The effect of segment length on some properties of thermoplastic poly(ester–siloxane)s

A series of novel thermoplastic elastomers, based on poly(dimethylsiloxane) (PDMS) as the soft segment and poly(butylene terephthalate) (PBT) as the hard segment, were synthesized by catalyzed two‐step, melt transesterification reactions of dimethyl terephthalate and methyl esters of carboxypropyl‐terminated poly(dimethylsiloxane)s (M?_n = 550–2170 g mol^?1) with 1,4‐butanediol. The lengths of both the hard and soft segments were varied while the weight ratio of the hard to soft segments in the reaction mixture was maintained constant (57/43). The molecular structure, composition and molecular weights of the poly(ester–siloxane)s were examined by ¹H NMR spectroscopy. The effectiveness of the incorporation of the methyl‐ester‐terminated poly(dimethylsiloxane)s into the copolymer chains was verified by chloroform extraction. The effect of the segment length on the transition temperatures (T_m and T_g) and the thermal and thermo‐oxidative degradation stability, as well as the degree of crystallinity and hardness properties of the synthesized TPESs, were studied. Copyright © 2003 Society of Chemical Industry     

Theoretical analysis of control properties for the brushless doubly fed reluctance machine

The brushless doubly fed induction machine (BDFIM) has been extensively researched for approximately 30 years, but a related machine [the brushless doubly fed reluctance machine (BDFRM)], has not. This was mainly due to the fact that reluctance rotor designs were not capable of generating saliency ratios large enough to make the BDFRM competitive with other machines. However, recent developments in reluctance rotors, spurred on by research into synchronous reluctance machines, has resulted in high saliency ratio rotors that are economic to build. This, together with the promise of higher efficiency and simpler control compared to the BDFIM, means that further investigation of the BDFRM is warranted. A relatively limited amount of work to date has been published on the BDFRM. This paper attempts to fill this void by presenting a theoretical analysis of some of the important control properties of the ideal BDFRM.     

Vertical ridge augmentation around dental implants using a membrane technique and autogenous bone or allografts in humans

This study investigated the effect on vertical bone regeneration of the addition of demineralized freeze-dried bone allograft or autogenous bone chips to a membrane technique. Twenty partially edentulous patients with vertical jawbone deficiencies were selected for this study. The patients were divided into two groups of 10 individuals. The 10 patients of Group A received 26 Br?nemark implants in 10 surgical sites. The 10 patients of Group B received 32 implants in 12 surgical sites. Fifty-two out of 58 implants (22 in Group A and 30 in Group B) extended 1.5 to 7.5 mm superior to the bone crest. Titanium-reinforced expanded polytetrafluoroethylene membranes were used to cover the implants and, before complete membrane fixation, demineralized freeze-dried bone allograft particles were condensed under the membrane in Group A, and autogenous bone chips were used in Group B. At the reentry after 7 to 11 months the membranes were removed and a small biopsy was collected from 11 sites comprehending the miniscrews. The clinical measurements from Group A demonstrated a mean vertical bone gain of 3.1 mm (SD = 0.9 mm, range 1 to 5 mm) with a mean percentage of bone gain of 124% (SD = 46.6%). The measurements from Group B showed a mean vertical bone gain of 5.02 mm (SD = 2.3 mm, range 1 to 8.5 mm) with a mean percentage of bone gain of 95% (SD = 26.8%). Histomorphometric analysis of the present study clearly demonstrated a direct correlation between the density of the pre-existing bone and the density of the regenerated bone. The mean percentage of new bone-titanium contact was from 39.1% to 63.2%, depending on the quality of the pre-existing bone. Both the clinical and histologic results indicate a beneficial effect of the addition of demineralized freeze-dried bone allograft or autogenous bone particles to vertical ridge augmentation procedures in humans.     

Analysis and design-optimization of LCC resonant inverter forhigh-frequency AC distributed power system

The analysis and design of an LCC resonant inverter for a 20 kHz AC distributed power system are presented. Several resonant converter topologies are assessed to determine their suitability for high efficiency power conversion, under resistive and reactive loads. Two LCC-resonant inverter designs were implemented. One with all switches operating with zero voltage switching (ZVS), and another with two switches operating with ZVS and two switches with zero current switching (ZCS). The experimental results are presented along with a performance comparison of the two versions     

MISR in-flight camera geometric model calibration and georectification performance

In order to facilitate a unique georectification approach implemented for Multi-angle Imaging SpectroRadiometer (MISR) data, specific calibration datasets need to be derived during flight. In the case of the spaceborne MISR instrument, with its unique configuration of nine fixed pushbroom cameras, continuous and autonomous coregistration and geolocation of image data are required prior to the application of scientific retrieval algorithms. In-flight-generated calibration datasets are required to (a) assure accuracy, (b) reduce processing load, and (c) support autonomous aspects of the processing algorithm. This paper describes the in-flight geometric calibration approach with the focus on the first year of activities and the georectification performance achieved.     

