Poisson’s ratio transition in strain crystallizing elastomer: from rubbery amorphous to semicrystalline

Abstract

The Poisson’s ratio (μ) of natural rubber (NR) undergoing strain-induced crystallization (SIC) during uniaxial stretching was investigated as a function of the applied stretch ($${\lambda }_{{||}}$$ λ ∣ ∣ ) over a broad $${\lambda }_{{||}}$$ λ ∣ ∣ range, encompassing the SIC onset stretch ($${{\lambda }_{{||}}}^{* }$$ λ ∣ ∣ * ≈ 4.1). Below $${{\lambda }_{{||}}}^{* }$$ λ ∣ ∣ * , μ remains near 1/2, indicating the incompressible behavior of NR in its fully rubbery amorphous state. However, once $${\lambda }_{{||}}$$ λ ∣ ∣ exceeds $${{\lambda }_{{||}}}^{* }$$ λ ∣ ∣ * , μ decreases as the degree of crystallinity (χc) increases. As $${\lambda }_{{||}}$$ λ ∣ ∣ increases from $${{\lambda }_{{||}}}^{* }$$ λ ∣ ∣ * to fracture stretch ($${\lambda }_{{||}}$$ λ ∣ ∣ ≈ 7.1), χc increases to 18%, and μ gradually decreases to 0.33. This reduction in μ reflects the transformation of the NR matrix from a rubbery amorphous state to a semicrystalline state. In fact, the true stress (force per cross-sectional area in the deformed state) at the fracture point, obtained via actual lateral contraction, is approximately 85% of that estimated under the assumption μ = 1/2. These findings provide a critical foundation for accurately modeling the mechanical behavior of strain-crystallizing elastomers.