Reentrant conformation transition in poly(N,N-dimethylacrylamide) hydrogels in water-organic solvent mixtures

Conformational changes in poly(N,N-dimethylacrylamide) (PDMA) networks swollen in aqueous solutions of organic solvents are studied both experimentally and theoretically. PDMA hydrogels of various charge densities were prepared by free-radical crosslinking copolymerization. Swelling behavior of the hydrogels was investigated in aqueous organic solvent mixtures as functions of solvent species and the concentration. With increasing volume fraction ? of acetone, tetrahydrofuran, or 1,4-dioxane in the aqueous solution, PDMA hydrogels exhibit reentrant conformation transition. During this transition, the gel first deswells in the range of ? between 0.4 and 0.9, and then rapidly reswells if ? is monotonically increased. The reswelling of the collapsed PDMA gel occurs in a narrow of ? above ?=0.97. It was shown that the reentrant transition in PDMA gels requires moderate hydrogen bonding organic solvents, so that the hydrophobic interactions between PDMA and the organic solvent dominate the swelling process. The results were interpreted using the theory of equilibrium swelling. The interaction parameters in the gel system as well as the partition parameter of the organic solvent between the gel and the solution phases were calculated.     

New copolymers of N,N-dimethylacrylamide with blocked isocyanates for oligonucleotide immobilization in DNA microarray technology

A family of copolymers of N,N-dimethylacrylamide containing blocked isocyanate functionalities is presented. The copolymers were characterized by DSC, GPC and ¹H NMR. Calorimetric analysis showed for any composition broad endothermal phenomena followed by a stronger exothermal peak, which can be attributed, respectively, to the deblocking and subsequent reaction of generated NCO groups.Characterization of glass slides coated with these polymers was done by contact angle measurements and atomic force microscopy. While the former method revealed minimal differences with formation of moderately hydrophilic surfaces, microscopic images showed a more homogeneous coating formation for the copolymer structure with 50% molar of blocked isocyanate. The efficiency of the coated substrates in the immobilization of amino functionalized oligonucleotides was successfully assessed through binding tests and analysis by confocal fluorescence microscopy.Finally, some model microarrays were fabricated by spotting and hybridization with complementary, fluorescently labelled targets was carried out. It resulted in surfaces coated with copolymers which show well defined circular spots with a fluorescence intensity higher than that obtained by slides treated by silanization, and based on the same immobilization chemistry.     

Experimental study of swelling and shrinking kinetics of spherical poly(N,N-diethylacrylamide) gel with continuous phase transition

The kinetics of swelling and shrinking of spherical particles of poly(N,N-diethylacrylamide) (DEAA) gel, prepared with static mixing technology, was investigated experimentally. The shrinking process was separated into late shrinking and initial shrinking by the skin layer effect. When the final temperature of temperature swing experiments was above the lower critical solution temperature (LCST) of polyDEAA, the apparent increase in the response rate constant (k_v) and the polymer diffusion constant (D) were due to reduction of the friction coefficient between polymer chains and water, arising from decrease in the water viscosity. When the final temperature was below the LCST, the main factor determining k_v and D was the change in the network structure of DEAA particles caused by the final temperature. The initial temperature affected the values of k_v and apparent diffusion constant D_app only in initial shrinking, in which the decrease in D with increase in the initial temperature came mainly from the spatial limitation.     

Rapid swelling and deswelling of thermoreversible hydrophobically modified poly(N-isopropylacrylamide) hydrogels prepared by freezing polymerisation

Rapid response thermally sensitive hydrophobically modified poly(N-isopropylacrylamide) hydrogels have been synthesised successfully using a two-step polymerisation method, the initial polymerisation being carried out at 20 °C, followed by polymerisation at −28 °C for 24 h. The results show that the swelling/deswelling rates of poly[N-isopropylacrylamide-co-(di-n-propylacrylamide)] P(NIPA-co-DPAM) hydrogels prepared by two-step polymerisation are much faster than for the same type of hydrogels prepared via conventional methods (30 °C for 24 h), i.e. the time for the former xerogel to absorb 70 and 90 wt% is just 30 and 240 min, respectively, compared to the latter xerogel which takes 1600 and 2500 min to absorb the same amounts of water. During deswelling (shrinking), the hydrogel loses 95 wt% water in 1 min, compared to a timescale for the corresponding cross-linked copolymers prepared by conventional methods of about 5 h for 50 wt% water loss. Scanning electron microscopy, and flotation experiments together with swelling ratio studies reveal that the polymeric network of the former hydrogel is characterised by an open structure with more pores and higher swelling ratio but lower mechanical strength compared to the latter hydrogels. Such rapid response hydrogels have potential applications in separation and drug release technologies for example.     

