Tuning the Thermoresponsive Behavior of Surface-Attached PNIPAM Networks: Varying the Crosslinker Content in SI-ATRP.

Sophia Thiele,Rebekah L N Hailes,John Andersson,Andreas Dahlin

doi:10.1021/acs.langmuir.0c03545

Sophia Thiele, Rebekah L N Hailes + Show 2 more

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https://doi.org/10.1021/acs.langmuir.0c03545

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Abstract

The synthesis and thermoresponsive properties of surface-attached poly(N-isopropylacrylamide)-co-N,N′-methylene bisacrylamide (PNIPAM-co-MBAM) networks are investigated. The networks are formed via SI-ARGET-ATRP (“grafting-from”) on thiol-based initiator-functionalized gold films. This method is reliable, well controlled, fast, and applicable to patterned surfaces (e.g., nanopores) for networks with dry thicknesses >20 nm. Surface-attached PNIPAM-co-MBAM gels are swollen below their volume phase transition temperature but above collapse without complete expulsion of water (retain ∼50 vol %). The swelling/collapse transition is studied using complementary SPR and QCMD techniques. The ratio between swollen and collapsed heights characterizes the thermoresponsive behavior and is shown to not depend on network height but to vary with MBAM content. The higher the proportion of the crosslinker, the lower the magnitude of the phase transition, until all responsiveness is lost at 5 mol % MBAM. The temperature range of the transition is broadened for more crosslinked PNIPAM-co-MBAM gels but remains centered around 32 °C. Upon reswelling, less crosslinked networks display sharp transitions, while for those containing ≥3 mol % MBAM, transitions remain broad. This tunable behavior persists for gels on nanostructured gold surfaces. Investigating PNIPAM-co-MBAM networks on gold plasmonic nanowell arrays is a starting point for expanding their scope as thermo-controlled nanoactuators.

Highlights

Poly(N-isopropyl acrylamide) (PNIPAM) is a well-studied thermoresponsive polymer due to its biocompatibility and biologically relevant lower critical solution temperature (LCST) of ∼32 °C in aqueous media.[1,2] Above this critical solution temperature, the unfavorable entropic contribution from the hydrophobic effect to the free energy of mixing dominates over exothermic hydrogen bonding, decreasing the solvent quality of water to the point where the polymer undergoes a sharp transition from a hydrated, extended coil conformation to a hydrophobic, collapsed structure
PNIPAM-co-MBAM networks were prepared on gold surfaces initiated with an ω-mercaptoundecyl bromoisobutyrate selfassembled monolayer (SAM) via SI-ARGET-ATRP
Looking toward applications on nanostructured surfaces,[54] we studied the behavior of PNIPAM-co-MBAM thermoresponsive gels on thin gold films containing plasmonic nanowell arrays with optical properties discussed in detail in a previous report by our group.[55]

Summary

Introduction

Poly(N-isopropyl acrylamide) (PNIPAM) is a well-studied thermoresponsive polymer due to its biocompatibility and biologically relevant lower critical solution temperature (LCST) of ∼32 °C in aqueous media (i.e., close to physiological temperature).[1,2] Above this critical solution temperature, the unfavorable entropic contribution from the hydrophobic effect to the free energy of mixing dominates over exothermic hydrogen bonding, decreasing the solvent quality of water to the point where the polymer undergoes a sharp transition from a hydrated, extended coil conformation to a hydrophobic, collapsed structure (which maximizes intra- and interchain interactions) By exploiting these responsive properties, PNIPAM hydrogels have been used in drug delivery systems,[3] cell culturing,[4] and artificial muscles.[5] PNIPAM brushes, formed by end-tethered chains on a variety of surfaces, have served as thermo-controlled nanoactuators (such as valves,[6] pumps,[7] and filters8) in microfluidic devices.[9,10] The extent of the thermally induced collapse of the polymer brushes depends on grafting density and molecular weight,[11,12] both of which can be controlled by surface-initiated ( termed “grafting-from”) atom transfer radical polymerization (SI-ATRP).[13−22] The LCST is known to remain essentially the same as for coils in solution (±1 °C) regardless of the grafting strategy.

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