Lower critical solution temperature LCST behavior

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). 

Figure 5. Cloud point phase diagram for physical blends of PVME and PS, showing lower critical solution temperature, LCST, behavior.

The lattice fluid equation-of-state theory for polymers, polymer solutions, and polymer mixtures is a useful tool which can provide information on equa-tion-of-state properties, and also allows prediction of surface tension of polymers, phase stability of polymer blends, etc. [17-20]. The theory uses empty lattice sites to account for free volume, and therefore one may treat volume changes upon mixing, which are not possible in the Flory-Huggins theory. As a result, lower critical solution temperature (LCST) behaviors can, in principle, be described in polymer systems which interact chiefly through dispersion forces [17]. The equation-of-state theory involves characteristic parameters, p, v, and T, which have to be determined from experimental data. The least-squares fitting of density data as a function of temperature and pressure yields a set of parameters which best represent the data over the temperature and pressure ranges considered [21]. The method,however,requires tedious experiments to deter-... 

In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend. 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

If the binodal and spinodal points are determined at various temperatures and are plotted together, a phase diagram such as the one shown in Figure 6.1 b may result. The temperature at which the binodal and spinodal curves merge together is the critical temperature. The phase diagram shown illustrates a case in which the miscibility gap occurs at temperatures above the critical temperature, and the system is said to exhibit a lower critical solution temperature (LCST) behavior. A system, on the other hand, may display an upper critical solution temperature (UCST) behavior, in which the miscibility gap occurs below the critical temperature. 

Certain principles mnst be obeyed for experiments where liquid-liquid equilibrium is observed in polymer-solvent (or snpercritical flnid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into acconnt the strong influence of polymer distribution functions on LLE, because fractionation occnrs dnring demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during dentixing leads to some effects by which the LLE phase behavior differs from that of an ordinary, strictly binary mixture, because a common polymer solution is a mnlticomponent system. Clond-point cnrves are measnred instead of binodals and per each individnal feed concentration of the mixtnre, two parts of a coexistence cnrve occnr below (for upper critical solution temperatnre, UCST, behavior) or above the clond-point cnrve (for lower critical solution temperature, LCST, behavior), i.e., produce an infinite nnmber of coexistence data. 

The miscible blend of poly(slyrene) (PS) and poly(viityl methyl ether) (PVME). The phase diagram is shown in Figure 18 (see Chapter 5, Transitions in Polymers). We recognize lower critical solution temperature (LCST) behavior above 152 °C. A plot of interaction parameter times temperature, xT, versus temperature, indicates clearly the LCST (Figure 19). We get also energy and excess entropy contribution to parameter x from this plot AU/R = - 43 K, S /R = -0.1 (Equation (23)). 

ROP and RAFT polymerization techniques were combined to synthesize multiarm star-block copolymers having PeCL inner blocks and PDMAEMA outer blocks. A hyperbranched polyester core was used as a multifunctional initiator. It was calculated that the functionality of the star-blocks was equal to 19. Temperature and pH-responsive micelles were obtained in aqueous solutions. Equilibrium between unimolecular and mulrimolecular micelles was observed at pH 6.58 by dynamic LS and TEM measurements. In low-pH solutions, the PDMAEMA chains were fully protonated and therefore highly stretched, leading to maximum Rh values. When the pH was increased, the micellar Rh decreased as a result of the deprotonation of the dimethylamine groups. PDMAEMA is also a temperature-sensitive polymer, as it exhibits lower critical solution temperature (LCST) behavior. It precipitates from neutral or basic solutions between 32 and 58 °C. At pH 6.58, the Rh values were found to decrease with increasing temperature, due to the gradual collapse of the PDMAEMA outer blocks. 

Goh, Paul, and Barlow [10] found that an alpha-methylstyrene-acrylonitrile (AMS-AN) copolymer at 50 mole fraction AN formed miscible polymer blends with poly(methyl methacrylate) (PMMA) and with poly(ethyl methacrylate) (PEMA). AMS-AN copolymer did not form miscible blends with polyacrylates or polyvinyl acetate. The miscible blends were found to exhibit lower critical solution temperature (LCST) behavior. 

When the curve delimiting the homogeneous and the heterogeneous zones is convex, the mixture is identified as exhibiting lower critical solution temperature (LCST) behavior. If a miscible A/B blend having a composition slightly different from the diluted extreme cases is heated above any temperature, the LCST curve demixes into two distinct phases, A and B. 

Phosphazenes can exhibit Lower Critical Solution Temperature (LCST) behavior. LSCT behavior is exhibited by a phosphazene that becomes less... 

DLS is also useful for determining phase behavior and micelle formation in block copolymers dissolved in ILs. The work by Lee et al. with poly(ethylene oxide)-h-poly(A-isopropylacrylamide) (PEO-h-PNIPAM) in [C2CjIM][BF ] and [C CjIM][BFJ shows how the lower critical solution temperature (LCST) behavior of PEO and the upper critical solution temperature (UCST) behavior of PNIPAM can be exploited to form PNIPAM-core micelles at low temperature and PEO-core micelles at high temperature. As can be seen in Figure 2.15, this system is thermally reversible allowing for potential applications as thermosensitive materials [8]. 

SI-IMP has been used for synthesis of different types of stimuli-responsive polymer brushes that are responsive to several external stimuli, such as pFI, temperature, and ionic strength [28,58-65]. Because materials interact with their surroundings via their interfaces, the ability to fashion soft interfacial layers and tune the range, extent, and type of physicochemical interactions across interfaces is central to a variety of applications. Rahane et al. carried out sequential SI-IMP of two monomers to create bilevel poly(methacrylic acid)-Woc/c-poly(N-isopropylacrylamide) (PMAA-b-PNIPAM) block copolymer brushes that can respond to multiple stimuli [28]. They observed that each strata in the bilevel PMAA-b-PNIPAM brush retained its customary responsive characteristics PMAA being a "weak" polyelectrolyte swells as pH is increased and the thermoresponsive PNIPAM block collapses as temperature is raised through the volume phase transition temperature due to its lower critical solution temperature (LCST) behavior. As a result of ions added to make buffer solutions of various pH and because of the effect of surface confinement, the swollen-collapse transition of the PNIPAM layer occurs at a... 

Polymers with lower critical solution temperature (LCST) behavior... 

For polymer blends exhibiting lower critical solution temperature (LCST) behavior, e.g., the system polystyrene/poly(vinyl methyl ether), one may anticipate the opposite behavior for purely phenomenological reasons. Interfacial tension should increase with increasing temperature in the two-phase region since the tie lines become longer with increasing temperature in that case... 

Using FRES and others techniques, Wang and Composto investigated wetting and phase separation in polymer blend films composed of deuterated poly(methyl methacrylate) (d-PMMA) and poly(styrene-raw-acrylonitrile) (h-SAN) at the critical concentration [176-180]. This blend is characterized by a lower critical solution temperature (LCST) behavior with Tlcst 160 °C and 0d-PMMA 0.5 [180]. In this... 

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