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.     

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.     

Cononsolvency of poly(N‐isopropylacrylamide) in methanol aqueous solution—insight by dielectric spectroscopy

Both water and methanol are good solvents for poly(N‐isopropylacrylamide) (PNIPAM), while PNIPAM does not dissolve in their mixed solvents, this phenomenon is called cononsolvency. Cononsolvency is closely related to many phenomena in life but so far, its mechanism is still controversial. In this work, the dielectric behavior of PNIPAM methanol aqueous solution was studied in the frequency of 40Hz–40GHz. From lower frequency to higher frequency, four relaxations were found. They are, respectively, from global chain motion, local motion of backbone, motion of side chain group, and the dipole orientation of the solvent molecule. The solvent dependence of dielectric parameters for the chain motion implied that the PNIPAM chain has undergone the coil‐globule‐coil transition. Dielectric analysis to microwave frequency showed that the volume of the bound solvent units on PNIPAM chain increases with the increasing methanol concentration, which suggested that the structure of solvation units bound on PNIPAM side chains undergo a changing process experience from water to water‐methanol cluster to the ternary methanol cluster. This work reveals the structure and dynamics of the PNIPAM chain and the solvent unit that involved in the solvation of PNIPAM, and provides some new insight into the cononsolvency phenomenon. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1227–1234     

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.     

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.     

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.     

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.     

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.     

The impact of the cononsolvency effect on poly (N-isopropylacrylamide) based microgels at interfaces

The paper addresses the effect of solid interfaces on the cononsolvency effect for poly(N-iso propylacrylamide) based microgels containing different contents of the co-monomer allyl acetic acid (AAA). The cononsolvency effect is studied by dynamic light scattering (DLS) in solution and with atomic force microscopy (AFM) at surfaces against different mixtures of water and organic solvent (ethanol, iso-propanol, and tetrahydrofuran). For the studies at interfaces, the microgels are spin coated on silicon wafers that are precoated with poly(allylamine hydrochloride) (PAH). The minimum in particle volume due to cononsolvency shows a pronounced shift from 10–20 % of organic solvent to 40–50 % after deposition at the Si/PAH wafer. The strong shift indicates an increase of water to organic solvent ratio within the gel at the surface with respect to the bulk solution. In order to understand the increase of water to organic solvent ratio, shrinking/reswelling AFM experiments for different spin-coating conditions and under ambient conditions are carried out. Spin coating from water instead from different solvent mixtures has no effect on the cononsolvency. In ambient conditions, the cononsolvency effect disappears     

Synthesis and study of the properties of stereocontrolled poly(N-isopropylacrylamide) gel and its linear homopolymer prepared in the presence of a Y(OTf)3 Lewis acid: effect of the composition of methanol-water mixtures as synthesis media

Poly(N-isopropylacrylamide) (PNIPAM) hydrogels and the corresponding linear homopolymers were synthesized in different methanol-water mixtures (x(m) = 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76, where x(m) is the mole fraction of methanol) in the presence of 0.1 M Y(OTf)(3) Lewis acid. The isotacticity (meso dyad (m), %) and cloud-point temperature of these homopolymers were gradually increased and decreased, respectively, with the increase in the x(m) values of the synthesis solvent mixtures. Moreover, the corresponding linear PNIPAM homopolymers prepared in the absence of Y(OTf)(3) showed an almost constant isotacticity of m = 45% and a cloud-point temperature of 33.0 °C. A SEM study revealed that the resulting hydrogels were highly porous except for the gels prepared at x(m) = 0 and 0.76. The swelling ratios of these hydrogels in water at different temperatures and in different methanol-water mixtures at 20 °C and the deswelling rate and the reswelling rate of these hydrogels were studied. All of these swelling results were compared with that of the corresponding gels prepared in the absence of a Lewis acid (Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Bhimireddi, R.; Rai, R.; Ray, B. J. Appl. Polym. Sci.2012, DOI: 10.1002/app.36318) and explained on the basis of the porosity of the gel, the state of aggregation and isotacticity of the PNIPAM chain segment, and the cononsolvency of the methanol-water mixture toward the PNIPAM chain segment.     

