Polymers, phase separated component

Phase separation occurs when AG rises above 0. This may be triggered by a rise in enthalpy (i.e., AH) or a decline in entropy (i.e., AS). To allow for the formation of a uniform network polymer, phase separation must be delayed until crosslinking is well enough advanced to prevent individual molecules from demixing. This delay is achieved by either reducing AH or by raising AS (in concert with T). The enthalpy factor (AH) is controlled by the difference in Hildebrand s solubility parameter (5) between the various reacting components, since... 

Polymer phase separation and crystallization, as introduced separately in the previous two chapters, have different molecular driving forces that can be simultaneously expressed by the use of the lattice model. Adjusting the corresponding driving forces, the mean-field theory can predict the phase diagrams, and at the meanwhile molecular simulations can demonstrate the complex phase transition behaviors of polymers in the multi-component miscible systems. 

In general, BCPs added as a component in a polymer blend compatibilize the interface of phase-separated components. As a result, the addition of a particular BCP can significantly alter the final morphology of the blend. However, before entering the analysis of the role of the BCP on the blend structure it is worth mentioning that, in addition to the chemical nature of the blocks involved, a crucial aspect on the addition of BCPs concerns the amount of BCP concentration within the blend (Fig. 6.2). We will limit our discussion here to the case of a BCP/homopolymer blend in the presence of air as the interface. At low concentrations of BCP in the blend (Fig. 6.2(a,b)) the BCP is present either in the bulk (dissolved in the polymer phase) or at the interface. In this concentration regime, the amount of BCP segregated at the interface is directly... 

Note that D and M are applied only to the overall or starting weight fi ac-tions hence the superscript o applied on them. Also, note that Du is a negative number, because the polymer phase separates from the solvent/precipitant. For simulation work, initial weight fraction of 7%/3%/90% were used for PMAA (component 1)/MAA (component 2)/water (component 3), respectively. The phase diagram, shown in Fig. 1.1.9 (Shi, 1997) for the PMAA/MAA/water system, indicates approximate values for the equilibrium compositions in the polymer-rich phase (c i = 12%, c s = 82%, c 2= 6%) and polymer-lean phase (c i =. 5%, c 3 = 97%, c 2 = 0.5%). 

A steady stream of encapsulation technologies continues to appear in the patent literature. Some are simply modifications or improvements of established technologies, whereas others are new technologies such as very low temperature casting (29), deposition of coating material from a supercritical fluid (30), and polymer phase separation induced by evaporation of a volatile solvent from a two-component solvent mixture (31). 

The lack of synchronous cross peaks between polystyrene and polyethylene bands indicates these polymers are reorienting independently of each other. Cross peaks appearing in the asynchronous spectrum (Figure 1-19) also verify the above conclusion. For an immiscible blend of polyethylene and polystyrene, where molecular-level interactions between the phase-separated components are absent, the time-dependent behavior of IR intensity fluctuations of one component of the sample... 

Conducting Polymer Blends, Composites, and Colloids. Incorporation of conducting polymers into multicomponent systems allows the preparation of materials that are electroactive and also possess specific properties contributed by the other components. Dispersion of a conducting polymer into an insulating matrix can be accompHshed as either a miscible or phase-separated blend, a heterogeneous composite, or a coUoidaHy dispersed latex. When the conductor is present in sufftcientiy high composition, electron transport is possible. 

The flow behavior of the polymer blends is quite complex, influenced by the equilibrium thermodynamic, dynamics of phase separation, morphology, and flow geometry [2]. The flow properties of a two phase blend of incompatible polymers are determined by the properties of the component, that is the continuous phase while adding a low-viscosity component to a high-viscosity component melt. As long as the latter forms a continuous phase, the viscosity of the blend remains high. As soon as the phase inversion [2] occurs, the viscosity of the blend falls sharply, even with a relatively low content of low-viscosity component. Therefore, the S-shaped concentration dependence of the viscosity of blend of incompatible polymers is an indication of phase inversion. The temperature dependence of the viscosity of blends is determined by the viscous flow of the dispersion medium, which is affected by the presence of a second component. 

We have seen above in two instances, those of liquid-liquid phase separation and polymer devolatilization that computation of the phase equilibria involved is essentially a problem of mathematical formulation of the chemical potential (or activity) of each component in the solution. 

Typically IPNs exhibit some degree of phase separation in their structure depending on how miscible the component polymers are. However, because the networks are interconnected such phase separation can occur only to a limited extent, particularly by comparison with conventional polymer blends. Polymer blends necessarily have to be prepared from thermoplastics IPNs may include thermosets in their formulation. 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

Polymer blends have been categorized as (1) compatible, exhibiting only a single Tg, (2) mechanically compatible, exhibiting the Tg values of each component but with superior mechanical properties, and (3) incompatible, exhibiting the unenhanced properties of phase-separated materials (8). Based on the mechanical properties, it has been suggested that PCL-cellulose acetate butyrate blends are compatible (8). Dynamic mechanical measurements of the Tg of PCL-polylactic acid blends indicate that the compatability may depend on the ratios employed (65). Both of these blends have been used to control the permeability of delivery systems (vide infra). 

This type of DNA condensation can be classified only formally as a product of DNA/poly-mer interactions, since no binding between these two components has been observed. Polymers that cause DNA condensation serve in this case as phase separation agents and concentrate DNA in the aqueous phase in high concentration. The presence of a certain amount of salt is required to overcome phosphate repulsion. 

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