Hyperbranched Polymers with Thermoresponsive Property Highly

Article pubs.acs.org/Langmuir

Hyperbranched Polymers with Thermoresponsive Property Highly Sensitive to Ions Xun-Yong Liu,† Xu-Ran Mu,‡ Yi Liu,‡ Hua-Ji Liu,‡ Yu Chen,*,‡ Fa Cheng,‡ and Shi-Chun Jiang*,§ †

School of Chemistry and Materials Science, Ludong University, Yantai 264025, Shandong, People’s Republic of China Department of Chemistry, School of Sciences, §Department of Polymer Materials Science and Engineering, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The salt effects on the water solubility of thermoresponsive hyperbranched polyethylenimine and polyamidoamine possessing large amounts of isobutyramide terminal groups (HPEI-IBAm and HPAMAM-IBAm) were studied systematically. Eight anions with sodium as the counterion and ten cations with chloride as the counterion were used to measure the anion and cation effects on the cloud point temperature (Tcp) of these dendritic polymers in water. It was found that the Tcp of these dendritic polymers was much more sensitive to the addition of salts than that of the traditional thermoresponsive linear polymers. At low anion concentration, the electrostatic interaction between anions and the positively charged groups of these polymers was dominant, resulting in the unusual anion effect on the Tcp of these polymers in water, including (1) Tcp of these dendritic polymers decreasing nonlinearly with the increase of kosmotropic anion concentration; (2) the chaotropic anions showing abnormal salting-out property at low salt concentration and the stronger chaotropes having much pronounced salting-out ability; (3) anti-Hofmeister ordering at low salt concentration. At moderate to high salt concentration, the specific ranking of these anions in reducing the Tcp of HPEI-IBAm and HPAMAM-IBAm polymers was PO43− > CO32− > SO42− > S2O32− > F− > Cl− > Br− > I−, in accordance with the well-known Hofmeister series. At moderate to high salt concentration, the specific ranking order of inorganic cations in reducing the Tcp of HPEI-IBAm polymer was Sr2+ ≈ Ba2+ > Na+ ≈ K+ ≈ Rb+ > Cs+ > NH4+ ≈ Ca2+ > Li+ ≈ Mg2+. This sequence was only partially similar to the typical Hofmeister cation series, whereas at low salt concentration the cation effect on Tcp of the dendritic polymer was insignificant and no obvious specific ranking order could be found.

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INTRODUCTION Specific ion effects on the chemical and physical properties of aqueous solutions of synthetic and natural macromolecules are ubiquitous.1−3 In many cases, the effectiveness of the ions more or less follows the well-known Hofmeister series4 that was first established to rank the salts according to their relative ability to precipitate certain proteins from an aqueous solution (a saltingout effect) versus to raise the solubility of certain proteins in aqueous solution (a salting-in effect). Up to the present date, many insights into the underlying mechanisms behind the Hofmeister series ordering have been gained; however, deeper understanding at the molecular level still seems to be elusive.5−7 One of the prevailing mechanisms for the Hofmeister series ordering is through ion-specific alterations in the hydrogen-bonding network of water.2,6−8 However, this idea has been challenged recently, and it is found that the properties of bulk water are not significantly perturbed by the dissolved ions.9−16 Recent experimental studies have proven © 2012 American Chemical Society

that the Hofmeister effect can be treated as an interfacial phenomenon, since ions can accumulate at the surface and interface.13,17−26 The accumulated ions can directly interact with the macromolecule and its immediately adjacent hydration shell that is located between macromolecules and the bulk solvent.1 Ions have been classified as either kosmotropes (waterstructure makers) or chaotropes (water-structure breakers) according to whether they are strongly hydrated or weakly hydrated with the interaction between waters as standard.6−8,27 Kosmotropic ions are of high charge density and bind the immediate water molecules more strongly than water binds itself, which usually exhibit the salting-out effect on proteins and macromolecules at moderate to high salt concentrations. Received: January 4, 2012 Revised: February 18, 2012 Published: February 22, 2012 4867

dx.doi.org/10.1021/la300046w | Langmuir 2012, 28, 4867−4876

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of thermoresponsive linear polymers and help to elucidate the phase transition process. Furthermore, proteins are usually in a charged state,28 whereas the majority of synthetic thermoresponsive linear polymers are in the neutral mode, by which only the interactions of salt with the neutral polar and nonpolar groups of proteins can be mimicked.29 Only recently, Zhang et al. employed positively and negatively charged thermoresponsive synthetic microgels to mimic the anions effect on proteins.41 However, no obvious specific anion effects could be observed in the phase transition of positively charged thermoresponsive microgels. Herein, we show that the positively charged thermoresponsive dendritic polymers employed are sensitive to the addition of salts, and both Hofmeister and anti-Hofmeister ordering can be found in this new system. It is known that both Hofmeister and antiHofmeister ordering phenomena have been found in protein systems;30,56,57 therefore, this thermoresponsive dendritic polymer system might be a better model to mimic the interactions among ions and positively charged proteins. The reason might be that the compact, folded proteins are more similar to the compact morphology of dendritic polymers than the normal loose-coil morphology of linear polymers.

Chaotropic ions are monovalent ions of low charge density and bind the immediate water molecules less strongly than water binds itself, which usually lead to the salting-in behavior of proteins and macromolecules in a certain range of salt concentrations. Anions have received more attention than cations since they have more pronounced ion-specific effects in various systems.2,3,6 The following is a typical Hofmeister series ordering of the anions for salting-out certain synthetic and natural macromolecules: CO32− > SO42− > S2O32− > F− > Cl− > NO3− > Br− > I− > ClO4− > SCN−.1,4,6 This ordering is usually similar, and only a slight variation in the sequences of the neighboring anions may exist in the different chemical and physical measurements of different systems. Moreover, the Hofmeister anion ordering can be correlated well with their hydration ability.28−31 As for Hofmeister cations, the influence of their ordering on the properties of synthetic and natural macromolecules cannot be correlated simply with their hydration ability. For instance, Na+, K+, and Cs+ have obvious difference in size, polarizability, and effect on water structure;27 however, they render no obviously different effect on the natural macromolecules including the uncharged acetyltetraglycine ethyl ester model peptide,32 collagen,3 gelatin,3,33 and so on. As the mimic of proteins, many synthetic thermoresponsive linear polymers and their hydrogels containing a large amount of amide groups have been studied concerning the effects of their salts on their phase transition in water.29,34−41 Similar to the natural proteins, anions have a pronounced influence on their properties and the rank ordering also follows the Hofmeister series. However, the effect of inorganic cations on their properties has not been studied in a systematic way since the selected cations exhibit similar influences.17,35,36 Kono's group reported their pioneering work on the thermoresponsive dendrimers whose lower critical solution temperature (LCST) could be tuned in a broad range by varying the molecular weight.42 Following their pioneering work, more kinds of thermoresponsive dendritic polymers were reported.43−51 Dendritic and linear polymers have significant differences in topology. Dendritic polymers usually have spheroid-like and compact structure, whereas linear polymers usually adopt the loose coil conformation.52,53 Therefore, the thermoresponsive dendritic polymer as a new member of the family of thermoresponsive polymers exhibits the obvious difference in properties compared with the traditional thermoresponsive linear polymer.42−44 Up to the present date, the systematic exploration of the salt effect on the properties of thermoresponsive dendritic polymers has never been done. Whether salts have a similar or different influence on thermoresponsive dendritic and linear polymers have never been illuminated. In this contribution, we systematically studied the effect of anionic and cationic ions on the thermoresponsive dendritic polymers containing large amounts of amide and amine groups. It was found that the phase transition temperature of the compact dendritic polymers exhibited higher sensitivity to ions than that of the traditional thermoresponsive linear polymers. This work might favor better understanding of the salt effect on the traditional thermoresponsive linear polymers, since linear polymers will experience a compact, crumpled state before aggregation.54,55 This compact, crumbled state resembles the morphology of dendritic polymers. Thus, thermoresponsive dendritic polymers might be used as the model to mimic the compact meso-state

