Solution properties of nafion in methanol/water mixture solvent

Dilute solution properties of Nafion in methanol/water (4/1 wt ratio) mixture solvent with Nafion concentrations ranging from 0.2 to 9.0 mg/ml was studied using membrane osmometer, viscoelasticity analyzer, and dynamic light scattering. Two aggregation processes were observed. The primary aggregation process causes formation of smaller sizes (∼10³ nm) rod-like aggregation particles, which can be dissociated into single molecular chains by dissolving Nafion in propanol/water mixture solvents, is attributed to the hydrophobic interaction of fluorocarbon backbone. The secondary aggregation process causes formation of larger aggregation particles (∼10⁴ nm), which can be dissociated into primary aggregation particles by mixing NaCl salt into Nafion/methanol/water solutions, is attributed to the ionic aggregation of primary aggregation particles which arise from the electrostatic attraction of Nafion side chain -SO₃⁻ ion pairs. Two critical concentrations were observed in this concentration regime, i.e. C∗∼around 1.0 mg/ml and C∗∗∼around 5.0 mg/ml in the present study, where transitions of Nafion aggregation conformations occur. C∗ is the concentration at which most of the Nafion primary rod-like perfluoro backbone aggregation particles aggregate to form secondary ionic aggregations. C∗∗ is the concentration at which the disordered segments of primary aggregation particles start to overlap and self-assemble.     

Novel polyion complex with interpenetrating polymer network of poly(acrylic acid) and partially protected poly(vinylamine) using N-vinylacetamide and N-vinylformamide

Poly(vinylamine), the simplest polycation with primary amines, was applied to interpenetrating polymer networks (IPN) with poly(acrylic acid). N-Vinylformamide (NVF) was employed for amino-protected monomers to control electrostatic balance. pH-responsivities of IPNs varied, depending on the hydrolysis conditions and acrylic acid (AAc) concentration of the second network. Poly(N-vinylacetamide)-co-poly(N-vinylformamide) (4/6, mol/mol) was employed for the first network, subsequently hydrolyzed with 50% amide groups, and the second network was polymerized with 0.25 mol L⁻¹ AAc, extremely shrunken hydrogels with polyion complex were formed at pH 7, showing that the controlled amount of highly active primary amines are available in IPN.     

Water-soluble polyelectrolyte complexes formed by poly(diallyldimethylammonium chloride) and poly(sodium acrylate-co-sodium 2-acrylamido-2-methyl-1-propanesulphonate)-graft-poly(N,N-dimethylacrylamide) copolymers

The formation of polyelectrolyte complexes (PECs) between the cationic homopolymer poly(diallyldimethylammonium chloride) (PDADMAC) and the anionic graft copolymers poly(sodium acrylate-co-sodium 2-acrylamido-2-methyl-1-propanesulphonate)-graft-poly(N,N-dimethylacrylamide) (P(NaA-co-NaAMPS)-g-PDMAM) was studied in aqueous solution in comparison with the PECs formed between PDADMAC and the graft copolymer backbone poly(sodium acrylate-co-sodium 2-acrylamido-2-methyl-1-propanesulphonate). The turbidimetric study of the PECs formed revealed that associative phase separation is prevented when the anionic polyelectrolyte is grafted with the nonionic hydrophilic poly(N,N-dimethylacrylamide) side chains. The PECs are formed through a charge neutralisation process and they adopt a compact structure, as shown by conductivity and viscometry measurements respectively. The water-insoluble PEC core seems to be stabilised by a hydrophilic PDMAM corona, leading to the formation of nanoparticles with a hydrodynamic radius of some decades of nanometers as determined by quasi-elastic light scattering measurements.     

