Effects of synthesis-solvent on swelling and elastic properties of poly(N-isopropylacrylamide) hydrogels

Thermosensitive N-isopropylacrylamide (NIPA) hydrogels were synthesized by a free radical copolymerization with N,N′-methylenebisacrylamide (MBAA) in four solvents: water, ethanol, acetone and N,N-dimethylformamide. The swelling and elastic properties of the hydrogels were affected by the synthesis-solvents; the hydrogels (e.g. NIPA/MBAA = 1000/50 mol/m³-pre-gel solution) synthesized in water have smaller swelling volume and larger shear modulus at 10 °C than those synthesized in amphiphilic solvents. The network structure of hydrogels was estimated in terms of the conversion and two sorts of effective crosslinking density based on the Flory theory and the concentration of crosslinker. The hydrogels synthesized in water can have the microscopic inhomogeneous network arising from the entanglement of polymer chains, while the hydrogels synthesized in amphiphilic solvents can have the homogeneous network arising from the polymer concentration lower than the pre-gel solution and can be similar in network structure to the lightly crosslinked hydrogel synthesized in water.     

Thermo-responsive extraction of cadmium(II) ion with TPEN-NIPA gel. Effect of the number of polymerizable double bond toward gel formation and the extracting behavior

N,N,N′,N′-(Tetrakis-2-pyridylmethyl)ethylenediamine (TPEN) derivatives bearing the different number (1-4) of a double bond moiety on the pyridine ring are synthesized and subjected to copolymerization with N-isopropylacrylamide in the presence of AIBN. The obtained poly(TPEN-NIPA) gels show thermo-responsive swelling/shrinking behaviors and are employed for the extraction of cadmium(II) ion from the aqueous solution to examine the relationship of the gel characteristics and the extraction performance. The polymer gels composed of the TPEN derivative bearing three or four double bonds exhibit temperature-dependent change of swelling and shrinking in water. These gels extract Cd^II ion efficiently from the aqueous solution in the swelling state at 5 °C, while little extraction was observed at 45 °C with shrinking.     

Rapid swelling and deswelling in cryogels of crosslinked poly(N-isopropylacrylamide-co-acrylic acid)

Thermally sensitive hydrogels of poly[N-isopropylacrylamide (NIPA)-co-acrylic acid (AA)] hydrogels with N,N-methylene bisacrylamide (BIS) as crosslinker have been synthesized via a two-step procedure in which, the initial polymerisation is conducted for various times at 18 °C, this step being followed by polymerisation for one fixed time at −22 °C. The gravimetrically determined rates of swelling/deswelling for these materials termed “cryogels” prepared by this two-step polymerisation are much higher than those for the same type of hydrogel prepared via conventional methods (30 °C for 24 h). For example the time for the former xerogel to take up 70% of its final water content at 25 °C is just 18 min, compared with a time 300 min for the latter to attain the same uptake of water. During deswelling (shrinking) at 50 °C, which is above the lower critical temperature, the hydrogel loses 60 and 90 wt% water in 1 and 10 min respectively, compared to a timescale for the corresponding crosslinked copolymers prepared by conventional methods of about 100 min for 50 wt% water loss. A third type of hydrogel was made by a cold treatment (CT), for which the hydrogel prepared by conventional polymerization was stored in the frozen state. The swelling rate of these CT xerogels was the same as that for xerogels prepared by conventional polymerization, but the deswelling rate of the former was higher than that of the latter; for example, during deswelling, a loss of 90% water is attained within a few minutes.Scanning electron microscopy, digital photographs and flotation experiments together with swelling ratio studies reveal that the polymeric network of cryogel produced by the two-step polymerization method is characterized by an open structure with more pores and higher swelling ratio but lower mechanical strength compared to the conventional hydrogels. Such rapid response hydrogels have potential applications in separation and drug release technologies for example.     

