Crystalline Transformation of Chitosan from Hydrated to Anhydrous Polymorph Via Chitosan Monocarboxylic Acid Salts

ABSTRACT

Spontaneous removal of monocarboxylic (formic, acetic, propionic or butyric) acids accompanying dehydration of the corresponding chitosan salts was observed from X-ray fiber diffraction diagrams obtained during the storage of these salts for a given period of time. The first three salts were prepared by immersing a tendon chitosan (a hydrated crystal) in an aqueous solution of respective monocarboxylic acid and 2-propanol. The salts showed similar fiber patterns not only to one another but also to the “Eight-fold” polymorph of the original chitosan, indicating that they are Type II salts, hydrated crystals, where the backbone chitosan molecule takes up an eight-fold helical conformation. The temperature required for the salt formation depended on the hydrophobicity of the acid, e.g., the chitosan formic acid salt could be prepared at room temperature, whereas, formation of the propionic acid salt was carried out at 4 °C. All the acids spontaneously evaporated accompanied by dehydration during storage of the salts, resulting in formation of anhydrous crystalline chitosan. Removal of the monocarboxylic acids was accelerated by increasing the hydrophobicity of the acid: at 100% relative humidity approximately 3 months for the formic, 1 month for the acetic and 3 weeks for the propionic acid salts. In the case of butyric acid the anhydrous polymorph of chitosan was obtained immediately after the reaction, suggesting that the water removing action of this acid was too fast to detect a Type II salt by the present X-ray method. The anhydrous crystals of chitosan were irreversibly prepared by annealing a hydrated crystal in water at a high temperature, such as 240 °C, leading to a little loss of orientation and to thermal decomposition of the sample specimen to some extent. But it was found that, through Type II salts of monocarboxylic acids, the hydrated crystals of chitosan can be dehydrated even at room temperature without any loss of orientation and decomposition of the chitosan specimen.     

Binding cellulose and chitosan via click chemistry: Synthesis,characterization, and formation of some hollow tubes

A novel cellulose‐click‐chitosan polymer was prepared successfully in three steps: (1) propargyl cellulose with degrees of substitution (DS) from 0.25 to 1.24 was synthesized by etherification of bamboo Phyllostachys bambusoide cellulose with propargyl chloride in DMA/LiCl in the presence of NaH. The regioselectivity of propargylation on anhydrous glucose unit determined by GC‐MS was in the order of 2 >> 6 > 3; (2) the functional azide groups were introduced onto the chitosan chains by reacting chitosan with 4‐azidobenzoic acid in [Amim]Cl/DMF and the DS ranged from 0.02 to 0.46; (3) thus, the cellulose‐click‐chitosan polymer was obtained via click reaction, that is, the Cu(I)‐catalyzed Huisgen 1,3‐dipolar cycloaddition reaction, between the terminal alkyne groups of cellulose and the azide groups on the chitosan backbone at room temperature. The successful binding of cellulose and chitosan was confirmed and characterized by FTIR and CP/MAS ¹³C NMR spectroscopy. TGA analyses indicated that the cellulose‐click‐chitosan polymer had a higher thermal stability than that of cellulose and chitosan as well as cellulose–chitosan complex. More interestingly, some hollow tubes with near millimeter length were also observed by SEM. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Crosslinking of chitosan fibers with dialdehydes: Proposal of a new reaction mechanism

A highly deacetylated chitosan from shrimp with a degree of deacetylation of 95.28 ± 3.03% was prepared and spun into a monofilament fiber using a solution of 4% (w/v) chitosan in 4% (v/v) aqueous acetic acid. Samples of the spun fibers were immersed in aqueous solutions containing glutaraldehyde and glyoxal, and subsequently washed and dried. When the concentration of crosslinking agent was varied at room temperature over a constant time of 1 h, dry mechanical properties improved up to a point after which increasing concentrations resulted in degradation. Immersion time was also varied between 1 and 60 min at 25.8°C, and temperature was varied between 25.8 and 70.0°C, at fixed concentrations of both glyoxal and glutaraldehyde. It was demonstrated that mechanical improvements might be rendered at higher temperatures over lesser times. However, it was also shown that at higher temperatures, fiber mechanical properties would begin to diminish. Chitosan films were subjected to similar treatments in aqueous crosslinking solutions. Fourier transform infrared data (FTIR) on the films suggest that some interaction is occurring between the glutaraldehyde or glyoxal and the amine group on the chitosan backbone. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1079–1094, 1999     

