Investigating the Interphase in Hydroxyl-Terminated Polybutadiene (HTPB) Composites via Dynamic Mechanical Analysis 35 Fig. 4 Max tanδ height vs average particle size in HTPB/glass bead composites in the binder. Without proper dispersion, a cluster of particles can effectively behave as a larger particle and reduce the overall surface area of the filler. Additionally, not all samples exhibited a distinct secondary peak at warmer temperatures. Specifically, the P4000, P4000SC, and P-0040-SC samples. With the suspected agglomeration issues, it is unknown if this is representative behavior or due to the lack of particle dispersion. The HTPB binder also appears to exhibit an underlying secondary tanδ peak. This suggests that there is a second relaxation process occurring in the binder alone, but it is impossible to determine if the introduction of filler affects the second relaxation process in the HTPB or creates a new relaxation process from creation of an interphase. Furthermore, looking at the uncoated beads in figure 2b there is no clear trend in the trace heights above Tg.Wewould expect to see a correlation with particle size as discussed above, but any trends remain unclear even if the P4000 and P0040 samples are disregarded. However, a trend exists in the traces of the coated beads in figure 2d if sample P0040-SC is excluded. Sample P4000-SC is still suspected to be an outlier due to agglomeration concerns, but there appears to be a direct relationship between particle size and the height of the second tanδ peak as we should expect. Comparing the coated and uncoated, the coated beads consistently exhibited a higher second tan δ peaks. The uncoated and coated P4000 and P0040 samples did not exhibit identifiable peaks, but the coated beads still showed higher tanδ traces beyond the Tg. Interestingly, it seems there is another tan δ peak for the P0280 and P0080 glass beads around 30 ◦C. The underlying cause is currently unknown but implies a potential third relaxation process for these systems. EMG fitting The tan δ curves were modelled after baseline correction with EMG functions shown in equation 1. The initial purpose of the EMG modelling was to quantify the peak heights, widths, and areas, and use Tci calculate the Eaof the relaxation processes. It was found that some systems could be modelled adequately with two EMG functions, while others required three. Optimization schemes for data sets requiring three EMG functions often yielded multiple analytical solutions for all parameters. Figure 5a shows an example of the range of EMG fitting solutions. Up to a 50◦C difference inTc2 andTc3could occur when fits had multiple analytical solutions. Additionally, some traces could be modelled adequately with two or three EMG functions as demonstrated in figure 5b. In these cases, the model with three EMG functions did not appear to accurately assign Tc2 to the second tan δ peak due to over fitting, but the models with two EMG function did. Lastly, most of the second tanδ peak height differences were erased with the baseline correction function that minimized differences for Ai between samples. Analysis of the EMG function parameters andTci values for Ea calculations is not reported for reasons just mentioned.
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