## 2 Results and Discussion
### Application Testing of Controlled Morphology Hyperbranched Polymers
#### 2.2.2 Film Formation
However, the errors associated with the block-like system became very small in step 7 (D), indicating that a uniform film developed more rapidly than for the semi-block-like HB. The latter did not become uniform until step 11 (F). Once 90 \({}^{\circ}\)C was attained, the average film thickness was very similar for the two HB formulations. The more rapid block-like polymer film formation may be due to its smaller molecular size, allowing both quicker diffusion to the surface and/or better surface packing. The Stribeck curves for each test formulation, which were produced by the MTM apparatus, were then inspected. The results for the pure base oil are shown in Figure S8, as the 120 \({}^{\circ}\)C measurements could not be performed to prevent damage to the analytical device. The observed trend was an increase in the dimensionless friction coefficient as the speed of rotation decreased. This is because, at slower speeds, the surfaces are less able to entrain the test material; rather, it is forced out of the intersurface space due to the force applied between the surfaces. At 60 \({}^{\circ}\)C, the friction coefficient fluctuated considerably, but did exhibit an increasing trend, with a maximum coefficient of 0.38 being achieved. Meanwhile, at 90 \({}^{\circ}\)C, the friction rose to a maximum of 0.46, at which point the test was stopped to prevent apparatus damage. It was hypothesised that a decrease in the viscosity of the oil at higher temperatures resulted in lower entrainment of the oil, leading to greater wear. The \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Step No**} & **Maximum** & **Minimum** & **Mean Thickness** & **Standard** & **Measurement** \\  & **Thickness [nm]** & **Thickness [nm]** & **[nm]** & **Deviation [nm]** & **Temperature [\({}^{\circ}\)C]** \\ \hline
**A (Zero Point)** & 6.0 & \(-10.0\) & 0.7 & 1.7 & 60 \\ \hline
**B (Step 3)** & 218.0 & \(-3.0\) & 194.9 & 44.6 & 60 \\
**C (Step 5)** & 227.0 & 1.0 & 194.4 & 46.6 & 60 \\
**D (Step 7)** & 223.0 & 80.0 & 200.1 & 12.1 & 90 \\
**E (Step 9)** & 227.0 & 118.0 & 201.0 & 9.9 & 90 \\
**F (Step 11)** & 223.0 & 75.0 & 204.4 & 8.8 & 120 \\
**G (Step13)** & 220.0 & 118.0 & 201.4 & 13.3 & 120 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the film thickness data obtained from interferometry for a 1 wt% block-like HB polymer/base oil formulation. The blue shading indicates that the measurement was conducted after film buildup (i.e., following a period of constant temperature and rolling speed). The orange shading indicates that the measurement was conducted after a Stribeck curve measurement was conducted. influence this viscosity change has on the final performance of the HB-formulated systems will require further study. Thus, to better understand how it influences the mechanisms at each boundary, mixed or hydrodynamic lubrication was achieved. However, this more detailed study is outside the scope of this initial work programme and is the subject of an ongoing study. By comparing the data from the HB polymer formulations exhibited, a marked decrease in the friction coefficient is observed with the addition of both HB polymer types, see Figure 8.  
Even at low entrainment speeds, the HB systems outperform the pure base oil, e.g., at 60 \({}^{\circ}\)C, the semi-block-like HB results (Figure 8, left) peaked at a coefficient of friction of 0.14, compared to 0.38 for the oil without a polymer. Similarly, the 120 \({}^{\circ}\)C Stribeck curve results in an increased friction between the two surfaces, as would be expected. However, for the semi-block-like polymer, the expected trend of increasing friction coefficient with increasing temperature is reversed at 90 \({}^{\circ}\)C, where the friction decreases with increasing temperature. By cross-referencing this observation to the film thickness data in Table 4, this decrease in friction is accompanied by a significant increase in the average film thickness of \(\sim\)16 nm at this point, compared to the average 60 \({}^{\circ}\)C value.