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

Abstract

Crystallization studies at quiescent and shear states in isotactic polypropylene (iPP) containing nanostructured polyhedral oligomeric silsesquioxane (POSS) molecules were performed with in situ small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). DSC was used to characterize the quiescent crystallization behavior. It was observed that the addition 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 significantly reduced at a POSS concentration of 30 wt %, which suggests a retarded growth mechanism due to the molecular dispersion of POSS in the matrix. In situ SAXS was used to study the behavior of shear-induced crystallization at temperatures of 140, 145, and 150°C in samples with POSS concentrations of 10, 20, and 30 wt %. The SAXS patterns showed scattering maxima along the shear direction, which corresponded to a lamellar structure developed perpendicularly to the flow direction. The crystallization half-time was calculated from the total scattered intensity of the SAXS image. The oriented fraction, defined as the fraction of scattered intensity from the oriented component to the total scattered intensity, was also calculated. The addition of POSS significantly increased the crystallization rate during shear compared with the rate for the neat polymer without POSS. We postulate that although POSS crystals have a limited role in shear-induced crystallization, 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 during shear. The addition of POSS decreased the average long-period value of crystallized iPP after shear, which indicates that iPP nuclei are probably initiated in large numbers near molecularly dispersed POSS molecules.

Effect of Silica Nanoparticle on the Quiescent and the Shear Induced Crystallization Behaviors of Poly(trimethylene terephthalate)

“…Moreover, they have revealed that higher POSS concentrations have resulted in faster crystallization rates during shear. 28 Their results are similar with those of ARES in this study. To estimate the crystallization process of pure PTT and the nanocomposites variation of G 0 with time at frequencies of 1 and 3 rad/s at 202 C was measured by a rotational rheometer, ARES.…”

“…Fu et al have developed in situ SAXS technique with synchrotron radiation that is suitable for monitoring morphological changes during shear induced crystallization in real time. 28 However, shear induced crystallization behavior also can be examined by using some rheological instruments. 29,30 Isothermal crystallization experiment on the PTT and the nanocomposites is carried out by some rotational type rheometers to trace the shear induced crystallization behavior of the samples.…”

Polyethylene/polyhedral oligomeric silsesquioxanes composites: Electrical insulation for high voltage power cables

“…In PP-based systems, increased Tons and Tc values and accelerated crystallization resulted from the addition of 3-30 wt% OmPOSS. On the other hand, decreased Tons and Tc values and retarded crystallization were found with the addition of 3-6 wt% OibPOSS and 3-10 wt% IoPOSS (Fu et al, 2001;Pracella et al, 2006). These results show that the influence of POSS on the crystallization behavior of polymers depends, at least to a certain degree, on the type of polymer concerned, as well as on the type and concentration of POSS.…”

Morphology and crystallization kinetics of polyethylene/long alkyl-chain substituted Polyhedral Oligomeric Silsesquioxanes (POSS) nanocomposite blends: A SAXS/WAXS study

“…In Figure 3B SAXS data for pure PE and the blends is given. For pure PE a scattering maximum at ~200 Å, representing the long period (L p ) is observed, which relates to the overall average length-scale of crystalline and amorphous layer thicknesses [11,38,45]. From the SAXS data for the two blends PE-C 8 and PE-C 12 , the 1 st order peaks at 26 Å and 36 Å respectively are visible indicating that the pure POSS crystals coexist in the PE matrix.…”

“…modulus, strength, glass transition temperature, mechanical performance and thermal stability [5,6,[11][12][13][14][15]. Changes in the physical properties of the host polymer are usually attributed to the molecular aggregation of the POSS molecules themselves which then serve to reinforce the polymer network by acting as nucleating agents or plasticisers, which can influence the final crystalline morphology by either retarding or increasing the host polymer's crystallisation process.…”