Reaction Control in Quiescent Systems of Free-Radical Retrograde-Precipitation Polymerization

Free-radical retrograde-precipitation polymerization, FRRPP in short, is a novel polymerization process discovered by Dr. Gerard Caneba in the late 1980s. The current study is aimed at gaining a better understanding of the reaction mechanism of the FRRPP and its thermodynamically-driven features that are predominant in controlling the chain reaction. A previously developed mathematical model to represent free radical polymerization kinetics was used to simulate a classic bulk polymerization system from the literature. Unlike other existing models, such a sparse-matrix-based representation allows one to explicitly accommodate the chain length dependent kinetic parameters. Extrapolating from the past results, mixing was experimentally shown to be exerting a significant influence on reaction control in FRRPP systems. Mixing alone drives the otherwise severely diffusion-controlled reaction propagation in phase-separated polymer domains. Therefore, in a quiescent system, in the absence of mixing, it is possible to retard the growth of phase-separated domains, thus producing isolated polymer nanoparticles (globules). Such a diffusion-controlled, self-limiting phenomenon of chain growth was also observed using time-resolved small angle x-ray scattering studies of reaction kinetics in quiescent systems of FRRPP. Combining the concept of self-limiting chain growth in quiescent FRRPP systems with spatioselective reaction initiation of lithography, microgel structures were synthesized in a single step, without the use of molds or additives. Hard x-rays from the bending magnet radiation of a synchrotron were used as an initiation source, instead of the more statistally-oriented chemical initiators. Such a spatially-defined reaction was shown to be self-limiting to the irradiated regions following a polymerization-induced self-assembly phenomenon. The pattern transfer aspects of this technique were, therefore, studied in the FRRP polymerization of N-isopropylacrylamide (NIPAm) and methacrylic acid (MAA), a thermoreversible and ionic hydrogel, respectively. Reaction temperature increases the contrast between the exposed and unexposed zones of the formed microgels, while the irradiation dose is directly proportional to the extent of phase separation. The response of Poly (NIPAm) microgels prepared from the technique described in this study was also characterized by small angle neutron scattering.

Model based experimental investigation on Powered Gait Orthosis (PGO)

Research on rehabilitation showed that appropriate and repetitive mechanical movements can help spinal cord injured individuals to restore their functional standing and walking. The objective of this paper was to achieve appropriate and repetitive joint movements and approximately normal gait through the PGO by replicating normal walking, and to minimize the energy consumption for both patients and the device. A model based experimental investigative approach is presented in this dissertation. First, a human model was created in Ideas and human walking was simulated in Adams. The main feature of this model was the foot ground contact model, which had distributed contact points along the foot and varied viscoelasticity. The model was validated by comparison of simulated results of normal walking and measured ones from the literature. It was used to simulate current PGO walking to investigate the real causes of poor function of the current PGO, even though it had joint movements close to normal walking. The direct cause was one leg moving at a time, which resulted in short step length and no clearance after toe off. It can not be solved by simply adding power on both hip joints. In order to find a better answer, a PGO mechanism model was used to investigate different walking mechanisms by locking or releasing some joints. A trade-off between energy consumption, control complexity and standing position was found. Finally a foot release PGO virtual model was created and simulated and only foot release mechanism was developed into a prototype. Both the release mechanism and the design of foot release were validated through the experiment by adding the foot release on the current PGO. This demonstrated an advancement in improving functional aspects of the current PGO even without a whole physical model of foot release PGO for comparison.

SO2 emissions at Volcan de Colima, 2003-2007

Volcán de Colima has been continuously erupting since the onset of dome growth in 1998. This period of unrest has had 4 prominent periods; 1998-1999, 2003, 2004-2005, and the current dome growth that began in February of 2007. Each of these episodes was marked by lava extrusion forming a dome and lava flows, followed by explosions that destroyed the dome. The Correlation Spectrometer (COSPEC) was used to determine SO2 emission rates on 164 days from May 2003 to February 2007, using both stationary ground based scans and some flight traverses. Scans were separated into the categories of explosive degassing and passive, or background degassing. These scans show variation in the SO2 flow rate from below detection limit (~3 t/d depending on environmental conditions) during background, passive emissions to a peak of 2949 t/d (34 kilograms/second) during an explosion on 9 October, 2004. Both passive and explosive degassing increased when there was lava extrusion in 2004 and with the increased explosive activity in 2005. These two different processes of degassing wax with each other when activity increases and wane together as well, indicating a parallel cyclicity in the volcanic eruption and degassing rates, where the conduit partially seals (pressurizes) between explosions. Colima’s gas and eruptive behavior is compared to similar systems such as Santiaguito and Soufrière Hills, Montserrat. About 2/3 of Colima’s SO2 degassing, amounting to 1.3 x 105 tonnes in 3.74 yrs has come in short lived small (VEI=0-1) vertical explosions that occurred at the rate of 100-3000explosions/ month, and the remaining third has occured in continuous passive degassing. Colima emits sulfur at a rate equivalent to about 0.04 to 0.08 wt % S, similar to other andesitic convergent plate boundary volcanoes. There has been an explosive destruction of the dome in every cycle for that past 5 years, and it is assumed that the current dome which began growth in February, 2007 (just at the end of this study) will be destroyed. Higher emission rates seen in the quiescence of 2006 may have eased the pressure at the time, resulting in the slow effusion of the current dome and lack of explosivity.