Strong and Lightweight Stereolithographically 3D-Printed Polymer Nanocomposites with Low Friction and High Toughness.

Strong and lightweight polymer nanocomposites with low friction, high toughness, and complex shapes were obtained for the first time through an affordable stereolithographic 3D printer, using low amounts of TiO2 nanoparticles. Tridimensional solid structures (i.e., tensile bars, compressive test specimens, gyroid-type structures, and dense lattices) were obtained. Herein, we found that the compressive stress, compressive strain, yield strength, and toughness corresponding to 3D-printed polymer nanocomposites were simultaneously increased-which is uncommon-using low amounts (0.4 wt.%) of TiO2 nanoparticles. Furthermore, we obtained lightweight cylindrical structures exhibiting high resistance to compression with a low friction coefficient (µ~0.2), and the printability of complex and hollow structures was demonstrated.

Tuning HAuCl4/Sodium Citrate Stoichiometry to Fabricate Chitosan-Au Nanocomposites.

Nanocomposite engineering of biosensors, biomaterials, and flexible electronics demand a highly tunable synthesis of precursor materials to achieve enhanced or desired properties. However, this process remains limited due to the need for proper synthesis-property strategies. Herein, we report on the ability to synthesize chitosan-gold nanocomposite thin films (CS/AuNP) with tunable properties by chemically reducing HAuCl4 in chitosan solutions and different HAuCl4/sodium citrate molar relationships. The structure, electrical, and relaxation properties of nanocomposites have been investigated as a function of HAuCl4/sodium citrate molar relation. It was shown that gold particle size, conductivity, Vogel temperature (glass transition), and water content strongly depend upon HAuCl4/sodium citrate relationships. Two relaxation processes have been observed in nanocomposites; the α-relaxation process, related to a glass transition in wet CS/AuNP films, and the σ-relaxation related to the local diffusion process of ions in a disordered system. The ability to fine-tune both α- and σ-relaxations may be exploited in the proper design of functional materials for biosensors, biomaterials, and flexible electronics applications.

Novel gigahertz frequency dielectric relaxations in chitosan films.

Molecular relaxations of chitosan films have been investigated in the wide frequency range of 0.1 to 3 × 10(9) Hz from -10 °C to 110 °C using dielectric spectroscopy. For the first time, two high-frequency relaxation processes (in the range 10(8) to 3 × 10(9) Hz) are reported in addition to the low frequency relaxations α and ß. These two relaxation processes are related to the vibrations of OH and NH2/NH3(+), respectively. The high-frequency relaxations exhibit Arrhenius-type dependencies in the temperature range 10 °C to 54 °C with negative activation energy; this observation is traceable to hydrogen bonding reorientation. At temperatures above the glass transition temperature (54 °C), the activation energy changes from negative to positive values due to breaking of hydrogen bonding and water loss. Upon cooling in a sealed environment, the activation energies of two relaxation processes are nearly zero. FTIR and XRD analyses reveal associated structural changes upon heating and cooling. These two new high-frequency relaxation processes can be attributed to the interaction of bound water with OH and NH2/NH3(+), respectively. A plausible scenario for these high-frequency relaxations is discussed in light of impedance spectroscopy, TGA, FTIR and XRD measurements.