Synthesis of star polymer by means of the living polymerization of [(o-trifluoromethyl)phenyl]acetylene using a MoOCl4-based catalyst and the linking method

A star polymer was synthesized by addition of 1,4-diethynyl-2,5-dimethylbenzene as linking agent (30 °C, 24 h) after living polymerization of [(o-trifluoromethyl)phenyl]acetylene (o-CF₃PA) with MoOCl₄-n-Bu₄Sn-EtOH catalyst (in anisole, 30 °C, 20 min; [Mo]=10 mM, [P^∗]/[Mo]=40%, [o-CF₃PA]₀=200 mM). The M_n values of the living and star polymers were 8.1×10³ and 5.3×10⁴, respectively, according to gel permeation chromatography, while these values determined by multi-angle laser light scattering (MALLS) were 7.8×10³ and 2.5×10⁵. The M_w/M_n and arm number of the star polymer were 1.04 and 29, respectively, according to MALLS. The molecular weight and arm number of star polymer increased with increasing linking agent concentration and polymerization temperature.     

Flocculation of montmorillonite by some hydrophobically modified polycations containing quaternary ammonium salt groups in the backbone

The flocculating efficiency of some hydrophilic/hydrophobic cationic polyelectrolytes on montmorillonite suspension in water was investigated as a function of both the polycation structure and the flocculation parameters: polycation/montmorillonite contact time, suspension pH, polycation dose, and temperature. Cationic polyelectrolytes with quaternary ammonium salt groups in the backbone and hydrophobic side chains (hexyloxypropyl, PCA₅H₁ and PCA₅H₂; and decyloxypropyl, PCA₅D₁, respectively) were used as flocculants. The flocculation was time dependent. A negative influence of the stirring time and of the abrupt variation of pH from basic to acidic on the separation efficiency was evidenced. The increase of the polycation dose from 0.58 mg polycation/g montmorillonite up to 1.74 mg/g montmorillonite showed a positive influence on the sedimentation of montmorillonite particles for all the polycations taken into account. The influence of the polycation structure was reflected in the lower turbidity found in the case of the polycation PCA₅D₁ compared with that of PCA₅H₁, at the same concentration; this reflects the positive influence of the hydrophobicity increase on the suspension separation. A common characteristic for all the polycations is that, for more than about 60 min of settling, the lowest turbidity was found at 25°C and the highest at 5°C. The turbidity found at 15°C was in between, both before and after 60 min of settling time. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 84: 871–876, 2002; DOI 10.1002/app.10371     

Effect of the alkyl group on the synthesis and the electrochemical properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids

The effect of the alkyl side group on the synthesis and the electrochemical properties of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR_1ATFSI) ionic liquids (ILs) is reported. The investigation was focused on the PYR_1ATFSI ionic liquid family because of the interesting electrochemical properties of the members with propyl and butyl side chains. Side alkyl groups (A = C_nH_2n+1 with n ranging from 1 to 10) of different length and structure were used for the synthesis of PYR_1ATFSI materials. NMR and DSC have shown that the ionic liquids were correctly synthesized with the exception of the compounds with tertiary side chains. Most of the materials exhibited a conductivity higher than 10⁻³ S cm⁻¹ already at 12 °C. In the molten state a moderate conductivity decrease was observed with increasing the length and the branching of the side chain (C₂H_2n+1) group according with the change of viscosity of the ionic liquids. Most of the PYR_1ATFSI samples exhibited an electrochemical stability window exceeding 5 V.     

Radically crosslinked branched poly(N-allylethylenimine) with lithium triflate as a solid state polymer electrolyte

