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.     

Swelling behavior of poly(acrylamide)/clay nanocomposite hydrogels in acrylamide aqueous solution

The swelling behaviors of poly(acrylamide) (PAAm)/clay nanocomposite hydrogels (hereinafter abbreviated as NC gels) in acrylamide (AAm) aqueous solution have been investigated. As‐prepared PAAm/clay hydrogels (S‐M gels) were posttreated by immersing them in AAm aqueous solution. It was found that the swelling ratio of the NC gels increased greatly when the concentration of the solution is below a critical concentration (c*), whereas the gels were disintegrated in the solution when the concentration of the solution is above the c*. Some disc‐like particles were found in the AAm solution accompanying with the unusual swelling behaviors. This unusual swelling behavior is resulted from the change of network structure of the NC gels in AAm aqueous solution, which was further convinced by transmission electron microscopy and element analyses. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Investigation of ZnO‐release behavior of poly(N‐isopropylacrylamide‐co‐maleic acid)/ZnO composite hydrogels by differential pulse polarography

Poly(N‐isopropylacrylamide‐co‐maleic acid)‐zinc oxide [P(NIPA/MA)/ZnO] composite hydrogels, containing a definite amount of ZnO, were prepared from N‐isopropylacrylamide (NIPA) and maleic acid (MA) monomers with 0–31.15 mol % MA in aqueous saturated zinc oxide solutions by radiation‐induced polymerization and gelation using γ rays from a ⁶⁰Co source. The amounts of released ZnO from these composite hydrogels were determined by differential pulse polarography (DPP) using the characteristic peaks obtained at about –1,000, ?1,050, and –1,300 mV at pH 2.0, 5.5, and 7.05, respectively. It was found that the ZnO‐release behavior of P(NIPA/MA)/ZnO composite hydrogels depended strongly on the MA content and pH of the medium.     

Influence of the initiator system on the spatial inhomogeneity in acrylamide‐based hydrogels

The effect of the initiator system used in the gel preparation on the spatial inhomogeneity in poly(acrylamide) (PAAm) and poly(N,N‐dimethylacrylamide) (PDMA) hydrogels was investigated by static light scattering and elasticity measurements. The hydrogels were prepared by free‐radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N‐dimethylacrylamide (DMA) with N,N′‐methylenebisacrylamide as a crosslinker. Two different redox‐initiator systems, ammonium persulfate (APS)–N,N,N′,N′‐tetramethylethylenediamine (TEMED) and APS–sodium metabisulfite (SPS), were used to initiate the gelation reactions. Compared to the APS–TEMED redox pair, no significant scattered light intensity rise was observed during the crosslinking polymerization reactions initiated by the APS–SPS system. It was found that both PAAm and PDMA gels are much more homogeneous when the APS–SPS redox pair was used as the initiator. The results are explained by the formation of shorter primary chains as well as the delay of the gel point in APS–SPS initiated gel formation reactions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3228–3237, 2007     

Radiation polymerization and concentration separation of P(NIPA‐co‐AMPS) hydrogels

Extraction or concentration with temperature‐sensitive hydrogels is a novel separation technology. In this study, N‐isopropylacrylamide (NIPA) was synthesized by acrylonitrile and isopropanol. Poly(N‐isopropylacrylamide) (PNIPA) and copolymer of NIPA and 2‐acrylamide‐2‐methylpropane sulfonate [P(NIPA‐co‐AMPS)] hydrogels were prepared by radiation polymerization. Dependence of their swelling behavior on temperature was studied. Effects of radiation dose on polymerization, feed composition on thermoresponse, electrolyte on relative swelling ratio, and swelling and deswelling kinetics were investigated. The experimental results showed that P(NIPA‐co‐AMPS) hydrogels with low content of AMPS/NIPA (1–5 %), prepared at a radiation dose‐rate of 1 kGy/h and total dose of 30–40 kGy, could enhance the swelling ratio of PNIPA hydrogels significantly and raise the phase‐transition temperatures. P(NIPA‐co‐AMPS) hydrogels produced under optimum conditions were used to concentrate aqueous bovine serum albumin (BSA, M = 69 000 g mol^?1) solution. When aqueous BSA concentration was below 5 %, the separation efficiency was more than 80 % with low cost and low energy consumption. Copyright © 2005 Society of Chemical Industry     

Swelling and elasticity of poly (N‐isopropylacrylamide‐co‐4‐vinyl benzene sulfonic acid sodium salt) hydrogels