Properties of nitrocellulose‐coated and polyethylene‐laminated chitosan and whey films

Chitosan (chitosan acetic acid salt) and whey (65% protein) films were coated with a nitrocellulose lacquer or laminated with polyethylene to enhance their water resistance and gas barrier properties in humid environments. The barrier properties were measured by the Cobb₆₀ test and water‐vapor (100% relative humidity) transmission and oxygen (90% relative humidity) permeability tests. Mechanical properties were obtained with tensile tests. Packaging properties were studied with crease and folding tests. The Cobb₆₀ test revealed that the coated films were resistant to liquid water, at least for a short exposure time, if the coating thickness was at least 10–17 μm. Water‐vapor transmission rates comparable to those of polyethylene‐laminated films were obtained for coated chitosan at a coating thickness of 5–7 μm. The coated films possessed low oxygen permeability despite the high humidity. Coated films dried for 3 weeks showed oxygen permeabilities at 90% relative humidity that were similar to values for dry ethylene‐co‐vinyl alcohol at 0% relative humidity. The lacquer partly penetrated the whey films, and this led to excellent adhesion but poor lacquer toughness. The lacquer coating on chitosan was tougher, and it was possible to fold these films 90° without the coating fracturing if the coating thickness was small. The coated whey films were readily creasable. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 985–992, 2001     

Effect of organic and inorganic acids on chitosan/glycerophosphate thermosensitive hydrogel

The influence of organic and inorganic acids on chitosan/glycerophosphate (CS/GP) hydrogel has been investigated by dissolving chitosan in different acids. The results of gelation showed that all of the chitosan dissolved in monovalent acid solutions (i.e., formic, acetic, propionic, butyric, isobutyric, lactic, nitric, hydrochloric, and chloroacetic acid), when neutralized by GP solution, could transform into hydrogel after 2–5 min at 37 °C, while those dissolved in multivalent acids failed in gel formation. The inner structures of CS/GP hydrogels prepared with monovalent acids depended on the ionic strength and chain length of acids. Morphological examination by scanning electron microphotography demonstrated that large pores occurred during the gel-forming process, and the aperture size was also related to different acids. The cytotoxicity studies indicated that CS/GP systems prepared by dissolving chitosan in tested acids except chloroacetic acid were nontoxic to mouse embryonic fibroblasts and Hela cells.     

Opposite trends in thermodynamic compatibility between copolyamide and chitosan in their binary blend

Gibbs energy, enthalpy, and entropy of mixing in binary blends of chitosan with ter‐copolyamide 6/66/610 at ambient conditions have been determined over the entire concentration range using thermodynamic cycle based on dissolution of individual polymers and their blends of different composition in a common solvent – formic acid. Experimental procedure included stepwise equilibrium vapor sorption of glacial formic acid on the cast films and isothermal microcalorimetry of dissolution of these films in liquid glacial formic acid at 25 °C. Formic acid appeared to be a very good solvent for individual polymers and their blends. Flory‐Huggins interaction parameter determined from sorption isotherms was negative and varied from ?2.56 to ?1.79 depending upon blend composition. The enthalpies of dissolution of individual polymers and their blends were strongly exothermic and varied from ?200 to ?40 Joule/g. Independent thermodynamic cycles for Gibbs free energy and enthalpy remarkably revealed similar trends in concentration dependence of different thermodynamic functions of mixing between chitosan and copolyamide. At high chitosan content, the binary blend is characterized by large and negative values of Gibbs free energy, enthalpy, and entropy of mixing that provide high polymer compatibility. On the contrary, at high copolyamide content the blends are incompatible and are characterized by positive values of enthalpy, entropy, and Gibbs free energy of mixing. Such complicated thermodynamic behavior is the result of the superposition of strong molecular interactions (H‐bonds) between polymers in the blend and isothermal fusion of copolyamide crystallites. Thermodynamic analysis correlates well with the data obtained by polarized microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2603–2613, 2007     