Branched poly(N-allylethylenimine) (BPAEI), a solid state polymer electrolyte host, was synthesized by allylation of branched poly(ethylenimine) (BPEI). Allylation was essentially complete with the 2 and 1° nitrogen atoms of BPEI being mono-allylated and di-allylated, respectively, and with little or no quaternization. BPAEI can be radically cross-linked with and without lithium trifluoromethanesulfonate (LiTf) present to form free-standing, homogeneous, minimally hygroscopic films. BPAEI has a glass transition temperature (T_g) of −65 °C, as measured by differential scanning calorimetry (DSC), which increases with the concentration of initiator upon cross-linking using V-50 (2,2-azobis(2-amidino-propane) dihydrochloride) to −15 °C at a 10:1 nitrogen to initiator molar ratio (N:initiator). BPAEI with 20:1 N:Li⁺ (molar ratio) LiTf has a T_g of −48 °C, which increases with the concentration of radical initiator upon cross-linking using V-50 to 3 °C at 10:1 N:initiator. At compositions near 60:1 N:initiator, an unusual decrease in the rate at which T_g changes with cross-linking was observed, both with and without LiTf present, indicating that some undefined morphological changes occur. The effect of this morphological change resulted in the highest Ac conductivities at 60:1 N:initiator for all LiTf concentrations studied. At 20:1 N:Li⁺ LiTf and 60:1 N:initiator, the room temperature Ac conductivity was 1×10⁻⁸ S/cm which increased to 1×10⁻⁵ S/cm at 80 °C, the highest conductivity observed in the concentration ranges studied. Infrared spectroscopy (IR) showed that the concentrations of the individual ionic species present were largely independent of either LiTf concentration or cross-linking density, suggesting that changes in ion mobility, likely resulting from morphological changes, substantially control the ionic conductivity.     

Empirical determination of equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers

An equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers has been determined based on the empirical functions of thermal pressure coefficient γ_V with respect to volume at constant pressure. The experimental data of PVT over wide range of temperature and pressure published by Anderson and Swenson and Syassen and Holzapfel for rare gas solids and Olabisi and Simha and Zoller for semi-crystalline polymers are used to evaluate γ_V. The function of γ_V with respect to volume determined at constant pressure is given by where V₀ is the volume at 0 K, A, ? and c are constants. The function of internal pressure P_i = γ_VT − P with respect to temperature at constant pressure is determined by converting the function of γ_V(V) to a function of temperature γ_V(T). An empirical equation of state for zero internal pressure determined by pressure P^∗, volume V^∗ and temperature T^∗ at which P_i = 0 is expressed by P^∗V^∗/RT^∗=C^∗−D^∗V^∗ for rare gas and semi-crystalline polymer where C^∗ and D^∗ are constants. The practical meaning of the equation of state for P_i = 0 in the semi-crystalline polymers has been discussed.     

Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids

Various ionic liquids (ILs) were prepared via metathesis reaction from two kinds of 1-(2-hydroxyethyl)-3-methyl imidazolium ([HEMIm]⁺) and N-(2-hydroxyethyl)-N-methyl morphorinium ([HEMMor]⁺) cations and three kinds of tetrafluoroborate ([BF₄]⁻), bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) and hexafluorophosphate ([PF₆]⁻) anions. All the [HEMIm]⁺ derivatives were in a liquid state at room temperature. In particular, [HEMIm][BF₄] and [HEMIm][TFSI] showed no possible melting point from −150 °C to 200 °C by DSC analysis, and their high thermal stability until 380-400 °C was verified by TGA analysis. Also, their stable electrochemical property (electrochemical window of more than 6.0 V) and high ionic conductivity (0.002-0.004 S cm⁻¹) further confirm that the suggested ILs are potential electrolytes for use in electrochemical devices. Simultaneously, the [HEMMor]⁺ derivatives have practical value in electrolyte applications because of their easy synthesis procedures, cheap morpholinium cation sources and possibilities of high Li⁺ mobility by oxygen group in the morpholinium cation. However, [HEMMor]⁺ derivatives showing high viscosity usually had lower ionic conductivities than [HEMIm]⁺ derivatives.     

Theoretical studies of the electronic structures and optical properties of stable blue-emitting polymer based on 4H-cyclopenta-[def]-phenanthrene

