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.     

Copolymer hydrogels based on N-isopropylacrylamide and itaconic acid

In this study, the swelling behaviour of copolymer hydrogels of N-isopropylacrylamide (NIPAM) and itaconic acid (IA) in response to temperature and pH value of the external media was studied. The equilibrium degree of swelling for PNIPAM and PNIPAM/IA copolymers was greater at 25 °C than at 37 °C. The degree of swelling was low at low pH values. As the degree of ionization increased above the nominal pK_a values of IA, the increased hydrophilicity resulted in larger degrees of swelling. At 37 °C, the PNIPAM hydrogel and some copolymers show anomalous swelling behaviour, i.e. the overshooting effect, in buffered solutions of certain pH values. A swelling-deswelling study showed that the deswelling process of the hydrogels was faster then the swelling process. According to dynamic swelling studies, the diffusion exponent and the diffusion coefficient both increase with increasing content of IA.     

Super porous organic-inorganic poly(N-isopropylacrylamide)-based hydrogel with a very fast temperature response

Porous organic-inorganic (O-I) hydrogels showing a very fast temperature response, including very fast reswelling were prepared: only 6 s are needed for 72% deswelling (gel collapse) as well as for 72% reswelling. Both deswelling and reswelling are practically complete in 14 s. The gels were prepared from N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (BAA) and tetramethoxysilane (TMOS) by simultaneous radical polymerization and hydrolytic polycondensation of TMOS. The syntheses were carried out at temperatures below the lower critical solution temperature (LCST) of poly(NIPA) in two steps: during the first stage the temperature was held at T = +15 °C and during the second the temperature was lowered below the freezing point of the reaction mixture, T = −18 °C. The ice crystals, which grew during the second stage, served as the pore-forming agent. The best samples were obtained if the second stage was started shortly before the gel point of the reaction mixture. The introduction of the inorganic phase (silica) is necessary for the ability of fast reswelling and also results in a strong improvement of the hydrogels' mechanical properties, while the maximum swelling degree remains nearly unaffected.     

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.     

Synthesis and properties of thermosensitive, crown ether incorporated poly(N-isopropylacrylamide) hydrogel

A crown ether derivative (4′-allyldibenzo-18-crown-6, CE) was covalently incorporated into the network of temperature sensitive poly(N-isopropylacrylamide) (PNIPA) hydrogels by copolymerization in a mixed solvent of water and tetrahydrofuran (H₂O/THF). The poly(N-isopropylacrylamide-co-4′-allyldibenzo-18-crown-6) (poly(NIPA-co-CE)) hydrogels exhibited dramatically faster deswelling rates than normal PNIPA hydrogels at a temperature (50 °C) above their lower critical solution temperatures. The effect of the solvent component ratio in the mixed solvent during the copolymerization on the swelling properties of the poly(NIPA-co-CE) hydrogel was investigated. The thermosensitive poly(NIPA-co-CE) hydrogels have potential applications in the extraction of cations and separation of chiral drugs.     

Unusual swelling behavior of polymer-clay nanocomposite hydrogels

The swelling behavior and the elastic properties of nanocomposite hydrogels have been investigated. The hydrogels were prepared by free-radical polymerization of the monomers acrylamide (AAm), N,N-dimethylacrylamide (DMA), and N-isopropylacrylamide (NIPA) in aqueous clay suspensions at 21 °C. Laponite with a radius of gyration in distilled water of 20 nm was used as clay particles in the hydrogel preparation. The reactions with AAm monomer were carried out in the presence of the chemical crosslinker N,N′-methylenebis(acrylamide) (BAAm). It was found that the volume of nanocomposite hydrogels immersed in water rapidly increases and attains a maximum value after about one day. Surprisingly, further increase in the swelling time results in the deswelling of the gels until they reach a limiting swelling ratio after about 5 days. This unusual swelling behavior is observable only when the clay concentration in the hydrogel is above the overlap threshold c^∗. Swelling measurements combined with the elasticity tests show that the effective crosslink density first decreases, but then increases with increasing time of swelling of the hydrogels. The results were explained in terms of the rearrangements of the highly entangled polymer chains and clay particles during the gel volume change.     