Rapid swelling and deswelling in cryogels of crosslinked poly(N-isopropylacrylamide-co-acrylic)

Thermally sensitive hydrogels of poly[N-isopropylacrylamide (NIPA)-co-acrylic(AA)] hydrogels with N,N-methylene bisacrylamide (BIS) as crosslinker have been synthesised via a two-step procedure in which, the initial polymerisation is conducted for various times at 18 °C, this step being followed by polymerisation for one fixed time at −22 °C. The gravimetrically determined rates of swelling/deswelling for these materials termed “cryogels” prepared by this two-step polymerisation are much higher than those for the same type of hydrogel prepared via conventional methods (30 °C for 24 h). For example the time for the former xerogel to take up 70% of its final water content at 25 °C is just 18 min, compared with a time 300 min for the latter to attain the same uptake of water. During deswelling (shrinking) at 50 °C, which is above the lower critical temperature, the hydrogel loses 60 and 90 wt.% water in 1 and 10 min respectively, compared to a timescale for the corresponding crosslinked copolymers prepared by conventional methods of about 100 min for 50 wt.% water loss. A third type of hydrogel was made by a cold treatment (CT), for which the hydrogel prepared by conventional polymerisation was stored in the frozen state. The swelling rate of these CT xerogels was the same as that for xerogels prepared by conventional polymerisation, but the deswelling rate of the former was higher than that of the latter; for example, during deswelling, a loss of 90% water is attained within a few minutes.Scanning electron microscopy, digital photographs and flotation experiments together with swelling ratio studies reveal that the polymeric network of cryogel produced by the two-step polymerisation method is characterised by an open structure with more pores and higher swelling ratio but lower mechanical strength compared to the conventional hydrogels. The polymerisation was taking place on moderate freezing condition and the hydrogel was stored in a frozen state and subsequent thawing of polymer to be very useful the acceleration the response rate of this kind hydrogels. Such rapid response hydrogels have potential applications in separation and drug release technologies for example.     

Radiation induced synthesis and swelling characterization of thermo-responsive N-isopropylacrylamide-co-ionic hydrogels

Two-component thermo-responsive hydrogels poly(N-isopropylacrylamide)-co-vinylbenzyltrimethylammonium chloride (NIPA-co-VBT) and poly(N-isopropylacrylamide)-co-p-sodium styrene sulphonate (NIPA-co-SSS) were prepared by using high energy gamma radiation. The gels were characterized by Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and sol-gel analysis. The presence of ionic monomers in NIPA leads to lower gel content. Introduction of ionic components in the matrix enhanced swelling extent but caused slower volume transition. The swelling studies in alcohol indicated that swelling extent was function of polarity of the swelling medium and all gels followed the order water > methanol > ethanol > iso-propanol. In mixed co-solvents (water-alcohol), they exhibit complex reentrant behavior. The co-polymer gels containing VBT swelled faster and to higher extent than those containing SSS. The dynamic swelling studies indicated that diffusion of water in PNIPA gel shifts from Fickian for PNIPA to anomalous for NIPA-co-ionic gels also the mean swelling time (MST) decreases for gels containing ionic monomers.     

Swelling–shrinking behaviors of poly(N-isopropylacrylamide) and poly(N-n-propylacrylamide) gels prepared by chemical and radiation crosslinking methods

The swelling and shrinking kinetics of thermosensitive gels based on N-isopropylacrylamide (NiPAAm) and N-n-propylacrylamide (NnPAAm) were studied. Four gels cylindrical in shape were prepared by two different methods: γ-ray irradiation to aqueous solutions of poly(NiPAAm) (PNiPAAm) or poly(NnPAAm) (PNnPAAm) and redox polymerization of NiPAAm or NnPAAm monomer using N,N′-methylenebisacrylamide as a crosslinker. There were a few differences in the swelling kinetics among these gels. However, a marked difference was observed in the shrinking processes, the rate of which was faster in the order of radiation-crosslinked PNiPAAm gel > radiation-crosslinked PNnPAAm gel > chemically crosslinked PNnPAAm gel > chemically crosslinked PNiPAAm gel. This difference was discussed in terms of the microscopic structure of the gels, which was studied by light scattering techniques. It was found that the static inhomogeneities frozen in the chemically and radiation-crosslinked gels play a key role in their shrinking kinetics.     