Geometries, ionization potentials (IPs), electron affinities (EAs) and optical properties of two series of π-conjugated oligomers (2,6-(4,4-bis(2-ethylthexyl)-4H-cyclopenta-[def]-phenanthrene))_nCPPn (2,6-(4,4-bis(2-ethylthexyl)-8,9-dihydro-4H-cyclopenta-[def]-phenanthrene))_nHCPPn (n = 1-4) were studied theoretically. The ground and the excited state geometries were optimized by B3LYP and CIS methods with 6-31G^∗ basis sets, respectively. The absorption and the emission spectra were calculated by TD-B3LYP method. The lowest-lying absorption is assigned to π → π^∗ transition, and the fluorescence can be described as originating from the ¹[ππ^∗] excited state. IPs, EAs, H-L gaps, absorption and emission properties of PCPP (n = ∞) and PHCPP (n = ∞) were obtained by extrapolation method. The fact that the lowest-lying absorption and the emission of PCPP are blue-shifted compared with those of PHCPP, can be interpreted by the smaller effective repeating units of PCPP. The extra absorption band at 289 nm of PCPP is contributed by the π → π^∗ transition involving the extra π-conjugation CC bond.     

Synthesis and properties of polyacetylenes carrying N-phenylcarbazole and triphenylamine moieties

Novel acetylene monomers containing N-phenyl-substituted carbazole (Cz) and triphenylamine (TPA) groups, namely, 3-ethynyl-9-phenylcarbazole (1) and p-(N,N-diphenylamino)phenylacetylene (2) were synthesized, and polymerized with several Rh-, W-, and Mo-based catalysts. Poly(1) and poly(2) with high number-average molecular weights (15?500-974?000) were obtained in good yields (77-97%), when [(nbd)RhCl]₂-Et₃N (nbd = norbornadiene) was used as a catalyst. The polymers exhibited UV-vis absorption peaks derived from the Cz and TPA moieties at 250-350 nm and polyacetylene backbone above 350 nm. The UV-vis absorption band edge wavelengths of the polymers were longer than those of the corresponding monomers. Poly(2) exhibited a UV-vis absorption peak at a longer wavelength than poly(1) did, which indicates that poly(2) has main chain conjugation longer than that of poly(1). The molecular weights and photoluminescence quantum yields of the polymers obtained by the polymerization using [(nbd)RhCl]₂-Et₃N were larger than those of the Rh⁺(nbd)[η⁶-C₆H₅B⁻(C₆H₅)₃]-based counterparts. The cyclic voltammograms of the polymers indicated that they had clear electrochemical properties; the onset oxidation voltage of poly(1) was higher than those of N-alkyl-substituted Cz derivatives. The polymers showed electrochromism and changed the color from pale yellow to blue by application of voltage, presumably caused by the formation of charged polaron at the Cz and TPA moieties. The temperatures for 5% weight loss of the polymers were around 350-420 °C under air, indicating the high thermal stability.     

Influence of metal coordination on conductivity behavior in poly(butadiene-acrylonitrile)-CoCl2 system

The metal complex formation and the electrical properties of amorphous solid polymer electrolytes, based on poly(butadiene-acrylonitrile) copolymer (PBAN) and CoCl₂, have been studied over the homogeneity region of the system limited by the CoCl₂ concentration of 1.89 mol kg⁻¹. It has been found that ionic conductivity is carried out by the unipolar anion transfer at lower CoCl₂ concentrations (up to 0.10 mol kg⁻¹). As the CoCl₂ concentration increases, electronic conductivity appears in addition to ionic conductivity, and the former becomes dominant, starting from 0.38 mol kg⁻¹. It has been shown that the nature of charge carriers is determined by the composition of metal complexes formed by CoCl₂ and the macromolecular solvent PBAN. At lower concentrations, the [Co₂L₂Cl₄]⁰ dimers are the predominant species (L being macromolecule side groups CN), and their dissociation is followed by the formation of mobile Cl⁻ anions and immobile binuclear [Co₂Cl₃]⁺ complexes. As CoCl₂ concentration increases, polynuclear [Co_nL₂Cl_2n]⁰ (n > 2) complexes appear (L being CN and CC groups of PBAN). Specific features of chemical bonds in π-complexes of transition metals result in the appearance of electronic charge carriers. The abrupt increase in conductivity observed at the highest CoCl₂ concentration is connected with the formation of a percolation network of polynuclear [Co_nL₂Cl_2n]⁰ complexes.     

