Higher critical solution temperature

A difference in volume between cyclic and linear polymers leads to different solution properties of the cyclic polymers compared to the linear polymers, such as higher GPC elution volume [233], lower intrinsic viscosity [234], lower translational friction coefficient [235], neutron scattering functions more rapid decrease of second virial coefficient with molecular weight [236], and higher critical solution temperature [237]. 

Equations (19) and (20) are valid in theta solvent. The more compact structure and the lack of chain ends result in different chemical and physical properties of cyclic polymers, including lower translational friction coefficients, higher glass transition temperatures [167], faster crystallization [168], higher refractive index [169], higher density [170], higher critical solution temperature [167], and lower intrinsic viscosity [167, 171, 172]. 

Aqueous systems of ethylene oxide-propylene oxide-ethylene oxide triblock copolymers have been the subject of many studies, primarily in the termination of the phase diagrams. In this case the solvent is a precipitant for tte middle block, and a solvent for the outer blocks. Poly(ethylene oxide)-water systems show lower and higher critical solution temperatures, however, so that associates are formed in an extremely complex way with the formation of spherical and cylindrical micelles with various ways of ordering, depending concentra-... 

Fig. 2.2 Liquid immiscibility. The guaiacol (A) + glycerol (B) system happens to have a closed miscibility loop. The (phase) coexistence curves are shown on the left-hand side (a) for lower temperatures, at which a lower critical solution temperature (LCST), = 40°C, is seen, and on the right-hand side (b) for higher temperatures, where a UCST, Tcs = 82°C, is seen. The compositions of the A-rich phases" and the B-rich phases are shown at 50°C and 70°C, respectively.

One of the Interesting features of these binary solutions, and of many microemulsions, is their tendency to unmix at higher temperature. For example triethylamine-water mixtures unmix into nearly pure triethylamine and nearly pure water at 18.5 C similarly 2-butoxyethanol has a lower critical solution temperature at 49 C. 

Thus, the PEO segment actually becomes hydrophobic at higher temperatures. This temperature-dependent change converts the amphiphilic block copolymer to a water-insoluble hydrophobic polymer (Topp et al. 1997 Chung et al. 2000). The temperature at which the polymer exhibits this transition is called its lower critical solution temperature (LCST). In addition to PEO, substituted poly(A -isopropyl acrylamide) (PNIPAM Chart 2.1) exhibits temperature sensitivity, where the LCST can be tuned by varying the alkyl fimctionahty. The guest encapsulation combined with the temperature-sensitive precipitation of the polymers has been exploited to sequester and separate guest molecules from aqueous solutions (Fig. 2.4). 

The instance we have considered here, that of a polymer in a poor solvent, results in an upper critical solution temperature (UCST) as shown in Figure 2.33. This occurs due to (a) decreased attractive forces between like molecules at higher temperatures and (b) increased solubility. For some systems, however, a decrease in solubility can occur, and the corresponding critical temperature is located at the minimum of the miscibility curve, resulting in a lower critical solution temperature (LCST). This situation is illustrated in Figure 2.34. 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

The phase behaviour of many polymer-solvent systems is similar to type IV and type HI phase behaviour in the classification of van Konynenburg and Scott [5]. In the first case, the most important feature is the presence of an Upper Critical Solution Temperature (UCST) and a Lower Critical Solution Temperature (LCST). The UCST is the temperature at which two liquid phases become identical (critical) if the temperature is isobarically increased. The LCST is the temperature at which two liquid phases critically merge if the system temperature is isobarically reduced. At temperatures between the UCST and the LCST a single-phase region is found, while at temperatures lower than the UCST and higher than the LCST a liquid-liquid equilibrium occurs. Both the UCST and the LCST loci end in a critical endpoint, the point of intersection of the critical curve and the liquid liquid vapour (hhg) equilibrium line. In the two intersection points the two liquid phases become critical in the presence of a... 

Figure 2 illustrates the temperature dependence of the swelling degree as a function of precursor polymer type. Methylcellulose (MC), hydroxypropyl-methylcellulose, type E (HPMC-E) and hydroxypropylmethylcellulose, type K (HPMC-K) gels have comparable effective crosslink densities of about 2 x 10 5 mol/cm3 (as determined from uniaxial compression testing), while the crosslink density of the hydroxypropylcellulose (HPC) gel is about half this [52]. The transition temperature for each gel is within several degrees of the precursor polymer lower critical solution temperature (LCST), except for the MC gel, which has a transition temperature 9 °C higher than the LCST. The sharpness of the transition was about 3%/°C, except for the HPC gel transition, which was much sharper - about 8%/°C. 

