GRADATION OF MICROCAPSULE WALL POROSITY BY DEPOSITION OF POLYMER MIXTURES (EUDRAGIT RL AND EUDRAGIT RS) - PHASE-SEPARATION OF POLYMER MIXTURES AND EFFECTS OF EXTERNAL MEDIA AND CONDITIONS ON RELEASE

With the aim of increasing flexibility in controlling release from mic rocapsules, mixtures of wall polymers varying in porosity were investi gated by phase separation. Eudragit RL and RS (polymethylmethacrylate linear backbone polymers) mixtures differing in polar substituent cont ent and porosity were used as the wall material and were deposited usi ng a non-solvent addition method. Release rates increased with polar g roup content of the mixtures, using theophylline, potassium dichromate or sodium chloride as model core materials. Theophylline release rate had the same relationship to polar group content as found earlier for urea permeation of cast mixed-polymer films. Release was generally ac celerated in these systems when the external medium contained sodium l auryl sulphate as a wetting agent but not consistently, decreasing une xpectedly for RL-theophylline microcapsules. Localized dissolution of core substance was visible microscopically during release from single microcapsules. The release rate was sensitive to agitation intensity o nly at low wall to core ratios. Temperature change revealed only a sin gle release mechanism for sodium chloride by Arrhenius equation treatm ent. Puffer ions penetrated coatings readily, changing theophylline re lease rates and providing clear evidence of diffusion via a pore-capil lary mechanism.