Enhancement of Porosity and Surface Roughness of Cured Phenolic Resin by Ion Implantation
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ENHANCEMENT OF POROSITY AND SURFACE ROUGHNESS OF CURED PHENOLIC RESIN BY ION IMPLANTATION R.L. Zimmerman*, D. Ila*, C.C. Smith*, A.L. Evelyn*, D.B. Poker and D.K. Hensley *Center For Irradiation of Materials, Alabama A&M University, Normal, AL 35762 Oak Ridge National Laboratory, SMAC, Oak Ridge, TN, 37831 ABSTRACT We present recent results using ions such as C, O, Si, Fe, Zn, and Au at energies between 100 keV to 10 MeV to increase the roughness and porosity of the partially and fully cured precursor phenolic resins. The fully cured phenolic resin is called Glassy Polymeric Carbon (GPC). GPC is chemically inert, biocompatible and useful for medical applications, such as heart valves and other prosthetic devices. Ion implantation enhances biological cell/tissue growth on, and tissue adhesion to, prosthetic devices made from GPC. We have previously shown that increased porosity of GPC is also useful for drug delivery devices. The porosity of the ion implanted partially and fully cured precursor phenolic resins was measured by introducing lithium from a molten LiCl salt into each sample. By using Li(p,2α) nuclear reaction analysis (NRA) we measured the concentration of Li retention in the pre- and post-implanted samples. The surface roughness was measured using optical microscopy. The curing process was monitored using micro-Raman microscopy. We have correlated the NRA measurements of increased pore availability with the observations of increased surface roughness. INTRODUCTION Glassy Polymeric Carbon (GPC) shows increasing promise as a biomaterial [1-5] owing to its electrical, mechanical and chemical properties, which include exceptional biocompatibility. GPC is made from cured phenolic resins (resol), figure 1a, in an inert environment. After curing at 60 °C, figure 1b, the resin is pyrolyzed at low temperature rates to avoid changing shape or disruption due to volatile decomposition products [1,2]. Heat treatment to 550 °C produces a conducting material due to hydrogen release and conjugation of the aromatic rings forming graphene planes in random arrangement.
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Figure 1. Structures of a) resol (C7H8O2), b) the polymer which forms at 130°C by losing water and c) a graphene plane and loss of hydrogen (d) as heat treatment is increased beyond 700°C. O4.8.1
Figure 2. Schematic of the structure of Glassy Polymeric Carbon heat treated to 2500°C (right) according to Ref. [1]. The cross sectional dimensions of the ribbon like structures of graphene layers are of the order of 10 nm and the pores, which represent about 30% of the volume, are closed such that the material is completely impermeable, unlike the schematic shown here. The transmission electron micrograph at left and Raman spectroscopy (not shown), suggest that amorphous carbon is present and is responsible for eliminating pore connectivity. Although the material is almost as hard as diamond, no diamond like structure has been detected. For heat treatment at 650 °C, the material presents the highest available porosity due to an open connectivity
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