Chemical and property modification of silk with dibasic acid anhydrides

In an attempt to provide information for improving silk fiber and fabric properties including crease recovery and yellowing, silk modification has been studied by using succinic and glutaric anhydrides. Glutarylation has been found to protect silk fibers from yellowing caused by the high energy radiation and to be more effective on increasing the crease proofing. The silk fabrics did not reduce the tensile properties such as strength and elongation at break even after the chemical modification with dibasic acid anhydrides. Thermal properties of the fibers remained unchanged in spite of the succinylation or glutarylation. From all the considerations of the survey of dye uptake, it would seem that the ester crosslinks are easily formed much more by the glutarylation than by the succinylation. The recovery values and yellowness indices of the modified silk fabrics and the measuring results of thermal and thermomechanical properties are explicable, taking into account of the different reactivity of the functional groups induced by the chemical modification with the dibasic acid anhydrides.     

Chemical modification of silk with aromatic acid anhydrides

Bombyx mori silk fibers were chemically modified by acylation with aromatic acid anhydrides, such as phthalic and o-sulfobenzoic anhydrides. We examined the reactivity of these modifying agents toward silk fibers, the physical and thermal properties, and the dyeing behavior with acid and cationic dyes. The o-sulfobenzoic anhydride was more reactive toward silk fibroin than phthalic anhydride. The amount of both basic and acidic amino acid residues decreased after modification with aromatic acid anhydrides. The moisture regain of silk treated with phthalic anhydride remained almost unchanged, while that of the samples modified with o-sulfobenzoic anhydride increased linearly as the weight gain increased. Chemically modified silk fabrics showed improved crease recovery behavior, even though phthalic anhydride seemed more effective at comparatively low weight gain. The modification of silk with o-sulfobenzoic anhydride caused a drastic a reduction of acid dye uptake and enhanced the affinity of silk for cationic dye. Silk fibers did not show any significant change in thermal behavior, regardless of the modification with o-sulfobenzoic anhydride. Silk fibers modified with phthalic anhydride showed on differential scanning calorimetry (DSC) curves a minor and broad endothermic peak at around 210°C, attributed probably to the breaking of the crosslinks formed between adjacent fibroin molecules.     

Chemical modification of Bombyx mori silk with epoxide EPSIB

Bombyx mori silk fabrics were chemically modified by EPSIB (a multifunctional silicone‐containing epoxy crosslinking agent). The reactivity of the epoxy groups with silk fibroin was studied by using amino acid analysis. The physical properties of the modified silks such as resiliency (both wet and dry), moisture regain, dyeing behaviors, and solubility in a mixture solvent (C₂H₅OH × CaCl₂ : H₂O = 2 : 1 : 8, molar ratio) were examined. The modified silk fabrics exhibited a significantly improved resiliency, a small increase in moisture regain and whiteness, and a slightly decreased tensile strength. The contents of Serine, Trosine, Lysine, and Histidine decreased linearly as the wet crease recovery angle (CRA) increased. The solubility in the mixture solvent also decreased as the wet CRA increased. The changes of physical properties, especially the wet CRA, were mainly due to the presence of stable cross‐links between silk fibroin and epoxy groups. The DSC and TGA analyses showed that EPSIB‐modified silk fibroin had a higher thermal stability compared with the control. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3579–3586, 2004     

Physical properties of silk fibers treated with ethylene glycol diglycidyl ether by the pad/batch method

This paper deals with the epoxide treatment of silk fabrics by the pad/batch method. The optimum reaction conditions, i.e., NaOH concentration, and reaction temperature were 2.5 g/L and 30°C, respectively. A weight gain of 8.5% was attained at a reaction time of 6 h. This value slightly increased to 10% after 24 h. The reactivity of tyrosine and basic amino acid residues was dependent on the reaction time and did not significantly differ from the results of epoxide-treated silk fiber by the conventional method in tetrachloroethylene. The moisture regain slightly decreased at 4% weight gain and then increased with the epoxide content, exceeding the value of the untreated control. The crease recovery of the epoxide-treated silk fabrics measured in the wet state was significantly improved, whereas that in the dry state was almost unchanged. The rate of photoyellowing of the epoxide-treated silk fabrics by the pad/batch method was reduced significantly compared with that of the untreated control. Among the mechanical properties, elongation at break and tensile modulus remained unchanged, whereas the tensile strength slightly increased following the epoxide reaction. The thermal properties were evaluated by DSC and TGA and on the basis of the dynamic viscoelastic measurements. The DSC curve of the epoxide-treated sample showed a slight increase of the decomposition temperature of silk fibroin. The rate of weight loss determined by TGA remained unchanged regardless of the chemical modification, whereas the peak of loss modulus became broader and shifted to lower temperature. The X-ray diffractograms showed that the crystalline structure of silk fibers was not affected by the reaction with epoxides. © 1993 John Wiley & Sons, Inc.     