Diffusion of poly(ethylene glycol) and ectoine in NIPAAm hydrogels with confocal Raman spectroscopy

The diffusion behavior of poly(ethylene glycol) (PEG) in N-isopropylacrylamide (NIPAAm) hydrogels was investigated using confocal Raman spectroscopy with regard to temperature (25°C, 30°C and 35°C), PEG concentration (10 and 40?wt.%), PEG molecular weight (2,000 and 12,000?g/mol) and addition of the compatible solute ectoine (0.1 and 2?wt.%). Swelling and shrinking of the gels was observed by means of confocal Raman spectroscopy. The swelling behavior of NIPAAm gels in aqueous solutions of PEG and ectoine was found to resemble the swelling behavior in pure water with regard to temperature, i.e., the gel shrinks with increasing temperature. However, the presence and concentration of PEG and ectoine influence the swelling behavior by lowering the volume phase-transition temperature of the gel and facilitating shrinking. In some cases, a re-swelling of the gel was observed after the initial shrinking at the onset of PEG diffusion, which can be explained by PEG changing the chemical potential in the gel phase as it diffuses into the sample allowing the water to re-enter. The expulsion of water from the gel during shrinking and the so-caused increase of PNIPAAm and PEG concentrations in some cases led to the PEG diffusion seemingly being faster in more shrunken gels despite of their higher diffusion resistance.     

Dynamic deswelling studies of poly(N-vinyl-2-pyrrolidone/itaconic acid) hydrogels swollen in water and terbinafine hydrochloride solutions

Deswelling kinetics of water and terbinafine hydrochloride adsorbed poly(N-vinyl-2-pyrrolidone/itaconic acid) P(VP/IA) hydrogels were investigated. Hydrogels were prepared by irradiating the ternary mixture of VP/IA and cross-linking agent ethylene glycol dimethacrylate (EGDMA) in water by γ-rays at ambient temperature. Hydrogels swelled in pure water and terbinafine hydrochloride (TER-HCl) solutions at room temperature and deswelling or water loss were investigated between 4 and 45 °C temperature range and on human skin. The influence of IA content, % swelling, temperature and TER-HCl content on the water loss from gel matrix were investigated. Induction time for 80% water loss from hydrogel systems are found to increase from 9.6 to 21.2 h by increasing IA content in the gel system at 25 °C and decreased by 11 h with addition of TER-HCl in the gel system. Kinetic analyses had shown that the basic properties affecting the water loss behavior of these hydrogels are the IA and TER-HCl content and temperature of the medium.     

Synthesis and functionalities of poly(N-vinylalkylamide). VI. A novel thermosensitive hydrogel crosslinked poly(N-vinylisobutyramide)

A novel thermosensitive poly(N-vinylisobutyramide)(polyNVIBA) hydrogel was prepared by the copolymerization of N-vinylisobutyramide (NVIBA) with butylene-bis-NVA(B-BNVA) as a crosslinker in a high yield. The swelling transition behavior was examined in comparison with poly(N-isopropylacrylamide)(polyNIPAAm) hydrogel. The resulting polyNVIBA hydrogel clearly showed a swelling transion in water at ca. 41°C. To control the transition temperature (T_t) of the gel, crosslinked copolymers of NVIBA and N-vinylacetamide (NVA) were prepared and compared with copolymers of N-isopropylacrylamide(NIPAAm) and NVA. The incorporation of NVA led to a higher swelling transition temperature. T_t of poly(NVIBA-co-NVA) gels was almost the same as those in water-soluble poly(NVIBA-co-NVA). The responses for a swelling transition of polyNVIBA and poly(NVIBA-co-NVA) gels were sharp in comparison to polyNIPAAm gels. PolyNVIBA and poly(NVIBA-co-NVA) gels desorbed 98% of water above T_t. The characteristic and the mechanism of the phase transition on the hydrogels were discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3377–3384, 1997     

Lower critical solution temperature control and swelling behaviour of physically crosslinked thermosensitive copolymers based on N-isopropylacrylamide

