Lower critical separation phenomena

It is well known that the possible causes for the presence of lower critical separation phenomena in polymer solutions have been associated with differences in free volume between the solvent and the macromolecular solute in the vicinity of the solvent critical point. Another possible reason for the appearance of lower critical phenomena in aqueous polymer solutions has been identified with the circumstance that at moderate temperatures hydrophobic interactions among polymer chains could be developed. In the later case, the behaviour of phase separation should be related with the existence of hydrogen bonding as well. 

As mentioned previously (see Section 1.3.5) the binary M-X system shows a phase separation phenomenon in which the phase decomposes into two phases, having lower and higher concentrations of vacancies, below the critical temperature f, under the condition < 0, i.e. there is an attractive force between vacancies. In Section 1.3.5 it was not possible to refer to the details of those structures, because the model was less than simple. In any case, it can be safely said that if s < 0, vacancies cluster at low temperatures (from a thermodynamic point of view). Here let us briefly review the non-stoichiometry of 3d transition metal monoxides Mj- O, and then discuss the Fej system as a typical example of the clustering of vacancies in detail. 

The importance of the excess entropy of mixing in aqueous mixtures explains why many of these systems show phase separation with a lower critical solution temperature (LCST). This phenomenon is rarer—though not unknown—in non-aqueous mixtures (for an example, see Wheeler, 1975). The conditions for phase separation at a critical temperature can be expressed in terms of the excess functions of mixing (Rowlinson, 1969 Copp and Everett, 1953). 

Of all stimuli-responsive polymers, temperature-responsive polymers are the best known and most studied. Among those, polymers that exhibit a lower critical solution temperature (LOST) have found the widest applicability [14]. The LOST is a fascinating phenomenon found for various polymer solutions. Polymer solutions often exhibit both an LCST and an upper critical solution temperature (UCST). For the LCST, at temperatures below the LCST the polymer is completely miscible in the solvent, whereas at temperatures above the LCST a phase separation occurs. In fact, the most investigated temperature-responsive polymer featuring a LCST in water is poly(N-isopropylacrylamide) (pNlPAm). The LCST of pNlPAm is 32°C,... 

Polymer blends may be characterized in terms of the temperature dependence of the Flury-Huggins interaction parameter (j)- In the case of an upper critical solution temperature (UCST) blend, / decreases with temperature, and the blend remains miscible. For phase separation to occur in a UCST blend, the temperature must be lower than the critical solution temperature. In the case of a lower critical solution temperature (LCST) blend, x increases with temperature, and thus phase separation occurs above the critical solution temperature. The ability of CO2 to mimic heat means that miscibility is enhanced in the case of UCST blends, and for the case of LCST blends the miscibihty is depressed. Ramachandrarao et al. [132] explained this phenomenon by postulating a dilation disparity occurring at higher CO2 concentration as a result of the preferential affinity of CO2 to one of the components of the blend, inducing free-volume and packing disparity. 

While most of the work described in this monograph emphasizes the two-phased nature of IPNs and related materials, it is interesting to explore more deeply the characteristics of phase separation in polymer/polymer systems. Of key importance, McMaste/ " showed that most polymer/polymer phase diagrams are expected to exhibit a lower critical solution temperature (LOST). This means that as the temperature is raised the polymer pair becomes less mutually soluble, and phase separates. This effect is not immediately predicted by equations (2.1) and (2.2), which suggest the usual phenomenon of an upper critical solution temperature (UCST). 

Upper Critical Solution Temperature n The maximum temperature for phase separation of polymer-solvent solutions to occur (Flory-Huggins theory) also, phase separation occurs when the temperature is raised until a lower critical solution temperature is reached, the phenomenon is explained by the free-volume theories of polymer solutions (Kamide K, Dobashi T (2000) Physical chemistry of polymer solutions. Elsevier, New York). 

Early studies by Bates et al. [106,107] and by Sakurai et al. [87] reveal that these systems exhibit an upper critical solution temperature (UCST), i.e., undergo phase separation upon cooling. Subsequent investigations have focused on the combined influence of isotope effects and the blend microstructure on the miscibility patterns of these random copolymer binary mixtures. In particular, a series of systematic SANS experiments by Jinnai et al. [53] demonstrate that the UCST phase behavior that had previously been observed for these systems [87,106,107] remarkably converts to a lower critical solution temperature (LCST) phase separation with an increase in the vinyl content of the HPB component when the vinyl content of the DPB component remains flxed. This phenomenon cannot be explained by the traditional extension of FH theory to random copolymers since this theory is derived under the assumption that the individual Xap are of purely energetic origin. Thus, the FH random copolymer theory [88] is capable, at most, of predicting the conversion of a UCST phase separation system into a completly miscible system. 

When the polar additive nonylic acid was added into hexade-cane liquid, the contact ratio becomes much smaller than that of pure hexadecane, which is shown in Fig. 39. For hexa-decane liquid, the critical speed to reach zero contact ratio is 50 mm/s, which is much higher than that of mineral oil 13604 because of its much lower viscosity. Flowever, when nonylic acid was added into the hexadecane liquid, the critical speed decreased from more than 50 mm/s to 38 mm/s. The same phenomenon can be seen in Fig. 39(h) which shows the comparison of oil 13604 and that added with 1.8 %wt. nonylic acid. The addition of polar additive reduces the contact ratio, too, but its effect is not as strong as that in hexadecane liquid because the oil 13604 has a much larger viscosity. Therefore, it can be concluded that the addition of polar additives will reduce the contact ratio because the polar additives are easy to form a thick boundary layer, which can separate asperities of the two rubbing surfaces. 

From Fig. 3, it may be seen that lowering of the temperature to 60-70 will cause separation of amylopectin. In general, this phase separation takes the same route as that for the amylose (except for the peculiar, morphological phenomena of the latter). As crystallization is much slower for the branched fraction of starch, the critical temperature of phase separation is sufficiently high to permit the existence of a coherent, liquid phase for short periods of time. The fact that freshly obtained amylopectin precipitate is soluble in cold water, whereas, after several hours, it is completely insoluble in cold water can only be interpreted as being the result of crystallization. In accordance with this conclusion, it is to be noted that this phenomenon is perfectly reversible. 

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