Gelation via cationic chelation/crosslinking of acrylic‐acid‐based polymers

The gelation characteristics of acrylic‐acid‐based polymers in the presence of a range of cationic species, namely Ca²⁺, Mg²⁺ and Al³⁺, were investigated using in situ rheological measurements during photo‐polymerisation. Fourier transform mechanical spectroscopy was used to identify the gel point, using the Winter–Chambon criteria which allow the gel point to be pinpointed by establishing the sample spanning network and quantitatively determining stiffness, relaxation exponent, gel stiffness and fractal dimensions. The results showed that the gelation processes were greatly influenced by the type of cationic species that was used in the syntheses. At the gel point, more open network clusters were formed when Al³⁺ cations were used instead of Ca²⁺ cations or Mg²⁺ cations, all relating to chloride salts. Although the concentrations of the chelating/crosslinking aluminium species affected the kinetics of the gelation, the critical gel characteristics were hardly affected. Also the solubility of the chosen aluminium salt was shown to dictate the crosslinking rates and the properties of the critical gels. The extents of the reactions and the types of network formed at the gel point and beyond indicated that reactions between the Al³⁺ ions and COOH sites, from growing poly(acrylic acid) molecular chains, differ from those exhibited by Mg²⁺ and Ca²⁺ ions. All of the chelation/crosslinking reactions met the criteria of low mutation number (N_mu), such that in all cases N_mu ? 1. © 2019 Society of Chemical Industry     

Radiation‐induced crosslinking of acetylene‐impregnated polyesters. II. Effects of preirradiation crystallinity,molecular structure,and postirradiation crosslinking on mechanical properties

Electron beam‐irradiated crosslinking has been studied in a series of acetylene‐impregnated polyesters and amorphous copolyesters, including poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(cyclohexane dimethylene terephthalate) (PCDT), and poly(cyclohexane dimethylene terephthalate‐co‐ethylene terephthalate) (P(CDT‐co‐ET)) having 29 and 60 wt % ethylene terephthalate (ET). The extent of crosslinking was observed by gel fraction measurements and was found to be significantly influenced by the aliphatic chain content of the polyesters (PET < PBT < PCDT). In addition, as the preirradiation crystallinity of the polyesters was reduced, the extent of acetylene‐enhanced crosslinking was greatly raised. Decreases in the postirradiation crystalline melting temperature and degree of crystallinity were observed in all the polyesters, using differential scanning calorimetry measurements. Particularly significant findings have been the shift in the glass‐transition temperatures (Tg) to higher temperatures and the decrease in loss tangents at higher temperatures, both of which confirm that crosslinking has taken place. The storage moduli (E′) in the rubbery plateau region of PCDT and P(CDT‐co‐ET) were significantly affected by irradiation dose. Increased network tightness in postirradiated PBT and PCDT films was also inferred from melt‐rheology measurements, in which stress relaxed more slowly following a stepped strain. Improvements in the mechanical properties of the irradiated polyesters and copolyesters were clearly evidenced by the increased modulus at higher temperatures, observed using dynamic mechanical thermal analysis and melt‐rheology methods. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4476–4490, 2006     

Arrays of polyacrylamide hydrogels using a carbodiimide‐mediated crosslinking reaction

This study introduces a radical‐free approach for generating polyacrylamide (PAM) hydrogels with no toxic residues remaining in the networks. Acrylamide and bisacrylamide, which are neurotoxins, are not used during the hydrogel synthesis and only nontoxic side products are generated. This is achieved using a gentle carbodiimide‐mediated crosslinking (CMCL) reaction that does not require complex initiation systems and is effective in the presence of oxygen. This overcomes some of the key limitations related to PAM hydrogel synthesis using free‐radical routes and maintains the advantages of synthetic hydrogels over biopolymers. In addition, the CMCL reaction allows for accurate placement of functional groups, which controls hydrogel structure and performance including mechanical strength, swelling capacity, and hydrophobic balance. This flexibility is demonstrated through the synthesis and rheological characterization of a library of structurally diverse hydrogels as well as spherical hydrogels. PAM‐based hydrogels are used extensively in a broad number of applications, and this study demonstrates the applicability of this method as a nontoxic and radical‐free complementary alternative route that can generate structures analogous to those prepared using free‐radical routes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40416.     