In this contribution we have developed thermosensitive polymer matrices based on N-isopropylacrylamide (NIPAAm). Preparation of the hydrogels involved photopolymerisation of a combination of NIPAAm, 1-vinyl-2-pyrrolidinone (NVP) and distilled water, in appropriate amounts and contained a UV-light sensitive initiator called 1-hydroxycyclohexylphenylketone. As NIPAAm monomer could be readily dissolved in mixtures of liquid NVP and distilled water, the use of organic solvents was not required in the polymerisation process. Furthermore, chemical crosslinking agents are not needed in the synthesis. By alternating the feed ratio, hydrogels were synthesised to have lower critical solution temperatures (LCST) in the vicinity of 37 °C. This ability to shift the phase transition temperature of the gels provides excellent flexibility in tailoring transitions for specific uses. The transition temperature of the pseudo gels was established using cloud point measurement and modulated differential scanning calorimetry (MDSC). The chemical structure of the xerogels was characterised by means of Fourier transform infrared spectroscopy (Ftir), while swelling experiments in distilled water indicate that the swelling and dissolution behaviour of the gels is strongly temperature dependent.     

The synthesis, characterisation, phase behaviour and swelling of temperature sensitive physically crosslinked poly(1-vinyl-2-pyrrolidinone)/poly(N-isopropylacrylamide) hydrogels

Physically crosslinked complexes of polyvinyl pyrrolidinone-poly (N-isopropylacrylamide) (PVP-PNIPAAm) were prepared by photopolymerisation from a mixture of the monomers 1-vinyl-2-pyrrolidinone and N-isopropylacrylamide. IR spectroscopy and calorimetry were used to characterise the resulting xerogels. By alternating the monomer feed ratio, copolymers were synthesised to have their own distinctive lower critical solution temperature (LCST). The transition temperature of the gels was established using cloud point measurement and modulated differential scanning calorimeter (MDSC). This ability to shift the phase transition temperature of the copolymers provides excellent flexibility in tailoring transitions for specific uses. Swelling experiments were performed on the copolymer disks in distilled water at varying temperatures to establish the behaviour of the gels above and below phase transition temperature. The results obtained show that below transition temperature, the gels are water soluble but above this temperature they are slightly less water soluble; significantly less water soluble; or water insoluble; depending on the composition and LCST of the gel.     

Effect of filler networking on the response of thermosensitive composite hydrogels

Several composite hydrogels of poly(N-isopropylacrylamide) (pNIPAAm) with sodium montmorillonite (NaMM) have been synthesized using a fixed polymer/NaMM ratio (4:1 wt./wt.), but various monomer concentrations, in order to obtain hydrogels with different degrees of swelling, and thus different clay contents in the swollen state. For comparison, unfilled pNIPAAm gels have been also prepared at the same concentrations. The equilibrium swelling behaviour of the gels has been studied both in the swollen and in the shrunk state. In the swollen state, the polymer volume fraction increases with the initial monomer concentration C₀. In the shrunk state, the polymer fraction in pNIPAAm hydrogels is dependent on the specimen size and on C₀, whereas in the composite gels a constant polymer content is observed. When subjected to stepwise heating from 25 to 45 °C, unfilled gels undergo only poor deswelling. By contrast, complete deswelling takes place in composite gels. The latter show half-shrinking times varying over two orders of magnitude, depending on the monomer concentration and on the procedure followed to disperse NaMM, which determine the overall dispersion state of the filler, as evidenced by transmission electron microscopy (TEM). In particular, TEM observations show clay networking above a percolation threshold near 2.5 wt.% of NaMM. The effect of the incorporation of clay on the response to thermal stimuli is discussed in terms of the ability of NaMM to hinder the hydrophobic association of pNIPAAm segments and in terms of its dispersion state. It is suggested that, above the percolation threshold, NaMM forms a hydrophilic, physical network, through which water can flow also above the volume transition temperature, where pNIPAAm acquires a hydrophobic character.     

