Salt Effects in the Cononsolvency of Poly(N‐isopropylacrylamide) Microgels

Poly(N‐isopropylacrylamide) (PNIPAM) is well known to exhibit reentrant behavior or cononsolvency in response to the composition of a mixed solvent consisting of water and a low‐chain alcohol. Since the solvent structure plays an important role in this phenomenon, the presence of structure‐breaking/structure‐making ions in solution is expected to have a dramatic effect on the cononsolvency of PNIPAM. The present work examines the way that the presence of different salts can modify the reentrant‐phase diagram displayed by a cationic PNIPAM microgel in the mixed ethanol/water solvent. The effects of four Hofmeister anions—SO₄^2?_, Cl^?, NO₃^? and SCN^?—with different abilities to modify the solvent structure are analyzed. The species with kosmotropic or structure‐making character show a clear competition with ethanol for the water molecules, intensifying the nonsolvency of the PNIPAM with the EtOH volume fraction (?_e). However, striking results are found with the most chaotropic or structure‐breaking anion, SCN^?. In contrast to what happens in water‐rich solutions, the presence of SCN^? in alcohol‐rich solvents enhances the solubility of the polymer, which macroscopically results in the microgel swelling. Moreover, this ion displays great stabilizing properties when ?_e> is 0.2. These results have been explained by considering how chaotropic or structure‐breaking ions interact with water and ethanol molecules.     

Quantification of solvent water and hydration dynamic of thermo‐sensitive microgel by dielectric spectroscopy

The hydration of natural or synthetic macromolecules is of fundamental importance in our understanding of their structure and stability. Quantification of hydration water can promote the understanding to many complex biological mechanisms such as protein folding, as well as the dynamics and conformation of polymers. An approach to quantification of solvent water was developed by dielectric spectroscopy. Dielectric behaviors of PNIPAM microgels with different crosslink density distribution were measured in the range of 0.5–40 GHz and 15–50. An obvious relaxation process caused by free and bound water was found. Dielectric parameters of free and bound water show that the crosslink density distribution does not affect the volume phase transition temperature of microgels, but significantly influence the orientation dynamics of the solvent water. We found that the three kinds of microgel can be distinguished by the dielectric parameters of the bound water. In addition, the number of water in and outside microgel during the volume phase transition process was quantitatively calculated for the first time. This study provides the possibility for the quantification of water in complex biological process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1859–1864     

Methanol‐induced change of the mechanism of the temperature‐ and pressure‐induced collapse of N‐Substituted acrylamide copolymers

The solvation of two differently composed linear statistical copolymers from N,N‐diethyl acrylamide (DEAm) and N‐isopropyl acrylamide is studied by Fourier‐transform infrared spectroscopy. The solvent is changed from pure water to mixtures with methanol to investigate the cononsolvency effect. Furthermore, the influence of temperature and pressure is studied. The IR results are interpreted by sub‐band fitting of the amide I' band and quantum–chemical calculations. There are significant differences between the two copolymers that cannot be explained by a weighted superposition of the homopolymer spectra. An excess of DEAm units leads to a high number of nonsolvated side chains already in pure water. This high number is reached for equimolar copolymers only when methanol is added. The mechanism of the temperature‐ or pressure‐induced phase transition changes upon methanol addition for both copolymers. Generally, the phenomena are deduced to cooperativity at equimolar composition that is perturbed by methanol. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 532–544     

Cononsolvency of poly-N-isopropyl acrylamide (PNIPAM): Microgels versus linear chains and macrogels

Poly-N-isopropyl acryl amide (PNIPAM) is swollen in both pure water and pure methanol but collapses in mixtures of these solvents. In this review, this cononsolvency of PNIPAM in water/methanol mixtures is discussed. Experimental studies of linear PNIPAM chains and macrogels are compared to microgels. Theoretical studies are presented based on molecular dynamics simulation and quantum mechanical calculations as well as semi-empirical models. The different explanations for the cononsolvency available in the literature are introduced. Experiments show that all PNIPAM species collapse and re-swell at comparable methanol fraction in the mixture. Cross-linker density of macrogels and microgels has only slight influence on cononsolvency, whereas chain length of linear chains has a significant influence. Microgels provide advantages to study cononsolvency by en'abling a broader experimental approach. Furthermore, multi-sensitive microgels can be prepared, which contain compartments sensitive to different stimuli.     