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EXPERIMENTAL SECTION

Materials. Isobutyric anhydride (98%) was purchased from Alfa Aesar and used without further purification. Hyperbranched polyethylenimine, HPEI10K (Aldrich, Mn = 104g/mol, Mw/Mn = 2.5), was dried under vacuum prior to use. Benzoylated cellulose tubing (MWCO 1000) was purchased from Sigma and used as received. Triethyl amine (A.R., TEA) was dried over CaH2 and distilled before use. All the salts (spectra pure grade) were purchased from Tianjin Guangfu Fine Chemical Research Institute and used as received. Deionized water was double-distilled before use. Tris(2aminoethyl)amine was purchased from Aldrich and was distilled under reduced pressure before use. Tris(2-di(methylacrylate)aminoethyl)amine was synthesized according to the method of Dvornic.58 Pseudo generation 4 HPAMAM (HPAMAM4, Mw = 1.36 × 104, PDI = 1.4) was synthesized according to the literature.58−60 Instrumentation. 1H NMR spectra were recorded on a Varian INOVA 500 MHz spectrometer. The chemical shifts are given in parts per million (ppm). Light transmittance of the polymer solution was measured on a temperature-controlled Purkinje General (China) T6 UV/vis spectrophotometer at 660 nm, and the heating rate was 0.5 °C/2 min. The cloud-point temperature (Tcp) was defined as the temperature corresponding to the initial break points in the resulting transmittance versus temperature curve. The temperature error is ±0.1 °C. General Procedure for the Synthesis of HPEI-IBAm. The preparation of HPEI-IBAm is similar to that reported previously,43 but with a little modification. Under nitrogen atmosphere, isobutyric anhydride (27.45 g, 0.1735 mol) was added dropwise to the mixture of HPEI10K (12.77 g, 0.2169 mol of terminal groups) and triethyl amine (19.31 g, 0.1910 mol) in 50 mL of chloroform at room temperature with vigorous stirring. Subsequently, the reaction mixture was kept at room temperature for 24 h. Finally, the reaction temperature was raised to 72 °C for 2 h to finalize the reaction. The chloroform was removed under vacuum and the residue was dissolved in 50 mL of methanol. Five grams of potassium carbonate was added to the solution and the mixture was stirred at room temperature for 4 h. After centrifugation, the solution was concentrated to ca. 10 mL and then purified by dialysis against methanol using a benzoylated cellulose membrane (MWCO 1000 g/mol) for 2 days. Finally, the methanol solvent was removed under vacuum, and the product was dried in vacuum for 24 h. 1 H NMR (CDCl 3 ): δ (ppm) = 1.08 ((CH3)2CHCON-); 2.20−3.90 ((CH3)2CHCON-, ethylene protons of HPEI polymeric backbone). 4868


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Figure 1. Chemical structures of thermoresponsive hyperbranched HPEI-IBAm and HPAMAM-IBAm polymers.

Table 1. Structural Parameters of Thermoresponsive Hyperbranched Polymers and Their Phase Transition Behavior in Deionized Water polymer

dendritic core

degree of substitutiona (%)

Mn(NMR)b /104

N(IBAm)c

concentration at LCST (mg/mL)

LCST/°C

ΔT1/2d/°C

P1 P2 P3

HPEI HPEI HPAMAM

81 78 92

1.95 1.89 1.28

138 132 44

16 20 20

24.6 27.4 49.9

1.1 1.3 1.7

a Degree of substitution is the ratio of IBAm groups to the reactive amines of HPEI and HPAMAM, calculated from the 1H NMR spectrum of the corresponding HPEI-IBAm and HPAMAM-IBAm polymer. bMolecular weights were calculated from the 1H NMR spectra. cN(IBAm) represents the average number of IBAM groups per polymer. dΔT1/2 represents the sharpness of the phase transition and is defined as half the values of temperature difference of final break point with initial break point in the transmittance versus temperature curves.

Synthesis of IBAm Terminated HPAMAM (HPAMAM4-IBAm). Under nitrogen atmosphere, isobutyric anhydride (1.523 g, 9.624 mmol) was added dropwise to the mixture of HPAMAM4 (1.637 g, 8.020 mmol of primary amines) and triethyl amine (1.071 g, 10.59 mmol) in 20 mL of chloroform at 0 °C with vigorous stirring. Subsequently, the reaction mixture was kept at room temperature for 24 h. Finally, the reaction temperature was raised to 65 °C for 2 h to finalize the reaction. After cooling down to room temperature, the salt produced was filtered off. Volatiles in the filtrate were removed under vacuum, and the residue was dissolved in 20 mL of methanol. One gram of potassium carbonate was added to the solution and the mixture was stirred at room temperature for 4 h. After filtration, the solution was concentrated to ca. 10 mL and then purified by dialysis against methanol using a benzoylated cellulose membrane (MWCO 1000 g/mol) for 2 days in order to remove low-molecular-weight impurities. Finally, the methanol solvent was removed under vacuum, and the product was dried under vacuum for 24 h. The degree of substitution of IBAm groups is 92%. 1H NMR (CDCl3): δ = 1.10 ((CH3)2CHCON-); 2.15−3.75 ((CH3)2CHCON-, ethylene protons of HPAMAM moieties). Preparation of the Aqueous Salt Solutions of Polymers. Different amounts of salts were added to 3.0 mL of aqueous solution of polymers. The mixture was shaken gently. After the salt had dissolved completely, the clear solution was used for the cloud point measurement.

