Anomalous dilute solution properties of segmented polydimethylsiloxane-polyurea copolymers in isopropyl alcohol

Preliminary characterization of amphiphilic segmented copolymers of polydimethylsiloxane and urea ‘hard blocks’ was conducted by measuring isopropyl alcohol (primarily) dilute solution viscosities via capillary viscometry. The traditional data analysis techniques, which provide for extrapolation of intrinsic viscosities from these experiments, revealed that increasing concentrations of polymer produced lower reduced viscosities rather than the expected higher values. A very approximate data fit reveals negative Huggins and Kraemer constants from these analyses, which are highly unusual. In a solvent such as DMF, a similar polymer having poly(tetramethylene oxide) and urea blocks and measured with identical conditions exhibited the expected behavior, showing increasing reduced viscosities over concentrations in the same range. However, the non-linearity of the data is suggestive of much more complex hydrodynamic, or supramolecular, interactions that are not clarified by the initial research.     

Rheology and processing of BaSO4‐filled medical‐grade thermoplastic polyurethane

The rheological and processing behavior of BaSO₄‐fllled medical‐grade (additive‐free) thermoplastic polyurethanes (TPUs) was investigated. The rheology and the single‐screw and twin‐screw extrusion processing of filled TPU were found to be complicated by the moisture and the air entrained into the suspension upon the incorporation of the filler BaSO₄. It was observed that the 20 vol% BaSO₄‐filled TPU exhibits decreased, rather than increased, shear viscosity and elasticity (as manifested through smaller storage modulus values in small‐amplitude oscillatory shear flow) in comparison to unfilled TPU in the temperature range of 190°C to 200°C at which these materials are usually processed. The moisture remaining in the BaSO₄ will hydrolyze the polymer, leading to a decrease in the molecular weight of the TPU. However, the moisture contained in the filler itself is not sufficient to explain the significant reductions in viscosity and elasticity of the suspension upon the compounding of the BaSO₄. It is shown that the major factor giving rise to the reductions in elasticity and the viscosity of the suspension of TPU and BaSO₄ is the air (and the moisture air contains) entrained into the extruder during the feeding of the BaSO₄. Air, carried with the filler into the extruder, is entrained into the suspension to hydrolyze the TPU and to further impart a foamy structure under typical processing conditions to significantly reduce the shear viscosity and the elasticity of the suspension in comparison to unfilled TPU. Polym. Eng. Sci. 44:1941–1948, 2004. © 2004 Society of Plastics Engineers.     

Measurements and model calculations of activation cross sections for 232Th(n,2n)231Th reaction between 13.57 and 14.83 MeV neutrons

In this study, activation cross sections were measured for the reaction of ²³²Th(n,2n)²³¹Th (T_1/2 = 25.5 h) by using neutron activation technique at six different neutron energies from 13.57 and 14.83 MeV. Neutrons were produced via the ³H(²H,n)⁴He reaction using SAMES T-400 neutron generator. Irradiated and activated high purity Thorium foils were measured by a high-resolution γ-ray spectrometer with a high-purity Germanium (HpGe) detector. In cross section measurements, the corrections were made for the effects of γ-ray self-absorption in the foils, dead-time, coincidence summing, fluctuation of neutron flux, low energy neutrons. For this reaction, statistical model calculation, which the pre-equilibrium emission effects were taken into consideration, were also performed between 13.57 and 14.83 MeV energy range. The cross sections were compared with previous works in literature, with model calculation results, and with evaluation data bases (ENDF/B-VII, ENDF/B-VI, JEFF-3.1, JENDL-4.0, JENDL-3.3, and ROSFOND-2010).     

Fumed silica filled poly(dimethylsiloxane-urea) segmented copolymers: Preparation and properties

Novel fumed silica filled thermoplastic poly(dimethylsiloxane-urea) (TPSU) segmented copolymers were synthesized and characterized. TPSU copolymers were prepared from a cycloaliphatic diisocyanate, aminopropyl terminated PDMS oligomers with number average molecular weights of 3,200, 10,800 and 31,500 g/mol and 2-methyl-1,5-diaminopentane chain extender. Two different types of fumed silica HDK H2000 (hydrophobic) and HDK N20 (hydrophilic) were utilized and incorporated into silicone-urea copolymers in amounts of 1-60% by weight. Influence of the silica type (hydrophilic versus hydrophobic), amount of silica loading and the PDMS soft segment molecular weight on the morphology, tensile properties and modulus-temperature behavior of the nanocomposites were determined. Major observations of this study were: (i) under the blending conditions used, incorporation of silica does not seem to interfere significantly with the hydrogen bonding between urea groups, (ii) incorporation of silica does not affect the glass transition temperature of PDMS, (iii) incorporation of silica influences the tensile and thermomechanical properties of silicone-urea segmented copolymers significantly, (iv) average molecular weight of the PDMS soft segment in the silicone-urea copolymer plays a critical role on the improvement of the tensile properties of the fumed silica/TPSU composites.     

