pH-Responsive Core−Shell Particles and Hollow Spheres\nAttained by Macromolecular Self-Assembly

Abstract

According to our “block-copolymer-free” strategy for self-assembly of polymers, noncovalently connected\nmicelles (NCCM) with poly(ε-caprolactone) (PCL) as the core and poly(acrylic acid) (PAA) as the shell in\naqueous solutions were attained due to specific interactions between the component polymers. The micellar\nstructure was then locked in by the reaction of PAA with diamine. Afterward, hollow spheres based on\nPAA network were obtained by either core degradation with lipase or core dissolution with dimethylformamide of the cross-linked micelles. The cavitation process was monitored by dynamic light scattering,\nwhich indicated a mass decrease and size expansion. The hollow structure is confirmed by transmission\nelectron microscopy observations. The resultant hollow spheres are pH- and salt-responsive: there is a\nsubstantial volume increase when pH changes from acid to base, and vice versa. The volume change takes\nplace dramatically over the pH-range from 5.8 to 7.5. Furthermore, this volume−pH-dependence is found\nto be completely reversible provided the effect of ionic strength is excluded. The volume change can be\nadjusted by changing the shell thickness and the cross-linking degree of the hollow spheres. The salt effect\non the hollow sphere size depends on pH: with increasing salt concentration the size shows an increase,\na decrease, and a little change in acidic, basic, and neutral media, respectively.