EVALUATION OF A STATISTICAL-MODEL FOR THE FORMATION OF POLY[ACRYLOYL HYDROXYETHYL STARCH] MICROSPHERES

Purpose. To characterize the network structure of Poly(Acryloyl Hydrox yethyl Starch) (Ac-HES) microspheres and test the theoretical model an d the hypothesis that the rate of swelling of microspheres is inversel y related to the extent of crosslinking. Methods. Microspheres were pr epared with varying degrees of derivatization (DD) and molar ratios (M R) and subjected to the characterization of matrix structure by dynami c and equlilibrium swelling studies utilizing direct microscopic obser vation and the Flory-Rehner equation. The dependence of average molecu lar weight between crosslinking Mc, on DD and MR were compared to test the validity of the model. Results. Study of the dependence of Mc on the microspheres preparation parameters, DD and MR, showed that at con stant MR, the Mc decreased with DD, while at constant DD, the Mc initi ally decreased with MR to a minimum, and then increased with MR, compl ying with the model prediction. Dynamic swelling of microspheres showe d a monotonical increase to equilibrium size featured by two time vari ables, Tp and Teq, that were dependent on Mc; this permitted a concept ual view of the general structure of the Ac-HES microspheres. The Mc, which was more accurately determined by the weight method (as opposed to volume method), was independent of the size of microspheres althoug h there was evidence of variation among particles within a batch. Conc lusions. The results validated the model in describing the polymerizat ion/crosslinking reaction of the Ac-HES microsphere system and suggest ed that Mc is the principal factor in controlling release.