Enhancing mechanical strength of hydrogels via IPN structure

In this investigation, polyacrylamide (PAAm) as the flexible network is introduced to enhance the mechanical strength of hyaluronic acid–gelatin (HA–Gel) hydrogels by interpenetrating polymer network (IPN). The structure, mechanical property, and rheology property of the IPN hydrogels are investigated. It is found that the compressive strength of the HA–Gel/PAAm IPN hydrogels has increased five times higher than that of HA–Gel hydrogels. Rheological test demonstrates that elastic moduli (G′) and viscous moduli (G″) of HA–Gel/PAAm IPN hydrogels increase 100 times higher than those of HA–Gel hydrogels. Moreover, the HA–Gel hydrogels are fractured under the low compressive stress, whereas HA–Gel/PAAm IPN hydrogels are not broken under the high compressive stress. It is envisioned that the IPN hydrogels will be an effective approach to enhance the mechanical strength and broaden the range of hydrogels' applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44503.     

High‐temperature resistant superabsorbent based on poly(acrylic acid) with triallylammonium chloride as crosslinking agent

A novel high‐temperature resistant superabsorbent was prepared by solution polymerization of partially neutralized acrylic acid (AA), using triallylammonium chloride as crosslinker, potassium persulfate as initiator. The factors that influence the water‐absorbing capacity at 25 and 200°C such as mass concentration of monomer, mass ratio of crosslinker to AA, mass ratio of initiator to AA, and neutralization degree were investigated. The structure of the superabsorbent was characterized by Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy. The optimum conditions were obtained and the swelling ratios in distilled water and 1 wt % of NaCl solution could reach 841 and 74 g/g at 300°C, respectively. The superabsorbent also showed high swelling rate and good salt resistance. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41243.     

EDTA and pH‐sensitive crosslinking polymerization of acrylic acid, 2‐acrylamidoglycolic acid,and 2‐acrylamide‐2‐methyl‐1‐propanesulfonic acid

Network formation was monitored by shear storage modulus (G′) during free radical crosslinking polymerization to investigate the effects of pH and ethylenediaminetetraacetic acid (EDTA; a complex agent). Three types of acrylic monomers, acrylic acid (AAc), 2‐acrylamidoglycolic acid (AmGc), and 2‐acrylamido‐2‐methyl propanesulfonic acid (AmPS), were polymerized in the presence of a crosslinking agent. The ratio of crosslinking agent (methylene bis‐acrylamide; MBAAm) to monomer was varied as: 0.583 × 10^?3, 1.169 × 10^?3, 1.753 × 10^?3, and 2.338 × 10^?3. G′ of the hydrogel in crosslinking polymerizations of AAc and AmPS was effectively increased by addition of EDTA, which was not the case for the crosslinking polymerization of AmGc. The order of magnitude of G′ differed based on the acidity of monomer. The maximum values of G′ in crosslinking polymerizations of AAc, AmGc, and AmPS were ～20,000 Pa, 6000 Pa, and 400 Pa, respectively. G′ varied linearly with the molecular weight between crosslinks (M_wc). pH and EDTA‐complex affected the rate of intramolecular propagation during crosslinking polymerization. Our results indicated that G′ was primarily affected by the following factors in the order: (1) acidity of monomer, (2) M_wc, and (3) physical interactions induced by pH and EDTA. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41026.     

Kinetics and crystallization in pH‐sensitive free‐radical crosslinking polymerization of acrylic acid

Free‐radical crosslinking polymerization and crystallization of acrylic acid (AAc) were investigated by shear storage modulus (G′) measurements in pH 2, as well as in pH 6 and pH 10, by varying the molar ratio of crosslinking agent (N,N′‐methylene bis‐acrylamide; MBAAm) to AAc (0.583 × 10^?3, 1.169 × 10^?3, 1.753 × 10^?3, and 2.338 × 10^?3). Our results showed that the pre‐gelation time was the same at pH 2, regardless of the concentration of MBAAm. The propagation time was determined by the initial feed concentration of AAc, and the length of the linear curve in the propagation was proportional to the concentration of MBAAm. The Avrami exponent (n), as an indicative of growing pattern of an infinite molecule, in the crystallization was increased in proportional to the concentration of MBAAm, and generally low at pH 2. In the deceleration phase, n was observed near 1.0 throughout the all specimens. These results indicated that (1) the length of the pre‐gelation period was determined by the ionization of AAc (or pH), (2) the polymerization rate of AAc was not affected by the concentration of MBAAm, and (3) the inhomogeneity of hydrogel was determined by the growing pattern of infinite molecule in propagation phase. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42195.     

