Shape fixing temperature

Abstract This chapter introduces three sets of T -type shape memory polyurethanes. The thermal properties and morphological structure of polymers are shown to vary with chemical composition. The morphology of the T -SMPUs is thus correlated to shape memory properties. The influences of several important thermo-mechanical conditions including deformation temperature, deformation strain, and shape fixing temperature and time on shape memory properties are demonstrated. 

Influences of shape fixing temperature on shape memory polymers... 

Chapter 1 briefly introduces the mechanisms of the shape memoiy effect and shape memoiy polymers, summarizing key research in the subject. It also introduces one of the most important shape memory polymers, shape memory polyurethane, and its unique properties. Since a melting transition temperature (T ) or a glass transition temperature (T can act as the switch transition for shape memory polyurethane, it can be called either a T -type or a T -type shape memoiy polyurethane. Chapters 2 and 3 discuss T -type and T -type shape memoiy polymers. The effects of deformation temperature, deformation amplitude, shape fixing temperature and pre-deformation are discussed in terms of their influences on shape memoiy properties. 

We can sample the energy density of radiation p(v, T) within a chamber at a fixed temperature T (essentially an oven or furnace) by opening a tiny transparent window in the chamber wall so as to let a little radiation out. The amount of radiation sampled must be very small so as not to disturb the equilibrium condition inside the chamber. When this is done at many different frequencies v, the blackbody spectrum is obtained. When the temperature is changed, the area under the spechal curve is greater or smaller and the curve is displaced on the frequency axis but its shape remains essentially the same. The chamber is called a blackbody because, from the point of view of an observer within the chamber, radiation lost through the aperture to the universe is perfectly absorbed the probability of a photon finding its way from the universe back through the aperture into the chamber is zero. 

The complete curve for the response of an uncross-linked polymer at a fixed temperature, depicted here, covers so many decades of time that it has only been measured at a single temperature on a very few low-molecular-weight polymers. The experimental results seen in the literature are actually a composite of data taken at several temperatures over a limited time scale. The effect of a temperature rise is to translate the main transition in the curve of Figure 5A to the left, toward shorter time, with essentially no change in shape. 

The possible preparation of InAs by crystallization from the melt depends also on the liquidus shape (especially in the In-rich region). A summary of previous liquidus measurements was reported by De Winter and Pollack (1986) who employed a source dissolution method based on the equilibration, at a fixed temperature, of a known quantity of high-purity indium with single crystals of InAs, the weight loss of which was determined. The experiments were carried out under a flux of hydrogen purified via permeation through palladium. 

Typically, adsorption isotherms are generated using a batch experiment at a fixed temperature and a fixed feed composition. These experiments include exposing a known amount of adsorbent to a known concentration of adsorbate at a constant temperature. Once equilibrium is established, the net adsorbate concentration in the liquid is measured. This process is repeated at multiple adsorbate concentrations and temperatures. A plot of adsorbate loading (g adsorbate/g adsorbent) versus adsorbate concentration reveals the adsorption isotherm with the shape of the isotherm determining the suitability of a particular adsorbent for a particular system [20]. 

For any given potential and dipole function, at a fixed temperature, the classical and quantum profiles (and their spectral moments) are uniquely defined. If a desymmetrization procedure applied to the classical profile is to be meaningful, it must result in a close approximation of the quantum profile over the required frequency band, or the procedure is a dangerous one to use. On the other hand, if a procedure can be identified which will approximate the quantum profile closely, one may be able to use classical line shapes (which are inexpensive to compute), even in the far wings of induced spectral lines a computation of quantum line shapes may then be unnecessary. 

Useful insights into the kinetics of a phase transformation that proceeds by nucle-ation and growth can be obtained by observing the fraction transformed, , under isothermal conditions at a series of different temperatures. This is usually done by undercooling rapidly to a fixed temperature and then observing the resulting isothermal transformation. The kinetics generally follows the typical C-shaped behavior described in Exercise 18.4. If a series of such curves is obtained at different temperatures, the time required to achieve, for example, ( = 0.01, 0.50, and... 

Thixotropic Fluids. Thixotropic fluids are characterized by a decrease in their viscosity with time at a constant shear rate and fixed temperature. When shear rate is steadily increased from 0 to a maximum value and then immediately decreased toward 0, a hysteresis loop is formed, as shown in Figure 3. The shape of the hysteresis loop is also a function of the rate by which the shear rate, 7, is changed. Oil-well drilling muds, greases, and food materials are examples of thixotropic fluids. 

