Unified model of association-induced lower critical solution temperature phase separation and its application to solutions of telechelic poly(ethylene oxide) and of telechelic poly(N-isopropylacrylamide) in water

The authors present a model describing the coexistence of hydrophobic association and phase separation with lower critical solution temperature (LCST) in aqueous solutions of polymers carrying short hydrophobic chains at both chain ends (telechelic associating polymers). The LCST of these solutions is found to decrease along the sol/gel transition curve as a result of both end-chain association (association-induced phase separation) and direct hydrophobic interaction of the end chains with water. The authors relate the magnitude of the LCST decrease to a hydration cooperativity parameter sigma. The LCST decreases substantially (approximately 100 K) in the case of random hydration (sigma=1), whereas only a small shift (approximately 5-10 K) occurs in the case of cooperative hydration (sigma=0.3). The molecular weight dependence of the LCST drop is studied in detail in each case. The results are compared with experimental observations of the cloud points of telechelic poly(ethylene oxide) solutions, in which random hydration predominates, and of telechelic poly(N-isopropylacrylamide) solutions, in which cooperative hydration prevails.     

Micelle-mediated extraction

The extraction technique based on phase separation in aqueous micellar solutions is reviewed. The technique has now been utilized for separation and preconcentration of metal chelates, organic compounds, and proteins. Additionally, the phase behavior of the micellar solutions and recent advances in the phase separation technique are also described. In the extraction of metal chelates, distribution equilibria are considered. In contrast to conventional solvent extraction, the distribution of metal chelates into a condensed surfactant phase (surfactant-rich phase) was dependent on metal ions. Proteins were extractable into the surfactant-rich phase according to their hydrophobicity. The recent use of affinity ligands and water-soluble polymers for controlling extractability of proteins are also introduced.     

Thermogelation of amphiphilic poly(asparagine) derivatives

Thermoresponsive sol–gel transition polymers based on biodegradable poly(amino acid) were synthesized by the reaction of poly(succinimide) with dodecylamine and amino alcohols. The introduction of the hydrophobic amine into the thermoresponsive poly(amino acid)s induced the sol–gel transition in phosphate buffer saline. The effects of the side chain structure, molecular weight, concentration of the polymer, and the additives (inorganic salts and urea) in the solution on the thermoresponsive behaviors were systematically investigated. A relationship between the lowest critical solution temperature (LCST) in the dilute solution and the viscosity reduction of the concentrated solution upon heating was observed. The present poly(amino acid)s showing a thermoresponsive sol–gel transition in aqueous solutions possess immense potential as an injectable biodegradable hydrogel system for various biomedical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Unusual phase behavior in mixtures of poly(ethylene oxide) and ethyl alcohol

Poly(ethylene oxide) (PEO), soluble in both aqueous and organic solvents, is one of the most intriguing polymers. PEO solution properties have been extensively studied for decades; however, many of the studies have focused on specific properties, such as clustering, of PEO in aqueous solutions, and the behavior of PEO in organic solvents has not been adequately explored. The results presented here demonstrate that PEO crystallizes into a lamellar structure in ethyl alcohol after the mixture is quenched to room temperature from a temperature above the crystal melting point. Above the melting temperature, PEO completely dissolves in ethyl alcohol, and the mixture exhibits regular polymer solution thermodynamic behavior with an upper critical solution temperature (UCST) phase diagram. Remarkably, the UCST phase boundary is significantly below the melting temperature, and this indicates that the system undergoes a crystallization process before the phase separation can occur upon cooling and, therefore, possesses an unusual phase transition. The phase transition from the crystalline state to the miscible solution state is reversible upon heating or cooling and can be induced by the addition of a small amount of water. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 557–564, 2006     

聚N-异丙基丙烯酰胺在十二烷基硫酸钠水溶液中的线团球体转变

高分子链在溶液中,当溶剂由良变劣时,将发生线团-球体转变,而同时由于链间聚集造成的相分离,使得很难真正观察到热力学意义上稳定的高分子单链紧缩球体。表面活性剂的加入有助于阻止高分子链间发生聚集,在不同的外部条件下,表面活性剂同高分子链将形成形态迥异且又复杂有趣的复合结构。本文主要讨论了聚N-异丙基丙烯酰胺在十二烷基硫酸钠水溶液体系中的线团-球体转变行为,以及聚N-异丙基丙烯酰胺与十二烷基硫酸钠分子间的相互作用及结构模型。     

溶液中聚(N-异丙基-2-甲基丙烯酰胺)热相变可逆性的杂合二维相关红外光谱研究

基于聚(N-异丙基-2-甲基丙烯酰胺)(PNiPMA)在升温和降温两个过程中测得的变温红外光谱构筑样本-样本杂合二维相关(Hybrid 2D Correlation)光谱,揭示其在热诱导相变过程中初始组分的恢复程度、相转变温度以及转变速率等物理参数的可逆性.     

