Influence of nanoparticles on the rheological behaviour and initial stages of crystal growth in linear polyethylene

We demonstrate the role of broad molecular weight distribution (MWD) polyethylene (PE), in the presence of nanoparticles of different aspect ratios and binding efficiency with the polymer, in the formation of shish-kebab structures under a shear protocol using time- resolved small-angle X-ray scattering (SAXS). The results indicate a scattered intensity in the form of streaks at the equator while maxima in the meridian confirm the presence of an oriented structure in the polymer. The SAXS facilitated the probing of the steady growth of the obtained shish -kebab structures at an isothermal crystallization temperature (136 °C). The study reveals the influence of nanoparticles (single walled carbon nanotubes (SWNTs) and zirconia) in chain orientation. The presence of nanoparticles promotes the high degree of orientation, where shish is formed along the flow direction and kebab perpendicular to it. A higher degree of chain orientation is observed in the presence of SWNTs compared to zirconia nanoparticles. The SWNTs present in a small concentration (< 0.6 wt.-%) are aligned in the flow direction, which leads to an increase in shish length as estimated from Ruland's streak analysis. The stable shishes in the early stages of crystallization suppress the nucleation barrier for further crystallization. Compared to the polymer without nanoparticles the shish length increases in the presence of zirconia, however, the increase in shish length is much more pronounced in the presence of SWNTs compared to zirconia nanoparticles. The nano- particles favor the orientation fraction as deduced from the integrated intensity of scattering at the equator and meridian in the patterns. Absence of a plateau in the low frequency region of the polymer-SWNT composites suggests the non-existence of network formation. Nevertheless, comparing the storage modulus at two different temperatures (142 and 160 °C), suggests a strong temperature dependence and difference in adsorption energy of the two nanoparticles. © 2009 WILY-VCH Verlag GmbH & Co. KGaA.