Solvation characteristics of a model water‐soluble polymer

The model water‐soluble polymer poly(ethylene oxide) was used to investigate solvation characteristics in mixtures of d‐water (deuterated water) and d‐alcohols (deuterated alcohols). Three d‐alcohols have been used: d‐methanol, d‐ethanol, and d‐ethylene glycol. Small angle neutron scattering was used to monitor the solvation properties of poly(ethylene oxide) in the d‐solvent mixtures. Nonideal solvent mixing was observed throughout. Solvent mixtures were found to be more effective solvating agents than individual solvents. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3195–3199, 2006     

Hydration and phase separation of temperature-sensitive water-soluble polymers

Collapse of a poly(N-isopropylacrylamide)(PNIPAM) chain upon heating and phase diagrams of aqueous PNIPAM solutions with very flat LCST phase separation line are theoretically studied on the basis of cooperative dehydration(simultaneous dissociation of bound water molecules in a group of correlated sequence),and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods.The transition becomes sharper with the cooperativity parameterσof hydration.Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves(LCST cononsolvency) with the mole fraction of methanol due to the competition is calculated and compared with the experimental data.Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends(telechelic PNIPAM) are theoretically and experimentally studied.The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation),and separate from the coil-globule transition line.Associated structures in the solution,such as flower micelles,mesoglobules and higher fractal assembly,are studied by USANS with theoretical modeling of the scattering function.     

水和甲醇混合溶剂氢键相互作用及对高分子链溶解能力的理论研究

用密度泛函理论对水和甲醇混合溶剂体系的氢键结构进行了详细研究.通过构象和频率分析发现在水团簇中五聚体和六聚体环状结构最为稳定,同时发现一个全新的特征,即甲醇分子能与水五聚体和六聚体形成双氢键.根据各相互作用的稳定化能,分析了水和甲醇混合溶剂对PNIPAM溶解能力的影响,并对实验现象给予了合理解释.     

Conformation of adsorbed layers of polyNIPAM on silica in a binary solvent

The conformation of poly( N-isopropylacrylamide) chains adsorbed at a silica interface was studied as a function of concentration in the methanol-water binary solvent mixture. Both water and methanol are good solvents for PNIPAM; however, in certain mixtures cononsolvency is induced by a lowering of the LCST. This led to a decrease in the extent of the PNIPAM layer away from the interface as measured using the colloidal probe technique in the poor solvent region. At low methanol concentrations but still in the good solvent region capillary bridging between the silica surfaces with adsorbed PNIPAM layers was observed due to the increased methanol concentration in this interfacial region over that of the bulk. Furthermore, adsorption measurements showed that PNIPAM adsorbed only weakly to the silica interface with a low surface excess on the order of 0.23 mg/m (2), which allowed study of the behavior of the immobilized PNIPAM chains under highly dilute conditions using the quartz crystal microbalance. As the concentration of methanol increased toward the phase transition boundary, a slight contraction followed by an expansion of the PNIPAM was observed, which is in agreement with previous predictions from theory for polymers in solution.     