Upper critical solution temperature—type cononsolvency of poly(N,N-dimethylacrylamide) in water—organic solvent mixtures

The behaviour of linear poly(N,N-dimethylacrylamide) (PDMAM) chains was studied by turbidimetry and viscometry in mixtures of water with the polar organic solvents methanol, dioxane and acetone. The swelling-deswelling behaviour of PDMAM gels in the same solvent mixtures was also investigated. Contrary to the behaviour in water-methanol mixtures, in water-dioxane and water-acetone mixtures a significant shrinkage of polymer chains and deswelling of polymer gels, followed by phase separation, was observed for high organic solvent fractions. Cononsolvency phenomena were found to be temperature-dependent, as demixing occurred upon decreasing temperature. This upper critical solution temperature (UCST) phase separation behaviour in mixed solvents was studied by turbidimetry and compared to the well-known lower critical solution temperature (LCST) behaviour of poly(N-isopropylacrylamide) (PNIPAM) in similar solvents mixtures.     

疏水改性温敏水凝胶的合成及应用研究进展

N-异丙基丙烯酰胺（NIPAM）类水凝胶是典型的温敏水凝胶,通常含有亲水性酰胺基和疏水性异丙基,具有随温度变化而发生可逆溶胀/收缩的特殊性质,作为一种新型的智能材料得到广泛的应用。本文主要论述了NIPAM类疏水改性温敏水凝胶的合成,在骨架中引入疏水单体可以改善其疏水特性,同时提高其温度敏感性,使其在药物释放、物质分离及生物医用材料等领域具有独特的应用价值。目前对疏水改性温敏水凝胶的理论研究尚浅,仍需拓展其在实际方面的应用,今后可考虑改善疏水单体的官能团结构提高疏水性能,合成更具温度响应性和环境友好性的智能温敏水凝胶,拓展其在催化、水处理、生物化工等领域的广泛应用。     

Characterization of anion diffusion in polymer hydrogels used for wastewater remediation

Ion exchange systems for the removal of nutrient pollutants, even at extremely low concentrations, from wastewater effluents are a major environmental need. This work reports on the features of batch sorption processes for the ultimate removal and recovery of reactive phosphorus, nitrogen, and sulfur anions from aquaculture production wastewater effluents. The sorbent used was a crosslinked polyamine (PAA·HCl) polymeric hydrogel. The PAA·HCl hydrogels were prepared by chemically crosslinking aqueous solutions of linear PAA·HCl chains with epichlorohydrin (EPI). The nutrient anion binding capacity of the gels was studied as a function of various processing parameters. Lab scale batch and experiments showed maximum anion removal, from aquaculture wastewater, with regard to by 99%, by 70%, and by 95%. diffusivities of 16, 10, and 8×10⁻⁶ cm²/s were measured at pH levels of 5.5, 7.0, and 8.5.     

Synthesis and characterisation of stimuli-responsive poly(N,N′-diethylacrylamide) hydrogels

Stimuli-responsive poly(N,N′-diethylacrylamide) gels were prepared by free radical polymerisation in aqueous solution, using N,N-methylenebisacrylamide as crosslinking agent. The gels were compared with the corresponding poly(N-isopropylacrylamide)-based gels. In particular, the swelling ratio of both gel types including the effect of the crosslinker content, their swelling and deswelling kinetics, their permeability and finally their drug (insulin) storage and controlled release ability were compared. In spite of the similarity in the monomer/crosslinker ratio, the deswelling kinetics and the critical temperatures (ca. 30-32 °C in pure water), some differences could be observed. Compared to poly(N-isopropylacrylamide)-based gels, poly(N,N′-diethylacrylamide)-based gels show a broader phase transition temperature interval, a more pronounced dependency of the swelling ratio on the crosslinker content, slower reswelling kinetics, a higher ingress percentage for dextran standards ranging from 5 to 70 kD, but lower ingress percentages for proteins (BSA, insulin) and much faster drug (insulin) release kinetics. While a non-linear release kinetic was observed in the case of the poly(N-isopropylacraylamide)-based gels both in water and in PBS (phosphate buffered saline), this was not the case for the poly(N,N′-diethylacrylamide)-based gels.     

