## 3 Results and Discussion
### Roughness  
Figure 7 shows the roughness measurements obtained by the AFM, where the root mean square values measured after the surface treatments are plotted against the treatment time. To characterize the pristine surface, the measured Rq roughness values for all the samples were arithmetically averaged to be 4.83 nm, with a standard deviation of 0.84 nm. Figure 7 shows that roughness increased with treatment time. This increase in roughness was common among all the treatment conditions, and the trend is supported in other studies [13]. The treatment conditions did, however, influence the magnitude of change in surface roughness. For example, the sample treated at the plume's base at the lowest flow rate of 1 L/min had the lowest increase in roughness, namely, of only slightly greater than three times the pristine value. This is much smaller than the nearly seven times and over 10 times increase measured for the samples treated at the plume's base and tip, respectively, at the higher 2.5 L/min flow rate. To accompany the roughness data, AFM images have been included in Figure 8 to show the progression of the surface of the polymer with successive plasma treatments. All the images in Figure 8 are 5 m \(\times\) 5 m scans of the sample treated at the base position with a flow rate of 2.5 L/min. Figure 8a shows a uniform surface with few irregularities. The fine directional scratches on the surface are from the polishing process. Figure 8b shows a slight increase in the number of irregularities, as a few started to appear throughout the scan, though this increase was not very substantial when compared to that of further treatment times. This was significant because after the 1-min treatment, the roughness was hardly changed, while there was a substantial increase in the wettability of the surface. As the treatment progressed, a substantial visual difference began to occur after the 5-min treatment, as can be seen in Figure 8c, with small inhomogeneities appearing more often throughout the scan. These inhomogeneities further intensified after the 20-min treatment, as shown in Figure 8d. Figure 7: Rq roughness measurements measured by AFM and analyzed by NanoScope Analysis (_NanoScope Analysis (32 bit)_, version 1.50 (Build B72.115792); Bruker Corporation: 2015.). The increase in roughness with plasma treatment was caused by the bombardment of the surface of the polymer with the plasma species and the scission of the UHMWPE polymer chains [13; 24]. Additionally, the sporadic nature of the inhomogeneities found with the AFM images shows the uneven nature of the plasma treatment [13]. The circular divots on the surface of the polymer can be explained further in depth with an investigation of the sputtering effects of the plasma at the polymer surface, which was also seen in reference [13] with a plasma treatment on the polycarbonate. The sputtering of a polymer surface happens when particles with sufficient energy collide with the surface and detach small pieces or atoms [25; 26]. The reason that material is removed is that, after polymer chain scission, the free radicals formed recombine into new lower molecular weight species and because the oxidation that takes place on the surface, the particles become more fragile and it is easier for pieces to be removed [4; 13]. Another surface effect that plasma treatment could cause, besides the creation and removal of lower molecular weighted species, is the forming of higher molecular weight species by crosslinking [13]. This second process, however, was probably not observed much in this study because of the negligible change in hardness, as discussed later. Because the sputtering process requires a certain amount of energy to remove pieces of the surface, it makes sense that with the lower flow rate, and therefore, a smaller amount of kinetic energy, that not as much of this sputter effect would have occurred. This explains the lower magnitude of increase in the surface roughness of the sample treated with the lower 1 L/min flow rate.