http://hdl.handle.net/1721.1/7835 Search the Channel search http://dspace.mit.edu/simple-search http://hdl.handle.net/1721.1/30256 Title: Critical thickness in silicone thermosets <br/> <br/>Authors: Deopura, Manish, 1975- <br/> <br/>Abstract: Critical thickness effects are utilized to achieve high fracture toughness in brittle polymers. The postulate of critical thickness, which is: "Macroscopically brittle polymers deform in a ductile fashion below a critical dimension" is validated for silicone thermosets and polystyrene using novel "direct" methods by measuring failure strain in thin films. A discussion on polymer intrinsic deformation mechanisms is presented. Using these intrinsic deformation mechanisms as bases, it is argued that all polymers are ductile at the molecular level. Accordingly, it is suggested that polymer properties below a certain length scale (defined as critical thickness) are dominated by intrinsic deformation characteristics. Two analytical models have been developed which predict critical thickness based on the physical properties of the polymer. The first model is based on an energy criterion according to which crack initiation does not take place if the crack driving force is less than the crack resistance. Such a condition for a brittle polymer is achieved at the critical thickness. The second model is based on a mechanics criterion according to which a minimum film thickness (critical thickness) is required for typical fracture features like crazes to exist within it. Further, using these theoretical models as a basis, the effect of network density, temperature and strain rate on critical thickness is discussed. It is also shown that fracture strain is the characteristic material property to measure film toughness. A variety of silicone thermosets are studied to demonstrate engineering applicability of critical thickness.; (cont.) The selected polymers include a commercial laminate poly-phenyl- methyl-silsesquioxane resin, an experimental high temperature poly-methyl- silsesquioxane resin and an optical polysiloxane resin. In addition to silicone thermosets, polystyrene is studied as a reference polymer. A bending technique has been developed in order to determine failure strain of thin films (on substrates) of these materials. Using this technique, the failure strain is evaluated as a function of film thickness. Further, from a plot of failure strain as a function of film thickness the critical thickness is determined. For polystyrene, a critical thickness value of approximately 0.1 um is observed. The strain to failure of polystyrene films below the critical thickness is >15%, a marked increase over bulk material fracture strain (-2%). For each of the silicone thermosets, a range of curing temperatures is investigated to determine the influence of curing temperature on critical thickness. For the poly-phenyl-methyl-silsesquioxane resin, the optimum thin film properties are observed when it is cured at 225 ⁰C. The critical thickness is observed to be -5 gm with a strain to failure of -13% (bulk strain to failure <2%). Molecular engineering of the poly-phenyl-methyl-silsesquioxane by modifying the chemical structure using functionalized PDMS is shown to increase critical thickness to 0lm. For this PDMS modified poly-phenyl-methyl-silsesquioxane, critical thickness values have been determined over a range of test temperatures from -40 ⁰C to 75 ⁰C. Results indicate that the test temperature does not influence the critical thickness. For the poly-methyl- silsesquioxane, the critical thickness is observed to be greater than 2.51um with a strain to failure of 15% when cured at 250 ⁰C.; (cont.) Two optical polysiloxane resins are studied. The first resin, commercially called the PF-1202 resin is studied for a single cure temperature and is observed to have a critical thickness of -0.2-0.3um. The properties of the second resin, named MP-101, are studied for a range of cure temperatures. The best performance, with 8% strain to failure and a 0.5um critical thickness, is observed for a 50 ⁰C cure temperature. Infrared spectroscopy measurements in reflection mode are carried out to compare orientation effects in thin (below critical thickness) vis-A-vis thick (above critical thickness) films for polystyrene, poly-phenyl-methyl-silsesquioxane and PDMS modified poly-phenyl-methyl-silsesquioxane. It is observed that both the thick as well as thin films do not exhibit any substantial orientation. A correlation between molecular orientation and fracture properties cannot be made. Infrared spectroscopy has also been used to determine the "nature" of strain (elastic or plastic) these thin films (below critical thickness value) exhibit when stretched to values higher than their bulk counterparts. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (leaves 147-151). http://hdl.handle.net/1721.1/30254 Title: Mechanical characterization and in vivo operation of an implantable drug delivery MEMS device <br/> <br/>Authors: Li, Yawen, 1972- <br/> <br/>Abstract: The goal of this thesis was to advance an implantable drug delivery MEMS (MicroElectroMechanical Systems) device developed in our laboratory. This device was designed to locally deliver multiple substances in complex release profiles in order to maximize the effectiveness of drug therapies. It consists of an array of microreservoirs etched into a silicon substrate. Different types and dosages of drugs can be contained in these reservoirs capped by thin gold membranes. The drug release is achieved by the application of a small anodic potential on the gold membrane in a chloride containing medium (such as the body fluid). The gold membrane will corrode and disintegrate so that the drug contained within the reservoir is free to diffuse into the surrounding medium. Previous researchers have demonstrated in vitro and in vivo release of tracer molecules as well as a radiolabled chemotherapeutic agent (carmustine, or BCNU) from the device. However, systematic characterization of the mechanical and electrochemical behavior of gold membranes on the drug delivery device was necessary in order to achieve more reliable device performance and to demonstrate efficacy of BCNU delivered from the MEMS device against an experimental tumor model. A bulge test apparatus was constructed to characterize the mechanical properties of gold membranes. Uniform pressure was applied from underneath the gold membrane and the membrane deflection was measured using optical interferometry. Analyzing the deflection and pressure data allowed extraction of the elastic modulus and residual stress of the gold membrane.; (cont.) Gold membranes with in-plane sizes ranging from 20 to 200pim showed lower modulus (126-168 GPa) than bulk (111) single crystal gold (189 GPa). But their yield strength (317-351 MPa) was higher than the bulk value. An in situ experimental setup was constructed to observe the electrochemical disintegration process of the gold membranes. Real time images recorded from a CCD camera showed non-uniform corrosion