Thermal and swelling studies of hydrophobically modified polyacrylamide hydrogels

Hydrophobically modified polyacrylamide hydrogels were prepared by polymerizing acrylamide, using n‐butyl acrylate and 2‐ethyl hexyl acrylate as comonomers and dimethyl formamide as a solvent. We report here for the first time that the Alfred Q‐e reactivity ratios are matching with the experimental value (elemental analysis result) for the terpolymer hydrogel systems. Differential scanning calorimetry (DSC) thermograms of the hydrogel samples revealed two endothermic transitions, one near 0°C, corresponding to the melting transition of ice and another above ambient temperature. This high temperature transition is due to the rearrangement of water molecules engaged in hydrophobic hydration surrounding the hydrophobic groups (alkyl side chains). This depends on the type of hydrophobic groups and the gel compositions. Swelling and deswelling studies of the gels were performed using various solvents such as water and isopropyl alcohol. An abnormally high swelling and deswelling behavior was observed in the homopolymer gels and the gels with low amount of hydrophobic comonomers. This could be attributed to the more intramolecular cross‐linked structure formed during synthesis in organic solvent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of boronate-containing copolymers of N,N-dimethylacrylamide, their interaction with poly(vinyl alcohol) and rheological behaviour of the gels

Cross-linking of polyvinylalcohol (PVA) by boronate-containing copolymer of N,N-dimethylacrylamide (DMAA, 1) was studied and compared to cross-linking of PVA by borate buffers in weakly alkaline solutions. The copolymer of M_w=19,000 g mol⁻¹ containing 9 mol% N-acryloyl-m-aminophenylboronic acid (NAAPBA, 2) was prepared by free radical polymerization of the monomers, exhibiting copolymerization constants r₁=0.84 and r₂=2.2. Due to multipoint interaction of the copolymer with PVA via monodiols, the intermolecular cross-linking required for seven-fold and 10-fold lower boron concentrations as compared to borate buffers of pH 8.6 and 7.5, respectively. In rheological measurements, PVA-copolymer gels exhibited storage moduli (G′_max) comparable to those of PVA-borate gels prepared at 7.5-fold higher boron concentration and the same pH 8.6, what testified to the similar concentration of cross-links in the gels. Therefore, DMAA-NAAPBA copolymer is a more effective cross-linker of PVA than borate. The PVA-copolymer gel exhibited much higher relaxation time (97 s) compared to PVA-borate gels (≤20 s) indicating a longer lifetime of junction zones. The ‘shape stability’ of the gel is suggested to originate in the structure of junctions, containing several boronate-diol complexes, between the macromolecules of PVA and the copolymer.     

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with a hydrophobic comonomer

Hydrogels were prepared by free radical polymerisation in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with di-n-propylacrylamide (DPAM), di-n-octylacrylamide (DOAM) or di-dodecylacrylamide (DDAM) as hydrophobic comonomer. N,N-methylene bisacrylamide (BIS) and glyoxal bis(diallyacetal) (GLY) were used as crosslinkers. A series of copolymers with three different comonomer contents was synthesised and for some polymers three different crosslinker concentrations were employed. The swelling equilibrium of these hydrogels was studied as a function of temperature, hydrophobic comonomer species and content in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS). In pure water the gels showed a discontinuous volume phase transition at 33 and 30 °C for PNIPA and hydrophobically modified PNIPA copolymeric hydrogels, respectively. The swelling ratio r and the transition temperature (LCST) increased at low temperatures with the addition of SDS, this is ascribed to the conversion of non-ionic PNIPA gels into polyelectrolyte gels through the binding of SDS. At SDS concentration below 0.5 wt%, gels exhibited a single discontinuous volume transition at 36-38 °C. However, for SDS concentration above 0.5 wt%, two discontinuous volume transitions at 36-40 and 70 °C were observed. Additionally, the replacement of BIS by the novel octafunctional crosslinker glyoxal bis(diallylacetal) (GLY) yielded an increase in the swelling ratio.     

Fluorescence polarization and rheological studies of the poly(N-vinyl-2-pyrrolidone) hydrogels produced by UV radiation

Poly(N-vinyl-2-pyrrolidone) hydrogels produced by direct ultraviolet irradiation of PVP aqueous solution leads to crosslinking through pyrrolidinone moiety photolysis. Generally, hydrogel physical properties, like crosslinking density, pore size, swelling capacity, storage and loss moduli are obtained by swelling and rheological tests. However, relations between anisotropy obtained by fluorescence polarization studies and these properties have not been addressed for hydrogel systems. In this work we show that there is a correlation between the data obtained from anisotropy and rheological experiments, since both of them are related with crosslinking density of the hydrogels. These results reveal that fluorescence polarization spectroscopy is a promising tool for understanding the structure of hydrogels.     