the functional groups of hyperbranched polyamidoamine (HPAMAM) comprise many primary amine and amide groups. Controlling the degree of substitution (DS) of isobutyramide (IBAm) in the reactive amino groups of HPEI and HPAMAM will result in hyperbranched thermoresponsive HPEI-IBAm and HPAMAM-IBAm polymers, respectively.43,61 Both HPEI-IBAm and HPAMAM-IBAm polymers employed here have large amounts of polar amide and amine groups and nonpolar isopropyl units (Figure 1), and their structural parameters interpreted from 1H NMR are listed in Table 1. The cloud point temperature (Tcp) of the solutions of HPEIIBAm and HPAMAM-IBAm polymers was determined by a temperature-controlled UV−vis spectrometer at 660 nm. Figure 2 shows the typical temperature dependence of the light transmittance of the aqueous polymer solutions. The Tcp is defined as the temperature corresponding to the initial break point of the curve in Figure 2. It is clear that the Tcp decreases upon raising the polymer concentration at the beginning, but levels off when the polymer concentrations reach about 16−20 mg/mL. The lowest Tcp at certain polymer concentration is regarded as the LCST of the polymers. The sharpness of phase transition (ΔT1/2), defined as half the values of temperature difference between the final and initial break points in the transmittance versus temperature curves, is concentrationdependent (inset of Figure 2). It is clear that increasing the polymer concentration results in sharper phase transition first, and the ΔT1/2 values become nearly constant when the Tcp of

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RESULTS AND DISCUSSION Hyperbranched polyethylenimine (HPEI) contains a large amount of primary, secondary, and tertiary amine groups, while 4869


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Figure 2. Typical influence of temperature on the light transmittance of different concentrations of thermoresponsive hyperbranched polymer in deionized water (inset: effect of the concentration of polymer in deionized water on the cloud-point temperature and the sharpness of phase transition).

the polymer solution is close to the LCST. The ΔT1/2 values are around 1.0 °C for these polymers at the LCST, which are comparable with those of the well-known thermoresponsive linear poly(N-isopropyl acrylamide) (PNIPAM).34 The parameters relating to the phase transition behaviors of the employed HPEI-IBAm and HPAMAM-IBAm polymers in deionized water are also listed in Table 1. Anion Effect on the Tcp of the Aqueous Solutions of HPEI-IBAm and HPAMAM-IBAm. We first studied the anions’ effect on the Tcp of the aqueous solutions of HPEIIBAm and HPAMAM-IBAm. Eight sodium salts with anions having different charge number and charge density were chosen. From Figure 3, it can be seen that the Tcp values of HEI-IBAm and HPAMAM-IBAm can be modulated to different extents by the addition of different inorganic anions. At the moderate to high salt concentration, only I− exhibits the significant salting-in effect that is embodied by the increase of Tcp in certain salt concentration regions, whereas the other anions, F−, Cl−, Br−, CO32−, SO42−, S2O32−, and PO43−, show the obvious salting-out effect that is embodied with the decrease of Tcp. The specific ranking of inorganic anions in reducing the Tcp of both HPEI-IBAm and HPAMAM-IBAm polymers in water is as follows:

Figure 3. Influence of different sodium salts on the cloud point of (A) HPEI-IBAm (Polymer 2 in Table 1) and (B) HPAMAM-IBAm (Polymer 3 in Table 1) in deionized water (polymer concentration is fixed at 20 mg mL−1).

Mw/Mn = 1.6; 0.25 wt %, LCST = 39 °C) possessing higher LCST is influenced by the anions more significantly than PNIPAM (Mn = 7.5 × 103, Mw/Mn = 2.3 or Mn = 4.9 × 104, Mw/Mn = 1.8; 0.25 wt %, LCST = 32 °C) having lower LCST.36 For instance, concentrations of ca. 0.3 M Na2SO4 lowered the LCSTs of linear PNIPAM and PVIBAm around 10 and 12 °C, respectively, while NaI elevated their LCSTs maximally about 2 (ca. 0.3 M) and 3 °C (ca. 0.5 M), respectively. The LCST of HPEI-IBAm employed here is only 27.4 °C, lower than the respective 32 and 39 °C of PNIPAM and PVIBAm. However, its Tcp was much more sensitive to anions than those of PNIPAM and PVIBAm. For example, 0.3 M Na2SO4 could lower the Tcp of HPEI-IBAm about 19 °C, while 0.3 M NaI could enhance the Tcp of HPEI-IBAm by ca. 7 °C. As for HPAMAM-IBAm with higher LCST (LCST = 49.9 °C), its phase transition temperature is more sensitive to the added anion. For instance, 0.3 M Na2SO4 reduced the Tcp about 35 °C, and 0.3 M NaI elevated the Tcp ca. 4 °C. From the above results, it is clear that with 0.3 M or less Na2SO4 and NaI the Tcp of HPEI-IBAm and HPAMAM-IBAm can be modulated in the range of about 28 and 39 °C, respectively, whereas the phase transition temperatures of linear thermoresponsive PNIPAM and PVIBAm polymers can only be varied around 12 and 15 °C, respectively, which is much less than those shown by these thermoresponsive dendritic polymers. Thus, we tentatively conclude that the phase transition temperature of

PO4 3 − > CO32 − > SO4 2 − > S2O32 − > F > Cl− > Br− > I−

This sequence is in accordance with the well-known Hofmeister series for the biopolymers and synthetic watersoluble polymers.2,3 From this ordering, it can be known that the Tcp of these hyperbranched polymers can be lowered efficiently by the inorganic anions with high charge number. Among the inorganic anions with the same charge number, the one with the highest charge density has the strongest ability to reduce the Tcp of the polymer, such as F− and CO32−. The efficiency of salts in altering the Tcp of the aqueous solutions of thermoresponsive dendritic polymers was compared with that of the traditional thermoresponsive linear polymers. It has been found that more hydrophilic polymers having higher LCST are more salt-sensitive.37 This conclusion can also be verified by the fact that poly(N-vinyl isobutyramide) (PVIBAm, Mn = 1.0 × 104, Mw/Mn = 2.3 or Mn = 6.6 × 104, 4870


