Nanocomposite Thermoplastic Polymer Sheets for Medical Applications: Influence of Processing Conditions on Material Properties

The thermoplastic nanocomposites prepared in this work are intended to be used as mouldable sheets in the medical field. They will be used for biomedical applications such as prostheses, splint materials for physical rehabilitation, and immobilization masks in radiotherapy. The increase in bending stiffness resulting from the addition of nanofiller particles to the polymer can result in thinner and easier to deform plates, improving, e.g., the X-ray transparency, the production cost, and the patient's comfort. In this work, nanocomposite materials are produced focusing on nanocomposites based on two thermoplastic polymer materials, namely poly (?-caprolactone) (PCL) and polypropylene (PP). The nanofillers used are layered silicates (nanoclays), nanowhiskers and carbon nanotubes. The most important factor in achieving the superior properties of these hybrid materials is to realize a good dispersion of the nano-additive throughout the polymer matrix. The assessment of the quality or level of dispersion of the nanofiller particles is one of the main issues in this work. For each polymer matrix, the best nanofiller was determined first, being the nanofiller resulting in the highest increase in bending stiffness. For PCL, the nanoclay containing the most polar organic modifier (Cloisite 30B) resulted in the biggest increase in bending stiffness. Using a nanofiller loading of 5 wt.% (inorganic fraction) resulted in a doubling (100%) of the bending modulus compared to the pure PCL material. For PP, the increase in bending stiffness was less pronounced (40%) when adding nanoclay (Bentone 108) as reinforcing agent. In order to attain a better result, blends of PP and PP grafted with maleic anhydride were used. This resulted in a bigger increase in bending stiffness, around 70% compared to the pristine material. After defining the best nanofiller-polymer combinations, the morphology or dispersion of these nanocomposite samples was studied. This study was done using microscopy techniques (Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM)) and Scattering techniques (Wide Angle X-ray Diffraction (WAXD)) as conventionally done, but also using thermal analysis techniques (Modulated Temperature Differential Scanning Calorimetry (MTDSC), rheometry, and others (electroconductivity). The excess in heat capacity during quasi-isothermal crystallization, measured with MTDSC, can be correlated to the dispersion quality of the nanofiller particles and thus to the improvement in mechanical properties. The solid-like behaviour observed with rheometry is linked to the formation of a percolating network of the nanofiller particles when well-dispersed throughout the polymer matrix. SEM and TEM allow a clear visualization of the obtained morphology of a nanocomposite sample, whereas WAXD provides information on the intercalation distance in between the clay platelets in the polymer matrix. Addition of well-dispersed carbon nanotubes to a polymer matrix can render a (semi-)conductive material. The electroconductivity can be used to examine the dispersion quality of the nanotubes, since they need to be well distributed in the entire polymer matrix to form an interconnecting (percolating) conductive network. The percolation threshold for the best carbon nanotubes is situated around 0.5 to 0.75 wt.%. Simulation of the packing of nano platelets shows that at the percolation threshold no network can be formed if the particles are randomly packed and the inter particle distance would be in the order of a few nanometers. If particles would be linked by polymer chains over a distance of about the random