Influence of the molecular architecture of low‐density polyethylene on the texture and mechanical properties of blown films

Abstract

Three types of low-density polyethylene materials were investigated with respect to the influence of the molecular architecture on the mechanical and use properties of blown films. The materials were a branched polyethylene synthesized by free-radical polymerization under high-pressure (HP-LDPE), a linear ethylene-hexene copolymer (ZN-LLDPE) produced by low-pressure Ziegler-Natta catalysis, and an ethylene-hexene copolymer (M-LLDPE) from metallocene catalysis. The extrusion and blowing conditions were identical for the three materials, with a take-up ratio of 12 and a blow-up ratio of 2.5. The blown films displayed a decreasing puncture resistance in the order M-LLDPE, ZN-LLDPE, and HP-LDPE. In parallel, the tear resistance of the films became increasingly unbalanced in the same order of the polymers. The morphological study showed an increased anisotropy of the films in the same polymer order, the crystalline lamellae being increasingly oriented normal to the take-up direction. This texturing caused a detrimental effect on the mechanical properties of the films, notably increasing the capacity for crack propagation. The phenomenon was ascribed to the kinetics of chain relaxation in the melt that governed the ability of the chains to recover an isotropic state from the flow-induced stretching before crystallization. The puncture resistance was examined in terms of both texture and strain-hardening capabilities.

The effect of temperature on the viscoelastic response of semicrystalline polymers

“…The average distance between short branches along the main chain is about 7 nm. 12 This value perfectly agrees with the average segment length found in [13] for metallocenecatalyzed LLDPE (under the assumptions that sidechains are segregated from crystallites, and only a small amount of them may be included into crystallites as defects 6 ). The histograms reported in [13] reveal a wide distribution of segment lengths, with a relatively high probability of segments ranging up to 30 nm (which, in turn, implies a wide distribution of lamellar thicknesses in LLDPE).…”

“…The average size of lamellae and their curvature, as well as the type of their organization into spherulites, are strongly affected by crystallization conditions, molecular weight, and degree of chain branching. 12 LLDPE is a random copolymer produced by polymerization of ethylene in the presence of alkenes (butene, octene, octadecene, hexene, etc.). The average distance between short branches along the main chain is about 7 nm.…”

“…The histograms reported in [13] reveal a wide distribution of segment lengths, with a relatively high probability of segments ranging up to 30 nm (which, in turn, implies a wide distribution of lamellar thicknesses in LLDPE). The average radius of crystallites is strongly affected by the content of shortchain branches (composed mainly of butyl side groups with minor amounts of ethyl, methyl, and hexyl side groups 12 ), and it changes from 2.5 to 12 m depending on the crystallization conditions. 8 The average lamellar thickness ranges from 8.5 to 10 nm.…”

The effect of temperature on the viscoelastic behavior of linear low-density polyethylene

“…The average distance between short branches along the main chain is about 7 nm, [2]. This value coincides with the average segment length found in [3] for metallocene-catalyzed LLDPE.…”

Structure of blown films of polyethylene/polybutene‐1 blends

“…The super-molecular structure, including the crystallinity, and the morphology and orientation of the crystals of blown films of polyethylene (PE) have been extensively studied [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Typically, blown films of PE are semicrystalline with a maximum degree of crystallinity, which depends on the molecular architecture/branching-characteristics.…”