Polymers, phase separated

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

As is being discussed, polymers used to prepare micelles exhibit a LCST that can be deLned as the temperature at which the polymer phase separates (Heskinsand Guillet, 1968). Below the LCST, the polymer/micelle is soluble, but it precipitates at temperatures above the LCST. The diameter of these micelles rapidly rises at temperatures above the LCST, due to hydrophobic interactions that result in the aggregation of the micelles (Kohori et al., 1998). This effect of temperature on size was shown to be reversible, since the micellar architecture was maintained after lowering the temperature below the LCST (Chung et al., 1999). 

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... 

Abstract This article summarizes a large amount of work carried out in our laboratory on polysiloxane based Interpenetrating Polymer Networks (IPNs). First, a polydimethylsiloxane (PDMS) network has been combined with a cellulose acetate butyrate (CAB) network in order to improve its mechanical properties. Second, a PDMS network was combined with a fluorinated polymer network. Thanks to a perfect control of the respective rates of formation of each network it has been possible to avoid polymer phase separation during the IPN synthesis. Physicochemical analyses of these materials led to classify them as true IPNs according to Sperling s definition. In addition, synergy of the mechanical properties, on the one hand, and of the surface properties, on the other hand, was displayed. 

Explicitly developed are models of several theoretical multiphase distributions, with corresponding depth-profile results on thin-film plasma polymers, phase-separated block copolymers, and chemical reactions on fiber surfaces. Ion impact is treated from three points of view as an analytical fingerprint tool for polymer surface analysis via secondary ion mass spectroscopy, by forming unique thin films by introducing monomers into the plasma, and as a technique to modify polymer surface chemistry. 

Polymers such as dextrans could provide many desirable properties (e.g., high 7g and T ) to the freeze-dried formulation. Therefore, it is essential that future research address the theoretical and practical aspects of protein/polymer phase separation and develop the mechanistic insight to prevent this phenomenon during lyophilization. Also, as part of this effort, it is important to discern why other polymers (e.g., PVP and BSA) that protect labile proteins apparently do not phase-separate from the protein during lyophilization. 

Fundamental aspects of coacervation have been thoroughly covered for some time through the classical studies of Bungenberg de Jong and Kruyt for ionic systems, and by Dobry and Boyer-Kawenoki for non-ionic systems. The basic thermodynamic conditions for polymer-solvent interactions and polymer phase separation have been nicely described by Flory. In the following, polymer phase separation processes will be briefly considered from mechanistic and thermodynamic points of view. 

Unless the solubility parameters of both polymers are very similar, the polymers will be incompatible, leading to polymer phase separation, as X2, 3 > (Zc)2,3-... 

The theoretical treatment of polymer phase separation based on polymer-polymer repulsion requires an extension of the x-parameter concept on two polymers in a common solvent. For this case, Scott defined the critical conditions for phase separation, provided that the rather common conditions apply that Zi,2 - Z13I < 1 and < yTJ < X2... 

A very frequently described family of polymers subjected to simple coacervation are cellulose derivatives, particularly ethyl cellulose (EC). ° While most cellulose ethers are soluble in water, EC and the cellulose esters are insoluble or only partly soluble in water, e.g., as a function of pH. For coacervation of EC, toluene is a preferred good solvent and cyclohexane a poor solvent. Gradual addition of cyclohexane to a solution of EC desolvates the polymer. Alternatively, EC can be dissolved in hot cyclohexane cooling to room temperature induces polymer phase separation. In both these cases, the coacervate film or droplets can be hardened by exposing the coacervate to a large volume of cyclohexane, whereby physical cross-links are formed. 

The CPC, which was originally developed for separating blood lymphocytes, has evolved into several useful instruments for separations of cells, macromolecules, and small molecular weight compounds. Among those, the type-J multilayer CPC is most extensively utihzed for high-speed CCC separations of natural and synthetic products. The utility of the type-J CPC may be extended to the polymer phase separation of macromolecules and cell particles with a spiral disk assembly currently being developed in our laboratory. 

The effect of polymers on microemulsions phase behavior has been reported by Hesselink and Faber (8). They have described the surfactant-polymer phase separation in terras of the incompatibility of two different polymers in a single solvent, considering the microemulsion as a pseudo-polymer system. The effect of polymers on the phase behavior of micellar fluids has been recently studied by Pope et al. (9) and others (10,11). 

Exciplex states only exist at the interface between the two dissimilar polymers in the blend. Reducing the density of these interfaces in the polymer blend is expected to reduce the amount of exciplex observed. For example, annealing mobilizes the polymers and causes the film to move closer to thermodynamic equilibrium, i.e. the two polymers phase separate and the density of heterojunction sites decreases. Indeed, we observe that the amount of exciplex emission is reduced by the annealing treatment [30]. 

Parachutes, matrioshka structures, and necklaces result when the polymer phase separates from the surfactant bilayer during or after the polymerization process. Two mechanisms have been proposed to describe this phenomenon polymer rearrangement and preferential polymerization [12]. Polymer rearrangement occurs when growing or terminated polymer molecules initially dis-... 

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