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

TL;DR

This study reveals that semiflexible polymer glasses with $N_e = C_ ext{infty}$ can exhibit unexpected ductility, forming stable crazes and fracturing at larger strains than classical models predict, due to Kuhn segment stretching.

Contribution

The paper demonstrates through molecular dynamics simulations that classical formulas underestimate the ductility of semiflexible polymer glasses by ignoring Kuhn segment stretching effects.

Findings

Semiflexible polymer glasses can form stable crazes with larger extension ratios.

Fracture strains are higher than classical predictions due to Kuhn segment stretchability.

Classical models fail to account for the mechanical response of these glasses.

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 $\lambda_{\rm craze}$ 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.