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

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 ( $${\lambda }_{{||}}$$ ) over a broad $${\lambda }_{{||}}$$ range, encompassing the SIC onset stretch ( $${{\lambda }_{{||}}}^{* }$$ ≈ 4.1). Below $${{\lambda }_{{||}}}^{* }$$ , μ remains near 1/2, indicating the incompressible behavior of NR in its fully rubbery amorphous state. However, once $${\lambda }_{{||}}$$ exceeds $${{\lambda }_{{||}}}^{* }$$ , μ decreases as the degree of crystallinity (χc) increases. As $${\lambda }_{{||}}$$ increases from $${{\lambda }_{{||}}}^{* }$$ to fracture stretch ( $${\lambda }_{{||}}$$ ≈ 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. The Poisson''s ratio (μ) of natural rubber undergoing strain-induced crystallization was studied across a broad stretch range. Below the crystallization onset stretch ( $${\lambda }_{{||}}$$ * ≈ 4.1), μ remains near 0.5, indicating incompressibility. Beyond $${\lambda }_{{||}}$$ *, μ decreases with increasing crystallinity, reaching 0.33 at fracture ( $${\lambda }_{{||}}$$ ≈ 7.1), as the matrix transforms from amorphous to semicrystalline. The true stress at fracture, accounting for lateral contraction, is approximately 85% of the estimated value assuming μ = 0.5. These insights are vital for modeling the mechanical behavior of strain-crystallizing elastomers.