Wen Hong Ruan

A strategy for significant improvement of strength of semi-crystalline polymers with the aid of nanoparticles

With the addition of 1 wt% nanosilica, oriented polypropylene (PP) shows ultra-high tensile strength at break (≈ 320 MPa, stronger than unidirectional glass fiber (∼60 wt%)/PP composites and a low-carbon steel) through solid-state drawing strategy. When nano-SiO2 is present in the drawn PP, the aligned macromolecular chains in amorphous regions can be tied by the well distributed nanofillers to share the stress together. Above the critical content of nanoparticles or drawing ratio, the nanoparticles form a percolated network throughout the matrix, facilitating stress transfer in the amorphous phases during tensile test. Additionally, the nanoparticles favor microfibrillation of the polymer matrix mainly constituted by the crystalline phases. As a result, the high strength covalent bonds of macromolecules in both the amorphous and crystalline phases are brought into full play. Although successful application of adding nanofillers to strengthen polymers is widely spread in rubbers and gels, the increase in strength of semi-crystalline polymers by low aspect ratio nano-inclusion is insignificant up to now. The work is believed to open an avenue for reinforcing semi-crystalline polymers by nanoparticles.

Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer

Nanocomposites consisting of ethylene-propylene block copolymer filled with nano-silica (pre-treated by ball milling aided graft polymerisation) were prepared by a conventional compounding technique. The mechanical performance of the nanocomposites and the morphological changes induced by the addition of the nanoparticles were investigated. It was confirmed that the copolymer chains were chemically bonded to the silica particles during mechanochemical grafting in the ball mill. Morphology observations revealed that strong interfacial interaction between the grafting polymer (i.e., poly(butyl acrylate)) and the matrix (i.e., ethylene-propylene block copolymer) is critical for bringing the reinforcing effect of the nanoparticles into play. Owing to the enhanced interfacial interaction, the grafted nanoparticles exhibited a nucleating effect and improved the crystallinity of the polymer matrix. In addition, the particles also had a toughening effect on the amorphous polypropylene phase because of entanglements between the grafting polymer and the matrix. Insufficient interaction between the nanoparticles and ethylene-propylene rubber phase of the copolymer matrix actually introduces restraints. As a result, the tensile strength and modulus of the nanocomposites can be significantly increased by using low loadings of the treated nanoparticles. The decrease in the notched Charpy impact strength was insignificant in comparison to that of conventional micron-scale inorganic particles filled composites. The technical route proposed is therefore feasible for fabricating polymer composites with inorganic nanoparticles.

Improvement of creep resistance of polytetrafluoroethylene films by nano-inclusions

To improve creep resistance of directional polytetrafluoroethylene (PTFE) films, epoxy grafted nano-SiO2 is mixed with PTFE powder before sintering and calender rolling. The aligned macromolecular chains (especially those in amorphous region) of the composite films can be bundled up by the nanoparticles to share the applied stress together. In addition, incorporation of silica nanoparticles increases crystallinity of PTFE and favors microfibrillation of PTFE in the course of large deformation. As result, PTFE films exhibit lower creep strain and creep rate, and higher tensile strength and hardness. The work is believed to open an avenue for manufacturing high performance fluoropolymers by nano-inclusions.

Performance Improvement of Nano-silica/Polypropylene Composites through in-situ Graft Modification of Nanoparticles during Melt Compounding

Abstract To prepare inorganic nanoparticles filled polymer composites, reactive monomer was added to the ingredients prior to manufacturing. The results showed that in-situ graft polymerization of butyl acrylate onto nano-silica occurred during melt compounding with polypropylene matrix. As a result, dispersion of the grafted nanoparticles in the polymer became much more homogeneous than in the case of untreated version, and filler/matrix interaction was enhanced due to the intimate adhesion among the components. Both tensile performance and impact toughness of the composites were improved at rather low filler loading. Compared to the twostep approach developed in our lab, by which nanoparticles are treated by graft polymerization firstly and then mixed with polymer melt, the current one-step method is simpler and able to provide the composites with much higher notched impact resistance.

Preparation of Nano-Silica/Polypropylene Composites Using Reactive Compatibilization

Nano-sized silica particles were pre-grafted with poly(glycidyl methacrylate) (PGMA) by solution free-radical polymerization. When these grafted silica nanoparticles were melt compounded with polypropylene (PP), reactive compatibilization effect was perceived due to the chemical bonding between the grafted PGMA and amine functionalized PP, which led to a significant increase of tensile strength and notch impact strength of PP at rather low filler content. Accordingly, compatibility of each kind of the functionalized PP with grafted SiO2 nanoparticles was evaluated through investigating the mechanical properties, crystallization behavior and rheological performance of the composites. The results show that the reactive compatibilization is capable of providing stronger interfacial adhesion.