Communication: Relaxed 2/1-Helical Conformation of Type II Chitosan has a Tetrasaccharide Motif

Chitosan, the N-deacetylated chitin, has two crystal forms. One is hydrated and the other is anhydrous. The main chain in both is known to adopt an extended ribbon-like 2/1 helix with about 10.4 Å c-repeat per disaccharide.^1,2 This is the well known canonical structure of (1→4) linked cellulose, chitin and mannan. Among them, chitosan has a remarkable ability to form specific complexes with organic and inorganic acids in addition to transition and post-transition metal ions.^3-7 The complexes studied so far are classified into two groups: the above canonical structure (Type I), and an “8-fold” (8/3 or 8/5) helix⁴ with long 40 Å c-repeat (Type II). The structural details of the latter are yet unknown. The complexes containing monocarboxylic acids, such as formic, acetic and propionic acids, belong to Type II; interestingly, they all undergo a spontaneous removal of the acids accompanying water molecules.⁸ As a result, the Type II complex in the solid state changes to the anhydrous form which corresponds to Type I.² Therefore, the Type II helix must be characterized properly in order to understand the mechanism of crystalline transformation of chitosan from the hydrated to the anhydrous form via chitosan/monocarboxylic acid complexes. Based on recent X-ray diffraction analysis, in this communication, we propose that Type II chitosan has a relaxed 2/1-helical conformation that exploits a tetrasaccharide rather than a monosaccharide as its asymmetric motif.     

Core–shell structure PEO/CS nanofibers based on electric field induced phase separation via electrospinning and its application

Core–shell structured PEO‐chitosan nanofibers have been produced from electric field inducing phase separation. Chitosan, a positive charged polymer, was dissolved in 50 wt % aqueous acetic acid and the amino group on polycation would protonize, which would endow chitosan electrical properties. Chitosan molecules would move along the direction of the electric field under the electrostatic force and formed the shell layer of nanofibers. Preparation process of core – shell structure is quite simple and efficient without any post‐treatment. The core–shell structure and existence of chitosan on the shell layer were confirmed before and after post‐treatment by TEM and further supported by SEM, FTIR, XRD, DSC, and XPS studies. Blending ratio of PEO and chitosan, molecular weight of chitosan for the mobility of chitosan are thought to be the key influence factors on formation of core–shell structure. Drug release studies show that the prepared core–shell structure nanofibers has a potential application in the biomedical fields involving drug delivery. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2298–2311     

Heterogeneous chloroacetylation of chitosan powder in the presence of sodium bicarbonate

Because of the importance of the chloroacetyl group to carbohydrate synthesis, the objective of this work is to disclose a method that has been found useful for the heterogeneous chloroacetylation of chitosan powder in which sodium bicarbonate is used as the base for the neutralization of the acid byproduct. A series of reactions were conducted to determine the more optimal conditions under which to perform acylation. The three varied aspects of the reaction were the acylating reagent (chloroacetyl chloride and chloroacetic anhydride), the solvent (methylene chloride and N,N‐dimethylformamide), and the temperature (0 or 44 °C). According to Fourier transform infrared (FTIR), the chitosan powder being refluxed in methylene chloride in the presence of chloroacetic anhydride constituted the best conditions. By incorporating these conditions and increasing the amount of the base, we obtained a chloroacetylated chitosan powder that, characterized by FTIR, solid‐state cross‐polarity/magic‐angle spinning ¹³C NMR, and elemental analysis, had degrees of N‐ and O‐chloroacetylation of 0.32 and 0.15, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4174–4181, 2001     

