Lower Critical Solution Temperature Phenomenon Research Articles

Molecular Dynamics Calculations for the Temperature Response of Poly(alkylated tri(ethylene oxide)isocyanate) Aqueous Solution

Aqueous solutions of conventional temperature-responsive amphiphilic polymers undergo a coil–globule conformational transition around the lower critical solution temperature (LCST) that causes the polymer surfaces to become hydrophobic and the polymers to aggregate together. Isocyanate polymers with alkylated oligo(ethylene oxide) side chains are expected to have rigid main chains and, thus, do not undergo the coil–globule structural transition, but they have recently been reported to exhibit temperature-responsive properties. In this study, molecular dynamics was used to calculate the agglomeration tendencies of two chains of poly(alkylated tri(ethylene oxide)isocyanate) (PRTEOIC, where R = methyl (Me) or ethyl (Et)) in aqueous solution to elucidate the LCST phenomenon in the absence of coil–globule conformational transition. Our MD simulations showed that aggregation also occurs in rod polymers. Furthermore, we found that both (PMeTEOIC)2 and (PEtTEOIC)2 showed parallel agglomeration of the two molecular chains with increasing temperature, but only (PMeTEOIC)2 showed a metastable T-shaped agglomeration in the middle temperature range. The crossing-point temperature (TCRP) at which the density of the first hydrophobic hydration shell around the sidechain alkyl group equals the bulk water density is a useful indicator for predicting the LCST of rod polymers with dense side chains terminated by alkyl groups.

Controlling the Chain Folding for the Synthesis of Single-Chain Polymer Nanoparticles Using Thermoresponsive Polymers

Synthetic polymer single-chain nanoparticles (SCNPs) are an emerging new class of nanomaterials that possess similar folded structures as natural proteins. However, most SCNPs reported so far are p...

Lower critical solution temperature (LCST) phase behaviour of an ionic liquid and its control by supramolecular host-guest interactions.

Lower critical solution temperature (LCST) phase behaviour of an imidazolium-based ionic liquid is reported, which can be controlled by concentration, the choice of cation, anion and solvent, and by supramolecular host-guest complex formation. Molecular dynamics simulations provide insight into the molecular basis of this LCST phenomenon. This thermo-responsive system has potential applications in cloud point extraction processes.

Patchy supramolecules as versatile tools to probe hydrophobicity in nanoglobular systems.

We describe precise supramolecules that enable the evaluation of the effective hydrophobicity of amphiphilic or "patchy" nanoglobular systems. These supramolecules exhibit the lower critical solution temperature phenomenon, which provides a quantitative measure of their effective hydrophobicity. Specifically, two isomeric 8-aryl-2'-deoxyguanosine derivatives with a transposed pair of methylene groups self-assemble into hexadecameric nanoglobular supramolecular G-quadruplexes (SGQs) that show large differences in their transition temperatures as determined by turbidity and differential scanning calorimetry studies. Molecular modeling studies suggested that differential clustering of the hydrophobic patches on the surface is responsible for the striking differences between the two isomeric supramolecules.

In situ observations of liquid–liquid phase separation in aqueous MgSO4 solutions: Geological and geochemical implications

A previously unknown liquid–liquid phase separation in vapor-saturated aqueous MgSO4 solutions containing 1.19–19.36mass% of MgSO4 was observed in fused silica capillary capsules during heating at temperatures above 259°C. Under these conditions, we observed that MgSO4-rich droplets were separated from the original aqueous MgSO4 solutions during heating, and these two coexisting liquid phases homogenized during cooling. The newly discovered liquid–liquid phase separation in MgSO4 solutions was characterized by a lower critical solution temperature phenomenon, which was considered to be a macro-scale chemical property of polymeric mixtures. In situ Raman spectroscopic investigations identified a distinctly new ν1(SO42-) mode at ∼1020cm−1 in the MgSO4-rich droplets; the new ν1(SO42-) mode was predicted to be present in MgSO4 polymer(s) in aqueous solutions. As mentioned above, both the phase behavior and relevant Raman spectra indicate the existence of polymer(s) in MgSO4 solutions. The recognition of the liquid–liquid phase separation and polymerization of MgSO4 in aqueous MgSO4 solutions is important for the experimental investigation of thermochemical sulfate reduction (TSR), because (1) the emergence of the MgSO4-rich droplets will substantially increase the local MgSO4 concentration, which is not representative of the geologic environments where TSR occurs; and (2) the formation of various ion pairs and MgSO4 polymers makes the mechanism of TSR far more complex than that occurring at relatively low temperatures (i.e., <200°C).

