Effects of polyhedral oligomeric silsesquioxane reinforced polypropylene (PP) nanocomposites on the thermal, mechanical, and optical properties of PP

Polymeric hybrids and nanocomposites containing polyhedral oligomeric silsesquioxanes (POSS) - general formula (RSiO1.5)n where R is an organic group – as dispersed inorganic phase offer the opportunity to design advanced materials with enhanced thermal and mechanical performances. The incorporation of POSS or its derivatives into polymers can lead to some improved properties, such as the increase in use temperature, oxidation resistance, surface hardening and mechanical strengths, as well as reductions in flammability, heat evolution, and viscosity during processing. It has been pointed out that the morphology, the thermal stability and tensile properties of thermoplastics (polyolefins, polyesters, polyamides, etc.) containing POSS can be largely affected by the chemical structure, concentration and dispersion degree of POSS nanoparticles. It is possible to obtain dispersion of POSS particles to the scale of individual molecules with size approximately 1-2 nm. In view of this, POSS molecules are nearly equivalent in size to most polymer segments and, therefore, potentially can exert dramatic reinforcement effects. In contrast to other nanofillers, such as layered silicates which are platy and carbon nanotubes which are cylindrical, the shape of the POSS molecules is isotropic. In addition, POSS molecules can be easily functionalized by chemically altering the R groups. This presents opportunities for copolymerization or grafting reactions with functional groups on polymer chains and for defining and controlling the nature of interactions between POSS molecules and the polymer chains. Polypropylene (PP) is a versatile member of polyolefins and this thermoplastic polymer is used in a wide variety of applications including packaging, textiles, stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers and automotive components. PP has been used as the matrix material in a commercial form because of its many attractive properties, such as low weight, relatively low cost, excellent heat distortion temperature (above 100 °C) and its recycling ability. But its toughness, strength and stiffness are not sufficient for applications as an engineering plastic. To reinforce PP, polymer nanocomposite technique by adding rigid polymers, is used. In this study, 1, 3, 5, 8, 10 wt-% POSS containing PP nanocomposites PP/POSS nanocomposites (PP/POSS NC) were prepared by melt blending via extrusion. Methyl-POSS reinforced polypropylene (PP) nanocomposite was used as POSS carrier material (or as a masterbatch) and the effects of methyl-polyhedral oligomeric silsesquioxanes (methyl-POSS) on the rheological, and thermal behavior of polypropylene (PP) were investigated. Rheological studies of PP/POSS NC pellets were operated by using parallel plate rheometry at three different temperatures. For all samples, it can be observed that complex viscosity (η*) decreases with increasing frequency, both storage modulus (G') and loss modulus (G'') progressively increase with increasing frequency. Complex viscosity showed large increase with reduction of temperature at high frequency. The increase of frequency causes the restriction of chain movements and a stiffer behavior was observed. Thermal behaviors of nanocomposites were characterized by DSC and TGA. The fusion enthalpy of the nanocomposites (obtained by peak integration) decreases with increasing of POSS content. 5% and 10% weight loss temperatures obtained from TGA curves. It was observed that increasing of POSS content increased these degredation temperatures and also the thermal stability of nanocomposites. The prepared PP/POSS nanocomposites and PP pellets were also subjected to melt spinning process by using capillary rheometer with 6 take up speed and 3 plunger speed to produce 97 fiber samples. Draw down ratio (DDR=Ddie2/Dfiber2) of fibers were calculated and the effects of draw down ratio on the strength, and polymer chain orientation were investigated by mechanically, optically and morphologically. Mechanical properties of prepared fiber samples measured by universal testing machine. It was observed that POSS composition and DDR of the nanocomposites had a pronounced effect on the tensile properties of these materials. Tensile data shows that the modulus increase with increasing of take up speed and decreasing by increasing of POSS content. Addionally, the maximum load, tensile strain at maximum load, and tensile strain at break decreases with increasing of take up speed. Morphological characterizations were made by using polarized optical microscope (POM) and small angle X-ray scattering (SAXS). Birefringence data of fibers calculated by using POM images and DDR of samples. For all fibers, increasing of DDR lead to increase birefringence. The other result was that increasing of POSS content caused the decrease of birefringence values with the comparison of neat PP. SAXS pattern clearly showed that isotropy of fibers increases with increasing of POSS content. On the other side, increasing of take up speed increases anisotropy. Increasing of take up speed also increased the fiber orientation. Furthermore, it was observed that high POSS content fibers have more isotropic structure. In consideration of this observation it is clearly seen that POSS content effects the morphology of fibers significantly.