Effect of naturally occurring crosslinking junctions on green strength of natural rubber

The effect of the naturally occurring crosslinking junctions on green strength of natural rubber, isolated from Hevea brasiliensis, was investigated by using rubber extracted from Parthenium argentatum Gray (Guayule) as a model. Guayule rubber and natural rubber were characterized through nuclear magnetic resonance spectroscopy and size exclusion chromatography. The non‐rubber components of Guayule rubber and natural rubber were characterized by Kjeldahl method and Fourier transform infrared spectroscopy. It was found that Guayule rubber contains a much higher amount of fatty acids and their esters while it contains no proteins. The gel content, determined by swelling method, was related to a number of naturally occurring crosslinking junctions of Guayule rubber and natural rubber. The outstanding green strength of natural rubber was attributed to the effect of naturally occurring crosslinking junctions, when stress–strain curve and tensile properties of unvulcanized Guayule rubber were compared with those of unvulcanized natural rubber. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis,characterization, and swelling behaviors of chitosan‐g‐poly(acrylic acid)/poly(vinyl alcohol) semi‐IPN superabsorbent hydrogels

A series of granulated semi‐interpenetrating polymer network (semi‐IPN) superabsorbent hydrogels composed of chitosan‐g‐poly(acrylic acid) (CTS‐g‐PAA) and poly(vinyl alcohol) (PVA) were prepared by solution polymerization using ammonium persulfate (APS) as an initiator and N,N′‐methylenebisacrylamide (MBA) as a crosslinker. The effects of reaction conditions such as the concentration of MBA, the weight ratio of AA to CTS, and the content of PVA on water absorbency were investigated. Infrared (IR) spectra and differential scanning calorimetry (DSC) analyses confirmed that AA had been grafted onto CTS backbone, and PVA semi‐interpenetrating into CTS‐g‐PAA networks. SEM analyses indicated that CTS‐g‐PAA/PVA has improved porous surface and PVA was uniformly dispersed in CTS‐g‐PAA network. The semi‐IPN hydrogel containing 10 wt% PVA shows the highest water absorbency of 353 and 53 g g^?1 in distilled water and 0.9 wt% NaCl solution, respectively. Swelling behaviors revealed that the introduction of PVA could improve the swelling rate and enhance the pH stability of the superabsorbent hydrogel. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and characterization of chitosan‐modified polymethyl methacrylate latex particles

Stable chitosan‐modified polymethyl methacrylate (PMMA) latex particles were prepared by using 2,2′‐azobis(2‐amidinopropane) dihydrochloride (V‐50) as the cationic initiator. The polymerization rate (R_p) is controlled by the V‐50 concentration ([V‐50]) and R_p is less sensitive to the chitosan concentration ([C]) used in the synthesis work. The reaction system follows Smith–Ewart Case III kinetics due to the relatively large particles produced. The zeta potential data show that the isoelectric point (pI) of the latex particles is 10.7. The amounts of V‐50 (C_V‐50) and chitosan (C_c) ultimately incorporated into the particles correlate reasonably well with [V‐50] and [C], respectively. At pH 7, the quantity of the negatively charged bovine serum albumin (BSA, pI = 4.8) adsorbed on the positively charged chitosan‐free particles (Q) via the electrostatic interaction increases with increasing C_V‐50. However, Q is relatively insensitive to changes in C_c. This result implies that only the outermost region of the hairy chitosan‐modified particles is available for adsorption of the relatively large protein species. Colloidal stability shows a significant influence on the BSA adsorption process. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1489–1499, 1999     

Synthesis and characterization of electroactive epoxy‐based interpenetrating polymer network gel

Epoxy/poly(N‐isopropylacrylamide) interpenetrating polymer network gels were prepared by varying the excess amine content in the matrix (0.4–0.6 equivalent). All the samples were characterized for mechanical properties and swelling in distilled water. The topography of polymer network was characterized by atomic force microscopy. The 0.5 equiv. excess amine sample exhibited optimum properties. Studies on swelling at different pH and electroactivity in different aqueous solution were performed. The bending angle observed during first 1 min was 1–5° at 3–10 V and a maximum of 25° in 5 min at 20 V for 0.5 equiv. excess amine in NaCl solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Modified chitosan III,superabsorbency, salt‐ and pH‐sensitivity of smart ampholytic hydrogels from chitosan‐g‐PAN