Preparation of polypyrrole‐graft‐poly(N‐isopropylacrylamide)/silver nanocomposites from pyrrolyl‐capped macromonomer by AgNO3 and their stimuli responsibility of light emission

Pyrrolyl‐capped poly(N‐isopropylacrylamide) macromonomers (Py‐PNIPAM) were prepared through reversible addition‐fragmentation‐transfer polymerization with benzyl 1‐pyrrolylcarbodithioate as chain‐transfer agent. Polymerizations of Py‐PNIPAM with/without pyrrole using AgNO₃ as oxidizing agent and dimethylforamide as solvent resulted in graft copolymers of polypyrrole‐graft‐poly(N‐isopropylacrylamide) (PPy‐g‐PNIPAM) as well as silver nanoparticles, leading to the formation of PPy‐g‐PNIPAM/silver nanocomposites. The resulting nanocomposites were soluble in water when the content of PPy was low, and when the molar ratio of Py/Py‐PNIPAM increased to 30, the resulting products became insoluble in water. The resulting nanocomposites had special optical properties because of PPy as well as the temperature‐responsible PNIPAM. The chemical structure and composition of nanocomposite were characterized by ¹H nuclear magnetic resonance spectroscopy, gel permeation chromatograms, fourier transform infrared spectroscopy, and X‐ray diffraction. Their optical properties were characterized by UV–vis and fluorescence spectroscopy. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6950–6960, 2008     

Enhanced mobility and environmental stability in all organic field‐effect transistors: The role of high dipole moment solvent

Low‐operating voltage, high mobility, and stable organic field‐effect transistors (OFETs) using polymeric dielectrics such as pristine poly(4‐vinyl phenol) (PVP) and poly(methyl methacrylate) (PMMA), dissolved in solvents of high dipole moment, have been achieved. High dipole moment solvents such as propylene carbonate and dimethyl sulfoxide used for dissolving the polymer dielectric enhance the charge carrier mobilities by three orders of magnitude in pentacene OFETs compared with low dipole moment solvents. Fast switching circuits with patterned gate PVP‐based pentacene OFETs demonstrated a switching frequency of 75 kHz at input voltages of |5 V|. The frequency response of the OFETs is attributed to a high degree of dipolar‐order in dielectric films obtained from high‐polarity solvents and the resulting energetically ordered landscape for transport. Remarkably, these pentacene‐based OFETs exhibited high stability under bias stress and in air with negligible shifts in the threshold voltage. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1533–1542     

Solvatochromism and preferential solvation of Brooker's merocyanine in water–methanol mixtures

The excitation energy of Brooker's merocyanine in water–methanol mixtures shows nonlinear behavior with respect to the mole fraction of methanol, and it was suggested that this behavior is related to preferential solvation by methanol. We investigated the origin of this behavior and its relation to preferential solvation using the three‐dimensional reference interaction site model self‐consistent field method and time‐dependent density functional theory. The calculated excitation energies were in good agreement with the experimental behavior. Analysis of the coordination numbers revealed preferential solvation by methanol. The free energy component analysis implied that solvent reorganization and solvation entropy drive the preferential solvation by methanol, while the direct solute–solvent interaction promotes solvation by water. The difference in the preferential solvation effect on the ground and excited states causes the nonlinear excitation energy shift. © 2017 Wiley Periodicals, Inc.     

Effect of binary block‐selective solvents on self‐assembly of ABA triblock copolymer in dilute solution

We have studied the self‐assembly of the ABA triblock copolymer (P4VP‐b‐PS‐b‐P4VP) in dilute solution by using binary block‐selective solvents, that is, water and methanol. The triblock copolymer was first dissolved in dioxane to form a homogeneous solution. Subsequently, a given volume of selective solvent was added slowly to the solution to induce self‐assembly of the copolymer. It was found that the copolymer (P4VP₄₃‐b‐PS₃₆₆‐b‐P4VP₄₃) tended to form spherical aggregate or bilayer structure when we used methanol or water as the single selective solvent, respectively. However, the aggregates with various nanostructures were obtained by using mixtures of water and methanol as the block‐selective solvents. The aggregate structure changed from sphere to rod, vesicle, and then to bilayer by changing water content in the block‐selective solvent from 0 to 100%. Moreover, it was found that the vesicle size could be well controlled by changing the copolymer content in the solution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1536–1545, 2008     

