Interaction energy and chain conformation tuned by London dispersion and steric effects within hetero-molecular π electron complex
Authors: Zhang Baode , Zaric Snezana , Gofman Iosif , Heck Barbara , REITER Günter 
Published: February 2024

The interaction energy, stability and conformation, nanostructure of atom and molecule complexes with van der Waals bonding are often determined by the interplay between attractive London dispersion forces and repulsive forces due to the Pauli principle. London dispersion dominates the binding energy between two atoms and molecule complexes, chemical reactivity, molecular recognition, self-assembly, nanostructure, heterostructure with delocalized π electrons within nanocomposites as an attractive forces, while steric effects, usually, as a repulsive forces due to bulky groups in the organic chemical structure, or side chain of a polymer main chain, weaken the binding energy depending on the electronic resonance, size and position of the substitute groups at the molecules, or polymer backbone. However, due to the complicate interplay between the London dispersion and steric effects within molecules nanostructures, and heteromolecule complexes with van der Waals bonding, the complete understanding of the nature of the interplay on mechanics of remains a challenge within nanocomposites, such as a hetero molecules complexes, in particular, heterostructure with delocalized electrons. In this research, aromatic polyimides (PI) and carbon nanotubes (CNT), with delocalized electrons, were chosen as building blocks as two components in the hetero delocalized electron nanostructures. In order to compare the substituent groups on the interplay of London dispersion and steric effects, two polyimides have the same diamine part, only different in the linkage substituents between two phenyl rings of dianhydride part. The linkages are ether bond (C-O-C) and hexafluoroisopropylidene (-C(CF3)2), respectively. The one linked with atom O is named OPI, another one linked with (CF3)2 is named FPI. By changing the substitute groups from ether group to hexafluoroisopropylidene C-(CF3)2 groups on the PI monomer backbone to tailor the steric effects, the interaction energy and chain conformation between PI and CNT were studied experimentally and theoretically. Surprisingly, the two polyimide/CNT nanocomposites show distinct failure mode from CNT pull-out failure to CNT yielding, which was judged from local fracture surface morphology and stress-strain curves. The two kind of morphology indicates obvious different interfacial interaction energy and chain conformation between each PI and CNT within two nanocomposites. In order to explain the experimental results, accurate calculation of the interaction energy and chain conformations of each PI upon CNT were performed by symmetry adapted perturbation theory (SAPT) and molecular dynamic simulation (MDS). Each PI monomer was divided into four parts along the backone, respectively. The interaction energy was calculated at B3LYP-D3/6-31G* level with SAPT. In the case of OPI, carbon nanotube and the polyimide monomer encounter less steric interaction with CNT as the flexible ether linkage group on the backbone with rotational freedom are placed at the PI backbone, the monomer adopt a parallel conformation with carbon nanotube to obtain the maximum binding energy which driven by London dispersion; while in the case of FPI and CNT system, the two components encounter more steric interaction as the C(CF3)2 groups are placed at the in the dianhydride part of FPI backbone. The competition between steric effects and London dispersion leads to substantial steric strain in the dianhydride part of FPI backbone, which was accompanied by a considerable departure of the polymer conformation from the strain-free molecule with the same number of atoms by rotating and bending of bonds with a related increase in energy. The FPI monomer adopts a tilting conformation on the carbon nanotubes, which weaken the interaction energy between polyimide molecule and CNT. Further MDS of the interaction of polyimide chains with carbon nanotubes reveal that OPI chain helically wraps the nanotube surface, while FPI chains fail to wrap around the CNTs. The different preferred conformation of two PI chains around CNT agree well with the morphology of the quite different failure surface of two nanocomposites. Our analysis suggests that the interplay between London dispersion and steric effects in hetero π electron complexes contributes to the interaction energy and polymer chain conformation around CNT, which dictates the fracture morphology at interfaces between polyimde molecules and carbon nanotube at nanoscale, consequently governs the mechanical behavior of nanocomposites at macroscale when load is applied. This research is helpful to design nanocomposites by tailoring the interplay of London dispersion and steric effect at nanoscale to control the mechanics at macroscale. The work is of significance to reach the level of hierarchical complexity found in biological organism and developing strategies mimicking Nature to synthesize human designed bio-inorganic composite material. Even though the complexity of biological organism is difficult to achieved, the research provides a further insight into fundamental mechanism possibly governing in biological architecture.