Preparation and characterization of chitosan films,crosslinked with symmetric aromatic dianhydrides to achieve enhanced thermal properties

This study has developed a new generation of crosslinked chitosan‐based films using symmetric aromatic dianhydrides as crosslinking agents. The formation of the dianhydride‐crosslinked chitosan hydrogel films was confirmed using Fourier transform infrared and solid‐state ¹³C NMR spectral analyses. The films obtained from these derivatives were characterized by their thermal, swelling and hydrophilic properties. The results showed that introducing a cyclic imide moiety into the chitosan matrices played a significant role in enhancing the thermal properties of these chitosan films. It was found that even at high levels of substitution, thermal stability of the studied chitosan derivatives was improved, in spite of a reduction in crystallinity. Heterocyclic imide linkages produced networks that were insoluble in both acidic and alkaline media but allowed swelling in aqueous media. An increase in the hydrophobicity of the chitosan film surfaces was observed after introduction of the cyclic imide moiety. These engineered films produced noteworthy results concerning their thermal and swelling properties. There is a need to further investigate these films for drug delivery and biomaterials applications. © 2014 Society of Chemical Industry     

Synthesis and characterization of biodegradable TPP/genipin co-crosslinked chitosan gel beads

Novel chitosan gel beads were synthesized by a coupled ionic and chemical co-crosslinking mechanism. Tripolyphosphate (TPP) and a naturally occurring crosslinking reagent, genipin, which has been used in herbal medicine, were employed, respectively, as an ionic and a chemical crosslinkers to prepare the chitosan-based networks of gel beads. The competitive crosslinking of chitosan with ionic crosslinker (TPP) and chemical crosslinker (genipin) was characterized by FTIR, UV and EDAX spectroscopy (X-ray energy dispersion) spectroscopy. The variation of characteristic peak of genipin observed from UV spectroscopy and the characteristic peak of tripolyphosphate in crosslinked chitosan-based networks observed from FTIR spectroscopy suggests that the co-crosslinking mechanism is dependent on the pH of TPP/genipin co-crosslinker. The energy profiles of carbon and phosphorus estimated from confirms that chemical crosslinking dominates the co-crosslinking reaction at higher pH condition (pH 7.0 and 9.0) and ionic crosslinking dominates the co-crosslinking reaction at lower pH condition (pH 1.0, 3.0 and 5.0). The pH-dependent ionic/chemical co-crosslinking mechanism shows an obvious effect on the swelling property and enzymatic degradation behavior of prepared chitosan networks. These results reveal that the ionic/chemical co-crosslinked chitosan networks may be suitable for biomedical applications.     

Properties of hydrophilic chitosan network membranes by introducing binary crosslink agents

Summary In this investigation chitosan (CS) was crosslinked using mixtures of sulfosuccinic acid (SSA) and glutaraldehyde (GA) as the binary crosslinking agents to form hydrophilic chitosan network membranes. GA and SSA improve the tensile strength and contribute to hydrophilicity of the membranes, respectively. The membranes prepared by varying the crosslinking agent ratios are also characterized using FT-IR, X-ray diffraction, and tensile testing, and their swelling ratio and thermal properties were measured. Experimental results reveal that the contact angle of the membrane decreases from 84.54° to 69.83° and the maximum stress rises from 39.62 MPa to 133.66 MPa as the increase of the binary crosslinking agent content. These resultant membranes not only maintain its hydrophilicity and but also enhance the mechanical strength.     

The effect of PEG(400)DA crosslinking agent on swelling behaviour of acrylamide-maleic acid hydrogels

Summary Acrylamide (AAm)-maleic acid (MA) hydrogels with different monomer ratios were prepared in an aqueous solution by radical polymerization using a new crosslinking agent, polyethyleneglycol (400) diacrylate (PEG (400) DA) and also using N, N’-methylenebisacrylamide (NMBA) for comparison purpose. The hydrogels were characterized by acid group content and FTIR spectroscopy. Their swelling and deswelling behavior were studied as a function of maleic acid content, pH and crosslinking agent. Incorporation of MA in hydrogels significantly increased their swelling ability and their equilibrium degree of swelling (EDS) values changed between 100–269 g water/g polymer in water while AAm polymers swollen less (22–23 g water/g polymer). The AAm hydrogels showed Fickian type diffusion but the all AAm-MA hydrogels showed non-Fickian type behavior. Their swelling degree increased with the increase of pH of the external medium and an instantaneous increase was observed near a pH value of 7.0. The use of PEG (400) DA instead of NMBA as a crosslinking agent also increased the swelling rate and capacity of the gel. The swelling constant (K) and the diffusion coefficient (D) of hydrogels were also increased. The swelling-deswelling cycles in acidic-basic solutions showed that they could be used as pH responsive gels without any decrease in swelling capacity.     

