Mechanical and transport properties of drawn low‐pressure low‐density polyethylene

Abstract

The new low density polyethylene (LDPE) obtained under low pressure in the reactor, the so-called low pressure low density polyethylene (LLDPE), with a great many short branches seems to replace the conventional LDPE with long branches obtained in the reactor under high pressure (high pressure LDPE = HLDPE). It has a higher tensile strength, impact strength, elastic modulus, elongation at break, and resistance to heat and stress cracking. The replacement goes on in film extrusion, blow molding, and wire and cable coating. The material seems also to be a good mixing component in polymer blends. ' The most obvious physical differences between the short and long branch LDPE are the higher crystallinity and the narrower molecular weight distribution, higher melting temperature (122O instead of 108"), and a narrower melting range in the former than in the latter case.The thicker and more perfect crystals are the main cause of these changes. The higher melting point, however, means a higher processing temperature that may be more harmful to the material and require a higher energy input.The increased technical interest in this type of material prompted us to investigate the mechanical and transport properties of Dowlex 2045, a Dow Chemical made LLDPE with a density 0.9139 g/cm3 that corresponds to a, = 0.414. This is less than the density 0.9152 g/cm3 of the formerly investigated conventional HLDPE with a, = 0.423. The ratio 10-35 of side chain ends CH3 per 1000 CH2 on the backbone and side chains of the molecule indicates a large number of side chains. According to the producer the side chains are much shorter than in the conventional HLDPE. (Certain commercial material is identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.)The enormous elastic modulus of the crystals as compared with that of the amorphous component makes the mechanical properties of the semicrystalline polymer depend primarily on the fraction of taut tie molecules crossing the amorphous layers and on the mechanical properties of these layers. Although they are in the rubbery state, the amorphous molecules are so intimately bound to the crystals that the properties of the amorphous component substantially deviate from those of a completely amorphous rubber. In particular the specific volume of the amorphous component changes upon straining. Instead of the shear modulus G one must introduce the about 1000 times higher expansion modulus K of the rubber.The best method for the investigation of the amorphous component is the measurement of the transport properties. In first approximation they depend on the fractional free volume. It increases with the deformation if the specific volume does the same. The plastic deformation of drawing. however, reduces quite substantially the specific volume and hence the transport pr...

Transport and mechanical properties of iPP-sPP fibers

“…Clearly evident is a smooth decrease of diffusion up to λ = 6, and then a sharp drop between λ = 6 and λ = 8. The decrease of Do with λ, already found in polyethylene, both high density24 and low density,25–27 and in polypropylene,28, 30 can be explained very well in terms of structural changes caused by the drawing process. According to Peterlin's model, the original unoriented lamellar structure is gradually transformed into a highly oriented microfibrillar structure.…”

“…Transport properties are particularly suited to an investigation of the amorphous phase: as a matter of fact, they are related to the fraction and to the thermodynamic state of this phase and can give valuable information about its role in the deformation process and in final properties. Previous investigations of the uniaxial deformation of linear high‐density polyethylene,24 low‐density polyethylene,25–26 linear low‐density polyethylene,27 and isotactic polypropylene,28–30 have revealed the usefulness of transport properties for this purpose.…”

Effect of Molecular Orientation on Polymer Free Volume Distribution: An Atomistic Approach

“…It is known that a denser packing of amorphous polymer resulting from drawing or stretching reduces its fractional free volume. The consequence of this behavior is lowering of diffusivity and gas permeability. − The recent experimental data from Shelby et al, however, suggested chain alignment causes an increase in the number of larger, more elliptical free volume holes in amorphous polymers, while the polymer density increases with stretching. , In the present study, we applied MD simulations using the united atom model to quantitatively study the effect of molecular orientation induced by extensional strains on the free volume distribution in polymers. We also investigated whether the increase in the number of larger, more elliptical free volume regions (“holes” or “cavities”) with stretch is observable using our computational approach.…”

“…The consequence of this behavior is lowering of diffusivity and gas permeability. [14][15][16] The recent experimental data from Shelby et al, however, suggested chain alignment causes an increase in the number of larger, more elliptical free volume holes in amorphous polymers, while the polymer density increases with stretching. 17,18 In the present study, we applied MD simulations using the united atom model to quantitatively study the effect of molecular orientation induced by extensional strains on the free volume distribution in polymers.…”

Sorption and diffusion of toluene in isotropic and oriented linear polyethylene

“…For a commercial LLDPE (Dowlex 2045) these authors found that the diffusion coefficient and its concentration dependence tend to level off at the highest draw ratio of 8. 5 They suggested that for LLDPE the critical draw ratio for complete transformation of the original lamellar material into the microfibrillar structure is about 8, consistent with the HDPE r e~u l t s .~ Similar studies have been made on the sorption and diffusion of toluene in oriented polypropylene (PP).18 With increasing draw ratio both the s o r p tion and diffusion coefficient decrease steadily, but no abrupt drop occurs at the draw ratio corresponding to the complete transformation into the fibrous structure. However, the most striking difference between the behavior of the CH2C12-HDPE and toluene-PP systems is that while the concentration coefficient y for the former system shows a twentyfold increase between A = 8 and 9, the y value for diffusion of toluene in PP drops by 40% as A increases from 1 to 18.…”