Spectral Insights into Gelation Microdynamics of PNIPAM in an Ionic Liquid

Hydrogen Bonding Competition Mediated Phase Separation with Abnormal Moisture-Induced Stiffness Boosting

Moisture usually deteriorates polymers' mechanical performance owing to its plasticizing effect, causing side effects in their practical load-bearing applications. Herein, a simple binary ionogel consisting of an amphiphilic polymer network and a hydrophobic ionic liquid (IL) is developed with remarkable stiffening effect after moisture absorption, demonstrating a complete contrast to water-induced softening effect of most polymer materials. Such a moisture-induced stiffening behavior is induced by phase separation after hydration of this binary ionogel. Specifically, it is revealed that hydrogen (H)-bonding structures play a dominant role in the humidity-responsive behavior of the ionogel, where water will preferentially interact with polymer chains through H-bonding and break the polymer-IL H-bonds, thus leading to phase separation structures with modulus boosting. This work may provide a facile and effective molecular engineering route to construct mechanically adaptive polymers with water-induced dramatic stiffening for diverse applications.

Effect of Imidazolium Nitrate Ionic Liquids on Conformational Changes of Poly(N-vinylcaprolactam)

Detailed information about molecular interactions and conformational changes of polymeric components in the presence of ionic liquids (ILs) is essential for designing novel polymeric ionic liquid-based biomaterials. In biomaterials science and technology, thermoresponsive polymers (TRPs) are widely viewed as potential candidates for the fabrication of biorelated medical devices. Here, we synthesized thermoresponsive poly(N-vinyl-caprolactam) (PVCL) polymer and investigated the effects of imidazolium-based ILs (1-ethyl-3-methyl imidazolium nitrate and 1-butyl-3-methylimidazolium nitrate) with common anion and different cations on the phase transition behavior of PVCL aqueous solution. The impact of ILs on the phase transition behavior of PVCL was monitored by using UV-visible absorption spectra, steady-state fluorescence spectroscopy, thermal fluorescence spectroscopy, and temperature dependent dynamic light scattering. Results showed significant changes in the absorbance, molecular interactions, agglomeration, and coil to globule transition behaviors of PVCL in the presence of two ILs. PVCL aqueous solution showed significant conformational changes after the addition of ILs.

Intrinsically stretchable sheath-core ionic sensory fibers with well-regulated conformal and reprogrammable buckling

Fiber-based intrinsically stretchable ionic conductors are appealing in the emerging imperceptible sensing devices with an ultrabroad working range, yet still suffer from a low strain sensitivity (gauge factor generally smaller than 3) as compared to electronic conductors. To circumvent this issue, here we report downsized superelastic sheath-core ionic sensory fibers with a fingerprint-like conformal buckling architecture. By delicately regulating the intermolecular interactions in the ionogel core and fluoroelastomer sheath as well as adjusting holding strains, the transparency and buckling density of the fiber are finely modulated. Heat to above 60 °C would further erase the formed buckles, enabling the encoded information of the fiber to be reprogrammable. Importantly, introducing conformal buckles are shown to bring about two-fold sensitivity enhancement of the sensory fibers, leading to a considerably high gauge factor of 10.1 for ionic strain sensors. Furthermore, the water droplet control ability of the buckled fibers with tunable wettability is also demonstrated, contributing to the versatility of the present sensory fibers which may gain promising applications in integrative electronics, optics, and microfluidics.

Reversible Ion-Conducting Switch by Azobenzene Molecule with Light-Controlled Sol-Gel Transitions of the PNIPAm Ion Gel

