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). 

The coacervation of tropoelastin plays a crucial role in the assembly into elastic fibers. This coacervation is based on the LCST behavior of tropoelastin, which causes tropoelastins structure to become ordered upon raising the temperature. The loss of entropy of the biopolymer is compensated by the release of water from its chain [2, 18, 19]. This release of water results in dehydration of the hydrophobic side chains, and this is the onset of the self-assembly leading to the alignment of tropoelastin molecules. 

Inspired by the elastin-based side-chain polymers, Lemieux et al. prepared elastin-based stimulus-responsive gold nanoparticles. To this end, they capped gold particles with a layer of a single repeat of thiol-functionalized VPGVG peptides (Fig. 17a). These nanoparticles showed LCST behavior, which was modulated by varying the pH of the solution [131]. 

The TIPS technique is largely applied to yield a wide variety of polymer blends deriving from an UCST or LCST behavior [65] and has also proven to be useful for the preparation of porous thermoplastic polymers by the use of a phase separating solvent [41,42]. 

Becer CR, Hahn S, Fijten MWM, Thijs HML, Hoogenboom R, Schubert US (2008) Libraries of methacrylic acid and oligo(ethylene glycol) methacrylate copolymers with LCST behavior. J Polym Sci Part A Polym Chem 46 7138-7147... 

Figure 5 shows the cloud points of the aqueous polymer solutions of NNPA and NIPA measured with a differential scanning calorimeter. From the figure it was confirmed that the thermoshrinking behavior of the gels resulted from the LCST behavior of the corresponding polymer solutions. 

Miscible blends of high molecular weight polymers often exhibit LCST behavior (3) blends that are miscible only because of relatively low molecular weights may show UCST behavior (11). The cloud-point temperatures associated with liquid—liquid phase separation can often be adequately determined by simple visual observations (39) nevertheless, instrumented light transmission or scattering measurements frequendy are used (49). The cloud point observed maybe a sensitive function of the rate of temperature change used, owing to the kinetics of the phase-separation process (39). 

Figures 8.2a and b describe a UCST behavior, while Fig. 8.2c represents an LCST behavior. It is interesting to note that in the case of an UCST behavior the cloud-point curve will usually intersect the vitrification curve, while this may not be the case for an LCST behavior.

In Fig. 8.4 an UCST behavior is represented. The corresponding situation for an LCST behavior is a shift of the cloud-point curve to lower temperatures as conversion increases. A similar description of the phase-separation process is valid also for the LCST case. 

Volume of Mixing. In general terms, exothermic interaction effects tend to diminish the volume of a mixture whereas entropic effects act in the opposite way. For miscible polymers, therefore, one expects a negative volume of mixing. This has been confirmed experimentally for different miscible polymers with LCST behavior, e.g. for miscible 50/50 blends of polystyrene and poly(2-chloro-styrene) the volume change AVM/V at 130°C has been reported to be about... 

Schematic representations of the respective miscibility regions, when only LCST behavior occurs, are shown in Fig. 4. As far as the authors are aware, a window of immiscibility has not yet been reported.

SAN AN content 12wt%, PVME content in the blend 80 wt%, LCST behavior for all miscible blends [37]... 

In this section we would like to deal with the kinetics of the liquid-liquid phase separation in polymer mixtures and the reverse phenomenon, the isothermal phase dissolution. Let us consider a blend which exhibits LCST behavior and which is initially in the one-phase region. If the temperature is raised setting the initially homogeneous system into the two-phase region then concentration fluctuations become unstable and phase separation starts. The driving force for this process is provided by the gradient of the chemical potential. The kinetics of phase dissolution, on the other hand, can be studied when phase-separated structures are transferred into the one-phase region below the LCST. 

Phase dissolution in polymer blends. The reverse process of phase separation is phase dissolution. Without loss of general validity, one may assume again that blends display LCST behavior. The primary objective is to study the kinetics of isothermal phase dissolution of phase-separated structures after a rapid temperature-jump from the two-phase region into the one-phase region below the lower critical solution temperature. Hence, phase-separated structures are dissolved by a continuous descent of the thermodynamic driving force responsible for the phase separation. The theory of phase separation may also be used to discuss the dynamics of phase dissolution. However, unlike the case of phase separation, the linearized theory now describes the late stage of phase dissolution where concentration gradients are sufficiently small. In the context of the Cahn theory, it follows for the decay rate R(q) of Eq. (29) [74]... 

The appearance of LCST behavior, not only in polymer solutions but also in polymer blends, is due to a major factor that is neglected in the theory, i.e., fired volume. This can be fixed (e.g., by putting holes on the lattice), but that gets us beyond the scope of an introductory treatment. 

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-... 

Figure 5 shows the temperature dependence of the surface tension. The differences between calculated values and the experimental ones do not exceed ca. 1 mN m-1. An adjustable parameter is not used by assuming that the k does not vary with the temperature and is fixed at 0.5, a theoretical value for both PS and PVME. This indicates that the simulated equation-of-state parameters for the component polymers are reasonable. It has been known that the LCST behaviors are originated from the specific interactions between components and/or the finite compressibility of mixture and that the phase separation is entropically... 

In order to explain phase separation on heating, I.e., LCST behavior, the effect of volume changes on mixing must be considered. This effect Is described by equation-of-state theories such as that developed by Flory and co-workers (30). The free volume contributions to the free energy are unfavorable and increase with temperature. 

Melt flow, however, also affects the phase separation, usually enhancing the miscibility for partially miscible blends that show LCST behavior. From Lyngaae-Jorgensen work (46) one may derive the following relation between the shear stress and the change in the spinodal temperature, T, ( ),... 

Fig. 8 pH-dependent LCST behavior of a triblock copolymer containing elastin side chains, a Chemical structure b turbidity measurements performed at different pH (1, 2 and 3) for a triblock copolymer consisting of a PEG block of Mn = 1000 g/mol and elastin blocks with n = 11. Reprinted with permission from [41]. Copyright 2005 American Chemical Society... 

A third type of phase diagram, such as the one shown for molecular weight equal to 19,800, is called hourglass type and has been observed for only a few systems. Most polymer solutions only exhibit the UCST and LCST behavior. As the temperature increases for UCST and as the temperature decreases for LCST, the difference between... 

Very extensive data are available for polymer blends (including UCST/LCST behavior, heats of mixing, etc.), bnt, to onr knowledge, these are not systematically reviewed in any current database. 

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