Bridging Redox Species-Coated Graphene Oxide Sheets to Electrode for Extending Battery Life Using Nanocomposite Electrolyte

Substituting conventional electrolyte for redox electrolyte has provided a new intriguing method for extending battery life. The efficiency of utilizing the contained redox species (RS) in the redox electrolyte can benefit from increasing the specific surface area of battery electrodes from the electrode side of the electrode-electrolyte interface, but is not limited to that. Herein, a new strategy using nanocomposite electrolyte is proposed to enlarge the interface with the aid of nanoinclusions from the electrolyte side. To do this, graphene oxide (GO) sheets are first dispersed in the electrolyte solution of tungstosilicic salt/lithium sulfate/poly(vinyl alcohol) (SiWLi/Li2SO4/PVA), and then the sheets are bridged to electrode, after casting and evaporating the solution on the electrode surface. By applying in situ conductive atomic force microscopy and Raman spectra, it is confirmed that the GO sheets doped with RS of SiWLi/Li2SO4 can be bridged and electrically reduced as an extended electrode-electrolyte interface. As a result, the RS-coated GO sheets bridged to LiTi2(PO4)3//LiMn2O4 battery electrodes are found to deliver extra energy capacity (∼30 mAh/g) with excellent electrochemical cycling stability, which successfully extends the battery life by over 50%.

Graft Polymerization of p-Vinylphenylsulfonylhydrazide onto Nano-Silica and its Effect on Dispersion of the Nanoparticles in Polymer Matrix

To promote dispersion of nano-silica in polypropylene (PP), a polymerizable foaming agent p-vinylphenylsulfonylhydrazide was synthesized and grafted onto the nanoparticles via free-radical polymerization. The results of thermogravimetric analysis (TGA) showed that the sulfonyl hydrazide groups of poly(p-vinylphenylsulfonylhydrazide) acquired the desired thermal decomposition ability, which might be related to their internal oxidation-reduction. Electron microscopy observation indicated that the grafted nanoparticles exhibit greatly improved dispersion in PP owing to the fact that the sulfonyl hydrazine groups on the grafted polymer inside the agglomerates decomposed like blowing agent to form polymer bubbles, leading to rapid inflation of the surrounding matrix that pulled apart the agglomerated nanoparticles during melting mixing.

In Situ Crosslinking Induced Structure Development and Mechanical Properties of Nano-Silica/Polypropylene Composites

In our previous works, a double percolation mechanism of stress volumes was proposed to explain the special effects generated by small amounts of grafted nanoparticles. Accordingly, it is inferred that strengthening nanoparticle agglomerates and enhancing nanoparticles/polymer matrix interfacial interaction are the key issues to improve mechanical performance of the matrix polymer. To confirm this idea, in-situ crosslinking was adopted to prepare nanocomposites by adding reactive monomers and crosslinking agents during melt compounding of nano-silica with polypropylene (PP). It was found that the grafted polymer chains were successfully crosslinked and chemically bonded to the nano-silica forming crosslinked networks. Meanwhile, matrix molecular chains penetrated through the networks to establish the so-called semi-IPN structure that interconnected nanoparticles by the networks and improved filler/matrix interfacial interaction. As a result, the tiny nanoparticles were well distributed in the matrix and the toughening and reinforcing effects of the nanoparticles on the matrix were brought into play at rather low filler loading, as evidenced by mechanical performance tests. Besides, β-crystal was detected in the nanocomposites experienced in-situ crosslinking reaction.

Preparation and Properties of Nano-Silica Filled Self-Reinforced Polypropylene

To improve the properties of polypropylene (PP), a new route that combines nanoparticles filling with self-reinforced technique was applied in this work. That is, nano-silica particles were firstly modified by graft polymerization to increase interfacial interaction between nanoparticles and matrix. Then the grafted nanoparticles were melt-compounded with PP producing composites sheets, and the sheets were stretched under a temperature slight lower than the melt point of PP at a constant velocity. Finally, the stretched sheets were film-stacked with random PP copolymer by a special designed mold and were hot pressd at different processing temperature (T=150-175°C) and holding pressure (2.0-5.0MPa) under constant holding time of 5min. The resultant self-reinforced nanocomposite are much stronger and stiffer than the unfilled polymer as characterized by mechanical test. The results show that the optimum processing conditions for hot consolidation are 160°C and 2.5MPa. Addition of nanoparticles increases crystallinity of PP, and induces the formation of craze and cause much more surrounding matrix polymer to involve in large-scale plastic deformation, which might ensure an overall improvement of mechanical properties.

Drawing-Induced Dispersion of Nanoparticles and its Effect on Structure and Properties of Thermoplastic Nanocomposites

To prepare polymer nanocomposites with enhanced performance, well dispersion of nanoparticles in matrices is necessary. In this work a new route that combines graft pre-treatment and drawing technique with melt mixing was applied. That is, nano-SiO2 particles were firstly modified by graft polymerization and then the grafted nanoparticles were melt-compounded with polypropylene (PP) producing composites filaments via drawing. Finally, the filaments were injection molded into bulk materials. The resultant PP based nanocomposites are much tougher than the unfilled polymer as characterized by either static or dynamic test, besides showing a simultaneous increase in strength and stiffness. Morphology studies indicated that drawing induced extension and separation of the grafted nano-silica agglomerates in PP matrix during making the filaments are frozen to a certain extent after nanocomposites manufacturing. In this way, the nanoparticles are well distributed in the matrix and correlated with each other throughout the entire composites, which might ensure an overall improvement of mechanical properties. Besides, β-crystal of PP developed in the drawing process can be retained in the nanocomposites, which also contributes to the toughening of PP. In view of these, the proposed drawing aided dispersion of nanoparticles might also be applicable to the preparation of other nanoparticles/polymer composites.