Cononsolvency-induced micellization of pyrene end-labeled diblock copolymers of N-isopropylacrylamide and oligo(ethylene glycol) methyl ether methacrylate

Pyrene end-labeled double hydrophilic diblock copolymers, poly(N-isopropylacrylamide)-b-poly(oligo(ethylene glycol) methyl ether methacrylate) (Py-PNIPAM-b-POEGMA), were synthesized via consecutive reversible addition-fragmentation chain transfer polymerization using a pyrene-containing dithioester as the chain transfer agent. These diblock copolymers molecularly dissolve in pure methanol and water, but form well-defined and nearly monodisperse PNIPAM-core micelles in an appropriate mixture of them due to the cononsolvency of PNIPAM block. 1H NMR, laser light scattering, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the cononsolvency-induced PNIPAM-core micelles. When the volume fraction of water, phi water, in the methanol/water mixture is in the range of 0.5-0.8, the sizes of micelles are in the range of 20-30 nm in radius for Py-PNIPAM50-b- POEGMA18. At phi water = 0.5, the formed micelles possess the highest overall micelle density and the largest molar mass. The effects of varying the block lengths of Py-PNIPAM-b-POEGMA diblock copolymers on the structural parameters of PNIPAM-core micelles have also been explored. Although we can observe the immediate appearance of bluish tinge upon mixing the diblock copolymer solution in methanol with equal volume of water (phi water = 0.5), which is characteristic of the formation of micellar aggregates, the whole micellization process apparently takes a relatively long time to complete, as revealed by monitoring the time dependence of fluorescence emission spectra. The excimer/monomer fluorescence intensity ratios, IE/IM, continuously decrease with time and then reach a plateau value after approximately 20 min. The decrease of IE/IM after the initial formation of pseudo-equilibrium micelles should be ascribed to the structural rearrangement and further packing of PNIPAM segments within the micelle core, restricting the mobility of pyrene end groups and decreasing the probability of contact between them. Compared to the conventional cosolvent approach employed for the micellization of block copolymers in selective solvents, the reported cononsolvency-induced unimer-micelle-unimer transition of Py-PNIPAM-b-POEGMA in methanol/water mixtures has been unprecedented.     

Effect of freeze-drying and rehydrating treatment on the thermo-responsive characteristics of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microspheres

Investigations on the effect of freeze-drying and rehydrating treatment on equilibrium volume changes and on the thermo-response rate of poly(N-isopropylacrylamide) (PNIPAM) microspheres were carried out. The experimental results showed that freeze-drying and rehydrating treatment had nearly no effect on the low critical solution temperature and equilibrium volume changes of PNIPAM microspheres. Furthermore, when the PNIPAM microspheres were frozen in only liquid nitrogen through rapid cooling, the response rate of PNIPAM microspheres to environmental temperature change was nearly not affected by the treatment, which was surprisingly different from the macroscopic hydrogel. The dimension effect was responsible for this phenomenon. The micron-sized PNIPAM microsphere itself has a much quicker response rate compared with the bulky hydrogel because the characteristic time of gel deswelling is proportional to the square of a linear dimension of the hydrogel.     

Fabrication of novel thermo-responsive electrospun nanofibrous mats and their application in bioseparation

To fulfill the development of biotechnology and biomedicine, environmental-responsive polymer materials are wanted for isolation and purification of biomolecules. Herein, a novel thermo-responsive poly(methyl methacrylate) (PMMA)/poly(N-isopropylacrylamide) (PNIPAM) blend nanofibrous mat was developed, which can adsorb and release a model solute, bovine serum albumin (BSA), through the way of hydrophilicity–hydrophobicity transition behavior of PNIPAM. The uniform bead-free electrospun nanofibers were obtained from the homogeneous PMMA solution in the presence of different amount of PNIPAM. Scanning electron microscopy (SEM) analysis showed that the electrospinnability of PMMA was improved by the addition of PNIPAM, and the diameter of resultant nanofibers could be modulated by controlling the amount of PNIPAM. The thermo-responsive swelling behavior of the blend nanofibrous mats was reversible and reproducible by changing environmental temperature across the lower critical solution temperature (LCST) of PNIPAM. Moreover, the separation property of the blend nanofibrous mats was found to be related to the amount of PNIPAM as well as the concentration of BSA. As for a better separation effect, the nanofibers with higher content of PNIPAM were favorable.     

