| Summary: | The development of high-performance polymers/composites/nanocomposites has been one of main concerns nowadays, especially considering the consumption of finite resources. Silicone, which can be made from sands, has been widely used as high-performance polymer since applications in World War Ⅱ. In order to push the limit of silicones, cyclosiloxane polymer was developed to improve the thermal stability. However, lack of knowledge on cyclosiloxane polymer has provided little insight on structure-properties relationship.
This thesis aims to study the structure-properties relationship of cyclosiloxane polymer, reinforce cyclosiloxane polymer by additives and nanofillers, and explore applications of cyclosiloxane polymer/composites/nanocomposites.
Through a review of the literature, lack of knowledge about cyclosiloxane polymer was addressed. The kinetics study was performed to better understand and control the curing process of hydrosilylation reaction between cyclosiloxanes. To better understand the structure-properties relationship, the structure of cyclosiloxane was firstly identified. It was found that cyclosiloxane polymer is a layered structure in nano-size but isotropic in bulk.
After identifying the structure, possible reinforcement methods were proposes. By adding vinyl terminated polydimethylsiloxane (PDMS), which can be covalently bonded to the layered structure, a simultaneously improved toughness, and stiffness was observed. It is proposed that PDMS exists as two phases in cyclosiloxane polymer. The homogeneously dispersed PDMS chains anchor different layered structures together, forming a 3D connected network, contributing to both toughness and stiffness while the phase separated PDMS particles only contribute to toughness, deteriorate other mechanical properties.
Graphene oxide (GO) was basally modified and used to toughen the cyclosiloxane polymer matrix. A 153 increase in toughness with only 0.8 wt% graphene loading was observed. ‘Double side tape’ toughening mechanism is proposed to explain the toughening behavior and formation of micro-cracks. The increase of thermal conductivity when allylamine modified GO (GO-AA) was added into cyclosiloxane polymer is also explained by ‘double side tape’ effect of GO-AA nanosheets, which act as a staircase for phonon transfer and contribute to thermal conductivity.
Based on Thermal interface materials (TIMs) considerations, cyclosiloxane polymer was used as matrix and multi-walled carbon nanotubes (MWCNTs) were used as thermally conductive fillers. MWCNTs were modified by silica coating and/or siloxanes with double bonds before incorporating into cyclosiloxane polymer. The resulting increase in thermal conductivity was explained by alleviating modulus mismatch by silica coating, covalent bonds between the matrix and MWCNTs fillers, as well as dispersion. The insulate behavior of MWCNT-cyclosiloxane polymer nanocomposites except raw MWCNTs indicated possible candidates for TIMs.
In summary, cyclosiloxane polymer was studied from structure to properties, as well as the relationship between them. Based on the layered structure, possible reinforcement methods were carried out, and the corresponding mechanisms are proposed. Finally, MWCNTs-cyclosiloxane polymer nanocomposites were fabricated toward TIMs.
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