Mechanical and Rheological Behavior of Hybrid Cross‐Linked Polyacrylamide/Cationic Micelle Hydrogels

In this work, a hybrid cross‐linked polyacrylamide (PAM)/cationic micelle hydrogel is fabricated by introducing the cationic micelles into the chemically cross‐linked PAM network. The cationic micelles act as the physical cross‐linking points through the strong electrostatic interaction with anionic initiator potassium persulfate. Thereafter, in situ free radical polymerization is initiated thermally from the cationic micelle surface to form the hybrid cross‐linked network. The synergistic effect between chemical and physical cross‐link endows the hydrogel with excellent mechanical and recoverable properties. The resulting hydrogel exhibits tensile stress of 481 kPa and fracture toughness of 1.65 MJ m⁻³. It is found that the chemical cross‐linking can inhibit the hysteresis of the hybrid hydrogel, exhibiting good elasticity in the tensile loading–unloading test. Moreover, dynamic rheological measurements show that the hybrid hydrogels possess fewer defects of network and exhibit excellent self‐recovery behavior. Thus, this investigation provides a different view for the design of new high elastic and tough hydrogels containing hybrid physical and chemical cross‐linking networks.     

Properties of hydrogels of atactic poly(vinyl alcohol)/NaCl/H2O system and their application to drug release

In order to prepare cost‐effective physically cross‐linked hydrogels including food salt sodium chloride, samples, were prepared with various concentrations of NaCl and respective atactic poly(vinyl alcohol) (a‐PVA), and were evaluated. It had been observed that hydrogels containing NaCl concentration (9–11 wt%) along with a‐PVA concentration 9–5% respectively exhibited higher melting points (91.5–95.1 °C). A higher melting point characterizes the hydrogel composition of a system like a‐PVA(7%)/NaCl(11%)/H₂O. The swelling degree of this hydrogel was found to be comparatively better at 37 °C than at any other temperature studied here. However, irregular Fickian swelling was found at this temperature. The UV light absorption maximum at 362–364 nm and minimum at 351 nm for this hydrogel had been found as evidence of physical cross‐linking. A drug, theophylline was loaded by solvent‐sorption and feed‐mixture dissolving methods. The feed‐mixture dissolving method is better than solvent sorption because of high drug loading, comparatively low fraction release rate and more sustained‐release of drug than that of solvent‐sorption. Theophylline was released twice as fast from the hydrogel after solvent‐sorption drug loading (3 h) than from that which used the feed‐mixture dissolving method (6.5 h). Theophylline‐loaded hydrogels of this system (feed‐mixture dissolving) were then prepared at high temperature (60 °C) thawing for 6 h followed by chilling at 0.4 °C for 3 h as one cycle. And the drug release behaviour and mass transfer were found almost the same as for chilling (24 h at 0.4 °C)–thawing (48 h at room temperature). Drug release behaviour was studied as apparently irregular Fickian diffusion (Higuchi Matrix Dissolution Model). © 2002 Society of Chemical Industry     

Genipin‐cross‐linked chitosan‐based hydrogels: Reaction kinetics and structure‐related characteristics

The main aim of this work is the synthesis and characterization of cross‐linked chitosan systems. Chitosan hydrogels can be prepared by physical or chemical cross‐linking of polymer chains. Chemical cross‐linking, leading to the creation of hydrogel networks possessing improved mechanical properties and chemical stability, can be achieved using either synthetic agents or natural‐based agents. In this work, the cross‐linker Genipin, a naturally derived compound, was selected because of the lower acute toxicity compared to many other commonly used synthetic cross‐linking reagents. In particular, the chemical stabilization of chitosan through genipin cross‐linking molecules was performed and characterized by calorimetric analyses (differential scanning calorimetry), swelling measurements in different pHs, and ionic strength. The reaction kinetics was carried out by means of rheological measurements, and both the activation energy (E_a) and the reaction order (m) were calculated. The hydrogel analyses were carried out at different concentrations of genipin (GN1 and GN2). The results were used to evaluate the possibility to use the chemical cross‐linked chitosan–genipin hydrogel for biomedical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42256.     