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thermoresponsive dendritic polymers is more sensitive to the added salts than that of thermoresponsive linear ones. This phenomenon can be explained as follows: Compared with the loose random coil conformation of linear polymer, the spheroid-like structure of dendritic polymers is more compact. It has been known that the conformation of thermoresponsive linear polymer in aqueous solution experiences several sizerelated conformation variations upon altering the solution temperature through the LCST.54,55 The typical size-related conformations include the loose random coil conformation (temperature is far below LCST) and the crumpled coil conformation (temperature is near LCST). Linear thermoresponsive polymer adopts the coil conformation below its LCST. Upon addition of salt, some hydration water molecules are expelled from polymer chains, leading the coiled polymer chains to shrink and adopt a global conformation first. When the salt concentration is low enough, the surface of the spheroid-like conformation of the linear polymer is still hydrated. The addition of more salts will remove the residual hydrated water, resulting in aggregation of the polymers. The conformation of dendritic polymers resembles the crumpled coil conformation of linear polymers. Thus, the amount of salt for the conformation variation from loose coil to soluble crumpled coil shape is not required in the dendritic polymer system, resulting in higher sensitivity of dendritic polymers to the salt addition. The higher sensitivity of dendritic polymers to the salts would be expected to discriminate the salt ordering more accurately, especially for the less-discriminated cations. Since the conformation of dendritic polymers resembles one of several conformations of linear polymers, the systematic study of the salt effect on dendritic polymers might also supply some useful information to understand the salt function at the crumpled coil conformation stage of linear polymers. Ions can be classified as kosmotropes or chaotropes according to their hydration ability. Kosmotropes are of high charge density and bind the immediately water molecules more strongly than water binds itself, while chaotropes are monovalent ions of low charge density and bind the immediate water molecules less strongly than water binds itself. Sodium and chloride ions are usually used as the dividing species between these two types of behavior for cations and anions, respectively. It has been known that kosmotropic anions normally reduce the LCST of thermoresponsive linear polymers in the range of all the salt concentrations, and the decrease of LCST was usually linearly dependent on the concentration of kosmotropic anions. However, from Figure 4 it can be seen clearly that the Tcp variation of both HPEI-IBAm and HPAMAM-IBAm polymers does not completely follow a linear relationship with the concentration of the kosmotropic anion PO43−. The curves can be divided into two linear parts, and the linear segment existing at the low anion concentration has a sharper slope. The same phenomenon is also observed from the other kosmotropic anions, such as CO32−, SO42−, S2O32−, F−, and Cl−. With respect to the chaotropic anions of I− and Br−, unusual anion effect can also be found in both HPEI-IBAm and HPAMAM-IBAm polymer systems at low salt concentration (Figure 5). For instance, the well-known salting-in anion of I− exhibits an abnormal salting-out effect at low salt concentration. Moreover, its salting-out ability is much stronger than that of the well-known strong salting-out anions of F− and SO42− (Figure 6). In this low-salt-concentration region, anti-

Figure 4. Typical plots of cloud point temperature of HPEI-IBAm and HPAMAM-IBAm vs kosmotropic anion concentration (PO43−).

Hofmeister phenomena exist since the salting-out order becomes inverse among I−, Br−, and Cl−. It is known that the effect of anions on the LCST of PNIPAM can be explained by three interactions between anions and the polymer.29 First, the hydration waters that are hydrogen-bonded with the polar amide groups can be affected by the anions through the hydrogen bonding with the hydrogen atoms of hydration waters. Second, these anions can accumulate in the hydration waters wrapped on the hydrophobic isopropyl side chains of the polymer and increase their surface tension. Third, the anions may directly interact with the polar amide groups that have a kind of resonance providing the amide nitrogen with a partial positive charge and the carbonyl with a partial negative charge. The first interaction mainly exists between the kosmotropic anions and the hydration waters around polar amides, since kosmotropic anions have a stronger water manipulation ability. The main function of kosmotropic anions is to dehydrate the amide groups, which leads to the salting-out of the polymers thereby lowering the Tcp. In the second interaction, anions, especially the chaotropes, can effectively destabilize the hydrophobic hydration of the polymer by increasing the surface tension.62,63 This phenomenon is analogous to the water solubility of small alkanes, which become increasingly less soluble as salt is added.64 Therefore, the second interaction also results in the salting-out of the polymers thereby lowering the Tcp. Until now, the third interaction can only be observed for the most weakly hydrated anions (stronger chaotropes). The enthalpically favorable anion−polymer interaction results in the salting-in of the polymers, thereby elevating the Tcp since it will enhance the hydrophilicity of the polymers.29 If only these three interactions existed between anions and the thermoresponsive dendritic polymers employed here, it would be difficult to explain the unusual nonlinear dependence of phase transition temperature on kosmotropic anion concentration and the anti-Hofmeister phenomenon of chaotropic anions at the low salt concentration. The complexity of the curves in Figures 3−5 may be correlated with the abundance of amino groups of HPEI-IBAm and HPAMAM-IBAm polymers. When they are dissolved into the neutral deionized water, their amino groups will be partially quaternized due to the weak acid−base interaction.65 Thus, the systems studied here contain the fourth interaction originating from the electrostatic interaction between ammonium cations with anions. The ion-pairing between ammonium cations and anions shall screen the electrostatic repulsion inside and among 4871


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Figure 5. Plots of cloud point temperature of HPEI-IBAm and HPAMAM-IBAm vs kosmotropic anion concentration.

Figure 6. Influence of different sodium salts on the cloud point of (A) HPAMAM-IBAm and (B) HPEI-IBAm at low salt concentration.

3A where the HPEI-IBAm polymer has close phase transition temperature to the linear PNIPAM. The data abstracted from the modeling curves were compared with those from the linear PNIPAM. The model for the anion influence on the PNIPAM’s LCST can be expressed by eq 1 that includes a constant, a linear term, and a Langmuir isotherm29

the polymers, thereby promoting the salting-out behavior. It is well-known that electrostatic interaction is much stronger than other noncovalent interactions and the ion-pairing between ammonium cations of polymer and anions can be saturated. On the basis of these two points, it can be deduced that at the beginning a majority of the anions shall prefer to interact with the positive groups of polymers. After being close to the ionpairing saturation, the other noncovalent interactions between anions and the hydration waters around the neutral polar groups and hydrophobic units start to become dominant. Therefore, the sharper decrease of Tcp at low kosmotropic anion concentration in Figure 4 implies that the electrostatic interaction between kosmotropic anions and the ammonium cations of HPEI-IBAm and HPAMAM-IBAm polymers can more efficiently lower the Tcp than the other interactions. Furthermore, the anti-Hofmeister phenomenon of chaotropic anions at the low salt concentration of Figure 5 indicates that anions with more chaotropic property can more efficiently reduce the Tcp through ion-pairing with the ammonium cations of the polymers. Modeling Changes in the Tcp with Added Anions. The anion influence on the PNIPAM’s LCST has been successfully modeled by Cremer group.29 On the basis of the model established by them, we attempted to model the plots of Figure

