Andre Lee

Thermal and Viscoelastic Property of Epoxy-Clay and Hybrid Inorganic-Organic Epoxy Nanocomposites

The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface and chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. In this article, we investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers. Glass transitions of these nanocomposites were studied using differential scanning calorimetry (DSC). Small-strain stress relaxation under uniaxial deformation was examined to provide insights into the time-dependent viscoelastic behavior of these nanocomposites. Since the size of the POSS macromer is comparable to the distance between molecular junctions, as we increase the amount of POSS macromers, the glass transition temperature Tg as observed by DSC, increases. However, for an epoxy network reinforced with clay, we did not observe any effect on the Tg due to the presence of clay reinforcements. In small-strain stress relaxation experiments, both types of reinforcement provided some enhancement in creep resistance, namely, the characteristic relaxation time, as determined using a stretched exponential relaxation function increased with the addition of reinforcements. However, due to different reinforcement mechanisms, enhancement in the instantaneous modulus was observed for clay-reinforced epoxies, while the instantaneous modulus was not effected in POSS–epoxy nanocomposites.

A New Method to Reduce Cure-Induced Stresses in Thermoset Polymer Composites, Part III: Correlating Stress History to Viscosity, Degree of Cure, and Cure Shrinkage

Non-thermoelastic effects such as cure shrinkage of a polymer can play a role in residual stresses in composite parts. Studies have shown that cure shrinkage can place significant stresses on fibers. Therefore, the cure cycle of 3501-6 epoxy resins was modified to change its cure shrinkage characteristics to minimize the stresses. New cure strategies were developed using volumetric dilatometry, differential scanning calorimetry, dielectric cure monitoring, and a unique single fiber stress test method. Cure cycles were modified to balance the resins thermal expansion with its cure shrinkage. In some cases, a region of constant volume was achieved for a short time. However, the cure shrinkage eventually dominated over thermal expansion in all cycles as the polymer gelled. Changing the cure cycle affected the degree of cure at the point where the fiber/matrix interface developed as well as the amount of cure shrinkage occurring afterwards. A higher degree of cure at this point leads to longer stress relaxation time. Furthermore, less cure shrinkage at this point leads to less stress on the fibers. Also, slow heating rates allow more time for the polymer to relax and relieve stresses caused by cure shrinkage. Finally, a cure cycle that minimizes stresses due to cure shrinkage has been demonstrated.

Mechanical characterization of Sn‐3.5Ag solder joints at various temperatures

Deformation studies on eutectic Sn‐Ag solder (Sn‐3.5Ag in wt percent) joints were carried out at a range of temperatures using a rheometric solids analyzer (RSA‐III). Various performance parameters were evaluated with this equipment by subjecting geometrically realistic solder joints to shear loading at various temperatures (25, 75, 100, 125, and 150°C) with a nominal joint thickness of ∼100 μm and 1×1 mm solder joint area. Mechanical properties such as shear stress versus simple shear‐strain relationships, peak shear stress as a function of rate of simple shear‐strain and testing temperature, and creep parameters were evaluated to gain a better understanding of the parameters contributing to thermomechanical fatigue.

Isochoric and isobaric glass formation : Similarities and differences

Pressure-volume-temperature (PVT) studies were performed on a glass-forming polymer, poly(carbonate) (PC), under both isobaric and isochoric (constant volume) conditions. An isochoric glass transition was observed and the formation points were found to be consistent with those obtained isobarically. Although the isobaric and isochoric responses were, as expected, the same in the rubbery state, the glassy state values were found to be different and dependent upon the glass formation history. The isobaric data exhibited larger changes in going from the rubber to the glass, hence a stronger glass transition, than did the isochoric data. Inserting the experimental values for the thermal expansion coefficient α and isothermal compressibility β, into appropriate thermodynamic relations, measures of the strength of each transition are defined. Strength estimates based on literature values of α and β are compared to the experimental measures of the isochoric and isobaric transitions. In addition, both the isobaric and isochoric PVT results were analyzed in terms of the Fox and Flory free volume theory which assumes that the glass transition is an iso-free volume state. While the isobaric results were consistent with the Fox and Flory theory, the isochoric results were not consistent with the idea of an iso-free volume glass transition.

Synchrotron x-ray microscopy studies on electromigration of a two-phase material

Basic issues involving movement of conductive constituents and microstructural evolution from high current density in single phase materials are well documented. Recently, electromigration of conductive constituents in multiphase materials has gained attention due to the necessity of employing such alloys for interconnects in microelectronics. Reported studies on these alloys using complicated industrial geometry suffer from contributions such as current crowding. Hence a basic understanding on operative mechanisms during electromigration in multiphase alloys cannot be gained from these studies. Consequently, several mechanisms proposed from these studies involve fitting parameters and not well-understood complex diffusional processes. A joint configuration designed to avoid current crowding and associated local Joule heating is suitable for evaluating electromigration induced microstructural events. Synchrotron x-ray microscopy has provided information regarding two- and three-dimensional crystallographic orientations and strain fields in such joints, aiding the development of a basic understanding of electromigration in two-phase alloys.Basic issues involving movement of conductive constituents and microstructural evolution from high current density in single phase materials are well documented. Recently, electromigration of conductive constituents in multiphase materials has gained attention due to the necessity of employing such alloys for interconnects in microelectronics. Reported studies on these alloys using complicated industrial geometry suffer from contributions such as current crowding. Hence a basic understanding on operative mechanisms during electromigration in multiphase alloys cannot be gained from these studies. Consequently, several mechanisms proposed from these studies involve fitting parameters and not well-understood complex diffusional processes. A joint configuration designed to avoid current crowding and associated local Joule heating is suitable for evaluating electromigration induced microstructural events. Synchrotron x-ray microscopy has provided information regarding two- and three-dimensional crystallographi...

