A. Calderone

Supramolecular organization in block copolymers containing a conjugated segment: a joint AFM/molecular modeling study

Abstract The solid-state supramolecular organization of block copolymers containing one π-conjugated block and one non-conjugated block is elucidated with a joint experimental and theoretical approach. This approach combines atomic force microscopy (AFM) measurements on thin polymer deposits, which reveal the typical microscopic morphologies, and molecular modeling, which allows one to derive the models for chain packing that are most likely to explain the AFM observations. The conjugated systems considered in this study are based on aromatic building blocks (i.e. phenylene, phenylene ethylene, fluorene, or indenofluorene), substituted with alkyl groups to provide solubility; they are attached to non-conjugated blocks such as polydimethylsiloxane, polyethylene oxide, or polystyrene. Films are prepared from solutions in solvents which are good for both blocks, in order to prevent aggregation processes in solution. Therefore, the morphology observed in the solid state is expected to result mostly from the intrinsic self-assembly of the chains, with little specific influence of the solvent. In such conditions, the vast majority of compounds show deposits made of fibrilar objects. Closer examination of single fibrils on the substrate surface indicates that the objects are ribbon-like, i.e. their width is significantly larger than their height, with typical dimensions of a few tens of nanometers and a few nanometers, respectively. These results suggest that a single type of packing process, governed by the π-stacking of the conjugated chains, is at work in those block copolymers. This prevalence of such a type of packing is supported by the theoretical simulations. Molecular mechanics/dynamics calculations show that the conjugated segments tend to form stable π-stacks. In these assemblies, the block copolymer molecules can organize in either a head-to-tail or head-to-head configuration. The former case appears to be most likely because it allows for significant coiling of the non-conjugated blocks while maintaining the conjugated blocks in a compact, regular assembly. Such supramolecular organization is likely responsible for the formation of the thin, ‘elementary’ ribbons, which can further assemble into larger bundles. The issue of chain packing in fluorene-based systems has been modeled separately, since in these compounds, the alkyl groups attached to sp 3 -hybridized sites inherently accommodate out of the plane of the conjugated backbone, which can disturb the chain packing. Various possibilities of chain packing have been explored, starting from short alkyl substituents and extending the size of the side groups to n -octyl. The calculations indicate that, when in zig-zag planar conformation, linear alkyl side groups can orient in such a way that close π-stacking of the conjugated chains is preserved. In contrast, branched alkyl groups are too bulky to allow close packing of the conjugated backbones to take place. This difference is consistent with the presence or absence of fibrilar structures observed in thin deposits of the corresponding polymers; it can also account for the differences observed in the optical properties.

Highly Regular Organization of Conjugated Polymer Chains via Block Copolymer Self-Assembly

The microscopic organization in thin films of block copolymers containing either polyparaphenylene, or polyphenyleneethynylene rigid segments covalently bound to a flexible polydimethylsiloxane or polystyrene sequence was investigated. Atomic force microscopy and theoretical modeling based on molecular mechanics/molecular dynamics calculations were combined. The typical morphology obtained revealed the presence of bright elongated structures.

Electronic structure of molecular van der Waals complexes with benzene: Implications for the contrast in scanning tunneling microscopy of molecular adsorbates on graphite

We investigate the electronic structure of molecular model systems in order to improve our understanding of the nature of the contrast, which is observed in the scanning tunneling microscopy (STM) imaging of organic adsorbates on graphite. The model systems consist of a benzene molecule, representing the substrate surface, interacting with various molecules representing alkyl chains, oxygen- and sulfur-containing groups, fluorinated species, and aromatic rings. We perform quantum-chemical calculations to determine the geometric structure, stability, and electronic structure of these molecular complexes and analyze the theoretical results in relation with experimental STM data obtained on monolayers physisorbed on graphite. It appears that the STM contrast can be correlated to the energy difference between the electronic levels of the substrate and those of the adsorbate. Finally, we observe that the introduction of a uniform electric field in the quantum-chemical modeling can enhance the electronic intera...

Dynamics in Physisorbed Monolayers of 5-Alkoxy-isophthalic Acid Derivatives at the Liquid/Solid Interface Investigated by Scanning Tunneling Microscopy

Monolayers of isophthalic acid derivatives at the liquid/solid interface have been studied with scanning tunneling microscopy (STM). We have investigated the dynamics related to the phenomenon of solvent co-deposition, which was previously observed by our research group when using octan-1-ol or undecan-1-ol as solvents for 5-alkoxy-isophthalic acid derivatives. This solvent co-deposition has now been visualized in real-time (two frames per second) for the first time. Dynamics of individual molecules were investigated in mixtures of semi-fluorinated molecules with video-STM. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to analyze the data obtained for the mixtures under investigation. Upon imaging the same region of a monolayer for a period of time we observed that non-fluorinated molecules progressively substitute the fluorinated molecules. These findings illustrate the metastable equilibrium that exists at the liquid/solid interface, between the physisorbed molecules and the supernatant solution.