As binary PPE/SAN blends form the reference systems and the starting point for the foaming analysis, their miscibility will be considered first. As demonstrated in the literature [41, 42], both miscibility and phase adhesion of PPE/SAN blends are critically dependent on the composition of SAN, more precisely on the ratio between styrene and acrylonitrile (AN). Miscibility at all temperatures occurs up to 9.8 wt% of AN in SAN, whereas higher contents above 12.4 wt% lead to phase separation, independent of the temperature. Intermediate compositions exhibit a lower critical solution temperature behavior (LCST). Taking into account the technically relevant AN content SAN copolymers between 19 and 35 wt%, blends of SAN and PPE are not miscible. As the AN content of the SAN copolymer, selected in this work, is 19 wt%, the observed PPE/SAN blends show a distinct two-phase structure and an interfacial width of only 5 nm [42],... 

Fig. 8 Schematic of different chain conformations and the coil-to-globule transition of NIPAM-co-VP copolymers prepared at two temperatures, respectively, lower and higher than the lower critical solution temperature of PNIPAM homopolymer [56]...

Fig. 9 Temperature dependence of the ratio of average radius of gyration to average hydrodynamic radius () °f copolymer NIPAM-co-VP chains prepared at two different temperatures, respectively, lower and higher than the lower critical solution temperature of PNIPAM homopolymer. The weight average molar masses of NIPAM-co-VP/60/5 and NIPAM-co-VP/30/5 are 2.9 x 106 and 4.2 x 106 g/mol, respectively [56]...

Using this approach, hydrophilic (neutral or ionic) comonomers, such as end-captured short polyethylene oxide (PEO) chains (macromonomer), l-vinyl-2-pyrrolidone (VP), acrylic acid (AA) and N,N-dimethylacrylamide (DMA), can be grafted and inserted on the thermally sensitive chain backbone by free radical copolymerization in aqueous solutions at different reaction temperatures higher or lower than its lower critical solution temperature (LCST). When the reaction temperature is higher than the LOST, the chain backbone becomes hydrophobic and collapses into a globular form during the polymerization, which acts as a template so that most of the hydrophilic comonomers are attached on its surface to form a core-shell structure. The dissolution of such a core-shell nanostructure leads to a protein-like heterogeneous distribution of hydrophilic comonomers on the chain backbone. 

In Fig. 3.3a and b, it was possible to observe the maximum value of ttc at the different temperatures that the monolayer can reach. At higher temperatures, rrc increases, PVP exhibits a low critical solution temperature (LCST) in water [45]. The PVP in 0.55 M aqueous Na2S04 exhibits a lower LCST at 301 K. For soluble amphiphilic monolayers, such as those of PVP, when the temperature is changed the loss of monolayer material must be considered due to the solubilization in the subphase. 

Figure 13.15 is drawn for a single constant pressure equilibrium phase compositions, and hence the locations of the lines, change with pressure, but the general nature of the diagram is the same over a range of pressures. For the majority of systems the species become more soluble in one another as the temperature increases, as indicated by lines CG and DH of Fig. 13.15. If this diagram is drawn for successively higher pressures, the corresponding three-phase equilibrium temperatures increase, and lines CG and DH extend further and further until they meet at the liquid/liquid critical point Af, as shown by Fig. 13.16. The temperature at which this occurs is known as the upper critical solution temperature, and at this temperature the two liquid phases become identical and merge into a single phase.

The system with which we have begun our investigations is the styrene-dimethylsiloxane system. The dimethylsiloxane blocks should be considerably less compatible with polystyrene blocks than either polybutadiene or polyisoprene since the solubility parameter of dimethylsiloxane is much farther from that of polystyrene than are the solubility parameters of polybutadienes or of polyisoprenes (17), no matter what their microstructure. Furthermore, even hexamers of polystyrene and of polydimethylsiloxane are immiscible at room temperature and have an upper critical-solution temperature above 35°C (18). In addition, the microphases in this system can be observed without staining and with no ambiguity about the identity of the phases in the transmission electron microscope (TEM) silicon has a much higher atomic number than carbon or oxygen, making the polydimethylsiloxane microphases the dark phases in TEM (19,20). 

In SAS, a compressed gas is added to a polymer solution. The upper critical solution temperature (UCST) and lower critical solution temperature (LCST) of that solution are shifted to higher and lower temperatures respectively until they finally merge to one region of immiscibilty over the whole temperature range. This process can be used for solvent recovery in solution polymerisation processes as well as for molecular weight fractionation of polymers. 

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