Upper Critical Solution Temperature Phase Research Articles

Lipidation alters the phase-separation of resilin-like polypeptides.

Biology exploits biomacromolecular phase separation to form condensates, known as membraneless organelles. Despite significant advancements in deciphering sequence determinants for phase separation, modulating these features in vivo remains challenging. A promising approach inspired by biology is to use post-translational modifications (PTMs)-to modulate the amino acid physicochemistry instead of altering protein sequences-to control the formation and characteristics of condensates. However, despite the identification of more than 300 types of PTMs, the detailed understanding of how they influence the formation and material properties of protein condensates remains incomplete. In this study, we investigated how modification with myristoyl lipid alters the formation and characteristics of the resilin-like polypeptide (RLP) condensates, a prototypical disordered protein with upper critical solution temperature (UCST) phase behaviour. Using turbidimetry, dynamic light scattering, confocal and electron microscopy, we demonstrated that lipidation-in synergy with the sequence of the lipidation site-significantly influences RLPs' thermodynamic propensity for phase separation and their condensate properties. Molecular simulations suggested these effects result from an expanded hydrophobic region created by the interaction between the lipid and lipidation site rather than changes in peptide rigidity. These findings emphasize the role of "sequence context" in modifying the properties of PTMs, suggesting that variations in lipidation sequences could be strategically used to fine-tune the effect of these motifs. Our study advances understanding of lipidation's impact on UCST phase behaviour, relevant to proteins critical in biological processes and diseases, and opens avenues for designing lipidated resilins for biomedical applications like heat-mediated drug elution.

Determination of phase diagram of polyolefin blend nanocomposites by rheological approach: Influence of rheological method and nanoparticles on shifting the phase transition boundary

AbstractIn this research, by using different rheometric methods, the temperatures of phase transition for upper critical solution temperature phase diagram of polyethylene (PE) and poly(ethylene‐co‐vinyl acetate) (EVA) blends and filled systems with organically‐modified nanoclay (ONC) were measured. Since the PE/EVA‐based mixtures are widely applied in the film and foam industries, the evaluation of the phase behavior of these mixtures is of great importance. Microscopic observations show that ONC nanoparticles have a compatibilization role on the biphasic microstructure of PE/EVA. ONC nanoparticles have a remarkable impact on the viscoelastic properties of the EVA phase, leading to a reduction in the asymmetry of the viscoelastic properties of the two polymeric phases. The findings of this work focused on the isochronal dynamic temperature sweep tests on the well‐mixed and phase‐separated nanofiller‐filled systems with different cooling/heating rates indicated that these series of experiments did not have the necessary efficiency to determine the phase separation boundary and spinodal temperatures in thermodynamically equilibrium conditions. By using Han plot and dynamic temperature sweep tests at the minimum possible heating rate, 0.25 K/min, it was proved that by adding nanoparticles to PE/EVA blends, the area of well‐mixed region of the phase diagram did not show a significant increment. The pinning effects of nanoparticles on the polymeric chains and correspondingly, the reduction of molecular dynamics mainly affect the phase separation phenomenon.

Phase behavior for the poly(2-ethyl-2-oxazoline) + supercritical DME + alcohol and carbon dioxide + 2-ethyl-2-oxazoline mixtures at high pressure

The experimental isotherms of 2-ethyl-2-oxazoline in supercritical carbon dioxide SCO2) were reported up to a pressure and temperature of 17.48 MPa and 373.2 K, respectively. The unbroken mixture critical line starting from pure SCO2 to pure 2-ethyl-2-oxazoline with a pressure maximum suggests the type I behavior in the system. The theoretical investigation is conducted with the help of Peng-Robinson equation of state (PR EOS) and satisfactory agreement is observed between predicted and experimental results. The solubility of poly(2-ethyl-2-oxazoline) in supercritical DME (dimethyl ether) is examined experimentally which demonstrates UCST (Upper Critical Solution Temperature) phase behavior for poly(2-ethyl-2-oxazoline) concentration range from 3.9 wt% to 5.0 wt%. Furthermore, the experiments are performed to determine the effect of co-solvents on the phase behavior of poly(2-ethyl-2-oxazoline) + DME + alcohol solutions. The phase shift from UCST to LCST (Lower Critical Solution Temperature) is noted for poly(2-ethyl-2-oxazoline) + DME + alcohol (methanol and ethanol) with rising alcohol concentration. Moreover, for poly(2-ethyl-2-oxazoline) + DME + alcohol (1-propanol., 1-butanol., 1-pentanol, and 1-octanol.), UCST phase behavior is observed for all concentrations of the alcohols. The drastic depression in the cloud point curve in the presence of a medium-chain alcohols (1-propanol to 1-octanol) is probably due to the association of alcohol with the polymer molecules.

