Development of Mechanically Robust Ultra-thin Polymer Films Using Covalent Layer by Layer Assembly of Epoxy Compounds

Abstract

Nanofabrication of two component epoxy adhesives via covalent linkage was carried out using Layer by Layer (LbL) multilayer assemblies, adopting a dipping as well as alternate spraying-dipping technique for the deposition onto pre-activated silicon or quartz substrates and gold nanoparticles (Au-NPs). Dipping technique was employed for the curing of cresol novolac epoxy resin (CNER), phenol epoxy novolac resin DEN-438® (PNER) and Araldite MY-720 with poly(ethylenimine) (PEI) and tetraethylenepentamine (TEPA) on silicon and quartz surfaces. Thus the LbL film architectures obtained for various adsorption times and polymer concentrations were (PEI/CNER)n, (PEI/PNER)n, (PEI/MY-720)n, PEI(CNER/TEPA)n/CNER, PEI(PNER/TEPA)n/PNER and PEI(MY- 720/TEPA)n/MY-720 (where n = number of layer pairs deposited). The classical conditions of polyelectrolyte multilayer build-up for covalent LbL assembly were optimized for the construction of multilayers having linear growth increment with respect to the number of layers chemisorbed. The thickness of each layer pair was measured using an ellipsometer and found in the range of 1 to 4 nm depending on the epoxy compound used. The multilayer films so prepared were quite homogeneous and highly reproducible. UV-Visible spectroscopy was also employed to monitor the chemisorption of UV active chromophores. The optimised epoxy-amine network layers thus formed by covalent LbL assembly of epoxy resins were then applied onto Au-NPs films of the architecture (PAH/Au-NPs)5. These epoxy protected Au-NPs films having architecture (PAH/Au-NPs)5/(PEI/CNER)10 and (PAH/Au-NPs)5/(PEI/PNER)10 were tested for their mechanical robustness with the help of a rubbing machine. The surface morphology of the rubbed samples was studied by AFM, although certain grooves appeared, but there is no significant difference in overall film thickness before and after rubbing test. So, epoxy protected Au-NPs film proved to be quite strong to endure 60 rubbing cycles as compared to virgin Au-NPs film which were mechanically much weak. The adsorption process was further optimised to get fast curing process by employing various accelerators, increasing the polymer concentration, decreasing the adsorption time and also by reducing the number of layer pairs. Lupasol-HF, proved to be an exceptional curing agent after dialysis (to get narrow but high molar mass PEIdia), for the curing of various epoxy resins at room temperature. The spraying of PEIdia (40 mg mL-1) for 10 s followed by dipping for 10 min in epoxy solution (100 mg mL-1) greatly enhanced the speed of covalent LbL adsorption process. Although curing of these films at elevated temperature resulted in ultimate robustness with no loss in thickness after 20 rubbing cycles, yet room temperature curing was also employed for a specified time period by storing the films in air tight containers. The epoxy-amine film thickness for the protection of Au-NPs was found to be 10 nm for CNER and 6 nm for PNER. The ellipsometer data revealed that after more than 60 rubbing cycles, the epoxy protected Au-NPs film lost ca. 6% of initial film thickness. Moreover, the study has proved to be an economical preparation of more effective covalent LbL assemblies, both in terms of cost and time. Therefore, the epoxy-amine network has great potential to protect the underlying weak Au-NPs films and many such future applications.