## 3 Results and Discussion
### Finite Element Analysis for \(h\) = 1 \(\upmu\)m with Three Different Orientations  
To investigate the influence of plastic zone, FEA analysis was performed based on experimental data. Only two indents with different orientation are considered in ABAQUS due to its high computational resources. The von Mises stress distributions with different orientations and selected separation for depth, \(h\) = 1 \(\upmu\)m, are presented in Figure 10. The figure shows the top view and cross-section, cut through the center of the max depth as indicated with a dotted line in the images in the first row. In the last row, a von Mises stress vs. depth taken from FEA cross-section at the center between two indents is provided. The depth in this case means the distance taken from the very top surface going towards the sample bulk. Similar analysis was done for \(h\) = 5 \(\upmu\)m (Figure S4.1) and 10 \(\upmu\)m (Figure S4.2), given in Supporting Information S4. As shown in Figure 10, for a nominal separation of 5, the two indentations and their plastic zones overlap significantly and von Mises stress is affected by both indents. The difference of hardness and modulus values of PS when compared to those obtained at recommended distances (30 times the maximum indent depth) is only about 4% and 2.7%, respectively. The difference of 3.5% for \(H\) holds for _d/h_ = 10-15, for modulus it drops to 2%. Hardly any deviation (\(<\) 0.5%) is measured at _d/h_ = 20 and above. As shown in Figure 10, the stress contours of two indents start to separate at _d/h_ = 15, yet there is still a low von Mises stress at the regions present. The von Mises stress would overlap until the separation between indents reaches at least 19 for all three different orientations. The stress plots shown in the last row of Figure 10, also indicate that constant low stresses in PS can be achieved at separations above 20 at maximum depth of 1 \(\upmu\)m independent on indent orientation. Another two depths, \(h\) = 5 \(\upmu\)m and 10 \(\upmu\)m, with different orientation are also investigated and presented in Supporting Information S4. Interestingly, the FEA is somewhat different. In those cases, the plastic zone and large stress overlap is observed till _d/h_ = 7-8. Although the indentations are away from each other with a distance of 10 times maximum depth, it provides that the von Mises stress would overlap until the distance between indentations reaches 15 times the maximum indentation depth. The von Mises stress plots indicate that the stress stays stable from the distance above 15. The difference in hardness values changes from 6% at _d/h_ = 5, to below 1% at _d/h_ = 10 and above. While modulus difference values change from \(\sim\)10% at _d/h_ = 5, to 3% at _d/h_ = 10, and below 1% at _d/h_ > 15 and above. Comparing PS properties at various depth, it was observed that at maximum depth of \(h\) = 1 \(\upmu\)m, the difference of the hardness values is higher than that of modulus. This might be explained as strain hardening affect, as well as specific surface effects like ageing. Furthermore, FEA analysis clearly indicates that there is still a very strong von Mises stress overlapping up to a separation of 15. Based on the experimental and FEA results, _d/h_ should be at least 15 to ensure \(H\) and \(Er\) data consistency for low depths. When the indent depth increases, the difference in modulus values is higher that than of hardness, for both \(h\) = 5 \(\upmu\)m and 10 \(\upmu\)m. In this case, \(H\) values become constant at _d/h_ = 10 when indents are fully separated and plastic zones are hardly overlapping (Supporting Information S4, Figures S4.1 and S4.2), but modulus will be affected until _d/h_ = 15. As discussed in Section 3.1, overlap of plastic zone might not be the correct criterion for determining minimum separation, however in this case, FEA analysis and experimental results agree that separation of 15 provides the most constant results. Polymers are viscoelastic materials, meaning that material behavior is time-dependent.