Traditional manufacture of ceramics is based on the use of fine natural raw materials which are capable of producing easily worked plastic mass (paste) with water. The formed bodies maintain their shape. The products are strengthened and the shape fixed by firing at temperatures which were attainable even in the primitive kilns. Kaolin and clays are the main raw materials which exhibit these required properties. 

The solvent vapor pressure is not zero and changes with the composition of the solution at a fixed temperature. If the mole fraction of solvent (Xi) is 1, then the vapor pressure is P°, the vapor pressure of pure solvent at the temperature of the experiment. When Xi approaches 0 (giving pure solute), the vapor pressure i of the solvent must go to 0 also, because solvent is no longer present. As the mole fraction Xi changes from 1 to 0, i drops from P° to 0. What is the shape of the curve ... 

Three-dimensional (3-D) plot of the shape memory cycle for (a) a shape memory polymer (SMP) and (b) vulcanized natural rubber. The star indicates the start of the experiment (initial sample dimensions, temperature, and load). Both the SMP and the rubber were deformed under constant loading rate at constant temperature. The deformation step was then followed by a cooling step under constant load. At low temperature, the load was removed and shape fixing was observed for the SMP, but an instant recovery was seen for natural rubber. Shape recovery of the primary equilibrium shape was obtained by heating the SMP. (Adapted from Liu, C., Qin, H., and Mather, P. T. 2007. Review of progress in shape-memory polymers, journal of Materials Chemistry 17 1543-1558. Copyright Royal Society of Chemistry. Reproduced with permission.)... 

In summary, the test results show that cold compression is an effective and efficient method for programming. It is found that the pre-strain level must be larger than the yielding strain of the SMP in order to fix a temporary shape at temperatures below Tg. It is also found that a longer stress relaxation time leads to a larger shape fixity ratio. The upper bound of the shape fixity is determined by the differcnee between the pre-strain and the springback, which is the ratio of the relaxed stress over the relaxed modulus. 

Pbnm structure under static conditions. Simulations were performed in the canonical ensemble (fixed temperature and cell shape and size) temperature was maintained with a Nose (1984) thermostat. The time step was 1 fs, and the simulations were run for 1.6 ps. 

Carbon molecular sieves were prepared from activated carbons by coke deposition from the thermal cracking of propylene. The heat treatment was carried out in a smaller diameter U-shaped fixed bed reactor made of an A inch 316 SS tube. A 2 kW vertical electric furnace was used for heating the reactor, while the design of gas flow and temperature control instruments in the activation system permitted also their use in the smaller reactor system. The... 

The isobaric-isothermic ensemble is characterized by a fixed number of atoms (N), a fixed pressure (P), and a fixed temperature (T). This method is applicable to periodic systems only. The unit cell vectors are allowed to change, and the pressure is adjusted by adjusting the volume (the size and shape of the unit cell). Several methods are available to control pressure. Those of Berendsen et al. (1984) and Anderson (1980) only vary the size of the unit cell, whereas that of Parrinello and Rahman (1982) allows both the cell volume and its shape to change. NPT is the ensemble of choice when the correct pressure, volume, and densities are important in the simulation. This ensemble can also be used during equilibration to achieve the desired temperature and pressure before changing to the constant-volume or constant-energy ensemble when data collection starts. 

Figure 13.1 The critical behavior of a pure substance. Below the critical temperature, at a fixed temperature, a decrease in volume results in a transition to a liquid state in the region AB in which the two phases coexist. The envelope of the segments AB for the family of isotherms has the shape BCD. Above critical temperature there is no such transition. The gas becomes more and more dense, there being no distinction between gas and liquid phases. Following the path shown by the arrows, it possible to go from a gas to a liquid state without going through a transition...

So far, the results have demonstrated that nucleation is characterized decisively by non-steady-state effects. The typical shape of the isotherms illustrated in Fig. 2.9 is the result of the continuous approach of the non-steady-state nucleation rate to the equilibrium nucleation rate at any fixed temperature. The different shape of isotherms shows that the high-temperature and... 

On the other hand, when the temperature is below r, s, the switching phase (light grey in Fig. 7.4) will fix the chains keeping the temporary shape. When temperature is below 7) s it is possible to release the stress. 

The stress-strain test is the procedure more commonly reported in the scientific literature to characterize the shape memory effect (Chen et al., 2009 Liu et al., 2007). It can be represented in a two-axis system, the variables of which are stress and strain for a fixed temperature. A more efficient representation of this test represents these variables in a three axis system, by adding the temperature axis. This will allow observing the temperature behaviour and locating the transition temperature. Figure 7.5 shows a typical stress-strain test in a three axis system. The complete shape memory test is constituted by a four-step q cle ... 

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