Lower Critical Solution Temperature Behavior Research Articles

A Coordinating Small Organic Molecule with Tunable Lower Critical Solution Temperature for Efficient Management of Solar Radiation.

Structurally well-defined small molecules with lower critical solution temperature (LCST) behavior offer enormous prospects for fine-tuning their phase transition properties to be "on-demand" applied in the specific scene but are still underexplored. Herein, a novel amphiphilic small LCST molecule is rationally designed and synthesized. The molecule, namely TG, features a conjugation of multiple short ethylene glycol (EG) chains with the functional coordinating terpyridine (Tpy) moiety. The molecule TG demonstrates excellent LCST behavior down to 0.05× 10-3 m in a water solution. And a cloud point Tcp = 30.9°C with a very short thermal hysteresis ΔT = 0.2°C and good reversibility can be achieved when c = 0.1× 10-3 m. The excellent LCST properties of TG have enabled its successful performance as the smart window for solar radiation management with the ∆Tlum, ∆TIR, and ∆Tsol being 83.6%, 49.1%, and 67.2%, respectively. Moreover, the presence of Tpy moiety in TG enables its coordination with Ru3+ and the resulting complex also exhibits modulated LCST behavior with different concentration-dependent Tcp. These studies would provide novel small-molecule-based scaffolds for constructing better solar radiation management systems as well as other thermal-responsive smart materials.

Preparation and performance evaluation of intelligent plugging agent for thermo‐sensitive water‐based drilling fluid

AbstractIn the process of unconventional oil and gas exploitation, the invasion of a large amount of drilling fluid into the formation is one of the most critical reasons leading to wellbore instability. A temperature‐sensitive polymer (AOS) was synthesized from acrylamide, oligo (ethylene glycol) methyl ether methacrylate (OEGMA), and sodium p‐styrene sulfonate by emulsion polymerization. The material is a solid‐free fluid with good fluidity at room temperature, after injection into the formation, the fluid is gradually transformed into hard gel above the lower critical solution temperature (LCST) (90°C), thus blocking micro‐fractures and improving wellbore stability. Infrared and NMR characterization confirmed the successful preparation of the target product. The transmittance of AOS with ambient temperature was measured by UV–vis spectrophotometer, and the LCST behavior of AOS was verified by rheological and viscoelastic measurements. The interaction mechanism between AOS and sodium bentonite was evaluated according to the American Petroleum Institute standard. The results show that after aging at 180°C, the filtration loss is reduced by 93%, and the medium permeability (100, 200 mD) formation is effectively plugged, with a plugging efficiency is as high as 74.7%.

Poly(N-isopropylacrylamide)-based hydrogel carrier containing Prussian blue for thermally controlled release

A hydrogel consisted of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-AA) and Prussian blue (PB) was synthesized for a cargo carrier by controlling the release of organic dye under near infrared (NIR) light. The AA moiety of the hydrogel provided chemical bonding with Fe (III) ions for further PB formation. Biocompatible, NIR-responsive PB generated heat and the heat was transferred to the hydrogel. The NIPAAm moiety in the hydrogel was shrunken because of the low critical solution temperature behavior of the hydrogel, and the hydrogel released the loaded dye with 62%. These results suggest dye-loaded hydrogel as a prospective platform for light-triggered carriers.

Rheological study on lower critical solution temperature behavior of organo-soluble cyano-substituted p-aramid in isotropic phase

Rheological study on lower critical solution temperature behavior of organo-soluble cyano-substituted p-aramid in isotropic phase

Host-guest assembly triggered lower critical solution temperature behavior with reversible photo-regulation

The less frequent lower critical solution temperature (LCST) phenomenon has attracted extensive attention due to the foresighted application in smart materials. Nevertheless, the existing LCST-type materials are mainly dominated by a few polymers with specific moieties due to the damped hydration at a higher temperature and the lack of reversible regulations of the corresponding temperature sensitivity. Herein, an unanticipated and reversibly photo-responsive LCST behavior was uncovered in the BSC4/Azo-4 assembly solution. Such behavior arose from the host-guest binding between BSC4 and Azo-4 in an aqueous solution since neither the guest nor host alone exhibited an LCST property. Ascribing to the distinct π-π stacking between the trans- and cis-isomers of Azo-4, a reversible regulation of LCST behavior was achieved by irradiating UV and blue light alternately. Our study demonstrated the complexity and possibility of supramolecular self-assembly, opening a new avenue in the fabrication and adjustment of LCST-type materials in the future.