The mechanical properties and the swelling behavior of hydrogels based on N‐isopropylacrylamide (NIPA) and 4‐vinylbenzenesulfonic acid sodium salt (styrene sulfonate, SSA) monomers were investigated. The mole fraction of SSA in the comonomer feed varied between 0 and 1, whereas the crosslinker ratio was fixed at 1/85. Both the swelling and the elasticity data of the hydrogels swollen in water show that they are in the non‐Gaussian regime. The exponents found for the charge density dependence of the equilibrium swelling ratio as well as for the volume dependence of the gel elastic modulus are in good agreement with the predicted course of the non‐Gaussian elasticity of swollen hydrogels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 135–141, 2004     

Photografted polymeric networks based on N‐isopropylacrylamide: Depth profiling by infrared spectroscopy

The study presents the application of infrared spectroscopy in attenuated reflection geometry with variable angle of incidence (VA‐ATR‐FTIR) in analysis of the in‐depth distribution of several chemical species in photografted layers. Two types of networks based on N‐isopropylacrylamide (NIPA) and one interpenetrated network of NIPA and N,N‐dimethylacrylamide (DMA) were produced by UV‐induced graft polymerization on polypropylene surfaces. The NIPA‐g‐PP samples were obtained in two different UV irradiation conditions: under broad band irradiation and using soft UV light (λ > 300 nm). NIPA‐co‐DMA‐g‐PP has been obtained using λ > 300 nm. VA‐ATR‐FTIR spectroscopy revealed the distribution of NIPA and DMA units across the thickness of the probed layer, according to the network type and photografting conditions. The spectral analysis of NIPA‐g‐PP reveals the influence of irradiation conditions, particularly the UV‐B radiation, on the coupling of monomers. For the NIPA‐co‐DMA‐g‐PP sample, a slight agglomeration of DMA units near the surface has been observed, which is maybe related to the more reactive character of DMA. According to the nonhomogenous distribution of the NIPA and DMA units inside the grafted layer, the surface contribution can be separated from the bulk one. The depth profile of several chemical species has been finally constructed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46048.     

Environmentally sensitive hydrogels: N‐isopropyl acrylamide/Acrylamide/ Mono‐, Di‐, Tricarboxylic acid crosslinked polymers

Environmentally sensitive hydrogels responsive to various stimuli such as temperature, pH, ionic strength of the medium and the solvent were prepared by using N‐isopropyl acrylamide (NIPAM), acrylamide (AAm) and monomers that have various number of carboxylic acid (XA) functionality using N,N′‐methylene bisacrylamide (Bis) as crosslinker. Hydrogels were prepared via free radical polymerization reaction in aqueous solution. P(NIPAAm‐co‐AAm) and p(NIPAAm‐co‐AAm)/XA hydrogels that contain monoprotic crotonic acid (CA) exhibit a lover critical solution temperature (LCST) at 28°C, whereas p(NIPAAm‐co‐AAm)/IA (IA:itaconic acid), and P(NIPAAm‐co‐AAm)/ACA (ACA:acotonic acid) hydrogels exhibit a lover critical solution temperature at 30.7°C and 34.4°C, respectively. Spectroscopic and thermal analyses were performed for the structural and thermal characterizations of the prepared hydrogel. The swelling experiments as equilibrium swelling percentages by gravimetrically were carried out in different solvents, at different solutions temperature, pH, and ionic strengths to determine their effects on swelling characteristic of hydrogels. POLYM. ENG. SCI., 55:843–851, 2015. © 2014 Society of Plastics Engineers     

Swelling–deswelling kinetics of poly(N‐isopropylacrylamide) hydrogels formed in PEG solutions

A series of poly(N‐isopropylacrylamide) (PNIPA) hydrogels was prepared by free‐radical crosslinking copolymerization of N‐isopropylacrylamide (NIPA) and N,N′‐methylenebisacrylamide (BAAm) in aqueous solutions of poly(ethylene glycol) of molecular weight 300 g/mol (PEG). The amount of PEG in the polymerization solvent, the crosslinker (BAAm) content, and the gel preparation temperature (T_prep) were varied in the gelation experiments. The hydrogels were characterized by the equilibrium swelling and elasticity tests as well as by the measurements of the deswelling–reswelling kinetics of the hydrogels in response to a temperature change between 25 and 48°C. The rate of deswelling of the swollen gel increases while the rate of reswelling of the collapsed gel decreases as the amount of PEG in the polymerization solvent is increased or as the crosslinker content is decreased. The T_prep effect on the swelling kinetics of the hydrogels was only observed if the PEG content of the polymerization solvent is less than 20%, which is explained with the screening of H‐bonding interactions in concentrated PEG solution. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 37–44, 2006     

