecular weight, and to a much lesser degree on molding conditions; it appeared to be independent of comonomer. The overall crystallinity, as determined by both x‐ray and density methods, and crystallite perfection was increased by a slower crystallization rate, but was independent of molecular weight. The tensile impact strength (energy to rupture) was found to increase with molecular weight, and decrease on the introduction of comonomer. But the effect of thermal history gave rise to results which bore no clear‐cut relation to molecular and morphological parameters. The value of the instrumented tensile impact test is shown by the fact that these apparent anomalies can now be resolved by separating the elastic and plastic portions of the deformation. An increased level and perfection of crystallinity results in higher load bearing properties accompanied by a reduction in duration of impact. It is primarily the duration, rather than the peak load, which is affected by molecular weight. The main effect produced by changing from a type L (long) tensile impact specimen to a type S (short) was to reduce considerably the duration of the plastic region while simultaneously raising the maximum elastic load; this is thought to be associated with a more localized deformation and therefore a higher rate of strain.

The melting of defect polymer crystals

Melting of imperfect crystals is treated, using the methods of irreversible thermodynamics. A reproducible limiting heating path of zero entropy production from the metastable solid to the supercooled molten state is described. Experimental evidence shows that this ‘equilibrium’ heating path can often be approximated by heating fast enough so that reorganization and recrystallization in the crystalline aggregates are minimized, but not so fast that superheating is experienced. Expressions for the melting of perfect homopolymer lamellae, defect copolymer lamellae, and for the experimental maximum melting point of copolymers are discussed. For polyethylene a self-consistent set of data based on melting experiments carried out under zero entropy production heating is given. By this method the equilibrium maximum melting point for polyethylene homopolymer is found to be 142°C, while the surface free energy of a (001)-surface of polymer lamellae is about 75 erg/cm 2.