Temperature-Responsive Deep Eutectic Solvents for Highly Selective Separation of 5-Hydroxymethylfurfural from Reaction Mixture

5-Hydroxymethylfurfural (HMF) is a key biobased platform compound for the production of high-value chemicals and fuels. However, separation and purification of HMF is currently the bottleneck for HMF production. In this work, a novel class of temperature-responsive deep eutectic solvent (DES)–water systems have been explored to selectively separate HMF from reaction mixtures. The fructose dehydration reaction was conducted in homogeneous DES–H2O systems, and then, the produced HMF was migrated to the DES-rich phase while the unreacted fructose was retained in the H2O-rich phase upon cooling. Thus, selective separation of HMF was realized. It was found that water–water and DES–DES interactions predominate over DES–water interactions during cooling, thus resulting in upper critical solution temperature phase separation. Under optimal conditions, the partition coefficient and separation factor of HMF could reach up to 78 and 609, respectively, which is about 5 and 13 times those reported in ionic liquid-based aqueous biphasic systems. Furthermore, HMF could be removed from the DES-rich phase, giving a high recovery efficiency of 69%, which is much higher than the best value reported in the literature. At the same time, DES was regenerated and reused in the next process, and no obvious decrease in partition coefficient of HMF was observed after six cycles.

Synthesis of a dual UCST-type thermosensitive and acid-degradable nanogel based on poly(N-acryloyl glycinamide) and a ketal-containing crosslinker

In the present study, we designed a dual upper critical solution temperature (UCST) type thermosensitive and acid-degradable nanogel via UV-light initiated RAFT-mediated polymerization-induced thermal self-assembly (PITSA). The dual stimuli-triggered nanogel was prepared with N-acryloyl glycinamide as the precursor monomer of a hydrogen-bonding UCST-type thermosensitive polymer, a new pH-sensitive ketal-containing crosslinker, and poly(poly(ethylene glycol) methyl ether acrylate) acting as both a macromolecular chain transfer agent and a surfactant, called macro-transurf. The resulting spherical nanogel exhibits dual thermo- and pH-sensitive properties, as determined by UV–Vis spectrophotometry and/or dynamic light scatterring measurements at different values of pH (4, 6 and 9) and temperature (5 and 55 °C). The nanogel displays a UCST phase transition at 41 °C in buffer solution (pH = 9) and is degraded rapidly (25 and 45 min) in acidic environments (pH 4 and pH 6).

Preparation of a thermo-responsive drug carrier consisting of a biocompatible triblock copolymer and fullerene.

A triblock copolymer (PEG-b-PUEM-b-PMPC; EUM) comprising poly(ethylene glycol) (PEG), thermo-responsive poly(2-ureidoethyl methacrylate) (PUEM), and poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) blocks was synthesized via controlled radical polymerization. PEG and PMPC blocks exhibit hydrophilicity and biocompatibility. The PUEM block exhibits an upper critical solution temperature (UCST). PMPC can dissolve hydrophobic fullerenes in water to form a complex by grinding PMPC and fullerene powders. Fullerene-C70 (C70) and EUM were ground in a mortar and phosphate-buffered saline (PBS) was added to synthesize a water-soluble complex (C70/EUM). C70/EUM has a core-shell-corona structure, whose core is a complex of C70 and PMPC, the shell is PUEM, and corona is PEG. The maximum C70 concentration dissolved in PBS was 0.313 g L-1 at an EUM concentration of 2 g L-1. The C70/EUM hydrodynamic radius (Rh) was 34 nm in PBS at 10 °C, which increased due to the PUEM block's UCST phase transition with increasing temperature, and Rh attained a constant value of 38 nm above 36 °C. An anticancer drug, doxorubicin, was encapsulated in the PUEM shell by hydrophobic interactions in C70/EUM at room temperature, which can be released by heating. The generation of singlet oxygen (1O2) from C70/EUM upon visible-light irradiation was confirmed using the singlet oxygen sensor green indicator. Water-soluble C70/EUM may be used as a carrier that releases encapsulated drugs when heated and as a photosensitizer for photodynamic therapy.

