Biodegradable poly(asparagine) grafted with poly(caprolactone) and the effect of substitution on self-aggregation

Micelle-like aggregates formed with amphiphilic graft copolymers, poly(asparagine)-g-poly(caprolactone) (PAsn-g-PCL) were prepared through 'the precipitation and dialysis method' and their self-aggregation phenomena were investigated by light scattering, fluorescence probing, and TEM. The strong hydrophobic interaction of associated PCL grafts would facilitate the primary aggregate formation in a bimodal size distribution and significantly reduced CAC (critical aggregation concentration) with respect to DS (degree of substitution). Further, the size of aggregates increased as DS reduced apparently. The graft copolymer having a higher DS formed the more rigid or compact hydrophobic core in self-aggregates with polarization as well as smaller aggregates. When introduced in a basic condition at 37 °C, the effective diameter of aggregates increased and the scattering intensity reduced due to the degradation of PCL as time changed. When incubated in aqueous solution at 4 °C, the sample of DS 4.2 maintained without any size change for up to 20 days, while that of DS 1.2 increased up to the equilibrium diameter. But, the rapid growth of turbid state at DS 6.0 was observed within 2 weeks. After 25 days, the effective diameter of aggregates increased irrespective of DS, becoming a little turbid, and attributed to the swelling and degradation of hydrophobic domains with various morphogenic changes.