Memory-effect phenomena

Memory-effect phenomena occur because the typical relaxation times for polymer solutions correspond to times readily accessed experimentally. A disturbance or fluctuation takes place in a polymer solution, and the response to the initial impulse lasts long enough to be experimentally significant. In principle, memory-effect phenomena are equally found for simple liquids and small-molecule solutions. However, characteristic relaxation times for a small-molecule liquid are 

Memory effects are revealed by experiments in which a complex fluid is subject to a time-dependent shear rate. Included under this rubric are measurements of the startup stress when a constant shear rate is suddenly imposed on an initially stationary system, and the stress when a system subject to some nonzero constant rate of strain suddenly has the rate of strain increased, decreased, or reversed. A prominent feature in measurements of stress on sudden imposition of a large rate of strain is stress overshoot, in which the stress first increases to a value much larger than its steady-state value, and then relaxes back to its steady-state value. Contrariwise, if the shear rate applied to a polymer fluid is held constant for a long time and then suddenly reduced, the stress may show undershoot the stress declines to a value well below its steady-state value and then increases back to its steady-state value. Related features have been seen for N. Bird, et al. also note measurements on responses to superposed flows, in particular the combination of a constant rate of shear flow with an oscillatory shear parallel or perpendicular to the constant shear(7). Bird, et al. further assert that multiple oscillations around the steady-state stress are sometimes observed before the steady state is attained. Recent studies involving step strains or oscillatory shear superposed on steady shear are reported by Li and Wang(8). 

System relaxation times have been determined from the relaxation of the stress after abrupt cessation of shear flow. Representative applications of the approach are found in Takahashi, etal. 9), who examined 355-3840 kDa polystyrenes in benzyl- -butylphthalate, at concentrations identified as showing dilute-solution behavior for the steady-state compliance Je and semidilute behavior for the zero shear viscosity. The stress relaxation after shear cessation, identified as the transient viscosity, decreased exponentially with time except at the shortest times studied, leading to an identification of an observed longest relaxation time Xm, whose c and M dependences were determined. 

Studies of stress overshoot on sudden imposition of constant rate of strain include Osaki, etal. 0) and Inoue, etal. ). Osaki, etal. also report the time dependence of N. Inoue, et al. note a potential artifact perturbing stress measurements, namely shear-induced phase separation. Representative experiments on double strain rates are presented by Oberhauser, et al. 2) and by Wang and Wang(13). The observed stress has a complex time dependence including overshoot and undershoot  

Oberhauser, et al. also present orientation angle and birefringence from optical flow birefringence measurements. Tapadia and Wang(14) and Boukany, et a/. (15) interpret the overshoot and undershoot processes in terms of fluids that have become habituated to a particular rate of shear, so when the shear rate is changed they are initially in the state pertaining to the former shear rate, and only gradually achieve the state characteristic of the new steady-state shear rate. 

---

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 observations of the memory effect phenomenon summarized in Table 3.3 have been explained by two opposing hypotheses ... 

Some Experimental Observations of the Memory Effect Phenomenon... 

Although the evidence of the memory effect phenomenon is plentiful, and clearly not in question, there have been only a limited number of direct molecular-level investigations to verify the above hypotheses. Furthermore, the results of these investigations have so far presented opposing conclusions on which hypothesis is correct. 

Stdckel, D. The Shape Memory Effect Phenomenon, Alloys, and Applications, Nitinol Devices Components, Inc., Freemont, CA, 2000. 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

A very important electrochemical phenomenon, which is not well understood, is the so-called memory effect. This means that the charging/discharging response of a conducting polymer film depends on the history of previous electrochemical events. Thus, the first voltammetric cycle obtained after the electroactive film has been held in its neutral state differs markedly in shape and peak position from subsequent ones [126]. Obviously, the waiting time in the neutral state of the system is the main factor determining the extent of a relaxation process. During this waiting time, which extends over several decades of time (1-10 s), the polymer slowly relaxes into an equilibrium state. (Fig. 13) After relaxation, the first oxidation wave of the polymer appears at more... 

Regeneration Many LDH materials show a unique phenomenon called memory effect, which involves the regeneration of the layered crystalline structure from their calcinated form when the latter is dispersed in an aqueous solution containing suitable anions [96]. This property is often used to synthesize and modify LDH with different types of intercalating anions. The regeneration property shown by LDH is extensively reported in the literature [97, 98]. 

