Thermodynamics and Kinetics of Phase Separation

The effect of various kinetic conditions on the phase separation in semi-IPNs produced from PU and PS was studied using FTIR by He, Widmaier, and Meyer [293]. To obtain semi-IPNs of PU/PS mass ratio 30 70, the mixtiu e of initial components was stored at room temperature for different periods of time t (5 min, 15 min, and 2 h), and then the temperature was elevated up to 343 K and the process was performed until the full disappearance of the double bonds. 

It was discovered that at t = 5 and 15 min, PU network and PS are formed simultaneously but with different rates, whereas at f = 2 h the formation of PS proceeds in a fully cured PU network. Various kinetic conditions of PU formation affect the phase separation estimated from the data of the hght transmission as a function of time. The system loses its transparency by simultaneous formation of PU and PS (t = 5 min) when the conversion degree of PS is about 0.2. By forming the semi-IPN in the f = 2 h regime the system stays transparent, whereas for f = 15 min a trend toward diminishing transmission of light is observed at the latest stages of IPN formation. 

Introduction of preliminary polymerized PS into PU in an amount of 0.5-1.0% initiates phase separation in the system in the first few minutes. It was also found that increasing MM of PS diminishes the time of the onset of phase separation. Thus, the authors reach a conclusion that if the reaction mixture is separated into two phases before gelation of PU, the semi-IPNs formed are turbid, but if separation begins after PU gelation transparent IPNs are formed. The thermodynamic reasons for these effects are not clear. 

It can therefore be inferred that simultaneously proceeding chemical processes of formation of polymer molecules which constitute the IPNs and physical processes of microphase separation occur under nonequilibrium conditions [103]. In this case the microphase structure of semi-IPNs and the kinetics of their formation become interrelated (see Sect. 4). The effect of the reaction conditions on the morphology of simultaneous PU/PMMA IPNs has also been estabUshed in [294], and for poly(dimethylsiloxane-urethane)/PMMA IPNs in [295]. It was shown that depending on the kinetic conditions, both compatible and incompatible IPNs could be formed with well-matched rates of cross-finking reaction. Incompatible IPNs are formed by a much slower cross-linking reaction producing phase-separated IPNs. We believe that in the present case, because of the method of determination, one should talk not about true compatibility, but about the dependence of apparent compatibility on reaction kinetics. 

The first attempt to describe theoretically the processes of phase separation during the reaction of formation of semi-IPNs has been done in the works [296,297]. Semi-IPNs based on PS and a reactive epoxy monomer based on DGEBA with a stoichiometric amount of 4,4 -methylenebis(2,6-diethylaniline) were studied experimentally. Thermodynamic analysis of the phase separation proceeding during the curing reaction was performed that considered the composition dependence of the interaction parameter x(T, 2) (where T is the temperature and 2 is the voliune fraction of PS) and the polydispersity of both polymers. The latter is especially important, hi this analysis, x(T, p2) was considered as the product of two functions, one depending on the temperature [D(T)] and the other depending on the composition [B( )]. For the initial mixture (before the reaction) the cloud point curves showed upper critical solution temperature behavior and the dependence x(r, 2) on the composition was determined from the threshold point. 

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Thermodynamics and kinetics of phase separation of polymer mixtures have benefited greatly from theories of spinodal decomposition and of classical nucleation. In fact, the best documented tests of the theory of spinodal decomposition have been performed on polymer mixtures. 

In a dynamic and cross-linkable system, such as the curing of a thermoset that contains a thermoplastic, the phase separation is more complicated than nonreaction system. The phase separation is controlled by the competing effects of thermodynamics and kinetics of phase separation and cure rate of thermoset resin (i.e. time dependent viscosity of the system). 

Utracki, L.A. (1994) Thermodynamics and kinetics of phase separation, in Interpenetrating Polymer Networks, (eds D. Klempner, L.H. Sperling, and L.A. Utracki), American Chemical Society, Washington, DC, pp. 77-123. 

Our investigations of this system center on four aspects. First, to establish that the compatible mixtures indeed represent the thermodynamically stable states at room temperature second, to find the upper phase boundary of the mixtures third, to explore the use of nuclear magnetic resonance to study molecular motion which allows us to draw conclusions about the scale of homogeneity of mixing and fourth, to study the morphology and kinetics of phase separation, in particular, spinodal decomposition associated with LCST. 

The objective of this review is to characterize the excimer formation and energy migration processes in aryl vinyl polymers sufficiently well that the excimer probe may be used quantitatively to study polymer structure. One such area of application in which some measure of success has already been achieved is in the analysis of the thermodynamics of multicomponent systems and the kinetics of phase separation. In the future, it is likely that the technique will also prove fruitful in the study of structural order in liquid crystalline polymers. 

The basic, macroscopic theories of matter are equilibrium thermodynamics, irreversible thermodynamics, and kinetics. Of these, kinetics provides an easy link to the microscopic description via its molecular models. The thermodynamic theories are also connected to a microscopic interpretation through statistical thermodynamics or direct molecular dynamics simulation. Statistical thermodynamics is also outlined in this section when discussing heat capacities, and molecular dynamics simulations are introduced in Sect 1.3.8 and applied to thermal analysis in Sect. 2.1.6. The basics, discussed in this chapter are designed to form the foundation for the later chapters. After the introductory Sect. 2.1, equilibrium thermodynamics is discussed in Sect. 2.2, followed in Sect. 2.3 by a detailed treatment of the most fundamental thermodynamic function, the heat capacity. Section 2.4 contains an introduction into irreversible thermodynamics, and Sect. 2.5 closes this chapter with an initial description of the different phases. The kinetics is closely link to the synthesis of macromolecules, crystal nucleation and growth, as well as melting. These topics are described in the separate Chap. 3. 

The second chapter, by D. Vollmer (Germany), brings a quantitative comparison of experimental data and theoretical predictions on thermodynamic and kinetic properties of microemulsions based on nonionic surfactants. Phase transitions between a lamellar and a droplet-phase microemulsion are discussed. The work is based on evaluation of the latent heat and the specific heat accompanying the transitions. The author focuses on the kinetics of phase separation when inducing emulsification failure by constant heating. The chapter is a comprehensive, detailed study of all the aspects related to the phase separation phenomenon in microemulsions. 

Many of the studies that draw comparisons between gel-derived and conventional ceramics examine phase changes, including crystallization and liquid-liquid phase separation. This section provides a brief review of the thermodynamics and kinetics of those processes. 

Phase Transformations discusses the thermodynamics and kinetics of crystallization and phase separation. 

Compatibilized blends with addition of nanoparticles can become an alternative for conventional compatibilized blends containing block copolymers. Addition of oragnoclays to polymer blend affects multiple features thermodynamic phase behavior of the blend, the kinetics of phase separation and also the morphology formed in the two-phase region. Hemmati et al. proved that incorporation of organoclay evidently enhances the miscibility of PE and ethylene-vinyl acetate [EVA] phases in the amorphous regions of nanocomposites. In addition, the studies revealed that nanofiller influences the diffusion of polymer chains, which contributes to... 

For simultaneous semi-IPNs made from PU and PS, the kinetics of phase separation was studied using optical microscopy completed by image analysis [90]. The development of a nodular structure was observed. A thermodynamic approach has allowed us to establish that the diameter of the phase-separated species was the result of the competition between the kinetics of network formation and the kinetics of phase separation. The mechanism of phase separation was not discussed. 

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