Electrosynthesis and characterization of an electrochromic material from poly(1,6‐bis(2‐thienyl)pyrene) and its application in electrochromic device

Electrochemical polymerization of 1,6‐bis(2‐thienyl)pyrene (BTP) could be achieved in acetonitrile/dichloromethane (ACN/DCM) (1:1, by volume) solution containing sodium perchlorate (NaClO₄) as a supporting electrolyte. The resulting polymer poly(1,6‐bis(2‐thienyl)pyrene) (PBTP) were characterized by cyclic voltammetry, UV–vis spectroscopy, and scanning electron microscopy. The resulting polymeric film has distinct electrochromic properties and exhibits three different colors under various potentials. Moreover, the PBTP film showed reasonable optical contrast (DT %) at 700 nm is found to be 29% and satisfactory response time is measured as 1.3 s. An electrochromic device (ECD) based on PBTP and poly(3,4‐ethylenedioxythiophene) was also constructed and characterized. This ECD has these qualities: quick switching time, reasonable contrast, and good optical memories and redox stability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39770.     

TEOS as an improved alternative for chitosan beads cross‐linking: A comparative adsorption study

In this work the use of tetraethoxysilane (TEOS) for cross‐linking of chitosan hydrogel beads was studied at the level of 1 mmol TEOS per gram of chitosan. They were compared with glutaraldehyde and epichlorohydrin cross‐linked beads. The hydrogels were characterized by FTIR, SEM, water content, nitrogen content, and their point of zero charge. The performance of the anionic dye Remazol Black (RB) and the cationic Cd(II) adsorptions was assessed in order to characterize the sorbate–sorbent interaction. Adsorption experimental data were analyzed using two‐ and three‐parameter isotherm models along with the evaluation of mean adsorption energy and standard free energy. The adsorption was observed to be pH dependent. The uptake rate of RB and Cd(II) showed that the three type of beads followed a similar kinetic behavior. For both sorbates the TEOS cross‐linked beads showed the higher maximum adsorption capacity, followed by epichlorohydrin and glutaraldehyde cross‐linked beads. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41005.     

Development of a thermosensitive grafted drug delivery system—synthesis and characterization of NIPAAm‐based grafts and hydrogel structure

A thermosensitive grafted hydrogel was investigated for heating‐activated drug release. The hydrogel was created by grafting oligomers of N‐isopropylacrylamide‐co‐acrylamide (AAm) to a poly(2‐hydroxyethyl methacrylate), or PHEMA, hydrogel. N‐Isopropylacrylamide‐co‐AAm oligomers were synthesized with a range of compositions to raise the lower critical solution temperature (LCST) above physiological temperature. PHEMA hydrogels with these thermosensitive grafts were synthesized by free‐radical solution polymerization, using an acrylated version of the oligomers. The oligomers were characterized for their molecular weight, LCSTs, and rate of response to a change in temperature. With the flexibility in tuning their properties by varying reaction parameters, these oligomers present possibilities in several fields, including drug delivery. The impact of cross‐linking agent type and the amount and presence of grafts on the polymer network structure was found by determining the hydrogel mesh sizes. PHEMA gels cross‐linked with methylenebisacrylamide had larger mesh sizes than those cross‐linked with ethylene glycol dimethacrylate. Increasing amounts of cross‐linking agent decreased mesh sizes. LCSTs exhibited by oligomers were slightly lower than those exhibited by polymer gels of the same composition. The grafting reaction was found to have only a slight impact on the hydrogel mesh size. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of poly(thiophen‐3‐yl acetic acid 4‐pyrrol‐1‐yl phenyl ester‐co‐N‐methylpyrrole) and its application in an electrochromic device

Copolymer of thiophen‐3‐yl acetic acid 4‐pyrrol‐1‐yl phenyl ester (TAPE) with N‐methylpyrrole (NMPy) was synthesized by potentiostatic electrochemical polymerization in acetonitrile–tetrabutylammonium tetrafluoroborate solvent–electrolyte couple. The chemical structures were confirmed via Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), and UV–vis spectroscopy. Electrochromic and spectroelectrochemical properties of poly(TAPE‐co‐NMPy) [P(TAPE‐co‐NMPy)] were investigated. Results showed that the copolymer revealed color change between light yellow and green upon doping and dedoping of the copolymer, with a moderate switching time. Furthermore, as an application, dual‐type absorptive/transmissive polymer electrochromic device (ECD) based on poly(TAPE‐co‐NMPy) and poly(3,4‐ethylene dioxythiophene) (PEDOT) have been assembled, where spectroelectrochemistry, switching ability, stability, and optical memory of the ECD were investigated. Results showed that the device exhibited good optical memory and stability with moderate switching time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1988–1994, 2006     

