The effects of reprocessing cycles on the structure and properties of polystyrene/Cloisite 15A nanocomposites

This work focuses on the effect of repeated processing cycles on the structure and properties of PS/Cloisite 15A (5% w/w) nanocomposite through a series of eight extrusion cycles using a co-rotating intermeshing twin screw extruder. For comparison, neat PS was also reprocessed as a reference material. Changes in the nanostructure were studied by wide angle X-ray scattering (WAXS) and rheological measurements. Molecular weight change was determined by size exclusion chromatography (SEC). Chemical changes were studied by Fourier transform infrared spectroscopy (FT-IR). Furthermore, the mechanical properties were investigated by a nanoindentation test method, whereas colorimeter techniques were used to analyze yellowness during reprocessing. The results of the WAXS analysis show that the d(001) peak of the PS nanocomposite shifts to lower angles with increasing the number of reprocessing cycles indicating intercalated-delaminated structure. The rheological data reveal an enhancement of G' in the melted state with reduction of the terminal slope of the nanocomposite samples over the PS matrix, thus confirming the occurrence of strong interaction between the silicate sheets and the polymer and their tendency to form a three dimensional superstructure. Further, in contrast to neat PS which undergoes a large reduction of molecular weight with increasing the number of cycles due to chain scission: in PS nanocomposites, the SEC data shows however that the degradation involves two mechanisms: chain scission as the most prevalent one, with the probability of occurrence of some crosslinking after 8 reprocessing cycles as demonstrated by the increase in both (M) over bar (w) and (M) over bar (n). Repeated reprocessing globally yield colour changes in both the neat PS and PS nanocomposites whereas, the chemical structure and nanomechanical properties remain stable during reprocessing for PS nanocomposite. (C) 2011 Elsevier Ltd. All rights reserved.