The effect of the aging period on the martensitic transformation and kinetic characteristic of at % Cu<Subscript>68.09</Subscript>Al<Subscript>26.1</Subscript>Ni<Subscript>1.54</Subscript>Мn<Subscript>4.27</Subscript> shape memory alloy

In this work, the effect of aging period on the characteristic transformation temperatures, thermodynamic parameters and structural variations of CuAlNiMn shape memory alloys were investigated. Aging was performed at above the austenite finish temperature of the un-aged specimen (120°C) for six different retention times, namely 1h, 2h, 3h, 4h, 5h and 6h. The changes in the transformation temperatures were examined by differential scanning calorimetry at different heating/cooling rates. The aging period was found to have an effect on the characteristic austenite and martensite transformation temperatures and thermodynamic parameters such as the enthalpy and entropy of alloys. High-temperature order-disorder phase transitions were determined using a differential thermal analysis, which showed that all the un-aged and aged specimens had an A2 → B2, B2 → L2₁ and an L2₁ → 9R, 18R transition. The structural analysis of the un-aged and aged specimens was performed through X-ray diffraction measurements at room temperature. The intensities of the diffraction peaks varied according to the aging time.     

Measurements of activation cross-sections for the F(n, α)N reaction for neutrons with energies between 13 and 15 MeV

In this study, activation cross-sections were measured for the ¹⁹F(n, α)¹⁶N reaction at six different neutron energies from 13.5 and 14.9 MeV. The fast neutrons were produced via the ³H(d,n)⁴He reaction on SAMES T-400 neutron generator. The cyclic activation technique was used. Induced gamma activities were measured by a high-resolution gamma-ray spectrometer with high-purity germanium (HpGe) detector. Measurements were corrected for gamma-ray attenuations, random coincidence (pile-up), dead time and fluctuation of neutron flux. Results were compared with the previous works.     

FTIR investigation of the influence of diisocyanate symmetry on the morphology development in model segmented polyurethanes

Novel segmented polyurethanes with hard segments based on a single diisocyanate molecule with no chain extenders were prepared by the stoichiometric reactions of poly(tetramethylene oxide)glycol (M_n=1000 g/mol) (PTMO-1000) and 1,4-phenylene diisocyanate (PPDI), trans-1,4-cyclohexyl diisocyanate (CHDI), bis(4-isocyanatocyclohexyl)methane (HMDI) and bis(4-isocyanatophenyl)methane (MDI). Time dependent microphase separation and morphology development in these polyurethanes were studied at room temperature using transmission FTIR spectroscopy. Solvent cast films on KBr discs were annealed at 100 °C for 15 s and microphase separation due to self organization of urethane hard segments was followed by FTIR spectroscopy, monitoring the change in the relative intensities of free and hydrogen-bonded carbonyl (CO) peaks. Depending on the structure of the diisocyanate used, while the intensity of free CO peaks around 1720-1730 cm⁻¹ decreased, the intensity of H-bonded CO peaks around 1670-1690 cm⁻¹, which were not present in the original samples, increased with time and reached saturation in periods ranging up to 5 days. Structure of the diisocyanate had a dramatic effect on the kinetics of the process and the amount of hard segment phase separation. While PPDI and CHDI based polyurethanes showed self-organization and formation of well ordered hard segments, interestingly no change in the carbonyl region or no phase separation was observed for MDI and HMDI based polyurethanes. Quantitative information regarding the relative amounts of non-hydrogen bonded, loosely hydrogen bonded and strongly hydrogen bonded and ordered urethane hard segments were obtained by the deconvolution of CO region and analysis of the relative absorbances in CO region.     

Corrosion of ceramics for vinasse gasification in supercritical water

Supercritical water gasification (SCWG) is a very efficient process to convert wet biomass into energetic gases. Unfortunately, SCWG reactor may strongly corrode due to the addition of temperature, pressure and the presence of corrosive species. In the present paper, the corrosion of various ceramic materials in subcritical and supercritical water (SCW) gasification process was studied in a batch reactor. We compare the corrosion in distillated water and the corrosion in sugar beet slurry that will be gasified under supercritical conditions. The experimental temperatures were 350 °C and 550 °C and the pressure was 25 MPa. Technical ceramics (SiC, alumina, Y stabilized zirconia, Si₃N₄, BN, aluminosilicate, cordierite-mullite) show poor capability to sustain corrosion whereas graphite and glassy carbon are the highest performance materials in our working conditions.