Toward an understanding of thermoresponsive transition behavior of hydrophobically modified N-isopropylacrylamide copolymer solution

Poly(N-isopropylacrylamide-co-vinyl laurate)(PNIPAAm-co-VL) copolymers were prepared at various feed ratios via conventional radical random copolymerization. The formation, composition ratios and molecular weight of copolymers were examined. The thermoresponsive behaviors of PNIPAAm and PNIPAAm-co-VL solutions at low and high concentrations were intensively investigated by turbidity measurement, Micro-DSC, temperature-variable state fluorescence, ¹H NMR and dynamic light scattering (DLS). Several important results were obtained that (1) incorporation of PVL results in much lower and broader LCST regions of the copolymer solutions, and facilitates the formation of hydrophobic microdomains far below LCST, causing a pronounced aggregation in solutions (2) temperature-variable ¹H NMR spectra shows that during the phase transition, the ‘penetration’ of PNIPAAm into the hydrophobic core is a process accompanied with a transition of isopropyl from hydration to dehydration as well as a self-aggregation of hydrophobic chains at different temperature stages (3) according to the ¹H NMR spectra of polymer solutions obtained at varied temperatures, the microdomains from hydrophobic VL moieties have a different accessibility for isopropyl groups and the entire chains during phase transition (4) temperature-variable DLS demonstrates that the temperature-induced transition behavior of copolymers is supposedly divided into three stages: pre-LCST aggregation (<20 °C), coil-globule transition at LCST (20-25 °C) and post-LCST aggregation (>25 °C).     

Dynamic swelling of hydrogels based on random terpolymers of N-isopropylacrylamide, methacrylic acid and poly(ethylene glycol) macromonomer

A series of pH-responsive hydrogels based on N-isopropylacrylamide (N-iPAAm), methacrylic acid (MAA) and poly(ethylene glycol) monomethyl ether monomethacrylate macromonomer (PEGMEMA), P(N-iPAAm-co-MAA-co-PEGMEMA) random terpolymers, were synthesized and their swelling behaviour studied as a function of both monomer composition and previous swelling treatment. The swelling kinetic curves were followed using gravimetric, photographic and magnetic resonance imaging (MRI) techniques, which provide spatial and temporal resolution. The swelling behaviour was non-Fickian at pH 7, being this fact more relevant when the samples were pre-soaked in pH 2 solution. Low pH promotes hydrogen bond arrangements that disrupt at pH 7, where sigmoidal swelling curves were observed. The sigmoidal shape of the curves increases as well as the swelling time with increasing N-iPAAm/PEGMEMA ratio. This indicates that hydrogen bond arrangements between MAA and N-iPAAm are stronger that those formed by MAA and PEGMEMA. The influence of the polymer composition on the hydrogen bond arrangements was also studied from the swelling kinetics curves at different pH media, observing that the swelling rate, the swelling curve shape and the whole amount of water absorbed were clearly dependent on this parameter.     

Direct UV photocrosslinking of poly(N-vinyl-2-pyrrolidone) (PVP) to produce hydrogels

Hydrogels for biomedical purposes, made from synthetic polymers as starting materials and free of co-adjuvant molecules, have been produced almost exclusively by high-energy radiative processes. On the other hand, UV photocrosslinking of such materials has been used in conjunction of monomers and/or photoinitiators. This work was addressed to the analysis of poly(N-vinyl-2-pyrrolidone) (PVP) submitted to direct photocrosslinking in aqueous solution, using low pressure Hg lamp (λ_em=254 nm). The process efficiency was evaluated, and the properties of the hydrogel formed were determined. The product thus formed has similar micro- and macroscopic properties, as compared to hydrogels produced by high-energy radiation and presents no cytotoxicity. These results demonstrated the viability of using this method as a versatile alternative to hydrogel production, broadening the possibility of its production where high-energy radiation facilities are not available.     

