Breaking Strong Alkynyl-Phenyl Bonds: Poly(para-phenylene ethynylene)s under Mechanical Stress

Abstract

Stronger chemical bonds withstand higher mechanical forces; thus, the rupture of single bonds is preferred over the rupture of double or triple bonds or aromatic rings. We investigated bond scission in poly(dialkylp-phenylene ethynylene)s (PPEs), a fully conjugated polymer. In a scalebridging approach using electron-paramagnetic resonance spectroscopy and gel permeation chromatography of cryomilled samples, in combination with density functional theory calculations and coarse-grained simulations, we conclude that mechanical force cleaves the sp−sp 2 bond of PPEs (bond dissociation energy as high as 600 kJ mol −1 ). Bond scission primarily occurs in shear bands with locally increased shear stresses. The scission occurs in the middle of the PPE chains. Breaking sp−sp 2 bonds into free radicals thus is feasible but requires significant mechanical force and an efficient stress concentration.