Phase separation in polymer systems

This growing demand for polymer blends has generated a need for a better understanding of the thermodynamics of miscibility and phase separation in polymer systems. This in turn has generated tremendous interest in techniques that can be used to characterize the thermodynamics of polymer-polymer systems. 

Liquid-liquid phase separations in polymer systems from a molecular... 

The turbidity spectrum method is u.sed for a wide range of problems in the physical chemistry of polymers and colloids, biophysic.s, and biochemistry, including the study of phase separation in polymer systems. 

In a number of cases, analysis of the thermal effects enables one to reveal additional iiiforiiialion of the state diagram the presence of a glass transition curve, the identification of the phase separation regions of both the amorphous and crystalline types in the same system (van Emmerik and Smolders, 1973b). However, the thermal effects of phase separation in polymer systems often turn out to be insignificant for reliable determination of the phase separation region (Papkov, 1981). 

As was mentioned above, the CPC is of great importance in the study of phase separation in polymer systems as one of the few experimental methods for recording pha.s<> separation. 

For kinetic difTiculties, the phase separation in polymer systems seldom finishes with phase equilibrium. This is the reason why only several systems ar< known for which the binodals have been traced using the determined polymer concentration in the coexisting pha.ses, and the critical concentration has been evalnaterl by extrapolating the phase-volume ratio r —t 1. 

Careful experimental measurements of the binodal curves in the system polystyrene-Hine-thylcyclohcxane by Dobashi ct al. (1980ab) were of great importance for a further development of the theory of phase separation in polymer systems. The investigators strove for the complete equilibrium of two liquid phases in each experiment and then determined... 

The identification of mixed types of phase separation in polymer systems causes great difficulties. Important information for phase analysis of a system with a mixed type of phase separation regions can be given by differences in the kinetics of phase separation of different nature. 

The following detaiils of phase separation in the system FEO-l-water indicate how difficult it is to identify phase separation in polymer systems. 

Chapter 6 occupicsi the most mophase separation, including Wunderlich s (1973, 1976, 1980) fundamental monographs. Nevertheless, the matter of this chapter, in its relation to th< others, must play a pphase separation in polymer systems. This chapt< r also reports th< results of application of the turbidity spectrum method to phase analysis of some systems with a crystallizing polymers poly(vinyl alcohol) -I- water and poly(ethylenc oxide) -I- water, wliosc treatment by well-established metlio[Pg.853]

New topics, including phase separations in polymer systems, electrokinetics of charged permeable surface coatings, and polymer brush coatings to control adsorption and adhesion of particles... 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

DMA can be applied to a wide range of materials using the different sample fixture configurations and deformation modes (Table 23.1) [10,11]. This procedure can be used to evaluate by comparison to known materials (a) degree of phase separation in multicomponent systems (b) amount type, and dispersion of filler (c) degree of polymer crystallinity, (d) effects of certain pretreatment and (e) stiffness of polymer composites [8,11]. 

An example of this case is a vinyl (A2 ) - divinyl (A4) polymerization. The assumption of an ideal polymerization means that we consider equal initial reactivities, absence of substitution effects, no intramolecular cycles in finite species, and no phase separation in polymer- and monomer-rich phases. These restrictions are so strong that it is almost impossible to give an actual example of a system exhibiting an ideal behavior. An A2 + A4 copolymerization with a very low concentration of A4 may exhibit a behavior that is close to the ideal one. But, in any case, the example developed in this section will show some of the characteristic features of network formation by a chainwise polymerization. 

PHASE SEPARATION IN AQUEOUS SYSTEMS CONTAINING HIGH POLYMERS... 

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. 

The microstructure of the multiphase media is often the product of phase transitions, e.g. (i) capillary condensation in the porous media, (ii) phase separation in polymer/polymer and polymer/solvent systems, (iii) nucleation and growth of bubbles in the porous media, (iv) solidification of the melt with a temporal three-phase microstructure (solid, melt, gas), and (v) dissolution, crystallization or precipitation. The subject of our interest is not only the topology of the resulting microstructured media, but also the dynamics of its evolution involving the formation and/or growth of new phases. 

These results demonstrate that nonadsorbing polymer can induce phase separations in colloidal systems with the nature of the phases depending primarily on the ratio of the particle and polymer sizes. Since the strength of the attraction is not necessarily a monotonic function of the polymer concentration, e.g., because of penetration of the free polymer into a grafted layer, both destabilization and restabilization are possible. 

For simple coacervation induced by non-solvent addition in aqueous systems, ethanol, acetone, dioxane, isopropanol, and propanol are the most preferred to cause polymer desolvation and phase separation. In organic systems, mainly non-polar solvents. 

When the appropriate precautions are taken the method appears particularly suited for measuring very low tensions 10 mN m sometimes even as low as 10 mN m ). Such ultralow tensions are for example encountered in micro-emulsion systems and in just phase-separated polymeric or micellar solutions. For phase-separated colloid-polymer systems de Hoog and Lekkerkerker ) even reported values down to a few pN m , reproducibly being obtained after implementing a number of methodical improvements. (Alternatives for low tensions are the sessile and pending (micro-) drop but these do not usually go below 10 mN m ) Commercial apparatus are nowadays available. A variant proposed by Than et al. J employs a thin rod in the axis of the cylinder, to reduce spin-up time and suppress drift. Another variant, proposed by Kokov, analyses the centrifugal field required to squeeze liquid out of an orifice" ). 

Studies of various kinds of equilibria provide a wealth of information about polymer systems. Classical thermodynamics, which is concerned with the macroscopic properties of a system and the relations that hold between them at equilibrium, form a sufficient basis for description of these equilibria in polymer systems. We shall consider in a major part of this chapter methods of study of polymer solutions that deal with equilibria and can be fully described by thermodynamic relations. These include vapor pressure, osmotic pressure, and phase separation in polymer-solvent systems. 

Although any of the designs mentioned above will provide the location of phase boundaries (versus temperature and pressure), it is also important to know the compositions of the two phases in equilibrium. Note that while tie lines (lines connecting phases in equilibrium on T-x or p-x diagrams) are horizontal for simple binary mixtures, this is not true for phase separation in multicomponent systems (most notably polymer-fluid systems where the polymer sample contains chains of various lengths). Consequently, ports which allow withdrawal of samples following phase separation and equilibration are an important feature of view cells. Such ports also allow for the measurement of partition coefficients of solutes between, for example, aqueous and CO2 phases. 

We have also reported a new series of asymmetric and color-tunable PPV derivatives, including copolymers, that were developed in order to overcome the phase separation in polymer-blending systems with MEH-PPV units. In order to improve the performance of PLEDs, the bulky dimethyldode-cylsilylphenyl group was introduced into the mcfa-position of the phenyl substitutent, which inhibits the intermolecular interaction between the... 

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