Development of MechanicaUy 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 deposit ion 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-43S® (PNER) and Araldite MY-nO with poly(ethylenim ine) (PEl) and tetraethylenepcntamine (TEPA) on silicon and quartz surfaces. Thus the LbL film architectures obtained for variolls adsorption times and polymer concentrations were (PEI/CNER)n. (PEIIPNER)n. (PEl/MY -720)", PEI(CNERffEPA)"ICNER, PEI(PNERffEPA)"IPNER and PEl (MY - 720ffEPA)rlMY-nO (where n = number of layer pairs deposited). The classical condit ions of polyelectrolyte multilayer build-up for covalent LbL assem bly were optim ized for lhe construction of multi layers havi ng linear growth increment with respect to the number of laye rs chem isorbed. The thickness of each layer pair was measured using an ellipsometer and found in the range of 10 to 40 nm depending on the epoxy compound used. The multilayer films so prepared were quite homogeneous and highly rcproducible. UV-Visible spectroscopy was also employed to monitor the chemisorption of UV active chromophores. The optimised epoxy-amine network layers thus fonn ed by covalent LbL assembl y of epoxy res ins were then appl ied onto Au-NPs films of the architecture (pA I-IIAu-NPs)s. These epoxy protected All-NPs films hav ing archi tecture (PAI-i/All-NPs)5/(PEI/CNER)1O and (PAJ-I/Au-NPs)s!(PEUPNER)1O 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 di ffe rence in overall fi lm th ickness before and after ru bbing test. So, epoxy protected Au-NPs film proved to be quite strong to endure 60 rubbing cycles as compared to virgin Au-N Ps film which were mechan ically much weak. The adsorpt ion process was further optimised to get fast curing process by employing variolls acce lerators, increasing the polymer concentrat ion, 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 PEldl.). for the curing of various epoxy resins at room temperature. The spraying of PEldi3 (40 mg mL-I ) for 10 s followed by dipping for 10 min in epoxy solution (100 Illg mL·I ) great ly enhanced the speed of cova lent LbL adsorpt ion process. Although curing of these films at elevated temperature resu lted 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~allline 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 rubbi ng cycles, the epoxy protected Au~NPs film lost ca. 6% of ini tial film thickness, Moreover, the study has proved to be an econom ical preparation of more effect ive covalent LbL assemblies, both in tenns of cost and time. Therefore, the epoxy~a min e network has great potential to protect the underlying weak Au-NPs fi lms and many sLich future appl icati ons.