LCST and UCST

Hence, both UCST and LCST shift to higher temperatures with increasing pressure. However, if AVM > 0 in the gap between UCST and LCST then the gap diminishes with increasing pressure. As a rule, taken from the theory, positive volumes of mixing are likely when the gap between LCST and UCST is sufficiently small. In other words, an increasingly positive volume of mixing is unfavorable for miscibility of polymers and leads ultimately to phase instability. 

For the system PMMA/PVDF one can estimate the volume of mixing according to Eq. (22). As the key-point, the system exhibits both LCST and UCST. The critical points are reported to be about at 325 and 140 °C for 50/50 blends [11], These data can be used to calculate, from Eqs. (18) and (19), the quantities XAB and p. 

Fig. 7. Miscibility door for 50/50 blends of PB and SB as a function of copolymer composition. The circles refer to experimentally determined LCSTs and UCSTs. The curve was calculated using the equation-of-state theory discussed in Sect. 2.1. Miscibility occurs to the left of the curve. Inside the dashed area, solution cast films are transparent [39]...

Fig. 20. Phase diagram for cellulose diacetate fraction-2-butanone systems, showing LCST and UCST 57 >...

Huggins theory but differs in one important respect in that it allows the lattice to have some vacant sites and to be compressible. Thus the compressible lattice theory is capable of describing volume changes on mixing as well as LCST and UCST behaviours. As with the theory of Flory and his co-workers, (which is proportional to the change in energy that accompanies the formation of a 1-2 contact from a 1-1 and a 2-2 contact) is obtainable from experimental values of heats of mixing. 

Recent attempts to prepare 26 by RAFT, however, failed [153]. Double hydrophilic block copolymers of NIPAM and 23e [154], as well as of N,N-diethylacrylamide and 23b [155], were prepared with the CTA benzyl dithiobenzoate, and exhibit LCST and UCST behavior in water. The new polymer 51 is also part of amphiphiUc di- and triblock copolymers [152]. Diblock copolymers with poly(ethylene glycol) methyl ether acrylate, dimethylacry-lamide, or 4-vinylstyrene sulfonate are macrosurfactants with a switch-able hydrophobic block. Triblock copolymers containing additionally 4-vinylbenzoic acid differ in the nature of the hydrophilic part [152]. Near-monodisperse block copolymers of N,N-dimethacrylamide and 49a were synthesized in different ways via macro-CTAs of both monomers as the first step. Such sulfobetaine block polymers form aggregates in pure water but are molecularly dissolved after addition of salt [152,156,157]. 

Chen, S.-J., Economou, I.G., and Radosz, M., Phase behavior of LCST and UCST solutions of branchy copolymers experiment and SAET modeling. Fluid Phase Equilibria, 83, 391-398, 1993. 

One can go on and examine more complex functions fl T) that cross the borderline many times the corresponding T(x) diagram would have as many LCST and UCST as the number of times the function crosses the borderline. Theoretically, there is no limit to the number of LCST and UCST, the only question is whether there are real mixtures for which pAAB will have this kind of complex behavior. 

Figure 16.2 Experimental phase diagram of the sPP/EPDM blend as determined by a combination of DSC and light scattering techniques, exhibiting the combined LCST and UCST together with the melting-point depression. The UCST curve was determined after the blends were homogenized in the single phase below the T, but above the crystallization temperatures. The symbols represent the experimentally determined points and the lines are drawn by hand or polynomial hts to guide the eyes. (From Reference (6) with permission from Elsevier.)...

Figure 15.4 Effect of various variables on LCST and UCST shown schematically in a pressure-temperature diagram. Source Adapted with permission from Seiler M. Chem Eng Tech 2002 2 237 [9]. Copyright 2002 Wiley-VCH Verlag GmbH Co. KGaA.

Type VI systems are eomposed of eomplex molecules with hydrogen bonding or other strong intermoleeular forees and result in behavior where LCST and UCST are at temperatures well removed from gas-hquid eritieal temperature of the more volatile component. 

The phase behavior of polymer blends that is mixtures of two chemically different polymers is experimentally well accessible in a window which is bounded at high temperatures by thermal decomposition temperature, of the polymer components and at low temperatures by the glass transition temperature, r, of the system. This is shown schematically, in Figure 16. When LCST and UCST merge, we have immiscibility or heterogeneity of the blend (see Figure 17). 

