Microencapsulation using poly(DL-lactic acid). II: Effect of polymer molecular weight on the microcapsule properties.

Poly(DL-lactic acid) (DL-DPA) of three different molecular weights, 20,500; 13,300 and 5200, was used to prepare microcapsules containing differing contents of phenobarbitone (PB), as a reference core. A water/oil (W/O) emulsion evaporation method was used. The effect of polymer molecular weight on the particle size, 'encapsulation efficiency', morphology, density, thermal behaviour and swelling property has been reported. A general trend towards lowering the mean microcapsule size, both by volume and population, was observed with respect to lower polymer molecular weight. The gross morphology of the microsapsule surface, encapsulation efficiency and density were unaffected by variations in polymer molecular weight. Differential scanning calorimetric analysis of the microcapsules showed a lowering of glass transition temperature after microencapsulation. The melting endotherm for phenobarbitone also indicated the presence of crystalline drug in the microcapsule matrix. These microcapsules were found to swell in the aqueous environment and the mean size increased linearly with time. However, the rate of swelling was higher with low molecular weight polymer and also depended on core loading.