In situ injectable poly(γ‐glutamic acid) based biohydrogel formed by enzymatic crosslinking

Enzymatic crosslinking was developed to prepare in situ forming poly(γ‐glutamic acid) (γ‐PGA) based hydrogel in this study. First, the precursor of poly(γ‐glutamic acid)–tyramine (γ‐PGA–Ty) was synthesized through the reaction of carboxyl groups from a γ‐PGA backbone with tyramine. The structure of the grafted precursor was confirmed by ¹H‐NMR and Fourier transform infrared spectroscopy. After that, the crosslinking of the phenol‐containing γ‐PGA–Ty precursor was triggered by horseradish peroxidase in the presence of H₂O₂; this resulted in the formation of the γ‐PGA–Ty hydrogels. The equilibrium water content, morphology, enzymatic degradation rate, and mechanical properties of the hydrogels were characterized in detail. The data revealed that the well‐interconnected hydrogels had tunable water contents, mechanical properties, and degradability through adjustments of the composition. Furthermore, cell experiments proved the biocompatibility of the hydrogels by 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay. These characteristics provide an opportunity for the in situ formation of injectable biohydrogels as potential candidates in cell encapsulation and drug delivery. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42301.     

Thermoresponsive nanocomposite hydrogels with high mechanical strength and toughness based on a dual crosslinking strategy

The practical application of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are severely limited by their poor mechanical properties. Herein, we reported a series of dual crosslinked (DC) PNIPAM hydrogels with superior mechanical properties prepared by simple copolymerization of N-isopropylacrylamide and sodium acrylate (SA) in the laponite RDS suspension, following by a soaking process in multivalent metal cations (e.g., Ca²⁺, Al³⁺, Fe³⁺) aqueous solutions to form ionic coordination interactions with  COO groups of copolymer side chains. The effect of laponite RDS, AANa (sodium acrylate), and metal cation (e.g., Fe³⁺) concentrations on the mechanical properties and deswelling properties of the DC hydrogels are evaluated. The DC hydrogel prepared with 10 w/v% laponite RDS, 0.25 mol/L AANa and 0.45 mol/L Fe³⁺ possesses the best mechanical properties (ca. 1.1 MPa of tensile strength, 9.1 MPa of compression strength at 80% of compression strain, 1.4 MPa of elastic modulus and 1.3 MJ/m³ of toughness). Moreover, we also discovered that the DC hydrogels crosslinked by Fe³⁺ showed better mechanical properties due to the larger charge and ion radius of Fe³⁺.     

Particle number and size of polystyrene‐dependent micellar crosslinking polymerization of acrylic acid

The effect of the number and size of polystyrene particles and the concentration of ammonium persulfate used as the initiator on the micellar crosslinking polymerization of acrylic acid was studied by real‐time monitoring of the storage modulus (G ′), the damping factor (tanδ), and the ratio of the complex modulus (G*) to the maximum G* (G*_max) during 1 h of polymerization. The molar ratio (5.83 × 10^?4) of N,N′‐methylenebis‐acrylamide to acrylic acid was fixed. Polystyrene particles were prepared by emulsifier‐free emulsion polymerization. The diameter of the particles ranged from 233 to 696 nm. The results show that crosslinking polymerization was most effective when 1.31 × 10¹² particles were incorporated into the system, while crosslinking polymerization was less effective in the particle‐filled system than in the unfilled polymerization system if the particle number was 50% lower or higher. Crosslinking was also more effective with the use of uncrosslinked firmer and larger particles at the fixed particle number, except for the anomalous behavior observed with 696 nm polystyrene particles. Increasing the feed concentration of the initiator resulted in more efficient crosslinking up to a limiting concentration of 0.765 mg mL^?1 (the molar ratio of initiator to monomer was 8.52 × 10^?4). When this initiator concentration was doubled, the rate of increase of G ′ in the deceleration phase was slower after the network was formed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42851.     

Fast responsive poly(acrylic acid‐co‐N‐isopropyl acrylamide) hydrogels based on new crosslinker

Novel dual temperature‐ and pH‐sensitive poly(acrylic acid‐co‐N‐isopropylacrylamide), AA/NIPAAm, hydrogels were successfully prepared by chemical crosslinking with crosslinkers. Copolymers of AA/NIPAAm were crosslinked in the presence of different mol % of N,N‐methylene bisacrylamide (MBA) and melamine triacrylamide (MAAm) as crosslinkers by bulk radical polymerization. The resultant xerogels were characterized by extracting the soluble fractions and measuring the equilibrium water content. Lower critical solution transition temperatures (LCST) were measured by DSC. The properties of crosslinked AA/NIPAAm series are evaluated in terms of compositional drift of polymerization, heterogeneous crosslinking, and chemical structure of the relevant components. Soluble fractions of the crosslinked networks were reduced by varying the MAAm and MBA concentrations. The influence of environmental conditions such as temperature and pH on the swelling behavior of these polymeric gels was investigated. The swelling behaviors of the resulting gels show pH sensitivity. The prepared MAAm type AA/NIPAAm hydrogels exhibited a more rapid deswelling rate than MBA type AA/NIPAAm hydrogels in ultra pure water in response to abrupt changes from 20°C to 50°C. The results of this study provide valuable information regarding the development of dual stimuli‐sensitive hydrogels with fast responsiveness. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Evaluation of pH‐sensitive hydrogels to control the permeability anisotropy of oil reservoirs