UCST-Type Copolymer through the Combination of Water-Soluble Polyacrylamide and Polycaprolactone-Like Polyester.

Copolymers of acrylamide (AAm) and hydrophobic monomers are known as non-ionic polymers to show an upper critical solution temperature (UCST) in aqueous solution. By utilizing cyclic ketene acetals and carrying out the radical ring opening polymerization along with copolymerization with AAm, a UCST polymer, poly(acrylamide-co-2-methylene-1,3-dioxepane) (P(AAm-co-MDO)), is obtained. The AAm/MDO content results in the shift of UCST phase transition temperature. The present work shows how the combination of water-soluble PAAm and polycaprolactone-like structured PMDO develops a new type of thermoresponsive polymer which can be expected for bio-related applications.

Interplay between Glass Formation and Liquid-Liquid Phase Separation Revealed by the Scattering Invariant.

The interplay of the glass transition with liquid-liquid phase separation (LLPS) is a subject of intense debate. We use the scattering invariant Q to probe how approaching the glass transition affects the shape of LLPS boundaries in the temperature/volume fraction plane. Two protein systems featuring kinetic arrest with a lower and an upper critical solution temperature phase behavior, respectively, are studied varying the quench depth. Using Q we noninvasively identify system-dependent differences for the effect of glass formation on the LLPS boundary. The glassy dense phase appears to enter the coexistence region for the albumin-YCl3 system, whereas it follows the equilibrium binodal for the γ-globulin-PEG system.

Unification of lower and upper critical solution temperature phase behavior of globular protein solutions in the presence of multivalent cations.

In globular protein systems, upper critical solution temperature (UCST) behavior is common, but lower critical solution temperature (LCST) phase transitions are rare. In addition, the temperature sensitivity of such systems is usually difficult to tune. Here we demonstrate that the charge state of globular proteins in aqueous solutions can alter their temperature-dependent phase behavior. We show a universal way to tune the effective protein interactions and induce both UCST and LCST-type transitions in the system using trivalent salts. We provide a phase diagram identifying LCST and UCST regimes as a function of protein and salt concentrations. We further propose a model based on an entropy-driven cation binding mechanism to explain the experimental observations.

Effect of Temperature on the Cononsolvency of Poly(N-isopropylacrylamide) (PNIPAM) in Aqueous 1-Propanol

The wide range of applications of poly(N-isopropylacrylamide) (PNIPAM) is based on the temperature dependence of its coil-to-globule transition, which strongly relies on the solvent. Here, we focus on the cononsolvency effect of PNIPAM oligomers in aqueous 1-propanol mixtures that is studied by molecular dynamics simulations of single chains and membrane-like arrangements. The complete phase diagram is sketched from the radius of gyration of the simulated oligomers, and it is compared to that obtained from the hydrodynamic radius of PNIPAM microgels, finding a good agreement. At the water-rich region, the decrease of the lower critical solution temperature (LCST) with increasing cosolvent concentration is independent of the polymer length and concentration. In this region, the radius of gyration of our simulated oligomer is strongly temperature dependent and a coil–globule transition temperature is easily captured. Conversely, at the alcohol-rich region, simulations show a monotonically increasing radius of gyration of the oligomer with alcohol concentration, a radius that is practically independent of temperature. This finding is in line with a polymer phase separation, showing an upper critical solution temperature (UCST) for a small cosolvent concentration window, which depends on the polymer length and concentration. Hence, in this case, polymer–polymer effective interactions are key to phase separation instead of its single chain conformation, contrasting with the coil–globule transition. Indeed, we find a soft-coil like structure for the simulated oligomer around a propanol molar fraction of 0.24, which is close to the mixture composition where the UCST phase transition is detected. Finally, in line with an UCST scenario, our simulated membrane turns unstable at high temperature and alcohol concentration.

Synthesis and Properties of Upper Critical Solution Temperature Responsive Nanogels.

