Our research team has developed a unique approach to research in polymer nanocomposites: we synthesize custom-made layered materials (clays and other inorganic solids) with surface chemical groups tailored for compatibility with PET or other target polymers.
Some of our synthetic platelets, including metal phosphonates, (Figure 2, left) and hectorite, are composed of stacked platelets with face surfaces covered by covalently-bound organic functional groups (phenyl, carboxyl, alkyl, and others). Other synthetic layered materials, including layered perovskites and magadiite (Figure 2, center and right), can be exfoliated in water and subsequently functionalized with desired organic groups. In principle, the organic groups on the platelet faces can be tailored to maximize compatibility with target polymers, leading to the ability to exfoliate high aspect ratio platelets in hydrophobic polymers like PET. In thermodynamic terms, the polymer prefers intimate contact with a hydrophobically-modified platelet surface (via covalently attached organic groups), rather than with the more hydrophilic “native” platelet surface. This approach also offers other advantages, including ability to synthesize platelets with enormous aspect ratios (length-to-thickness), and tighter control over composition and purity compared to natural clays.
In the area of PET-based nanocomposites, our team is one of a few academic groups in the nation having a complete system for making PET nanocomposites by in situ polymerization. We exfoliate platelets into water or ethylene glycol (or mixtures), and then introduce these platelet suspensions directly into bishydroxyethyl terephthalate (BHET, the monomer for PET). Elevated temperature and vacuum strip out water and excess glycol, followed by batch polymerization to produce PET-platelet nanocomposites. These materials are ground to a uniform granule size and then subjected to a second “solid-state” polymerization to raise the PET molecular weight. We use melt flow indexing to estimate PET molecular weight relative to commercial PET samples. The whole process typically takes a minimum of three days to produce about 150 g of PET-platelet nanocomposite.
Once we have synthesized a PET nanocomposite, we use the melt flow indexer to estimate the degree of polymerization and to produce a thick fiber sample. We measure mechanical properties via DMA and barrier properties via gravimetric water uptake. After installation of our microcompounder and film line in the second quarter of 2007, we will be able to produce PET nanocomposite film and fiber samples and subject them to uniaxial extension. We will then be able to assess the gas barrier performance of these nanocomposites using our gas permeation system.
Polystyrene Nanocomposites for Energy Storage.
We recently received over $900,000 in funding from the Air Force to explore applications of polymer nanocomposites as high performance thin film capacitors. To address the Air Force's need for new high energy density storage systems for pulse power applications, this project will develop an entirely new class of light weight capacitors based on advanced polymer-platelet nanocomposite dielectrics (PPNDs). We will synthesize high permittivity inorganic platelet materials, covalently functionalize the platelet surfaces with organic groups, and prepare polymer nanocomposites incorporating these materials. In collaboration with researchers at Wright-Patterson AFB, we will characterize the electrochemical performance of these materials, evaluating the impact of functionalized platelets on dielectric material properties and prototype capacitor performance.
NSF Partnership for Innovation
This NSF-funded award ($600,000 over three years) will establish the Polymer Nanocomposites Manufacturing Partnership (PNMP), a joint effort of USC and polymer manufacturing companies located in or near South Carolina. The goal of the PNMP is to foster innovation in polymer manufacturing through (1) basic research in synthesis and characterization of layered (nano)materials, routes to their incorporation in polymer nanocomposites, and accelerated methods for evaluating nanocomposite performance; (2) joint University/industry development of polymer nanocomposite technology for near-term commercialization; and (3) workforce development through student involvement in research, cross-disciplinary education, and industrial internships. Specifically, the PNMP set up as many as four cross-disciplinary research teams of students, faculty, and industry representatives, each focused on the interests of one industrial partner. The PNMP includes four confirmed industrial partners: Eastman, Michelin, and PBI Performance Products, and MeadWestvaco.