Chemical and electronic aspects of metal/conjugated polymer interfaces. implications for electronic devices

The chemical nature and the electronic structure of metal/conjugated polymer interfaces are investigated in the context of polymer-based light-emitting diodes. We consider the interaction of low-workfunction metals (Al, Ca) with the surface of conjugated polymers or model oligomer molecules with a combined experimental and theoretical approach. The early stages of the interface formation are followed with X-ray and ultraviolet photoelectron spectroscopies and the experimental data are compared to the results of quantum chemical calculations. The reactions of Al and Ca with the organic surface are found to be fundamentally different: while the former forms new covalent bonds onto the polymer backbone, the latter tends to dope the conjugated system. Both types of reaction are expected to modify drastically the electronic properties of the polymer semiconductor.

The aluminum/polyethylene terephthalate interface: A joint theoretical and experimental study

The aluminum/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum‐chemical calculations on short molecular model systems interacting with a few Al atoms. The geometric structure of the organometallic complex is optimized and its stability as well as the changes in charge density due to Al bonding are evaluated. The theoretical results are compared to experimental x‐ray photoelectron spectroscopy data collected during the early stages of interface formation. In particular, the evolution of the polymer core level peaks is paralleled to the Al‐induced modification of the charge density on the model molecules. Al is found to react preferentially with the ester group by forming covalent bonds with the oxygen and/or the carbon atom of that group.

A joint theoretical and experimental study of the aluminium/polyethylene terephthalate interface

Abstract The aluminium/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum-chemical calculations on short molecular model systems interacting with a few aluminium atoms. These results are compared to experimental X-ray photoelectron spectroscopy data collected during the early stages of interface formation.

Conjugated polymer chains self-assembly: a new method to generate (semi)-conducting nanowires?

Abstract The morphology of thin films of conjugated polymers and block copolymers (containing one conjugated segment and one (or two) non-conjugated segments) is analysed with a combination of atomic force microscopy data and molecular mechanics/dynamics calculations. The rigid character of the conjugated segment induces specific morphological characteristics, with respect to the situation usually found in coil-coil copolymers. The copolymers organise as very long, thin objects with constant width and height in the nanometre range (nanoribbons). Decreasing the amount of polymer on the substrate allows the observation of isolated nanoribbons. The comparison of the experimental data with the theoretical modelling indicates that the ribbons are made of highly regular stacks of conjugated chains surrounded by coiled non-conjugated segments.

Theoretical modeling of the geometric and electronic structure of polymer/graphite interfaces

Abstract This work deals with the microscopic description of the electronic and geometric structures at the interface between hydrocarbon polymers, such as polyethylene or polystyrene, and a carbon surface, i.e., graphite or carbon black. In order to understand the nature of the local interactions at the interfaces, we performed theoretical calculations, based on both quantum-mechanical ab initio techniques and empirical molecular mechanics techniques. The interfaces between polyethylene and graphite and between polystyrene and graphite are modeled by considering the alkane/benzene and ethylbenzene/benzene complexes, respectively.

Geometric and electronic structure of alkane/benzene, ethylbenzene/benzene, and alkane/ethylbenzene complexes : Towards the characterization of polymer alloy composites

Abstract As a first step towards the theoretical investigation of polymer alloy composites formed by the dispersion of carbon black particles throughout a polyethylene/polystyrene blend, we discuss here the results of ab initio Hartree—Fock quantum-chemical calculations, including correlation effects via second-order Moller—Plesset perturbation theory, on small model systems. These are chosen to simulate the various interfaces that appear in the composites: the interfaces between the polymers and the graphitic surface of carbon black are modeled by the complexes formed by benzene (taken as the substrate) with methane, ethane, propane, and ethylbenzene while the polymer/polymer interface is modeled by the propane/ethylbenzene system. The functionalization of the benzene substrate with hydroxyl, amine, carboxylic, or quinoid groups is also investigated in order to determine the influence of such moieties that can be present on the carbon surface. Our main goal by studying these model systems is to characterize the primary binding interaction sites and to evaluate the energies involved in the bonding; our results can also serve as a reference for molecular mechanics calculations on more extended systems.