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Nowadays, research into environmentally friendly, renewable materials is the focus of materials science. One of the best candidates for these purposes is polylactic acid (PLA), whose properties are determined mainly by its D-lactide content. PLA is often plasticized to achieve proper toughness. Our aim was to investigate the combined effects of plasticizers and D-lactide content on PLA. We investigated two different plasticizers: oligomeric lactide acid (OLA) and dioctyl adipate (DOA). An internal mixer was used to prepare the compounds, and then sheets were prepared by hot pressing. After mechanical and morphological analyses, we found that tensile strength and modulus of neat PLAs and PLA-OLA compounds decreased almost linearly with increasing D-lactide content. The mechanical properties of PLA-DOA compounds depended far less on D-lactide content than in the case of PLA-OLA compounds. Plasticizers promote the crystallization of crystallizable PLAs by their chain mobilizing effect—we obtained a higher crystalline fraction. The latter effect reduces the impact of the plasticizing effect of plasticizers in the product. The compatibility and dispersibility of plasticizers also have a significant effect on the properties of the materials. OLA is more compatible with PLA than DOA, which resulted in better plasticization, but caused more defects in the crystallites, thus reducing the crystalline melting temperature, and so the processing temperature of the compound containing plasticizers.

abstract: In this study the creep behaviour and its dependence on moisture content in injection moulded hybrid composites reinforced with carbon fibres and carbon nanotubes was investigated. The tensile tests and the scanning electron microscope images confirmed that the presence of carbon fibres helped the uniform distribution of carbon nanotubes in the matrix. In the composites reinforced only with carbon fibres, the creep rate was lower than in any of the composites reinforced with only carbon nanotubes. Synergistic effects were observed when carbon nanotubes were added to the composite besides the carbon fibres: the creep rate was even lower and decreased with the increase in nanotube content. In the composites only reinforced with carbon nanotubes, crystallinity decreased, and moisture absorption increased with increasing nanotube content, but in the hybrid composites, the decrease in crystallinity and moisture absorption was much lower, thus the disadvantageous effect of moisture on the creep rate was much smaller in the hybrid composites.

In this study, we developed electrically conductive nano- and hybrid composites with a poly(lactic acid) (PLA) matrix for different melt processing technologies. We used short carbon fiber and multi-walled carbon nanotube reinforcements to enhance electric conductivity. We prepared the composite compounds with twin-screw extrusion; then the compounds were processed via injection molding and fused filament fabrication. We showed that electric conductivity only slightly increased by when only carbon nanotubes were added to the PLA matrix. However, when carbon fibers were added to the nanocomposites, the higher shear during melt mixing helped the uniform dispersion of the carbon nanotubes, resulting in a highly conductive reinforcement network in the composite. On the other hand, the hybrid reinforcement resulted in higher viscosity, making melt processing difficult and the material also became more brittle. Therefore, we added an oligomeric lactic acid plasticizer to the hybrid composites, and produced specimens by injection molding and 3D printing. The tensile strength increased by 140% and the elongation at break increased by 56%, and at the same time, the electrical conductivity of the material remained at a high level.