In vivo and in vitro blood compatibility of polyelectrolyte complexes formed between cellulose derivatives

The blood compatibility of cellulosic polyelectrolyte complexes (PECs) and the effect of excess charge in PEC on the blood compatibility were examined in detail by both in vivo and in vitro blood tests. For this purpose, two types of quaternary ammonium cellulose derivatives were prepared by treating cellulose or hydroxyethyl cellulose with glycidyl trimethylammonium chloride. In vivo blood tests were made by implanting the polymer-coated suture into a jugular and femoral vein of a dog. In vitro blood tests include the measurement of whole blood coagulation time on polymer-coated glass tubes, platelet adhesion measurements using a column packed with polymer-coated glass beads, and a measurement of activation of the intrinsic coagulation system. It was found that among the PECs examined, the PECs containing quaternary ammonium derivatives as polycation components have an excellent blood compatibility. The experiments on the effect of excess charge in PEC revealed that (i) the relative coagulation time of whole blood is almost independent of the mole ratio of polycation to polyanion component within the mole ratios examined, being in good agreement with those by in vivo blood tests, but (ii) platelet adhesion increases with increasing the mole ratio of polycation/polyanion in the PEC, and (iii) the activation of the intrinsic coagulation system increases with decreasing the mole ratio.     

Self-association and microenvironment of random amphiphilic copolymers of sodium N-acryloyl-l-valinate and N-dodecylacrylamide in aqueous solution

The present paper reports on the syntheses and association behavior of two random copolymers of sodium N-acryloyl-l-valinate and N-dodecylacrylamide in buffered (pH 8.0) aqueous solution containing 0.1 M NaCl. Surface tension and viscosity results showed pronounced amphiphilic nature of the copolymers in aqueous solution at pH 8.0. Steady-state fluorescence studies using pyrene and N-phenyl-1-naphthylamine as probe molecules suggested microdomain formation through interpolymer association above a critical concentration called ‘critical aggregation concentration’ (CAC) as low as ca. 10⁻³ g L⁻¹. The local polarity of the hydrophobic domain formed in aqueous solution was estimated from steady-state fluorescence spectra of pyrene. The microviscosity of the domains was evaluated using 1,6-diphenyl-1,3,5-hexatriene as a fluorescent probe using steady-sate fluorescence depolarization and time-resolved fluorescence method. Dynamic light scattering technique was performed over a wide range of concentration to determine hydrodynamic size of the aggregates. It was observed that both copolymers retain rather open conformation in dilute solutions having polymer concentrations less than CAC. However, with increase in concentration the intermolecular association becomes favorable towards the formation of more compact aggregates. The transmission electron microscopic images of both copolymers at a concentration above CAC revealed spherical aggregates of uniform diameter (∼50 nm).     

Rechargeable Li/LiFePO4 cells using N-methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI electrolyte incorporating polymer additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (∼0-∼30 °C).     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Supercritical CO2-assisted preparation of ibuprofen-loaded PEG-PVP complexes

Stoichiometric ratios of poly(ethylene glycol) (PEG, M_w = 400) with poly(vinylpyrrolidone) (PVP, M_w = ±3.1 × 10⁴ and M_w = 1.25 × 10⁶ M_w) were prepared from ethanol cast solutions and in supercritical CO₂. The complex formation was studied via glass transition (T_g) analysis obtained from differential scanning calorimetry (DSC) thermograms. PEG-PVP blends were also loaded with ibuprofen. The molecular dispersion of ibuprofen, mechanism of interaction, the effect of CO₂ pressure and temperature and ageing of blends were also analysed with DSC, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray diffraction (XRD). T_g analysis indicated that supercritical CO₂ can facilitate the formation of stoichiometric PEG-PVP complexes. Processing of PEG-PVP blends with ibuprofen results in the molecular dispersion of ibuprofen mainly bonded to PVP carbonyl groups, without significant disruption of the PEG-PVP complex. Increasing process pressure results in extraction of some PEG fractions. Post-processing ATR-FTIR shifts in ibuprofen-PEG-PVP complexes is greater with supercritical CO₂ processing. These shifts are mainly attributed to atmospheric moisture absorption. Overall it was shown that, ibuprofen-loaded PEG-PVP complexes can be prepared from supercritical CO₂ processing showing similar characteristics to such complexes prepared from solution casting.     