Designing Lower Critical Solution Temperature Behavior into a Discotic Small Molecule

Design and analysis of amphiphilic small molecules exhibiting lower critical solution temperature (LCST) behavior is reported. 2,3,6,7,10,11-Hexakis[2-(N,N-dialkylamino)ethoxy] triphenylenes containing hydrophilic groups attached at their discotic core were prepared, and the LCST behavior of their solutions was studied using fluorescence spectrophotometry and 1H NMR spectroscopy. 1H NMR spin−lattice relaxation times were used to assess the rotational mobility of molecules below and above the clouding point. The impact on the LCST of supramolecular π−π stacking forces introduced by the triphenylene (TP) core was studied. The operation of the LCST phenomenon was found not to depend significantly on stacking of TP moieties. This process in small molecular species offers several advantages over the polymer-originated phenomenon. For instance, enabling an analogue of the LCST transition in dye molecules might allow the design of novel optical devices by permitting previously unavailable specific aggregated states.

Glucose permeable poly (dimethyl siloxane) poly ( N-isopropyl acrylamide) interpenetrating networks as ophthalmic biomaterials

Poly (dimethyl siloxane) (PDMS) has been widely used as a biomaterial in ophthalmic and other applications due to its good compatibility, high mechanical strength, excellent oxygen permeability and transparency. However, for use as an artificial cornea, contact lens and in other applications, modifications with hydrophilic functional groups or polymers are necessary to improve wettability for tear protein and mucin interactions and to improve glucose permeability for cellular health. Poly ( N-isopropyl acrylamide) (PNIPAAM) is a biocompatible and hydrophilic polymer that has been extensively studied on controlled drug release applications due to its lower critical solution temperature (LCST) phenomenon. In the current work, a composite interpenetrating network (IPN) of PDMS and PNIPAAM was formed to generate polymers with oxygen and glucose permeability as well as improved wettability compared to PDMS homopolymers and greater mechanical strength than PNIPAAM homopolymers. Transparent vinyl and hydroxyl terminated PDMS/PNIPAAM IPNs (PDMS-V and PDMS-OH IPNs, respectively) were successfully synthesized. Transmission electron microscopy images verified the structure of the IPNs. Surface analysis suggested that PNIPAAM was present on the surface as well as in the bulk material. PDMS-OH IPNs generated from a PDMS-OH matrix cured in the presence of solvent had the highest glucose permeability at 10 −7 cm 2/s, comparable to that of the native cornea. The LCST phenomenon remained in these materials, although changes were not as abrupt as with pure PNIPAAM. These results suggest that these materials may be further developed as ophthalmic biomaterials or for controlled drug-release applications.

Reversibility of thermodynamic lower critical solution temperature phenomenon in miscible poly(ε-caprolactone)/poly(benzyl methacrylate)

Differential scanning calorimetry and optical microscopy were performed to examine the reversibility of phase separation at above the lower critical solution temperatures in a miscible poly(e-caprolactone) (PCL)/poly(benzyl methacrylate) (PBzMA) blend system. Upon heating, phase separation occurred via a binodal nucleation and growth (NG) mechanism at above 240 °C, which is a lower critical solution temperature (LCST). The pattern of phase domains suggests that the phase separation was meta-stable. Interestingly, the LCST phase separation was found to be readily reversible to original homogeneity upon cooling at regularly accessible rates. A major factor may be that the temperature window between the LCST curve and blend Tg curve is wide, resulting in a convenient temperature range for the polymer chains to kinetically reorganize to a state favored by the thermodynamic conditions.

