Biomedical thermoplastic polyurethane nanocomposites: Structure - property relationships

Thermoplastic polyurethanes (TPUs) are excellent potential materials for the construction of implantable medical components due to their exceptional mechanical properties and biocompatibility. In this thesis, the viability of siloxane-based TPU (ElastEont E5-325) nanocomposites was investigated as new insulation materials for implantable and electrically active medical devices. The effect of the organically modified nanosilicate (organosilicate) addition on the underlying TPU morphology and mechanical properties was thoroughly studied. Results demonstrated that, nanofiller aspect ratio, surface modification, nanofiller concentration and processing route affect the morphology and the mechanical properties of the TPU. Specifically, the TPU incorporating low aspect ratio organohectorite and high aspect ratio organofluoromica with the most hydrophobic modification (75% DODMAC / 25% choline) demonstrated the best dispersive and distributive mixing, hence resulted in an overall best profile of mechanical and thermomechanical properties. This finding supports the importance of polarity matching between the TPU and organosilicate in enhancing silicate layer dispersions in the TPU, thus enabling enhanced thermal and mechanical properties of the nanocomposites. Furthermore, it was found that the hydrophobic low aspect ratio organohectorite acts as a very potent interfacial compatibilizer. At 2 wt % loading, the resulting nanocomposite displayed vastly superior mechanical properties to both soft silicone and host TPU. It was hypothesized that the hydrophobic low aspect ratio organohectorite serves to provide more cohesive hard microdomains and thus creep resistance and dimensional stability. Interestingly, at a higher (4 wt %) loading of organohectorite, gross morphological changes in the TPU microdomain texture were observed, which adversely effected the mechanical properties of the TPU. Further, the effect of single versus dual surface modification on the morphology and properties of the TPU was investigated. It was demonstrated that the incorporation of dual modified organofluoromicas as nanofillers enhances the tensile strength, toughness and tear strength of the TPU. Therefore, it was concluded that the presence of dual surfactants, which form regions of higher and lower surface energy on the layered silicate surface, enables molecular interactions between the organofluoromica with both TPU hard and soft segment populations. In addition, the presence of a second choline-based surfactant with reactive nOH functionality led to the formation of positively charged TPU chain end groups as a result of trans-urethanization reactions during high temperature compounding, thus introducing labile lgrip-slip-grip-slipr interactions between the TPU and the nanofiller. These molecular interactions contributed to a reduced level of stiffening, while at the same time enhancing the TPU toughness. The increased in the creep resistance and retardation in the stress relaxation of the TPU proved that the dual modified organofluoromica also serves to enhance the dimensional stability of the TPU. In terms of nanocomposite processing methods, melt processing resulted in better exfoliated and dispersed organofluoromica in the TPU matrix as compared to the solvent casting, which was due to the higher shear forces associated with twin-screw extrusion. High energy milling of a dual-modified high aspect ratio fluoromica was shown to markedly improve the exfoliation and dispersion of the organofluoromica in the ElastEont E5-325 TPU, and thus the mechanical properties of the resulting nanocomposites. In vitro mechanical performance demonstrated the retention of these promising properties of the TPU/organofluoromica nanocomposites under hydrated conditions, further highlighting the potential of these novel nanocomposites to be further developed for biomaterials applications.