Polyethylene Microtubes from Silica Fiber‐based Polyethylene Composites Synthesized by an In Situ Catalytic Method

Abstract

Most composite materials that consist of a thermoplastic matrix and a particulate filler are prepared by mechanical mixing of the components above the melting temperature of the polymer. Both organic (e.g., poly-p-(phenylene terephthalamide): PpPTA, polyethylene: PE) and inorganic (carbon, glass) fibers are extensively used as fillers for polymer-based composite materials; in particular, silica fibers combine the advantages of high tensile and compressive strengths with low cost, when compared to other competing fibers. PE is one of the polymers most often used as a matrix because of its chemical resistance to solvents, low friction coefficient, hydrophobicity, and biocompatibility. However, the mechanical mixing of components does not allow a fine and homogeneous dispersion of the filler, and the resulting material is usually characterized by poor mechanical properties. In order to improve the properties of the resulting composite, a direct connection of the polymer with the surface of the filler is highly desirable. The polymerization filling technique (PFT) has proved very efficient in this regard. This method, initially investigated in Ziegler–Natta polymerization and more recently developed for metallocene catalysis applied to a broad range of microfillers (e.g., kaolin, silica, carbon nanotubes, and graphite), consists of anchoring a polymerization catalyst directly onto the filler surface, followed by its activation with a cocatalyst. The polymer chains grow directly from the catalytic species upon in situ addition of the monomer, and for this reason the polymer is formed in very close contact with the filler surface. Such a technique leads to a much more uniform filler distribution and a considerably enhanced interfacial adhesion, even at high filler content, compared to conventional mechanical techniques. In this work we present an original method to realize silicamicrofiber-based PE composites, which relies upon the in situ polymerization of ethylene from the gas phase, catalyzed by active Cr species anchored onto the surface of the silica microfibers. As a result, silica microfibers are homogeneously coated and interconnected by the PE chains grown in situ, forming a promising composite material without the need for post-synthesis processing. With respect to the PFT technique mentioned above, this “microprocessing” method is much cleaner because it does not require the use of cocatalysts and solvents. In particular, we demonstrate that by properly tuning the reaction conditions, it is possible to move from wellisolated to exclusively interconnected homogeneously PEcoated silica fibers, which finally constitute a real microcomposite material. The possibility of realizing a well-defined PE coating on silica microfibers can be used in an inverse way, i.e., by considering the silica microfibers as templates for the realization of PE microtubes. Here, we demonstrate that removal of the internal silica core from the composite material indeed results in the formation of PE microtubes. These preliminary results open the way towards the realization of PEbased microand nanodevices by means of a simple and tunable catalytic approach. In this sense, this work presents an example of how new materials with a controlled morphology can result from combining the rational design of polymerization catalysts with nanotechnology. The silica fibers used as fillers (shown in Scheme 1a) are characterized by a Brunauer–Emmet–Teller (BET) surface area of 3.6 m g (from N2 adsorption experiments), a diameter in the 5–10 lm range, and a relatively rough surface morphology (see the scanning electron microscopy (SEM) images reported in Fig. 1a). The fibers were, in accordance with the standard procedure normally used for Phillips-type systems, impregnated with a solution of H2CrO4, dried at room-temperature (RT), and then calcined in O2 at 650 °C, resulting in anchored Cr species (step a→b in Scheme 1). The in situ polymerization reaction was conducted by dosing C2H4 on a CO prereduced system (Scheme 1c) in static conditions at 350 °C (ethylene vapor pressure P(C2H4) = 500 Torr), for a time ranging in the 5–12 h interval (steps c→d and c→e in Scheme 1). It is worth mentioning that the system that results from the impregnation of the silica fibers with H2CrO4 (Scheme 1b) can be considered as a model version of the Phillips catalyst because the crystalline nature of the fibers and their low surface area should, in principle, allow the grafting of well-isolated and homogeneous Cr species. The presence of a PE coating and the morphology of the resulting material were investigated by means of microRaman spectroscopy and SEM, respectively. SEM images of the silica microfibers before (Scheme 1a) and after 5 h of in situ C2H4 C O M M U N IC A IO N