Polyacrylonitrile (PAN) grafted chitosan was prepared by ceric‐initiated graft polymerization of acrylonitrile onto chitosan in a homogenous medium. The copolymer chitosan‐g‐PAN product was then hydrolyzed to yield a novel smart hydrogel (H‐chitoPAN) with superabsorbing properties. The influence of add‐on values as well as temperature and time of hydrolysis of the initial chitosan‐g‐PAN on swelling behavior of the hydrogel was evaluated in water and various salt solutions. The swelling kinetics of the superabsorbing hydrogel was studied as well. The hydrogels exhibited ampholytic and pH‐sensitivity characteristics. Several sharp swelling changes were observed in lieu of pH variations in a wide range (pH 2–13). The swelling variations were explained according to swelling theory based on the hydrogel chemical structure. Superabsorbency, pH‐ and salt‐sensitivity of the chitosan‐based hydrogel was briefly compared with the classical starch‐based superabsorbent, H‐SPAN. The pH‐reversibility and on–off switching behavior of the intelligent H‐chitoPAN hydrogels makes them good candidates for considering as potential drug carries. Copyright © 2004 John Wiley & Sons, Ltd.     

Site specificity of the (alpha)C--H bond dissociation energy for a naturally occurring beta-hairpin peptide-An ab initio study

A naturally occurring beta-hairpin peptide (PDB ID 1UAO) was used as a model to study the backbone oxidation of a protein with ab initio calculation at the B3LYB/6-31G(d) without any constraints. The (alpha)C--H bond dissociation energy of three different glycyl radicals located at different sites on the beta-hairpin peptide was calculated to evaluate the site specificity of backbone oxidation. The molecular and electronic structures of these glycyl radicals were analyzed to rationalize this site specificity. The overall molecular structure of the alpha-H abstracted beta-hairpin peptide remained almost unchanged with the exception of the local conformation of the attacked residue. However, the (alpha)C--H bond strength varied dramatically among these different sites.     

Kinetic study of chitosan‐tripolyphosphate complex reaction and acid‐resistive properties of the chitosan‐tripolyphosphate gel beads prepared by in‐liquid curing method

Chitosan gel beads were prepared using an in‐liquid curing method by the ionotropic crosslinking with sodium tripolyphosphate. Crosslinking characteristics of the chitosan‐TPP beads were improved by the modification of in‐liquid curing mechanism of the beads in TPP solution. Chitosan gel beads cured in pH value lower than 6 were really ionic‐crosslinking controlled, whereas chitosan gel beads cured in pH values higher than 7 were coacervation‐phase inversion controlled accompanied with slightly ionic‐crosslinking dependence. According to the result, significantly increasing the ionic‐crosslinking density of chitosan beads could be achieved by transferring the pH value of the curing agent, TPP, from basic to acidic. The swelling behavior of various chitosan beads in acid appeared to depend on the ionic‐crosslinking density of the chitosan‐TPP beads that were deeply affected by the curing mechanism of the beads. The mechanism of chitosan‐TPP beads swollen in weak acid was chain‐relaxation controlled, while the mechanism of chitosan‐TPP beads swollen in strong acid seem to be not only chain‐relaxation but also chain‐scission controlled. Chitosan‐TPP beads prepared in acidic TPP solution decreased the chain‐scission ability due to the increase of ionic crosslinking density of the beads. By the transition of curing mechanism, the swelling degree of chitosan‐TPP beads was depressed, and the disintegration of chitosan‐TPP beads would not occur in strong acid. The mechanism of ionic‐crosslinking reaction of chitosan beads could be investigated by an unreacted core model, and the curing mechanism of the chitosan beads is mainly diffusion controlled when higher than 5% of chitosan was employed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1551–1564, 1999     

Synthesis of well‐defined chitosan‐based tricomponent copolymers for constructing multifunctional delivery systems

