## 2 Results and Discussion
### Application Testing of Controlled Morphology Hyperbranched Polymers
#### 2.2.2 Film Formation

Table 3: Summary of the film thickness data obtained from interferometry for a pure base oil solution. The blue shading indicates that the measurement was conducted after film buildup (i.e., following a period with no change in temperature or rolling speed). The orange shading indicates that the measurement was conducted after a Stribeck Curve measurement was conducted. Figure 5: Plots of the film thickness (nm) for a pure oil formulation. (**A–D**) **Top**: two-dimensional scatter plots for each step, where the data has been binned, as indicated in the legend. (**A\({}^{\prime}\)–D\({}^{\prime}\)**) **Bottom**: three-dimensional surface plots for each step. The labels (**A–D**) correlate across the two plots and with the labels in Figure 4.  
This conclusion was supported by the graphical visualisation of the data provided in the supporting information (SI) document is Figure S3. The Stribeck measurement (rolling data) was performed between steps 3 and 5, (entry B and C), and after step 7 (entry D). The data showed that the average film thickness decreased from 71 nm at entry B to 54 nm at entry C during the experiment, due to the changing rolling speed. However, the decrease in average thickness between entry C and D of ~50 nm corresponds to an increase in the test temperature from 60 to 90 \({}^{\circ}\)C. This is considerably greater than that caused by the Stribeck measurement. Meanwhile, the interferometry results of the comparative experiment, which utilised a 1 wt% solution of the semi-block-like HB polymer in oil, are shown in Figure 6.  
With this formulation, measurements could be conducted at a temperature of 120 \({}^{\circ}\)C (Figure 6F,G) without having to abandon the test due to excessive wear. Comparing the semi-block-like polymer solution data to that of the pure base oil, it was also apparent that there was very little wear occurring between the surfaces. This was shown by the lack of elongation of the contact point. The interferometry images also got steadily darker as the different rotation and temperature regimes were performed. This was indicative of the formation of a tribological film at the contact point between the surfaces and this film has been visualised in Figure 7.  
Both the 2D scatter plot (top) and the 3D surface plot (bottom) indicated that a significant film formed rapidly, with the majority of the measured film having a thickness of >200 nm in plot Figure 7B. From this step, the film thickness continued to grow and become more uniform as the test progressed, with very little variation in film thickness seen after plot F. This indicated that varying rolling speed and increasing temperature both caused an increase in the thickness of the film formed, when the semi-block-like HB polymer was incorporated into the formulation. The film thickness summary for this test is shown in Table 4 and visualised in Figure S4, which corroborates that the observed formation of a protective, robust film was observed. Initially, at step 3 (entry B), the average film thickness was 180 nm. The film then grew to a peak average thickness of 202 nm at step 13 (entry G) at 120 \({}^{\circ}\)C. Furthermore, at this point, the calculated errors (standard deviations) reduced significantly from ~35 to ~3 nm, indicating little variation in the film thickness across the wear scar. Comparing these results to the pure base oil data in Table 3, it was noted that the trend of decreasing film thickness was reversed. With the base oil formulation, the surface contact increased due to the loss of the film at the contact point. Thus, this was the root cause of the wear for the pure oil case. Rather, with the semi-block-like formulation, the thin film was found to grow with increasing temperatures and rotation regimes (i.e., after Stribeck measurements). This demonstrated that the addition of this HB copolymer resulted in the formation of a thin surface film, which was not worn down with an increasing temperature and applied force between the two surfaces.