T = T0 + c[M] +

Bmax KA[M] 1 + KA[M]

(1)

T0 is the LCST of PNIPAM in the absence of salt, and [M] is the molar concentration of salt. The constant, c, has unit of temperature/molarity. The second term of eq 1 depicts the interactions between anions and hydration waters around polymers (the first and second interactions mentioned above) that lead to the linear decrease of phase transition temperature of polymer following the increase of anion concentration. This term is necessary for both kosmotropic and chaotropic anions. The third term is a Langmuir isotherm term depicting the direct ion binding to the polar groups of neutral polymer (the third interaction mentioned above). KA is the binding constant of the anion to the polymer. Bmax is the increase in the phase transition temperature due to direct ion binding at saturation. This term is only necessary for chaotropic anions, not for kosmotropic anions. The fourth interaction between ammo4872


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Table 2. Fitted Values for c, Bmin, Kd, b, KA, and Bmax from Tcp Measurements of HPEI-IBAm (P2) as well as Literature Values29 of Linear PNIPAM (LP) for the Anions

a

c (°C/mol)

Bmin (°C)

anion

P2

LP

b (M−1)

KB (M−1)

KA (M−1)

P2

LP

Bmax (°C) P2

LP

PO43− CO32− SO42− S2O32− F− Cl− Br− I−

−82.1(2.3)a −57.4(2.0) −59.2(2.6) −52.7(16.3) −27.9(8.3) −11.1(0.1) −6.6(2.9) −7.7(4.1)

 −25.1 −18.3 −16.3 −9.0 −5.3 −3.7 −1.5

3.73(0.16) 3.68(0.14) 2.41(0.19) 3.48(0.56) 3.19(0.43) 3.10(0.38) 5.8(15.2) 6.0(25.4)

−1113(746) −4613(2.6E6) 18.0(4.9) 26.7(19.6) 9.6(13.4) 9.1(0.8) −4(313) −6(2655)

28.7(50) 0.016(93) 670(240) 359(167) 131(56) 95(36) 98(259) 593(2367)

      9(39) 14 (26)

      2.7 4.3

      9(23) 199(34)

      0.7 1.1

The data in the parentheses represents the standard error.

Figure 7. (A) Residual cloud-point temperature data from Figure 3A after subtracting the ion-pairing term in eq 2. (B) Residual cloud-point temperature data from Figure 3A after subtracting the ion-pairing term in eq 3.

measures of the effectiveness for a specific anion to associate with positive charges of the dendritic polymer and thereby attenuate the electrostatic repulsion between the charged macromolecules. KB is the binding constant of the anion to the positive groups of the polymer. The curves of kosmotropic anions including chloride in Figure 3A were modeled by eq 2, and the curves of chaotropic anions (Br− and I−) were modeled by eq 3. All the curves could be fitted well by eqs 2 and 3, respectively (see figures in Supporting Information). The fitted values are provided in Table 2 along with the values for linear PNIPAM. The standard error of the fitted data is in the corresponding parentheses. It can be seen that the standard errors of most of the fitted constants are very big, such as the constants KB, Bmin (from eq 3), b, KA, and Bmax. This is normal since the number of degrees of freedom (the number of fitted parameters) of eqs 2 and 3 is very high (four and six fitted constants in eqs 2 and 3, respectively). Therefore, it is difficult to interpret these parameters with such high standard errors and it is also difficult to make any solid conclusions with these parameters. However, most of the constant c data in Table 2 have acceptable standard errors. Therefore, only the constant c data in Table 2 are interpreted and discussed in this paper. The constant c represents the capability of the anions to disturb the hydration water around the polar and nonpolar groups of the polymer. The more negative this value is, the stronger the capability of the anion to disturb the ordered hydration waters, which leads to the Tcp decrease of polymer. According to constant c in Table 2 with the consideration of the upper and lower limits of the standard error, the specific ranking of

nium cations of polymer and anions is also a saturation phenomenon and can be modeled by a Langmuir isotherm.30 Because electrostatic neutralization is involved, an exponential factor must be added.66 Since the third and fourth interactions have opposite impact on the phase transition temperature, a new Langmuir-type isotherm term suitable for depicting the electrostatic neutralization must be used. This new term was supposed to contribute to the unusual nonlinear dependence of phase transition temperature on anion concentration and the anti-Hofmeister phenomenon at the low salt concentration. Since unusual effects exist in both kosmotropic and chaotropic anions, this new term will be required in the modeling of both kosmotropic and chaotropic anion effects. Therefore, the influence of kosmotropic and chaotropic anions on the Tcp of HPEI-IBAm was modeled using eqs 2 and 3, respectively. T = T0 + c[M] −

T = T0 + c[M] +

Bmin KB[M] e−b[M] 1 + KB[M] e−b[M]

(2)

Bmax KA[M] B K [M] e−b[M] − min B 1 + KA[M] 1 + KB[M] e−b[M] (3)

The constant, Bmin, has units of temperature and represents the maximum decrease in Tcp when all of the positive charges in the polymer have anions associated with them. The constant, b, has units of reciprocal molarity and is an electrostatic interaction factor that is related to the surface potential of the dendritic polymer.30 The values of both Bmin and b are 4873