Suppression of inhomogeneities in hydrogels formed by free-radical crosslinking copolymerization

Network microstructures of poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) hydrogels were compared by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebis(acrylamide) as a crosslinker. During the formation of PAAm gels, the reaction time dependence of the scattered light intensity exhibits a maximum at a critical reaction time, while in case of PDMA gels, both a maximum and a minimum were observed, corresponding to the chain overlap threshold and the gel point, respectively. This difference in the time-course between the two gelling systems is due to the late onset of gelation in the DMA system with respect to the critical overlap concentration. Compared to the AAm system, no significant scattered light intensity rise was observed during the crosslinking polymerization of DMA. It was shown that, regardless of the crosslinker ratio and of the initial monomer concentration, PDMA gel is much more homogeneous than the corresponding PAAm gel due to the shift of the gelation threshold to the semidilute regime of the reaction system. The results suggest that the spatial gel inhomogeneity can be controlled by varying the gel point with respect to the critical overlap concentration during the preparation of gels by free-radical mechanism.     

Environmental responses of poly(N‐isopropylacrylamide‐co‐glycidyl methacrylate derivatized dextran) hydrogels

A series of intelligent hydrogels (poly(NIPA‐co‐GMA‐Dex)) were synthesized by copolymerization of N‐isopropylacrylamide (NIPA) and glycidyl methacrylate derivatized dextran (GMA‐Dex) in aqueous solution with different ratios. Their swelling behaviors at different temperatures and in different pH and ionic strengths, and their mechanical properties were studied. It has found that poly(NIPA‐co‐GMA‐Dex) hydrogels are temperature‐, pH‐, and ionic strength‐sensitive associated with the roles of the component PNIPA and GMA‐Dex, respectively. Most significantly, poly (NIPA‐co‐GMA‐Dex) hydrogels exhibit simultaneously good swelling properties and mechanical properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2435–2439, 2005     

Cephazoline sodium release from poly(N‐isopropyl acrylamide‐co‐N,N‐dimethylacrylamide) hydrogels

Hydrogels are hydrophilic polymers that swell to an equilibrium volume in the presence of water, preserving their shape. The dynamic swelling behavior of poly(N‐isopropylacrylamide‐co‐N,N‐dimethylacrylamide) [poly(NIPA‐co‐DMA)] copolymers at 37°C was investigated. It was observed that the swelling degree in the copolymers decreases with the N‐isopropylacrylamide content. In addition, the liberation mechanism was found to be Fickian. Diffusion coefficients according to Fick′s law as a function of the N‐isopropylacrylamide concentration and results of the release process are reported. The kinetics of cephazoline sodium release from poly(NIPA‐co‐DMA) hydrogels with different compositions was studied. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3433–3437, 2004     

Hydrophobically modified nanocomposite hydrogels with self-healing ability

Several strategies have been developed in the past two decades to increase the mechanical performance of the hydrogels, and to generate self-healing function within the polymer network. Here, we combine two of these strategies to create hydrophobically modified nanocomposite (NC) hydrogels with high mechanical strength and self-healing efficiency. The hydrogels were prepared by in situ copolymerization of N,N-dimethylacrylamide and n-octadecyl acrylate (C18A) in the presence of 2 w/v % Laponite clay nanoparticles in an aqueous solution of worm-like sodium dodecyl sulfate micelles. Incorporation of hydrophobic C18A segments into the gel network significantly increases both the storage and loss moduli of NC hydrogels indicating increasing elasticity and energy dissipation. An improvement in the mechanical performance and self-recoverability of NC hydrogels was also observed after hydrophobic modification. The compressive fracture stress and Young's modulus increase with increasing amount of C18A, and they become 9 ± 1 MPa and 30 ± 2 kPa, respectively, at 4 mol % C18A. Incorporation of hydrophobic segments also provides a larger energy dissipation under large strain as compared to the traditional NC hydrogels providing a self-healing efficiency of 90 ± 10% in mechanically strong NC hydrogels. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48853.     

