Browsing by Author "Cavaye, Hamish"

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    Nitrated cross-linked b-cyclodextrin binders exhibiting low glass transition tempratures
    (Wiley, 2018-09-04) Luppi, Federico; Cavaye, Hamish; Dossi, Eleftheria
    Polymeric binders such as β‐cyclodextrins (βCDs) are used to bind with other constituents of energetic formulations and to prevent accidental ignition. One of the advantages of βCDs is the ability to tune their properties by chemical modification. Here, we synthesised nitrated cross‐linked βCDs (βNCXCDs) to produce new binders for energetic formulations. The cross‐linking of βCD with non‐toxic triethylene glycol diglycidyl ether (TEGDGE, X=T) and poly(ethylene glycol) diglycidyl ethers (PEGDGE, X=P) yielded soft, water soluble oligomeric compounds (βCXCDs) which can improve the processability of energetic formulations and contribute to their desensitisation. When the PEGDGE cross‐linker was used, lower glass transition temperatures were achieved, which extended the operative range of the βCPCD binder to −20 °C. The analogous nitrated systems (βNCXCDs) were therefore synthesised using a 1 : 1 (v/v) ratio of 98 % sulfuric acid/100 % nitric acid or 100 % fuming nitric acid, increasing their solubility in acetone and tetrahydrofuran. The nitrated derivatives were characterised by decomposition temperatures (200 °C) and energies (up to 1750 J g−1) comparable to nitrocellulose. Moreover, the glass transition of the inert βCXCDs at low temperatures (<0 °C) was conserved in the corresponding nitrated βNCXCDs, ensuring the desensitisation of energetic compositions even at low temperatures. This is the first time that nitrated derivatives of βCD with glass transition temperatures below 0 °C have been reported, suggesting such derivatives could make suitable replacements for nitrocellulose and other binders in energetic formulations.
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    Understanding and controlling the glass transition of HTPB oligomers
    (Royal Society of Chemistry, 2021-04-07) Dossi, Eleftheria; Earnshaw, Jacob; Ellison, Laurence; Rabello dos Santos, Gabriella; Cavaye, Hamish; Cleaver, Douglas J.
    In this paper, we use a combination of experiment and simulation to achieve enhanced levels of synthetic control on the microstructure of the much-used binder material hydroxyl terminated polybutadiene (HTPB). Specifically, we determine the appropriate combination of initiator, temperature and solvent required to dial in the relative contents of trans, cis and vinyl monomeric units. When an alkylithium initiator (TBDMSPLi) is used, the vinyl content increases from 18% to >90% as the polymerization solvent is switched from non-polar to polar. Further, in non-polar solvents, the vinyl content increases from 18% to 40% with decreasing polymerization temperature. The glass transition temperature, Tg, is shown to be strongly affected by the microstructure, covering the very wide range of −95 °C to −25 °C. The Tg values of HTPB oligomers with high vinyl content are exceptionally high (−25 °C) and can be associated with their aliphatic backbones with pendant side-groups structures. The experimental indications that intramolecular degrees of freedom have a dominant effect on Tg are confirmed by Molecular Dynamics simulations. These simulations identify crankshaft flips of main-chain sub-sections as the key mechanism and relate this to the vinyl content; the frequency of these rotations increases by an order of magnitude, as the vinyl content is reduced from 90% to 20%. The generic mechanistic understanding gained here into what constitutes a “good binder” material is readily transferrable to the potential identification of future candidate systems with very different chemistries.