Swelling kinetics of a hydrogel of poly(ethylene glycol) and poly(acrylamide‐co‐styrene)

A hydrogel incorporating the hydrophilic polymer poly(ethylene glycol) and a copolymer of acrylamide and styrene was synthesized, and the dynamics of the water‐sorption process were studied. The effects of the composition of the hydrogel and the temperature of the swelling medium were investigated with respect to the water‐sorption characteristics of the hydrogel, and the kinetic parameters, including the swelling exponent and diffusion constant, were evaluated. The hydrogel was also judged for the antithrombogenic property of its surface. The experimental findings were explained on the basis of the core–shell polymeric structure of the hydrogel. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1419–1428, 2002     

Multiple Shape Memory,Self‐Healable,and Supertough PAA‐GO‐Fe3+ Hydrogel

A multiple shape memory and self‐healing poly(acrylic acid)‐graphene oxide‐Fe³⁺ (PAA‐GO‐Fe³⁺) hydrogel with supertough strength is synthesized containing dual physically cross‐linked PAA network by GO and Fe³⁺. The first GO cross‐linked hydrogel can be reversibly reinforced by immersing in FeCl₃/HCl and pure water and softened by immersing in HCl. The tensile strength is 2.5 MPa with the break strain of 700%. Multiple shape memory capability is found depending on this unique feature, the hydrogel can be fixed in four temporary shapes by adjusting the immersing time in FeCl₃/HCl and pure water, and recovered in sequence by immersing in HCl. This hydrogel also exhibits perfect self‐healing behavior, the cut as‐prepared hydrogel is almost completely healed by immersing in FeCl₃/HCl. Besides, the hydrogel shows enhanced electrical conductivity with the presence of GO and Fe³⁺. This supertough hydrogel provides a new way to design soft actuators.

     

Reinforcing biopolymer hydrogels with ionic‐covalent entanglement hydrogel microspheres

Microscopic hydrogel spheres can be used to improve the mechanical properties of conventional hydrogels. We prepared ionic‐covalent entanglement (ICE) hydrogel microspheres of calcium cross‐linked gellan gum and genipin cross‐linked gelatin using a water‐in‐oil emulsion‐based processing technique. The method was optimized to produce microspheres with number average diameter 4 ± 1 µm. These ICE microspheres were used to reinforce gelatin hydrogels and improve their compressive mechanical properties. The strongest microsphere reinforced hydrogels possessed a compressive mechanical stress at failure of 0.50 ± 0.1 MPa and a compressive secant modulus of 0.18 ± 0.02 MPa. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40557.     

Reduction of postsurgical adhesion formation with CM‐chitosan hydrogel barriers prepared by using γ‐irradiation

To evaluate the efficiency of carboxymethylchitosan (CM‐chitosan)‐based hydrogels as barriers for reducing postsurgical adhesions, CM‐chitosan was synthesized to simplify the hydrogel‐making process, and the CM‐chitosan solutions were cross‐linked by using γ‐ray irradiation to create the desired hydrogels instead of using chemical cross‐linking reagents. The prepared CM‐chitosan hydrogels were characterized by a FTIR spectroscopy, swelling behavior, gel‐fraction content,and mechanical property such as gel strength of a hydrogel and the results showed a good swelling behavior and mechanical properties. Also, the radiation‐induced CM‐chitosan hydrogels significantly reduced and inhibited the postsurgical adhesions in the rat models. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A Composite Thermo‐Responsive Membrane System for Improved Controlled‐Release

A novel composite thermo‐responsive membrane system for improved controlled‐release is successfully developed. The membrane is composed of a porous membrane with grafted poly(N‐isopropylacrylamide) (PNIPAM) gates acting as functional valves, and a cross‐linked PNIPAM hydrogel inside the reservoir acting as the solute carrier. The thermo‐responsive controlled‐release characteristics of the proposed system are studied when the ambient temperature is continuously increased from 20 to 45 °C (across the LCST of PNIPAM) at a constant rate of 1.5 °C/min. The experimental results show that the prepared system exhibits significantly better performance for thermo‐responsive controlled‐release than single‐functional systems currently in existence, due to the cooperative action of the gating membrane and the inner cross‐linked hydrogel. Furthermore, due to the distinctive composite architecture, the proposed system can overcome some inherent disadvantages of current systems, such as the drug security problem of the reservoir‐type systems and the mechanical strength problem of the hydrogel matrix‐type systems. The system proposed in this study provides a new mode for thermo‐responsive controlled‐release.     