聚异丙基丙烯酰胺高分子在甲醇水溶液中溶解性的拉曼光谱研究

通过测量-13℃(低于低临界溶解温度(LCST))时聚异丙基丙烯酰胺(PNIPAM)高分子在甲醇水溶液中的拉曼光谱非一致效应(NCE),试图从PNIPAM与溶剂分子间的相互作用角度理解PNIPAM的溶解性.通过比较甲醇水溶液中加入PNIPAM前后甲醇分子C-O伸缩所对应的NCE变化,我们认为:甲醇摩尔分数(x)在1.0-0.90范围内,PNIPAM优先吸附甲醇分子;x=0.80-0.50时,PNIPAM优先吸附水分子;而x=0.50-0.20时,PNIPAM破坏了甲醇与水所形成的三元环稳定结构.进一步比较加入PNIPAM或其单元结构--异丙基丙酰胺(NIPPA)对甲醇水溶液NCE的影响,发现PNIPAM通过链段间的疏水协同作用吸附了甲醇分子.我们认为在甲醇水溶液的低浓度区间,这种协同作用破坏了甲醇与水形成的三元环团簇结构,而当温度升高时这种结构又重新形成,导致了PNIPAM在甲醇水溶液中的混致不溶现象.     

Mechanism of Conformational Transition of Silk Fibroin in Alcohol-water Mixtures

Circular dichroism, intrinsic fluorescence of protein and exogenous fluorescence probe of 8‐anilino‐1‐naphtha‐ lenesulfonic acid hemimagnesium salt (ANS) was used to investigate the mechanism of conformational change of silk fibroin (SF) in aqueous alcohol including methanol and ethanol. The conformational transition of SF from random coil to β‐sheet was found to be of a close relationship with the microstructure of the solvent. The alcohol‐water mixture at low concentration had little effect on the solvation of the peptide unit, as the inherent water structure was conserved. At high alcohol concentration, the transition from the tetrahedral‐like water structure to the chain‐like alcohol structure in the mixtures induced a β‐sheet conformation of SF, as a result of the formation of intramolecular hydrogen bond between the peptide units in order to eliminate the thermodynamic unfavorite from the contact to the solvent molecules. Meanwhile, the aggregating of hydrophobic side chains was decreased by the alcohol via the destruction of hydrogen bond network of water by alcohol and the binding of alcohol to hydrophobic group.     

Solvent induced amplification of the photoresponsive properties of α,ω-di-[4-cyanophenyl-4'-(6-hexyloxy)-azobenzene]-poly(N-isopropylacrylamide) in aqueous media

Irradiation at room temperature of α,ω-di-[4-cyanophenyl-4'-(6-hexyloxy)-azobenzene]-poly-(N-isopropylacrylamide) (Az(2)-PNIPAM) solutions in water/1,4-dioxane (6 mol% dioxane) reversibly converts a turbid suspension into a clear solution, demonstrating for the first time that cononsolvency of PNIPAM in mixed aqueous solvents in synergy with preferential chromophore solvation can act as actuators of responsive systems.     

Poly(acrylonitrile) ultrafiltration membranes. I. Polymer‐salt‐solvent interactions

Fourier transform infrared spectroscopy was used to study the interactions among LiCl, ZnCl₂, and AlCl₃ with N,N‐dimethylformamide (DMF) and poly(acrylonitrile) (PAN). It was observed that all three salts complex with DMF as well as PAN. The strength of the cation interaction with the >C?O oxygen of DMF was found to be higher than that with the ? CN group of PAN. The >C?O stretching frequency of DMF with ZnCl₂ was red shifted, indicating stronger complex formation compared with other two cations. With the addition of salt, the salt–DMF pseudo solvent was found to become a θ solvent for PAN compared with neat DMF. This change in PAN solvation power was primarily the result of DMF–salt complexation. As a result of the complexation, Mark‐Houwink constant a, was found to reduce from 0.75 (for pure DMF) to ～0.6 for DMF–salt solvents, indicating decreased PAN chain expansion. Comparison of intrinsic viscosity [η] values indicated that addition of salts to PAN–DMF solutions resulted in: (i) decrease in the DMF solvation power, which causes less expanded polymer coils, and (ii) increased interpolymer chain entanglements via salt‐promoted chain association. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2061–2073, 2005     