Synthesis and solution properties of hydrophobically associating ionic polymers made from diallylammonium salts/sulfur dioxide cyclocopolymerization

Sulfur dioxide, N,N-diallyl-N-carboethoxymethylammonium chloride and the hydrophobic monomers N,N-diallyl-N-dodecylammonium chloride or N,N-diallyl-N-octadecylammonium chloride were cyclocopolymerized in dimethyl sulfoxide using azobisisobutyronitrile (AIBN) as the initiator to afford water-soluble cationic polyelectrolyte (CPE) having five-membered cyclic structure on the polymeric backbone. The CPE on acidic (HCl) hydrolysis of the pendent ester groups gave the corresponding cationic acid salt (CAS) which was converted to the anionic polyelectrolyte (APE) by treatment with sodium hydroxide. The solution properties of the CPE and APE containing varying amount of the hydrophobic monomers in the range 0-10 mol% were investigated by viscometric techniques. The polymers showed that concentration (C^∗_HA) of less than 1 wt% was required for the manifestation of hydrophobic association, and displayed significant hydrophobic association in salt (NaCl)-free as well as salt-added solutions.     

CO2 absorption properties of Brønsted acid-base ionic liquid composed of N,N-dimethylformamide and bis(trifluoromethanesulfonyl)amide

The solubilities of CO₂ and the liquid densities in a Brønsted acid-base ionic liquid, [DMFH][Tf₂N], composed of N,N-dimethylformamide (DMF) and bis(trifluoromethanesulfonyl)amide (HTf₂N) have been investigated at high pressures and at different temperatures. The results were compared with those in DMF and a typical 1-butyl-3-methylimidazolium analogue with the same anion, [BMIM][Tf₂N]. The mole fraction scaled solubilities of CO₂ in the three liquids showed a slight increase in the following order, DMF < [DMFH][Tf₂N] < [BMIM][Tf₂N], whereas more remarkable difference was observed in the volume scaled concentrations of CO₂, [BMIM][Tf₂N] < [DMFH][Tf₂N] « DMF, mainly due to the bulkiness of liquid entities.     

A study of thermoassociative gelation of aqueous cationic poly(N-isopropyl acrylamide) graft copolymer solutions

In this work thermoassociative gel formation of a new family of aqueous temperature-responsive copolymer solutions has been investigated. This was achieved using a cationic poly(N-isopropyl acrylamide) (PNIPAm) graft copolymer recently prepared [Liu R, De Leonardis P, Cellesi F, Tirelli N, Saunders BR. Langmuir 2008;24:7099]. The PDMA⁺_x-g-(PNIPAm_n)_y copolymers have x and y values that originate from the macroinitiator; the value for n corresponds to the PNIPAm arm length. DMA⁺ is quarternarized N,N-dimethylaminoethyl methacrylate. The copolymer solutions exhibited cloud point temperatures (T_clpt) of about 33 °C, which were not significantly affected by x/y ratio or the value for n. Thermoassociative gel formation occurred above T_clpt at copolymer concentrations (C_copol) greater than or equal to 4 wt.%. This is a reasonably low C_copol value and is a consequence of the graft copolymer architecture employed. We investigated the effect of temperature, C_copol and copolymer structure on gelation and gel elasticity using variable - temperature dynamic rheology. For PDMA⁺₃₀-g-(PNIPAm₂₁₀)₁₄ solutions at 39 °C it was found that G′ (elastic modulus) scales with C_copol according to G′ ∼ C_copol^3.85. The data suggested that a significant proportion of PNIPAm units is not directly involved in network formation. Thermoassociative gel formation and the gel properties for these systems appear to be governed by a balance between electrostatic repulsion involving the DMA⁺ units (favouring spatial extension of the copolymer backbones) and attractive hydrophobic interactions between PNIPAm side chains (favouring associative crosslink formation).