Hofmeister Effect on Thermo-responsive Poly(N-isopropylacrylamide)Hydrogels Grafted on Macroporous Poly(vinyl alcohol) Formaldehyde Sponges

In this work,the Hofmeister effects of nine kinds of anions at different concentrations on the lower critical solution temperature(LCST)of the macroporous thermo-responsive poly(N-isopropylacrylamide)grafted poly(vinyl alcohol)formaldehyde(PVF-g-PNIPAM)hydrogels are investigated with differential scanning calorimetry(DSC).Four kinds of anions with strong hydration,including CO3^2–,SO4^2–,S2 O3^2–,and F^–,and four kinds of anions with weak hydration,including Br^–,NO3^–,I^–,and ClO4^–,and Cl^–as a medium anion are systematically studied and found to demonstrate the effects of the residual hydroxyl groups and network structure of PVF on the LCST values of PVF-g-PNIPAM hydrogels in comparison with that of neat PNIPAM.On the one hand,the existence of hydroxyl groups on PVF backbone promotes the solubility of grafted PNIPAM due to their hydrophilicity and hydrogen-bond interactions with water.On the other hand,the network structure of as-prepared samples restricts free movements of grafted PNIPAM chains,which results in the increase of LCST values.In addition,the difference of grafting percentage also influences the variation of LCST values of PVF-g-PNIPAM hydrogels under salt concentration.     

Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.     

Effects of Hofmeister Anions on the LCST of PNIPAM as a Function of Molecular Weight

The effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated as a function of molecular weight and polymer concentration with a temperature gradient microfluidic device under a dark-field microscope. In solutions containing sufficient concentrations of kosmotropic anions, the phase transition of PNIPAM was resolved into two separate steps for higher molecular weight samples. The first step of this two step transition was found to be sensitive to the polymer's molecular weight and solution concentration, while the second step was not. Moreover, the binding of chaotropic anions to the polymer was also influenced by molecular weight. Both sets of results could be explained by the formation of intramolecular and intermolecular hydrogen-bonding between polymer chains. By contrast, the hydrophobic hydration of the isopropyl moieties and polymer backbone was found to be unaffected by either the polymer's molecular weight or solution concentration.     

Destruction of hydrogen bonds of poly(N-isopropylacrylamide) aqueous solution by trimethylamine N-oxide

Trimethylamine N-oxide (TMAO) is a compatible or protective osmolyte that stabilizes the protein native structure through non-bonding mechanism between TMAO and hydration surface of protein. However, we have shown here first time the direct binding mechanism for naturally occurring osmolyte TMAO with hydration structure of poly(N-isopropylacrylamide) (PNIPAM), an isomer of polyleucine, and subsequent aggregation of PNIPAM. The influence of TMAO on lower critical solution temperature (LCST) of PNIPAM was investigated as a function of TMAO concentration at different temperatures by fluorescence spectroscopy, viscosity (η), multi angle dynamic light scattering, zeta potential, and Fourier transform infrared (FTIR) spectroscopy measurements. To address some of the basis for further analysis of FTIR spectra of PNIPAM, we have also measured FTIR spectra for the monomer of N-isopropylacrylamide (NIPAM) in deuterium oxide (D(2)O) as a function of TMAO concentration. Our experimental results purportedly elucidate that the LCST values decrease with increasing TMAO concentration, which is mainly contributing to the direct hydrogen bonding of TMAO with the water molecules that are bound to the amide (-CONH) functional groups of the PNIPAM. We believed that the present work may act as a ladder to reach the heights of understanding of molecular mechanism between TMAO and macromolecule.     

Specific thermoresponsiveness of PMMA-block-PDMAEMA to selected ions and other factors in aqueous solution

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) is a functional polymer that presents positive charges at low and neutral pHs. Copolymers bearing PDMAEMA blocks usually show a thermal behavior in water marked by a lower critical solution temperature (LCST). This behavior suggests them as interesting “smart” materials for several applications such as water decontamination and many others. In the present work, the LCST of a poly(methyl methacrylate)-block-poly(2-(dimethylamino)ethyl methacrylate) (PMMA-b-PDMAEMA) synthesized by RAFT was measured in several conditions in aqueous solution. It was demonstrated that ionic strength modulates the LCST and this effect depends on the nature of the anion. For instance, weakly hydrated anions such as perchlorate make the LCST lower as the ionic strength gets higher, on the other hand, strongly hydrated ions such as sulfate have not shown a marked effect on LCST. Those results were understood as a consequence of the specific binding of each anion onto the positive-charged PDMAEMA chains, changing the interactions between the chains and water molecules, as well as affecting their pK _a and the ionic interactions. Also, as expected, the LCST is very dependent on pH. The overall results point to the opportunity of modulating very precisely the thermal behavior of PMMA-b-PDMAEMA systems by usual solution conditions such as the type of the anion present.     