Chitosan‐based tricomponent copolymers, chitosan‐g‐poly(ε‐caprolactone)‐(g‐poly(oligo(ethylene glycol) methacrylate)) (CS‐PCL‐POEGMA, CPP), are synthesized as multifunctional nanocarriers for antitumor therapy. 2‐Bromoisobutyric acid and PCL are first site‐specifically conjugated onto the hydroxy groups of chitosan backbone through conventional coupling chemistry to give CS‐PCL‐Br using sodium dodecyl sulfate–chitosan complex as an organosoluble intermediate. CPP‐PCL‐Br is further used for initiating the single electron transfer‐living radical polymerization of OEGMA in the mixed solvent of dimethyl sulfoxide and lactic acid, yielding CPP. One‐pot reaction of CPP with a small amount of NaN₃ under the catalysis of Cu(I)Br/tris‐(2‐dimethylaminoethyl)amine converts the bromo ends of POEGMA grafts to azide functionality, which is used for conjugation of folic acid targeting moiety via azide–alkyne click reactions. The resultant tricomponent copolymers can assemble into spherical micelles with the capacity of coincorporating indocyanine green and Doxorubicin through electrostatic and hydrophobic interactions, respectively. The dual‐agent‐loaded micelles display a combined effect for combating HepG2 cells when irradiated with near‐infrared laser. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Characterization and antibacterial activity of chitosan‐based composites with polyester

The effects of replacing poly(butylene succinate adipate) (PBSA) with acrylic acid‐grafted PBSA (PBSA‐g‐AA) on the structure and the properties of a PBSA/chitosan composite were investigated. The properties of both PBSA‐g‐AA/chitosan and PBSA/chitosan were compared using Fourier transform infrared (FTIR), ¹³C nuclear magnetic resonance (NMR), X‐ray diffraction (XRD), and an antibacterial activity test. With PBSA‐g‐AA in the composite, the compatibility with chitosan and, consequently, the properties of the composite became greatly improved due to the formation of ester and imide groups that conferred better dispersion and homogeneity of chitosan in the matrix. Composites containing PBSA‐g‐AA/chitosan exhibited superior mechanical properties due to greater compatibility between the two components. Moreover, chitosan enhanced the antibacterial activity of the composites. Composites of PBSA‐g‐AA or PBSA that contain chitosan have better antibacterial activity. The functionalized PBSA‐g‐AA/chitosan composites showed markedly enhanced antibacterial properties due to the carboxyl groups of acrylic acid, which acted as coordination sites for the chitosan phase, allowing the formation of stronger chemical bonds. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and properties of chitosan‐based thermo‐ and pH‐responsive nanoparticles and application in drug release

In this research, thermo‐ and pH‐responsive nanoparticles with an average diameter of about 50–200 nm were synthesized via the surfactant‐free emulsion polymerization. The thermal/pH dual responsive properties of these nanoparticles were designed by the addition of a pH sensitive monomer, acrylic acid (AA), to be copolymerized with N‐isopropylacrylamide (NIPAAm) in a chitosan (CS) solution. The molar ratio of CS/AA/NIPAAm in the feed was changed to investigate its effect on structure, morphology, thermal‐ and pH‐responsive properties of the nanoparticles. It was found that CS‐PAA‐PNIPAAm nanoparticles could be well dispersed in the aqueous solution and carried positive charges on the surface. The addition of thermal‐sensitive NIPAAm monomer affected the polymerization mechanism and interactions between CS and AA. The particle size of the nanoparticles was found to be varied with the composition of NIPAAm monomer in the feed. The synthesized nanoparticles exhibited stimuli‐responsive properties, and their mean diameter thus could be manipulated by changing pH value and temperature of the environment. The nanoparticles showed a continuous release of the encapsulated doxycycline hyclate up to 10 days during an in vitro release experiment. The environmentally responsive nanoparticles are expected to be used in many fields such as drug delivery system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2798–2810, 2009     

Facile synthesis of tetrahydrobenzoxanthenones via a one‐pot three‐component reaction using an eco‐friendly and magnetized biopolymer chitosan‐based heterogeneous nanocatalyst