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inorganic anions in lowering the Tcp of HPEI-IBAm nearly obeys the normal Hofmeister ordering. Moreover, most constant c values are more than 2-fold lower than those from linear PNIPAM. This more or less supports the experimental conclusion that the phase transition temperature of thermoresponsive dendritic polymers is more sensitive to the anions than thermoresponsive linear polymers. For kosmotropic anions, the corresponding fitted data of KB, Bmin, and b are put into eq 2. Subtracting the electrostatic interaction term of eq 2 and replotting the residual data reveal a linear relationship (Figure 7A). This further supports that the deviation from the linear relationship in the salt effect on the Tcp of HPEI-IBAm and HPAMAM-IBAm polymers originates from the electrostatic interaction of anions with the positive groups of polymers. As for chaotropic anions, the corresponding fitted data KB, Bmin, and b are put into eq 3. After subtracting the electrostatic interaction term of eq 3 and replotting the residual data, similar curves as those found in the PNIPAM system are observed (Figure 7B), where no decrease of phase transition temperature exists at low salt concentration. This further implies that electrostatic interaction dominates at the low salt concentration not only for kosmotropes, but also chaotropes. Cation Effect on the Tcp of the Aqueous Solution of HPEI-IBAm. The effect of inorganic cations on the phase transition of synthetic thermoresponsive polymers has not been studied in a systematic way since the selected cations exhibit a similar influence.17,35,36 However, in the protein systems, although the cations had much less significant effect than anions, some specific cation ordering was still generalized. The typical Hofmeister cation series found in the salting-out of collagen and gelatin proteins is as follows: NH4+ ≈ Rb+ ≈ K+ ≈ Na+ ≈ Cs+ > Li+ > Mg2+ > Ca2+ >Ba2+.33,67 The characters of this Hofmeister series are as follows: (1) except for Li+ the other alkaline cations and ammonium have similar salting-out ability; (2) the salting-out efficiency of Li+ is usually weaker than the other alkaline cations and ammonium, but stronger than the divalent alkaline earth cations; (3) among the alkaline earth cations, the one with higher charge density has stronger salting-out ability. For the study of cations’ effect on the Tcp of HPEI-IBAm (Polymer 1 in Table 1) aqueous solutions, ten inorganic chloride salts are utilized. In the region of low salt concentration ([Cl−] < 0.2M), the influence of various cations on the Tcp of HPEI-IBAm is insignificant, and thus no obvious specific ranking ordering can be drawn. However, the specific ranking order of cations appeared gradually with the further increase of the salt concentrations, especially when the chloride concentration is above 0.8 M (Figure 8). This further implies that, unlike the thermoresponsive linear polymers, the phase transition temperature of the thermoresponsive dendritic polymer is also sensitive to the added cations. At the low salt concentration ([Cl−] < 0.3 M), Sr2+ and Ba2+ do not show any special capacity to influence the Tcp compared with the other inorganic cations; however, further raising the concentration of Sr2+ or Ba2+ results in the precipitation of HPEI-IBAm polymers. Hence, at the moderate to high salt concentration the specific ranking order of inorganic cations in reducing the Tcp of HPEI-IBAm polymer is as follows:

Figure 8. Influence of different chloride salts on the cloud point of HPEI-IBAm (HPEI10K−IBAm0.81: the degree of substitution of IBAm group is 81%; polymer concentration is fixed at 16 mg mL−1).

This sequence shows only partial similarity to the typical Hofmeister cation series. The similarities are as follows: (1) except for Li+, the other alkaline cations have similar salting-out abilities; (2) the salting-out efficiency of Li+ is usually weaker than that of the other alkaline cations and ammonium. The obvious differences are as follows: (1) The salting-out efficiency of ammonium becomes weaker than that for the monovalent alkaline cations except Li+; (2) the salting-out efficiency of Li+ becomes weaker than that of the other divalent alkaline earth cations except Mg2+; (3) among the alkaline earth cations, the one with the highest charge density has weakest salting-out ability; (4) the salting-out efficiencies of Sr2+ and Ba2+ become much stronger than that of the monovalent cations.

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CONCLUSIONS The phase transition temperature of thermoresponsive dendritic polymers is more sensitive to the addition of salts (including anions and cations) than those of thermoresponsive linear ones. At low salt concentration, the electrostatic interaction between anions and the positive charges of the polymers is dominant since it is stronger than the other interactions, such as the interactions disturbing the order of hydration waters of the polymers and the binding interaction of anions to the neutral polar groups of polymer. This results in an unusual anion effect on the Tcp of the polymers, including the following: (1) Tcp of polymer decreases nonlinearly with the increase of kosmotropic anion concentration; (2) at low salt concentration, the chaotropic anions show abnormal salting-out properties and the stronger chaotropes have very pronounced salting-out ability; (3) anti-Hofmeister ordering exists at low salt concentration. At moderate to high salt concentration, the specific ranking of these inorganic anions in reducing Tcp of the thermoresponsive hyperbranched polymers is in accordance with the well-known Hofmeister series for the biopolymers and synthetic water-soluble polymers: PO43− > CO32− > SO42− > S2O32− > F− > Cl− > Br− > I−. At moderate to high salt concentration, the specific rank order of inorganic cations with chloride as the counterion in the salting-out of HPEI-IBAm polymer is as follows: Sr2+ ≈ Ba2+ > Na+ ≈ K+ ≈ Rb+ > Cs+ > NH4+ ≈ Ca2+ > Li+ ≈ Mg2+. This sequence shows only partial similarity to the typical Hofmeister cation series. Since the compact morphology of the thermoresponsive dendritic polymer resembles the dynamic state experienced by the thermoresponsive linear polymer, this work might interpret

Sr 2 + ≈ Ba 2 + > Na+ ≈ K+ ≈ Rb+ > Cs+ > NH 4+ ≈ Ca 2 + > Li+ ≈ Mg 2 + 4874


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(13) Gurau, M. C.; Lim, S.-M.; Castellana, E. T.; Albertorio, F.; Kataoka, S.; Cremer, P. S. On the Mechanism of the Hofmeister Effect. J. Am. Chem. Soc. 2004, 126, 10522−10523. (14) Boström, M.; Williams, D. R. M.; Ninham, B. W. Specific Ion Effects: Why Dlvo Theory Fails for Biology and Colloid Systems. Phys. Rev. Lett. 2001, 87, 168103. (15) Boström, M.; Williams, D. R. M.; Ninham, B. W. Specific Ion Effects: Why the Properties of Lysozyme in Salt Solutions Follow a Hofmeister Series. Biophys. J. 2003, 85, 686−694. (16) Boström, V.; Kunz, W.; Ninham, B. W. Hofmeister Effects in Surface Tension of Aqueous Electrolyte Solution. Langmuir 2005, 21, 2619−2623. (17) Chen, X.; Yang, T.; Kataoka, S.; Cremer, P. S. Specific Ion Effects on Interfacial Water Structure near Macromolecules. J. Am. Chem. Soc. 2007, 129, 12272−12279. (18) Guo, F.; Friedman, J. M. Charge Density-Dependent Modifications of Hydration Shell Waters by Hofmeister Ions. J. Am. Chem. Soc. 2009, 131, 11010−11018. (19) Jungwirth, P.; Tobias, D. J. Ions at the Air/Water Interface. J. Phys. Chem. B 2002, 106, 6361−6373. (20) Jungwirth, P.; Tobias, D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 2001, 105, 10468−10472. (21) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Experiments and Simulations of Ion-Enhanced Interfacial Chemistry on Aqueous NaCl Aerosols. Science 2000, 288, 301−306. (22) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M. Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563−566. (23) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259−1281. (24) Gopalakrishnan, S.; Liu, D.; Allen, H. C.; Kuo, M.; Shultz, M. J. Vibrational Spectroscopic Studies of Aqueous Interfaces: Salts, Acids, Bases, and Nanodrops. Chem. Rev. 2006, 106, 1155−1175. (25) Pegram, L. M.; Record, M. T. Partitioning of Atmospherically Relevant Ions between Bulk Water and the Water−Vapor Interface. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 14278−14281. (26) Liu, D. F.; Ma, G.; Levering, L. M.; Allen, H. C. Vibrational Spectroscopy of Aqueous Sodium Halide Solutions and Air-Liquid Interfaces: Observation of Increased Interfacial Depth. J. Phys. Chem. B 2004, 108, 2252−2260. (27) Collins, K. D. Charge Density-Dependent Strength of Hydration and Biological Structure. Biophys. J. 1997, 72, 65−76. (28) Collins, K. D. Ion Hydration: Implications for Cellular Function, Polyelectrolytes, and Protein Crystallization. Biophys. Chem. 2006, 119, 271−281. (29) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. Specific Ion Effects on the Water Solubility of Macromolecules: Pnipam and the Hofmeister Series. J. Am. Chem. Soc. 2005, 127, 14505−14510. (30) Zhang, Y.; Cremer, P. S. The Inverse and Direct Hofmeister Series for Lysozyme. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 15249− 15253. (31) Lagi, M.; Nostro, P. L.; Fratini, E.; Ninham, B. W.; Baglioni, P. Insights into Hofmeister Mechanisms: Anion and Degassing Effects on the Cloud Point of Dioctanoylphosphatidylcholine/Water Systems. J. Phys. Chem. B 2007, 111, 589−597. (32) Robinson, D. R.; Jencks, W. P. The Effect of Concentrated Salt Solutions on the Activity Coefficient of Acetyltetraglycine Ethyl Ester. J. Am. Chem. Soc. 1965, 87, 2470−2479. (33) Bello, J.; Riese, H. C. A.; Vinograd, J. R. Mechanism of Gelation of Gelatin. Influence of Certain Electrolytes on the Melting Points of Gels of Gelatin and Chemically Modified Gelatins. J. Phys. Chem. 1956, 60, 1299−1306. (34) Schild, H. G.; Tirrell, D. A. Microcalorimetric Detection of Lower Critical Solution Temperatures in Aqueous Polymer Solutions. J. Phys. Chem. 1990, 94, 4352−4356.