occurring first around the membrane edges. Bulge tests on the corroded membranes indicated a gradual loss of mechanical integrity of the gold membranes due to corrosion. The gold membrane disintegration probably occurred by a combination of membrane thinning through active dissolution and accumulation of plastic deformation due to the transient formation of a passive film on top of the gold membrane in each voltammetry cycle. Dense gold membranes with reproducible opening behavior are critical to the success of large scale in vivo studies and future commercial applications. Defects in the gold membranes led to premature leakage of BCNU, a small molecule drug. Wafers with sputtered gold membranes patterned by wet etching had a higher device yield and membrane quality than wafers with evaporated gold membranes patterned by lift off. The mechanical and electrochemical studies provided guidance to improve the operation reliability and reproducibility of the drug delivery device. In vivo release of BCNU from the drug delivery device was demonstrated in a rat flank model. Acute temporal release kinetics of 14C labeled BCNU in vivo was evaluated by analysis of the plasma 14C concentration using the accelerator mass spectrometry (AMS) technique.; (cont.)The in vivo 14C labeled BCNU release profile from the activated devices was similar to that of the in vitro and subcutaneously injected controls. The time to reach a steady-state plasma 14C concentration was on the order of one hour. Efficacy of BCNU delivered from the drug delivery device was demonstrated in a 9L rat flank tumor model. Co-formulation of PEG with BCNU led to complete and rapid release of payload in vivo. The retarding effect of BCNU on the tumor growth was dose dependent in the range of 0.67 - 2 mg. BCNU delivered from activated devices seemed to be as effective as equipotent injections of BCNU against the tumor growth. This tumor effect study provided preliminary efficacy validation of the drug delivery device as well as important dosage information for further efficacy evaluation of the BCNU/IL-2 combination therapy. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references. http://hdl.handle.net/1721.1/30249 Title: Scanning probe microscopy with inherent disturbance suppression using micromechanical systems <br/> <br/>Authors: Sparks, Andrew William, 1977- <br/> <br/>Abstract: All scanning probe microscopes (SPMs) are affected by disturbances, or mechanical noise, in their environments which can limit their imaging resolution. This thesis introduces a general approach for suppressing out-of-plane disturbances that is applicable to non-contact and intermittent contact SPM imaging modes. In this approach, two distinct sensors simultaneously measure the probe-sample separation: one sensor measures a spatial average over a large sample area while the other responds locally to topography underneath the nanometer-scale probe. When the localized sensor is used to control the probe-sample separation in feedback, the spatially distributed sensor signal reveals only topography. This technique was implemented on a scanning tunneling microscope (STM) and required a custom micromachined scanning probe with an integrated interferometer for the spatially averaged measurement. The interferometer design is unique to SPM because it measures the probe-sample separation instead of the probe deflection. A robust microfabrication process with a novel breakout scheme was developed and resulted in 100 % device yield. For imaging, an STM setup with optical readout was built and characterized. The suppression improvement over conventional SPM imaging was measured to be 50 dB at 1 Hz, in agreement with predictions from classical feedback theory. Images are presented as acquired with each sensor signal in several environments, and the interferometer images show remarkable clarity when compared with the conventional tunneling images.; (cont.) The out-of-plane noise floor with this technique on the home-built microscope was 0.1 i rms. The results of this work suggest that the resolution of STM and other SPM modes, notably tapping mode atomic force microscopy (AFM), can be substantially improved, allowing low noise imaging of nanoscale topography in noisy environments and potentially enabling repeatable atomic scale imaging in ambient conditions. <br/> <br/>Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.; Includes bibliographical references (p. 109-116). http://hdl.handle.net/1721.1/17033 Title: Chemistry of airborne particles from metallurgical processing <br/> <br/>Authors: Jenkins, Neil Travis, 1973- <br/> <br/>Abstract: Airborne particles fall into one of three size ranges. The nucleation range consists of nanoparticles created from vapor atom collisions. The decisive parameter for particle size and composition is the supercooling of the vapor. The accumulation range, which comprises particles less than 2 micrometers, consists of particles formed from the collision of smaller primary particles from the nucleation range. The composition of agglomerates and coalesced particles is the same as the bulk vapor composition. Coarse particles, the composition of which is determined by a liquid precursor, are greater than 1 micrometer and solidify from droplets whose sizes are controlled by surface, viscous, and inertial forces. The relationship between size and composition of airborne particles could be seen in welding fume, a typical metallurgical aerosol. This analysis was performed with a cascade impactor and energy dispersive spectrometry with both scanning electron microscopy (SEM-EDS) and scanning transmission electron microscopy (STEM-EDS). Other methods for properly characterizing particles were discussed. In the analysis, less than 10% of the mass of fume particles for various types of gas metal arc welding (GMAW) were coarse, while one-third of flux cored arc welding (FCAW) fume particles were coarse. Coarse particles had a composition closer to that of the welding electrode than did fine particles. Primary particles were not homogeneous. Particles larger than the mean free path of the carrier gas had the same composition as that of the vapor, but for particles 20 to 60 nanometers, smaller particles were more enriched in volatile metals than larger particles were. This was explained by the cooling path along the bubble point line of a binary phase diagram.; (cont.) Particles were not necessarily homogenous internally. Because nanoparticles homogenize quickly, they may form in a metastable state, but will not remain in that state. In this analysis, the presence of multiple stable immiscible phases explains this internal heterogeneity. The knowledge contained herein is important for industries that depend on the properties of nanoparticles, and for manufacturing, where industrial hygiene is important because of respirable particle by-products, such as high-energy-density metallurgical processing. <br/> <br/>Description: Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.; Vita.; Includes bibliographical references.