Poly(N-isopropylacrylamide) Response to Salt Concentration and Anion Identity: A Brush-on-Brush Study

Comparing polymer-surfactant complexes to polyelectrolytes

HYPOTHESIS

Understanding the complex interactions between polymers and surfactants is required to optimise commercially relevant systems such as paint, toothpaste and detergent. Neutral polymers complex with surfactants, forming 'pearl necklace' structures that are often conceptualised as pseudo-polyelectrolytes. Here we pose two questions to test the limits of this analogy: Firstly, in the presence of salt, do these polymer-surfactant systems behave like polyelectrolytes? Secondly, do polymer-surfactant complexes resist geometric confinement like polyelectrolytes?

EXPERIMENTS

We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR.

FINDINGS

At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective CMC. However, at high NaCl concentrations (e.g., 500 mM) no brush collapse was observed at all (non-zero) concentrations of SDS studied, contrary to what is seen for many polyelectrolytes. Study of the polymer-surfactant system under confinement revealed that the physical volume of surfactant dominates the structure of the strongly confined system, which further differentiates it from the polyelectrolyte case.

Supersoft Elastic Bottlebrush Microspheres with Stimuli-Responsive Color-Changing Properties in Brine

Solvent-free supersoft elastomer is highly desirable for building photonic structures with significant stimuli-responsive color changes. We report supersoft elastic porous microspheres with vivid structural colors obtained via self-assembly of amphiphilic bottlebrush block copolymers at the water/oil interface templated by ordered water-in-oil-in-water double emulsions. The porous structure is composed of cross-linked bottlebrush polydimethylsiloxane (PDMS) as the supersoft elastic skeleton and bottlebrush poly(ethylene oxide) (PEO) as the internal responsive layer. The obtained microspheres show large reversible volume changes through well-controlled dehydration or hydration of PEO in response to salt ions in an aqueous environment. As a result, full-spectrum colors are obtained dependent on different salt concentrations. In-situ observation of color reflection of a microsphere indicates a gradual structural transition from the outside to the inside corresponding to migration of water molecules and salt ions. Moreover, rod-like bottlebrush PEO exhibits an anion-induced salting-out behavior different from that of random coil polymers. The significantly responsive behaviors of bottlebrush block copolymer (BBCP) assemblies in the presence of salt ions primarily rely on the supersoft elastic skeleton of the porous structure, providing a facile route to the creation of stimuli-responsive photonic materials by low-cost self-assembly methods.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Two-Stage Collapse of PNIPAM Brushes: Viscoelastic Changes Revealed by an Interferometric Laser Technique

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.