Liquid–liquid equilibria of polymer solutions: Flory‐huggins with specific interaction

We have developed a new Flory‐Huggins model by adding a specific interaction parameter derived from a modified double‐lattice model for the Helmholtz energy of mixing for binary liquid mixtures. This model is very simple and could be easily integrated into engineering applications. Using this revised model, we can successfully describe the phase behavior of polymer solutions with an upper critical solution temperature (UCST), a lower critical solution temperature (LCST), both UCST and LCST, and a closed miscibility loop. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 162–167, 2010     

A new thermo‐responsive block copolymer with tunable upper critical solution temperature and lower critical solution temperature in the alcohol/water mixture

The multi‐thermo‐responsive block copolymer of poly[2‐(2‐methoxyethoxy)ethyl methacrylate]‐block‐poly[N‐(4‐vinylbenzyl)‐N,N‐diethylamine] (PMEO₂MA‐b‐PVEA) displaying phase transition at both the lower critical solution temperature (LCST) and the upper critical solution temperature (UCST) in the alcohol/water mixture is synthesized by reversible addition‐fragmentation chain transfer polymerization. The poly[2‐(2‐methoxyethoxy)ethyl methacrylate] (PMEO₂MA) block exhibits the UCST phase transition in alcohol and the LCST phase transition in water, while the poly[N‐(4‐vinylbenzyl)‐N,N‐diethylamine] (PVEA) block shows the UCST phase transition in isopropanol and the LCST phase transition in the alcohol/water mixture. Both the polymer molecular weight and the co‐solvent/nonsolvent exert great influence on the LCST or UCST of the block copolymer. By adjusting the solvent character including the water content and the temperature, the block copolymer undergoes multiphase transition at LCST or UCST, and various block copolymer morphologies including inverted micelles, core‐corona micelles, and corona‐collapsed micelles are prepared. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4399–4412     

UCST and LCST phase behavior of poly(trimethylene ether) glycol in water

We describe the initial studies of the complex aqueous phase behavior of poly(trimethylene ether) glycol (PO3G), a renewably sourced polyether glycol. Cloud point measurement revealed that a low molecular weight PO3G exhibits both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) in water in the temperature range between 30°C and 80°C. At low concentrations of PO3G, the polymer solutions exhibit LCST‐type phase behavior. In the intermediate concentration ranges, PO3G and water are immiscible. However, at higher concentrations of PO3G, the solutions show UCST‐type phase behavior. In addition, both the LCSTs and UCSTs can be easily tuned over a wide range by varying the amount of alcohol co‐solvents. These findings have potential applications in the design of personal care applications and in the development of thermosensitive “smart” materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Upper Critical Solution Temperature-Type Responsive Cyclodextrins with Characteristic Inclusion Abilities

Water-soluble and thermoresponsive macrocycles with stable inclusion toward guests are highly valuable to construct stimuli-responsive supramolecular materials for versatile applications. Here, we develop such macrocycles – ureido-substituted cyclodextrins (CDs) which exhibit unprecedented upper critical solution temperature (UCST) behavior in aqueous media. These novel CD derivatives showed good solubility in water at elevated temperature, but collapsed from water to form large coacervates upon cooling to low temperature. Their cloud points are greatly dependent on concentration and can be mediated through oxidation and chelation with silver ions. Significantly, the amphiphilicity of these CD derivatives is supportive to host-guest binding, which affords them inclusion abilities to guest dyes. The inclusion complexation remained nearly intact during thermally induced phase transitions, which is in contrast to the switchable inclusion behavior of lower critical solution temperature (LCST)-type CDs. Moreover, ureido-substituted CDs were exploited to co-encapsulate a pair of guest dyes whose fluorescence resonance energy transfer process can be switched by the UCST phase transition. We therefore believe these novel thermoresponsive CDs may form a new strategy for developing smart macrocycles and allow for exploring smart supramolecular materials.     

