Unusually Stable Hysteresis in the pH-Response of Poly(Acrylic Acid) Brushes Confined within Nanoporous Block Polymer Thin Films

Abstract

Stimuli-responsive soft materials are a highly studied field due to their wide-ranging applications; however, only a small group of these materials display hysteretic responses to stimuli. Moreover, previous reports of this behavior have typically shown it to be short-lived. In this work, poly(acrylic acid) (PAA) chains at extremely high grafting densities and confined in nanoscale pores displayed a unique long-lived hysteretic behavior caused by their ability to form a metastable hydrogen bond network. Hydraulic permeability measurements demonstrated that the conformation of the PAA chains exhibited a hysteretic dependence on pH, where different effective pore diameters arose in a pH range of 3 to 8, as determined by the pH of the previous environment. Further studies using Fourier transform infrared (FTIR) spectroscopy demonstrated that the fraction of ionized PAA moieties depended on the thin film history; this was corroborated by metal adsorption capacity, which demonstrated the same pH dependence. This hysteresis was shown to be persistent, enduring for days, in a manner unlike most other systems. The hypothesis that hydrogen bonding among PAA units contributed to the hysteretic behavior was supported by experiments with a urea solution, which disrupted the metastable hydrogen bonded state of PAA toward its ionized state. The ability of PAA to hydrogen bond within these confined pores results in a stable and tunable hysteresis not previously observed in homopolymer materials. An enhanced understanding of the polymer chemistry and physics governing this hysteresis gives insight into the design and manipulation of next-generation sensors and gating materials in nanoscale applications.

Isoporous Membranes from Novel Polystyrene-b-poly(4-vinylpyridine)-b-poly(solketal methacrylate) (PS-b-P4VP-b-PSMA) Triblock Terpolymers and Their Post-Modification

“…It is the most widely studied diblock copolymer to date for the fabrication of integral asymmetric membranes. However, the phase inversion technique was successfully applied also to other diblock copolymer systems polystyrene-b-poly(ethylene oxide) (PS-b-PEO) [33,34], polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) [35], polystyrene-b-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA) [36,37], polystyreneb-poly(methyl methacrylate) (PS-b-PMMA) [38], poly(α-methylstyrene)-b-poly(4-vinylpyridine) (PαMS-b-P4VP) [39], and also triblock terpolymer systems such as polyisoprene-b-polystyreneb-poly(4-vinylpyridine) (PI-b-PS-b-P4VP) [15], polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-b-PS-b-PDMA) [40], and polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) [41]. Although many different block copolymers were used for the SNIPS process to generate isoporous membranes featuring comparably polar surfaces, no approaches for the preparation of such membranes were reported using two neighbored hydroxyl groups (-OH) containing block copolymers.…”

Hydrogels with a Memory: Dual-Responsive, Organometallic Poly(ionic liquid)s with Hysteretic Volume-Phase Transition

“… 44 , 45 Such hysteretic volume-phase transitions in soft materials could be utilized for example in memory devices, where multiple states of the material can exist, preserving information on physical history in a single environment. 46 − 51 Few examples of pH-responsive polymer gels with hysteretic volume-phase transitions and strong bi-stability can be found in literature. 43 , 46 The organometallic PIL hydrogel presented here provides a unique example of a thermo-responsive hysteretic system.…”

Responsive filtration membranes by polymer self-assembly

“…Studies that utilize SNIPS of terpolymers that feature poly(acrylic acid) (PAA) segments exhibit even stronger and more interesting pH response [72][73][74] . These membranes also showed the ability to uptake copper ions by complexation, and permeability that is responsive to the concentration of copper ions 74 .…”