In stratified or fractured oil reservoirs, the oil recovery efficiency tends to be low as the injected fluid flows mainly through the matrix's most permeable regions, leaving behind part of the displaceable oil in the matrix's unswept zones. Given this issue, this study aims to evaluate the potential of applying seven commercial samples, based on poly(acrylic acid), to control the anisotropic permeability profile of reservoirs. To perform this study, first, continuous and oscillatory shear tests were conducted to characterize the hydrogel's rheological and viscoelastic behavior in various subsurface conditions (salinity, temperature, and/or pH value). Second, polymer dispersion elution tests were performed in a porous medium to evaluate the matrix's permeability reduction after treatment with hydrogels. The seven commercial samples were classified as pseudoplastic fluids at pH values ranging from 1 to 10. Under typical reservoir conditions, PAAr 70 (which has intermediate molar mass and intermediate number of crosslinks) was the only sample to behave as a strong gel (G′/G″ > 10). Elution tests confirmed that the PAAr 70 hydrogel gelified inside the consolidated sandstone plugs and reduced the matrix's permeability four‐fold. Therefore, samples based on poly(acrylic acid) with high crosslink density proved to be the most promising for controlling the anisotropic permeability profile of heterogeneous oil reservoirs. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40665.     

Radiation‐induced crosslinking and mechanical properties of blends of poly(lactic acid) and poly(butylene terephthalate‐co‐adipate)

Blends of poly(L ‐lactic acid) (PLLA) and poly (butylene terephthalate‐co‐adipate) (PBTA) were prepared at ratios of 50 : 50, 60 : 40, and 80 : 20 by melt blending in a Laboplastomill. Improved mechanical properties were observed in PLLA when it was blended with PBTA, a biodegradable flexible polymer. Irradiation of these blends with an electron beam (EB) in the presence of triallyl isocyanurate (TAIC), a polyfunctional monomer, did not cause any significant improvement in the mechanical properties, although the gel fraction increased with the TAIC level and dose level. Irradiation of the blends without TAIC led to a reduction in the elongation at break (E_b) but did not show a significant effect on the tensile strength. E_b of PBTA was unaffected by EB radiation in the absence of TAIC. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ethanol organosolv lignin as a reactive filler for acrylamide‐based hydrogels

Hydrogels based on acrylamide (AM) and ethanol organosolv lignin (EOL) with high swelling and good mechanically elastic properties were synthesized in an alkaline solution. EOL was used as a reactive filler for the preparation of AM‐based hydrogels. The impact of EOL addition on the physicochemical properties of AM‐based hydrogels was investigated using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy, and their mechanical properties were examined. The water swelling ratio of the prepared hydrogels increased with the increase of EOL content, and their maximum swelling ratio could reach up to 180. Mechanical measurements indicated that their tensile strength was highly dependent on the amount of EOL, and their elongation at break reached up to 1400%. The formation mechanism of EOL composite hydrogels was probably that most of AM was synthesized into the crosslinked poly(acrylic amide) network, and small quantities of AM was hydrolyzed to acrylic acid ions under alkaline condition. The chain transfer of free radicals from AM and/or AA to EOL molecules occurred in the polymerization process. With increasing EOL content in the hydrogels, an interpenetrating polymer network might be mainly formed by the hydrogen bonding between EOL and AA and/or AM molecules. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42638.     

Evaluation of two chemical crosslinking methods of poly(vinyl alcohol) hydrogels for injectable nucleus pulposus replacement

Low back pain caused by intervertebral disc degeneration is one of the most common spinal disorders among patients seeking medical treatment. The most common surgical treatments are spinal fusion and total disc arthroplasty, both of which are very invasive surgical procedures. Nucleus pulposus replacement is an earlier stage intervention for disc degeneration. One of the material classes being studied for this application is hydrogels: a three‐dimensional hydrated network of polymer(s), which mimics the mechanical and physiological properties of the nucleus. Poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), and poly(ethylene glycol) (PEG) hydrogels have previously been shown to be great candidate materials for injectable nucleus pulposus replacement, but have experienced issues with swelling and mass retention. The addition of chemical crosslinking to the PVA/PVP/PEG hydrogel system will allow tailoring of the swelling, mechanical, injectability, and mass loss properties of the hydrogel network. Two chemical crosslinking methods were evaluated for the PVA/PVP/PEG hydrogel system by characterizing the hydrogels with compression, swelling, and spectroscopy experiments. The results of these experiments led to the selection of the difunctional crosslinking strategy using PEG functionalized with terminal epoxide group (PEG diglycidyl ether) as the preferred crosslinking method. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40843.     