Fiber formation and physical properties of chitosan fiber crosslinked by epichlorohydrin in a wet spinning system: The effect of the concentration of the crosslinking agent epichlorohydrin

The wet spinning of chitosan fibers was studied with 2% acetic acid as the solvent, 10% aqueous sodium hydroxide as the nonsolvent, and a 4% chitosan solution as the polymer. This article describes the crosslinking of the chitosan fibers. Epichlorohydrin (ECH) was selected as a convenient base‐catalyzed crosslinking agent. The coagulation and crosslinking of the chitosan fibers occurred simultaneously in the coagulation bath. In this study, we investigated the effect of the concentration of the crosslinking agent, ECH, in the spinning dope on the structural, thermal, morphological, and mechanical properties (e.g., the tenacity, elongation, and work of rupture) of chitosan fibers. The tenacity of the chitosan fibers, especially the wet tenacity, was improved by crosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2054–2062, 2004     

Investigation of the preparation and physical properties of a novel semi-interpenetrating polymer network based on epoxised NR and PVA using maleic acid as the crosslinking agent

Natural rubber (NR) and its derivatives as renewable and biodegradable materials have attracted considerable attention because of the serious pollution problems caused by synthetic materials and a shortage of resources. A new semi-interpenetrating polymer network (semi-IPN) based on epoxidised natural rubber and polyvinyl alcohol containing maleic acid as a crosslinking reagent was synthesized and characterized by FTIR, XRD, SEM, swelling ratio in both distilled water and toluene, and mechanical properties. The curing time and dose of maleic acid were varied from 10 to 60 min, and from 10 to 60% (w/w), respectively. An IR spectroscopic study indicated the presence of an ester linkage at 1730 cm⁻¹ in maleic acid crosslinked with PVA in semi-IPN films. In addition, the crystalline content of PVA dramatically decreased after adding maleic acid in the semi-IPN, as observed from its XRD. The semi-IPNs exhibit good mechanical properties, thermal stability, characteristics of a polyvinyl alcohol–maleic acid polymer network. An SEM of the semi-IPNs containing maleic acid showed no phase separation, when compared with the sample prepared in the absence of maleic acid.     

改性季铵化壳聚糖阴离子交换膜的制备与表征

以壳聚糖（CS）为原料,2,3-环氧丙基三甲基氯化铵（GTA）为醚化剂,通过亲核取代反应制备了季铵化壳聚糖（QCS）,用傅立叶变换红外光谱（FT-IR）表征了产物的结构,结果表明产物结构与目标产物的结构相符。以QCS为原料,醋酸溶液为溶剂,通过加入交联剂戊二醛（GA）和荷正电聚苯乙烯（PS）微球,制备了一系列改性QCS阴离子交换膜,并对其含水率、溶胀度、离子交换量等性能进行了测定。结果表明：交联剂的加入可有效抑制QCS膜的形变,降低了其溶胀度,微乳液的加入会在一定程度上提高复合膜的离子交换能力,交联度为4%、微乳液用量为15%（v/v）时离子交换膜的含水率趋于稳定。     

Synthesis,characterization and swelling behavior of chitosan‐sucrose as a novel full‐polysaccharide superabsorbent hydrogel

A novel full‐polysaccharide hydrogel was prepared by crosslinking of chitosan with periodate‐oxidized sucrose. A tetraaldehyde molecule is synthesized via periodate oxidation of sucrose and then applied as a crosslinking agent to form a new hydrogel network. A mechanism for the superabsorbent hydrogel formation via reductive N‐alkylation was also suggested. The structure of the hydrogel was confirmed by FTIR spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). It is shown that crosslinking of chitosan can improve its thermal stability. The effects of crosslinker concentration, pH, and inorganic salt on the swelling behavior of the hydrogel were studied. The results indicate that the hydrogel has good pH sensitivity and pH reversible response. The smart hydrogels may have potential applications in the controlled delivery of bioactive agents and for wound‐dressing application © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

