Temperature memory effect

Melting endotherms Melting point depression Melting temperature, Memory effect Metal plating Metallocene catalyst... 

Sun, L. and Huang, W.M. (2010) Mechanisms of the multi-shape memory effect and temperature memory effect in shape memory polymers. Soft Matter, 6, 4403 1406. 

Note The detection of signals from previously measured samples in the mass spectrum of the actual analyte is usually termed memory or memory effect. It is caused by contaminations of ion source or sample introduction system. The best way to reduce memory effects is to use the lowest amount of sample necessary to produce good spectra, to keep ion source and ion source housing at elevated temperature, and to allow some minutes for pumping between subsequent measurements. 

LDHs can take up anion species from solution by three different mechanisms surface adsorption, interlayer anion-exchange and reconstruction of a calcined LDH precursor by the memory effect . The memory effect [130] of LDHs, discussed in 2.3 above, is one of their most attractive features as adsorbents for anionic species. Calcination allows the recycling and reuse of the adsorbents with elimination of organic contaminants as CO2 and water [ 131]. The main advantages of LDHs over traditional anionic exchange resins are their higher AEG values and the fact that LDHs are resistant to high temperature treatments. 

Dousova et al. [142] found that calcined Mg/Al LDHs were effective in removal of As (V) compounds from aqueous solutions at 293 K and neutral pH utilizing the memory effect . More than 70 % of As (V) compoimds were removed from aqueous solution at low sorbent-solution ratios. Parida et al. also studied the affinity of Mg/Fe LDHs toward the removal of inorganic selenite (SeOs ) from aqueous media [143]. The results indicated that the efficacy of SeOs removal increases with a decrease in either pH or temperature. 

Pd-catalyzed reactions proceed via 7i-allyl complexes, which at room temperature isomerize via a ti-o-ti rearrangement. As a consequence, branched as well as Z-and B-linear starting materials yield the same products, with memory effects being minimal at room temperature [16], The isomerization processes of (allyl)Ir complexes are usually slow, and accordingly any memory effects are pronounced. The configurational stability of (allyl) Ir intermediates of the Ir-catalyzed allylic substitution was studied by an investigation of substitutions at nonracemic allylic substrates (Scheme 9.5). 

Alkylimidazolinm tetraflnoroborates are, for example, ionic liquids at room-temperature that can provide an anion to stabilize an intermediate cation-radical with no possibility of nucleophilic attack on it. Ionic liquids have a huge memory effect, and their total friction is greater than that of conventional polar solvents. Thus, the total friction of l-ethyl-3-methylimidazolium hexafluoro-phosphate is about 50 times greater than that of AN (Shim et al. 2007). The solvent effects of ionic liquids on ion-radical ring closures deserve a special investigation. The ring closure reactions can be, in principal, controlled by solvent effects. 

The atomization temperature is usually chosen so as to give a rapid peak. It should not be so hot as to damage the tube unnecessarily or distil off involatile contaminants, or so cool as to lose sensitivity or create memory effects (although a tube clean, i.e. a high-temperature cycle, can be included in the programme). 

Figure 21-8 (a) Transversely heated graphite furnace maintains nearly constant temperature over its whole length, thereby reducing memory effect from previous runs. The i vov platform is uniformly heated by radiation from the outer wall, not by conduction. The platform is attached to the wall by one small connection that is hidden from view. [Courtesy Perkin-Bmer Corp., Norwalk, Cl] (to) Heating profiles comparing analyte evaporation from wall and from platform. [From W. Slavin, Atomic Absorption Spectroscopy, Anal. Chem. 1982,54,685A.]... 

Appropriate parameters should follow linear Arrhenius behavior as a function of temperature (69). The model must demonstrate oxidant pressure dependence (78), memory effects as a function of growth temperature (83), and substrate doping effects on oxidation (84). 

Just above the melting point the polymer is visually quite viscous and numerous observations have been made that the polymer exhibits a memory effect, that is to say, on recooling the melt crystallites will appear in the same sites where they had been before melting the polymer. Hartley, Lord and Morgan (1954) state It is reasonable to suppose that there will be a few localities in the crystalline polymer which have a very high degree of crystalline order, and therefore the melt can contain, even at considerable temperatures above the observed melting or collapse point, thermodynamically stable minute crystals of the polymer . Especially if the polymer has been irradiated so as to contain a few crosslinks as in irradiated polyethylene, then flow is inhibited and spherulites can be made to appear on recrystallization in the same sites that they had before the polymer was melted, Hammer, Brandt and Peticolas (1957). However, as mentioned above, the specific heat of irradiated polyethylene in the liquid state is identical with that of the unirradiated material, within the limits of experimental error. Dole and Howard (1957). 

There has been a general consensus among hydrate researchers that hydrates retain a memory of their structure when melted at moderate temperatures. Consequently, hydrate forms more easily from gas and water obtained by melting hydrate, than from fresh water with no previous hydrate history. Conversely, if the hydrate system is heated sufficiently above the hydrate formation temperature at a given pressure, the memory effect will be destroyed. Some experimental observations of the memory effect phenomenon are summarized in Table 3.3. 

The memory effect has important implications for the gas industry. For example, after hydrates initially form in a pipeline, hydrate dissociation should be accompanied by the removal of the water phase. If the water phase is not removed, the residual entity (i.e., residual structure, persistent crystallites, or dissolved gas) will enable rapid reformation of the hydrate plug. Conversely, if hydrate formation is desired, the memory effect suggests that hydrate formation can be promoted by multiple dissociation and reformation experiments (provided the melting temperature is not too high, or melting time is not too long). 

---