Exploring a simple, on-demanding method of manipulating ionic conduction of ionic liquids with large amplitudes is a challenging task. Here, a reversible ion-conducting switch was obtained based on photoswitchable sol-gel transitions. The device was successfully applied in an electronic circuit to switch it on/off. The ion gel was prepared by directly mixing following individual components: azobenzene (Azo), poly(N-isopropylacrylamide) (PNIPAm), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]). The mixture is denoted as Azo/PNIPAm/[C₂mim][NTf₂]. The framework of this gel structure was particularly designed as an analogue to the physical mode of control theory: sensor/amplification/action. Light-induced isomerization of Azo acts as the light sensor to trigger the macroscopic sol-gel transition of PNIPAm assemblies. Such transition works as the amplification, which significantly affects the ionic movements, resulting in high-amplitude switching behavior. A photoswitchable ionic conductive device was demonstrated as action in this paper. Under UV irradiation, the sol-like state of Azo/PNIPAm/[C₂mim][NTf₂] provided a higher ion conduction (around 1 mS/cm) while being exposed to visible light, and a lower ion conduction (0.04 mS/cm) was observed in the gel state. This photoswitchable ion conductivity device was integrated to a well-designed logic gate to switch circuits on or off. This confirms the possible practical application of the sol-gel device, which outputs stable and detectable electrical signals. The research here demonstrates a simple but effective strategy to control the ionic movements, which can be applied in optoelectronic devices. The principle can be used to design different types of molecular optoelectronic switches.

Solvation and diffusion of poly(vinyl alcohol) chains in a hydrated inorganic ionic liquid

While the behavior of polyelectrolyte chains in aqueous salt solutions has been extensively studied, little is known about polar polymer chains in solvents with extremely high concentrations of inorganic ions, such as those found in ionic liquids (ILs). Here, we report on expansion, solvation and diffusion of poly(vinyl alcohol), PVA, chains in dilute solutions of a hydrated inorganic IL phase change material (PCM), lithium nitrate trihydrate (LNH). This solvent has an extremely high concentration of inorganic ions (≈18 M) with a low concentration of water molecules largely forming solvation shells of Li⁺ and NO₃^- ions, as shown using ATR-FTIR spectroscopy. Diffusion and hydrodynamic size of PVA chains of different molecular weights in this unusual solvent were studied using fluorescence correlation spectroscopy (FCS). A higher scaling exponent obtained from the molecular weight dependences of the diffusion coefficients of PVA chains as well as a lower overlap concentration (c*) of PVA in LNH solutions as measured by FCS suggest an expansion of the polymer coils in this solvent. We argue that enhanced solubility of PVA in LNH solutions is likely a result of increased rigidification of polymer chains due to the binding of solvated Li⁺ ions, which is demonstrated using ⁷Li NMR spectroscopy. We believe that an understanding of solvation and ion-binding capability can offer crucial insight into designing polymer-based shape stabilization matrices for inorganic PCMs.

Azobenzene-Based Ionic Liquid Switches Phase Separation of Poly(N-isopropylacrylamide) Aqueous Solutions as a Molecular Trigger, Leading to UV Shutdown of Ionic Transport

This paper presents a light-switchable phase separation of poly(N-isopropylacrylamide) (PNIPAm) in aqueous solutions using an azobenzene-based ionic liquid as a molecular trigger. A different UV response was obtained with use of an azobenzene molecular trigger. A UV-induced aggregation of PNIPAm was observed in the current system, which is opposite to that reported for azobenzene-functionalized PNIPAm systems. According to in situ FT-IR, cis-isomers destabilized the hydrogen bonding between water and PNIPAm, contributing to a light-induced aggregation of PNIPAm. This polarity effect was quite similar to salting effects of hofmeister anions on clouding points. Furthermore, a UV shutdown of ion transport of the azobenzene-based ionic liquid was demonstrated with a significant reduction factor of 0.03 at ambient temperature, which can be potentially applied in biotechnology, optoelectronics, and ionic circuits.

