11th Annual Cohen-Register MicrosymposiumJune 11 & 12, 2003
Miscibility of Poly(vinyl chloride)/Poly(methyl methacrylate)/Dioctyl Phthalate Blends
In the past, many studies have investigated the potential for toughening semicrystalline and glassy polymers using hard filler particles. The premise for such research is that such composites could be stiffer, tougher, and cheaper than the base polymer. In general, these studies have looked at single homopolymers while varying filler content, type, surface properties, etc. We hope to add degrees of freedom to such research by using a miscible polymer blend as the matrix for a composite. PVC and PMMA were chosen due to their broad miscibility range and striking differences in mechanical behavior. The plasticizer DOP, known to be compatible with PVC, has been added to the system to widen the range of possible matrix behavior. In the current work, a significant portion of the PVC/PMMA/DOP phase diagram has been elucidated through thermal analysis techniques. The results show that although the system is miscible for most of the parameter space studied, PMMA-rich blends appear to exhibit limited miscibility.
Cavitated Block Copolymer Micellar Thin Films and their Applications in Carbon Nanotube Synthesis
Using a diblock copolymer of polystyrene-polyacrylic acid (PS-b-PAA), polymer thin films can be synthesized with hexagonal arrays of open surface cavities. This process involves spin casting a micellar solution of PS-b-PAA in toluene onto a silicon nitride substrate and produces arrays of spherical PAA nanodomains in a PS matrix. The thin films are then submerged in a weak alkaline solution, and the PAA nanodomains are cavitated as a result of swelling caused by the carboxylic acid groups. These acid groups can then be utilized to sequester metal ions by submerging the films in a metal salt solution. After the synthesis of these substrates using iron nanoparticles, the films will then be used as catalysts for the production of carbon nanotubes. In carbon nanotube production, many current catalysts consist of creating metal nanoparticles on a substrate with a lack of control of their size or spacing. The size and spacing of these metal nanoparticles on the substrate directly controls the size and spacing of the nanotubes formed. Using these block copolymer micellar thin films as carbon nanotube catalysts, we hope to be able to create carbon nanotubes in ordered arrays with controlled site density.
Patternable PEG-Grafted Poly(allylamine) for Fabrication of Bio-Array Templates
Newly designed poly(allylamine)-g-poly(ethylene glycol) polyelectrolyte graft copolymers has been synthesized in order to combine bio-functionality with the ability to pattern charged polyelectrolyte multilayer surfaces. Polymer-on-polymer stamping (POPS) techniques developed in our group have been used to create micron scale patterned regions on oppositely charged multilayer surface by direct stamping of these graft copolymers. As is well known, the long side chains of PEG effectively resist adsorption of antibodies or other proteins, and create a bio-inert area when patterned by POPS. On the other hand, a desired antibody can be covalently attached to the graft copolymer by introducing proper coupling agents at the PEG side chain. While simple adsorption of antibody onto graft copolymer patterned surfaces generates negative patterns of antibody proteins, the conjugation method using coupling agent-modified graft copolymer produces a positive antibody pattern. Patterns of antibody specific to the B cell receptor have directly led to a regular B cell array. Patterning two different antibodies on a surface and a consequent bi-cellular array of immunocytes are currently being investigated and tested for an application as an immunological sensor.
Morphological Effects of Tethered and Untethered Polyhedral
Oligosilsesquioxanes (POSS) on the Viscometric and Linear Viscoelastic
Properties of PMMA
Polyhedral oligosilsesquioxanes (POSS), hybrid organic-inorganic molecular composites with structure RxSixO1.5x, have shown promise in engineering plastics due to their improval of the thermal stability and abrasion resistance of soft materials without substantially increasing their viscosity. However, little is known about the effect POSS has on the viscometric properties of thermoplastics in the melt state. In this study, PMMA blended with - and copolymerized with - two distinct POSS molecules (R8Si8O12) containing isobutyl and cyclopentyl R-groups, respectively, was tested in steady shear flow. Three specific systems were analyzed: PMMA homopolymer (Mw = 140,000 g/mol) blended with low volume fractions of discrete POSS particles, MMA copolymerized with propylmethacryl-POSS (Mw = 310,000 g/mol), and the PMMA-POSS copolymer blended with POSS particles, to contrast the effect of tethered versus untethered POSS moieties on the polymer's rheological properties. The morphologies of each sample were characterized using X-ray diffraction and the scattering patterns are related to the rheological properties measured in steady shear and in small amplitude oscillatory shear. The change in zero shear-rate viscosity with volume fraction is notably different from what is observed using conventional macroscopic fillers. We also observed a strong time dependence of the storage and loss moduli at low frequencies for the PMMA-POSS copolymers due to structure development amongst the tethered POSS molecules. These results are compared with previously reported studies of POSS systems, and the scaling of the observed results with volume fraction is compared with predictions from percolation theory.
Rugate Interference Filters from Polyelectrolyte Multilayers
Our group has recently demonstrated the possibility of using the polyelectrolyte multilayer (PEM) deposition technique to create optical interference coatings. Such coatings are generally based on alternating layers of quarterwave optical thickness, and demonstrate interesting optical characteristics, such as tunable reflection and pass bands. The desire to extend this technique to filters possessing arbitrary variations in refractive index (rugate filters) is motivated by the ability to enhance the optical filter performance. Such enhancements can include narrowing the reflection bands and eliminating higher order reflectance lobes. This talk will discuss the unique way PEMs are suited to creating rugate filters through "digitalization" of the continuous index profile design, whereby many high-low index layers are used to approximate a smooth refractive index profile. I will present recent experimental results, as well as methods for modeling these filters as a means of speeding the design process and elucidating differences between theory and experiment.
Multilayer coextrusion is an advanced polymer processing technique that is capable of generating laminate materials with layer thicknesses ranging from millimeters to nanometers. Recent works have shown that coextruded multilayers, consisting of alternating layers of brittle PMMA and ductile PC, have hinted towards possessing enhanced energy dissipation qualities, through inter-layer cooperative yielding and deformation, as the number of individual layers increase. Our goal is multifaceted: to characterize these materials under small and high rate deformation using AFM, SEM, TEM, dynamic mechanical analysis and impact testing, develop a mechanistic understanding of energy dissipation in these materials, and to determine the effects of processing and material rheology on the nanostructures ballistic performance. We will present a current overview of the literature and present some initial investigations into the rheological behavior of multilayers.
Fabrication of Optical Devices from Multilayer Systems
Multilayer structures have been built from poly(acrylic acid) (PAA) and poly(allylamine hydrochloride (PAH)on various substrates. These multilayer films transformed into micro- and nanoporous structures upon the treatment of acid aqueous solutions at different pH value. Optical shutters have been fabricated by infiltrating mico-porous mutilayer films with liquid crystals. With micron size droplets of liquid crystal dispersed in the film, due to the difference of the refractive index between the liquid crystal and polymers, light passed through the film in the absence of an applied electric field was strongly scattered, giving a milky, translucent film. Application of an electric field across the film reduced the refractive index of the liquid crystal and placed the film in a highly transparent state.