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Title: Dynamic nanoindentation of various polymer nano-composites
Author: Stewart , Alistair
Awarding Body: University of Ulster
Current Institution: Ulster University
Date of Award: 2012
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There is a strong rationale for replacing traditional carbon fibre and glass fibre composites with nano-composites as smaller size and higher aspect ratio reinforcement can lead to improved mechanical properties, via improved load transfer from reinforcement to polymer matrix but also increased dissipation of mechanical energy within the material, an advantage for maximising toughness and fatigue resistance. This project focuses on using dynamic nanoindentation to characterise polymer nano-composites made with graphene and carbon nanotubes (CNT), namely, CNT-polymethylmethacrylate (PMMA) composites, CNT-polydymethylsiloxane (PDMS) and graphene-polyamide composites. Melt-processed PMMA nano-composites studied by optical microscopy show a gradual increase in aggregate size for the functionaLized CNT/PMMA composites, the results for the nonfu nctionalised CNT/PMMA composite showing a poorer dispersion of CNT. Thermogravimetric analysis (TGA) also indicates that the functionalized-CNT are better dispersed into the polymer. These results are consistent with those obtained by nanoindentation and Rockwell hardness testing; higher storage modulus, lower loss tangent and higher HR value for the composites made with functionalized-CNT, sign of a better interfacial load transfer. The addition of solvent, the use of tip sonication as well as the e NT aspect ratio were all found to have an influence on the CNT dispersion within in-situ cured CNTIPDMS nano-composites. TGA showed a lowering of the degradation temperature upon CNT addition, indicative of CNT inhibiting the cure process, in accordance with recently reported results. Nanoindentation resu lts indicate that the specimen made with the better dispersed CNT exhibit the larger storage modu lus. Generally, these stiffness values increase at low CNT content and then decrease, possibly because of the effect that CNT have upon the PDMS cross-linking. In these nanocomposites, the loss tangent is large, dominated by the elastomeric matrix and not sensitive to the interfacial energy dissipation. Finally. in situ-polymerised graphene-polyamide nano-composites were investigated. TGA showed that the graphene was oxidised in these composites. Optical microscopy indicates that the fine-dispersion of the reinforcement disappear at higher graphene content. Nanoindentation showed that the storage modulus peaked at low graphene content and then decreased at higher percentages, while tested at a larger scale (mm\ the mechanical properties of these composites only saturates with graphene content. This result may be due to the processing of the high graphene content specimen, made by solvent evaporation, during which aggregates may reform, as observed by optical microscopy. Overall, this investigation has shown that dynamic nanoindentation is a valuable tool for studying the dispersion of reinforcement in polymer nanocomposites.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available