Controlled Release of Farmazin from Thermosensitive Gels Based on Poly(N-vinylcaprolactam)

With the aim to prolong the effect of a wide-spectrum antimicrobial agent, Farmazin (tylosine tartrate), this drug was immobilized on a gel of N-vinylcaprolactam-sodium itaconate copolymer, which undergoes a phase transition on heating. The phase transition, accompanied by a change in volume, in aqueous solutions of the pure copolymer gel and that with the immobilized drug was studied.     

FT-IR and 2D-IR spectroscopic studies on the effect of ions on the phase separation behavior of PVME aqueous solution.

Thermosensitive phase transition behavior of poly(vinyl methyl ether) (PVME) in an aqueous solution and the effect of inorganic ions on the coil-globule transition have been investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) accessory. ATR-IR spectra of PVME aqueous solution indicate that in water-PVME-inorganic salts system, the phase separation temperature of PVME aqueous solution decreased with the increase of ion concentration and the increase of anion electronegativity. Meanwhile, two-dimensional infrared (2D-IR) measurements have been made to clarify the microcosmic conformational changes of PVME during the coil-globule transition. Results show that the conformation changes of main chains occur earlier than those of ether groups during heating. Furthermore, the 2D correlation spectroscopy of PVME aqueous solution during heating and the increase of concentration of potassium chloride have been studied. The features of 2D-IR spectra during heating did not change compared to the features of PVME aqueous solution during the increase of concentration of potassium chloride. This result implies that, although the addition of inorganic ions shifts the phase separation temperature, it does not alter the internal mechanism of the coil-globule transition of PVME.     

Precise synthesis of end‐functionalized thermosensitive poly(vinyl ether)s by living cationic polymerization

A series of poly(2‐methoxyethyl vinyl ether)s with narrow molecular weight distributions and with perfectly defined end groups of varying hydrophobicities was successfully synthesized by base‐assisting living cationic polymerization. The end group was shown to greatly affect the temperature‐induced phase separation behavior of aqueous solutions (lower critical solution temperature‐type phase separation) or organic solutions (upper critical solution temperature‐type phase separation) of the polymers. The cloud points were also influenced largely by the molecular weight and concentration of the polymer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Effect of heating rate on thermo-induced aggregation of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions

<正>The effects of heating rate on the aggregate behavior of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions were investigated in detail by laser light scattering and TEM.By employing two separate heating protocols,step-by-step heating at5 K/step and one-step jump,to heat the sample from 15℃to the selected temperature, we found that the heating rate only showed significant effect on the aggregates above the cloud point.The aggregate formed by step-by-step heating exhibited a much larger size and a broader size distribution than those formed by one-step jump heating.Moreover,neither of the aggregates were ideal micellar structures as indicated by the size and the R_g/R_h values.On the contrary,at temperatures below the cloud point where the block copolymer formed core-shelled micelles,the heating rate showed negligible effect on the size and size distribution of the micelles.Since the system underwent a phase separation above the cloud point,the heating rate effect could be reasonably explained by the phase separation mechanisms:the nucleation-and-growth mechanism in the metastable region and the spinodal decomposition mechanism in the unstable region.     

Evidence of water reorientation on model electrocatalytic surfaces from nanosecond-laser-pulsed experiments

The behavior of water at the interface formed between a quasi-perfect Pt(111) single-crystal electrode and an aqueous electrolyte solution is studied by means of the laser-induced temperature jump method. This method is based on the use of nanosecond laser pulses to suddenly increase the temperature at the interface. The measurement of the response of the interface toward the laser heating under coulostatic conditions provides evidence on the net orientation of water at the interface. Especially interesting is the study of the effect on the interfacial water caused by the selective deposition of foreign metal adatoms, because these bimetallic systems usually exhibit appealing electrocatalytic properties. The T-jump methodology shows that the surface composition strongly affects the interaction of water with the surface. The most representative parameter to characterize this interaction is the potential where water reorientation occurs; this potential shifts in different directions, depending on the relative values of the electronegativity of the adatom and the substrate. These results are discussed in the light of available information about the effect of adatom deposition on the work function and the surface potential of the modified surface. Finally, some implications on the enhancement of the electrocatalytic activity are briefly discussed.     

Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

Phase separation in aqueous solutions of polyethylaminophosphazene hydrochloride during heating

It is shown that aqueous solutions of polyethylaminophosphazene hydrochloride undergo phase separation during heating. This phenomenon is studied in detail at pH 3.5 (0.1 M citrate buffer) in relation to the composition of the system with the use of nephelometry and high-sensitivity DSC. The cloud points and the enthalpy of phase separation of the system are determined, and its phase diagram is constructed. The system features a lower critical solution temperature: w _2,cr = 7.3 × 10⁻⁴ and T _cr = 34.3°C. The enthalpy of phase separation is 12.8 ± 0.6 J/g of the polymer, regardless of the system composition. A new approach to the analysis of DSC data on the phase separation of aqueous solutions of polymers during heating is advanced on the basis of calorimetric parameters coupled with the data on the composition of coexisting phases. Through this approach, the main contribution to the heat effect of phase separation of the system under study is related to a change in the energy state of a polyethylaminophosphazene hydrochloride molecule as a result of its dehydration.     

Thermoresponsive hydrogels from symmetrical triblock copolymers poly(styrene-block-(methoxy diethylene glycol acrylate)-block-styrene)

A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20-40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel-sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP(n) about 450), gels had already formed at 3.5 wt % at low temperatures. The gel-sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.     

表面张力法研究温敏性接枝共聚物的微胶束化行为

用表面张力法研究了以水溶性可生物降解的葡聚糖为主链 ,具有温敏相变特性的聚 (N 异丙基丙烯酰胺 )为接枝链的葡聚糖 接枝 聚 (N 异丙基丙烯酰胺 ) (Dextran g PNIPAM)共聚物在水溶液中的胶束化行为 .研究结果表明Dextran g PNIPAM体系的微胶束化行为与共聚物结构和溶液体系的温度密切相关 ,接枝共聚物中PNIPAM含量越大 ,水溶液体系的温度越高 ,形成胶束的临界胶束浓度 (CMC)越小 .特别值得指出的是 ,无论水溶液的温度是否高于PNIPAM接枝链段的相变温度 (LCST) ,即PNIPAM链段由亲水性转变为疏水性的温度 ,Dextran g PNIPAM均呈现一个临界胶束浓度大 ,对该现象给予了解释 .     

Behavior of thermosensitive graft copolymer with aromatic polyester backbone and poly-2-ethyl-2-oxazoline side chains in aqueous solutions

Thermoresponsive graft copolymers with alkylene-aromatic polyester main chain and poly-2-ethyl-2-oxazoline side chains were synthesized. Two copolymer samples which differed in grafting density (0.5 and 0.7) were studied using dynamic and static light scattering and turbidimetry in aqueous solutions at concentration 0.0053?g?cm^?3. Hydrodynamic radii of scattering objects and their contribution to light scattering were obtained as a function of temperature in a wide temperature interval. Temperatures of phase separation were found out. Effect of grafting density on the copolymer behavior in aqueous solutions upon heating was determined. In particular, the phase separation temperature reduces with the decreasing grafting density.     

Phase‐transition characteristics of amphiphilic poly(2‐ethyl‐2‐oxazoline)/poly(ε‐caprolactone) block copolymers in aqueous solutions

Amphiphilic diblock and triblock copolymers of various block compositions based on hydrophilic poly(2‐ethyl‐2‐oxazoline) (PEtOz) and hydrophobic poly(ε‐caprolactone) were synthesized. The micelle formation of these block copolymers in aqueous media was confirmed by a fluorescence technique and dynamic light scattering. The critical micelle concentrations ranged from 35.5 to 4.6 mg/L for diblock copolymers and 4.7 to 9.0 mg/L for triblock copolymers, depending on the block composition. The phase‐transition behaviors of the block copolymers in concentrated aqueous solutions were investigated. When the temperature was increased, aqueous solutions of diblock and triblock copolymers exhibited gel–sol transition and precipitation, both of which were thermally reversible. The gel–sol transition‐ and precipitation temperatures were manipulated by adjustment of the block composition. As the hydrophobic portion of block copolymers became higher, a larger gel region was generated. In the presence of sodium chloride, the phase transitions were shifted to a lower temperature level. Sodium thiocyanate displaced the gel region and precipitation temperatures to a higher temperature level. The low molecular weight saccharides, such as glucose and maltose, contributed to the shift of phase‐transition temperatures to a lower temperature level, where glucose was more effective than maltose in lowering the gel–sol transition temperatures. The malonic acid that formed hydrogen bonds with the PEtOz shell of micelles was effective in lowering phase‐transition temperatures to 1.0M, above which concentration the block copolymer solutions formed complex precipitates. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2400–2408, 2000     

