Nandika D’Souza

Supercritical CO2 Processed Polystyrene Nanocomposite Foams

Polystyerene (PS) nanocomposite foams were prepared using CO2 supercritical fluid (SCF) as a solvent and blowing agent. PS was first in situ polymerized with 0, 1, and 3% Montmorillonite-Layered Silicate (MLS) mixtures, which were then compression-molded into thin laminates. The laminates were foamed in a batch process at temperatures and pressures within the range of 60–85 C and 7.6–12 MPa. Characterization was accomplished with scanning electron microscopy (SEM), Differential Scanning Calorimetry (DSC), and X-ray Diffraction (XRD). SEM images revealed the effects of different processing parameters on the foam’s cellular morphology, and also showed that the MLS layers were arranged in alignment with the foam cell walls. DSC data indicated that different concentrations of MLS have a notable effect on the glass transition temperature (Tg) of the composite, and that the foaming process itself alters the endothermic behavior of the material. XRD spectra suggested that the PS–MLS composite had an intercalated structure at both the 1 and 3% mixtures, and that the intercalation may be enhanced by the foaming process.

Creep and stress relaxation in a longitudinal polymer liquid crystal: Prediction of the temperature shift factor

The polymer liquid crystal PLC is the PET/0.6PHB copolymer; PET=poly(ethylene terephthalate), PHB=ρ-hydroxybenzoic acid (LC): 0.6=the mole fraction of PHB. This is a multiphase system with PHB-rich islands in a PET-rich matrix. Tensile creep compliance was measured isothermally from 20 °C to 160 °C in 10 °C intervals. Master curves were determined using the time–temperature superposition for 20 °C and for the glass transition temperature of the PET-rich phase TgPET=62 °C. Experimental values of the temperature shift factor aT as a function of temperature T agree in the entire T range with those from Eq. (7) relating aT to the reduced volume ṽ and the Hartmann equation of state Eq. (10). Values of aT(T) calculated from the Williams–Landel–Ferry (WLF) formula give very large errors below Tg. A control 14 months creep experiment agrees with the theoretical predictions from Eq. (7). Stress relaxation experiments were performed under the constant strain of 0.5% from 20 °C to 120 °C, again master curves were de...

Nylon 6 and Montmorillonite Layered Silicate (MLS) Nanocomposites

Reducing the high gas permeability of plastic packaging is of serious concern for a variety of applications in food packaging and microelectronics arena. In the development of high altitude scientific balloons, the potential for unmanned mission success is critically related to the improvement in helium barrier capability. Separately, the introduction of expandable smectite clays, such as montmorillonites, into polymers has resulted in improved barrier properties. Here we investigate the mechanical properties and helium permeability of dispersed montmorillonite-layered silicates (MLS) in nylon-6 nanocomposite films. A 20 wt% MLS in nylon-6 master batch was prepared in a twin-screw extruder and then mixed with nylon to produce nylon MLS nanocomposites with MLS content ranging from 1 to 5% for testing. The nylon nanocomposites were extruded into films. A highly exfoliated dispersion in different films was determined using X-ray diffraction. Nylon-6 nanocomposites showed formation of -crystalline form, which was absent in neat nylon-6. Differential Scanning Calorimetry (DSC) confirmed the crystallinity change and the role of MLS as an additional nucleation contributor. The glass transition and mechanical properties were related to the nucleation capabilities of MLS. Increase in the degree and type of crystallinity together with tortuous path development due to exfoliated dispersion were the primary reasons behind the improvement in gas barrier properties of exfoliated nylon-6 nanocomposites.

High sensitivity gas permeability measurement system for thin plastic films

We have developed a system to quantitatively measure the permeation of gases through thin flexible substrates with high sensitivity. The system consists of two chambers, a high pressure side and an ultrahigh vacuum (UHV) side, separated by the flexible sample to be analyzed. The system is calibrated using a combination of a National Institute of Standards and Technology traceable calibrated He leak and a variable aperture calibrated orifice. The base total pressure for the UHV side is 1–3×10−10Torr. The partial pressure of individual gases that we are studying is <10−10Torr. The sample to be measured is secured between the two chambers using two 2.75 conflat flanges, two copper gaskets, and two indium “O” rings. The key factors that impact sensitivity and quantification are (1) reducing the residual partial pressure of the gas of interest to as low a value as possible on the UHV side of the system, (2) sealing the plastic sample between the two chambers with no detectable gas leakage around the sample or ...

