Hybrid micellar network hydrogels of thermosensitive ABA triblock copolymer and polymer brush-grafted nanoparticles: Effect of LCST transition of polymer brushes on gel property

Abstract

This article presents a study of the effect of LCST transition of polymer brushes on properties of hybrid micellar network hydrogels of a thermosensitive ABA triblock copolymer and polymer brush-grafted nanoparticles (hairy NPs) with NPs initially residing outside the core of micelles. Four batches of thermosensitive polymer brush-grafted, 20 nm silica NPs with different lower critical solution temperatures (LCSTs) and a thermosensitive ABA triblock copolymer composed of a poly(ethylene oxide) central block and thermosensitive outer blocks (ABA-N) were prepared. The LCSTs of hairy NPs were significantly higher than the critical micellization temperature of ABA-N, resulting in the NPs being initially located outside the core of micelles. The effects of LCST transition of hairy NPs and NP-to-polymer mass ratio on properties of hybrid micellar hydrogels of ABA-N with a concentration of 10 wt% and hairy NPs were investigated by rheological measurements. For all hybrid hydrogels studied in this work, the dynamic storage modulus G′ exhibited a sharp increase on top of a plateau that was forming in the heating ramp at a temperature corresponding to the LCST transition of hairy NPs. This phenomenon was caused by the collapse of the brushes on NPs triggering the absorption of thermosensitive outer blocks of ABA-N molecules in the dangling and loop forms and the reorganization of the 3-D gel network structure, which increased the density of bridging chains and thus the G′. This explanation was supported by the results from fluorescence resonance energy transfer studies. The maximum values of G′ (G′max) of the gels containing higher LCST hairy NPs were noticeably lower compared with the gels with lower LCST hairy NPs and the gel with no NPs. The G′max of all hybrid hydrogels decreased gradually with the increase of the NP-to-polymer ratio.