the phase transition behavior of the traditional thermoresponsive linear polymer in the presence of salts. Furthermore, the compact morphology of dendritic polymers is more similar to that of the compact folded proteins; thus, the thermoresponsive dendritic polymer system might be a better model to mimic the interactions among ions and proteins than the normal thermoresponsive linear polymers with loose-coil morphology.

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ASSOCIATED CONTENT

S Supporting Information *

Fitted plots of different concentrations of sodium salts vs the cloud point of HPEI-IBAm (Polymer 2 in Table 1) in deionized water. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-27403475; fax: +86-22-27403475. E-mail: [email protected] (Yu Chen); [email protected] (Shi-Chun Jiang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (20804027, 20974077, 21074088).

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REFERENCES

(1) Zhang, Y.; Cremer, P. S. Interactions between Macromolecules and Ions: The Hofmeister Series. Curr. Opin. Chem. Biol. 2006, 10, 658−663. (2) Kunz, W.; Nostro, P. L.; Ninham, B. W. The Present State of Affairs with Hofmeister Effects. Curr. Opin. Colloid Interface Sci. 2004, 9, 1−18. (3) von Hippel, P. H.; Schleich, T. Ion Effects on the Solution Structure of Biological Macromolecules. Acc. Chem. Res. 1969, 2, 257− 265. (4) Kunz, W.; Henle, J.; Ninham, B. W. ‘Zur Lehre Von Der Wirkung Der Salze’ (About the Science of the Effect of Salts): Franz Hofmeister’s Historical Papers. Curr. Opin. Colloid Interface Sci. 2004, 9, 19−37. (5) Baldwin, R. L. How Hofmeister Ion Interactions Affect Protein Stability. Biophys. J. 1996, 71, 2056−2063. (6) Collins, K. D.; Washabaugh, M. W. The Hofmeister Effect and the Behaviour of Water at Interfaces. Q. Rev. Biophys. 1985, 18, 323− 422. (7) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. The Hofmeister Series: Salt and Solvent Effects on Interfacial Phenomena. Q. Rev. Biophys. 1997, 30, 241−277. (8) Leberman, R.; Soper, A. K. Effect of High Salt Concentrations on Water Structure. Nature 1995, 378, 364−366. (9) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347−349. (10) Kropman, M. F.; Bakker, H. J. Dynamics of Water Molecules in Aqueous Solvation Shells. Science 2001, 291, 2118−2120. (11) Batchelor, J. D.; Olteanu, A.; Tripathy, A.; Pielak, G. J. Impact of Protein Denaturants and Stabilizers on Water Structure. J. Am. Chem. Soc. 2004, 126, 1958−1961. (12) Smith, J. D.; Saykally, R. J.; Geissler, P. L. The Effects of Dissolved Halide Anions on Hydrogen Bonding in Liquid Water. J. Am. Chem. Soc. 2007, 129, 13847−13856. 4875