A random copolymer ((U/A10)165) bearing pendent ureido groups and a small amount (10 mol %) of primary amino groups exhibits an upper critical solution temperature (UCST). We prepared a diblock copolymer (PMPC20P(U/A10)165) composed of water-soluble poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and (U/A10)165 blocks via reversible addition-fragmentation chain-transfer radical polymerization with postmodification reaction. The subnumbers are the degrees of polymerization of each block. Although in water PMPC20P(U/A10)165 dissolves as a unimer above the UCST phase transition temperature ( Tp), it forms polymer micelles composed of dehydrated (U/A10)165 cores and hydrophilic PMPC shells. A nanogel was prepared by cross-linking the pendent primary amines in the micelle core using (hydroxymethyl)phosphonium chloride below Tp. NMR and light-scattering data indicated that the nanogel core shrinks upon dehydration below Tp and swells upon hydration above Tp. The nanogel can encapsulate guest molecules such as hydrophobic fluorescence probes and bovine serum albumin (BSA) below Tp mainly owing to hydrophobic interactions in the core. Encapsulated BSA can be held in the nanogel core below Tp and subsequently released above Tp.

Rheologically determined phase diagram of poly(ε‐caprolactone)/poly(styrene‐co‐acrylonitrile) blends: Role of ramp rate in dynamic measurements

ABSTRACTThe phase behavior of poly(ε‐caprolactone) (PCL)/poly(styrene‐co‐acrylonitrile) (SAN) blends, with a lower critical solution temperature (LCST)‐type phase diagram over a virtual upper critical solution temperature (UCST) one, was investigated through thermal analysis and dynamic rheological measurements as a function of ramp rate. The LCST phase diagram was detected rheologically from the observed slope changes in the dynamic temperature ramps of storage modulus (G′). The determined phase transition points along with the spinodal temperatures, which are estimated based on the theoretical approach of Ajji and Choplin's mean field theory, shift to higher temperatures by reducing the ramp rate. The detected shifts show a composition dependency. Far away from the critical point, the phase transition temperatures of PCL/SAN blends change more noticeably, which originates from the smaller magnitude of concentration fluctuations in the metastable region and the stronger competition between the phase dissolution and cooling rate. The Flory–Huggins interaction parameter (χ) was appropriately adjusted into the LCST phase boundary as a function of temperature and composition. The results confirmed that the adjusted χ has higher temperature dependency at larger ramp rate of the dynamic measurements. The experimentally combined LCST and UCST phase behavior were also verified by the compressible regular solution model. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47750.

Biodegradable zwitterionic nanoparticles with tunable UCST-type phase separation under physiological conditions.

Thermo-responsive polymeric nanoparticles (NPs) are emerging as a powerful tool in nanomedicine for the fabrication of advanced drug delivery systems. In addition to their size and biodegradation rate, phase separation of NPs upon application of a thermal stimulus provides an additional switch to control the rate of release of active components. Among the materials currently developed for biomedical applications, NPs stabilized by zwitterionic polymers are gaining increasing interest due to their high stability and ability to escape the body immune response. Yet, biodegradable zwitterionic NPs with temperature response under physiological conditions are currently not available. Here, we develop a new class of biodegradable zwitterionic NPs that exhibit UCST phase transition in the biological temperature range (T = 30-45 °C) and in physiological solution (i.e. 0.9% w/w NaCl). We design a strategy that relies on the self-assembly of block copolymers produced via reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization. These copolymers comprise a zwitterionic portion exhibiting an upper critical solution temperature (UCST) and a biodegradable hydrophobic block consisting of oligoesters functionalized with a vinyl group. This modular macromolecular architecture allows us to independently control a variety of NP properties by modifying the individual components of the copolymer. In particular, the zwitterionic block of the copolymers controls the UCST-type phase separation behavior, while the number of the oligoester repeating units governs the size of the NPs and the length of the oligoester dictates the degradation rate. After demonstrating the synthesis of highly controlled degradable NPs, we show the potential of this new class of materials in the context of drug delivery by controlling the release of a drug-mimic molecule upon temperature variations in a broad time range from few minutes to 20 hours.

Highly Ordered Polypeptide with UCST Phase Separation Behavior.

