Enhancing combustion performance of nano-Al/PVDF composites with β-PVDF

Solid energetic nanocomposites, consisting of reactive metal particles and oxidizers, find broad applications ranging from pyrotechnics to solid rocket propellants and solid ramjet fuels. In particular, nano-aluminum (n-Al) and polyvinylidene fluoride (PVDF) are attractive fuel and oxidizer materials, due to the high energy density of n-Al, and the high oxidizing potential and excellent mechanical properties of PVDF. PVDF is a semi-crystalline polymer and has three common crystalline phases, alpha (alpha), gamma (gamma), and beta (beta) phases. Many research works have focused on the chemical reactions between Al and PVDF, yet the effect of PVDF crystallinity on the Al/PVDF reaction is unknown. Here, we experimentally and computationally demonstrate that increasing the mass fraction of beta-phase PVDF from 2.5 to 25% in Al/PVDF composites substantially improves peak pressure by 90% (from similar to 34 to similar to 64 psi) and pressure rise rates by 300% (from 2.5 psi/ms to 10 psi/ms). This stems from the alignment of F atoms along one side of the beta-PVDF polymer chain, making it structurally conducive to reacting with Al particles to form strong Al-F interactions. This strong interaction leads to higher binding energy between, and hence higher reactivity in, beta-PVDF and Al. Our research provides a new method for enhancing the reactive performance of Al/PVDF composites by increasing the content of beta-PVDF. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.