pubs.acs.org/Macromolecules Published on Web 06/14/2010 r 2010 American Chemical Society Macromolecules 2010, 43, 6067–6074 6067 DOI: 10.1021/ma1007573 Morphology of Injection-Molded Isotactic Polypropylene/Silica Composites Prepared via in-Situ Sol-Gel Technology Xiaomin Zhu,* ,† Claudiu Melian, † Qizheng Dou, † Karin Peter, † Dan E. Demco, † Martin M€ oller, † Denis V. Anokhin, ‡ Jean-Marc Le Meins, ‡ and Dimitri A. Ivanov* ,‡ † DWI an der RWTH Aachen e.V. und Institut f € ur Technische und Makromolekulare Chemie der RWTH Aachen, Pauwelsstr. 8, D-52056 Aachen, Germany, and ‡ Institut de Sciences des Mat eriaux de Mulhouse, IS2M CNRS LRC 7228, 15 rue Jean Starcky, F-68057 Mulhouse, France Received April 8, 2010; Revised Manuscript Received June 4, 2010 ABSTRACT: We report on the semicrystalline morphology of injection-molded isotactic polypropylene (iPP)/silica composites prepared via in situ sol-gel technology using hyperbranched polyethoxysiloxane as a silica precursor. The microstructural analysis has been carried out with a combination of small- and wide- angle X-ray scattering and 1 H solid-state NMR spectroscopy. The in situ formed silica particles significantly alter the semicrystalline morphology of iPP by improving orientation of the “mother” crystals in the sample core simultaneously with an increase of the “daughter” crystal fraction. Moreover the particles induce formation of thicker crystalline domains and growth of β-crystals in the sample core. The observed morphological modifications can be accounted for by a 2-fold effect of the silica particles, i.e., their action as a nucleating agent of the β-phase, on the one hand, and as a lubrificant reducing the polymer melt viscosity, on the other. Proton spin-diffusion experiment using a double-quantum filter was found suitable for the measurement of the chain mobility gradient in the interface region. The results obtained indicate enhanced polymer chain mobility in the amorphous region in the presence of the silica particles. Finally, the morphology and chain dynamics of the iPP/silica composites are correlated to mechanical properties of the injection-molded samples. 1. Introduction Incorporation of inorganic micro- and nanoparticles into poly- mer matrices is a well-established approach for improvement of material properties 1-3 such as reinforcement of thermoplastic and thermosetting polymers, enhancement of thermal stability and fire resistance, and increase of barrier properties. As de- scribed by Einstein’s theory, the viscosity of a composite increases with volume fraction of the filler. 4 However, it has been recently demonstrated that ultrasmall nanoparticles homogeneously dis- tributed in the polymer matrix can effectively reduce the poly- mer melt viscosity. 5-7 The viscosity decrease was attributed in this case to increased free volume induced by the nanoparticles addition. By measuring the diffusion coefficient of nanoparticles in the polymer melt using dynamic X-ray scattering, it has been concluded that the nanoparticles diffuse faster than the entangled mesh and so do not participate in its dynamics; they merely occupy space to provide a constraint release mechanism via dilution. 8 The silica nanoparticles are among the most widely used additives and fillers for polymer materials. Because of the high hydrophilicity of silica and hydrophobic nature of most organic polymers, it is a big challenge to produce polymer/silica compo- sites with homogeneous particle distribution. The existing recipes for the polymer/silica composite preparation such as mixing in solution or melt, as well as solution sol-gel technology, either require a large amount of solvent or lead to significant par- ticle agglomeration. Recently, we proposed a new route for preparation of polymer/silica composites, which is compatible with industrial fabrication. 9 The process is based on a solvent-free in situ sol-gel technology employing a nonvolatile silica precur- sor polymer: hyperbranched polyethoxysiloxane (PEOS). 10,11 By following this route, the isotactic polypropylene (iPP)/silica composites were prepared in a twin-screw mini-extruder. Accord- ing to transmission electron microscopy (TEM) data, for small amounts of PEOS (e5 wt %) the procedure yields iPP/silica composites with particles smaller than 100 nm. For higher PEOS loadings, the particle size increases up to several micrometers. Importantly, the in situ generated nano- or microparticles are homogeneously dispersed within the polymer matrix. 9 Generally, injection molding is one of the most common processing techniques for the semicrystalline iPP. 12 In the course of injection molding, iPP undergoes flow-induced crystallization. The presence of additives can have a strong influence on this process and eventually affect the final morphology and properties of the resulting articles. 13 Recently, a combination of differential scanning calorimetry (DSC), small- and wide-angle X-ray scat- tering (SAXS/WAXS), and 1 H time-domain and frequency- domain nuclear magnetic resonance (NMR) measurements was employed to determine the rigid (crystalline), semirigid (interface), and mobile (amorphous) fractions in semicrystalline iPP as a function of annealing time and temperature. 14 The data on the iPP microstructure provided by proton spin-diffusion NMR 15 and SAXS allow to address the effect of annealing, 14 uniaxial stretching, and aging. 16,17 Solid-state NMR is a powerful tech- nique for investigation of morphology and chain dynamics of polymer materials. 15,18-20 The phase composition and size of different structural domains can be conventionally determined based on the wide-line NMR spectra and spin-diffusion measure- ments, respectively. In comparison to X-ray scattering, NMR can *To whom correspondence should be addressed: tel þ49-241-80- 23341, fax þ49-241-80-23301, e-mail zhu@dwi.rwth-aachen.de (X.Z.); tel þ33-389608807, fax þ33-389608799, e-mail dimitri.ivanov@uha.fr (D.A.I.).