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.