SIMPLE COACERVATION OF HYDROXYPROPYL METHYLCELLULOSE PHTHALATE (HPMCP) .1. TEMPERATURE AND PH DEPENDENCY OF COACERVATE FORMATION

The simple coacervation of hydroxypropyl methylcellulose phthalate (HP MCP) on the addition of 20% (w/w) sodium sulphate solution was investi gated as a function of the temperature and pH value of the aqueous pol ymer solution. Phase diagrams, quantitative investigations of the isol ated polymer-rich phases and charge density measurements served to cha racterize phase separation. The existence of coacervate and precipitat e phases of HPMCP was attributed to the chemical structure of the poly mer. Analogous to other polymers such as gelatin or cellulose acetate phthalate, HPMCP formed polymer-rich coacervate and precipitate phases from aqueous solutions following the addition of electrolyte solution s. Increased temperature and total electrolyte content had a synergist ic effect on phase separation. With increasing temperature the polymer content in the polymer-rich phase rose up to 23.6% (w/w) at 60 degree s C corresponding to a polymer yield of 63.7%. This was accompanied by the gelation of the coacervate phase which was attributed to the temp erature-dependent hydration of the methyl and hydroxypropyl substituen ts of the polymer. Moreover, a minimum pH value of the HPMCP solution was required for coacervate formation, otherwise the polymer was salte d out as a precipitate. Charge density measurements showed that this w as accompanied by the almost entire dissociation of the HPMCP carboxyl groups. They were hence responsible for the pH-dependent polymer hydr ation and essential to coacervate formation. The adjustment of an appr opriate pH value is therefore a prerequisite for the formation of coac ervate phases in the process of microencapsulation with HPMCP by simpl e coacervation. Owing to the temperature dependency of coacervate form ation, the temperature course required for the microencapsulation proc ess was found to run opposite to that described for cellulose acetate phthalate.