Polymer coacervation characterization

Polymer-polymer coacervates, in microcapsule form, can be characterized by a number of methods, though the most accurate involves a compression-based micromanipulated probe connected to a sensitive transducer.f The precision of such techniques can be as high as 10%. Generally, a 1 1 stoichiometry provides the most stable microcapsules. Other properties, including the sphericity, transparency, or membrane homogeneity are also often characterized. Novel methods based on analytical ultracentrifugation are particularly useful for 2D membranes. ... 

Ameodo, C., Benoit, J.P., and Thies, C. (1987). Characterization of complex coacervates used to form microcapsules. Polym. Mater. Sci. Eng., 57, 255-259. 

Cellulose acetate phthalate forms a pH-triggered phase transition system, which shows a very low viscosity up to pH 5. This system will coacervate in contact with the tear fluid (pH 7.4), forming a gel in few seconds and releasing the active ingredients over a prolonged period of time. The half-life of residence on the rabbit corneal surface was approximately 400 seconds compared to 40 seconds for saline. However, such systems are characterized by a high polymer concentration, and the low pH of the instilled solution may cause discomfort to the patient. 

The polymer employed to prepare microspheres must be characterized in terms of molecular weight and purity,however this topic is beyond the scope of this article. Characterization of the materials may have implications for the formation of the microspheres. The viscosity and film-forming properties of the polymers used should be known. Viscosity can affect the tendency to form microspheres, their size, and even their shape. Burgess and coworkers have shown that albumin-acacia coacervates do not form microcapsules under certain conditions of pH and ionic strength, if the viscosity of the coacervate phase is too high. Burgess and Carless developed a method to predict the optimum conditions for complex coacer-vation based on the charge carried by the two polymers involved. 

Negative values for X23 are less common than positive values. They commonly occur between pairs of polymers that possess polar substituents that interact favourably. These correspond to compatible polymers that are miscible and can exist in solution together. Of course, if the interaction between the polymers is sufficiently strong, molecular association may result in coprecipitation or the formation of coacervates. Such behaviour, characterized by the formation of phases containing comparable amounts of the two polymers, should not be confused with the incompatibility noted in the preceding section for positive values for X23- In the latter case, the polymer phases formed would contain predominantly one polymer (except in the vicinity of the critical point), not comparable amounts of both polymers. 

Prokop, A., Hunkeler, D., DiMari, S., Haralson, M. A., Wang, T. G. Water Soluble Polymers for Immunoisolation I Complex Coacervation and Cytotoxicity. Vol. 136, pp. 1-52. Prokop, A., Kozlov, E., Carlesso, G and Davidsen,J. M. Hydrogel-Based Colloidal Polymeric System for Protein and Drug Delivery Physical and Chemical Characterization, Permeability Control and Applications. Vol. 160, pp. 119-174. 

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