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

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 $\lambda_{\rm craze}$ and fracture stretch $\lambda_{\rm frac}$ predict that systems with $N_e = C_\infty$ have $\lambda_{\rm craze} = \lambda_{\rm frac} = 1$ and hence cannot be deformed to large strains. Using molecular dynamics simulations, we show that in fact such glasses can form stable crazes with $\lambda_{\rm craze} \simeq N_e^{1/4} \simeq C_\infty^{1/4}$, and that they fracture at $\lambda_{\rm frac} = (3N_e^{1/2} - 2)^{1/2} \simeq (3C_\infty^{1/2} - 2)^{1/2}$. We argue that the classical formulas for $\lambda_{\rm craze}$ and $\lambda_{\rm frac}$ fail to describe SPGs' mechanical response because they do not account for Kuhn segments' ability to stretch during deformation.