Polymethacrylamide—An underrated and easily accessible upper critical solution temperature polymer: Green synthesis via photoiniferter reversible addition–fragmentation chain transfer polymerization and analysis of solution behavior in water/ethanol mixtures

Within the group of stimuli-responsive, “smart” materials, upper critical solution temperature (UCST) polymers remain sparsely investigated. Thus, this work focusses on a vastly ignored UCST polymer: polymethacrylamide (PMAAm). A cost-efficient photoiniferter reversible addition–fragmentation chain transfer (RAFT) polymerization yielding narrowly dispersed (Đ < 1.1) PMAAm is presented. This PMAAm exhibits highly thermoreversible UCST-type phase transitions (PT) in water/ethanol mixtures (ethanol content: 17–35 wt%) which are investigated via temperature dependent dynamic light scattering (DLS). Apart from the ethanol content, the PT temperature is affected by polymer mass fraction and chain length and varies between 10–80 °C depending on the three mentioned parameters. Lastly, PMAAm's propensity towards amide hydrolysis and concomitant PT-suppression is investigated. Below temperatures of 40 °C, PMAAm solutions show no sign of amide hydrolysis for at least three days, however, if heated to 70 °C, the thermoresponsiveness gradually degrades within hours.     

Phase behavior in mixtures of a homopolymer and a block copolymer. 4. SALS and SAXS studies

Phase behavior of blends of poly(vinyl methyl ether) (PVME) with four styrene-butadiene-styrene (SBS) triblock copolymers, being of various molecular weights, architecture, and compositions, was investigated by small-angle light scattering. Small-angle X-ray scattering investigation was accomplished for one blend. Low critical solution temperature (LCST) and a unique phase behavior, resembling upper critical solution temperature (UCST), were observed. It was found that the architecture of the copolymer greatly influenced the phase behavior of the blends. Random phase approximation theory was used to calculate the spinodal phase transition curves of the ABA/C and BAB/C systems; LCST and resembling UCST phase behavior were observed as the parameters of the system changed. Qualitatively, the experimental and the theoretical results are consistent with each other. © 1996 John Wiley & Sons, Inc.     

Tunable UCST thermoresponsive copolymers based on natural glycyrrhetinic acid

Water-soluble thermoresponsive polymers present either upper critical solution temperature(UCST) or lower critical solution tempe rature(LCST) depending on the location of their miscibility range with water at high temperatures or at low temperatures.Compared with LCST polymers,the water-soluble UCST polymers are still less explored until now.In this work three copolymers of P(AAm-co-GAA) were synthesized by copolymerizing two acrylamide monomers,acrylamide(AAm) and acrylamide functionalized with natural glycyrrhetinic acid(GAA),using reversible addition-fragmentation chain transfer(RAFT) polymerization.These copolymers exhibited the typical UCST thermoresponsive behavior,and their phase transition temperatures could be easily tuned to around 37℃ for potential biological applications.Moreover,the UCST of P(AAm-co-GAA) can be adjusted not only by the content of glycyrrhetinic acid(GA) and polymer concentrations,but also by the host-guest interactions between GA and cyclodextrins(β-and γ-CD).The suitable value of UCST and the biocompatible nature of GA and CDs may endow these copolymers with practical applications in biomedical chemistry.     

Hydration Changes during Thermosensitive Association of a Block Copolymer Consisting of LCST and UCST Blocks

Summary: Thermosensitive association of a diblock copolymer consisting of poly(3‐dimethyl(methacryloyloxyethyl) ammonium propane sulfonate) (PdMMAEAPS), as an upper critical solution temperature (UCST) block, and poly(N,N‐diethylacrylamide) (PdEA), as a lower critical solution temperature (LCST) block, has been investigated by using IR spectroscopy. The ν(CO) and ν(SO) bands of the PdMMAEAPS block and the amide I band of PdEA block critically changed at the UCST and LCST, respectively, indicating that the segmental interaction of each block is altered at each transition.

The double temperature responsiveness of a UCST block and LCST block containing diblock copolymer. Micelles form at temperatures both below the UCST and above the LCST of the blocks.     

Liquid-liquid equilibria of polymer solutions: Applicability of extended Redlich-Kister expansion

We developed a simple and improved expression for the Helmholtz energy of mixing which uses a Taylor series of an exponential function based on extending the Redlich-Kister expansion. This model incorporates the chain-length dependence of polymers and specific interactions such as hydrogen bonds. The proposed model can accurately predict most phase diagrams of various binary polymer solutions including upper critical solution temperature (UCST), lower critical solution temperature (LSCT), both UCST and LCST, and closed miscibility loops. Our model fits experimental data of the complex phase behavior of polymer solutions well.     