Effect of cosolvents on the phase separation of polyelectrolyte complexes.

Evidence is shown that cosolvent mixtures control the coacervation of mixtures of oppositely charged polyelectrolytes. Binary and ternary solvent mixtures lead to non-monotonic solubility as a function of the average dielectric constants of the solvent mixtures. These data are rationalized by considering both electrostatic-driven phase separation and solvophobic-driven phase separation using group contribution effects on solubility parameters. These estimates are introduced into an effective Flory-Huggins interaction parameter within the framework of Voorn-Overbeek theory with variable dielectric constants and temperature dependences. Despite its simplicity, the model recovers salient experimental observations not only on their coacervate stabilities, but also on their lower critical solution temperature behaviors. These observations highlight the importance of weak van der Waals interactions in determining the phase behaviors of polyelectrolyte complexes relative to electrostatic correlations.

Unraveling the interplay of temperature with molecular aggregation and miscibility in TEA-water mixtures.

In the phase diagram of binary liquid mixtures, a miscibility gap is found with the concomitant liquid-liquid phase separation, wherein temperature is a key parameter in modulating the phase behavior. This includes critical temperatures such as the lower critical solution temperature (LCST) and upper critical solution temperature (UCST). Using a comprehensive approach including molecular dynamics (MD) simulation, graph theoretical analysis and spatial inhomogeneity measurement in an LCST-type mixture, we attempt to establish the relationship between the molecular aggregation pattern and phase behavior in TEA-water mixtures. At lower temperatures of binary liquid mixtures, TEA molecules tend to aggregate while simultaneously interacting with water forming a homogeneous solution. As the temperature increases, these TEA aggregates tend to self-associate by minimizing the interaction with water, which facilitates formation of two distinct liquid phases in the binary liquid. The spatial distribution analysis also reveals that the TEA aggregates compatible with water promote uniform distribution of water molecules, maintaining a homogeneous solution, while the water-incompatible ones generate isolation of water H-bond aggregates, leading to liquid-liquid phase separation in the binary system. This current study on temperature-induced molecular aggregation behavior is anticipated to contribute to a critical understanding of the phase behavior in binary liquid mixtures, including UCST, LCST, and reentrant phase behavior.

Preparation of Alginic Acid/Poly(2‐Oxazoline) Hybrid Gels and Their Use in Cell Preservation

AbstractA hybrid gel is developed aiming to extend the storage period for cell preservation compared with an alginate gel. The introduction of poly(2‐oxazoline)s into alginic acid promises to improve functional materials by changing their functional groups (e.g., poly(2‐isopropyl‐2‐oxazoline) has the lower critical solution temperature behavior). This study demonstrates chemical modification of a carboxylate of sodium alginate by 1,3‐diaminopropane‐terminated poly(2‐oxazoline)s using 3‐ethylcarbodiimide hydrochloride (EDC) and N‐hydroxy succinimide (NHS) as the condensation reagents for the aminations. The incorporated ratio of poly(2‐oxazoline)s side chains into the carboxylates estimated by 1H NMR is ca. 5–12% and the remaining pendent carboxylates are used for gelation by adding calcium salts (CaCl2). Gelation is observed within 30 min producing 93–99% gel fractions that swell in water. Similar gelation occurs in the cultivation medium of green‐fluorescence protein (GFP)‐encoded and COVID‐19 spike protein‐encoded DNA recombinant E.coli in both the absence and presence of isopropyl β‐D‐1‐thiogalactopyranoside (IPTG). Even in the presence of IPTG, fluorescence ascribed to GFP is not observed while the cells are kept at 25 °C for 3 weeks. After solvation by ethylenediaminetetraacetic acid (EDTA), fluorescence intensity at 508 nm is clearly observed and cell preservation at room temperature is thereby demonstrated.

Interionic bonding in aqueous phosphonium ionic liquid solutions exhibiting LCST behavior with high phase separation temperatures

Phosphonium-derived ionic liquids have emerged as a powerful class of thermoresponsive materials with sought after Lower Critical Solution Temperature (LCST) phase separation with applications across a broad range of fields. Herein, we report the spectroscopic examination of phosphonium salts bearing the general structure [X][P444n] wherein systematic structural variations were designed to gain a fundamental understanding behind the impact of alkyl chain length, anion size, and effective nuclear charge on their temperature-dependent phase separation behavior. Using variable temperature 1H, 31P, and 19F NMR, the bonding changes which drive LCST phase separation for the assembled thermoresponsive ILs were elucidated. For the first time, spectroscopic evidence revealed a delicate balance between the interdependence of alkyl chain length and anion size on phase separation that formed the basis of new LCST design principles.