Solvent‐ and counterion‐specific swelling behavior of poly(acrylic acid) gels

The collapse of alkali metal poly(acrylate) (PAAM) gels was investigated for various water/organic solvent mixture systems: methanol (MeOH), ethanol (EtOH), 2‐propanol (2PrOH), t‐butanol (tBuOH), dimethyl sulfoxide (DMSO), acetonitrile (AcN), acetone, tetrahydrofuran (THF), and dioxane. In order to ascertain the counterion specificity in the swelling behavior, four kinds of alkali metal counterions were used: Li⁺, Na⁺, K⁺, and Cs⁺. Remarkable solvent and counterion specificities were observed for every counterion species and every solvent system, respectively. For example, in aqueous EtOH the dielectric constants (D_cr) at which collapse occurred were in the order PAACs < PAALi < PAAK < PAANa. On the other hand, the D_cr at which PAALi gel collapsed increased in the order tBuOH < dioxane < THF < MeOH < 2PrOH < EtOH < acetone < AcN < DMSO, where the D_cr ranged from about 39 to about 67. This was in contrast to our previous observation for a partially quaternized poly(4‐vinyl pyridine) (P4VP) gel, which collapsed in a much narrower D_cr region in similar mixed solvents. The present solvent‐ and counterion‐specific collapses are discussed on the basis of solvent properties such as the dielectric constant and Gutmann's donor number and acceptor number of a pure solvent. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2791–2800, 2000     

Testing of the effect of parameters on the cononsolvency of random copolymer gels of N-isopropylacrylamide and N-ethylacrylamide in methanol-water mixed solvents by simple gravimetric method

Random copolymer gels of N-isopropylacrylamide (NIPAM) and N-ethylacrylamide (NEAM) were synthesized in using different monomer compositions in 1:1 methanol-water mixtures and in pure water. The samples were characterized by cononsolvency study in methanol-water mixtures at various temperatures by swelling ratio measurements. It is observed that with the variation of the temperature and monomer compositions in the synthesis reaction mixture, the cononsolvency properties varied significantly. But the variation in the solvents compositions by keeping monomer compositions fixed does not affect the cononsolvency much. All results can be explained on the basis of the differences in thermoresponsive character or difference in the hydrophilicity of PNIPAM and PNEAM in water, preferential adsorption of the solvents by polymer networks, solvent-solvent interactions, heterogeneity of the polymer networks synthesized at different solvent compositions and at different temperatures, difference in their morphological properties, etc.     

Synthesis and self‐assembly in water of coil‐rod‐coil amphiphilic block copolymers with central π‐conjugated sequence

The purpose of this study is to correlate the nano‐organization in water of coil‐rod‐coil amphiphilic block copolymers constituted of a conjugated segment to their optoelectronic properties. The ABA block copolymer structures, easily achieved via coupling reactions, are based on conjugated rod of dihexylfluorene and 3,4‐ethylenedioxythiophene units linked to two flexible poly(ethylene oxide) or poly[(ethylene oxide)‐ran‐(propylene oxide)] chains. These well‐defined copolymers exhibited a range of specific morphologies in water, a good solvent of coil blocks and a bad solvent of the conjugated rod. Particularly, vesicles and micelles with spherical, cylindrical, or elongated shape were noticed. Correlations were attempted to be established between the weight percent of the conjugated sequence contained in the copolymers, the morphology of the nanostructures obtained by self‐assembly in solution and the resulting optical properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4602–4616, 2008