Isotactic Polypropylene-Exfoliated Graphene Nanoplatelet (iPP-xGnP TM ) nanocomposites were prepared through an in-situ polymerization technique and compared to analogous composites prepared by melt compounding. In-situ preparation of iPP-xGnP nanocomposites was accomplished via single site metallocene polymerization of propylene within a toluene dispersion of xGnP nanoparticles. The in-situ prepared nanocomposites were compared to analogous nanocomposites prepared by melt compounding of commercial Ziegler-Natta iPP with xGnP. Optical microscopy showed the in-situ prepared nanocomposites demonstrated poorer xGnP dispersion compared to composites prepared by melt compounding. All xGnP-reinforced nanocomposites demonstrated increased crystallization temperature, as well as increases in mechanical strength and modulus, relative to neat iPP. However, the non-linear mechanical properties were found to be influenced by the both the preparation method and nanoparticle loading. Nanocomposites prepared by in-situ polymerization generally demonstrated superior ductility and fracture toughness compared to composites prepared by melt compounding. The results are discussed with regard to the preparation technique and xGnP loading. IntroductionPolyolefin nanocomposites offer opportunities to improve the properties of polyolefins with relatively small amounts of reinforcement. Compared to traditional fiber-reinforced composites, nanocomposites only require small reinforcement concentrations (< 2 vol %) to create property improvements. Polyolefin nanocomposites have shown property improvements such as M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 mechanical reinforcement, controlled gas permeability, and increased electrical conductivity when compared to the neat polyolefin resins [1].Many researchers strive to improve the mechanical properties of polyolefins using nanoscale reinforcement in order to create new and economical materials. For example, interest in the automotive industry has been directed toward developing reinforced polyolefins to replace engineering thermoplastic and metallic automotive materials, enabling cost and weight savings [2]. Polyolefin nanocomposites are ideal materials for this application due to the availability of low cost nanoscale reinforcements and polyolefin resins.Recently, graphene nanoplatelets (GNPs) have been investigated as nanoreinforcements for polyolefins [3,4]. The production of GNPs can be achieved by the thermal exfoliation of mineral graphite. Most notably, Drzal et al. developed an efficient method to produce Exfoliated Graphene Nanoplatelets (xGnP TM ) using acid intercalation followed by microwave assisted exfoliation [5,6]. These nanoplatelets are ideal nanoscale reinforcements due to their high aspect ratio, surface area, stiffness, thermal conductivity, and nucleation efficiency for crystallization of polyolefins [3,4,7,8]. Typically, polyolefin-GNP nanocomposites demonstrate improved modulus, strength, and higher crystallization temperature, along with decreased st...

Herein, a new process referred to as melt-mastication (MM) is used for the first time and evaluated for dispersing nanoparticles in polymers. Compared to a conventional melt processing (CMP) technique, MM produces higher mixing torque and therefore shear in the melt during processing, resulting in the fragmentation of micrometer-scale agglomerates of exfoliated graphene nanoplatelets (xGnP) in polypropylene. The efficacy of MM compared to CMP is evaluated using quantitative stereological techniques. Stereology reveals a correlation between the steady state process torque and the spatial size distribution of agglomerates.Finally, a mechanism for agglomerate fragmentation is proposed and discussed with respect to the results.

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This article reviews recent changes in the design of HIV-I vaccine. The safety information for current vaccines has been established, and future ethical considerations are reviewed. Of recent significance, gp120 envelope vaccines are being combined with canarypox vectors in an effort to elicit a broad immune response. This will probably be the aim of future research for an ever-growing problem. A total of 10 143 AIDS cases have occurred in the USA in the year 2000.(1) The international burden is grimmer. For example, approximately 13% of South Africans between 20 and 64 years old are HIV positive. Predictions estimate this number to rise to almost 30% by 2010.(2) With many different HIV serotypes, a worldwide effective HIV vaccine is not in the near future.(3) However, the scientific community has bolstered its effort by strengthening resources and developing national and international collaboration groups focused in developing a safe and effective HIV vaccine.