On the effect of crystallinity on the elastic properties of semicrystalline polyethylene

• Published in: Polymer engineering and science 2000-02, Vol.40 (2), p.330-335
• Main Author: Doyle, Michael J.
• Format: Article
• Language: English
• Subjects: Analysis; Applied sciences; Crystal lattices; Crystallization; Exact sciences and technology; Mechanical properties; Organic polymers; Physical sciences; Physicochemistry of polymers; Polyethylene; Properties and characterization; Strains and stresses; Stress relaxation (Materials); Stress relieving (Materials)
• ISSN: 0032-3888; 1548-2634
• DOI: 10.1002/pen.11166

The small strain elastic response of a semicrystalline polyethylene has been simulated with a simple finite element model comprising a composite microstructure of alternating amorphous and crystalline layers arranged in a multi‐layered spherical shell structure. The crystalline lamella is modeled as an isotropic solid having the experimentally determined modulus properties of bulk linear polyethylene and extrapolated to 100% crystallinity. The amorphous layer is modeled as an elastomeric solid with the modulus that of molten linear polyethylene extrapolated to room temperature. Assuming constancy of these phase properties, the finite element calculations of elastic modulus versus crystallinity fit quite well the experimental data of Crist for linear polyethylene above 55% crystallinity. At lower crystallinity the experimental modulus data, which are for copolymers, fall below the model calculations. Two possible causes are discussed: the loss of deformation constraint in lamellae of finite and decreasing width as crystallinity declines, and an expansion of the crystal lattice as a result of the crowding of the comonomer units excluded from the crystal lamella. Either effect would reduce both the modulus and the yield stress as is observed.