Linear Sulfur-Nylon Composites: Structure, Morphology, and Antibacterial Activity

Stabilization of sulfur in its linear form is highly challenging, but a recently developed inverse vulcanization method succeeded by reacting polysulfur terminals groups with organic cross-linkers. In the present work, linear sulfur chains were stabilized in a nylon matrix by forming an amine-polysulfur complex. Composites were prepared by coextruding nylon with various amount of sulfur (2.5-30 wt %) under an inert gas environment. Polysulfur chains produced under extrusion conditions react with terminal amine groups of nylon in the molten state and form the amine-polysulfur complex. The color of these composites changes from translucent to dark brown, which is due to formation of the sulfur-amide complex based on Fourier transform infrared spectroscopy. Furthermore, the presence of NH3+ and sulfur anion peaks in XPS spectra confirms the formation of this complex. Disappearance of the sulfur crystalline peak and absence of sulfur melt transition in X-ray diffraction and differential scanning calorimetry, respectively, suggests the conversion of crystalline sulfur into randomly arranged amorphous polysulfur. Scanning electron microscopy and transmission electron microscopy also show the homogeneous dispersion of sulfur due to the strong interaction with nylon. Dynamic mechanical thermal analysis and use of a universal testing machine showed reduced toughness with the increasing sulfur loading due to the interference of sulfur with the hydrogen bonding of nylon. The presence of sulfur in the composites exhibited excellent antimicrobial property. The nanocomposites can be processed into films, molds, and threads without any change in morphology. The one step preparation and processing of sulfur nanocomposites can also be extended to the other thermoplastic polymers, and the method can be easily scaled up to bulk quantity.