Abstract
Many temperature-responsive polymers exhibit a single-phase transition at the lower critical solution temperature (LCST). One exception is poly(N-isopropylacryamide) (PNIPAM). PNIPAM brush layers (51 ± 3 nm thick) that are end-grafted onto glass beads collapse in two stages. The viscoelastic changes of a PNIPAM brush layers were investigated with an interferometric laser method at different temperatures. This method is able to measure the two-stage collapse of beads coated with a polymer brush layer. When these beads are situated close to a hydrophilic glass surface, they exhibit Brownian motion. As this Brownian motion changes with temperature, the collapse of the polymer layer is revealed. The characteristic spectrum of the Brownian motion of beads is modeled by a damped harmonic oscillator, where the polymer layer acts as both spring and damping elements. The change of the Brownian motion spectrum with temperature indicates two transitions of the PNIPAM brush layer, one at 36 °C and one at 46 °C. We attribute the first transition to the LCST volume collapse of PNIPAM. Here, changes of the density and viscosity of the brush dominate. The second transition is dominated by a stiffening of the brush layer.
Highlights
Poly(N-isopropylacryamide) (PNIPAM) is a functional polymer that responds to changes in its environment
We used an optical interference method to measure the spectrum of the Brownian motion of a PNIPAM brush-coated bead sticking to a wall, i.e., trapped in a harmonic potential
Scanning force microscopy can provide information about the interface of a sample.[56−58] in this case, colloids need to be attached to the cantilever apex
Summary
Poly(N-isopropylacryamide) (PNIPAM) is a functional polymer that responds to changes in its environment. We associate the increase in transition temperature to confinement of the PNIPAM layer between two hydrophilic hard walls.[51,52] This confinement could be the reason for the slightly narrow temperature transition range that we observe compared to literature.[14,32,49,53] The second transition occurred at 10 °C above the LCST We attribute this transition to changes in the elastic properties of the brush. At the LCST transition, the brush collapses and the packing density increases At this temperature, we previously predicted that (βStokes·βd)−1 would decrease by a factor of 1.4 (Figure 3A). The dissipations in our measurements are dominated by the viscosity of the interfacial polymer layer, which is described by ηrel
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