The formation and characterization of polymeric materials precipitated by CO(2)-based spray processes.

摘要

Submicron polymeric micromaterials have been formed by two promising compressed fluid spray processes, i.e., Rapid Expansion of Supercritical Solutions (RESS) and Precipitation with a Compressed Fluid Antisolvent (PCA). To understand the formation mechanisms, novel experimental techniques and characterization methods have been developed to achieve further control over the product morphology.; RESS involves dissolving a polymer into a SCF solvent and then expanding across a fine throttling device to facilitate a rapid supersaturation. The formation mechanism has been clarified by holding the concentration of a semicrystalline fluoropolymer in CO{dollar}sb2{dollar} solution constant, obtaining the solution cloud point curves, varying the pre-expansion temperature above and below the cloud point, and changing the length to diameter (L/D) ratio of the nozzle. Unlike previous studies, CO{dollar}sb2{dollar} has been used as a solvent and the morphology has been explained in terms of the location of phase separation within the expansion nozzle.; In PCA, an organic solution is sprayed through an atomization nozzle into cocurrently flowing CO{dollar}sb2{dollar}. The liquid or supercritical CO{dollar}sb2{dollar} is an antisolvent for the solute (polymer), but is miscible with the solvent. To produce new morphologies and characterize the formation mechanism, three new concepts have been investigated: (1) the formation of metastable polymer blends, (2) the use of a coaxial atomization nozzle and (3) the stabilization of microparticles using surfactants.; By investigating the T{dollar}sb{lcub}rm g{rcub}{dollar} behavior of metastable polymer blends the kinetics of PCA phase separation have been characterized. Further control over the product morphology, and a better understanding of the hydrodynamic and mass transfer mechanisms has been provided with the development of the coaxial spray nozzle. With this nozzle, the atomization and suspension of the droplets has been controlled more effectively to produce more uniform particles, with far less agglomeration. Finally, flocculation and agglomeration has been minimized by adding a surfactant or polymer to CO{dollar}sb2{dollar} or the polymer solution to sterically stabilize the precipitating polymer microstructure. A novel PCA-turbidimetry apparatus has been developed to study "in-situ" latex stabilization and better understand the flocculation and sedimentation mechanisms.