Molecular weight influence on shape memory effect of shape memory polymer blend (poly(caprolactone)/ styrene‐butadiene‐styrene )

The shape memory effect (SME) does not only concern the macroscopic structure. It concerns also the polymer structure at morphological, macromolecular, and molecular scales. This effect may depend on different physicochemical properties like morphology heterogeneity, chain rigidity, steric hindrance, chain polarity, free volume, cross-linking or entanglement density, molecular shape and weight, and so on. Hence, finding the relationship between the SME and these properties is very important. This can help to obtain the knowledge about the phenomenon origin and mechanism. One of the basic polymer properties, which can have direct SME, may be the molecular weight (Mw). The question here is: If the Mw of a shape memory polymer (SMP) changes, for different reasons like degradation, what will be the effect of this change on its SME. In order to answer to this question, the investigation is focused on an SMP blend of 40% poly(ɛ-caprolactone) (PCL) and 60% styrene-butadiene-styrene (SBS). Then, enzymatic hydrolysis is performed on this blend to change its Mw. It is shown that this change is only related to the variation in the Mw of PCL. After that, different samples with a distinct average Mw are prepared and characterized by various experimental methods. Shape memory tests are performed on these blends, and the recovery rate (Rr) for each of them is determined. It is found that when Mw of PCL decreases, its degree of crystallinity, its glass transition, and its melting temperatures, corresponding to the PCL phase, increase. However, the elongation at break of the blend declines with the reduction in Mw. The tests show that the alteration in the blend's Mw influences its SME. Indeed, Rr of the (PCL/SBS) mixture drops with the decrease in Mw of PCL

Effect of Hydrogen on the Stress Relaxation of Aged NiTi Shape Memory Alloys

The susceptibility of the NiTi shape memory alloy to relaxation after the hydrogen charging in an aqueous solution has been investigated with respect to ageing during one to six days in air at room temperature. The orthodontic wires have been prepared by immersing in a 0.9% NaCl solution for 3 h, under applied current density of 10 A/m^2 and then relaxed with an imposed deformation in a fully austenite state of structure and in a state with 1/3 and 2/3 of the martensite volume fraction. Through the stress relaxation, the hydrogen-charged specimen has shown a significant decrease of the stress, compared to the non-immersed alloy, when the imposed deformation was located in the plateau of the austenite-martensite transformation. It was also found that a longer ageing period is important and the properties of the wires with longer stress relaxation are similar to the those of non-charged wires. Nevertheless, no difference has been detected between the as-received and the as-charged specimens when the imposed deformation was located in the elastic deformation region of the fully austenitic structure. This behavior is attributed to the effect of the gradient of absorbed hydrogen, existing between the surface and the center axis of the studied wires, which facilitates the mobility of the martensite bands during the stress relaxation

Thermal stability and hydrogen-induced softening in Zr57Al10Cu15.4Ni12.6Nb5 metallic glass

International audienceThe effects of hydrogen absorption on the structure, mechanical and thermal properties of Zr57Al10Cu15.4Ni12.6Nb5 amorphous metal were investigated. Structural analysis, using X-ray diffraction, showed that prominent peak shift towards lower angles (from 37.55 degrees to 37.47 degrees) after 1 h of hydrogen charging in 0.1 M NaOH solution which indicates a volume expansion of the amorphous structure. By means of nanoindentation tests, we found that hydrogen-induced, glass plasticity could occur in Zr-based metallic glass even for high Zr content (>50 at%). The thermal stability against crystallization, evaluated by differential scanning calorimetry (DSC), has been improved by hydrogen absorption. The onset crystallization temperature was increased by 6 degrees C to 10 degrees C due to slight hydrogen addition. The resistance to hydrogen embrittlement combined with high thermal stability gives rise to the possibility of the practical use of Zr57Al10Cu15.4Ni12.6Nb5 amorphous alloy as a good metal membrane operating at temperatures below 400 degrees C. (C) 2016 Elsevier B.V. All rights reserved

Influence of long term exposure to molten lead on the microstructure and mechanical behaviour of T91 steel

Specimens of T91 9%Cr martensitic steel were exposed to molten lead at 525°C under controlled oxygen partial pressure, for durations ranging from 6 hours to one month. At low oxygen concentration, a thin chromium oxide layer is formed; under higher concentration, the oxide layer becomes thicker and complex. Under all conditions in liquid lead, numerous porosities appear after a few days of exposure to lead in the sub-surface regions of the samples. From the residual deflection of bent samples, the stress relaxation is much faster in liquid lead than under gaseous atmosphere. However, the tensile properties remain unchanged after one month ageing in lead

Resistance to corrosion an embrittlement of T91 steel in stagnant Pb-Bi of eutectic composition

The resistance to corrosion and embrittlement of T91 steel in stagnant Pb-Bi eutectic was studied in the temperature interval 150-650°C. The degradation of the corrosion resistance manifested itself above 350°C, with Pb-Bi sticking at surface inclusions at 600°C and with localized corrosion effects visible at 650°C. Nevertheless, the specimens aged in Pb-Bi did not exhibit any degradation of mechanical properties during ex situ testing in air at room temperature. On the contrary, a liquid metal embrittlement (LME) effect was observed, when the tensile tests were camed out in situ in Pb-Bi in the temperature interval 300-400°C. The LME effect depends on temperature, strain rate, existence of stress concentrators and especially on the chemical composition of the cover gas. This effect was observed in Pb-Bi under He-4% H$_2$ cover gas, whereas almost no LME effect was detected in Ph-Bi under vacuum. A maximum reduction in energy to rupture (around 30%) was found for notched specimens tested in Pb-Bi under He-4% H$_2$ cover gas at a crosshead displacement rate of 6x10$^{-4}$ mm/s at 350°C