Sugden, IJ; (2014) Carboranes and their incorporation into siloxane polymers. Doctoral thesis , UCL (University College London).

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Abstract

This is a report detailing the computational investigation of copolymers containing carborane and siloxane monomers, to aid in the design an industrially relevant material for use in high neutron radiative environments. This includes determining the optimal carborane/siloxane ratio in the designed material with regards to macroscale physical properties; with experimentally determined values for pure siloxane phases reproduced, using classical methods. The investigation shows that increasing carborane content increases bulk modulus and decreases the thermal expansion coefficient, levelling off beyond 50% carborane content. It also includes the effect of including specific side groups to polymer strands in order to affect properties; for instance, it is seen that phenyl groups increase the flexibility of the polymer strands. Alongside this, the report includes the simulation of property “aging”; using classical crosslinking methods to model the effect of high energy ions travelling through the material following neutron capture events, and ab initio simulation of damage to the monomer. Unsurprisingly, crosslinking sees a reduction in flexibility, leading to an increase in bulk modulus and a decrease in the thermal expansion coefficient, whilst the changes in vibrational spectra as a result of neutron capture events are predicted: due to changes in bond strength and orbital structure, modes involved with cage elements move to a higher frequency, and B-H modes move to lower. Finally for the designed material, context is given by examining the current state of the art: solid boron carbide, and its remarkable resistance to radiation of several different forms, with experimental theories and mechanisms discussed. Carborane clusters are further examined in other technological areas: thermal rearrangements of single carboranes and metallo-carboranes, with the 40 to 145 kJ/mol (dependent on mechanism) difference between theoretical and experimental activation energies rationalised, and investigations of icosahedral boron cluster anions in lithium battery electrolytes, where a ≈17% improvement in lithium mobility is theorised.

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