Formation of Fe2+-phenanthroline complexes in the volume of hydrogel

The interaction of weakly crosslinked gels of poly(methacrylic acid) and a nonionic gel based on N-vinylcaprolactam and N-vinylimidazole with Fe²⁺ ions in aqueous solutions has been studied. A comparative study of the conformational state and of the absorption ability of the gels for their interaction with iron and ferroin ions has been performed. It has been shown that Fe²⁺ ions are efficiently absorbed by both poly(methacrylic acid) gels and the gels based on N-vinylcaprolactam and N-vinylimidazole. In this case, the poly(methacrylic acid) gels undergo contraction, while nonionic gels derived from N-vinylcaprolactam and N-vinylimidazole experience swelling. During the interaction of gels containing immobilized Fe²⁺ ions with phenanthroline, the efficient absorption of the complexon and the formation of Fe²⁺-phenanthroline complexes in the gel volume take place, thus inducing the contraction of gels. The interaction of poly(methacrylic acid) gels with ferroin is accompanied by the absorption of the complex, the formation of the tertiary complexes, and the collapse of the gel. The efficiency of formation of tertiary complexes in the gel volume is independent of their preparation procedure.     

Synthesis and characterization of thermoresponsive poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide-co-<Emphasis Type="Italic">N</Emphasis>-hydroxymethylacrylamide-co-2-hydroxyethyl methacrylate) hydrogels

Thermoresponsive hydrogels based on N-isopropylacrylamide, N-hydroxymethylacrylamide, and 2-hydroxyethyl methacrylate, poly(NIPAM–co-NHMAAm–co-HEMA), have been synthesized and their swelling—deswelling behavior studied as a function of NIPAM concentration, NIPAM/NHMAAm and NIPAM/HEMA mole ratio, and total monomer concentration. Copolymers varying in composition have been obtained by redox copolymerization of these three monomers. Temperature has been changed in the ranges from 4 to 70 °C at fixed pH and total ionic strength. Equilibrium swelling ratio, dynamic swelling ratio, and dynamic deswelling ratio were evaluated for all hydrogel systems. The equilibrium swelling ratios of the copolymeric gels decrease with increasing NHMAAm and HEMA content. The formation of the intermolecular hydrogen bonding between hydroxyl and amido groups decreases the hydrophilic group numbers of the gel and the affinity of the gel towards water decreases. The copolymer gels also showed rapid volume transitions with time. The time required for equilibrium shrinking increased with increasing NHMAAm and HEMA content in the gel.     

Synthesis of N,N-dialkylaminobenzonitriles and halo-(N,N-dialkyl)benzamidines by reaction of halobenzonitriles with lithium amides

3- and 4-N,N-Dialkylaminobenzonitriles and 4-chloro-(N,N-dialkyl)benzamidines were isolated by reacting 4-chlorobenzonitrile with hindered lithium amides under thermodynamic (0 °C) and kinetic control conditions (−78 °C), respectively. As previously reported, a benzyne mechanism seems to be confirmed since N,N-dialkylaminobenzonitriles are formed. Only benzamidines were isolated in fair to high yields at both 0 °C and −78 °C with non-hindered lithium amides. Exploitation and mechanistic rationale of the reaction of different halobenzonitriles are also reported.     

Studies on preparation and properties of NIPAAm/hydrophobic monomer copolymeric hydrogels

A series of thermoreversible copolymeric hydrogels with various molar ratios of N-isopropylacrylamide (NIPAAm) and hydrophobic monomers such as 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (OFPMA) and n-butyl methacrylate (BMA) were prepared by emulsion polymerization. The effect of hydrophobic monomer on the swelling behavior and mechanical properties of the present copolymeric hydrogels was investigated. Results showed that the equilibrium swelling ratio and critical gel transition temperature (CGTT) decreased with an increase of the content of hydrophobic monomer, but the gel strength of the gel increased with an increase of the content of hydrophobic monomer. Due to stronger hydrophobicity of OFPMA, the NIPAAm/OFPMA copolymeric hydrogels had lower swelling ratios and higher gel strengths than NIPAAm/BMA copolymeric gels.     