Rheological Study on the Thermoreversible Gelation of Stereo-Controlled Poly(N-Isopropylacrylamide) in an Imidazolium Ionic Liquid

The thermoreversible sol-gel transition for an ionic liquid (IL) solution of isotactic-rich poly (N-isopropylacrylamides) (PNIPAMs) is investigated by rheological technique. The meso-diad content of PNIPAMs ranges between 47% and 79%, and molecular weight (M_n) is ~35,000 and ~70,000 g/mol for two series of samples. PNIPAMs are soluble in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([BMIM][TFSI]) at high temperatures but undergo a gelation with decreasing temperatures. The transition temperature determined from G’-G” crossover increases with isotacticity, consistent with the previous cloud-point result at the same scanning rate, indicating imide groups along the same side of backbones are prone to be aggregated for formation of a gel. The transition point based on Winter-Chambon criterion is on average higher than that of the G’-G” crossover method and is insensitive to tacticity and molecular weight, since it correlates with percolation of globules rather than the further formation of elastic network (G’ > G”). For the first time, the phase diagram composed of both G’-G” crossover points for gelation and cloud points is established in PNIPAM/IL mixtures. For low-M_n PNIPAMs, the crossover-point line intersects the cloud-point line. Hence, from solution to opaque gel, the sample will experience two different transitional phases, either clear gel or opaque sol. A clear gel is formed due to partial phase separation of isotactic segments that could act as junctions of network. However, when the partial phase separation is not faster than the formation of globules, an opaque sol will be formed. For high-M_n PNIPAMs, crossover points are below cloud points at all concentrations, so their gelation only follows the opaque sol route. Such phase diagram is attributed to the poorer solubility of high-M_n polymers for entropic reasons. The phase diagram composed of Winter-Chambon melting points, crossover points for melting, and clear points is similar with the gelation phase diagram, confirming the mechanism above.

A simulation method for the phase diagram of complex fluid mixtures

The phase behavior of complex fluid mixtures is of continuing interest, but obtaining the phase diagram from computer simulations can be challenging. In the Gibbs ensemble method, for example, each of the coexisting phases is simulated in a different cell, and ensuring the equality of chemical potentials of all components requires the transfer of molecules from one cell to the other. For complex fluids such as polymers, successful insertions are rare. An alternative method is to simulate both coexisting phases in a single simulation cell, with an interface between them. The challenge here is that the interface position moves during the simulation, making it difficult to determine the concentration profile and coexisting concentrations. In this work, we propose a new method for single cell simulations that uses a spatial concentration autocorrelation function to (spatially) align instantaneous concentration profiles from different snapshots. This allows one to obtain average concentration profiles and hence the coexisting concentrations. We test the method by calculating the phase diagrams of two systems: the Widom-Rowlinson model and the symmetric blends of freely jointed polymer molecules for which phase diagrams from conventional methods are available. Excellent agreement is found, except in the neighborhood of the critical point where the interface is broad and finite size effects are important. The method is easy to implement and readily applied to any mixture of complex fluids.

Influence of lithium salt-induced phase separation on thermal behaviors of poly(vinylidene fluoride)/ionic liquid gels and pore/void formation by competition with crystallization

The thermal behavior of poly(vinylidene fluoride)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide/lithium bis(trifluoromethylsulfonyl)amide (PVDF/[C₂mim][TFSA]/LiTFSA) gels, prepared by cooling from the hot solution, was investigated with various concentrations of LiTFSA (C_LiTFSA). The peak melting temperature (T_m) of the gels shifted toward higher temperatures with increased C_LiTFSA. However, the thickness of lamellar crystal was found to decrease with the increase in C_LiTFSA, which meant that the increase in T_m was not caused by the thickening of lamellar crystal. Furthermore, we found the appearance of domains above T_m in the high C_LiTFSA region (≥20 wt%), which was a lithium ion-rich phase caused by the phase separation. Therefore, it is considered on the basis of Nishi–Wang equation that an increase in the interaction parameter with increasing C_LiTFSA toward the phase separation increased the T_m. The phase-separated domains competed with the subsequent crystallization, which resulted in the formation of micrometer-sized pores and nanometer-sized voids in the spherulites. Spectral measurements revealed that PVDF was not specifically solvated in the solution state above the crystallization temperature, while [TFSA]⁻ anion formed a complex with lithium ion irrespective of the PVDF content. These results led to the consideration that an increase in the interaction parameter might be caused by the strong interaction between lithium ion and [TFSA]⁻ anion to form the complex, which would also lower the interaction between PVDF and [TFSA]⁻ anion.