A new composite sorbent for water and dye uptake: Highly swollen acrylamide/2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid/clay hydrogels crosslinked by 1,4‐butanediol dimethacrylate

A novel type of highly swollen hydrogels based on acrylamide (AAm) with 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPS) and clay such as bentonite (Bent) crosslinked by 1,4‐butanediol dimethacrylate (BDMA) was prepared by free radical solution polymerization in aqueous media. Water uptake and dye sorption properties of polyelectrolyte AAm/AMPS hydrogels and AAm/AMPS/Bent composite hydrogels were investigated as a function of composition to find materials with swelling and sorption properties. FTIR analyses were made. Swelling experiments were performed in water and dye solution at 25°C, gravimetrically. Highly swollen AAm/AMPS and AAm/AMPS/Bent hydrogels were used in experiments on sorption of water‐soluble monovalent cationic dye such as Lauths violet “LV, (Thionin).” Swelling of AAm/AMPS hydrogels was increased up to 1,920–9,222% in water and 867–4,644% in LV solutions, while AAm hydrogels swelled 905% in water and swelling of AAm/AMPS/Bent hydrogels was increased up to 2,756–10,422% in water and 1,200–3,332% in LV solutions, while AAm/Bent hydrogels swelled 849% in water. Some swelling kinetic and diffusional parameters were found. Water and LV diffusion into hydrogels was found to be non‐Fickian in character. For sorption of cationic dye, LV into AAm/AMPS and AAm/AMPS/Bent hydrogel was studied by batch sorption technique at 25°C. The amount of the dye sorbed per unit mass removal effiency and partition coefficient of the hydrogels was investigated. The influence of AMPS content in the hydrogels to sorption was examined. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Behaviors of polyelectrolyte AAm/AMPS/bentonite composite hydrogels in uptake of uranyl ions from aqueous solutions

Uranyl ion (UO₂²⁺) sorption properties of polyelectrolyte composite hydrogels made by the polymerization of acrylamide (AAm) with 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPS) and clay such as bentonite (Bent) were investigated as a function of composition to find materials with swelling and uranyl ion sorption properties. Highly swollen AAm/AMPS hydrogels and AAm/AMPS/Bent composite hydrogels were prepared by free radical solution polymerization in aqueous solutions of AAm with AMPS as co‐monomer and two multifunctional crosslinkers such as ethylene glycol dimethacrylate (EGDMA) and 1,4 butanediol dimethacrylate (BDMA). Swelling experiments were performed in water at 25°C, gravimetrically. The influence of AMPS content in hydrogels was examined. Uranyl ion adsorption from aqueous solutions was studied by batch sorption technique at 25°C. The effect of uranyl ion concentration and mass of AMPS on the uranyl ion adsorption were examined. Finally, adsorption capacity (the amount of sorbed uranyl ion per gram of dry hydrogel) (q) was calculated to be 0.67 × 10⁻³–2.11 × 10⁻³ mol uranyl ion per gram for the hydrogels. Removal effiency of uranyl ions (RE%) was changed range 9.05–29.92%. The values of partition ratio (K_d) of uranyl ions was calculated to be 0.10–0.43 for AAm/AMPS hydrogels and AAm/AMPS/Bent composite hydrogels, respectively. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Swelling behavior and determination of diffusion characteristics of acrylamide–acrylic acid hydrogels

In this study, a random copolymer of acylamide and acrylic acid [poly(AAm‐co‐AA)] was prepared by a redox copolymerization method of their aqueous solutions. The effects of initial AAm/AA mole ratio, PEG 4000 content, and N,N′‐methylenebisacrylamide concentration on swelling behavior were investigated in water. Average molecular weights between crosslinks, percentage swelling, swelling equilibrium values, and diffusion/swelling characteristics (i.e., the structure of network constant, the type of diffusion, the initial swelling rate, swelling rate constant) were evaluated for every hydrogel systems. The hydrogels showed mass swelling capabilities in the range 789–1040% (for AAm/AA hydrogels), 769–930% (for AAm/AA hydrogels in the presence of PEG 4000), and 716–1040% (for AAm/AA hydrogels containing different concentrations of the crosslinker). The swelling capabilities of the hydrogels decreased with the increasing AA, PEG 4000, and crosslinker concentrations. The diffusion of water into AAm/AA hydrogels was found to be a non‐Fickian type. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1289–1293, 2004     

Trititanate‐nanosheet incorporated hydrogels: Self‐initiated photopolymerization and application in photodegradation of methylene violet

