Molecular Orientations at Buried Conducting Polymer/Graphene Interfaces

Molecular Structures of Poly(methyl methacrylate) at Different Buried Interfaces Revealed by Sum Frequency Generation Vibrational Spectroscopy

Silica or calcium fluoride (CaF2) substrate-supported poly(methyl methacrylate) (PMMA) thin films as insulating layers are commonly used in photoelectric/photovoltaic devices to improve the efficiency or stability of these devices. However, a comparative investigation of molecular structures at buried PMMA/silica and PMMA/CaF2 interfaces under thermal stimuli remains unexplored. In this study, we qualitatively and quantitatively revealed different molecular orderings and orientations of PMMA at two interfaces before and after annealing using sum frequency generation (SFG) vibrational spectroscopy. SFG vibrations were carefully assigned by using various deuterated PMMAs. SFG results indicated that, at the buried PMMA/silica interface, the side OCH3 groups were prone to lie down before annealing and tended to stand up after annealing. In contrast, the case was the opposite at the buried PMMA/CaF2 interface. The relative hydrophobicity/hydrophilicity of the two substrates and the developed hydrogen bonds upon annealing at the buried PMMA/silica interface, which is absent at the CaF2 surface, are believed to be the driving forces for different interfacial molecular structures. This study benefits the molecular-level understanding of the interfacial local structural relaxation of polymers at buried interfaces and the rational design of photoelectric/photovoltaic devices from the molecular level.

Role of Interfacial Interactions in the Graphene-Directed Assembly of Monolayer Conjugated Polymers

Development of graphene-organic hybrid electronics is one of the most promising directions for next-generation electronic materials. However, it remains challenging to understand the graphene-organic semiconductor interactions right at the interface, which is key to designing hybrid electronics. Herein, we study the influence of graphene on the multiscale morphology of solution-processed monolayers of conjugated polymers (PII-2T, DPP-BTz, DPP2T-TT, and DPP-T-TMS). The strong interaction between graphene and PII-2T was manifested in the high fiber density and high film coverage of monolayer films deposited on graphene compared to plasma SiO2 substrates. The monolayer films on graphene also exhibited a higher relative degree of crystallinity and dichroic ratio or polymer alignment, i.e., higher degree of order. Raman spectroscopy revealed the increased backbone planarity of the conjugated polymers upon deposition on graphene as well as the existence of electronic interaction across the interface. This speculation was further substantiated by the results of photoelectron spectroscopy (XPS and UPS) of PII-2T, which showed a decrease in binding energy of several atomic energy levels, movement of the Fermi level toward HOMO, and an increase in work function, all of which indicate p-doping of the polymer. Our results provide a new level of understanding on graphene-polymer interactions at nanoscopic interfaces and the consequent impact on multiscale morphology, which will aid in the design of efficient graphene-organic hybrid electronics.

A simple predictor of interface orientation of fluids of disk-like anisotropic particles and its implications for organic semiconductors

From classical molecular dynamics simulations, we identify a simple and general predictor of molecular orientation at solid and vapour interfaces of isotropic fluids of disk-like anisotropic particles based on their shape and interaction anisotropy. For a wide variety of inter-particle interactions, temperatures, and substrate types within the range of typical organic semiconductors and their processing conditions, we find remarkable universal scaling of the orientation at the interface with the free energy calculated from pair interactions between close-packed nearest neighbours and an empirically derived universal relationship between the entropy and the shape anisotropy and bulk volume fraction of the fluid particles. The face-on orientation of fluid particles at the solid interface is generally predicted to be the equilibrium structure, although the alignment can be controlled by tuning the particle shape and substrate type, while changing the strength of fluid-fluid interactions is likely to play a less effective role. At the vapour interface, only the side-on structure is predicted, and conditions for which the face-on structure may be preferred, such as low temperature, low interaction anisotropy, or low shape anisotropy, are likely to result in little orientation preference (due to the low anisotropy) or be associated with a phase transition to an anisotropic bulk phase for systems with interactions in the range of typical organic semiconductors. Based on these results, we propose a set of guidelines for the rational design and processing of organic semiconductors to achieve a target orientation at a solid or vapour interface.

Probing Covalent Interactions at a Silicone Adhesive/Nylon Interface

Covalent bonding is one of the most robust forms of intramolecular interaction between adhesives and substrates. In contrast to most noncovalent interactions, covalent bonds can significantly enhance both the interfacial strength and durability. To utilize the advantages of covalent bonding, specific chemical reactions are designed to occur at interfaces. However, interfacial reactions are difficult to probe in situ, particularly at the buried interfaces found in well-bonded adhesive joints. In this work, sum frequency generational (SFG) vibrational spectroscopy was used to directly examine and analyze the interfacial chemical reactions and related molecular changes at buried nylon/silicone elastomer interfaces. For self-priming elastomeric silicone adhesives, silane coupling agents have been extensively used as adhesion promoters. Here with SFG, the interfacial chemical reactions between nylon and two alkoxysilane adhesion promoters with varied functionalities (maleic anhydride (MAH) and epoxy) formulated into the silicone were observed and investigated. Evidence of reactions between the organofunctional group of each silane and reactive groups on the polyamide was found at the buried interface between the cured silicone elastomer and nylon. The adhesion strength at the nylon/cured silicone interfaces was substantially enhanced with both silane additives. SFG results elucidated the mechanisms of organo-silane adhesion promotion for silicone at the molecular level. The ability to probe and analyze detailed interfacial reactions at buried nylon/silicone interfaces demonstrated that SFG is a powerful analytical technique to aid the design and optimization of materials with desired interfacial properties.