Carbon dots: Fluorescence active, covalently conjugated and strong reinforcing nanofiller for polymer latex

Abstract This paper reports a unique approach to the synthesis of rubber nanocomposites in which functionalized carbon dots (cdots) are covalently bonded to functionalized rubber latex. In this approach, the cdots are functionalized with amine (-NH2) groups that are capable of chemically binding with the carboxylated styrene butadiene rubber (XSBR) latex possessing acid (-COOH) groups. Pristine cdots are characterized using powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy and high resolution transmission electron microscopy (HRTEM). The -COOH groups of XSBR latex has been covalently appended with the -NH2 groups of cdots using N-hydroxysuccinimide (NHS) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) mediated amide coupling. FTIR spectroscopy and XPS endorse the covalent conjugation of cdots with XSBR latex. Remarkable increase in the maximum tensile stress and elongation at break of XSBR has been observed by addition of very less amount cdots. XSBR containing 0.8 parts per hundred of rubber (phr) of cdots shows a maximum tensile stress value which is around 235% higher in comparison to neat XSBR. Dynamic mechanical analysis (DMA) shows the reduction in the tan δ peak height and increase in the tan δ peak temperature of XSBR by incorporation of cdots. Also, XSBR containing 0.8 phr of cdots shows an elastic modulus (E’) value which is around 70% higher in comparison to neat XSBR. The maximum degradation temperature ( T max ) and the onset of degradation temperature ( T i ) of XSBR containing 1 phr of cdots is around 10 °C and 8 °C higher when compared to neat XSBR. The prominent improvement in the overall properties of XSBR latex by addition of cdots has been attributed to the enhanced cross-linking introduced by the covalently conjugated cdots in XSBR matrix. XSBR-cdots nanocomposite has also been found display good photoluminescence properties due to the presence of cdots, which are confirmed by UV–Vis spectroscopy and fluorescence spectroscopy studies. Morphology of XSBR-cdots nanocomposites has been studied using HRTEM and atomic force microscopy (AFM).