Exposing the aqueous mixtures of acrylamide (AAm) monomers and exfoliated trititanate nanosheets (TiNs) to ultraviolet light achieved nanocomposite hydrogels TiNs–PAAm in a facile one‐step synthetic route. The incorporated semiconductor TiNs functioned as a photocatalytic crosslinker, initiating the in situ photopolymerization of monomers and hydrogen‐bonding crosslinking with the polymeric networks to form nanocomposite hydrogels. The resultant hydrogels exhibited good absorption behaviors of methylene violet (MV), and the hydrogel prepared from an AAm/TiNs ratio of 2.46 × 10² g/g exhibited a maximum uptake of 8.4 g MV solution for 1 g. The immobilized TiNs retained high photocatalytic activity in the recyclable photodegradation of MV. It indicated the nanocomposite hydrogels may be considered as a good candidate in wastewater treatment. POLYM. COMPOS., 37:2811–2819, 2016. © 2015 Society of Plastics Engineers     

Physicochemical and morphological properties of poly(acrylamide) and methylcellulose hydrogels: Effects of monomer,crosslinker and polysaccharide compositions

This article describes the physicochemical (mechanical and swelling) and morphological characterization of poly(acrylamide) and methylcellulose (PAAm‐MC) hydrogels synthesized with different formulations by the free radical polymerization method. The structure‐property relationship of the PAAm‐MC hydrogels is very important for application of these materials in different fields. Results showed that the properties of the PAAm‐MC hydrogels can be controlled by varying the acrylamide (AAm) and N′,N‐methylene‐bis‐acrylamide (MBAAm) concentrations and methylcellulose (MC) content. Increase of AAm and MBAAm concentrations causes a pronounced decrease in swelling degree (SD) values and porosity, and an increase in mechanical properties. Increasing the MC concentration caused an increase in SD values and porosity, but decrease in maximum load and modulus of elasticity because of the increase in the hydrogel hydrophilicity due to incorporation of hydroxyl groups from MC chains. PAAm‐MC hydrogels are excellent candidates for several applications, such as matrices for cell transplantation, controlled release (agrochemicals and drugs), tissue repair and regeneration. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Thermosensitive nanoparticles prepared from poly(N‐isopropylacrylamide‐acrylamide‐vinilpyrrolidone) and its blend with poly(lactide‐co‐glycolide) for efficient drug delivery system

Temperature‐responsive polymers have become increasingly attractive as carrier for the injectable drug delivery systems. In the present work, we have studied the preparation of poly(N‐isopropylacrylamide‐acrylamide‐vinilpyrrolidone) (NIPAAm‐AAm‐VP terpolymer) nanoparticulated terpolymer and its blend with poly(lactide‐co‐glycolide, PLGA; molar ratio of lactide/glycolid 1/3). Thermosensitive terpolymer, poly(NIPAAm‐AAm‐VP) was prepared by free‐radical polymerization in aqueous solution. The nanoparticles of poly(NIPAAm‐AAm‐VP) and its blend with PLGA containing naltrexone were prepared using the evaporation and w/o emulsion‐solvent evaporation methods, respectively. Nanoparticles prepared from terpolymer‐PLGA blend at low polymer concentration (5%) shows larger particle size (>300 nm) and higher drug content%. Various types of nanoparticles showed a burst release of less than 10% after 24 h . The results suggest that by regulating different variables, desired release profiles of naltrexone can be achieved using a blend of PLGA‐poly(NIPAAm‐AAm‐VP) nanoparticulate system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Physically Linked PVA/P(AAm‐co‐SMA) Hydrogels: Tough,Fast Recovery,Antifatigue, and Self‐Healing Soft Materials

The application of traditional chemically crosslinked hydrogels is often limited by poor mechanical properties because of their own inhomogeneous network and irreversible crosslinking bonds. Herein, physical interactions are applied to crosslink the interpenetrating network hydrogel, i.e., hydrogen bonding and crystalline domain for polyvinyl alcohol network, and hydrophobic interaction inside micelle for poly (acrylamide‐co‐stearyl methyl acrylate) [P(AAm‐co‐SMA)] network. In this gel network system, reversible energy dissipation mechanism is realized by dissociation and reassociation of weak interactions including hydrogen bonding and hydrophobic interaction inside the micelle. Strong crystalline domains serve as permanent crosslinking interactions to maintain network integrity under large extension. As a result, the synergy of weak and strong interactions leads to tough, antifatigue, fast recovery, and self‐healing properties of the hydrogel. This proposed strategy of achieving versatile hydrogels can broaden the use of hydrogels into load‐bearing applications.