Lithium salt-induced phase separation on thermal behaviors of PVDF/ionic liquid gels and pore/void structures formation by competition with crystallization.

Multiple interaction regulated phase transition behavior of thermo-responsive copolymers containing cationic poly(ionic liquid)s

Schematic illustration of the phase transition mechanism of the P(OEGMA-co-BVIm[SCN]) copolymer.

Two-dimensional correlation spectroscopy in polymer study

This review outlines the recent works of two-dimensional correlation spectroscopy (2DCOS) in polymer study. 2DCOS is a powerful technique applicable to the in-depth analysis of various spectral data of polymers obtained under some type of perturbation. The powerful utility of 2DCOS combined with various analytical techniques in polymer studies and noteworthy developments of 2DCOS used in this field are also highlighted.

Response mechanism of the phase transitions of poly(n-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) using infrared spectroscopy

The thermal and ionic effects on the phase transitions of poly(N-isopropylacrylamide) (PNIPAAm) and its copolymer with benzo-18-crown-6-acrylamide, poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (PNIPAAm-co-BCAm), were investigated using infrared (IR) spectral variations of methyl (CH3), C=O, and amine (NH) groups. Subsequently, perturbation correlation moving-window two-dimensional correlation infrared spectroscopy (PCMW 2D-IR) was applied to clarify the differences in the phase-transition mechanisms of the polymers. The dominant influence on the phase-transition mechanism of PNIPAAm is whether the anion is evenly distributed in the bulk solution. The results show that the phase transition shifts to a lower temperature with increasing barium chloride (BaCl2) concentrations. In addition, the effect of the anion on the chemical group is homogeneous upon heating. As a result, the relevant transition temperature ranges have remain approximately constant. In contrast, the dominant influence on the phase-transition mechanism of PNIPAAm-co-BCAm is the interactions of the polymer chains with barium ions (Ba(2+)). The hydrophilic BCAm-Ba(2+) complexes distributed in the PNIPAAm-co-BCAm chain prevent the water molecules from leaving the polymer chains, which leads to an increase in the transition temperature and the complicated variation of the transition temperature range, as environmental stimuli-response behavior, with increasing BaCl2 concentrations.

The dynamic process of radioactive iodine removal by ionic liquid 1-butyl-3-methyl-imidazolium acetate: discriminating and quantifying halogen bonds versus induced force

The halogen bonds vs. induced force of the dynamic process of iodine removal by ionic liquid is discriminated and quantified.

Temperature-controlled electrochemical switch based on layered double hydroxide/poly(N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly

In this paper we report the fabrication of layered double hydroxide (LDH) nanoparticles/poly(N-isopropylacrylamide) (pNIPAM) ultrathin films (UTFs) via the layer-by-layer assembly technique, and their switchable electrocatalytic performance in response to temperature stimuli was demonstrated. X-ray diffraction and UV-vis absorption spectroscopy indicate a periodic layered structure with uniform and regular growth of the (LDH/pNIPAM)(n) UTFs; an interaction based on hydrogen bonding between LDH nanoparticles and pNIPAM was confirmed by X-ray-photoelectron spectroscopy and Fourier transform infrared spectroscopy. Temperature-triggered cyclic voltammetry and electrochemical impedance spectroscopy switch for the UTFs was obtained between 20 and 40 °C, accompanied by reversible changes in surface topography and film thickness revealed by atomic force microscopy and ellipsometry, respectively. The electrochemical on-off property of the temperature-controlled (LDH/pNIPAM)(n) UTFs originates from the contraction-expansion configuration of pNIPAM with low-high electrochemical impedance. In addition, a switchable electrocatalytic behavior of the (LDH/pNIPAM)(n) UTFs toward the oxidation of glucose was observed, resulting from the temperature-controlled charge transfer rate. Therefore, this work provides a facile approach for the design and fabrication of a well-ordered command interface with a temperature-sensitive property, which can be potentially applied in electrochemical sensors and switching.