单宁酸对海藻酸钠/壳聚糖微球性能的影响

研究了单宁酸的引入对海藻酸钠/壳聚糖水凝胶在微球化和微胶囊化应用性能方面的影响。首先制备了单宁酸交联改性的海藻酸钠/壳聚糖水凝胶微球。利用傅里叶变换红外光谱分析了共混物分子结构间的相互作用,采用热重分析仪考察了微球热稳定性,并研究了单宁酸的加入对微球粒径、含水量和溶胀性的影响。结果表明由于单宁酸与海藻酸钠/壳聚糖之间的交联作用,它的加入改善了微球的热稳定性,增加了微球的粒径并提高了微球的含水量和溶胀率。在此基础上,单宁酸交联的海藻酸钠/壳聚糖水凝胶包封薄荷油进行微胶囊化应用,证明单宁酸交联的聚合物所制备的微胶囊,包封率更高,热稳定性更好。     

Chitosan–polyelectrolyte complexation for the preparation of gel beads and controlled release of anticancer drug. II. Effect of pH‐dependent ionic crosslinking or interpolymer complex using tripolyphosphate or polyphosphate as reagent

Chitosangel beads were prepared using an in‐liquid curing method by ionotropic crosslinking or interpolymer linkage with tripolyphosphate (TPP) or polyphosphate (PP). The ionic interaction of chitosan with TPP or PP is pH‐dependent due to the transition of “ladder‐loop” complex structures. Chitosan gel beads cured in a pH value lower than 6 of a TPP solution was a controlled homogeneous ionic‐crosslinking reaction, whereas chitosan gel beads cured in a lower pH PP solution was a nonhomogeneous interpolymer complex reaction due to the mass‐transfer resistance for the diffusion of macromolecular PP. According to the results of FTIR and EDS studies, it was suggested that significantly increasing the ionic‐crosslinking density or interpolymer linkage of a chitosan–TPP or chitosan–PP complex could be achieved by transferring the pH value of curing agent, TPP or PP, from basic to acidic. The swelling behavior of various chitosan beads in acid medium appeared to depend on the ionic‐crosslinking density or interpolymer linkage of the chitosan–TPP or chitosan–PP complex, which were deeply affected by the in‐liquid curing mechanism of the chitosan gel beads. By the transition of the in‐liquid curing mechanism, the swelling degree of chitosan–TPP or chitosan–PP beads was depressed and the disintegration of chitosan–TPP or chitosan–PP beads did not occur in strong acid. The drug‐release patterns of the modified chitosan gel beads in simulated intestinal and gastric juices were sustained for 20 h. These results indicate that the sustained release of anticancer drugs could be achieved due to the variation of the reaction mechanism of a chitosan–polyelectrolyte pH‐dependent ionic interaction. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1093–1107, 1999     

Tensile,Swelling, and Oxidative Degradation Properties of Crosslinked Polyvinyl Alcohol/Chitosan/Halloysite Nanotube Composites

Glutaraldehyde (GA) crosslinked polyvinyl alcohol (PVA)/chitosan (CS)/halloysite nanotube (HNT) composite films were prepared using a wet casting method. The tensile, morphology, thermal degradation, swelling, moisture, and oxidative degradation properties of crosslinked composite films were carried out. The presences of crosslinking in the composite films were confirmed by FTIR result. The tensile strength of the crosslinked composite films increased up to 0.5 wt% of HNTs loading. Increasing HNTs reduced the thermal degradation, swelling, and moisture properties of crosslinked composite films reduced with the increase of HNTs content. Results also indicated that the crosslinked composite films were degraded using Fenton reagent.     

Crosslinking and decomposition reactions of epoxide functionalized polynorbornene. Part I. FTIR and thermogravimetric analysis

The miniaturization of microelectronic devices has created a demand for new low‐dielectric‐constant materials to be used as insulating layers between metal interconnects. In this study, a functionalized polynorbornene consisting of a copolymer of decyl norbornene and epoxide norbornene has been investigated as a low‐temperature curing dielectric. Polynorbornenes possess properties that are attractive for microelectronics packaging; however, films of these polymers must be crosslinked in order to obtain the solvent resistance and low solvent swelling necessary for multilayer applications. Crosslinking of these materials was achieved by acid‐catalyzed cationic crosslinking of epoxide side groups. The reactions that occurred during higher temperature curing of epoxy functionalized norbornene films were studied using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis. Epoxide crosslinking and epoxide decomposition reactions were identified and studied as a function of temperature and time. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 568–577, 2003     

