Force-Induced Transition of π-π Stacking in a Single Polystyrene Chain.

Although π-π interactions have been studied for several decades, the quantification of the strength of π-π interactions in a macromolecule remains a big challenge. Herein, we utilize single-molecule atomic force microscopy and steered molecular dynamics simulations to study the π-π interactions in polystyrene (PS). It is found that in high vacuum, the single-chain mechanics of PS differs largely from that of polyethylene (PE). Accordingly, the strength of intrachain π-π interactions in PS is estimated to be 0.7 kcal/(mol stack), which is much lower than that in a small-molecule system (benzene dimer, 2-3 kcal/(mol stack)). Further study shows that in high vacuum, there are two types of π-π stacking in the single PS chain, i.e., the every-other-moiety (E) type and the adjacent-moiety (A) type. Upon force stretching, a transition from E-type to A-type π-π stacking can be observed.