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Electroless deposition of Ag on Urea-Formaldehyde micrsopheres

 

Jeff Thompson
Former Undergraduate Student in
Material Science and Engineering

 

The autonomic or “self-healing” polymers being developed at UIUC have many possible applications in aerospace, automotive, or any area where a part will be subject to large amounts of cyclic fatigue. These applications are based upon the ability of the material to heal seal small stress cracks as they form through the release of a monomer from capsules, which is subsequently polymerized. However, in order for heal to occur in high %, there must be sufficient non-degraded monomer inside of each sphere to fill in the stress cracks. As such, the focus of my research has been to develop a method of coating the spheres in order to increase the retention of monomer and to enhance the thermal stability of the monomer by providing a layer with some insulating properties. While no one has developed a system for this specific purpose, significant study of the optical properties of metal nanoshells has been pursued by Halas et al.¹. They report the successful growth of complete silver films on latex substrates, which has been the starting point for work being carried out.
The current self-healing system being pursued uses dicyclopentadiene (DCPD) monomer contained in a urea-formaldehyde (UF) sphere, which is polymerized by a Grubbs catalyst via ROMP. For initial studies, mineral oil filled UF spheres from 3M have been used for the purpose of developing the silver coatings on the spheres. Characterization of spheres has been carried out using a Hitachi S4700 SEM running at an accelerating voltage of 5kV to minimize charging. The basic idea behind the deposition of the Ag films is that the Ag⁺ is reduced from solution onto the UF shells via a variety of reducing agents in aqueous solution. Initially, a mirror plating solution from Peacock Labs that functionalizes the surface of the UF with Sn²⁺ then reduces the Ag⁺ with formaldehyde was used to grow rough lumps of Ag. This method has limited potential though, due to its inability to form continuous coatings as seen in Figure 1 below.

Seeking to increase coverage of Ag on the spheres, alternate methods were assessed, including one reported by Mayer which describes the growth of Ag films on carboxylated PS latex dispersions.² In that procedure, a more rapid reducing agent such as H₂PO₂^- was used to produce many small nuclei initially, followed by a slower reduction of Ag⁺ with formaldehyde. This method leads to lateral growth of the smaller initial nuclei present. While this method was unable to produce the complete coatings of Ag reported, it was discovered that carboxylation of the surface is important for thegrowth of numerous Ag nucleation sites on UF. Currently, the spheres are carboxylated by immersion in a saturated solution of diethyl ether and succinic anhydride before employing the electroless deposition chemicals from Peacock labs. This method has been successful in producing nearly complete coatings as seen in Figure 2 below.

Figure 2. Carboxylated spheres plated twice with Peacock solutions.

 

In the near future, the method developed using the 3M spheres will be applied to the DCPD filled spheres, in order to assess the deposition process in the environment of the actual self-healing system.

References

(1) Halas, N.J.; Jackson, J.B. J. Phys. Chem. B, 105, 2743-2746 (2001).
(2) Mayer, A.B.R.; Grebner, W.; Wannemacher, R. J. Phys. Chem. B, 104, 7278-7285 (2000).

 

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