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LCST polymers with UCST behavior

Marzieh Najafi, Mehdi Habibi, Remco Fokkink, Wim E. Hennink and Tina Vermonden

Published: 28 February 2021

Abstract

In this study, temperature dependent behavior of dense dispersions of core crosslinked flower-like micelles is investigated. Micelles were prepared by mixing aqueous solutions of two ABA block
copolymers with PEG B-blocks and thermosensitive A-blocks containing PNIPAM and crosslinkable moieties. At a temperature above the lower critical solution temperature (LCST), self-assembly of the polymers resulted in the formation of flower-like micelles with a hydrophilic PEG shell and a hydrophobic core. The micellar core was stabilized by native chemical ligation (NCL). Above the LCST, micelles displayed a radius of B35 nm, while a radius of B48 nm was found below the LCST due to hydration of the PNIPAM core. Concentrated dispersions of these micelles (Z7.5 wt%) showed glassy state behavior below a critical temperature (Tc: 28 1C) which is close to the LCST of the polymers. Below this Tc, the increase in the micelle volume resulted in compression of micelles together above a certain concentration and formation of a glass. We quantified and compared micelle packing at different concentrations and temperatures. The storage moduli (G0
) of the dispersions showed a universal dependence on the effective volume fraction, which increased substantially above a certain effective volume fraction of j = 1.2. Furthermore, a disordered lattice model describing this behavior fitted the experimental data and revealed a critical volume fraction of jc = 1.31 close to the experimental value of j = 1.2. The findings reported provide insights for the molecular design of novel thermosensitive PNIPAM nanoparticles with tunable structural and mechanical properties.

Full Access Link: Soft Matter