The Nano Shapes of Things to Come

By Jan H. Schut

Nanomaterials are invisible by definition. The traditional nanofillers—nanoclays, nanosilicas, nanoparticles of metal oxides, and carbon nanotubes—are almost inconceivably small, but not necessarily chemically complex. This year, however, the Society of Plastics Engineer’s annual Nanocomposites Conference, held March 5-7 at Lehigh University in Bethlehem, Pa. (http://tinurl,com/polymer-nanocomposites-2012), will introduce a plastics audience to a brave new world of synthetic nanomaterials with almost inconceivably complex chemical structures.

“Biobased nanocomposites will be introduced, as well as a new session on polymer nanocomposites for drug delivery,” says technical program chairman Raymond Pearson, a professor at Lehigh. Presentations also include new test results and photo evidence for some unusual new thinking on the melt behavior and direct melt processing of nanocomposites.

THE COMPLEXITY OF SYNTHETIC NANOS

Nanoparticles are being synthetically combined with organic macromolecules into complicated and precise structures with some crazy shapes and potent and specific functions. One that will be presented is shaped like a hollow cage with tassels at the corners, another like a scorpion. A third would look like a vine growing around a stick if you could see it.

Janis Matisons, research scientist at Gelest Inc., Morrisville, Pa. (www.gelest.com) will present “POSS Nanomaterials: Synthesis and Tailored Materials Properties.” Specialty POSS materials (polyhedral oligosilsesquioxanes) are new for Gelest, a maker of silanes, silicones and metal oxides, so new that they aren’t even listed among Gelest’s new products yet. POSS materials are nano-scale cage-like structures that combine ceramic and polymer molecules, first characterized in the 1940s. A single POSS particle is a silica cube 1.5 nanometers across, on each corner of which is attached a silicon atom with 1.5 oxygen atoms and a reactive functional group.

“If you modify POSS with methacrylate molecules in each corner of the cage, and add 0.02% of that POSS material to PMMA, you raise the resulting polymer Tg by 40 degrees C from 92 to 140,” Gelest’s Matisons notes. The POSS material also crosslinks the filled PMMA into a thermoset, though the PMMA remains clear. POSS also reduces flammability, lowers density and viscosity, increases moisture resistance, and yet is invisible to analysis by standard FTIR spectroscopy.

Gelest’s first is a liquid crystal POSS commercialized nine months ago, which can be blended into thermoplastics or thermosets. Gelest is also developing a new POSS material with photo-chromic groups attached to each corner of the silica cage, which isn’t commercial yet. These materials have unique photo-chromic liquid crystalline behavior and can be blended into polymeric coatings and emulsions.

Kathryn Uhrich, chemistry professor at Rutgers University in New Brunswick, N.J. (www.rutgers.edu) will present “Nanoscale Assemblies in Drug Delivery,” with the latest in the ongoing work of her laboratories at Rutgers on nanoscale biocompatible polymers called amphiphilic macromolecules. The patent-applied-for macromolecules (U.S. Patent Applic. # 20120022159 Jan. 26, 2012) contain mucic acid (a multihydroxylated saccharide) for reactive sites, aliphatic chains of different lengths to control hydrophobicity and aggregation behavior, and methoxy-terminated poly(ethylene glycol). These scorpion-like macromolecules have a hydrophilic head and hydrophobic tail, allowing them to aggregate head first into a ball, which can hold drug molecules.

In collaboration with Prabhas Moghe in bioengineering at Rutgers, Uhrich discovered that negatively charged, or anionic, nanoparticles of these scorpion-like macromolecules interact with the bad cholesterol, LDL. These new biopolymers are proven useful in keeping LDL from forming atherosclerotic plaques on vascular walls, at least in rats. The nanopolymer doesn’t interact with the good cholesterol HDL. In current work the amphiphilic macromolecules are being coated onto stents to reduce inflammation and plaque build-up from the injury done by the stent.

Kathryn Uhrich at Rutgers is an inventor of these synthetic scorpion-like macromolecules with hydrophilic head and hydrophobic tail. They form balls which can hold drug molecules. Blended into biopolymers, they could coat stents and prevent atherosclerotic plaque on vascular walls.

Daniel Roxbury, a research assistant at Lehigh, will present “Sequence-Specific Interactions between DNA and Single-Walled Carbon Nanotubes,” based on his dissertation, presented this spring under professor Anand Jagota of Lehigh. Roxbury’s work uses a known phenomenon that a single strand of DNA will wrap itself around a single-walled carbon nanotube like a vine around a pole, thus forming a water-dispersible hybrid molecule. Roxbury tested the strength of the bond between certain DNA sequences and carbon nanotubes and found that some DNA sequences bond 20 times more strongly to the CNTs than others. “Investigated through molecular dynamics simulation, this difference is attributed to variations in the secondary structure of the adsorbed DNA,” Roxbury explains. “These are the first theoretical indications that DNA-based single-walled CNTs can be selective at a molecular level.” The object is to create functionalized CNTs that can enter cells of the body for gene therapy.

Multiple strands of DNA wrap around a carbon nanotube like vines on a stick. Daniel Roxbury’s dissertation at Lehigh, presented at the Nanocomposites conference, finds that some DNA bonds better to the CNT than others. The goal of this complex nanoparticle is gene therapy.