Synthesis and drug‐release studies of low‐fouling zwitterionic hydrogels with enhanced mechanical strength

Reversible addition–fragmentation chain‐transfer polymerization was introduced to prepare a series of zwitterionic poly(hydroxyethyl methacrylate)‐g‐poly(sulfobetaine methacrylate) (PSBMA) hydrogels (HSGs) with different monomer feed ratios. Compared with PSBMA hydrogels, these hydrogels exhibited enhanced mechanical strengths. Then, the HSGs were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and swelling measurements. We found that the equilibrium swelling ratios, mechanical strengths, and drug‐release behaviors were significantly affected by the feed ratios of the gels. The hydrophilic tetracycline hydrochloride release results suggest that the hydrophilic drug release from the HSGs could be prolonged by the variation of the hydroxyethyl methacrylate amount in the gel networks. The bovine serum albumin adsorption data showed that the zwitterionic HSG with 18.2 wt % sulfobetaine methacrylate exhibited good protein‐resistance properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41041.     

Enzymatic and ionic crosslinked gelatin/K‐carrageenan IPN hydrogels as potential biomaterials

Hydrogels of a natural origin have attracted considerable attention in the field of tissue engineering due to their resemblance to ECM, defined degradability and compatibility with biological systems. In this study, we introduced carrageenan into a gelatin network, creating IPN hydrogels through biological methods of enzymatic and ionic crosslinking. Their gelation processes were monitored and confirmed by rheology analysis. The combination of biochemical and physical crosslinking processes enables the formation of biohydrogels with tunable mechanical properties, swelling ratios and degradation behaviors while maintaining the biocompatibilities of natural materials. The mechanical strength increased with an increase in carrageenan content while swelling ratio and degradability decreased correspondingly. In addition, the IPN hydrogels were shown to support adhesion and proliferation of L929 cell line. All the results highlighted the use of biological crosslinked gelatin‐carrageenan IPN hydrogels in the context of tissue engineering. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 10.1002/app.40975.     

Effect of crosslinking concentration on mechanical and thermodynamic properties in acrylic acid–co–methyl methacrylate hydrogels

The mechanical and thermodynamic properties of poly(acrylic acid‐co‐methyl methacrylate) hydrogels with varying crosslinker N,N′‐methylenebisacrylamide (NMBA) content are reported. A higher NMBA content generally led to a stronger and harder gel with lower water content. Swelling capacity decreased as the NMBA concentration increased between 0.5% and 2%, remaining constant beyond this range. The temperature changes of the partial molar Gibbs free energy of dilution and enthalpic and entropic contributions were examined. The thermodynamic parameters showed that swelling was an unfavorable and endothermic process. The freezing and nonfreezing water in the hydrogel was determined by differential scanning calorimetry (DSC). Freezing water content decreased with increasing crosslinker (NMBA) content, whereas the ratio of nonfreezing water to total water content increased with NMBA content because of the promoting of hydrophobic interactions in the hydrogels. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4016–4022, 2006     

Preparation and physical properties of hyaluronic acid‐based cryogels

Macroporous hydrogels based on hyaluronan (HA), a natural polysaccharide occurring in extracellular matrix, have attracted interest over many years owing to their numerous applications in the biomedical area. However, HA hydrogels produced so far suffer from low mechanical strength and slow rate of response against external stimuli, which limit their applications. Here, we prepared macroporous HA cryogels of high mechanical stability and fast responsivity from aqueous HA solutions at subzero temperatures using ethylene glycol diglycidyl ether as a crosslinking agent. HA cryogels are squeezable and no crack development was exhibited when compressed up to 80% strain. Depending on the synthesis parameters, the cryogels exhibit an elastic modulus between 0.2 and 2 kPa, and show fast swelling/deswelling behavior. The microstructure of the cryogels consists of large, interconnected pores on the order of 100 µm separated by thick pore walls, as observed by scanning electron microscopy and confocal scanning laser microscopy. © 2015 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. 2015 , 132, 42194.