Miscibility and phase behavior of water-dicarboxylic acid type ionic liquid mixed systems

Tetra-n-butylphosphonium type ionic liquids with fumarate anion and maleate anion exhibit different physico-chemical properties and different solubility to water in their cis and trans conformations; fumarate showed the usual upper critical solution temperature (UCST) behavior, whereas maleate had highly unusual lower critical solution temperature (LCST) behavior after mixing with water.     

Lower critical solution temperature type of phase separation in aqueous mixture of polyelectrolytes

Phase separation behavior in aqueous mixture of different polyelectorolytes having like charges has been investigated as functions of concentration and charge density. When the charge densities of both polyelectorolytes were equally high, the compatibility between different polyelectorolytes was relatively good and the phase separation behavior was a normal upper critical solution temperature (UCST) type. With decreasing the charge density of one polyelectorolyte keeping the charge density of another polyelectrolyte unchanged, the compatibility between different polyelectorolytes became poorer. When the charge density of one polyelectorolyte was lowered below a certain value, the phase separation behavior suddenly changed from the UCST type to a lower critical solution temperature (LCST) type.     

Phase behavior of poly(4‐tert‐butylstyrene‐stat‐4‐tert‐ butoxystyrene)/polyisoprene blends with competitive interactions

The phase behavior of statistical copolymers composed of (4‐tert‐butylstyrene) (B) and (4‐tert‐butoxystyrene) (O), abbreviated as s‐BO, with polyisoprene (I) was investigated by optical microscopic (OM) observation and small‐angle neutron scattering (SANS) measurements. It has been known that B/I blend shows lower critical solution temperature (LCST) type phase diagram, while O/I blend has upper critical solution temperature (UCST) type one. Several blends of s‐BOs having mol fraction of B, m_B, comparable to 0.50, with I showed both UCST and LCST type phase diagram. Furthermore, UCST type phase behavior was observed for blends having small m_B, while LCST type one was for that of large m_B at all used temperatures. Hence, the phase behavior of s‐BO/I blend can be understood as a result of the competition of two interactions having opposite temperature dependence. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2272–2280, 2009     

Phase Behaviour of Hydroxypropyl Cellulose/ Polyacrylamide Gels

Summary : Polysaccharide-based materials have gained nowadays a great importance in many fields of industry, e.g., in the pharmaceutical industry. Some characteristics of such compounds are their non-toxicity and the presence of a lower critical solution temperature (LCST) and/or an upper critical solution temperature (UCST). This work presents results on the synthesis and characterization of composites from hydroxypropyl cellulose (HPC) and polyacrylamide (PAAm). The ratio of HPC/PAAm was varied as well as the concentration of the crosslinker and the pH value of the reaction mixture. The compounds prepared were characterized by means of Fourier Transform Infrared spectroscopy (FTIR) and turbidity measurements. FTIR spectroscopy showed the presence of the main structural features of the precursors in the gels. The turbidity measurements of the gels in water showed the presence of a LCST in all samples. The LCST decreased when the quantity of HPC was increased in the gel and diminished also with increasing quantities of crosslinker and pH. The turbidity measurement showed the presence of an UCST when the concentration of the polymer was higher than 1% (w/w). This phenomenon is caused by the presence of the PAAm, which acted as precipitant and influenced over the entropy of the system and provoked phase separation.     

Parametric studies of interaction strengths in polymer/CO2 systems: discontinuous molecular dynamics simulations

Discontinuous molecular dynamics simulations are performed on homopolymer/solvent and surfactant/solvent systems. The homopolymer and surfactant molecules are modeled as freely jointed square-well chains. Solvent molecules are modeled as both hard spheres and square-well spheres. We explore how the various interaction parameters affect the types of phase behavior and micellization observed in the homopolymer/solvent and surfactant/solvent systems. Increasing the packing fraction of homopolymers in both hard-sphere solvents and square-well solvents increases the solvent's ability to dissolve homopolymers only when the segment-solvent interaction strength exceeds a critical value. Although only upper critical solution temperature (UCST) behavior is observed for homopolymers in hard-sphere solvents, both UCST and lower critical solution temperature (LCST) behavior are observed for homopolymers in square-well solvents, depending upon the interaction strengths and chain length. This indicates that it is necessary to account for the solvent-solvent attraction to model LCST behavior in supercritical CO2. Our simulation results on surfactants in hard-sphere solvents show that it is necessary to account for the interactions experienced by both the head and tail blocks in order to capture the essential features of surfactant/supercritical CO2 systems.     

