Biopolymer and biosurfactant-graft-calcium sulfate/polystyrene nanocomposites: Thermophysical, mechanical and biodegradation studies

Nanocomposites of polystyrene and CaSO4 nanorods with different loadings have been prepared by mixing pre-synthesized biopolymer (gum ghatti) and biosurfactant (rhamnolipid and surfactin) grafted CaSO4 nanorods into commercial polystyrene with the combination of solvent blending and extrusion molding from masterbatch. In this work, it is shown that surfactin-graft-CaSO4, (i) enhanced the dispersion of the nanorods within the matrix, (ii) increased the thermal stability and thermomechanical properties indicating good reinforcement, and (iii) resulted in higher biodegradation efficiency as compared to the nanocomposites filled with rhamnolipid-graft-CaSO4 or gum ghatti-graft-CaSO4. Biodegradation of polystyrene nanocomposites was assessed using a microbial inoculum (Rhodococcus pyridinivorans NT2) in a 28-day biodegradation assay. The viability of the isolate growing on the film surface was confirmed using a triphenyltetrazolium chloride reduction test. The viability was also correlated with a concomitant increase in the protein density of the biomass. The mechanical properties of nanocomposites were significantly reduced after the microbial degradation. Biodegradation of the materials was further confirmed by the decreased molecular weight through gel permeation chromatography, and changes in the chemical structure as verified by Fourier transform infrared spectroscopy. This was supported by the degraded surface of the films observed under filed emission scanning electron microscopy (FESEM). The effects of biodegradation on the thermal properties were studied through the differential scanning calorimetry and thermogravimetry. To improve our knowledge of polystyrene biodegradation, we used Raman spectroscopy to identify patterns of polystyrene biodegradation. In addition, the intermediates formed in the degradation process were monitored by Gas Chromatography-Mass Spectrometry and the biodegradation rate of the materials was determined by measuring the carbon dioxide (CO2) evolution. These results indicate that surface grafting of nano-CaSO4 with surfactin leads to optimal degradation of the nanocomposites.