Adsorption isotherms and dissipation of adsorbed poly(N-isopropylacrylamide) in its swelling and collapsed states

Poly(N-isopropylacrylamide) (PNIPAM) physisorbed on gold surfaces in aqueous solutions has been studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption isotherms of the polymer, that is, the adsorbed mass versus the concentration of PNIPAM in solution, show distinctly different behaviors at temperatures below and above a lower critical solution temperature (LCST). Below the LCST, PNIPAM forms a single compact layer in solutions with concentrations up to 100 ppm in weight; above the LCST, much thicker films of PNIPAM form in the same concentration range. Changes in the dissipation factor versus solvent concentration show a behavior similar to those in the isotherms. The difference in the adsorption behavior below and above the LCST can be qualitatively explained in terms of the conformation difference of the polymer in its swelling and collapsed states.     

Release property of temperature-sensitive alginate beads containing poly(N-isopropylacrylamide)

The graft copolymer (APN) of alginate and poly(N-isopropylacrylamide) (PNIPAM) were synthesized and APN beads were prepared by dropping the aqueous solution of the copolymer into an aqueous solution of Ca(2+) solution. Alginate chains were employed to play a role in forming beads by electrostatic interactions with a multivalent ion, Ca(2+). Grafted PNIPAM segments were adopted to act as a valve for the pores of the beads, since they exhibit the properties of thermal contraction and expansion. The percent of release of blue dextran from APN beads was higher at 40 degrees C than at 25 degrees C. The difference in the release between two temperatures became more distinguishable when the content of PNIPAM in APN beads is higher. Below lower critical solution temperature (LCST), the expanded PNIPAM would close the pores of the beads, resulting in a lower release rate. Above LCST, the thermally contracted polymer would open the pores, resulting in a higher release rate. The percent of release from APN beads were investigated when the temperature of the release medium is altered. The release rate was relatively low at 25 degrees C. The temperature, however, changed up to 40 degrees C, a marked increase in the release rate was observed. These trends were found to be reproducible when the temperature was repeatedly altered between 25 and 40 degrees C. As a result, a stepwise response to the temperature alteration was obtained.     

End-grafted low-molecular-weight PNIPAM does not collapse above the LCST

The interfacial properties of end-grafted temperature-responsive poly(N-isopropylacryamide) (PNIPAM) were quantified by direct force measurements both above and below the lower critical solution temperature (LCST) of 32 degrees C. The forces were measured between identical, opposing PNIPAM films and between a PNIPAM film and a lipid membrane. At the grafting densities and molecular weights investigated, the polymer extension did not change significantly above the LCST, and the polymers did not adhere. Below the LCST, the force-distance profiles suggest a vertical phase separation, which results in a diluter outer layer and a dense surface proximal layer. At large separations, the force profiles agree qualitatively with simple polymer theory but deviate at small separations. Importantly, at these low grafting densities and molecular weights, the end-grafted PNIPAM does not collapse above the LCST. This finding has direct implications for triggering liposomal drug release with end-grafted PNIPAM, but it increases the temperature range where these short PNIPAM chains function as steric stabilizers.     

Effects of Hofmeister anions on the aggregation behavior of PEO-PPO-PEO triblock copolymers

The effects of a series of Hofmeister anions on the phase behaviors of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer were investigated with an automated melting point system. Well hydrated anions and poorly hydrated anions interacted with the polymer differently and further affected the phase transition of the polymer. Poorly hydrated anions worked through changing the interfacial tension at the polymer/aqueous interface and in enhancing the polymer hydration by ion binding. The phase transition of the polymer in the presence of well hydrated anions correlated directly to the hydration entropy of the anions. As a consequence, the polymer showed a two-step phase transition in solutions containing poorly hydrated anions while displayed a single-step phase transition in the presence of well hydrated anions. The mechanisms of how ions interact with the polymer and further modulate its phase behaviors were discussed.     

