Hysteresis monomer

The model describes the characteristic stress softening via the prestrain-dependent amplification factor X in Equation 22.22. It also considers the hysteresis behavior of reinforced mbbers, since the sum in Equation 22.23 has taken over the stretching directions with ds/dt > 0, only, implying that up and down cycles are described differently. An example showing a fit of various hysteresis cycles of silica-filled ethylene-propylene-diene monomer (EPDM) mbber in the medium-strain regime up to 50% is depicted in Figure 22.12. It must be noted that the topological constraint modulus Gg has... 

We shall deal in this lecture with recent improvements in the elastomers synthesis, that should be able to cope with the above mentioned requirements, without resorting to important investments for new plants or to cumbersome feedstocks. The improvement of the elastomer synthesis relies upon new catalytic systems that allow a control of elastomer tacticity in order to achieve a strain induced crystallization, and suitable monomer combinations in order to minimize the hysteresis loss of the elastomer in a wide range of temperatures and frequencies. 

Zinc increases the rate of dimerization 21, 65, 66) however, Apple-bury and Coleman 48) showed that zinc is not necessary for dimerization to occur. Starting at a high pH and slowly lowering the pH they found that all of the zinc is lost by the time the pH reaches 4.0, yet the molecule though inactive is still dimeric. However, upon increasing the pH of a solution of monomers, the dimer reforms by pH 5.0, yet the zinc does not bind completely until pH 6.0. Also, the optical rotatory dispersion (ORD) spectrum is the same for dimer at pH 8.0 and monomer at pH 4.0, but a spectral change occurs for the monomer at pH 2.0 48, 67). Applebury and Coleman 48) reasoned that there must be a kinetic barrier which prevents any rapid equilibration of the dimer monomer system at intermediate pH values and that the same barrier exists in the hysteresis loop involved in the titration of carboxyl groups on the enzyme 21, 67). 

In summary, the rheological properties of these bis-urea solutions can be switched from a viscoelastic behavior (at low temperatures) to a purely viscous behavior (at high temperatures). Moreover, the transition has been shown to be fast, reversible (without hysteresis) and extremely cooperative the conversion of tubes into thin filaments occurs within a temperature range of 5 °C only [40]. This transition can be triggered by temperature, but also by a change in the solvent composition or by a change of the monomer composition. 

In some cases, it is desirable to have a pharmaceutical protein in an aggregated state because it is the bioactive form of the protein. An example of this is surfactant protein B (SP-B), a pulmonary surfactant protein necessary for normal lung function in neonatal infants. " The protein exists exclusively as a homodimer in which the monomers are linked by a disulfide bond. In studies investigating efficacy of the SP-B monomer compared with the dimer in transgenic mice, it was found that although the surfactant action was preserved in the monomeric form of the protein, altered lung hysteresis was noted. The authors concluded that SP-B dimerization is required for optimal lung function. 

A study of the dissociation of apoferritin in subunits by extremes of pH. was initiated for a number of reasons. The observation (152) that treatment with acetic acid produces stable subunits at pH 3.0 together with evidence of a pH dependent conformation change in apoferritin below pH 3.0 (159) and the observation that at pH 3 apoferritin is present only as 17.6 S monomer (R. R. Crichton and C.F. A. Bryce, unpublished work) suggested that at low pH apoferritin can be reversibly dissociated, and that a significant hysteresis occurs in reassociation of the subunits to apoferritin monomers. In the course of studies on subunit dissociation (109) and on oligomer formation from apoferritin monomers (152) it was shown that partial dissociation of apoferritin into 2—3 S subunits occurs at high pH. 

The dissociation of apoferritin at extremes of pH was examined by sedimentation velocity techniques (166). It was established that between pH 2.8—10.6 the apoferritin monomer ( 17S) was the only species that could be detected. Between pH 2.8—1.6 and 10.6—13.0 both monomer (17S) and a low molecular weight component (2—3S), presumed to be subunit, were detected. The dissociation follows a smooth sigmoidal curve (Fig. 7 a) in both cases with mid points, corresponding to equal amounts of 17 S and 2—3S component at 2.2—2.4 for the acid dissociation and 11.8—12.2 for the alkaline dissociation (the exact values are dependent on the buffer used). When apoferritin is completely dissociated into subunits at low pH (either by exposure to buffer of pH 1.6, or by treatment with 67% acetic acid) and is then dialysed into buffer of higher pH, reassociation does not take place in dilute glycine buffers until pH values in excess of that required to induce subunit dissociation (Fig. 7 a). The reassociation then follows a sigmoidal curve until complete reassociation to a 17S monomer is attained at pH 4.3. Although we have not as yet been able to follow reassociation completely in a more concentrated buffer (200 mM cf 10 mM) it is clear (Fig. 7 a) that subunit reassociation occurs at a much lower value and that the hysteresis observed between dissociation and reassociation is much less in the more concentrated... 

Figure 13. Hysteresis loop of proteoglycan monomer (-------) after 5 min (-...

As shown in Scheme 2.27, POEVE and PNIPAM have similar structures with both hydrophilic and hydrophobic parts in each monomer unit. Quite recently, our group examined another possibility for thermosensitive phase separation random copolymers of hydrophilic and hydrophobic monomers.Although random copolymers have been investigated previously, the achieved phase separation was broad, with hysteresis and low turbidity. By living cationic polymerization in the presence of added bases, our group successfully prepared random copolymers of IBVE and HOVE, both of which are typical hydrophobic and hydrophilic monomers, and which are not thermosensitive themselves. At low temperature, the polymers were soluble in water, but when the temperature was increased to a critical point, the transparent solution became opaque. The phase separation was quite sensitive (Scheme 2.27(a)) and the temperature of phase separation was governed by the monomer feed ratio. 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

Fig. 13.27 Achiral antiferroelectric. Voltage dependence of pyroelectric coefficient describing the double hysteresis loop (a) and dependence of the field-induced polarization on the content of a monomer in the polymer-monomer mixtures (b)...

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