Temperature, LCST

Fig. 1. Phase diagram for mixtures (a) upper critical solution temperature (UCST) (b) lower critical solution temperature (LCST) (c) composition dependence of the free energy of the mixture (on an arbitrary scale) for temperatures above and below the critical value.

The critical point (Ij of the two-phase region encountered at reduced temperatures is called an upper critical solution temperature (UCST), and that of the two-phase region found at elevated temperatures is called, perversely, a lower critical solution temperature (LCST). Figure 2 is drawn assuming that the polymer in solution is monodisperse. However, if the polymer in solution is polydisperse, generally similar, but more vaguely defined, regions of phase separation occur. These are known as "cloud-point" curves. The term "cloud point" results from the visual observation of phase separation - a cloudiness in the mixture. 

Reactive compatibilization can also be accomplished by co-vulcanization at the interface of the component particles resulting in obliteration of phase boundary. For example, when cA-polybutadiene is blended with SBR (23.5% styrene), the two glass transition temperatures merge into one after vulcanization. Co-vulcanization may take place in two steps, namely generation of a block or graft copolymer during vulcanization at the phase interface and compatibilization of the components by thickening of the interface. However, this can only happen if the temperature of co-vulcanization is above the order-disorder transition and is between the upper and lower critical solution temperature (LCST) of the blend [20]. 

The first elastomeric protein is elastin, this structural protein is one of the main components of the extracellular matrix, which provides stmctural integrity to the tissues and organs of the body. This highly crosslinked and therefore insoluble protein is the essential element of elastic fibers, which induce elasticity to tissue of lung, skin, and arteries. In these fibers, elastin forms the internal core, which is interspersed with microfibrils [1,2]. Not only this biopolymer but also its precursor material, tropoelastin, have inspired materials scientists for many years. The most interesting characteristic of the precursor is its ability to self-assemble under physiological conditions, thereby demonstrating a lower critical solution temperature (LCST) behavior. This specific property has led to the development of a new class of synthetic polypeptides that mimic elastin in its composition and are therefore also known as elastin-like polypeptides (ELPs). 

The phase transition temperatures (lower critical solution temperature, LCST) of the pol5miers were obtained from the change in the transmittance of their aqueous solutions (Figure 1). The aqueous solution of the obtained pol5uner was prepared and its transmittance at 500 nm was monitored with increase in the ambient temperature. Both of poly-NIPA and poly-NEA showed a sudden decrease in the transmittance at 37.5 and 69.2 °C, respectively. The result shown in Figure 1 clearly suggests the thermosensitivity of the pol5mers, and the obtained LCST values are close to those reported for poly-NIPA (34.8 °C) [8] and poly-NEA (72 °C) [9]. 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

In this study, adsorption behavior of water soluble polymers and their effect on colloid stability have been studied using polystyrene latices plus cellulose derivatives. As the aqueous solution of hydroxy propyl cellulose(HPC) has a lower critical solution temperature(LCST), near 50 °C(6 ), an increased adsorption and strong protection can be expected by treating the latices with HPC at the LCST. 

As reported previously [22-23], polyDMAEMA and PolyEAAm have lower critical solution temperatures (LCSTs) at 50° and 80°C, respectively. Interestingly, poly[DMAEMA-co-EAAm (or AAm)] exhibits the LCST between 0° and 50°C, depending on the copolymer composition. 

An interesting family of polymeric ligands show inverse temperature dependence of solubihty in water, i.e. they can be precipitated from aqueous solutions by increasing the temperature above the so-called cloud point. Typically these ligands contain poly(oxyalkylene) chains, but the phenomenon can be similarly observed with poly(N-isopropyl acrylamide) derivatives (e.g. 132) and methylated cyclodextrins, too. At or above their cloud points these compounds fall off the solution, due to the break-up and loss of the hydration shell which prevents aggregation and precipitation of their molecules. Conversely, upon cooling below this temperature (also called the lower critical solution temperature, LCST) these substances dissolve again. 

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.

Hydrogels synthesized from polymers and copolymers of N-isopropyl acrylamide (NIPAAm) shrink or swell as the temperature is raised or lowered through their lower critical solution temperature (LCST). 

Poly(N-isopropylacrylamide) (polyNIPAAM), formed by a free radical polymerization of N-isopropylacrylamide, is a water soluble, temperature sensitive polymer. In aqueous solution, it exhibits a lower critical solution temperature (LCST) in the range of 30-35 C depending on the concentration and the chain length of the polymer. Thus, as the solution temperature is raised above the LCST, the polymer undergoes a reversible phase transition characterized by the separation of a solid phase which redissolves when the solution temperature is lowered below the LCST. Its physicochemical properties have been investigated by several laboratories (1-3). 

LOST. We determined the lower critical solution temperature (LCST) of the polymer at various concentrations by visual observation of the temperature at which turbidity first appeared in a solution immersed in a silicone oil bath with the temperature raised at the rate of 3 C/hour. 

High polymers of N-isopropyl acrylamide (NIPAAM) exhibit a lower critical solution temperature (LCST) in phosphate buffered saline above 31 C precipitation occurs, with minimal concentration dependence. 

Lower Critical Solution Temperatures LCSTs were determined from plots of optical density at 600 nm versus temperature for 0.03% solutions of each polymer in PBS and were defined as the temperature at which Asoo = 0.1. Temperatures were raised at less than 0.3 C per minute and were measured with a thermometer that had been calibrated against an NBS primary standard thermometer. LCSTs for Figure 6 were determined from the cloud points of 0.01% solutions. 

Note A miscibility gap is observed at temperatures below an upper critical solution temperature (UCST) or above the lower critical solution temperature (LCST). Its location depends on pressure. In the miscibility gap, there are at least two phases coexisting. 

Fig. 11 Plots of the measured lower critical solution temperature (LCST) as a function of the theoretical average number of OEGMA475 units per chain for a series of P(Me02MA-co-OEGMA475) copolymers of various composition. Hydrophobic and hydrophilic molecular regions on the copolymer are indicated in red and blue, respectively. (Reprinted with permission from [76]. Copyright (2008) John Wiley Sons, Inc.)...

Table 4 Thermal and lower critical solution temperature (LCST) properties for the copolymers of the p(Amor-stot-HPA) library...

Considering the rather complicated processes that take place during dissolution it is not surprising that some systems show peculiar behavior. For example, while solubility generally increases with temperature, there are also polymers that exhibit a negative temperature coefficient of solubility in certain solvents. Thus, poly(ethylene oxide), poly(N-isopropylacrylamide), or poly(methyl vinyl ether) dissolve in water at room temperature but precipitate upon warming. This behavior is found for all polymer-solvent systems showing a lower critical solution temperature (LCST). It can be explained by the temperature-dependent... 

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