Crystallization studies of isotactic polypropylene containing nanostructured polyhedral oligomeric silsesquioxane molecules under quiescent and shearconditions

Crystallization studies at quiescent and shear states in isotactic polyprop ylene (iPP) containing nanostructured polyhedral oligomeric silsesquioxane (POSS) molecules were performed with in situ small-angle X-ray scattering ( SAXS) and differential scanning calorimetry (I)SC). DSC was used to charact erize the quiescent crystallization behavior. It was observed that the addi tion of POSS molecules increased the crystallization rate of iPP under both isothermal and nonisothermal conditions, which suggests that POSS crystals act as nucleating agents. Furthermore, the crystallization rate was signif icantly reduced at a POSS concentration of 30 wt %, which suggests a retard ed growth mechanism due to the molecular dispersion of POSS in the matrix. In situ SAXS was used to study the behavior of shear-induced crystallizatio n at temperatures of 140, 145, and 150 degreesC in samples with POSS concen trations of 10, 20, and 30 wt %. The SAXS patterns showed scattering maxima along the shear direction, which corresponded to a lamellar structure deve loped perpendicularly to the flow direction. The crystallization half-time was calculated from the total scattered intensity of the SAXS image. The or iented fraction, defined as the fraction of scattered intensity from the or iented component to the total scattered intensity, was also calculated. The addition of POSS significantly increased the crystallization rate during s hear compared with the rate for the neat polymer without POSS. We postulate that although POSS crystals have a limited role in shear-induced crystalli zation, molecularly dispersed POSS molecules behave as weak crosslinkers in polymer melts and increase the relaxation time of iPP chains after shear. Therefore, the overall orientation of the polymer chains is improved and a faster crystallization rate is obtained with the addition of POSS. Moreover , higher POSS concentrations resulted in faster crystallization rates durin g shear. The addition of POSS decreased the average long-period value of cr ystallized iPP after shear, which indicates that iPP nuclei are probably in itiated in large numbers near molecularly dispersed POSS molecules. (C) 200 1 John Wiley & Sons, Inc.