A chitosan‐based magnetic nanocomposite was synthesized by an eco‐friendly and simple procedure, and was characterized using scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray diffraction and Fourier transform infrared spectroscopy. It was then used for the efficient synthesis of tetrahydrobenzoxanthen‐11‐one derivatives via a one‐pot three‐component condensation of 2‐naphthol, various aldehydes and dimedone in ethanol. The catalyst was recovered easily and reused several times without significant loss of catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of crosslinker density on the polymer network structure in poly‐N,N‐dimethylacrylamide hydrogels

To investigate the effects of crosslinker density on the properties of hydrogels, compression tests, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Raman measurements were performed on poly‐N,N‐dimethylacrylamide hydrogels. The results of the compression tests showed that the Young's modulus increases as the crosslinker density increases. To understand the mechanism of the change in the mechanical properties, the structures of the polymer networks and water and the molecular vibrations were analyzed using SEM, DSC, and Raman methods. From the SEM images, it was found that the porosity estimated from the mesh size and cell density increases with increasing crosslinker density. In addition, the DSC and Raman results show that the thickness of the bound water increases as the porosity increases, although the density of the polymer chains in the porous wall remains nearly constant. The increase in the number density of polymer chains can be one of the mechanisms contributing to the increase in the mechanical strength of the hydrogels at lower crosslinker density below 5 mol %, as proposed by previous studies. At higher crosslinker density, however, the number density of polymer chains does not increase with increasing crosslinker density. The present results suggest that the bound water plays an important role in strengthening the hydrogel. The water structure may be one of the dominant factors governing the chemical and physical properties of hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1017–1027     

Characterization of ring‐opening polymerization of genipin and pH‐dependent cross‐linking reactions between chitosan and genipin

In this study, a novel chitosan‐based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent—genipin. The results showed that the crosslinking reactions were pH‐dependent. Under basic conditions, genipin underwent a ring‐opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ～ 88 monomer units). This ring‐opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C‐1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin‐crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin‐crosslinked chitosan gels. The ring‐opening polymerization of genipin and the pH‐dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan‐based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985–2000, 2005     

Chitosan‐based composite membranes containing chitosan‐coated carbon nanotubes for polymer electrolyte membranes

Considering the poor dispersion and inert ionic conduction ability of carbon nanotubes (CNTs), functionalization of CNTs is a critical issue for their application in polymer electrolyte membranes. Herein, CNTs were functionalized by the polyelectrolyte, chitosan (CS), via a facile noncovalent surface‐deposition method. The obtained CS‐coated CNTs (CS@CNTs) were then incorporated into the CS matrix and fabricated composite membranes. The CS coating can enhance the compatibility between CNTs and the matrix, thus ensuring the homogenous dispersion of CS@CNTs and effectively improved the mechanical properties of the composites. Moreover, the CS coating can make CS@CNTs act as an additional proton‐conducting pathway through the membranes. The CS/CS@CNTs‐1 composite shows the highest proton conductivity of 3.46 × 10⁻² S cm⁻¹ at 80°C, which is about 1.5‐fold of the conductivity of pure CS membrane. Consequently, the single cell equipped with CS/CS@CNTs‐1 membrane exhibits a peak power density of 47.5 mW cm⁻², which is higher than that of pure CS (36.1 mW cm⁻²).     

Synthesis of hydroxyl‐bearing polyurethanes from naturally occurring myo‐inositol

A route from naturally occurring myo‐inositol to hydroxyl‐bearing polyurethanes has been developed. The diol prepared from the bis‐acetalization of myo‐inositol with 1,1‐dimethoxycyclohexane was reacted with a rigid diisocyanate, 1,3‐bis(isocyanatomethyl)cyclohexane to afford the corresponding polyurethane, of which glass transition temperature (T_g) was quite high as 192 °C. The polyurethane contains side chains inherited from the acetal moieties of the diol monomer and was treated with trifluoroacetic acid to hydrolyze the acetal moieties and afford the target polyurethane functionalized with hydroxyl groups. The presence of many hydroxyl groups in the side chains, which can form hydrogen bonds with each other, resulted in a high T_g, 186 °C. In addition, the hydroxyl groups were reacted with isocyanates to achieve further side‐chain modifications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1358–1364     