制备CS-Ca2+-SAL中空凝胶球及其对碳纳米的缓释行为

将海藻酸钠溶液(SAL)滴入壳聚糖-钙(CS-Ca2+)分散溶液中，海藻酸钠与壳聚糖分子链之间发生物理交联的同时，也与钙离子进行同步交联，可一步制备毫米级壳聚糖-钙-海藻酸钠(CS-Ca2+-SAL) 水凝胶球。通过调控原料用量比例、滴加速度和方式，成功制备内部中空，球壁均匀，表面致密的凝胶球。以荧光性碳纳米点为标记物，考察凝胶球的溶胀性能和缓释性能。结果表明，凝胶球在中性水溶液中12h溶胀率可达26.09%，并能保持完整球形结构。溶胀性能受到pH的影响显著，处于pH=1.2的溶液体系中溶胀率最小，处于pH =6.8溶液环境中溶胀率最大。荧光凝胶球在pH=7.1的Tris-HCl缓冲液中的释放率实验结果表明，对碳纳米的释放模型最符合Hixon-crowell溶蚀方程，说明凝胶球的缓释机制以溶蚀为主，扩散为辅。     

Spray-Dried Chitosan Microspheres as a pDNA Carrier

Spray-dried chitosan microparticles with controlled size were prepared and crosslinked with the biocompatible reactant d,l-glyceraldehyde. Glutaraldehyde, the conventional cross-linking agent for proteins, was used as a control. The crosslinking degree was characterized through thermal analysis, X-ray diffraction, and infrared spectroscopy. In addition, the crosslinking effects were analyzed in terms of particle size, swelling capability, and surface charge. By properly choosing d,l-glyceraldehyde concentration and crosslink time, a high degree of control was achieved over the physical properties of particles. Yields around 78% as well as images obtained through fluorescence microscopy demonstrated the capability of the crosslinked chitosan microspheres to pDNA complexation.     

Preparation,characterization, and swelling and drug release properties of a crosslinked chitosan‐polycaprolactone gel

For applications in biotechnology to prepare biopolymers containing functional groups is essential. In addition, these materials have to be strong to provide physical support for practical applications. Recently, chitosan, polycaprolactone (PCL), and their various combinations were used for this purpose. In this work, we described the preparation and characterization of a new biodegradable polymeric gel containing chitosan and PCL. The gel preparation reactions were performed in suitable acetic acid solutions to obtain the products in high yields. A crosslinking agent was added to produce crosslinked gels. Swelling behavior of chitosan/PCL gels in different compositions was studied, and the results were compared. The chitosan/PCL gels show a rather large equilibrium swelling in water and in the phosphate buffered saline solution. Acrylic acid (AA) was added to these gels during preparation process to obtain a stable material for various applications. These polymeric gels were characterized by Fourier transform infrared. Their physical and morphological properties were investigated by using differential scanning calorimeter and scanning electron microscope techniques, respectively. Cell growth experiments indicate that chitosan, a positively charged polysaccharide, is not suitable for cell proliferation studies. On the other hand, the drug release studies were successful and, 59% of lidocaine, was released from a chitosan/PCL/AA hydrogel in buffer solution at pH = 7.4 at 37°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Chitosan/poly(vinyl alcohol) hydrogels for amoxicillin release

Chitosan and poly(vinyl alcohol)-based hydrogel films were synthesized using tartaric acid as a crosslinking agent. The films denoted as CVT were then characterized using Fourier transform infrared, Nuclear magnetic resonance, X-ray diffraction, and scanning electron microscopy analysis. TG/DTG and DSC analysis were also carried out for the determination of thermal properties of hydrogel films. Swelling properties of these hydrogel films were investigated at two different pHs and temperatures. The swelling behaviors of all samples were increased in acidic medium, while decreased in alkaline medium. The enzymatic degradation of the hydrogels was studied using lysozyme, and degradation rates were found to be parallel with the swelling ratio for CVT hydrogel. The hydrogels were also used for the amoxicillin release in KCl/HCl and PBS buffer solutions. The release behaviors of CVT hydrogel films were slower and can be controlled as compared with commercial drug release systems. CVT hydrogel films may be more appropriate for controlled release of amoxicillin.     

