Lower critical solution temperature states

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). 

For the hydrocarbon--CO2 systems studied here, at pressures above the critical pressure (7.383 MPa) and above the critical temperature (304.21 K) of C02 the isobaric x,T coexistence plots of liquid and vapor phases form simple closed loops. The minimum occurs at the lower consolute point or the Lower Critical Solution Temperature (LCST). Since pressure is usually uniform in the vicinity of a heat transfer surface, such diagrams serve to display the equilibrium states possible in a heat transfer experiment. 

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-... 

Polymer blends typically show a decrease in miscibility with increasing temperature. [27] McMaster has used a modified Flory equation of state thermodynamic model to show that the existence of a lower critical solution temperature (LCST) is caused mainly by differences in the pure component thermal expansion coefficients. 

Saraiva, A. et al.. Application of the van der Waals equation of state to polymers. IV. Correlation and prediction of lower critical solution temperatures for polymer solutions. Fluid Phase Equilibria, 115, 73-93, 1996. 

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,... 

The principal mechanism of temperature-sensitive polymers is the sharp transition from coil to globule in water on heating, indicating a change from a hydrophilic state (coil) below the lower critical solution temperature (LCST) to a hydrophobic state (globule) above the LCST. Representative temperature-sensitive polymers include A -isopropylacrylamide (NIPAAm), its copolymers (LCST 30-50°C) [108, 124-127], polyester block copolymers (20-100°Q [97, 128], and elastin-like polypeptides (27-40°C) [129-131]. To achieve both spatial and temporal control in conjunction with local temperature increases (2-5°C), the LCST of a given polymer can be tailored through its comonomer composition, hydrophilic-hydrophobic balance, stereochemistry [125-127,132], and the addition of salts and surfactants [133]. These thermosensitive polymers with controlled LCSTs (around body temperature) can be applied to specific applications (e.g., tumor treatment). 

More recently we utilized the well-known temperature-induced volume phase transition properties of poly(N-isopropylacrylamide) (PNIPAM) (73-75) to create novel CCA materials with variable sphere size and variable array periodicity (76). In water below 30°C, PNIPAM is hydrated and swollen, but when heated above its lower critical solution temperature ( 32 C) it undergoes a reversible volume phase transition to a collapsed, dehydrated state. The temperature increase causes the polymer to expel water and shrink into a more hydrophobic polymer state. 

Cite the following fact to illustrate the difficulties in the phase analysis of polymer systems. For the poly(vinyl alcohol)- -water system, some researchers propose a state diagram of amorphous phase separation with an upper critical solution temperature, others — ainor]>hons separation with a lower critical solution temperature about 100 C there are some who think that there is no region of amorphous separation below 150 -instead, they observe liquid-crystal phase separation. Such are the discrepancies on the basic question of thermodynamics ... 

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