Thermodynamic Behavior of Polyelectrolytes with the Lower Critical Solution Temperature (LCST) Phenomenon

A series of vinyl polymers with L-valine and L-leucine residues, and related copolymers with N-isopropylacrylamide, were studied in aqueous solution at different temperatures (25, 30 and 35°C) and at two ionic strengths (0.01 M and 0.1 M NaCl). The protonation behavior revealed great differences between the polymers that were attributed to the size of the hydrophobic lateral group. Macromolecular shrinkage, occurring above a critical degree of protonation β, was related to hydrophobic forces outweighing the electrostatic repulsions between COO – groups. Low salt concentrations increased the electrostatic potential while high temperatures increased the hydrophobic interaction at lower β. The release of fewer water molecules structured around the polymer chain, responsible for the lower critical solution temperature phenomenon, revealed lower entropy changes at higher temperatures. The reversible configuration of graft polymer chains instantly responded to changes in pH and temperature, modifying the water filtration rates through the pores of cellulose membranes.

Thermodynamic Behavior of Polyelectrolytes with the Lower Critical Solution Temperature (LCST) Phenomenon

A series of vinyl polymers with L-valine and L-leucine residues, and related copolymers with N-isopropylacrylamide, were studied in aqueous solution at different temperatures (25, 30 and 35°C) and at two ionic strengths (0.01 M and 0.1 M NaCl). The protonation behavior revealed great differences between the polymers that were attributed to the size of the hydrophobic lateral group. Macromolecular shrinkage, occurring above a critical degree of protonation β, was related to hydrophobic forces outweighing the electrostatic repulsions between COO – groups. Low salt concentrations increased the electrostatic potential while high temperatures increased the hydrophobic interaction at lower β. The release of fewer water molecules structured around the polymer chain, responsible for the lower critical solution temperature phenomenon, revealed lower entropy changes at higher temperatures. The reversible configuration of graft polymer chains instantly responded to changes in pH and temperature, modifying the water filtration rates through the pores of cellulose membranes.

Vinyl Polymers Containing L-Valine and L-Leucine Residues: Thermodynamic Behavior of Homopolymers and Copolymers with N-Isopropylacrylamide

Vinyl polymers carrying carboxyl groups and the groups (amido and isopropyl) present in the well-known thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) were studied for protonation reaction thermodynamics at different temperatures (25, 30, and 35 °C). The study was performed in aqueous solution (0.1 M NaCl) to elucidate the mutual influence of temperatures and pH on the mechanism responsible for phase separation of polymers having a lower critical solution temperature (LCST). The thermodynamic data (viscometry, basicity constants, enthalpy changes) for the protonation of carboxylate groups in homopolymers and copolymers with N-isopropylacrylamide (NIPAAm) showed that subtle conformational changes occurred at a critical degree of protonation (α). Beyond this critical α value, a larger endothermic effect was superimposed on that of the protonation of the COO - group. The phenomenon was ascribed to hydrophobic forces between isopropyl groups outweighing the repulsive electrostatic interactions of the polymer in the ionized, unfolded state. The enthalpy changes (-ΔH°) became larger as the NIPAAm content increased in the copolymers, and at higher temperatures the magnitude of this change dropped sharply. The critical α shifted to a lower value because higher temperatures enhanced hydrophobic interactions. The reduced amount of structured water molecules on the polymer, responsible for the LCST phenomenon, was revealed by the lower entropy change (AS°), that confirmed that the process is entropy-driven and based on a critical balance between hydrophobic and electrostatic forces.

Functional metallomacrocycles and their polymers. Part 31. Autoxidation of thiol by temperature-sensitive polymer catalyst containing cobalt(II) phthalocyanine complex

Copolymers (I-IV) of 2-acryloylamino-9,16,23-tri( t-butyl)phthalocyanine (APc( t-Bu) 3) and acrylamide derivatives were prepared and their catalytic activity for oxidation of 2-mercaptoethanol was examined. Copolymers (II and IV) of APc( t-Bu) 3 and N-isopropylacrylamide revealed a lower critical solution temperature phenomenon at 29°C in carbonate buffer, that is to say, the copolymers dissolving in buffer solution below 29°C separated out as precipitate above 29°C. The catalytic activity of polymer IV, which was prepared by metalation of copolymer II, markedly changed around 29°C. Copolymer IV served as a homogeneous catalyst of autoxidation of 2-mercaptoethanol below 29°C. In contrast, the catalytic activity drastically decreased above 29°C because of separation as precipitate.