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

Abstract

Observations are reported on linear low-density polyethylene in isothermal torsional oscillation and relaxation tests at various temperatures ranging from room temperature to 110 C. Constitutive equations are derived for the viscoelastic response of a semicrystalline polymer at small strains. The polymer is treated as an equivalent network of strands bridged by junctions (entanglements, physical cross-links on the surfaces of crystallites and lamellar blocks). The network is thought of as an ensemble of meso-regions with various potential energies for rearrangement of strands. Two types of meso-domains are introduced: active, where strands separate from temporary junctions as they are excited by thermal fluctuations, and passive, where detachment of strands is prevented by the surrounding macromolecules. The time-dependent behavior of the ensemble reflects separation of active strands from their junctions and merging of dangling strands with the network. Stress-strain relations are developed by using the laws of thermodynamics. The governing equations involve six material constants that are found by fitting the experimental data. The study focuses on the effects of (i) temperature, (ii) the deformation mode (torsion versus bending), and (iii) the loading program (oscillations versus relaxation) on the adjustable parameters. IntroductionThis paper is concerned with the effect of temperature on the viscoelastic behavior of injectionmolded linear low-density polyethylene (LLDPE) at isothermal deformations with small strains. The choice of this polymer for the investigation is explained by its numerous industrial applications: films for packaging, thin-wall molded components, pipes, irrigation and spray tubes, hoses, geomembranes, etc.Polyethylene is a semicrystalline polymer that contains three different crystallographic forms: monoclinic, hexagonal and orthorhombic. In injection-molded specimens, orthorhombic structures are mainly formed, whereas monoclinic and hexagonal crystallites are produced by polymerization under high pressure (over 400 MPa) and at high temperature, [1].LLDPE is a random copolymer prepared by polymerization of ethylene in the presence of 1-alkenes. 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. The histograms reported in [3] reveal a wide distribution of segment lengths, with a relatively high probability of segments ranging up to 30 nm, which results in a wide distribution of lamellar thicknesses in LLDPE. The average radius of crystallites is strongly affected by the content of short-chain branches, and it changes from 2.5 to 12 lm depending on the crystallization conditions, [4]. The average lamellar thickness ranges from 8.5 to 10 nm, [4].