Rheological study of transient networks with junctions of limited multiplicity. II. Sol/gel transition and rheology

Viscoelastic and thermodynamic properties of transient gels formed by telechelic associating polymers are studied on the basis of the transient network theory that considers the correlation among polymer chains via network junctions. The global information of the gel is incorporated into the theory by introducing elastically effective chains defined according to the criterion of Scanlan [J. Polym. Sci. 43, 501 (1960)] and Case [J. Polym. Sci. 45, 397 (1960)]. We also consider the effects of superbridges whose backbone is formed by several chains connected in series and containing several breakable junctions. The dynamic shear moduli of this system are well described in terms of the Maxwell model characterized by a single relaxation time and high-frequency plateau modulus. Near the critical concentration at the sol/gel transition, superbridges become infinitely long along the backbone, thereby leading to a short relaxation time tau for the network. It is shown that tau is proportional to the concentration deviation Delta near the gelation point. The plateau modulus G(infinity) increases as the cube of Delta near the gelation point as a result of the mean-field treatment, and hence the zero-shear viscosity increases as eta(0) approximately G(infinity)tau approximately Delta(4). The present model can explain the concentration dependence of the dynamic moduli observed for aqueous solutions of telechelic poly(ethylene oxide).     

Effect of ethanol on the micellization and gelation of pluronic p123

In certain applications copolymer P123 (E21P67E21) is dissolved in water-ethanol mixtures, initially to form micellar solutions and eventually to gel. For P123 in 10, 20, and 30 wt % aqueous ethanol we used dynamic light scattering from dilute solutions to confirm micellization, oscillatory rheometry, and visual observation of mobility (tube inversion) to determine gel formation in concentrated solutions and small-angle X-ray scattering (SAXS) to determine gel structure. Except for solutions in 30 wt % aqueous ethanol, a clear-turbid transition was encountered on heating dilute and concentrated micellar solutions alike, and as for solutions in water alone (Chaibundit et al. Langmuir 2007, 23, 9229) this could be ascribed to formation of wormlike micelles. Dense clouding, typical of phase separation, was observed at higher temperatures. Regions of isotropic and birefringent gel were defined for concentrated solutions and shown (by SAXS) to have cubic (fcc and hcp) and hexagonal structures, consistent with packed spherical and elongated micelles, respectively. The cubic gels (0, 10, and 20 wt % ethanol) were clear, while the hex gels were either turbid (0 and 10 wt % ethanol), turbid enclosing a clear region (20 wt % ethanol), or entirely clear (30 wt % ethanol). The SAXS profile was unchanged between turbid and clear regions of the 20 wt % ethanol gel. Temperature scans of dynamic moduli showed (as expected) a clear distinction between high-modulus cubic gels (G'max approximately 20-30 kPa) and lower modulus hex gels (G'max<10 kPa).     

Interfacial Microcompartmentalization by Kinetic Control of Selective Interfacial Accumulation

Reported here is a 2D, interfacial microcompartmentalization strategy governed by 3D phase separation. In aqueous polyethylene glycol (PEG) solutions doped with biotinylated polymers, the polymers spontaneously accumulate in the interfacial layer between the oil-surfactant-water interface and the adjacent polymer phase. In aqueous two-phase systems, these polymers first accumulated in the interfacial layer separating two polymer solutions and then selectively migrated to the oil-PEG interfacial layer. By using polymers with varying photopolymerizable groups and crosslinking rates, kinetic control and capture of spatial organisation in a variety of compartmentalized macroscopic structures, without the need of creating barrier layers, was achieved. This selective interfacial accumulation provides an extension of 3D phase separation towards synthetic compartmentalization, and is also relevant for understanding intracellular organisation.