The relaxation phenomenon described herein is also called first-cycle effect, secondary break-in, waittime effect, memory effect, and hysteresis phenomenon which is in connection with the characteristic changes of the cyclic voltammograms obtained for -> polymer-modified electrodes [iii-vi]. 

Applications of Ni/MH batteries include computers, camcorders, cellular phones, communication equipment, variety of cordless consumer products, high rate long cycle life applications, electric vehicles (under development), and so on. Ni/MH batteries are more environmental friendly than Ni/Cd batteries, and they are easy to dispose. Disadvantages of Ni/MH batteries include lower rate capability, poorer charge retention, and less tolerance for overcharge than Ni-Cd batteries. Like Ni/Cd batteries, Ni/MH batteries are also subject to the memory effect a description of this phenomenon can be found in Sect. 7.9.2.2. 

When we compare r and AT of the first run with those of the subsequent average ones, we find that the memory effect is small, or even be recovered, in the solutions with a small AFP concentration. The recovering of memory effect tended to larger in higher AFP concentrations. This is a very interesting phenomenon, and has been reported to be unique for AFPs, although this effect should be confirmed by statistical analysis in future studies. 

Cooperativity is one of the most appealing and elusive facets of the spin-crossover phenomenon. It is a main aspect because discontinuity in the magnetic and optical properties along with thermal hysteresis confer to these systems potential memory effect. Nevertheless, because most of the spin-crossover systems are discrete in nature, cooperativity stems from assemblies of molecules held together by nonco-valent interactions and, consequently, difficult to control. 

The present chapter is organized in three sections the first section is devoted to basic theoretical backgroimd concerning the spin-crossover phenomenon, viz ligand field theory, thermodynamics and cooperativity the second section reports on some examples that we feel are particularly relevant to illustrate the main directions in which research on synthetic aspects of cooperativity is being directed finally, the third section describes three approaches, up to now reported, concerning spin bistability in supramolecular and molecular systems and memory effect. 

Neumann [2] first used the term molecular hysteresis to describe the phenomenon which was hysteresis on precipitation and dissolution of the polymers in pH changes of aqueous solutions. However, because segments of the molecules interact, as do molecules in condensed form, the hysteresis he describes is still a collective phenomenon. Frieden [3] proposed the idea of a hysteretic enzyme in 1970 and Hand and Carpenter [4] reported that phosphofructokinase, an important regulatory enzyme, could be a hysteretic enzyme. The concept is of a simple mechanism consisting of two parts fast binding between the enzyme and a substrate, and a slow conformational change in the complex enzyme. The resulting conformation is retained for a considerable time, behavior that constitutes a memory effect but not hysteresis. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

The original structure can be restored by rehydration. This phenomenon is called the memory effect of hydrotalcite, and suggests that dehydration induces a small displacement of the cations only and not complete rearrangement of structure. Dehydration is accompanied by a spectacular increase of surface area, as illustrated in Table 2, and gas phase rehydration with water-saturated nitrogen by a decrease to very low values of surface area and porosity [12]. 

A study of the memory effect , the phenomenon that makes EB-1 transform into ES-I and EB-11 into ES-11 upon doping, and reversibly so, has been undertaken as well [305]. RDF analysis reveals differences between EB-1 and EB-11 in the range 2.5 A, i.e. in intrachain coiTelations. This is attributed to differences in chain conformation. (Ring tilt angles and C—N—C zigzag... 

The phenomenon of shape memory effect in SMPs is brought about by large changes in elastic modulus, E, above and below the transition temperature. Figure 1.3 shows a typical modulus behaviour of SMPs with temperature. At a temperature above the transition, the polymer enters a rubbery elastic state, and hence the elastic modulus of the polymer is much reduced. Consequently the polymer can be easily deformed by application of an external force (Bar-Cohen, 1999 Liu et ah, 2007). If the material is allowed to cool below its transition temperature, under reasonable strain, its temporary deformation becomes hxed. At this stage, the polymer lacks its rubbery elasticity and displays a high modulus. This state is called the glassy state. This deformation can be recovered when the polymer is heated above the transition (Hu, 2007). 

---