Investigating the Interphase in Hydroxyl-Terminated Polybutadiene (HTPB) Composites via Dynamic Mechanical Analysis 39 compliant as HTPB and must be cooled to section, another consequence arose. We observe that the embedded particle— having the appearance a pristine flat cut optically at cryogenic temperatures—appeared to be protruding from the binder once the sample was brought to RT (see the top (red) trace of figure 8 for reference). We believe this phenomenon is a direct result of HTPB’s relatively large coefficient of thermal expansion to that of soda-lime silica glass of HTPB as it warms to RT, thus altering the prepared surface [24]. Other surface preparation approaches—cryo- and RT hand polishing and cryo- and RT broad beam ion milling—have been attempted to overcome this unforeseen consequence. For each mitigation strategy, we observe issues. Particle pull out (from hand polishing), charring of the HTPB (from ion milling), and metal re-deposition (from ion milling) significantly alter the surface and, in turn, irreversibly alter the interphase. At the time of this study, we are continuing to investigate other cross sectioning techniques (such as oscillating diamond knives, diamond wire saw, etc.) that will not induce damage and minimize the observed thermal expansion issue, while still preparing a flat surface for high quality AFM measurements. Fig. 8 Optical image of the surface of P4000 (4 wt. %) composite sample prepared via cryo-microtome. Cutting direction is from top to bottom of the image. Elongated vertical lines running through the sample are knife marks, an artifact based on knife damage. Colored circles and arrow highlight interesting features of the cross section, see main text Figure 8 shows a representative optical image of the cryo-microtome P4000 (4 wt. %) surface used for AFM analysis. We note key features of the cross sectioned surface to highlight the difficulties of sample preparation, and the obstacles faced in finding a viable bead/interphase to analyze. The cutting direction is from top to bottom of the image. The red circle in figure 8 shows a glass bead that is cut in half (and appears slightly raised from the HTPB binder) but does not exhibit the same knife marks as the surrounding HTPB sample (elongated vertical lines). Such an artifact can be indicative of a fractured glass bead instead of one that is cleanly cut. Highlighted in yellow in figure 8, we observe a vacancy in the HTPB where a glass bead was pulled out of the sample. The blue circle in figure 8 highlights a conglomerate of smaller glass beads that presents as a non-spherical cluster. Finally, we can observe a ‘lip’ behind some of the embedded beads (shown with the white arrow in figure 8 ) that correlates with the ‘back’ of the bead in terms of cutting direction. All of these artifacts allude to damage imparted by the cutting process, which will likely affect the measured interphase width. Nevertheless, we attempt AFM analysis on coated and uncoated P4000 composites at 4 wt. % loading. Two glass beads were identified in the P4000-SC sample and are denoted as bead A and bead B. Two glass beads are also identified in the
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