Rapid pH/temperature-responsive cationic hydrogels with dual stimuli-sensitive grafted side chains

Dual temperature- and pH-sensitive comb-type grafted cationic hydrogels are successfully synthesized by grafting polymeric chains with freely mobile ends, which are composed of both N-isopropylacrylamide (NIPAM) segments and N,N-dimethylamino ethyl methacrylate (DMAEMA) segments, onto the backbone of crosslinked poly(NIPAM-co-DMAEMA) networks. Equilibrium and dynamic swelling/deswelling properties of the prepared hydrogels responding to pH and/or temperature are investigated. The prepared hydrogels demonstrate a lower critical solution temperature (LCST) at about 34 °C and a pK_a value at about pH 7.3. At lower pH and lower temperature, both the swelling degree and the swelling rate of the comb-type grafted hydrogel are larger than those of the normal-type crosslinked hydrogel. The comb-type grafted poly(NIPAM-co-DMAEMA) hydrogel exhibits a more rapid deswelling rate than that of the normal-type hydrogel in response to a pH jump from 2.0 to 11.0 at a fixed temperature. The volume changes of the poly(NIPAM-co-DMAEMA) hydrogels are acute in a series of fixed buffer solutions with an abrupt increase of environmental temperature from 18 °C to a temperature higher than the LCST. The comb-type grafted poly(NIPAM-co-DMAEMA) hydrogels show quite fast shrinking behaviors in response to simultaneous dual temperature and pH stimuli. Drug-release in vitro from the prepared poly(NIPAM-co-DMAEMA) hydrogels is carried out when the environmental temperature and pH are changed synchronously. The results show that the model drug Vitamin B₁₂ is released much more rapidly from the comb-type grafted hydrogel than that from the normal-type hydrogel. The proposed dual temperature/pH-sensitive comb-type grafted cationic poly(NIPAM-co-DMAEMA) hydrogel in this study may find various potential applications, e.g., for fabricating rapid-response smart sensors, actuators, and chemical/drug carriers and so on.     

Dual thermo- and pH-sensitive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels with rapid response behaviors

Novel dual temperature- and pH-sensitive comb-type grafted poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AAc)) hydrogels were successfully prepared by grafting PNIPAM chains with freely mobile ends onto the backbone of a cross-linked P(NIPAM-co-AAc) network. The prepared comb-type grafted P(NIPAM-co-AAc) hydrogels exhibited a more rapid deswelling rate than normal-type P(NIPAM-co-AAc) hydrogels in ultrapure water in response to abrupt changes from 25 °C to 60 °C. The same was true in buffer solution with a pH jump from 7.4 to 2.0 at 25 °C. Unexpectedly, the comb-type grafted P(NIPAM-co-AAc) hydrogels showed abnormal shrinkage behaviors in a buffer solution when the temperature increased from 25 °C to 60 °C with a pH value fixed at 7.4 or 2.0. In a buffer solution of pH 7.4, when the environmental temperature jumped from 25 °C to 60 °C, the grafted comb-type hydrogels shrank slower than the normal-type hydrogels, while at pH 2.0, the gels shrank faster than the normal-type gels in the beginning, which was followed by a slower shrinking. Interestingly, the much quicker shrinkage of the comb-type grafted P(NIPAM-co-AAc) hydrogels was observed because of the cooperative thermo-/pH-responses when the simultaneous temperature and pH stimuli met from pH 7.4/25 °C to pH 2.0/60 °C. The results of this study provide valuable information regarding the development of dual stimuli-sensitive hydrogels with fast responsiveness.     

Superfast responsive ionic hydrogels with controllable pore size

A series of strong polyelectrolyte hydrogels was prepared from the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as the monomer and N,N′-methylene(bis)acrylamide (BAAm) as a crosslinker in aqueous solutions. The gel preparation temperature (T_prep) was varied between −22 and 25 °C. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at T_prep=−8 °C. The hydrogels prepared below −8 °C exhibit a discontinuous morphology consisting of polyhedral pores of sizes 30-50 μm, while those formed at higher temperatures have a non-porous structure. The pore size of the networks increased by decreasing the charge density of the hydrogels, while addition of low molecular weight salts into the gelation system reduced the size of the pores. Calculations based on the equilibrium between the ice and unfrozen gel phases in the reaction system at low temperatures explain the results of observations. It was also shown that the hydrogels formed below −8 °C exhibit superfast swelling properties as well as reversible swelling-deswelling cycles in water and acetone.     