Preparation of a multicolored reflective electrochromic display based on monodisperse polymeric microspheres with N‐substituted viologen pendants

This article describes the preparation of multicolored reflective electrochromic displays (R‐ECDs) through the control of the substituents on viologen molecules. Cyanophenyl, salicylic, and alkyl groups were introduced onto 4,4′‐bipyridine. The synthesized viologen moieties were reacted with methyl chloride functional groups on polymeric microspheres, which were prepared by seeded polymerization. Then, R‐ECD cells were fabricated by the placement of a mixture of the viologen‐modified polymeric microspheres and an electrolyte between sandwiched indium tin oxide glasses. The R‐ECDs showed an inherent white background without potential because the micrometer‐range particles scattered the light, and the viologen was in a bleached state. From spectrophotometric results and visual appearances, it was confirmed that the devices showed red, green, and blue colors with respect to cyanophenyl‐, salicylic‐, and alkyl‐substituted viologen pendants when 2.7 V was applied to the device. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of Na‐alginate/acrylamide hydrogel and its application in dye removal

Sodium‐alginate/acrylamide (Na‐alginate/AAm) cross‐linked hydrogel was prepared by using gamma irradiation. The gel content and the water absorbency behavior of the obtained hydrogel were investigated. The thermal and morphological properties of the prepared hydrogel were studied using thermogravimetric analysis and scanning electron microscopy. The thermal stability of Na‐alginate significantly changed when mixed with AAm. The adsorption of basic violet 7 dye on the prepared hydrogel was studied under different experimental conditions. The adsorption process for the basic dye was proven pH dependent. The maximum amount adsorbed (mg/g) of the Na‐alginate/AAm for the dye was found to be 78.1.0 mg/g at pH 9.0. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

PP/EMA TPV: Dynamic cross‐linking through an alcoholysis reaction

Dynamically cross‐linked polypropylene/ethylene‐methyl acrylate copolymer (PP/EMA) blends were prepared by reactive extrusion in a twin‐screw extruder. A transesterification reaction catalyzed by an organotin compound and involving the acrylic moieties of EMA with the hydroxyl functions of pentanediol was used to cross‐link the elastomeric EMA. The extent of cross‐linking was estimated through the insoluble content of the blend. The alcohol‐to‐acrylic ratio and the dibutyltin oxide content were both found to increase the extent of EMA cross‐linking. Processing conditions such as screw rotation rate and extrusion temperature profile also had large effects on the reaction extent. Conditions leading to improved pentanediol dispersion were found to generate larger insoluble fraction. Gel contents of EMA phase as high as 75% to 85% were obtained. Interestingly, the stress‐strain curves of the cross‐linked PP/EMA TPV are very well represented by a single master curve. Dynamically cross‐linked blends show systematically better tensile properties than the non‐reactive system. Blends with higher insoluble content show strain at break in the 650%–750% range. Material processability was maintained, even for the systems with high insoluble content.     

Synthesis and gelation of fluoroalkyl end‐capped copolymers containing glucosyl segments: Application to new fluorinated conductive polymer electrolytes

Fluoroalkyl end‐capped copolymers containing glucosyl segments were prepared by the copolymerizations of fluoroalkanoyl peroxides with 2‐glucosyoxyethyl methacrylate (GEMA) and comonomers such as acrylic acid (ACA) and methacrylate monomer‐containing poly(oxyethylene) units (PME). Under the non‐cross‐linked conditions, fluoroalkyl end‐capped GEMA–ACA and GEMA–PME copolymers were found to cause a gelation in dimethyl sulfoxide (DMSO), where the aggregations of end‐capped fluoroalkyl segments and the hydrogen‐bonding interaction between hydroxyl segments are involved in establishing a physical gel network, although the corresponding nonfluorinated GEMA copolymers could cause no gelation in DMSO. More interestingly, it was demonstrated that these fluorinated polymeric gelling electrolytes containing lithium salts exhibit a considerably high ionic conductivity of 10^?3 S/cm level at room temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2833–2838, 2002     

Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca²⁺ DN hydrogel is reported, consisting of a HPAAm network and a Ca²⁺ cross‐linked alginate network. The HPAAm/alginate‐Ca²⁺ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m^?3. HPAAm/alginate‐Ca²⁺ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.     

Electrochromic properties of some bis‐chalcone derivatives‐based nanofibers

Two bis‐chalcone derivatives, (2E,6E)‐2,6‐bis[(thiophen‐2‐yl)methylene]cyclohexanone ( C1 ) and (2E,6E)‐2,6‐bis[(furan‐2‐yl)methylene]cyclohexanone ( C2 )‐based electrochromic (EC) nanofibers were produced in the presence of poly(methyl methacrylate) (PMMA) as supporting polymer using the electrospinning technique. The scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy were used to examine morphology and chemical compositions of nanofibers before and after stability test. SEM images of the obtained smooth and bead‐free nanofibers before the stability test showed that both bis‐chalcone derivatives were homogeneously dispersed on the surface of the electrospun nanofibers. Nanofibers of bis‐chalcone derivatives were characterized with Fourier‐transform infrared spectroscopy. The electrochemical and EC properties of these bis‐chalcone derivatives were investigated. The C1 ‐PMMA nanofiber‐based electrochromic device (ECD) showed higher ΔT_max (41.47%) than that of the C2 ‐PMMA nanofiber‐based ECD (4.67%) during coloration/bleaching at 715 nm. The switching times for coloration and bleaching of C1 ‐PMMA nanofiber‐based ECD were found to be 4.42 and 1.12 s, respectively, and the coloration efficiency was 136.18 cm²/C. Repeated cyclic voltammograms and 1000 cycles of chronoamperometric measurements of the bis‐chalcone derivatives indicated that ECDs have long‐term redox stability. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46010.     

Microfocus small‐angle X‐ray scattering investigation of the skin–core microstructure of lyocell cellulose fibers

Skin–core microstructures in hydrated lyocell cellulose fibers were investigated using microfocus small‐angle X‐ray scattering (SAXS). Both single fibers and fiber cross sections were observed. In all the fibers studied, a thin skin layer was observed in which the voids were smaller in cross section and better oriented than those in the core. The division between skin and core may not be sharp. A draw ratio of 0.80 gave fibers with voids that were shorter, larger in cross section, and more misoriented than the fibers produced with a draw ratio of 1.68. In contrast, there was little difference in void structure between samples coagulated in water and in 25% amine oxide aqueous solution. The results are explained in the context of the spinodal decomposition, which is thought to occur during coagulation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2799–2816, 2002; DOI 10.1002/app.10256     

Graphene Oxide Hybrid Supramolecular Hydrogels with Self‐Healable,Bioadhesive and Stimuli‐Responsive Properties and Drug Delivery Application

Self‐healable hydrogels are promising soft materials with great potential in biomedical applications due to their autonomous self‐repairing capability. Although many attempts are made to develop new hydrogels with good self‐healing performance, to integrate this characteristic along with other responsive multifunctions into one hydrogel still remains difficult. Here, a self‐healable hybrid supramolecular hydrogel (HSH) with tunable bioadhesive and stimuli‐responsive properties is reported. The strategy is imparting graphene oxide (GO) nanosheets and quadruple hydrogen bonding ureido‐pyrimidinone (UPy) moieties into a thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) polymer matrix. The obtained GO–HSH hydrogel shows rapid self‐healing behavior and good adhesion to various surfaces from synthetic materials to biological tissue. In addition, doxorubicin hydrochloride (DOX) release profiles reveal the dual thermo‐ and pH‐responsiveness of the GO–HSH hydrogel. The DOX‐loaded hydrogel can further directly adhere to titanium substrate, and the released DOX from this thin hydrogel coating remains biologically active and has high capability to kill tumor cells.