Space-resolved thermal properties of thermoplastics reinforced with carbon nanotubes
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Composites comprising biobased poly(lactic acid) (PLA) and polyethylene (Bio-PE) were reinforced with multi-walled carbon nanotubes (MWCNTs). These nanocomposites were analyzed using space-resolved thermal analysis (TA) integrated with atomic force microscopy. The deflection temperature, which indicates thermal-induced expansion and thermal transitions of the composite, was monitored by nanoscale TA (nanoTA) utilizing the displacement of a cantilever in contact with the material. Results were compared to bulk electrical, mechanical and thermal properties. Electrical conductivity was detected at lower MWCNT loadings for PLA than for Bio-PE (at 2.5 vs. 5 mass%). Maximal electrical conductivity of 27 S m−1 for PLA and 0.7 S m−1 for Bio-PE-based samples was reached at 10 mass% MWCNT loading. Tensile behavior combined with thermogravimetric analysis indicated strong MWCNT–Bio-PE interactions, in contrast to PLA. The glass transition and melting temperature measured by differential scanning calorimetry (DSC) were not changed by the increase in MWCNT loading. Increased deflection temperature was registered by bulk heat deflection measurements on Bio-PE, but not for PLA. The thermal transitions obtained by nanoTA at the nanoscale were in the same temperature range as the first transitions observed upon temperature ramp in DSC (e.g., glass transition and melt temperatures of PLA and Bio-PE, respectively). Remarkably, thermal expansion was detected by nanoTA for PLA- and Bio-PE-based composites below electrical percolation threshold as well as an increase in PLA softening temperature. Space-resolved nanothermal analysis revealed thermal phenomena that are otherwise overlooked when bulk methods are applied.
KeywordsNanocomposite Thermoplastic Conductivity Nanothermal AFM Nanoindentation
We gratefully acknowledge the assistance provided by the Institute of Wood Technology and Renewable Materials, as well as the Institute of Natural Materials Technologies, both at BOKU, Tulln. Paul Patter and Prof. Emil J. W. List-Kratochvil from the NanoTecCenter Weiz are acknowledged for their support concerning the electrical conductivity measurements. We would like to thank the Kompetenzzentrum Holz GmbH for providing access to various measuring equipments, i.e., Lukas Sobczak and Christoph Jocham for their support during the experiments. We would also like to thank Prof. Dietmar Pum of the Department of Nanobiotechnology, BOKU. Finally, Dr. Soledad Peresin from VTT Technical Research Centre of Finland is thanked for the initiation of the nanothermal analysis work. NatureWorks® and C-polymers are acknowledged for providing the raw materials.
The presented study received substantial financial support from the Government of Lower Austria.
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