Critical solution temperature, effect

A. Effect of Third Component on Critical Solution Temperature. 195... 

Polymer Adsorption at the Lower Critical Solution Temperature and Its Effect on Colloid Stability... 

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. 

One very important property in solvent selection is the activity coefficient. Many techniqnes exist for estimating activity coefficients (Fredenslund et al., 1977). In addition to these detailed techniques, a number of simpler approaches have been found to be very effective. These include infinite dilntion activity coefficients (Thomas and Eckert, 1984), critical solution temperatures (Francis, 1944), and solubility parameters (Barton, 1983). In implementing the above system the authors chose to use a three term solubility parameter model. 

It is widely known that poly(N-isopropylacrylamide), poly(IPAAm), in water has a lower critical solution temperature (LCST) at 32 °C. LCST was originally observed in PEG solutions a long time ago. Rowlinson et al. [40] (1957) explained the lower consolute temperature for PEG in water in terms of negative entropies. The first paper on the LCST of poly(IPAAm) at about 31 °C was presented by Heskins and Guillet in 1968 [41]. They reported that aqueous solution of poly(IPAAm) showed phase separation above this temperature, and ascribed it primarily to an entropy effect on the basis of thermodynamical considerations. 

Interactions between different distant parts of the molecule tend to expand it, so that in the absence of other effects a would be greater than unity, but in solution in poor solvents interactions with the solvent tend to contract it. According to Flory s theory (18) these two tendencies will just balance so that a — 1 at a particular temperature T—0 (the theta temperature ), and at this temperature A2 =0 and further this temperature is the limit as Mn- go of the upper critical solution temperature for the polymer-solvent system in question. Quantities relating to T=0 will be denoted by subscript 0. Flory s theory implies that ... 

The volume change in these gels is not due to ionic effects, but rather to a thermodynamic phenomenon a lower critical solution temperature (LCST). The uncrosslinked polymer which makes up the gel is completely miscible with water below the LCST above the LCST, water-rich and polymer-rich phases are formed. Similarly, the gel swells to the limit of its crosslinks below the LCST, and collapses above the LCST to form a dense polymer-rich phase. Hence, the kinetics of swelling and collapse are determined mostly by the rate of water diffusion in the gel, but also by the heat transfer rate to the gel. 

Figure 2 illustrates the temperature dependence of the swelling degree as a function of precursor polymer type. Methylcellulose (MC), hydroxypropyl-methylcellulose, type E (HPMC-E) and hydroxypropylmethylcellulose, type K (HPMC-K) gels have comparable effective crosslink densities of about 2 x 10 5 mol/cm3 (as determined from uniaxial compression testing), while the crosslink density of the hydroxypropylcellulose (HPC) gel is about half this [52]. The transition temperature for each gel is within several degrees of the precursor polymer lower critical solution temperature (LCST), except for the MC gel, which has a transition temperature 9 °C higher than the LCST. The sharpness of the transition was about 3%/°C, except for the HPC gel transition, which was much sharper - about 8%/°C. 

In a blend of immiscible homopolymers, macrophase separation is favoured on decreasing the temperature in a blend with an upper critical solution temperature (UCST) or on increasing the temperature in a blend with a lower critical solution temperature (LCST). Addition of a block copolymer leads to competition between this macrophase separation and microphase separation of the copolymer. From a practical viewpoint, addition of a block copolymer can be used to suppress phase separation or to compatibilize the homopolymers. Indeed, this is one of the main applications of block copolymers. The compatibilization results from the reduction of interfacial tension that accompanies the segregation of block copolymers to the interface. From a more fundamental viewpoint, the competing effects of macrophase and microphase separation lead to a rich critical phenomenology. In addition to the ordinary critical points of macrophase separation, tricritical points exist where critical lines for the ternary system meet. A Lifshitz point is defined along the line of critical transitions, at the crossover between regimes of macrophase separation and microphase separation. This critical behaviour is discussed in more depth in Chapter 6. 

Orzechowski K (1999) Electric field effect on the upper critical solution temperature. Chem Phys 240 275-281... 

Define the upper and lower critical solution temperature. What is the effect of impurities on them (Meerut 2004)... 

Cowie, J. M. G. Maconnachie, A. Ranson, R. J., "Phase Equilibria in Cellulose Acetate-Acetone Solutions. The Effect of the Degree of Substitution and Molecular Weight on Upper and Lower Critical Solution Temperatures," Macromolecules, 4, 57 (1971). 

Zeman, L. Biros, J. Delmas, G. Patterson, D., "Pressure Effects in Polymer Solution Phase Equilibria. I. The Lower Critical Solution Temperature of Polyisobutylene and Poly-dimethylsiloxane in Lower Alkanes," J. Phys. Chem., 76, 1206 (1972). 

Miscible blends of poly(vinyl methyl ether) and polystyrene exhibit phase separation at temperatures above 100 C as a result of a lower critical solution temperature and have a well defined phase diagram ( ). This system has become a model blend for studying thermodynamics of mixing, and phase separation kinetics and resultant morphologies obtained by nucleation and growth and spinodal decomposition mechanisms. As a result of its accessible lower critical solution temperature, the PVME/PS system was selected to examine the effects of phase separation and morphology on the damping behavior of the blends and IPNs. 

Chen et al. [67,68] further extended the study of binary blends of ESI over the full range of copolymer styrene contents for both amorphous and semicrystalline blend components. The transition from miscible to immiscible blend behavior and the determination of upper critical solution temperature (UCST) for blends could be uniquely evaluated by atomic force microscopy (AFM) techniques via the small but significant modulus differences between the respective ESI used as blend components. The effects of molecular weight and molecular weight distribution on blend miscibility were also described. 

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

The interfacial tension between water and mercury is 426-427 dynes/cm. in absence of oxygen, but if measured in presence of air it varies between 375 and 427. The effect of pressure on interfacial tension varies with the pressure and may be positive (increasing a) or negative withp in lb./in.2 the values of (100/or)(do /d ) at about 5000 atm. are Hg/H2O+0 74, Hg/ether+1-23, water/ether—20-73, chloroform/water—0-73, carbon disulphide/water+2 37. The interfacial tension between two liquids vanishes at the critical solution temperature.4... 

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