Temperature responsive flocculation and solid-liquid separations with charged random copolymers of poly(N-isopropyl acrylamide)

Temperature-responsive random copolymers based on poly(N-isopropyl acrylamide) (PNIPAM) with 15 mol% of either acrylic acid or dimethylaminoethyl acrylate quaternary chloride were prepared. The effect of the charge and its sign were investigated in the solid-liquid separation of silica and alumina mineral suspensions. The results were compared to PNIPAM homopolymer of similar molecular weight. PNIPAM copolymers of the same charge as the particles (co-ionic PNIPAM) act as dispersants at both 25°C and 50°C. Flocculation occurs when counter-ionic PNIPAM facilitates selective aggregation and rapid sedimentation of minerals at both 25°C and 50°C. Adsorption and desorption studies showed that, unlike non-ionic PNIPAM, little desorption of the counter-ionic copolymers from the oxides occurred after cooling a suspension from 50°C to below the lower critical solution temperature. Thus, incorporation of counter-ionic charge into the temperature sensitive polymer PNIPAM was found to reduce the sediment bed consolidation upon cooling when compared to PNIPAM homopolymers. The lack of secondary consolidation upon cooling is attributed to attractive inter-particle forces, such as conventional polyelectrolyte flocculation mechanisms (bridging, charge neutralization or charge patch) which persist at both 25°C and 50°C when counter-ionic PNIPAM is used. On the other hand, it was possible to obtain rapid sedimentation with the counter-ionic PNIAPMs even when they were added to the suspension already at 50°C, a process which has not been possible with neutral PNIPAM homopolymers.     

Feasible Fabrication of Hollow Micro-vesicles by Non-amphiphilic Macromolecules Based on Interfacial Cononsolvency

Herein we present a new perspective showing that water-soluble liquids, when added to water, undergo transient emulsification before complete dissolution. Thus, non-amphiphilic macromolecules can self-assemble at the two-miscible-phase interface when cononsolvent effect appears. A representative case shown here is that when poly(N-isopropylacrylamide)(PNIPAm), prepared by aqueous radical polymerization, in methanol solution is added into water, the polymer chains rapidly self-assemble into hollow micro-vesicles based on the cononsolvency at water/methanol interface. This finding provides a subtle strategy to prepare hollow micro-vesicles by non-amphiphilic polymers without template participating. We proposed a new concept "interfacial cononsolvency" to describe the formation process. Due to the easy modification process, sugar-contained PNIPAm chains are synthesized by copolymerization. As an application example, it is shown that these sugar-contained PNIPAm chains can afford "sweet" micro-vesicles(containing glucose residues). And the "sweet" micro-vesicles can well mimick the protocells which are involved in the recognition of bacteria.     

Thermoresponsive polymer-stabilized silver nanoparticles

Silver nanoparticles (Ag NPs) stabilized by a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), have been synthesized by the reduction of silver ions with NaBH(4) in aqueous solutions. The obtained Ag NPs are very stable at room temperature due to the extended coil conformation of the PNIPAM chain at temperatures below its volume phase transition temperature ( approximately 32 degrees C). At higher temperatures (such as 45 degrees C) above the phase transition of PNIPAM, only minute aggregation between Ag NPs was observed, showing that the collapsed PNIPAM chains still retain the ability to stabilize Ag NPs. The PNIPAM-stabilized Ag NPs were then characterized as a function of the thermal phase transition of PNIPAM by UV-vis spectroscopy, dynamic light scattering, transmission electron microscopy, and cyclic voltammeter. Consistent results were obtained showing that the phase transition of PNIPAM has some effect on the optical properties of Ag NPs. Switchable electrochemical response of the PNIPAM-stabilized Ag NPs triggered by temperature change was observed.     