Spinodal Decomposition of Filled Polymer Blends: The Role of the Osmotic Effect of Fillers.

The reported work addresses the effect of fillers on the thermodynamic stability and miscibility of compressible polymer blends. We calculate the spinodal transition temperature of a filled polymer blend as a function of the interaction energies between the blend species, as well as the blend composition, filler size, and filler volume fraction. The calculation method relies on the developed thermodynamic theory of filled compressible polymer blends. This theory makes it possible to obtain the excess pressure and chemical potential caused by the presence of fillers. As a main result of the reported work, we demonstrate that the presence of neutral (non-adsorbing) fillers can be used to enhance the stability of a polymer blend that shows low critical solution temperature (LCST) behavior. The obtained results highlight the importance of the osmotic effect of fillers on the miscibility of polymer blends. The demonstrated good agreement with the experiment proves that this effect alone can explain the observed filler-induced change in the LCST.

Dense Monolayer Network Structures of Double Hydrophilic Hyperbranched Copolymers at the Air/Water Interface.

Influences of subphase pH and temperature on the interfacial aggregation behavior of two double hydrophilic hyperbranched copolymers of poly[oligo(ethylene glycol) methacrylate-co-(2-diisopropylamino)ethyl methacrylate] (P(OEGMA-co-DIPAEMA)) at the air/water interface are studied by the Langmuir film balance technique. Morphologies of their Langmuir-Blodgett (LB) films are characterized by atomic force microscopy (AFM). At the interface, P(OEGMA-co-DIPAEMA) copolymers tend to form a dense network structure of circular micelles composed of branching agent-connected carbon backbone cores and mixed shells of OEGMA and DIPAEMA segments (pendant groups). This network structure containing many honeycomb-like holes with diameters of 6-8nm is identified for the first time and clearly observed in the enlarged AFM images of their LB films. Under acidic conditions, surface pressure versus molecular area isotherms of the two copolymers in the low-pressure region show larger mean molecular area than those under neutral and alkaline conditions due to the lack of impediment from DIPAEMA segments. Upon further compression, each isotherm exhibits a wide pseudo-plateau, which corresponds to OEGMA segments being pressed into the subphase. Furthermore, the isotherms under neutral and alkaline conditions exhibit the lower critical solution temperature behavior of OEGMA segments, and the critical temperature is lower when the hyperbranched copolymer contains higher OEGMA content.

Thermally Reversible On‐Off Switching of Aggregation‐Induced Emission via LCST Phase Transition of Ionic Liquids in Water

AbstractA new phosphonium‐based ionic liquid is designed and synthesized using the 1,1,2,2‐tetraphenylethene‐derived anion ([P4444]4[TSPE]). Aqueous solutions of [P4444]4[TSPE] are obtained as transparent liquids (monophasic state), which suspended with increasing temperature. This behavior is found to exhibit a lower critical solution temperature (LCST) phase transition behavior. The LCST phase transition temperature can be controlled from 36 to 55 °C by changing the weight ratio between [P4444]4[TSPE] and water. When observing the LCST phase transition behavior of aqueous solutions of [P4444]4[TSPE] under excitation light with a wavelength of 365 nm, negligible photoluminescence in the monophasic state is observed at room temperature. Conversely, strong photoluminescence is observed in the suspended state when the temperature is above the LCST phase transition temperature. This strong photoluminescence is attributed to aggregation‐induced emission (AIE) that is induced by the restriction of the intramolecular rotations of the 4‐sulfophenyl groups against the ethylene core in the [TSPE] anion. This behavior is thermally reversible. This is the first example of on‐off switching of AIE achieved using the LCST behavior of ionic liquids.

Synthesis and Characterization of Thermoresponsive Chitosan-graft-poly(N-isopropylacrylamide) Copolymers.

