Chemical Adhesion of Organic Monomers at Reactive Sites on Oxidised Metal Surfaces
Final Report Abstract
Adhesion mechanisms play a crucial role for the function and properties at any polymer-metal contact. Major adhesives and organic coatings are based on reactive monomer mixtures which form the polymer structure right in contact with the metal substrate. The initial step of adhesive chemical bonding of organic monomer molecules on solids and the subsequent reaction step of a free monomer to an adhesively bonded one are to be understood in detail. This joint experimental and theoretical project discovers whether there are chemical adhesion mechanisms at work and how the polymerisation starts at the native metal contact under the influence of such mechanisms. Therefore, a dedicated Molecular Layer Deposition (MLD) device was designed and assembled and the protocols for the controlled deposition of monomer molecules from gas phase on metal surfaces were developed. Dipropylene glycol (DPG) and 4,4’-methylene diphenyl diisocyanate (4,4’-MDI) are selected as common monomers for polyurethane networks on natively oxidised surfaces of aluminium, copper and gold. These metals as well as polyurethanes are significant in many fields of technical application. A large part of the theoretical contribution to this project was to develop the theoretical machinery for realistic simulations of experimental infrared spectra. The ab initio quantum mechanical calculations of the conformational states and their eigenvibration spectra for 4,4’-MDI in gas phase demonstrated that the approach is accurate. Finite-cluster models of surface layers were produced whereby both crystalline and disordered, oxygen-containing structures could be created. The latter ones provide models for the real oxidic surfaces of the metals used in the experiments. The clusters are large enough to allow for the adsorption of the complete molecule and, still, allow accurate ab initio calculations even for the cluster with the adsorbed monomer. We devised a theoretical approach with which the cluster model for the oxidized metal surface plus the adsorbed monomer can be treated with accurate ab initio methods. An approach has been identified for the calculation of spectra that specifically takes the orientation of the monomers on the surfaces into account. Thereby, calculated spectra match closely the conditions of the experimental IR spectroscopy with polarised IR light. By comparing experiment and theory, it should be possible to identify the geometry of the adsorbed monomers on the oxidized metal surfaces. The monomer adsorption experiments with the monomers in the MLD device and subsequent IR spectroscopic characterisation of the monomer adsorption state proved that DPG interacts only weakly physically with the three metal surfaces. However, the MDI molecules form a tiny stable layer with adhesion via one of the isocyanate group with all three metals which we anticipate as a donor-acceptor interaction. Strength of adhesion is quite high on Al and Cu but much weaker on Au. In addition, some part of the isocyanate entities form urea species that exhibit different orientations on the metal surfaces and there is a residual part of free isocyanate groups left for polymerisation. Therefore, the MDI adsorption layer provided the right starting point for subsequent deposition of a DPG layer as the partner for polyurethane formation. Surprisingly this reaction faced some unexpected obstacles: Some DPG molecules reacted quite fast with isocyanate groups to PU fragments but the greater part of the alcohol desorbs during the 1st hour after deposition. Hence, a delicate balance of chemisorption and reactivity has to be established specifically for the given metal surface in order to produce a polymer layer. Future correlation with sophisticated quantum mechanical simulations for the adhering molecules can provide insight in the very nature of the proposed chemical adhesion mechanisms and on the partner site on the metal surfaces as well. In conclusion, the results of the project contribute to the fundamental questions of how chemical adhesion works and how it modifies the beginning of the step-growth polymerisation at a metal surface. That is an indispensable part of understanding the properties of the forming interface. The results pave an innovative way for studying chemical adhesion in organic-metal systems and they are most relevant for application issues in the technologies of adhesive bonding and ultra-thin polymer formation.
Publications
- Adhesion and stability of PU monomers on native metals: Adsorption from the gas phase, Proc. 5th World Conf. on Adhesion and Related Phenomena, Nara, Japan, (2014)
F. Fug, A. Petry, H. Jost, W. Possart
- Adhesion of polyurethane monomers on native metals: Adsorption from the gas phase, Proc. 10th Europ. Adhesion Conf., Alicante, Spain, (2014)
F. Fug, A. Petry, H. Jost, W. Possart
- Adsorption of polyurethane monomers from the gas phase on native metals: Adhesion and stability of the obtained layers, Proc. 5th World Conf. on Adhesion and Related Phenomena, Nara, Japan, (2014)
F. Fug, A. Petry, H. Jost, W. Possart
- Adsorption of polyurethane monomers from the gas phase on native metals: Infrared study of the obtained layers, Proc. 10th Europ. Adhesion Conf., Alicante, Spain, (2014)
F. Fug, A. Petry, H. Jost, W. Possart
- 4,4′-methylene diphenyl diisocyanate – Conformational space, normal vibrations and infrared spectra. Polymer
Volume 99, 2 September 2016, Pages 671-683
F. Fug, J. Vargas, K. Rohe, M. Springborg, C. Nies, W. Possart
(See online at https://doi.org/10.1016/j.polymer.2016.07.075)