Liquid-liquid equilibria and theta temperatures in homopolymer-solvent solutions from a perturbed hard-sphere-chain equation of state

The perturbed hard-sphere-chain (PHSC) equation of state is used to calculate liquid-liquid equilibria of binary nonpolar solvent/homopolymer systems exhibiting both an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST). Systems studied include polyisobutylene, polyethylene, and polystyrene solutions. Equation-of-state parameters of homopolymers are obtained by regressing the pressure-volume-temperature data of polymer melts. In polymer solutions, however, theory overestimates the equation-of-state effect which causes the LCST at elevated temperature. To correct the overestimated equation-of-state effect, an empirical adjustable parameter is introduced into the perturbation term of the PHSC equation of state. An entropy parameter is also introduced into the Helmholtz energy of the mixture to correlate quantitatively the dependence of critical temperatures on polymer molecular weight. For systems exhibiting a LCST, two adjustable parameters are required to obtain quantitative agreement of theoretical critical temperatures with experiment as a function of polymer molecular weight. For systems exhibiting both an UCST and a LCST, three adjustable parameters may be necessary. The need for so many empirical binary parameters is probably due to the oversimplified perturbation term which is based on the mean-field assumption. © 1996 John Wiley & Sons, Inc.     

Effect of aromatic substitution on phase behavior of blends of halogenated polystyrene and conventional polystyrene

Miscibility phase behavior in blends of poly(bromostyrene) with polystyrene (PS) has been investigated by means of time‐resolved light scattering, optical microscopy, and DSC. Cloud point phase diagrams of blends of conventional PS with poly‐(2‐bromostyrene) (P2BrS), poly‐(3‐bromostyrene), and poly‐(4‐bromostyrene) of comparable molecular weights were established by light scattering. Of particular interest is the fact that ortho, meta, and para substitutions in the styrenic aromatic rings of poly(bromostyrene) show profound effects on the composition–temperature phase diagrams of their blends with PS, exhibiting a lower critical‐solution temperature (LCST), an upper critical solution temperature (UCST), and combined LCST/UCST diagrams, respectively. Poly‐(2‐chlorostyrene) exhibits an LCST behavior very close to that of the P2BrS blend, suggesting that these types of halogen atoms may be inconsequential to phase behavior. A similar study has been extended to a PS blend containing commercial brominated PS (66 mol % bromine substitution) to determine what location of bromine substitution is crucial for miscibility enhancement in the flame‐retardant brominated PS blends. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1605–1615, 2001     

Perturbation of critical solution temperatures by impurity doping

An expression is developed for the variation of the critical solution temperature of a binary liquid system when a third component (dopant) is added, using an extension of the regular solution theory. The model can be used for UCST, LCST and for closed loop systems and has the correct features in the limiting cases.     

Mesophases in a Gel from Hydroxypropyl Cellulose/Polyacrylamide

The objective of the present work is to analyze the different mesophases observed in a gel synthesized from hydroxypropyl cellulose (HPC) and polyacrylamide (PAAm) and its phase behavior in water. Hydroxypropyl cellulose is a material derived from cellulose it is non toxic and degradable. HPC is widely used in pharmaceutics, food additives, stabilizer, thickener, etc., HPC is approved by the FDA for use in the food industry. HPC has the characteristic that it forms liquid crystals, depending on the solvent and on the concentration of the polymer. In this work a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST) were observed, the resulting gel showed anisotropic, nematic and cholesteric phases in water, the phase formed depended on the concentration of the polymer.     

Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.     

Measuring LCSTs by novel temperature gradient methods: evidence for intermolecular interactions in mixed polymer solutions

Herein we describe studies of molecular interactions in thermoresponsive polymers as they go through phase transitions in aqueous solutions. By using our recently reported linear temperature gradient setup for studying the effects of temperature on chemical processes, we demonstrate the ability to probe lower critical solution temperature (LCST) behavior with excellent precision. This method also provides a simple and convenient way to assay the LCST of solutions containing more than one polymer and follow the clouding kinetics of polymer mixtures in real time.