Insight into the effect of methylated urea on the phase transition of aqueous solutions of poly(N‐isopropylacrylamide) by microcalorimetry: Hydrogen bonding and van der Waals interactions

The effect of different functional groups of methylated urea on the phase transition of poly(N‐isopropylacrylamide) (PNIPAM) aqueous solutions has been studied by a high‐sensitivity differential scanning calorimetry. The results reveal that with the addition of osmolytes with N? H groups, the enthalpy change increases with the number of DSC cycles, presumably due to the gradual formation of hydrogen bonds with dehydrated C?O groups of PNIPAM at high temperature. Moreover, with the addition of tetramethylurea (TMU) without hydrogen bond donor groups, the enthalpy change of PNIPAM solution remains unchanged with the number of DSC cycles and decreases with the TMU concentration, suggesting that the van der Waals interactions between TMU and isopropyl groups of PNIPAM and the weakening of hydrophobic interactions between isopropyl groups play a dominant role in the effect of TMU on the phase transition of PNIPAM. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1145–1151     

Effect of chain architecture on the phase transition of star and cyclic poly(N‐isopropylacrylamide) in water

The heat‐induced phase transition of aqueous solutions of Poly(N‐isopropylacrylamide) (PNIPAM) in water is examined for a four‐arm PNIPAM star (s‐PNIPAM), a cyclic PNIPAM (c‐PNIPAM), and their linear counterparts (l‐PNIPAM) in the case of polymers (1.0 g L^?1) of 12,700 g mol^?1 < M_n < 14,700 g mol^?1. Investigations by turbidity, high‐sensitivity differential scanning calorimetry (HS‐DSC), and light scattering (LS) indicate that the polymer architecture has a strong effect on the cloud point (T_c: decrease for s‐PNIPAM; increase for c‐PNIPAM), the phase transition enthalpy change (ΔH decrease for s‐PNIPAM and c‐PNIPAM), and the hydrodynamic radius of the aggregates formed above T_c (R_H: c‐PNIPAM < s‐PNIPAM < l‐PNIPAM). The properties of s‐PNIPAM are compared with those of previously reported PNIPAM star polymers (3 to 52 arms). The overall observations are described in terms of the arm molecular weight and the local chain density in the vicinity of the core of the star, by analogy with the model developed for PNIPAM brushes on nanoparticles or planar surfaces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2059–2068.     

Temperature-modulated photoluminescence of quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) grafted with thermoresponsive poly( N-isopropylacrylamide) chains have been prepared. As the temperature increases, PNIPAM chains shrink and aggregate so that the QDs exhibit enhanced fluorescence emission. At a temperature around the lower critical solution temperature (LCST) of PNIPAM, the fluorescence exhibits a maximum intensity. Our experiments reveal that the fluorescence emission is determined by the interactions between QDs as a function of the interdot distance. The optical interdot distance for the maximum luminescence intensity is approximately 10 nm. The chain length of PNIPAM also has an effect on the luminescence. Short PNIPAM chains are difficult to associate, leading to a large interdot distance, so that the luminescence intensity changes slightly with temperature.     

NMR study of phase separation in D2O/ethanol solutions of poly(N-isopropylmethacrylamide) induced by solvent composition and temperature

¹H and ¹³C NMR spectroscopies were applied to investigate phase separation in solutions of poly(N-isopropylmethacrylamide) (PIPMAm) in D₂O/ethanol (EtOH) mixtures induced by solvent composition (cononsolvency) and temperature. Effects of EtOH content in D₂O/EtOH mixtures and temperature on the appearance and extent of the phase separation were characterized. Differences in mesoglobules formed during the phase separation induced by cononsolvency and temperature were found. For temperature-induced phase separation, ¹³C spin-spin relaxation times showed that besides the free EtOH expelled from the PIPMAm mesoglobules, there are also EtOH molecules bound in these mesoglobules. On the other hand, virtually no bound EtOH molecules were detected for mesoglobules formed as a consequence of the cononsolvency. For PIPMAm random copolymers containing negatively charged methacrylate units the phase separation induced by solvent composition was not observed.