Characterization and mechanical properties of foamed poly(ɛ-caprolactone) and Mater-Bi blends using CO2 as blowing agent

Biodegradable foams are a key area of growth for packaging applications. Starch-based materials have been a successful environmentally degradable polymer. However, making foams out of these materials has been challenging due to their chemical composition and consequent low miscibility in CO2. We examine the potential for developing biodegradable foams with one component being a starch-based polymer by introducing it into polycaprolactone. Polycaprolactone has thus far shown very high-expansion ratios for foams with supercritical CO2. Blends with increasing amounts of starch-based materials were processed and foamed using isothermal treatments with supercritical CO2. Characterization of the samples was done using X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. The melting enthalpies and temperatures of the starch-based materials phase decreased with decreasing starch-based materials indicating some an influence of polycaprolactone on the starch-based materials crystallinity. Foaming, however, caused a reversal in this effect with the foamed melting points similar to the pure components. Micrographs of the samples from the scanning electron microscopy revealed that the cell size of the foams reduced with the increase in starch-based materials concentration. Mechanical tests—tensile, compression, shear, and impact—were performed on the foamed samples. The results indicate a valuable approach to foaming materials that are compostable but not CO2 miscible through blending with a highly foamable polymer such as polycaprolactone.

Influence of Bath Composition at Acidic pH on Electrodeposition of Nickel-Layered Silicate Nanocomposites for Corrosion Protection

Nickel-layered silicates were electrochemically deposited from acidic bath solutions. Citrate was used as a ligand to stabilize nickel (II) ions in the plating solution. The silicate, montmorillonite, was exfoliated by stirring in aqueous solution over 24 hours. The plating solutions were analyzed for zeta-potential, particle size, viscosity, and conductivity to investigate the effects of the composition at various pHs. The solution particles at pH 2.5 (−22.2 mV) and pH 3.0 (−21.9 mV) were more stable than at pH 1.6 (−10.1 mV) as shown by zeta-potential analysis of the nickel-citrate-montmorillonite plating solution. for the films ranged from −0.32 to −0.39 V with varying pH from 1.6 to 3.0. The films were immersed in 3.5% NaCl and the open circuit potential monitored for one month. The coatings deposited at pH 3.0 were stable 13 days longer in the salt solution than the other coatings. X-ray diffraction showed a change in the (111)/(200) ratio for the coatings at the various pHs. The scanning electron microscopy and hardness results also support that the electrodeposition of nickel-montmorillonite at pH 3.0 (234 GPa) had improved hardness and morphology compared to pH 2.5 (174 GPa) and pH 1.6 (147 GPa).

Influence of CO2 Blowing Agent on Porous Bioresorbable Stent Structure

Bioresorbable stents with limited functional lifetimes and with drug delivery capabilities are desired. Various methods have been investigated to induce porosity in bioresorbable polymeric stent fibers, thereby to permit increased drug reservoir capacity versus polymer-coated metal stents. We developed microporous surface layers on PLLA fibers to serve as the drug reservoir, but found that impurities, the use of chemicals, and multiple step procedures associated with our, and other published methods limited utility. Thus we investigated theoretically attractive CO2 blowing methods, in which gas under pressure and temperature induces porosity. We report the results of initial studies of CO2-induced porosity in PLLA stent fibers.Copyright

Metal Matrix Composite Coatings of Cupronickel Embedded with Nanoplatelets for Improved Corrosion Resistant Properties

The deterioration of metals under the influence of corrosion is a costly problem faced by many industries. Therefore, particle-reinforced composite coatings are being developed in different technological fields with high demands for corrosion resistance. This work studies the effects of nanoplatelet reinforcement on the durability, corrosion resistance, and mechanical properties of copper-nickel coatings. A 90 : 10 Cu-Ni alloy was coelectrodeposited with nanoplatelets of montmorillonite (Mt) embedded into the metallic matrix from electrolytic baths containing 0.05, 0.10, and 0.15% Mt. X-ray diffraction of the coatings indicated no disruption of the crystal structure with addition of the nanoplatelets into the alloy. The mechanical properties of the coatings improved with a 17% increase in hardness and an 85% increase in shear adhesion strength with nanoplatelet incorporation. The measured polarization resistance increased from 11.77 kΩ·cm2 for pure Cu-Ni to 33.28 kΩ·cm2 for the Cu-Ni-0.15% Mt coating after soaking in a simulated seawater environment for 30 days. The incorporation of montmorillonite also stabilized the corrosion potential during the immersion study and increased resistance to corrosion.

An Experimental Study on the Mode I Fracture Behavior of Hexagonal Honeycombs

Cellular materials have a high strength-to-weight ratio, which is good for lightweight structural applications. In order to accelerate the commercial use of cellular materials in the structural applications, a well understanding of fracture behavior of cellular materials is required in terms of structural integrity. The objective of the study is to develop a predictive model on fracture behavior of orthotropic cellular polymers, prepared from an additive manufacturing method, under the mode-I macroscopic loading. The constituent material’s fracture properties are obtained from the three-point bending test of samples with a notched crack. Using the base material’s properties, a model on fracture toughness of hexagonal honeycombs with a varying cell angle is developed using a linear elastic fracture mechanics (LEFM) model of a cell wall material. Numerical and experimental tests will follow to validate the model.Copyright