Electrochemically and electrochromically stable polyimides bearing tert-butyl-blocked N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units

A new class of electrochemically active polyimides with di-tert-butyl-substituted N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units was prepared from N,N-bis(4-aminophenyl)-N′,N′-bis(4-tert-butylphenyl)-1,4-phenylenediamine and various aromatic tetracarboxylic dianhydrides via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical or thermal cyclodehydration. Most of the polyimides are readily soluble in many organic solvents and can be solution-cast into tough and amorphous films. They had useful levels of thermal stability, with relatively high glass-transition temperatures (276-334 °C), 10% weight-loss temperatures in excess of 500 °C, and char yields at 800 °C in nitrogen higher than 60%. Cyclic voltammograms of the polyimide films cast on the indium-tin oxide (ITO)-coated glass substrate exhibited two reversible oxidation redox couples at 0.70-0.74 V and 1.05-1.08 V vs. Ag/AgCl in acetonitrile solution. The polyimide films revealed excellent stability of electrochromic characteristics, with a color change from colorless or pale yellowish neutral form to green and blue oxidized forms at applied potentials ranging from 0.0 to 1.3 V. These anodically coloring polymeric materials exhibited high optical contrast of percentage transmittance change (Δ%T) up to 44% at 413 nm and 43% at 890 nm for the green coloration, and 98% at 681 nm for the blue coloration. After over 50 cyclic switches, the polymer films still exhibited good redox and electrochromic stability.     

Conductance measurements of some electrolytes in N, N-dimethylformamide and in binary mixtures of N, N-dimethylformamide containing some organic bases and acetonitrile

Equivalent conductances of Bu₄ NBPh₄ Bu₄ NNO₃ NI, KI, NaBPh₄, Ph₄ PCl and AgNO₃ in N, N-dimethylformamide (DMF) and in binary mixtures of DMF containing 1 mol % of pyridine (Py), α-picoline (α-Pic), β-picoline (β-Pic), γ-picoline (γ-Pic) and acetonitrile (AN) have been measured at 25°C. Limiting ion conductances have been obtained by the method of Fuoss and coworkers [J. Solution Chem. 3, 45 (1974); ibid. 4 (1975)]. The actual solvated radii of ions, calculated using limiting ion conductances show that Ag⁺ possesses very strong preferential solvation by all the bases and by AN, while Na⁺ and K⁺ do not have preferential solvation by these solvents. DMF, though, is more basic than AN, but it does not show preferential solvation for Ag⁺ while AN does.     

Interpolymer complexes between hydrophobically modified poly(methacrylic acid) and poly(N-vinylpyrrolidone)

The formation and structure of interpolymer complexes between amphiphilic multi-block copolymers—poly(dimethylsiloxane-N-vinylpyrrolidone) and poly(dimethylsiloxane-methacrylic acid)—were studied using potentiometric and conductometric titration, viscometry and fluorescence spectroscopy. The ratio of hydrophobic/hydrophilic groups in the copolymer and the molecular weight of each sequence were varied in order to establish the influence of these parameters on the interactions between the components. The complexation between pairs of block copolymers, of block copolymers and homopolymers as well as between homopolymer pairs was considered. The complexes were formed through the hydrogen bonding. They have a compact structure with a non-stoichiometric composition of pyrrolidone to methacrylic acid groups ratio around 0.6-0.7, with the exception of complexes formed between pairs of homopolymers and of copolymers with the shortest siloxane block. The difference between the new complexes and the ones formed from homopolymers, with equimolar composition, is explained by the spatial non-complementarity of the copolymers having a ‘flower-like’ structure in aqueous solutions.     

The electrochemical oxidation of N, N′-diphenylbenzidine in acid,neutral and base acetonitrile solutions

N, N′-diphenylbenzidine and its oxidation products, the corresponding semiquinoneimine and diphenoquinonediimine, dissolve as neutral molecules, monocations,or dications, respectively, depending on the acidity or basicity of the acetonitrile solution containing lithium perchlorate. Equilibrium constants were determined for various protolytic and redox reactions from spectrophotometric and electrochemical measurements. In neutral solution, oxidation of the benzidine to the diphenoquinonediimine is followed by synproportionation to the semiquinoneimine. The rate constant was determined by spectrophotometry at the rotating disc electrode with optically transparent ring. In acid solutions, the benzidinium cation is oxidized to the semiquinoneimine dication which after deprotonization oxidizes the semiquinoneimine cation yielding the stable final products, the diphenoquinonediimine dication and the benzidinium cation. Deprotonization of the benzidinium cation is very slow, but the rate constant for deprotonization of the benzidinium dication could be measured. In alkaline solutions, the semiquinoneimine cation produced by oxidation of the benzidine is deprotonized by the base and oxidizes the resulting neutral semiquinoneimine yielding the benzidine and the diphenoquinonediimine cation which is further deprotonized to the neutral diphenoquinonediimine as the final products. The rate constant for deprotonization of the semiquinoneimine is larger for diethylamine as the base than for 2, 6-lutidine.     

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.