United-Atom Molecular Dynamics Simulations of Strain-Induced Crystallization in Polyisoprene Melts.

Stretching polyisoprene (PI) beyond a certain strain level causes strain-induced crystallization, resulting in nanocrystallite formation and toughening of the material. Although scattering experiments have confirmed the existence of nanocrystallites, their distribution and spontaneous formation remain unclear. Here, we present united-atom molecular dynamics (UA-MD) simulations that capture the spontaneous formation of nanocrystallites in stretched PI melts from an oriented state. Using molecular-level snapshots based on our PI-crystal local order parameter and two-dimensional scattering patterns, we detect the grown nanocrystallites. We further show that tiny nanocrystallites formed during rapid stretching can either grow or disappear during the accessible time scale of the subsequent isothermal crystallization stage. Importantly, our simulations demonstrate that the threshold for nanocrystallite growth correlates better with the initial segmental orientation than with the strain level alone. Our UA-MD approach provides molecular-level insights relevant to nanocrystallite formation and self-reinforcement in cross-linked PI networks, offering guidance for the design of tough soft materials.