Langmuir

Article

(35) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Effect of Additives on Phase Transition of N-Isopropylacrylamide Gels. Langmuir 1992, 8, 687−690. (36) Suwa, K.; Yamamoto, K.; Akashi, M.; Takano, K.; Tanaka, N.; Kunugi, S. Effects of Salt on the Temperature and Pressure Responsive Properties of Poly(N-Vinylisobutyramide) Aqueous Solutions. Colloid Polym. Sci. 1998, 276, 529−533. (37) Bloksma, M. M.; Bakker, D. J.; Weber, C.; Hoogenboom, R.; Schubert, U. S. The Effect of Hofmeister Salts on the LCST Transition of Poly(2-Oxazoline)s with Varying Hydrophilicity. Macromol. Rapid Commun. 2010, 31, 724−728. (38) Shechter, I.; Ramon, O.; Portnaya, I.; Paz, Y.; Livney, Y. D. Microcalorimetric Study of the Effects of a Chaotropic Salt, KSCN, on the Lower Critical Solution Temperature (LCST) of Aqueous Poly(NIsopropylacrylamide) (PNIPA) Solutions. Macromolecules 2010, 43, 480−487. (39) Mori, T.; Fukuda, Y.; Okamura, H.; Minagawa, K.; Masuda, S.; Tanaka, M. Thermosensitive Copolymers Having Soluble and Insoluble Monomer Units, Poly(N-Vinylacetamide-co-methyl acrylate)S: Effect of Additives on Their Lower Critical Solution Temperatures. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2651− 2658. (40) Okamura, H.; Morihara, Y.; Masuda, S.; Minagawa, K.; Mori, T.; Tanaka, M. Effects of Salts and Copolymer Composition on the Lower Critical Solution Temperature of Poly(Methyl 2-acetamidoacrylate-comethyl methacrylate) Solutions. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1945−1951. (41) Hou, Y.; Yu, C.; Liu, G.; Ngai, T.; Zhang, G. Effects of Anions on the Aggregation of Charged Microgels. J. Phys. Chem. B 2010, 114, 3799−3803. (42) Haba, Y.; Harada, A.; Takagishi, T.; Kono, K. Rendering Poly(Amidoamine) or Poly(Propylenimine) Dendrimers Temperature Sensitive. J. Am. Chem. Soc. 2004, 126, 12760−12761. (43) Liu, X.; Cheng, F.; Liu, H.; Chen, Y. Unusual Salt Effect on the Lower Critical Solution Temperature of Hyperbranched Thermoresponsive Polymers. Soft Matter 2008, 4, 1991−1994. (44) Haba, Y.; Kojima, C.; Harada, A.; Kono, K. Comparison of Thermosensitive Properties of Poly(Amidoamine) Dendrimers with Peripheral N-Isopropylamide Groups and Linear Polymers with the Same Groups. Angew. Chem., Int. Ed. 2007, 46, 234−237. (45) Aathimanikandan, S. V.; Savariar, E. N.; Thayumanavan, S. Temperature-Sensitive Dendritic Micelles. J. Am. Chem. Soc. 2005, 127, 14922−14929. (46) Jia, Z.; Chen, H.; Zhu, X.; Yan, D. Backbone-Thermoresponsive Hyperbranched Polyethers. J. Am. Chem. Soc. 2006, 128, 8144−8145. (47) Shen, Y.; Kuang, M.; Shen, Z.; Nieberle, J.; Duan, H.; Frey, H. Gold Nanoparticles Coated with a Thermosensitive Hyperbranched Polyelectrolyte: Towards Smart Temperature and pH Nanosensors. Angew. Chem., Int. Ed. 2008, 47, 2227−2230. (48) Liu, H.; Chen, Y.; Shen, Z. Thermoresponsive Hyperbranched Polyethylenimines with Isobutyramide Functional Groups. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1177−1184. (49) Guo, Z.; Zhang, Y.; Huang, W.; Zhou, Y.; Yan, D. Terminal Modification with 1-Adamantylamine to Endow Hyperbranched Polyamidoamine with Thermo-/pH-Responsive Properties. Macromol. Rapid Commun. 2008, 29, 1746−1751. (50) Kojima, C.; Yoshimura, K.; Harada, A.; Sakanishi, Y.; Kono, K. Synthesis and Characterization of Hyperbranched Poly(Glycidol) Modified with Ph- and Temperature-Sensitive Groups. Bioconjugate Chem. 2009, 20, 1054−1057. (51) Tono, Y.; Kojima, C.; Haba, Y.; Takahashi, T.; Harada, A.; Yagi, S.; Kono, K. Thermosensitive Properties of Poly(amidoamine) Dendrimers with Peripheral Phenylalanine Residues. Langmuir 2006, 22, 4920−4922. (52) Gao, C.; Yan, D. Hyperbranched Polymers: From Synthesis to Applications. Prog. Polym. Sci. 2004, 29, 183−275. (53) Newkome, G. R.; Moorefield, C. N.; Vögtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 2001.

(54) Wu, C.; Wang, X. H. Globule-to-Coil Transition of a Single Homopolymer Chain in Solution. Phys. Rev. Lett. 1998, 80, 4092− 4094. (55) Wang, X. H.; Qiu, X. P.; Wu, C. Comparison of the Coil-toGlobule and the Globule-to-Coil Transitions of a Single Poly(NIsopropylacrylamide) Homopolymer Chain in Water. Macromolecules 1998, 31, 2972−2976. (56) Boström, M.; Tavares, F. W.; Finet, S.; Skouri-Panet, F.; Tardieu, A.; Ninham, B. W. Why Forces between Proteins Follow Different Hofmeister Series for pH above and Below pI. Biophys. Chem. 2005, 117, 217−224. (57) Finet, S.; Skouri-Panet, F.; Casselyn, M.; Bonneté, F.; Tardieu, A. The Hofmeister Effect as Seen by SAXS in Protein Solutions. Curr. Opin. Colloid Interface Sci. 2004, 9, 112−116. (58) Dvornic, P. R.; Hu, J.; Meier, D. J.; Nowak, R. M.; Parham, P. L. Hyperbranched Polyurea, Polyurethane, Polyamidoamine, Polyamide or Polyester; U. S. Patent Appl. 6534600-B2, 2002. (59) Pérignon, N.; Mingotaud, A.-F.; Marty, J.-D.; Rico-Lattes, I.; Mingotaud, C. Formation and Stabilization in Water of Metal Nanoparticles by a Hyperbranched Polymer Chemically Analogous to PAMAM Dendrimers. Chem. Mater. 2004, 16, 4856−4858. (60) Pérignon, N.; Marty, J.-D.; Mingotaud, A.-F.; Dumont, M.; Rico-Lattes, I.; Mingotaud, C. Hyperbranched Polymers Analogous to PAMAM Dendrimers for the Formation and Stabilization of Gold Nanoparticles. Macromolecules 2007, 40, 3034−3041. (61) Liu, Y.; Liu, X.-Y.; Liu, H.-J.; Cheng, F.; Chen, Y. Synthesis of Thermoresponsive Hyperbranched Polyamidoamine and the Molecular Weight, pH and Anion Sensitive Thermoresponsive Properties Thereof. Macromol. Res. 2012, 20 [Online Early Access] DOI: 10.1007/s13233-012-0079-1. (62) Melander, W.; Horvath, C. Salt Effects on Hydrophobic Interactions in Precipitation and Chromatography of Proteins: An Interpretation of the Lyotropic Series. Arch. Biochem. Biophys. 1977, 183, 200−215. (63) Jarvis, N. L.; Scheiman, M. A. Surface Potentials of Aqueous Electrolyte Solutions. J. Phys. Chem. 1968, 72, 74−78. (64) Morrison, T. J.; Billett, F. The Salting-out of Non-Electrolytes. Part II. The Effect of Variation in Non-Electrolyte. J. Chem. Soc. 1952, 3819−3822. (65) Borkovec, M.; Koper, G. J. M. Proton Binding Characteristics of Branched Polyelectrolytes. Macromolecules 1997, 30, 2151−2158. (66) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: San Diego, 1991. (67) von Hippel, P. H.; Wong, K. Y. The Effect of Ions on the Kinetics of Formation and the Stability of the Collagen-Fold. Biochemistry 1962, 1, 664−674.

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