30 J. G. Tramell et al. mobile than the bulk binder due to interaction with the particulate (i.e. tightly bound). The outer layer has higher mobility than the inner layer, but still less than that of the bulk binder (i.e. loosely bound). The reduction in molecular mobility and the sizes of the respective interphase regions depends on the polymer matrix used, and the particle’s surface chemistry, volume fraction, particle size, particle size distribution, and quality of dispersion within the binder [1]. It is understood that the polymer matrix influences the overall mechanical behavior of the composite, but there is a lack of understanding of the how the interphase region between the HTPB binder and filler contribute to the overall mechanical behavior. Dynamic mechanical analysis (DMA) is a technique that may provide insight into the properties of the interphase. DMA provides information of the viscoelastic properties of a material, and specifically in this study tan δ (i.e. loss factor) is investigated. The tan δ represents the ratio between a material’s loss modulus (energy dissipation) and storage modulus (energy storage). The tanδ provides insights into a material’s ability to dampen vibrations or absorb impacts. A peak in a tan δ trace indicates that the material is exhibiting a significant energy loss. This energy loss is primarily attributed to increased molecular mobility within the material and is often associated with a specific relaxation process (e.g. the glass transition temperature). In filled HTPB energetic systems, many have reported the presence of two distinct tanδ peaks, which suggests the presence of two distinct relaxation processes [2–6]. The first tan δ peak at the coldest temperature is broadly accepted to describe the glass transition temperature (Tg) of the bulk HTPB binder. However, there are several explanations of the origin of the second-broader tan δ peak that occurs at warmer temperatures. The second tan δ peak is commonly attributed to a separate relaxation process solely within the binder. The most frequent explanation attributes the second peak to the hard and soft segments of the HTPB binder. Another explanation ascribes this to the presence of sol-gel content in the polymer network (i.e. non-crosslinked chains) [3–5]. Despite previously published explanations for the additional tan δ peak in filled HTPB systems, other filled composite systems often link this second peak to the presence of an interphase region [1]. Although not universally applicable to every filled system, the interphase relaxation mechanism seen in DMA data for many other systems warrants consideration as the underlying mechanism of the second tanδ peak in highly filled HTPB systems [1, 7–9]. Bashir discusses several alternative and complimentary approaches to DMA that other investigators have used to characterize the interphase region such as nuclear resonance spectroscopy (NMR), atomic force microscopy (AFM), Raman spectroscopy, and dielectric relaxation spectroscopy [1]. AFM is perhaps the most attractive option since the technique can measure the widths and modulus of the interphase regions and could be useful to correlate interphase characteristics to tanδ peak heights and widths from DMA data. The selected approach of this work is to systematically change the particle size of filler and modify the particles’ surface chemistry, both of which should change the volume fraction of interphase region [1]. Changes to the effective interphase volume are expected to create changes in DMA tan δ traces and AFM measurements. By pairing DMA and AFM measurements, it may be possible to determine if DMA can infer additional information on the interphase region in filled HTPB systems (e.g. the interphase size). Materials A typical HTPB binder system was selected as the polymer matrix. Table 1 summarizes the components of the HTPB system. Soda-lime silica glass spheres (glass beads) were selected as the filler instead of energetic particles to simplify testing logistics and estimation of total surface area of the solids in the system. The weight percent of glass beads was chosen to generate a system with 50 volume % of filler. The filler content for all mixes was fixed at 50 volume % and the sizes of the glass beads were varied. Table 1 Filled HTPB composition Ingredient Function Weight% Poly bdR⃝ R45HTLO Prepolymer 18.41 Isodecyl pelargonate (IDP) Plasticizer 6.138 Dibutyltin dilaurate (DBTDL) Catalyst 0.04242 Ethyl 702 Antioxidant 0.1564 Isophorone diisocyanate (IPDI) Curative 1.766 Soda-lime silica glass sphere Filler 73.49 Soda-lime silica glass should have a high level of interaction with the binder due to the surface silanol groups at the beads’ surface. The silanol groups can react with the IPDI and create a covalent bond between the filler and the polymer network.
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