Manipulating phase separation structures of thermoresponsive polymers will enhance the usefulness of structure-controllable materials in fields such as drug delivery and tissue engineering. However, behaviors of upper critical solution temperature (UCST) have been less investigated so far, despite the importance of UCST. Here, we examined two citrulline-based polypeptides, poly(d-ornithine- co-d-citrulline) (PdOC) and poly(dl-ornithine- co-dl-citrulline) (PdlOC), to investigate how stereoregularity of the polypeptides influences UCST behavior, in addition to poly(l-ornithine- co-l-citrulline) (PlOC) previously studied. Homochiral PlOC and PdOC showed phase separation temperatures ( Tps) higher than that of racemic PdlOC. Moreover, PdlOC underwent liquid to coacervate phase separation at Tp, whereas PlOC and PdOC underwent liquid to solid-like aggregation transitions. From a structural point of view, circular dichroism and small-angle X-ray scattering measurements revealed that homochiral PlOC and PdOC polypeptides formed α-helical structures and assembled into a regular hexagonal lattice upon phase separation. Interactions between the pendent ureido groups of homochiral POCs appear to play pivotal roles in helical folding and assembly into the hexagonal structure. In addition, Tp change in response to biodegradation was confirmed for both PlOC and PdlOC. The biodegradability was considerably influenced by phase-separated structures. These findings of UCST-type POCs in this study would provide important insights into structure-controllable and thermoresponsive biomaterials.

Preparation of Highly Porous Polymer Membranes with Hierarchical Porous Structures via Spinodal Decomposition of Mixed Solvents with UCST Phase Behavior.

The predominant method to prepare polymer membranes is based on phase inversion. However, this method always leads to a dense skin with low porosity when normal polymers are used. Using the self-assembly of certain block copolymers, it is possible to prepare uniform pores with high porosity, but the prices of these polymers are too high to be afforded in practical applications. Here, we report a novel strategy to prepare highly porous and asymmetric polymer membranes using the widely used poly(vinylidene fluoride) (PVDF) as a prototype. The method combines spinodal decomposition with phase inversion utilizing mixed solvents that have the unique upper critical solution temperature phase behavior. The spinodal decomposition generates a thin surface layer containing a high density of relatively uniform pores in the mesoporous range, and the phase inversion generates a thick bulk layer composed of macrovoids; the two types of structures are interconnected, yielding a highly permeable, selective, and mechanically strong porous membrane. The membranes show an order of magnitude higher water permeance than commercial membranes and efficient molecular sieving of macromolecules. Notably, our strategy provides a general toolbox to prepare highly porous membranes from normal polymers. By blending PVDF with cellulose acetate (CA), a highly porous PVDF/CA membrane was prepared and showed similarly high separation performance, but the higher hydrophilicity of CA improved the membrane flux in the presence of proteins.

Rational Design of UCST-type Ureido Copolymers Based on a Hydrophobic Parameter.

Thermoresponsive polymers exhibiting upper critical solution temperature (UCST)-type behavior under physiologically relevant conditions have potential as biomaterials. The phase separation temperature ( Tp) of the UCST-type polymers can be increased by copolymerization with hydrophobic comonomers. Quantitative index parameters that could be used to rationally tune the Tp are lacking, however. We have reported that ureido copolymers such as poly(allylamine- co-allylurea) (AM-PU) and poly(l-ornithine- co-citrulline) exhibit UCST-type solution behavior under physiologically relevant conditions. In this study, AM-PU was hydrophobized by acylation. Tp of AM-PU can be regulated in a wide temperature range from about 20 to 80 °C or even higher by 20 mol % acylation with acetyl, propionyl, isobutanoyl, or pivaloyl groups, implying considerable impacts of hydrophobic groups on UCST phase separation. We observed a liner relationship between Tp and the hydrophobic parameter, log P, of the acyl groups. Furthermore, the acylation significantly reduced the influence of serum components on Tp by eliminating interactions of copolymers with serum components such as proteins and lipids. Acylation also abolished pH dependence of Tp which had been observed for unmodified AM-PU. Owing to the simple relationship between log P and Tp and the inertness of the acylated copolymer to serum components and pH changes, it is possible to rationally design copolymers exhibiting UCST-type behavior at a desired temperature under biological conditions.

