Phase Behavior of Polymer Blends

Preparation of vertical stratified blend films relies upon an understating and control of the phase separation process. The Flory-Huggins theory expresses a 

The phase separation in blend thin films also proceeds by the well-described bulk mode of nucleation and growth and spinodal decomposition. However, the conditions of phase separation during the formation of a blend thin film are significantly 

In practice, it is not easy to control the phase separation process in polymer blend films. The phase separation process in the polymer blends is very complicated, and the final morphology in the blend films is highly sensitive to many factors, such as solvent evaporation rate, solubility parameter, film-substrate interaction, surface tension of each component, and film thickness. Thus, the vertical phase separation can take place only under very extreme conditions [4B, 41], and, alternatively, lateral phase separation is more typical than vertical phase separation because the forces that contribute to the formation of lateral structures minimize the interface area. 

Therefore, there are not many reports of vertically separated semiconduc-ting/insulating polymer blends. However, the achievements in the limited reports have displayed the versatility of possible applications of semiconducting/insulating polymer blends in OTFT devices. 

One-Step Formation of Semiconducting and Insulating Layers in OTFTs 

As discussed above, polymer blends are often heterogeneous systems consisting of multiple phases. The system is in a thermodynamic equilibrium if the chemical potentials of the components are equal in all phases. The chemical potential is defined as a change in the Gibbs energy of the system induced by the addition of one molecule of component i, while the pressure, temperature and number of other molecules are kept constant (Eq. (3.15))  

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Kammer, H. W., Kressler, H. and Kummerioewe, C Phase Behavior of Polymer Blends - Effects of Thermodynamics and Rheology. Vol. 106, pp, 31-86. 

Phase Behavior of Polymer Blends Volume Editor Freed, K. 

Of course, nanocomposites are not the only area where mesoscale theories are being used to predict nanostructure and morphology. Other applications include—but are not limited to—block copolymer-based materials, surfactant and lipid liquid crystalline phases, micro-encapsulation of drugs and other actives, and phase behavior of polymer blends and solutions. In all these areas, mesoscale models are utilized to describe—qualitatively and often semi-quantitatively—how the structure of each component and the overall formulation influence the formation of the nanoscale morphology. 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

The phase behavior of polymer blends comprising amorphous polymers is experimentally well accessible in a window which is bounded at high temperatures by the thermal decomposition temperature of the polymer components and at low temperatures by the glass transition temperature of the system (cf. Fig. 1). Below the glass transition temperature the phase behavior can be estimated only tentatively. 

Finally, a challenging problem is to discuss the influence of hydrodynamic flow fields on the phase behavior of polymer blends. This is of fundamental interest and of technological importance as well since stresses and corresponding deformations are encountered during processing of blends. Extension of studies to blend systems under external flow is necessary for the better understanding of structure formation in polymer blends outside equilibrium. 

The first is to develop thermodynamic issues to understand the complex phase behavior of polymer blends. Experimental determination of miscibility regions provides the individual segmental interaction parameters necessary for predictions of various phase equilibria. 

In conclusion, the mean-field theory outlined above turns out to be a powerful tool for rationalizing the complex phase behavior of polymer blends, especially of random copolymer based blends, in terms of interaction, ftee-volume and size effects. 

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