Early stage of material movements in eutectic SnPb solder joint undergoing current stressing at 150°C

X-ray fluorescence spectroscopy was used to study movements of Sn and Pb in the eutectic SnPb solder joint undergoing electromigration with a current density of 104A∕cm2 at 150°C. During early stages of current stressing, Sn moves toward the anode faster than Pb. However, on continued application of current stressing, both Sn and Pb will continue to accumulate at the anode. Such accumulation of conductive species facilitates the formation of hillock with associated valley near the cathode.

Development of nanocomposite lead-free electronic solders

Inert, hybrid inorganic/organic, nano-structured chemicals, can be incorporated into low melting metallic materials, such as lead-free electronic solders to achieve desired level of performance. The nano-structured materials technology of polyhedral oligomeric silsesquioxanes (POSS), with appropriate organic groups, can produce suitable means to promote bonding between nano-reinforcements and the metallic matrix. The microstructures of lead-free solder with surface-active POSS tri-silanols were evaluated using scanning electron microscopy (SEM). Wettability of POSS-containing lead-free solders to copper substrate was also examined. Steady-state deformation of solder joints made of eutectic Sn-Ag solder with varying weight fraction of POSS of different chemical moieties were evaluated at a range of temperatures (25/spl deg/C, 100/spl deg/C, and 150/spl deg/C) using a Rheometric Solids Analyzer (RSA-III). Mechanical properties such as shear stress versus simple shear-strain relationships, peak shear stress as a function of rate of simple shear-strain and testing temperature were reported. The service reliability of joints made with these newly formulated nanocomposite solders was evaluated using a realistic thermomechanical fatigue (TMF) profile. Evolution of microstructures and residual mechanical property at different extend of TMF cycles were compared with joints made of standard, un-reinforced eutectic Sn-Ag solder.

Fracture of an epoxy polymer containing recycled elastomeric particles

The fracture behavior of elastomer-modified epoxy was investigated using compact-tension geometry. The elastomeric modifiers included a liquid carboxyl-terminated butadiene acrylonitrile and solid rubber particles of different sizes which were obtained from recycled automobile tires. When used with solid rubber alone, no significant improvement in the fracture toughness was observed. However, when used in combination with the liquid rubber modifier, it was observed that the fracture toughness of these hybrid epoxies was higher than that of those toughened with liquid rubber alone. This synergistic effect is explained in terms of crack deflection and localized shear yielding. Furthermore, we observed a slight improvement in the fracture toughness as the size of the solid rubber particles increased. Although using a combination of both reactive rubber liquids and solid rubber particles as toughening agents had been investigated previously, in this study, the solid rubber particles used were from recycled rubber tires. Therefore, we have clearly demonstrated an application of producing high-quality engineering epoxy systems using toughening modifiers that are relatively low in cost and created higher-value products for recycled solid rubber.

Aging in glasses subjected to large stresses and deformations

Abstract Physical aging studies near the conventional glass transition temperature Tg were made using a model epoxy glass. Non-linear viscoelastic responses were measured after quenching the samples from above Tg to below it. The physical aging response in creep in simple extension was studied as a function of stress magnitude and in stress relaxation in torsion as a function of deformation magnitude. In each test the time, t ∗ , for the aging time shift factor to approach a constant value was determined and found to be independent of the stress or deformation magnitude. The torsional experiments were performed in a torsional dilatometer and the volume recovery response during the physical aging experiment was measured in addition to the torque response. It was found that the underlying volume recovery kinetics were not changed by the mechanical stimuli, i.e. were independent of the applied deformation. These results are interpreted to mean that large mechanical stimuli do not alter the underlying thermodynamic state of the glass and aging is not ‘erased’ by the large stresses or deformations.

Identification and quantification of cis and trans isomers in aminophenyl double-decker silsesquioxanes using 1H–29Si gHMBC NMR

Cis and trans isomers of a series of double‐decker silsesquioxanes (DDSQ) were characterized by two‐dimensional NMR techniques. The 1H NMR spectra of these species have not previously been assigned to a degree that allows for quantification. Thus, 1H–29Si HMBC correlations were applied to facilitate 1H spectral assignment and also to confirm previous 29Si assignments for this class of silsesquioxanes. With the ability to identify all the pertinent resonances of the 1H NMR spectrum, 29Si NMR is no longer required for quantification and required only for characterization. This not only saves time and material but also provides a more accurate quantification, thus allowing for the ratio of cis and trans isomers present in each compound to be determined. A more accurate measure of the cis/trans ratio enables the investigation of its influence on the physical and chemical properties of DDSQ nanostructured materials. Copyright