Synthesis of chitosan‐based thermo‐ and pH‐responsive porous nanoparticles by temperature‐dependent self‐assembly method and their application in drug release

In this research, thermo‐ and pH‐responsive chitosan‐based porous nanoparticles were prepared by the temperature‐dependent self assembly method. The chitosan‐graft‐poly(N‐isopropylacrylamide) (CS‐g‐PNIPAAm) copolymer solution was prepared through polymerization of N‐isopropylacrylamide (NIPAAm) monomer in the presence of chitosan (CS) solution using cerium ammounium nitrate as the initiator. Then, CS‐g‐PNIPAAm solution was diluted by deionized water and heated to 40 °C for CS‐g‐PNIPAAm self‐assembly. After that, CS‐g‐PNIPAAm assembled to form micelles in which shell layer was CS. Crosslinking agent was used to reinforce the micelle structure to form nanoparticle. The molar ratio of CS/NIPAAm in the feed mixture was changed to investigate its effect on structure, morphology, thermal‐ and pH‐responsive properties of the nanoparticles. TEM images showed that a porous structure of nanoparticles was developed. The synthesized nanoparticles carried positive charges on the surface and exhibited stimuli‐responsive properties, and their mean diameter thus could be manipulated by changing pH value and temperature of the environment. The nanoparticles showed a continuous release of the encapsulated doxycycline hyclate up to 10 days during an in‐vitro release experiment. These porous particles with environmentally sensitive properties are expected to be utilized in hydrophilic drug delivery system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5126–5136, 2009     

A novel crosslinker for UV copolymerization of N-vinyl pyrrolidone and methacrylates to give hydrogels

Methacryloxyethyl vinyl carbonate, a novel crosslinker containing a vinyl carbonate and a methacrylate group which is capable of copolymerizing N-vinyl pyrrolidone and methacrylates, was prepared. With this crosslinker, N-vinyl pyrrolidone (NVP) and methacrylates can copolymerize efficiently under strictly UV conditions. It was found that this crosslinker was able to give hydrogels with higher water content, higher tear strength, and lower modulus, compared to a traditional dimethacrylate crosslinker. In addition, hydrogel lenses fabricated by UV curing of formulations using this crosslinker were found to be superior to those fabricated with established process in terms of optical quality. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1039–1046, 1997     

Proton transport through hydrated chitosan‐based polymer membranes under electric fields

Proton transport is essential in many areas of chemistry and biology and is especially important in the fields of proton exchange membrane fuel cells and biocompatible, protonic semiconductors. These devices make use of membranes to control the flow of protons for either the generation of energy or to more closely couple electronics and biology. In the present study, we make use of ab initio molecular dynamics simulations, including the effect of applied electric fields, to gain atomistic insight into the intrinsic conductivity of chitosan‐based polymers and demonstrate that chitosan does not act as a significant source of friction for the transport of protons while increasing the number of free ions. Published 2017.^? J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1103–1109     

Preparation and characterization of conductive chitosan–ionic liquid composite membranes

Anhydrous conductive membranes composing of a composite of chitosan (CS) and ionic liquids with symmetrical carboxyl groups were explored. Scanning electron microscope images revealed that porous composite membranes could be obtained by combining CS with different amounts of 1,4‐bis(3‐carboxymethyl‐imidazolium)‐1‐yl butane chloride ([CBIm]Cl). Fourier transform infrared and proton nuclear magnetic resonance confirmed that the formation of ammonium salts after CS was combined with [CBIm]Cl. The thermal property of CS–ionic liquid composite membranes was studied through thermogravimetric analysis. The anhydrous ionic conductivities of CS–[CBIm]X (X = Cl, Ac, BF₄, and I) composite membranes were measured using ac impedance spectroscopy at room temperature in N₂ atmosphere. The conductivities (0.4–0.7 × 10^?4 Scm^?1), found to be in the same range as semiconductors, were significantly higher than those of pure CS membrane (<10^?8 Scm^?1). In addition, the anhydrous conductivity of composite membrane based on CS–[CBIm]I at room temperature reached a level as high as 0.91 × 10^?2 Scm^?1 when iodine was doped. Copyright © 2011 John Wiley & Sons, Ltd.