Solute- and temperature-responsive "smart" grafts and supported membranes formed by covalent layer-by-layer assembly

Polymers like poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior. A variety of reports have shown that brush grafts of PNIPAM on surfaces exhibit similar temperature responsiveness. We recently described an alternative synthetic approach to such surfaces that affords surfaces with similar LCST-like behavior. We also noted how such surfaces' wettability can change in response to the identity and concentration of solutes. Here we show that this synthetic procedure can be extended to glass surfaces and to more complex surfaces present in porous glass frits. Functionalized glass surfaces exhibit solute-dependent wetting behavior analogous to that previously reported. We further show that the resulting responsive nanocomposite grafts on such frits exhibit "smart" responsive permeability with a greater than 1000-fold difference in permeability to water versus aqueous solutions of sodium sulfate. This "smart" permeability is ascribed to the solute-dependent wettability behavior of the responsive PNIPAM component of the nanocomposite graft, which is sensitive both to the identity and concentration of the solute anion and to temperature.     

Synthesis and characterization of temperature-sensitive cellulose-graft-poly(N-isopropylacrylamide) copolymers

A new series of cellulose-graft-poly(N-isopropylacrylamide)(cellulose-g-PNIPAM) copolymers were prepared by atom transfer radical polymerization(ATRP) of N-isopropylacrylamide monomers from a cellulose-based macro-initiator, which was homogeneously synthesized in an ionic liquid 1-allyl-3-methylimidazolium chloride(Amim Cl). The composition of cellulose-g-PNIPAM copolymers could be adjusted by altering the feeding ratio and reaction time. The resultant copolymers with relatively high content of PNIPAM segments(molar substitution of PNIPAM ? 18.3) were soluble in water at room temperature. Aqueous solutions of cellulose-g-PNIPAM copolymers exhibited clear temperature-sensitive behavior, and their sol-to-gel phase transition properties were investigated by dynamic light scattering(DLS) and UV measurements. Compared with pure PNIPAM, the cellulose-g-PNIPAM copolymers possessed higher lower critical solution temperatures(LCST) in a range from 36.9 ?C to 40.8 ?C, which are close to normal human body temperature, and could be tuned by adjusting the content of PNIPAM segments in copolymers. Spherical structure of cellulose-g-PNIPAM copolymers formed at temperatures above LCST and its morphology was observed by TEM and SEM. These novel cellulose-g-PNIPAM copolymers may be attractive substrates for some biomedical applications, such as drug release and tissue engineering.     

Molecular dynamics simulations of PNIPAM‐co‐PEGMA copolymer hydrophilic to hydrophobic transition in NaCl solution

Classical molecular dynamics simulations were carried out to investigate the hydrophilic to hydrophobic transition of PNIPAM‐co‐PEGMA close to its lower critical solution temperature (LCST) in 1 M NaCl solution. PNIPAM‐co‐PEGMA is a copolymer of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) methacrylate (PEGMA). The copolymer consists of 38 monomer units of NIPAM with two PEGMA chains attached to the PNIAPM backbone. The PNIPAM‐co‐PEGMA was observed to go through the hydrophilic?hydrophobic conformational change for simulations at temperature slightly above its LCST. Na⁺ ions were found to bind strongly and directly with amide O, even more strongly with the O atoms on PEGAMS chains, whereas Cl^? ions only exhibit weak interaction with the polymer. Significantly a novel caged stable metal‐organic complex involving a Na⁺ ion coordinated by six O atoms from the copolymer was observed after the PNIPAM‐co‐PEGMA copolymer went through conformational transition to form a hydrophobic folded structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

含环糊精的温度敏感性聚合物的合成及自组装

合成了侧基含环糊精的聚异丙基丙烯酰胺(PnipamCD), 该聚合物在水溶液中具有较高的最低临界溶解温度(LCST). 快速升温到溶液的LCST以上可形成球形胶束, 慢速升温到LCST以上可形成空心囊泡. 在PNIPAM的选择性溶剂中, PnipamCD形成棒状组装体.     

Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers

Inorganic anions strongly influence the electron transfer rate from the ascorbate to the ferrocene-terminated self-assembled monolayer (SAM) composed of 9-mercaptononyl-5'-ferrocenylpentanoate (Fc(CH2)4COO(CH2)9SH, MNFcP). At the 1 M concentration level of the supporting anion (sodium salt electrolyte), a more than 10-fold increase in the electrocatalytic oxidation rate constant of the ascorbate is observed in the following sequence: PF6-, ClO4-, BF4-, NO3-, Cl-, SO4(2-), NH2SO3- (sulfamate), and F-. The sequence corresponds to the direction of increasing hydration energy of the corresponding anion, suggesting that highly hydrated ions promote electrocatalytic electron transfer to the ferrocene-terminated SAMs, while poorly hydrated ions inhibit it. Fourier transform surface-enhanced Raman spectroscopy (FT-SERS), in combination with cyclic voltammetry, indicates the formation of surface ion pairs between the ferricinium cation (Fc+) and low hydration energy anions, while, on the contrary, no ion pairs were observed in the electrolytes dominated by the high hydration energy anions. Though it is evident that the ion-pairing ability of hydrophobic anions is directly responsible for the electrocatalytic electron transfer inhibition, an estimate of the free, ion-unpaired Fc+ surface concentration shows that it cannot be directly related to the electron transfer rate. This suggests that the principal reason of the anion-induced electron transfer rate modulation might be related to the molecular level changes of the physical and chemical properties as well as the structure of the self-assembled monolayer.     

PNIPAM chain collapse depends on the molecular weight and grafting density

This study demonstrates that the thermally induced collapse of end-grafted poly(N-isopropylacrylamide) (PNIPAM) above the lower critical solution temperature (LCST) of 32 degrees C depends on the chain grafting density and molecular weight. The polymer was grafted from the surface of a self-assembled monolayer containing the initiator (BrC(CH3)2COO(CH2)11S)2, using surface-initiated atom transfer radical polymerization. Varying the reaction time and monomer concentration controlled the molecular weight, and diluting the initiator in the monolayer altered the grafting density. Surface force measurements of the polymer films showed that the chain collapse above the LCST decreases with decreasing grafting density and molecular weight. At T > LCST, the advancing water contact angle increases sharply on PNIPAM films of high molecular weight and grafting density, but the change is less pronounced with films of low-molecular-weight chains at lower densities. Below the LCST, the force-distance profiles exhibit nonideal polymer behavior and suggest that the brush architecture comprises dilute outer chains and much denser chains adjacent to the surface.     

Interactions between temperature-sensitive hydrogel microspheres and granulocytes

Poly(N-isopropylacrylamide) (PNIPAM) has a low critical solution temperature (LCST) at 32°C in water and the hydrophilicity changes through the LCST. The microspheres whose surface was composed of PNIPAM exhibited phase transition behavior around 32°C. Therefore, the interactions between PNIPAM micropheres and granulocytes depended on the temperature. That is, the oxygen consumption and active oxygen production by cells in contact with PNIPAM-containing microspheres and adhesion of the microspheres to the cell surface were more enhanced above the LCST of PNIPAM than below it, whereas no significant temperature dependence of cell–microspheres interaction was observed in nonthermosensitive microsphere systems. It was suggested that the function of cells could be controlled with temperature using the temperature-sensitive microspheres.     

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.     

聚(N,N-二乙基丙烯酰胺)的合成及盐对其水溶液温敏性的影响

采用CuBr/2,2'-联二吡啶催化体系, α-溴代丙酸乙酯为引发剂, 甲醇为溶剂, 通过原子转移自由基聚合(ATRP)合成了分子量分布窄的聚(N,N-二乙基丙烯酰胺)(PDEAM). 用FT-IR、1H-NMR和凝胶渗透色谱(GPC)对其结构进行了表征; 利用透光率的测定研究了PDEAM水溶液浓度、盐以及表面活性剂对PDEAM水溶液低临界溶解温度(LCST)的影响. 结果表明: 随着PDEAM水溶液浓度的增大, LCST逐渐降低; NaCl、CH3COONa、KCl、Na2SO4及MgSO4使PDEAM水溶液的LCST降低, 降低程度与盐的种类和阴离子价数有关; 十二烷基磺酸钠(SDS)则使PDEAM水溶液的LCST升高.     

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N‐isopropylacrylamide)

The lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N‐isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18‐kDa sample had a 0.6 °C higher LCST. A 56‐kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl‐containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with M_w values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006     

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.