Delay spread measurements for the digital cellular channel inToronto

The paper describes a set of measurements performed in the Toronto area, in order to assess the impact of multipath propagation on the performance of the TIA IS-54 digital standard. Five existing cells were systematically surveyed: two in downtown Toronto, two in the suburbs, and one in a suburban/rural area. A sweeping correlator apparatus with a 0.1 μs resolution and 910 MHz carrier frequency, and an omnidirectional antenna was used. In one of the cells, the measurements were repeated with a 60 degree beamwidth directional antenna. A fairly sophisticated thresholding technique was applied in order to reduce the impact of noise. Generally, the measurements with the omnidirectional antenna exhibit multipath propagation with considerably smaller excess delays than some reported, but are consistent with earlier results obtained in the U.S. and Europe. Sectorization was found to considerably reduce the multipath effects. Very large delay components appear to be due mostly to the combination of large transmitter-receiver distances and a large degree of shadowing, and could probably be avoided to a large extent by standard cellular engineering techniques. In view of these results, it seems that the long delay (high selectivity) problem has been somewhat overemphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated in the case of dense urban environments     

Novel approach in investigation of the poly(acrylic acid) hydrogel swelling kinetics in water

The swelling kinetics curves of structurally defined poly(acrylic acid) hydrogel in bidistilled water at temperatures: 25, 30, 35, 40, and 45°C were determined. The possibility of kinetically explaining the isothermal swelling process by applying the following models: reaction controlled by diffusion, first order chemical reaction kinetics, and second order chemical reaction kinetics, was investigated. It was found that kinetically explaining the swelling process using these methods was limited to only certain parts of the process. The swelling process in bidistilled water was described in full range assuming that the hydrogel's swelling rate was a kinetically controlled reaction by the rate of the movement of reactive interface of hydrogel. Based on that model, the kinetic parameters, activation energy (E_a) and preexponential factor (A), of the swelling process were determined to be E_a = 35 kJ/mol and lnA = 8.6. A possible mechanism of the investigated swelling process was discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Functionalized oligopolysiloxanes by heterogeneously catalyzed equilibration polymerization reactions

Summary A new general procedure for preparation of functionalized oligopolysiloxanes of predetermined molecular weight is described. It utilizes heterogeneously catalyzed siloxane equilibration polymerization reactions which do not require troublesome and sometimes difficult post-preparative work-up procedures usually encountered with the well known homogeneously catalyzed corresponding reactions. The method is described using as example the preparation of , -telechelic vinyldimethylsiloxy-oligopolydimethylsiloxanes from octamethylcyclotetrasiloxane and 1,3-divinyltetramethyldisiloxane, but reference to the preparations of trimethylsiloxy-, dimethylsiloxy-and carboxypropyldimethylsiloxyoligopolydimethylsiloxanes, oligopolymethylhydridosiloxanes or their copolymers is also made.     

On the critical molecular chain length of polydimethylsiloxane

Because a study of the results reported for the chain dimensions of polydimethylsiloxane (PDMS), critical for the onset of this polymer's non-Newtonian flow behavior, obtained from the viscosity-molecular weight relationships available from the literature, clearly revealed that considerable differences exist between the reported data, a detailed analysis of these data was performed together with an additional examination of 10 new PDMS samples that were selected so as to have molecular weights that would fill the gaps observed in the polymer viscosity–chain length relationship constructed from the accepted literature data. The results obtained were analyzed by using several different procedures integrated into a recently described comparative method that could allow for determination of what is called the most realistic critical value, Z_wc. The latter was determined as 930 PDMS main-chain atoms, which corresponds to this polymer's degree of polymerization of 464.5 and the weight-average molecular weight of 34,500. It is not only shown that after elimination of some clearly erroneous data points from the previously reported relationships the obtained critical chain-length values could very well fit the earlier relationships, but also that appropriate “master” relationships were constructed including 48 pairs of the old and 10 pairs of the new data points. It is suggested that this relationship be accepted as the best-fit viscosity–polymer chain-length dependence for PDMS, and it is pointed out that the obtained PDMS critical chain-length value ranks this polymer's macromolecules as the most flexible of the corresponding long-chain molecules presently known. © 1993 John Wiley & Sons, Inc.