Preparation and characterization of pH‐sensitive hydrogels based on chitosan,itaconic acid and methacrylic acid

Novel pH‐sensitive hydrogels based on chitosan, itaconic acid and methacrylic acid were prepared in two steps. Chitosan was first ionically crosslinked with itaconic acid, after which a free radical polymerization and crosslinking of the chitosan/itaconic acid network was performed by adding methacrylic acid and a crosslinker in order to achieve better mechanical properties and tunable swelling. The samples were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, X‐ray diffraction, dynamic mechanical analysis and the swelling ratios of the hydrogels at various pH values (2.0–8.0). The hydrogel composition is found to have a great impact on the hydrogel structure, mechanical and thermal properties, morphology and swelling kinetics. The highly porous morphology of the gels is probably connected with the bulky chitosan/itaconic acid network which reduces the degree of crosslinking in the second step of the synthesis due to steric hindrances. The gels demonstrate substantial change in buffer absorbency with change of pH, low for acid buffers and the higher for pH values above 6 where the swelling is considerably slow, thus suggesting their strong candidature for use as oral drug‐delivery systems in the lower parts of the gastrointestinal tract and for drugs that require longer release times. Copyright © 2010 Society of Chemical Industry     

Hybrid ternary organic-inorganic films based on interpolymer complexes and silica

Homogeneous transparent hybrid films consisting of chitosan (CHI), poly(monomethyl itaconate) (PMMI) and silica were obtained indicating the absence of microphase separation. These ternary hybrid materials are very interesting since materials with high functionality can be obtained presenting different properties from those of the starting materials but with the advantage of preserving the inherent property of each component. The inorganic phase was prepared by sol-gel process of tetraethoxysilane (TEOS). Most of the amine groups from CHI (pK_b 7.7) are quaternized in the acidic medium used in the preparations (pH=2), where a physical crosslinking via hydrogen bonding could occur through carboxyl groups from PMMI. Silica gel obtained from TEOS has been intercalated as a very fine dispersion in the polymer complex formed between CHI and PMMI. Transmission electron microscopy and atomic force microscopy was used to examine the homogeneity of the ternary polymer hybrids (CHI/PMMI/SiO₂), obtained as self-supported films. The results support the nanometer scale dispersion of the phases. Porous silica films with high BET area were obtained by calcination of the hybrid films. The mean pore diameter of these silica films corresponds to the dimension of the polymer domains observed in the pristine hybrid films. Moreover, it was found that the swelling behavior of the samples was influenced by the organic and inorganic phases, where the inorganic phase tends to diminish the swelling.     

Immobilization of bacteriocin nisin into a poly(vinyl alcohol) polymer matrix crosslinked with nontoxic dicarboxylic acid

This work is focused on novel methodology of poly(vinyl alcohol) crosslinking by non‐toxic dicarboxylic acid, glutaric acid. The cross‐linked system was used as a matrix for immobilization of bacteriocin nisin. Effect of the crosslinking degree on physico‐chemical, morphology, mechanical, and thermal properties of poly(vinyl alcohol) films were investigated by using swelling test, Fourier transform infrared spectroscopy, scanning electron microscopy, stress–strain analysis, differential scanning calorimetry, and thermogravimetry. Release profile of the nisin from the cross‐linked poly(vinyl alcohol) was studied by high performance liquid chromatography. Antibacterial activity of the prepared systems was tested by agar diffusion test and dilution and spread plate technique. Results showed suitability of glutaric acid as effective crosslinking agent of poly(vinyl alcohol) that acts synergistically with bacteriocin nisin against the tested Gram‐positive and Gram‐negative bacterial strains. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43674.     

pH-sensitivity of hydrogels based on complex forming chitosan: Polyether interpenetrating polymer network

This article deals with a novel, semi-interpenetrating polymer network (semi-IPN), composed of crosslinked chitosan (cr-CS) with glutaraldehyde (GA) and polyether. The pH responsibility of swelling data shows that swelling reaches a maximum at pH = 3.19 and a minimum at pH = 13. The concentration of chitosan acetic acid solution, the amount of crosslinking agent, and polyether, have effects on the swelling behavior of the network. Stimulating-response of the semi-IPN, induced by abrupt changes of pH between 1 and 13, is discussed as well. © 1993 John Wiley & Sons, Inc.