## 4 Discussion
The effects seen through surface energy was previously explained through change in surface chemistry of the NTAPP-treated sample due to the reactive chemical species produced from it [12; 22]. In this study, XPS was used to consider changes in surface chemistry which has been previously known to be caused by NTAPP treatment [12; 20; 22]. Indeed, XPS results showed that there was a change in surface chemistry of epoxy resin following NTAPP treatment such as formation of C=O (Figure 6a) or C-OH (Figure 6c) chemical functional group and a decrease in intensity that corresponded to C-C chemical functional group (Figure 6a). Such formation of oxidative layer with higher binding energy along with reduction of carbon composition has been known to result in an increase in surface energy [12; 34; 35], which explains the results seen here. Hence, the XPS results explain the results obtained in this study as NTAPP treatment caused formation of specific chemical structure on the surface allowing improved surface energy and consequently improved adhesion between epoxy resin-based fiber post and core resin. The change in surface chemistry and consequent change in surface energy was then correlated with bonding strength between epoxy resin and core resin. The results showed a significant increase in shear bond strength following NTAPP treatment on epoxy resin, and this was in agreement with previous studies that evaluated the bond strength of polymer and composite materials after atmospheric pressure plasma treatment [11; 36; 37]. In a similar study by [PERSON] et al. who used ethylenediamine (EDA) plasma treatment to improve the bond strength between resin cement and epoxy resin-based fiber post [11], the bonding strength between two materials were compared following EDA plasma treatment, silane treatment, hydrofluoric acid treatment or silane, and hydrofluoric acid combined treatment. Although the results were different to our study as the bonded materials were different to our study (core resin compare to resin-based cement) as well as the experimental set-up, it was evident that none of silane treatment, hydrofluoric acid treatment or silane, and hydrofluoric acid combined treatment resulted in more improved bond strength than the untreated control, while only EDA plasma treatment resulted in significant improvement. The study concluded that the contact angle may not be the sole factor for improving bonding strength, while chemical functionalities may play an important role. Such findings were in agreement with other studies. [PERSON] et al. reported that vacuum-based plasma treatment using nitrogen resulted in increased tensile-shear bond strength between the fiber post and core resin [10] and the possible reason for such improved adhesion was due to the result of functional groups induced on the surfaces of the fiber post. Also, [PERSON] and [PERSON] demonstrated that the amine and imine species were induced as functional groups on the surface by nitrogen plasma treatment polystyrene [38]. The majority of these functional groups usually contained terminal nitrogen (primary amine or imine), which [PERSON] et al. hypothesized induced terminal nitrogen functional groups on the epoxy resin-based fiber posts surfaces treated with nitrogen supplied plasmas would react with functional groups in the composite core build-up material and therefore result in improvement in shear bond strength [10]. Indeed, our results have also demonstrated that the peak corresponding to the amine groups (402.0 eV) increased following NTAPP treatment for 1 min (Figure 6b). Additionally, this study compared the shear bond strength of NTAPP-treated commercially available post to core resin with that of conventional hydrofluoric acid or silane-treated post. The hydrofluoric acid treatment is often used as the surface treatment of fiber post as it increased the number of exposed glass fibers and it is often used in conjunction with silane to increase the surface area available for reaction and forms the stronger bond between fiber post and core resin [7].