Thermoresponsive chitosan-graft-poly(N-isopropylacrylamide) (CS-g-PNIPAAm) copolymers of different composition were synthesized by free-radical polymerization of chitosan (CS) and N-isopropylacrylamide (NIPAAm) in aqueous solution using potassium persulfate (PPS) as an initiator. By changing the molar ratio of CS:NIPAAm from 1:0.25 to 1:10 graft copolymers with a CS backbone and different amounts of PNIPAM side chains were prepared. The chemical structure of the obtained CS-g-PNIPAAm copolymers was confirmed by FTIR and 1H NMR spectroscopy. 1H NMR spectra were also used to calculate the content of attached PNIPAAm side chains. Moreover, the lower critical solution temperature (LCST) behavior of synthesized copolymers was assessed by cloud point, differential scanning calorimetry and particle size measurements. The aqueous solutions of copolymers containing ≥12 molar percent of PNIPAAm side chains demonstrated LCST behavior with the phase separation at around 29.0-32.7 °C. The intensity of thermoresponsiveness depended on the composition of copolymers and increased with increasing content of poly(N-isopropylacrylamide) moieties. The synthesized thermoresponsive chitosan-graft-poly(N-isopropylacrylamide) copolymers could be potentially applied in drug delivery systems or tissue engineering.

Clickable Polyprolines from Azido-proline N-Carboxyanhydride.

Polyproline is a material of great interest in biomedicine due to its helical scaffold of structural importance in collagen and mucins and its ability to gel and to change conformations in response to temperature. Appending of function-modulating chemical groups to such a material is desirable to diversify potential applications. Here, we describe the synthesis of high-molecular-weight homo, block, and statistical polymers of azide-functionalized proline. The azide groups served as moieties for highly efficient click-grafting, as stabilizers of the polyproline PPII helix, and as modulators of thermoresponsiveness. Saccharides and ethylene glycol were utilized to explore small-molecule grafting, and glutamate polymers were utilized to form polyelectrolyte bottlebrush architectures. Secondary structure effects of both the azide and click modifications, as well as lower critical solution temperature behavior, were characterized. The polyazidoprolines and click products were well tolerated by live human cells and are expected to find use in diverse biomedical applications.

Effect of heterocyclic substituents on the thermo/pH/CO2-responsiveness of multistimuli-responsive homopolymers

A series of novel thermo/pH/CO2-triple responsive homopolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. These homopolymers contain ethoxy groups, and the end substituents are pyrrolidine, piperidine, N-methylpiperazine, morpholine and thiomorpholine, respectively. The effects of different heterocyclic substituents on the responsiveness of temperature, pH and CO2 were studied. For temperature-response, it was found that the lower critical solution temperature (LCST)-type cloud point (CP) temperature decreased as the number of carbon atoms increases, while the solubility of the polymers changed greatly with the introduction of oxygen, nitrogen and sulfur atoms. And the CP of the polymers can be adjusted by molecular weight, concentration and salt. Furthermore, some of these polymers present upper critical solution temperature (UCST) behavior in alcohols. For pH-response, the initial pH of the polymer solution is different, their critical pH changed accordingly, and is affected by temperature; in addition, changing the pH of solution can also affect the CP. For CO2-response, all of the polymers were completely soluble after pumping with CO2, and their LCST behavior recovered after N2 was pumped in, but there was a gap with the initial CP. The study of these properties reveals the influence of different heterocyclic substituents on the responsiveness of polymers, which is of great significance for understanding the stimulation response properties of polymers and provides ideas for the design of new stimuli-responsive polymers.

Polyacrylamide-Based Block Copolymer Bearing Pyridine Groups Shows Unexpected Salt-Induced LCST Behavior

Thermal-responsive block copolymers are a special type of macromolecule that exhibit a wide range of applications in various fields. In this contribution, we report a new type of polyacrylamide-based block copolymer bearing pyridine groups of polyethylene glycol-block-poly(N-(2-methylpyridine)-acrylamide; Px) that display distinct salt-induced lower critical solution temperature (LCST) behavior. Unexpectedly, the phase-transition mechanism of the salt-induced LCST behavior of Px block copolymers is different from that of the reported LCST-featured analogues. Moreover, their thermo-responsive behavior can be significantly regulated by several parameters such as salt species and concentration, urea, polymerization degree, polymer concentration and pH values. This unique thermal behavior of pyridine-containing block copolymers provides a new avenue for the fabrication of smart polymer materials with potential applications in biomedicine.

High Molecular Weight Polyproline as a Potential Biosourced Ice Growth Inhibitor: Synthesis, Ice Recrystallization Inhibition, and Specific Ice Face Binding.