Fabrication of porous collagen/chitosan scaffolds with controlling microstructure for dermal equivalent

Two sets of homemade apparatus have been utilized to fabricate collagen/chitosan porous membranes by quenching its acetic solution and subsequently extracting the solvent with ethanol. The influence of chitosan concentration on the surface morphology of the collagen/chitosan membranes was studied using a quenching cold plate (apparatus 1). The pore size was enlarged along with an increase in the chitosan content, accompanied with the emergence of a sheet‐like microstructure. Due to the large thermal conductivity of the membrane‐forming platform (stainless steel), collagen/chitosan membranes prepared using apparatus 1 at freezing temperature between ?60 to ?20 °C present similar pore size (2–4 nm) and surface morphology. However, a large difference in pore size is generated using apparatus 2 (membrane preparation in a cold ethanol bath) and using a membrane‐forming platform of poor thermal conductivity (polymethylmethacrylate), e.g. ～10 to 20 μm at freezing temperature of ?60 to ?40 °C, and 265 μm at ?20 °C accompanied with the transformation from fiber‐ to sheet‐dominated morphology. The spongy collagen/chitosan membranes with pore sizes ranging from tens to hundreds of micrometers and porosity higher than 95%, which could be used as dermal regeneration template, have thus been fabricated. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermal modification of chitosan films as a way to control their transport properties

Russian Journal of Applied Chemistry - The possibility of using heat treatment (～120°C) for controlling the drug release from chitosan films prepared from acetic acid solutions and...     

Polymer electrolyte membranes based on p‐toluenesulfonic acid doped poly(1‐vinyl‐1,2,4‐triazole): Synthesis,thermal and proton conductivity properties

Throughout this work, the synthesis, thermal as well as proton conducting properties of acid doped heterocyclic polymer were studied under anhydrous conditions. In this context, poly(1‐vinyl‐1,2,4‐triazole), PVTri was produced by free radical polymerization of 1‐vinyl‐1,2,4‐triazole with a high yield. The structure of the homopolymer was proved by FTIR and solid state ¹³C CP‐MAS NMR spectroscopy. The polymer was doped with p‐toluenesulfonic acid at various molar ratios, x = 0.5, 1, 1.5, 2, with respect to polymer repeating unit. The proton transfer from p‐toluenesulfonic acid to the triazole rings was proved with FTIR spectroscopy. Thermogravimetry analysis showed that the samples are thermally stable up to ～250 °C. Differential scanning calorimetry results illustrated that the materials are homogeneous and the dopant strongly affects the glass transition temperature of the host polymer. Cyclic voltammetry results showed that the electrochemical stability domain extends over 3 V. The proton conductivity of these materials increased with dopant concentration and the temperature. Charge transport relaxation times were derived via complex electrical modulus formalism (M*). The temperature dependence of conductivity relaxation times showed that the proton conductivity occurs via structure diffusion. In the anhydrous state, the proton conductivity of PVTri1PTSA and PVTri2PTSA was measured as 8 × 10^?4 S/cm at 150 °C and 0.012 S/cm at 110 °C, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1016–1021, 2010     

High temperature creep behavior of phosphoric acid‐polybenzimidazole gel membranes

This work investigates the effects of polymer solids content and macromolecular structure on the high temperature creep behavior of polybenzimidazole (PBI) gel membranes imbibed with phosphoric acid (PA) after preparation via a polyphosphoric acid (PPA) mediated sol‐gel process Low‐solids, highly acid‐doped PBI membranes demonstrate outstanding fuel cell performance under anhydrous, ambient pressure, and high temperature (120–200 °C) operating conditions. However, PBI membranes are susceptible to creep under compressive loads at elevated temperatures, so their long‐term mechanical durability is a major concern. Here, we report results for the creep behavior of PBI membranes subject to compression at 180 °C. For para‐ and meta‐PBI homopolymers, increasing polymer solids content results in lower creep compliance and higher extensional viscosity, which may be rationalized by increasing chain density in the sol‐gel network. Comparing various homo‐ and copolymers at similar solids loading, differences in creep behavior may be rationalized in terms of chain–chain and chain‐solvent interactions that control macromolecular solubility and stiffness in the PA solvent. The results demonstrate the feasibility of improving the mechanical properties of PA‐doped PBI membranes by control of polymer solids content and rational design of PBI macromolecular structure. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1527–1538     