Hyperbranched Multiarm Copolymers with a UCST Phase Transition: Topological Effect and the Mechanism.

A novel thermoresponsive hyperbranched multiarm copolymer with a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] core and many poly(acrylamide- co-acrylonitrile) (P(AAm- co-AN)) arms was for the first time synthesized through a reversible addition-fragmentation chain-transfer polymerization. These copolymers show reversible, sharp, and controlled temperature-responsive phase transitions at the upper critical solution temperature (UCST) in water and electrolyte solution. It is the first report on the hyperbranched copolymers with a UCST transition. Two series copolymers with variable AN content (series A) and variable arm length (series B) were synthesized to study the influence of molecular structure on the UCST transition. It was found that the UCST of copolymers could be raised by increasing the AN content or decreasing the arm length. Most interestingly, the amplification effect of the hyperbranched topological structure leads to a broad change of the UCST from 33.2 to 65.2 °C with the little change of AN content (5.9%). On the basis of variable temperature nuclear magnetic resonance, dynamic light scattering, and transmission electron microscopy, a UCST transition mechanism, in combination with hydrophilic/hydrophobic balance and multimicelle aggregate (MMA), was proposed. This work enriches the UCST copolymer topology and may extend the knowledge on the structure-activity relationship as well as the mechanism of the UCST polymers.

Micellar Self-Assembly of Recombinant Resilin-/Elastin-Like Block Copolypeptides.

Reported here is the synthesis of perfectly sequence defined, monodisperse diblock copolypeptides of hydrophilic elastin-like and hydrophobic resilin-like polypeptide blocks and characterization of their self-assembly as a function of structural parameters by light scattering, cryo-TEM, and small-angle neutron scattering. A subset of these diblock copolypeptides exhibit lower critical solution temperature and upper critical solution temperature phase behavior and self-assemble into spherical or cylindrical micelles. Their morphologies are dictated by their chain length, degree of hydrophilicity, and hydrophilic weight fraction of the ELP block. We find that (1) independent of the length of the corona-forming ELP block there is a minimum threshold in the length of the RLP block below which self-assembly does not occur, but that once that threshold is crossed, (2) the RLP block length is a unique molecular parameter to independently tune self-assembly and (3) increasing the hydrophobicity of the corona-forming ELP drives a transition from spherical to cylindrical morphology. Unlike the self-assembly of purely ELP-based block copolymers, the self-assembly of RLP-ELPs can be understood by simple principles of polymer physics relating hydrophilic weight fraction and polymer-polymer and polymer-solvent interactions to micellar morphology, which is important as it provides a route for the de novo design of desired nanoscale morphologies from first principles.

Hydrogen bonding induced UCST phase transition of poly(ionic liquid)-based nanogels

In this study, novel thermo-responsive nanogels were facilely fabricated via ternary cross-linking copolymerization of ionic liquid (IL)-based monomers and cross-linker in methanol. To achieve the thermo-responsive performance of nanogels, monomers with hydroxyl or carboxyl group which can contribute to hydrogen (H)-bonding interaction is the key point of this strategy. The thermo-responsive behavior of poly(ionic liquid)s (PIL)-based nanogels were testified using dynamic light scattering (DLS), turbidimetric method, and differential scanning calorimetry (DSC) measurements. The results demonstrated that these novel nanogels presented reversible upper critical solution temperature (UCST) phase transition with the temperature changing. And their UCSTs could be tuned via the feed ratio of IL-based monomers and cross-linker. The nanogels were also measured using scanning electronic microscopy (SEM), thermo-gravimetric analyses (TGA) and Fourier transform infrared spectroscopy (FT-IR) techniques. In addition, a possible mechanism of thermo-responsive performance was suggested based on the above results. As a result, our findings provide an efficient and applicable platform for the fabrication of thermo-responsive polymeric nanomaterials.

Upper critical solution temperature (UCST) phase transition of halide salts of branched polyethylenimine and methylated branched polyethylenimine in aqueous solutions.

The upper critical solution temperature (UCST) phase transition of halide salts of branched polyethylenimine (PEI) and methylated branched polyethylenimine (MPEI) is first reported in aqueous solutions. In particular, iodide counter-ions can introduce UCST properties in MPEI. The importance of the counter-ion composition of MPEI for UCST transition is discussed in detail.