HOT AND COLD AT THE SAME TIME

In the latest thinking on how plastics melt, Richard Wool, professor of chemical engineering at the University of Delaware, in Newark (www.udel.edu), will present his recent theory of “Twinkling Fractal Nanoscale Applications to Biobased Materials” for the first time to a plastics audience. Wool first published his unusual theory of molecular behavior during glass transition three years ago. He describes a wild molecular dance that goes on in the Tg of amorphous polymers. It amounts to a different state of matter, which he calls a heterogeneous solid. In this heterogeneous solid state, solid clusters of molecules stand up like little fingers and vibrate wildly in pools of liquid molecules. The solid fingers give off vibrational energy as they “jump” into the liquid state. Using an Atomic Force Microscope, Wool has actually captured images that show this vibrating semi-solid fractal structure in polystyrene. The vibrational frequency and life of the solid clusters depends on their size in nanometers. His theory isn’t just entertaining—he compares watching the phenomenon to watching square dancing figures at a barn dance. It has practical applications. One will be to use it to predict how nano materials can raise the melt temperature and other properties of biopolymers.

This Atomic Force Microscope image of the “twinkling fractal structure” of PS at Tg of 15 C shows clusters of solid molecules vibrating wildly in a pool of liquid. Richard Wool of the Univ. of Delaware uses the theory to predict how nano materials will alter biopolymer properties.

Katsuyuki Wakabayashi, assistant professor of chemical engineering at Bucknell University, Lewisburg, Pa. (www.bucknell.edu) will present “Solid-State Shear Pulverization for Polymer Nanocomposites: A Chilled Twin-Screw Extrusion Process,”a potential new application for a cold twin-screw extrusion process, originally developed in Russia in the 1970s. Wakabayashi was a postdoctoral researcher under John Torkelson, a professor at Northwestern University, Evanston, Ill. (www.northwestern.edu), who collaborated with Klementina Khait, a retired professor at Northwestern, who brought the technology here from Russia. Solid-state shear pulverization (SSSP) uses a twin-screw extruder modified with cooling so that polymer pellets or flakes are subjected to intense shear and compression without melting, resulting in powders with altered polymer chain structures. SSSP has been tried for several applications from compatibilizing polymer blends for recycling to de-bundling nanomaterials for better dispersion, but it hasn’t so far been commercialized, probably because of its high energy requirement.

Now Wakabayashi has tried to make the process more energy efficient by combining the cold and hot steps in the same extruder to produce the nanomaterial and nanocomposite directly in-line. “The challenge is to combine the SSSP step in line with extrusion,” Wakabayashi says. He converted a 25-mm diameter twin-screw lab extruder with 34:1 L/D and six temperature zones, so that zones 1, 2, and 3 were for the cold SSSP process, cooled by recirculating an ethylene glycol/water mixture through a chiller. Zone 4 was at medium temperature and zones 5, 6 and the die section had a hot temperature profile for compounding the mixture. These sections were heated with electrical elements. Both plastic and filler were hopper-fed into zone 1. Wakabayashi first presented his cold-hot extrusion at ASIATEC 2011 in Tokyo in February, 2011, but at that time hadn’t done the full trials. This will be the first presentation of the results of the experiments. He will report successful results with nanocomposites of LLDPE with commercially available montmorillonite clay and show good dispersion and exfoliation of the nanoclay.

Another interesting new “direct” extrusion approach to nanocomposites is presented by Joseph Golba, lead scientist at PolyOne Corp., Avon Lake, Ohio (www.polyone.com) in “Nanocomposites via In-Situ Polymerization… Is This the Way to Go?” It presents the first report on PolyOne’s work exploring the technology of in situ polymerization of nylon with montmorillonite nano clay and also delves into some of the early intellectual property on in situ polymerization of nanoclay composites. PolyOne initially tried batch hydrolytic in-situ polymerization of nylon 6-clay nanocomposites, which produced “products of exceptional quality,” Golba says. “Independent x-ray diffraction analysis at the University of Akron (www.uakron.edu) judged the dispersion of clay in the nylon 6 to be about the best observed to date.” But overall process productivity was too low.

So PolyOne then explored reactive extrusion based on anionic in situ polymerization of nylon 6-clay nanocomposites, using a catalyst and activator originally designed for the nylon RIM (reaction injection molding) thermosetting process, which polymerizes at 150-160 degrees C, well below the melt temperature of nylon. “When you use these catalysts at higher melt extrusion temperatures for nylon of 240-260 degrees C, the chemistry won’t give you typical linear-chain nylon,” PolyOne’s Golba explains. “It produces very high molecular weight and branched chains.”  However, alternative chemistries have been identified. The IKV Institut fur Kunststoffverbeitung at RWTH Aachen University in Germany (www.ikv-aachn.de) two years ago at ANTEC 2009 also reported extrusion-based in situ polymerization of nanonylons, using an unidentified activator and catalyst from Brueggemann Chemical Group, Heillbronn, Germany (www.brueggemann.com).

The paper is entitled “Polyamide 6-Nanocompounds Made via In-Situ Polymerization from Clay and Caprolactam in a Twin-Screw Extruder” by Walter Michaeli and Bernd Rothe of the IKV.

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