Uranium adsorption studies on hydrothermal carbon produced by chitosan using statistical design method

A new hydrothermal cross-linking chitosan (HCC) was prepared by hydrothermal reaction at a mild temperature (180 °C) to diminish the solubility in acid solution, and the amine groups were almost retained during the hydrothermal reaction characterized by FT-IR and the dissolution rate in pH 3 solution decreased from 89.6 to 12.6 %. The effects of initial pH, contact time, initial concentration and temperature on the sorption capacity are discussed using 24 full-factorial central composite design using response-surface methodology. The HCC showed the highest uranium sorption capacity at initial pH of 7.92 and contact time of 273.6 min with 24 full-factorial central composite design and the maximum adsorption capacity was 273 mg/g. The adsorption process could be well defined by the Langmuir isotherm and the thermodynamic parameters, ?G°(298 K), ?H° and ?S°, demonstrated shown that the sorption process of U(VI) onto HCC was feasible, spontaneous and endothermic in nature.     

Nanostructure and properties of sulfonated polyarylenethioethersulfone copolymers as proton exchange fuel cell membranes

A systematic investigation of properties and nanostructure of sulfonated polyarylenethioethersulfone (SPTES) copolymer proton exchange membranes for fuel cell applications has been presented. SPTES copolymers are high temperature resistant (250 °C), and form tough films with excellent proton conductivity up to 170 ± 5 mS/cm (SPTES 70 @ 85 °C, 85%RH). Small angle X‐ray scattering of hydrated SPTES 70 revealed the presence of local water domains (diameter ～5 nm) within the copolymer. The high proton conductivity of the membranes is attributed to the formation of these ionic aggregates containing water molecules, which facilitate proton transfer. AFM studies of SPTES 70 as a function of humidity (25–65%RH) showed an increase in hydrophilic domains with increasing humidity at 22 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2813–2822, 2007     

An investigation on dispersion of carbon nanotubes in chitosan aqueous solutions

The dispersion of carbon nanotubes (CNT) in water of different pH and in chitosan aqueous solution of three acids, acetic acid, formic acid, and hydrochloric acid, was investigated. Chitosan was soluble in water of pH ≤3 and could well disperse untreated CNT and acid-treated CNT, both of which had poor dispersion in water of pH ≤3. With the presence of 0.1 wt% chitosan in solution, particle sizes of the CNT dispersion were found to decrease with increasing COOH contents on the CNT. Particle sizes of CNT, untreated and acid-treated, in the three acidic aqueous solutions were found to increase with increasing chitosan contents in solutions. Among the three acids, hydrochloric acid gave the smallest particle size of the CNT dispersion. Without chitosan, the dispersibility of the acid-treated CNT in aqueous solutions of three acids was in the order of acetic acid > formic acid > hydrochloric acid.     

Ionic conductivity and tensile properties of hydroxyethyl and hydroxypropyl chitosan membranes

Hydroxyethyl chitosan and hydroxypropyl chitosan were prepared through the reaction of alkali‐chitosan with 2‐chloroethanol and propylene epoxide, respectively. Fourier transform infrared and ¹³C NMR measurements were made to examine the substitution on the chitosan unit. According to a comparison of the peak areas between the modified chitosan and unmodified chitosan and the integration of peak areas of ¹H NMR spectra, for both modified chitosans, the maximum degree of substitution was less than 25%. The ionic conductivity and mechanical properties of modified chitosan membranes were investigated. In comparison with the unmodified chitosan membrane, hydrated hydroxyethyl and hydroxypropyl chitosan membranes with a higher degree of substitution showed an increase in ionic conductivity of about one order of magnitude; moreover, the crystallinity of hydroxyethyl and hydroxypropyl chitosan membranes was remarkably reduced, and their swelling indices increased significantly. However, these modified membranes did not exhibit significant changes in their tensile strength and breaking elongation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1379–1397, 2004     

