Unexpected ductility in semiflexible polymer glasses with $N_e=C_{\infty }$

Abstract

Semiflexible polymer glasses (SPGs), including those formed by the recently synthesized semiflexible conjugated polymers (SCPs), are expected to be brittle because classical formulas for their craze extension ratio ${\lambda }_{\mathrm{craze}}$ and fracture stretch ${\lambda }_{\mathrm{frac}}$ predict that systems with $N_e=C_{\infty }$ have ${\lambda }_{\mathrm{craze}}={\lambda }_{\mathrm{frac}}=1$ and hence cannot be deformed to large strains. Using molecular dynamics simulations, we show that in fact such glasses can form stable crazes with ${\lambda }_{\mathrm{craze}}\simeq N_e^{1/4}\simeq C_{\infty }^{1/4}$, and that they fracture at ${\lambda }_{\mathrm{frac}}=(3N_e^{1/2}-2{)}^{1/2}\simeq (3C_{\infty }^{1/2}-2{)}^{1/2}$. We argue that the classical formulas for ${\lambda }_{\mathrm{craze}}$ and ${\lambda }_{\mathrm{frac}}$ fail to describe SPGs' mechanical response because they do not account for Kuhn segments' ability to stretch during deformation.

Accepted for publication in PRL