Lower critical solution temperature structure

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). 

Temperature-sensitive polymers, depending on polymer structure and polymer-polymer interactions, generally exhibit two behaviors, lower critical solution temperature (LCST) [31] and upper critical solution temperature (UCST). Phase diagrams for these behaviors are presented in Figure 9. 

Poly(vinyl methyl ether), PVME, is a thermo-sensitive polymer. The aqueous solution has a Lower Critical Solution Temperature (LCST) of 37 °C. Therefore, PVME is soluble in water below its LCST, but insoluble above its LCST. When an aqueous solution of PVME is irradiated with y-rays the solution becomes PVME hydrogel [18, 19]. The gel shows thermo-sensitivity similar to the solution, and swells below 37 °C and shrinks above this temperature. It is important to form a fine porous gel structure to obtain quick response gels. There are two methods for the purpose. One is a method using micro-phase separation by heating. The other is a method using micro-phase separation by blending of polymer solutions. 

Macrophase separation after microphase separation has been observed in an AB block copolymer/homopolymer C blend (Hashimoto et al 1995). Blends of a PS-PB starblock copolymer (75wt% PS) and PVME homopolymer were prepared by solvent casting. Binary blends of PS and PVME exhibit a lower critical solution temperature (LCST), i.e. they demix at high temperatures. The initial structure of a 50% mixture of a PS-PB diblock and PVME shown in Fig. 6.20(a) consists of worm-like micelles. Heating led to macrophase separation as evident... 

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],... 

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. 

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 phase behavior is similar to that of a lower critical solution temperature (LCST), hence it is different from the above systems. The HPC/water system is an interesting model system because of the rich variety of phase structure 01 the material. HPC is a semicrystalline polymer in the solid state (7), but exhibits thermotropic liquid crystalline character at elevated temperatures below the melting point (8). It shows isotropic phase in dilute solutions, but forms an ordered liquid crystalline phase with cholesteric structure in concentrated solutions (4). 

To achieve desirable macroscopic properties, one would like to control the degree of miscibility of a blend. Heat treatment/annealing is a simple technique to modify a phase structure of a blend. Morphological changes induced by heat treatment can also affect NMR observable (see Section 10.3.2.1). Furthermore, as shown in Section 10.2.1, several blends exhibit a lower critical solution temperature (LCST) phase diagram. Such a blend phase-separates at temperatures above its LCST temperature. The compositional fluctuation during the phase-separation process is examined in Section... 

Poly(7V-isopropylacrylamide) (PNlPAAm) is a well-known thermo-responsive polymer and exhibits a lower critical solution temperature (LCST) of 32°C in water. It assnmes a random coil structure (hydrophilic state) below the LCST and a collapsed globnlar stractnre (hydrophobic state) above. Because of this sharp reversible transition, this polymer finds a vast array of applications,... 

Poly(iV-isopropylacrylaniide) (PNIPAAm) is by far the most prominent example of a thermally responsive polymer. It undergoes a phase transition at the lower critical solution temperature (LCST), resulting in a strong decrease in hydration of the polymer. For polymer brushes, this behavior is reflected in a collapse of the structure above the LCST. Because of the proximity of the LCST (32°C) to the body temperature, PNIPAAm is considered an interesting candidate for drug release systems. 

Addition of an anti-solvent to a polymer solution causes the polymer solution to split into a polymer-rich phase and a solvent-rich phase. When a non-solvent is added the overall density of the original solvent becomes lower, which decreases the Lower Critical Solution Temperature (LCST) of the solution. A liquid-liquid phase-split thus occurs without raising the temperature. A low-molecular weight anti-solvent like CO2, propane or ethane can effectively decrease the LCST of the polymer solution (6) and thus induce a liquid-liquid phase-split. It is due to this effect that Gas Anti-Solvent precipitation of polymers has focused on the production of polymer particles with a specific size, structure or shape such as micro tubes (7) or micro balloons (8). Phase separation phenomena in PPE solutions during the formation of polymer membranes by the addition of a conventional anti-solvent have been described by (9). 

Polymer chains with both hydrophobic and hydrophilic groups interact with water to make organized structures of water molecules around the hydrophilic groups. This lowers the energy of the system, which is typical at lower temperatures. However, at higher temperatures, these water structures breakdown as the water molecules tumble more chaotically, and the polymer chains are forced together to make a solid material. The temperature at which the polymer chains precipitate out of the solution into a solid mass is the lower critical solution temperature (LCST). 

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