Thermosensitive permeation from side‐chain crystalline ionomers

Poly(stearyl methacrylate‐co‐methacrylic acid) (SMA) and its sodium ionomer (SMI) were synthesized and the permeability of the model drug through the SMA and SMI films was measured. The side‐chain crystalline structure for the dried and hydrated SMA, SMI was investigated using DSC and WAXS. The side‐chain crystalline structure of the hydrated SMI was much more stable than that of the hydrated SMA at room temperature. The temperature‐sensitive phase transition of the side‐chain crystalline structure for the hydrated SMI was also studied by the temperature variable WAXS experiment. The temperature‐sensitive permeation of the hydrophilic model drug through SMI was observed around 20 °C, whereas the drug permeation through SMA was almost constant within the temperature range studied. The change of drug permeability through the SMA and SMI films with temperature seems to be associated with the side‐chain crystalline structure of the polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 823–830, 2000     

Dielectric relaxations of chitosan: The effect of water on the α‐relaxation and the glass transition temperature

In this work thermal relaxations of chitosan are reported by using a novel methodology that includes subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content. When all these aspects are taken into account, two relaxations are clearly revealed in the low frequency side of the impedance data. We focus on the molecular motions in neutralized and non‐neutralized chitosan analyzed by dielectric spectroscopy in the temperature range from 25 to 250 °C. Low and high frequency relaxations were fitted with the Havriliak and Negami model in the 10^?1 to 108 Hz frequency range. For the first time, the low frequency α‐relaxation associated with the glass‐rubber transition has been detected by this technique in both chitosan forms for moisture contents in the range 0.05 to 3 wt % (ca. 18–62 °C). A strong plasticizing effect of water on this primary α‐relaxation is observed by dielectric spectroscopy and is supported by dynamic mechanical analysis measurements. In the absence of water (<0.05 wt %) the α‐relaxation is obscured in the 20–70 °C temperature range by a superposition of two low frequency relaxation processes. The activation energy for the σ‐relaxation is about 80.0–89.0 kJ/mol and for β‐relaxation is about 46.0–48.5 kJ/mol and those values are in agreement with that previously reported by other authors. The non‐neutralized chitosan possess higher ion mobility than the neutralized one as determined by the frequency location of the σ‐relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2259–2271, 2009     

Synthesis and properties of poly(thiourethane)s having soft oligoether segments

Polyaddition of bis(five‐membered cyclic dithiocarbonate), 2,2‐bis[4‐(1,3‐thioxolane‐2‐one‐4‐yl‐methoxy)phenyl]propane ( 1 ), with diamines having soft oligoether segments and property of the obtained poly(thiourethane)s were examined. Treatment of 1 with equivalent diamines in tetrahydrofuran at room temperature gave poly(thiourethane)s having a mercapto group in each unit, which were further treated with acetic anhydride and triethylamine to give the corresponding S‐acetylated poly(thiourethane)s in high yield. Exposing the mercapto group containing poly(thiourethane)s to benzoyl chloride and triethylamine provided the corresponding S‐benzoylated poly(thiourethane)s effectively. Thermal properties of the obtained polymers were evaluated by thermogravimetric analysis and differential scanning calorimetry. The obtained polymers showed 10 wt % loss temperature (T_d10) in the range from 230 to 274 °C, which was relatively high when compared with the T_d10 of an analogous polymer prepared from 1 and 1,6‐hexamethylenediamine. The polymers obtained here exhibited glass transition temperature (T_g) in the range from ?16 °C to 40 °C, which was much lower than the analogous polymer described above, probably due to the soft oligoether segments. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1076–1081