Synthesis and characterization of pH- and temperature-sensitive hydrogel of N-isopropylacrylamide/cyclodextrin based copolymer

A reactive β-Cyclodextrin (β-CD) based monomer carrying vinyl carboxylic acid functional groups was synthesized via reaction of β-CD with maleic anhydride (MAH) in N,N-dimethylformamide (DMF) at 80 °C. By copolymerization of the monomer with N-isopropylacrylamide (NIPA), a novel hydrogel, poly(NIPA-co-MAH-β-CD) with pH and temperature sensitivities plus molecular inclusion function, was obtained using free radical polymerization in aqueous solution. The hydrogel's composition was determined by element analysis and infrared spectroscopy. Equilibrium swelling ratio (ESR) of hydrogels was tested under different environment of pH, temperature and ionic strength. The results indicated that ESR of hydrogels presents marked variations following the change of experimental conditions used.     

On the water swelling behaviour of poly(N-isopropylacrylamide) [P(N-iPAAm)], poly(methacrylic acid) [P(MAA)], their random copolymers and sequential interpenetrating polymer networks (IPNs)

Thermo- and pH-responsive stimuli hydrogels based on N-isopropylacrylamide (N-iPAAm) and methacrylic acid (MAA) have been synthesized and their swelling behaviour studied as a function of composition, pH and temperature. Copolymers varying in composition have been obtained by copolymerizing these two monomers and interpenetrating polymer networks (IPNs) of P(MAA) and P(N-iPAAm) by the sequential method. Temperature and pH have been changed in the ranges from 25 to 40 °C and from 2 to 9, respectively. The swelling behaviour of the hydrogels depends on the nature of the polymer and the environmental conditions, namely pH and temperature. Copolymer gels under basic conditions exhibit higher degree of swelling than the homopolymer ones. The disruption of the complexes dominates the kinetic swelling of MAA enriched gels under basic conditions. The hydrogen bond formation between carboxyl and amide groups has been made clear through the dynamic swelling behaviour of copolymers under acidic conditions. IPNs reduce their ability to swell in water with increasing P(N-iPAAm) content because of the formation of hydrophobic interpolymer complexes through hydrogen bonding. Lower critical solution temperature occurs only in the enriched N-iPAAm copolymers under acidic conditions when the MAA carboxyl groups are unionized.     

Synthesis of rapid responsive gels comprising hydrophilic backbone and poly(N-isopropylacrylamide) graft chains by RAFT polymerization and end-linking processes

A thermosensitive poly(N-isopropylacrylamide) (PNIPAM) grafted gel, which comprises hydrophilic backbone and freely mobile PNIPAM graft chains, was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization and end-linking processes. Functional PNIPAM bearing dithiobenzoate end group (-C(S)S-R) was prepared first, and then it was reacted with divinyl compounds to obtain gel. In order to adjust the composition of the gels, two divinyl compounds, N,N-methylenebisacrylamide (BIS) and poly(ethylene glycol) diacrylate (PEGDAC), were used. The cross-linking polymerization mechanism was proposed. The swelling and deswelling kinetics of the hydrogels were measured. The gels exhibit rapid deswelling kinetics. At the same time, they show rapid swelling kinetics within 30 min, whereas a conventional PNIPAM-co-PEG-co-BIS gel with the same feed composition requires more than 10 h to reach swelling equilibrium.     

Freezing as a path to build macroporous structures: Superfast responsive polyacrylamide hydrogels

Macroporous polyacrylamide (PAAm) hydrogels were prepared from acrylamide monomer and N,N′-methylene(bis)acrylamide (BAAm) crosslinker in frozen aqueous solutions. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at a gel preparation temperature of −6 °C. The hydrogels prepared below −6 °C exhibit a heterogeneous morphology consisting of pores of sizes 10–70 μm, while those formed at higher temperatures have a non-porous structure. PAAm networks with largest pores were obtained at −18 °C. The pore size of the networks increased while the thickness of the pore walls decreased by decreasing the monomer concentration. The hydrogels formed below −6 °C exhibit superfast swelling and deswelling properties as well as reversible swelling–deswelling cycles in water and in acetone, respectively.     

