Article pubs.acs.org/Macromolecules
Enhancing Gelation of Doubly Thermosensitive Hydrophilic ABC Linear Triblock Copolymers in Water by Thermoresponsive Hairy Nanoparticles Bin Hu, Wenxin Fu, and Bin Zhao* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: A method is reported for enhancing the gelation of doubly thermosensitive hydrophilic linear ABC triblock copolymers in water using thermoresponsive polymer brush-grafted nanoparticles (hairy NPs). A linear ABC triblock copolymer (ABC-Q) composed of a hydrophilic, charged middle block, and two thermosensitive outer blocks with different LCSTs, LCSTA of the lower LCST A block and LCSTC of the higher LCST C block, and two batches of hairy NPs with distinct thermoresponsive properties were prepared. When the temperature was raised from 0 °C to above the LCSTA but below the LCSTC, ABC-Q self-assembled into micelles in water with the lower LCST A block forming the core; further heating to above the LCSTC triggered the collapse of the C block, producing a two-compartment 3-D network micellar hydrogel when the polymer concentration was sufficiently high. Rheological studies showed that adding thermoresponsive hairy NPs with a LCST similar to the LCSTC of ABC-Q led to a significant increase in dynamic storage modulus (G′). For 6 wt % aqueous solutions of ABC-Q, the maximum value of G′ (G′max) increased with increasing amount of hairy NPs; a 45% increase in G′max was observed at the NP-to-polymer mass ratio of 60:100. It is believed that hairy NPs acted as “seeds” to adsorb the collapsed C block of ABC-Q, promoting the formation of bridging chains among micellar cores and NPs and thus enhancing the gelation. In contrast, no benefit was observed when adding hairy NPs with a LCST much higher than LCSTC; the G′max exhibited little change with increasing NP-to-polymer mass ratio. Our explanations for the rheological observations were supported by fluorescence resonance energy transfer studies.
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INTRODUCTION Thermosensitive block copolymers hold great potential for a wide variety of applications,1−4 including viscosity modification of aqueous systems, site-specific drug delivery, and tissue engineering. The aqueous solutions of these block copolymers often exhibit intriguing thermally induced reversible transitions between free-flowing liquids and free-standing gels when the polymer concentration is above a critical value, i.e., critical gelation concentration (CGC). Such polymer gels are commonly called injectable hydrogels because they allow the use of syringe and needle for injection, a minimally invasive means of administration.1,2 Among various block copolymer architectures, ABA triblock copolymers composed of thermosensitive outer blocks and a hydrophilic middle block have been intensively studied for the hydrogel formation.5−12 When the temperature is raised to above the lower critical solution temperature (LCST) of the outer blocks, the copolymers self-assemble into flower-like micelles in dilute aqueous solution and 3-dimensional network micellar hydrogels in more concentrated solution.4 For many applications, these physically cross-linked hydrogels are preferred over thermoresponsive diblock copolymer systems because of the relatively low CGCs, typically
