Hydrogen bonded assembly of poly(acrylic acid)-block-poly(L-valine) in dilute solutions

The self-assembly of four hybrid poly(acrylic acid)-block-poly(L-valine) (PAA-b-PLVAL) block copolymers (i.e., PAA(40)-PLVAL(100), PAA(80)-PLVAL(100), PAA(80)-PLVAL(80), and PAA(80)-PLVAL(60)) was investigated. Formation of core-shell spherical micelles was observed, and it correlated with the extent of beta-sheet ordering in PLVAL hydrophobic domains. PAA(80)-PLVAL(60), having the lowest beta-sheet content (12%), existed only as random aggregates at pH < 5 and in an unaggregated state at high pH. The beta-sheet formation also produced an unusual pH dependence of the micelles, where the swelling of micelles occurred only within a narrow degree of deprotonation (alpha) range of between 0.15 to 0.2 and 0.4 to 0.5 for PAA(80)-PLVAL(80) and PAA(80)-PLVAL(100), respectively. An additional role of the length of the PAA segment was identified as providing sufficient shielding to the hydrophobic core, which prevents further association into large compound micelles, as was observed for PAA(40)-PLVAL(100).