Effect of structural constraint on dynamic self-assembly behavior of PNIPAM-based nonlinear multihydrophilic block copolymers

Abstract

The effects of structural constraint on the thermosensitivity and dynamic self-assembly behaviors of PNIPAM-based nonlinear multihydrophilic block copolymers (MHBCs) are investigated by a combination of calorimetric, NMR, FTIR, DLS and zeta potential measurements. Two novel miktoarm star MHBCs, (PNIPAM)2-(PVP-b-PAA)2 with PNIPAM segments constrained at one end and (PNIPAM-b-PAA)2-(PVP)2 with PNIPAM segments constrained at two ends, are mainly analyzed to make comparison. As we previously reported (Soft Matter, 2012, 8, 3980), in (PNIPAM)2-(PVP-b-PAA)2 the three polymeric segments have relatively independent phase behaviors during the formation of PNIPAM-core micelles. However, by introducing more structural constraints to PNIPAM segments, (PNIPAM-b-PAA)2-(PVP)2 exhibits a much smoother and weaker phase transition. Additional analysis by IR spectroscopy, in combination with perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2DCOS), indicates that PVP segments in (PNIPAM-b-PAA)2-(PVP)2 show phase transition-like behavior by participating in the self-association process of PNIPAM segments, and there are both unimers and small associates existing in the solution below the LCST due to the slight interaction between PVP and PNIPAM segments. Moreover, due to the presence of PVP segments in the core, the micelles formed by (PNIPAM-b-PAA)2-(PVP)2 should have a loosely organized superstructure, which may account for the weak phase transition and low degree of phase separation. Finally, we propose a mechanism of the self-assembly process of (PNIPAM-b-PAA)2-(PVP)2, which is further confirmed by DLS and zeta potential measurements.