Ice-binding proteins (IBPs) from extremophile organisms can modulate ice formation and growth. There are many (bio)technological applications of IBPs, from cryopreservation to mitigating freeze-thaw damage in concrete to frozen food texture modifiers. Extraction or expression of IBPs can be challenging to scale up, and hence polymeric biomimetics have emerged. It is, however, desirable to use biosourced monomers and heteroatom-containing backbones in polymers for in vivo or environmental applications to allow degradation. Here we investigate high molecular weight polyproline as an ice recrystallization inhibitor (IRI). Low molecular weight polyproline is known to be a weak IRI. Its activity is hypothesized to be due to the unique PPI helix it adopts, but it has not been thoroughly investigated. Here an open-to-air aqueous N-carboxyanhydride polymerization is employed to obtain polyproline with molecular weights of up to 50000 g mol-1. These polymers were found to have IRI activity down to 5 mg mL-1, unlike a control peptide of polysarcosine, which did not inhibit all ice growth at up to 40 mg mL-1. The polyprolines exhibited lower critical solution temperature behavior and assembly/aggregation observed at room temperature, which may contribute to its activity. Single ice crystal assays with polyproline led to faceting, consistent with specific ice-face binding. This work shows that non-vinyl-based polymers can be designed to inhibit ice recrystallization and may offer a more sustainable or environmentally acceptable, while synthetically scalable, route to large-scale applications.

Fluorescent Nanoassemblies in Water Exhibiting Tunable LCST Behavior and Responsive Light Harvesting Ability.

Responsive fluorescent nanomaterials have been received considerable attention in recent years. In this work, a bola-type amphiphilic molecule, CSO, was synthesized which contains a hydrophobic cyanostilbene core and hydrophilic oligo(ethylene glycol) (OEG) coils at both sides. The cyanostilbene group is aggregation-induced emission (AIE) active, while the OEG coils are thermo-responsive. As a result, the CSO molecules can self-assemble into blue-fluorescent nanoassemblies with lower critical solution temperature (LCST) behavior in aqueous media. It is noteworthy that the LCST behavior can be reversibly regulated with changes in concentration and the introduction of K+ . Intriguingly, fluorescence of CSO assembly shows a blue-shift upon heating. Finally, by employing CSO as a light capturing antenna and energy donor, an artificial light harvesting system with tunable emission and thermo-responsive characteristics was fabricated. This study not only demonstrates an integrated approach to create responsive fluorescent nanomaterials, but also shows great potential for producing luminescent materials and mimicking photosynthesis in nature.

PH-Responsive lower critical solution temperature behaviour of a dibenzo-24-crown-8 based low-molecular-weight gelator in water

A dibenzo-24-crown-8 based low-molecular-weight gelator can form a gel in aqueous media and show lower critical solution temperature behaviour, which can be switched off and on by adding base and acid.

Modification of Thermoresponsive Poly(<i>N</i>-Isopropylacrylamide) End-Group with Hydrophilic and/or Hydrophobic Compounds Tuned the LCST

Poly (N-isopropylacrylamide) (PNIPAAm) is one of the most well-known thermoresponsive polymers that exhibits a reversible coil-to-globule transition in aqueous solution at lower critical solution temperature (LCST) (32°C). PNIPAAm behave as an extended coil form in an aqueous solution below the LCST, meanwhile, above the LCST, it shrinks into a globule form. The LCST of PNIPAAm could be tune when it is chemically modified with hydrophilic and/or hydrophobic compound. In this study, modifications of PNIPAAm end-group with maleimide or phenyl maleimide compounds were prepared and their LCST behaviours were investigated. One end-group of synthesized poly (N-isopropylacrylamide)-chain transfer agent (PNIPAAm-CTA) was modified with maleimide or phenyl maleimide compound through aminolysis reaction to form PNIPAAm-Maleimide (PNIPAAm-M) and PNIPAAm-Phenyl maleimide (PNIPAAm-PhM). Maleimide is a hydrophilic compound, and phenyl maleimide is a slight hydrophobic compound were used in this study. The modification with hydrophilic compound will higher the LCST of PNIPAAm. The slight hydrophobic of phenyl maleimide compound will decrease the LCST. In this study, the successfulness of aminolysis process of PNIPAAm-CTA were determined through the fourier transform infrared (FTIR). Moreover, the LCST behavior of PNIPAAm-CTA, PNIPAAm-M and PNIPAAm-PhM were determined through light scattering intensity analysis. The results indicated that upon heating the solutions of PNIPAAm-CTA, PNIPAAm-M and PNIPAAm-PhM in 10 mM HEPES solution pH 7.4 at 25°C–40°C, PNIPAAm-CTA, and PNIPAAm-PhM solutions started to increase their light intensities at 35°C and PNIPAAm-M at 36°C, respectively. To conclude, modification of PNIPAAm end-group with hydrophobic and/or hydrophilic compound could tune their LCST.