The True Stress-True Strain Relationship in the Plastic Deformation of some Crystalline Polymers

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Abstract
The relationships between true tensile stress (σ) and true tensile strain (∊) for plastic deformation of high density polyethylene (HPDE), its blends with n-paraffin, and nylon 6 were formulated in an equation with generalized form. The product of logarithmic σ/σ∗ and logarithmic ∊/∊∗ is equal to a constant with negative value (– c). a∗ and ∊∗ are determined empirically by shifting the doubly logarithmic curves along with the both axes. Constant c is characteristic of polymer species, being independent of melt index, drawing temperature (30–110°C for PE, 80–160°C for nylon 6), deformation rate and blending ratio of paraffin to PE. The value of c is 0.384 for PE and 0.175 for nylon 6. Referring to the idealized system with c = 0, σ∗ is predicated as the critical yield stress for unfolding molecules from lamellar crystals and ∊∗ is the true strain giving the extended chain conformation, which amounts to stretch ratio of 240 for PE. The fiber structure was assumed to form with contraction of the extended chains thus obtained. The law of volume additivity of σ∗ holds for the blend system of HDPE and n-paraffin.

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