Phase diagram of a cellulose solvent: N-methylmorpholine-N-oxide-water mixtures

The phase diagram of the N-methylmorpholine-N-oxide-H₂O mixtures from 0 to 100% has been determined. Three crystalline hydrates have been identified, the already known monohydrate, a dihydrate and a hydrate composed of 8 water molecules per NMMO. The melting temperature of the 8H₂O-NMMO hydrate is −47 °C with a melting enthalpy of about 80 J/g. The region between 25 and 55% of water does not show any crystallisation, but a glass transition around −60 to −100 °C.     

Influence of the swelling history on the swelling kinetics of stimuli-responsive poly[(N-isopropylacrylamide)-co-(methacrylic acid)] hydrogels

The influence of the swelling history on the swelling behavior of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] P[(N-iPAAm)-co-(MAA)] random copolymers hydrogels synthesized by free radical polymerization in solution of N-iPAAm and MAA comonomers crosslinked with tetraethylene glycol dimethyl acrylate (TEGDMA) has been studied. The swelling behavior under pH 7 at 18, 29, 39 and 49 °C of this series of copolymers, previously soaked either at pH 2 or 7 has been investigated. The swelling kinetics of these two series of samples displays different behavior as function of the composition and temperature. However, the equilibrium swelling values only show slight dependences on the previous soaking pH and temperature. When samples are soaked at pH 7, then the swelling at pH 7 follows a first order kinetics, irrespective of the copolymer composition or the temperature at which the experiment has been carried out. In this case, the swelling process is very fast and depends only slightly on temperature. The first order rate constant increases with the MAA content in the hydrogel. Furthermore, the swelling rate of copolymer hydrogels soaked at pH 2, show strong dependence on composition and temperature. They follow an autocatalytic swelling kinetics due to the disruption of hydrogen bond arrangements. An initial slow water uptake is followed by an acceleration process, in which water molecules inside the gel help the next water molecules to come in. Two rate constants, a first-order rate constant and an autocatalytic one have been obtained from the kinetics analysis. They have revealed different temperature dependence which may be due to a balance between hydrophobic and hydrogen bond interactions. The temperature dependence of the swelling kinetics is stronger and more complex for copolymers treated under pH 2 than for copolymers soaked under pH 7.     

High optical transparency, low dielectric constant and light color of novel organosoluble polyamides with bulky alicyclic pendent group

Novel optically transparent, low dielectric and highly organosoluble alicyclic polyamides derived from bulky alicyclic diamine containing trifluoromethyl group on either side, 1,1-bis[4-(2-trifluoromethyl-4-aminophenoxy)phenyl]-4-tert-butylcyclohexane (BTFAPBC), were prepared. The polyamides were obtained in almost quantitative yields and showed inherent viscosity values between 0.55 and 0.72 dL g⁻¹ in DMAc solution. Most of the polyamides showed excellent solubility in polar solvents such as N-methyl-2-pyrrolidinone (NMP), N,N′-dimethyl acetamide (DMAc), N,N′-dimethyl formamide (DMF), pyridine, cyclohexanone, γ-butyrolactone and chloroform. The cut-off wavelength for polyamides ranged from 350 to 388 nm. Polyamides with alicyclic tert-butylcyclohexyl cardo and trifluoromethyl substituents exhibited low dielectric constants ranging from 3.29 to 3.98 (at 100 Hz) compared with commercially available polyamides [Amodel^®, 4.2-5.7 at 100 Hz]. Polyamides showed glass transition temperatures in the range of 244-266 °C and possessed a coefficient of thermal expansion (CTE) of 60-75 ppm °C⁻¹. Thermogravimetric analysis data showed that the polyamides were stable up to 430 °C and the 10% weight loss temperature was found to be in the range of 437-466 °C in nitrogen atmosphere. The polyamide films had a tensile strength in the range of 66-103 MPa, elongation at break in the range of 5-8%, and tensile modulus in the range of 1.5-2.2 GPa. Due to their properties, the polyamides could be considered as engineering plastic and photoelectric materials.     

Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d₀, at 20 °C was found to be strongly dependent on the preparation temperature, T_prep, where d and d₀ are the diameter of gel during observation and preparation, respectively. The value of d/d₀ was about 1.2 for T_prep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d₀ decreases stepwise to 1.46. This upturn in d/d₀ was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T_prep. Though the shrinking half-time, t_1/2, of gel was on the order of 500 min for T_prep≤20 °C, t_1/2 decreased to 2 min for T_prep≥26 °C. Hence, a rapid shrinking was attained by simply increasing T_prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.     

A study of hydrogel thermal-dynamics using Fourier transform infrared spectrometer

A rapid and reliable method was presented for studying hydrogel dynamics/kinetics. Two temperature-sensitive hydrogels, poly-N-isopropylacrylamide (poly(NIPAAm)) and the copolymer of N,N-diethylacrylamide and sodium methacrylate (molar ratio=97:3, poly(NDEAAm-co-MAA)) were synthesized. The thermal-behaviors of the gels were studied through the absorbance intensities of both swollen water and gel frame components, and the peak positions of amide band along heating/cooling pathways under dynamic Fourier transform infrared (FTIR) probing. The results showed that the lower critical solution temperature (LCST) of poly(NIPAAm) is about 33-35 °C, which is consistent with reported value of ∼34 °C. Compared to poly(NIPAAm), poly(NDEAAm-co-MAA) has relatively continuous volume phase transition, starting at ∼35 °C and a better thermal-reversibility with similar swelling and deswelling profiles over a larger temperature range (10-80 °C for poly(NDEAAm-co-MAA) vs. 10-33 °C for poly(NIPAAm)). The H-bonding water along phase transition was also studied, showing a less reversibility of poly(NIPAAm) compared to poly(NDEAAm-co-MAA). In addition, FTIR spectrometer was also used to study the volume changes of poly(NDEAAm-co-MAA) under variations in environmental salinity.     

Formation of macroporous poly(acrylamide) hydrogels in DMSO/water mixture: Transition from cryogelation to phase separation copolymerization

Poly(acrylamide) hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide and N,N′-methylenebis(acrylamide) at −18 ^oC in aqueous DMSO solutions of various composition. The hydrogels formed in the solvent mixture with less than 25% DMSO by volume have irregular large pores of about 10¹ μm in diameter, typical for macroporous networks created by the cryogelation technique. Non-porous hydrogels were obtained in solutions containing 25% DMSO, while at larger DMSO contents, the structure of the hydrogel networks consists of aggregates of microspheres, which looks as cauliflowers, typical for a macroporous network formed by reaction-induced phase separation mechanism. Swelling measurements show that fast responsive PAAm hydrogels can be obtained as the DMSO content in the mixed solvent is decreased or increased starting from 25 v/v%. The results were interpreted as the transition from cryogelation to the phase separation copolymerization due to the marked freezing point depression of the solvent mixture as well as due to the action of the mixed solvent as a poor solvating diluent at −18 ^oC. It was also shown that the initial temperature of the cryogelation reactions in water strongly affects the hydrogel properties. Hydrogels formed at an initial temperature T_ini of 0 ^oC were very tough and can be compressed up to about 100% strain without any crack development while those formed at T_ini = 21 ^oC were fragile.     

Functional bioengineering copolymers. II. Synthesis and characterization of amphiphilic poly(N-isopropyl acrylamide-co-maleic anhydride) and its macrobranched derivatives

Radical-initiated copolymerization of N-isopropyl acrylamide (NIPA) with maleic anhydride (MA) and macromolecular reactions of synthesized poly(NIPA-co-MA) with polyethyleneglycol (PEG with a methoxy chain end and molecular weight of 2000 g mol⁻¹) and polyethyleneimine (PEI with molecular weight of 2000 g mol⁻¹) have been studied as a way to obtain new reactive amphiphilic water-soluble polymers potentially useful as carriers for gene delivery. Structure, composition and thermal behaviour of synthesized copolymers and their macrobranched architectures are determined by FTIR, ¹H and ¹³C NMR spectroscopy, elemental (N content) and chemical (acid number) analysis and differential scanning calorimetry, differential thermal and thermal gravimetric methods. It is shown that synthesized copolymers with given composition have low critical solution temperature (LCST) in the range of 30.2-46.4 °C at pH values of 4.0-7.4, which suggest the possibility of their biomedical applications.