Impact absorption in shear-stiffening materials

Nowadays there is a huge demand for protective gear in the sports industry due to the growing popularity of extreme sports in view of their greater availability. The need for them is not limited to extreme sports but spreads wider to sports with a high risk of injuries, such as hockey, American football, skiing, skateboarding, cycling, etc. A new class of “smart” protective materials – shear-stiffening materials, is intended to replace outdated solutions in a view of their advantages. Specifically, self-healing ability (hence, recyclability and reusability), ability to spread out the impact over a large area, and most importantly instant stiffening response to external stimuli which is completely reversible. Presently, two types of shear stiffening materials exist – shear thickening fluid (STF) which is a liquid dense suspension, and shear-stiffening gel (SSG) which is a supramolecular polymer network. However, it remains unclear how does the impact energy is dissipated in them and how features of their stiffening mechanism (jamming – in STF and dynamic crosslinking – in SSG) contribute to impact absorption. Jamming has been found to play a major role in impact dissipation on the example of polypropylene glycol/fumed silica STF. For the first time, using novel optical in situ speed recording of impact it was found that jamming can be triggered at as low volume fraction as >7.2%. Analysis of the flow field during impact revealed that the front propagation speed is 3-5 times higher than the speed of the impactor rod, which rules out jamming by densification, showing that the growth of jamming front is triggered by shear. The main impact absorption begins when the jamming front reaches the boundary, creating a solid-like plug under the rod that confronts its movement. These results provide important insights into the impact absorption mechanism in fumed silica suspensions with a focus on shear jamming. It was discovered that the stiffening response of the best known representative of SSG – polyborosiloxane (PBS), can be controlled by the molecular weight of the precursor due to chosen straightforward condensation synthesis routine. Introduced boron cites through Si-O-B bridges behave as dynamic crosslinks and are responsible for shear-stiffening properties. The dynamic crosslinking density which was quantified by Si-O-B infrared band intensity was shown to define the efficiency of impact absorption. PBS demonstrated a linear increase in peak forces with a decrease in the number of Si-O-B bonds during the drop weight impact test. Therefore, the low molecular weight of a precursor, hence, a high number of dynamic crosslinks is a primary requirement for effective protection against low-velocity impact. The origins of dynamic crosslinking in PBS are in debate for many decades. Here, we show that it does not involve hydrogen bonding, dative bonding or formation/exchange of boron anhydrides as was proposed earlier but solely is the result of associative dynamic exchange of Si-O-B covalent bonds. The formation and breakage of Si-O-B bonds were found to have a low energy barrier, therefore, can be easily formed and exchanged at room temperature. It was also found that the viscoelastic properties of PBS are largely dependent on the B-O functionality of the boron compound. Trifunctional boron crosslinking through B-O bonds results in gelation whereas bifunctional phenylboronic acid provided only chain extension without any signs of gelation. Therefore, mechanical properties could be tuned by the right choice of boron functionality which results in n-functional dynamic covalent bonds (-Si-O)n-B.Doctor of Philosoph