Substituent effects on two-dimensional assembling and chain folding of rigid-rod polymer poly(p-phenyleneethynylene) derivatives on the solid/liquid interface

The two-dimensional assembling behavior of a series of poly(p- phenyleneethynylene) (PPE) derivatives having identical conjugated backbone but different substituents and degrees of polymerization has been studied using scanning tunneling microscopy (STM). Submolecularly resolved STM images reveal diverse forms of chain folding in the assembled monolayer, allowing the analysis of the effects of polarity and concentration of substituents on the assembling behavior and chain folding of PPE. Theoretical simulations have been performed to understand the structural effects on the chain folding characteristics on the surface. The submolecularly resolved STM images also enable the quasi-quantitative determination of molecular weight and number-averaged degree of polymerization. The histograms of the contour length measured from the STM images agree well with the theoretical Schulz-Zimm distribution, indicating the potential capacity of the STM method for the estimation of molecular weight distribution of rigid-rod polymers.