YAM Yamauchi, H. and Maeda, Y., LCST and UCST behavior of poly(V-isopropyl-aciylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy, J. Phys. Chem. B, 111, 12964, 2007. 

WUG Wu, G., Chen, S.-C., Zhan, Q., and Wang, Y.-Z., Well-defined amphiphilic biodegradable comb-like graft copolymers Their rmique architecture-determined LCST and UCST thermoresponsivity. Macromolecules, 44, 999, 2011. 

The X versus l/T plots from these systems also show an extremum as was the case with type IV blends however the sign of the curvature (C) is negative. The SPI(7)/(iPP [system 47b] blend exhibits such behavior as can be seen in Fig. 19.5(a). Like type IV blends, these systems can thus exhibit both LCST and UCST behavior. The difference is that UCST behavior is predicted at high temperatures and LCST behavior is predicted at low temperatures. The bino-dal and spinodal curves for such systems thus form closed loops and the two-phase region is restricted to the middle of the phase diagram. The calculated phase diagram for a SPI(7)/(iPP blend with V = 580 has these characteristics and is shown in Fig. 19.5(b). 

The X parameter in these systems is also parabolic in l/T and thus these systems are in many respects like type III blends. The difference is that the bottom of the parabola occurs at an experimentally accessible temperature. An example of such a system is HHPP/JSPI(7) [system 51] and the dependence of x on l/T is presented in Fig. 19.4(a). For a given 0, such behavior can lead to multiple solutions to Eqs. (19.9) and (19.10) that lie within the accessible temperature window. These systems can thus exhibit both LCST and UCST behavior. A HHPP/JSPI(7) blend with N = 5,700 is predicted to exhibit such behavior, and the calculated phase diagram is shown in Fig. 19.4(b). Single phase behavior is observed at intermediate temperatures and phase separation is observed toward the outer edges of the available temperature window. LCST behavior is predicted at high temperatures and UCST behavior is predicted at low temperatures. 

PS/PoCS PS/poly(o-chloro styrene) blends showed both LCST and UCST X12 was independent of 4> and T 3... 

Fig. 10. 27 Phase diagram showing LCST and UCST behavior for polymer blends (Courtesy Online resources)...

Figure 3. Phase behavior in polymer solutions. Systems dispalying hour- glass shaped region of immicibility (A and B) both LCST and UCST (C and D) only UCST (E) only LCST (F) complete miscibility (G) and an island of immiscibility (H).

Figure II. Model predictions of the phase diagram for 10 mass % solutions of polyethylene (M= 108,000) in binary solvent pentane + carbon dioxide with 20, 30 and 40 % carbon dioxide content at a system pressure of 15, 35 and 65 MPa. With increasing pressure, the systems shift from one showing an hour-glass shaped region of immiscibility to one showing both LCST and UCST, and finally to one showing only UCST. [Refs.32 and 33].

Fig. 1.1.1 Binary phase diagrams (polymer composition represented by the volume fraction, , vs temperature, T) of polymer/solvent systems exhibiting (a) both LCST and UCST phase envelopes and (b) the hourglass-shape phase behavior, when the polymer 2 molecular weight is greater them that of polymer 1 (or Mi > Mi)...

The miscibility loop [38-42] has one upper critical solution temperature (UCST) at its top and one lower critical solution temperature (LCST) at its bottom. The miscibility dome has an ordinary UCST. As the molecular weight is increased, the LCST of the loop and the UCST of the dome come closer. Figure 6.9(b) shows how the miscibility loop and dome merge. At a certain value of n (1670 for the parameters given in this figure) the LCST and UCST merge into a higher-order critical point, which... 

Rg. 6.14 Molecular-weight dependence of the LCST and UCST (Shultz plot). The critical temperatures are plotted against the reciprocal of DP. The hyper critical point (HCP) and double critical point (DCP) are indicated by arrows. (Reprinted with permission from Ref. [55].)... 

For random hydration, the LCST and UCST merge at a molecular a weight between n = 50 and 100, and the phase separation region turns into an hourglass. For cooperative hydration, the LCST curves are very flat up to high polymer concentration. Molecular weight effect is weak. 

Case IV Circular Phase Envelope LCST and UCST... 

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