Characterisation of N-vinyl-2-pyrrolidone-based hydrogels prepared by a Diels-Alder click reaction in water

N-vinyl-2-pyrrolidone-based hydrogels were prepared by the Diels-Alder reaction in water for the first time. Copolymers of N-vinyl-2-pyrrolidone(VP) and furfuryl methacrylate(FM) were synthesised by free radical polymerisation in toluene at 70 °C by using 2,2′-azobisisobutyronltrile as an initiator. Polymeric dienophile (PEG-AMI) was prepared from N-alaninyl maleimide (AMI) and poly(ethylene glycol) (PEG) by using N,N′-dicyclohexylcarbodiimide (DCC) as a dehydrating agent. The prepared dienes and dienophile were then dissolved in water and mixed, leading to gelation via Diels-Alder reaction after some time. The gelation times of different copolymers and PEG-AMI in different solvents and at different temperatures were measured by the vial inversion method, and the swelling behaviour of dried hydrogels was studied using a general gravimetric method. The gelation time was shorter in water than in organic solvents, and the gelation time decreased with the increase of temperature and FM content in copolymers. Conversely, the swelling ratios increased with the decrease of temperature and FM content in the copolymers. Disassembly experiments suggested that N,N-dimethylformamide (DMF) could accelerate the retro-DA reaction.     

Fast synthesis of temperature-sensitive PNIPAAm hydrogels by microwave irradiation

A novel method, microwave irradiation synthesis, is proposed for the preparation of thermo-sensitive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels. The PNIPAAm hydrogels were separately synthesized by using microwave irradiation method and water-bath heating method. Chemical groups, lower critical solution temperature (LCST) and surface morphology of these PNIPAAm hydrogels were characterized by FT-IR, DSC and SEM. Swelling ratios of the gels were measured gravimetrically in the temperature range from 10.0 to 60.0 °C. Results showed that (1) the use of microwave irradiation can greatly shorten the reaction time required for PNIPAAm hydrogel synthesis from several hours to several minutes in comparison with water-bath heating method, and obviously improve the yields of the PNIPAAm gels, which were up to 99% after a short reaction time; (2) SEM micrographs and textural measurement revealed that the gels synthesized using microwave irradiation had more porous structure, and their average pore sizes and specific surface areas were larger than those of the gels synthesized using water-bath heating method; and (3) the PNIPAAm hydrogels synthesized using microwave irradiation had much higher swelling ratios at 10.0 °C below the LCST, and had lower swelling ratio at 60.0 °C above the LCST compared to the hydrogels synthesized by water-bath method.     

Some properties of networks produced by the Diels-Alder reaction between poly(styrene-co-furfuryl methacrylate) and bismaleimide

Poly (styrene-co-furfuryl methacrylate) networks were prepared by the Diels-Alder (D-A) reaction in solution at 25 °C between the linear copolymer and bismaleimide (BM). The resultant crosslinked polymers were swollen to equilibrium in toluene at 25 °C and swelling properties were studied by gravimetric and dimensional measurements. The swelling behaviour of these organogels depended on the composition of the copolymer and the concentration of BM used. Shear and Young’s moduli and the effective crosslinking densities (ν_e) were determined by compression (stress)-strain measurements. The theoretical crosslinking density was higher than the ν_e for all the crosslinked copolymers. An endothermic peak without T_g was observed on the DSC curve on heating the dry crosslinked polymer. On reheating a T_g at ≈98 °C was found, which is attributed to presence of linear copolymer produced by the retro D-A reaction in the first heating. The thermal stability of a crosslinked copolymer under nitrogen and air showed differences with the stability of the linear copolymer. The increase in viscosity of the solution during the D-A reaction was followed with time, for initial linear copolymers of different molecular weights. It was found that onset of gelation increased to longer reaction times the lower the molecular weight of copolymer.     

Sol–gel synthesis of macro–mesoporous titania monoliths and their applications to chromatographic separation media for organophosphate compounds

We have developed a method of independently tailoring the macro- and mesoporous structures in titania (TiO₂) monoliths in order to achieve liquid chromatographic separations of phosphorous-containing compounds. Anatase TiO₂ monolithic gels with well-defined bicontinuous macropores and microstructured skeletons are obtained via the sol–gel process in strongly acidic conditions using poly(ethylene oxide) as a phase separator and N-methylformamide as a proton scavenger. Aging treatment of the wet gels in the mother liquor at temperatures of 100–200 °C and subsequent heat treatment at 400 °C allow the formation and control of mesoporous structures with uniform pore size distributions in the gel skeletons, without disturbing the preformed macroporous morphology. The monolithic TiO₂ rod columns with bimodal macro–mesoporous structures possess the phospho-sensitivity and exhibit excellent chromatographic separations of phosphorus-containing compounds.