Chemical modification of tussah silk with acid anhydrides

Tussah silk fibroin was chemically modified by acylation with aliphatic, aromatic, and hydrophobic acid anhydrides. The tussah silk fibers were pretreated by immersing them in a lithium thiocyanate (LiSCN) solution and then acylated in dimethylformamide (DMF) at elevated temperatures. Using this method, acylated tussah silk fibers with weight gains of 8–22% could be obtained. The pretreatment with LiSCN was necessary to promote the acylation. Without it, the reaction did not proceed. The optimum temperature and reaction time of the pretreatment was 55°C and 60 min, respectively. When examining the physical properties and the thermal behavior of both pretreated and acylated tussah silk, it was found that the mechanical properties and the position of the major DSC endothermic peak remained unchanged, regardless of pretreatment and acylation. The moisture regain of the pretreated tussah silk increased slightly while the moisture regain of the acylated silk decreased linearly with increasing weight gain. The chemical modification allows for a wide control of the tussah silk fiber's properties, making it possible to use tussah silk for the development and production of novel textile and biomaterials. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 382–391, 2000     

Chemical modification of wool fibers with acid anhydrides

Wool fibers were chemically modified by reaction with succinic and glutaric anhydrides. The weight gain (and acyl content) increased with increasing the reaction temperature (65–80°C) and time (1–2 h), attaining 18.9% (158.9 mol/10⁵ g) and 23% (163.9 mol/10⁵ g) for succinylated and glutarylated wool, respectively. Changes in the amino acidic pattern of acylated wool, i.e., decrease of basic amino acid residues and formation of ornithine, were observed by acid hydrolysis. The X‐ray diffraction profiles of modified wool fibers remained essentially unchanged, suggesting that the crystalline structure was not affected by reaction with acid anhydrides. The degree of molecular orientation of acylated wool slightly decreased, especially at high weight gain. The viscoelastic response of wool modified with succinic and glutaric anhydrides was characterized by a shift to a lower temperature of both the drop of the storage modulus and the peak of the loss modulus. These features are indicative of a higher mobility of the keratin chians in the amorphous and crystalline domains. In fact, it is suggested that the chemical agent diffused into the accessible parts of α‐crystallites, reaching the available reactive sites. This did not cause changes in the crystalline pattern of wool, but resulted in a different thermal behavior of fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1573–1579, 1999     

Chemical modification of silk with itaconic anhydride

Bombyx mori silk fibers were chemically modified by acylation with itaconic anhydride. The reactivity of the modifying agent toward silk fibroin was investigated on the basis of the amino acid analysis. We examined the physical properties, the structural characteristics, and the thermal behavior of modified silk fibers as a function of the weight gain. Silk fibers with a weight gain of 9%, corresponding to an acyl content of 68.9 mol/10⁵g, were obtained at the optimum reaction conditions for silk acylation (75°C for 3 h). The amount of basic amino acid residues (Lys, His, and Arg) decreased linearly as the weight gain increased. The alkali solubility increased proportionally with the weight gain, probably due to the dissolution of the modifying agent reacted with silk fibroin, and not to the degradation of the fibers induced by the chemical modification. The birefringence value, related to the molecular orientation, slightly decreased when the weight gain increased. The isotropic refractive index, associated with the crystallinity, increased when the weight gain ranged from about 5 to 7% and then remained unchanged. The moisture regain did not change regardless of the chemical modification, and the crease recovery behavior of modified silk fabrics did not show significant improvement. The thermal behavior of silk fibers was affected by the modification with itaconic anhydride. The decomposition temperature shifted up to 322°C, 10°C higher than the control silk fibers, suggesting a higher thermal stability induced by chemical modification.     

Chemical structure and dynamic mechanical behavior of silk fibers modified with different kinds of epoxides

The chemical reactivity of epoxide molecules toward silk fibroin was investigated by determining the rate of conversion of reactive amino acid residues. Significant differences were found between two different bifunctional epoxides, diglycidyl ethers of ethylene glycol (E) and resorcinol (R), the former reacting at a higher extent with arginine and tyrosine. The moisture regain decreased by reaction with epoxides, at a variable rate and extent, according to the hydrophobic/hydrophilic properties of epoxides. A two-step behavior was observed when moisture regain values relating to the silk content in modified silk, fibers were plotted as a function of the weight gain. Dynamic mechanical data showed that the major loss peak became broader and its temperature shifted to lower values following the increase of weight gain. The loss peak temperatures showed a linear relationship with the amount of weight gain. The fine structural changes induced by reaction with eposides will be discussed in terms of chemical and steric factors of the epoxides, as well as of epoxide location within the different structural domains of silk fibers. © 1994 John Wiley & Sons, Inc.     

Structure and physical properties of epoxide-treated tussah silk fibers

The structural characteristics and physical properties of epoxide-treated tussah silk fibers from Antheraea pernyi silkworm are discussed in relation to the increasing weight gain values. Ethyleneglycol diglycidylether (E) and glycerin diglycidylether (G) were used as modifying agents. The noticeably high weight gain values (about 140%) obtained were attributed to the catalytic effect of SCN^? anion absorbed by the fibers during the pretreatment under reduced pressure conditions. The amino acid analysis showed that epoxide G exhibited a slightly higher reactivity toward tyrosine, while arginine preferably reacted with epoxide E. The peak of loss modulus (E″) determined by dynamic viscoelastic measurements became broader and its position linearly shifted to lower temperature when the weight gain increased, and a minor peak appeared in the low-temperature region below 50°C. Differential scanning calorimetry (DSC) thermograms showed that the position of the decomposition peak of modified silk fibers shifted to lower temperature with increasing weight gain values. The minor and broad endothermic peaks, appearing in the reference sample at about 234 and 290°C, disappeared by epoxide treatment. X-ray diffraction patterns of tussah silk fibers suggested that the epoxide treatment does not affect directly the crystalline regions but causes a decrease of molecular orientation in the amorphous regions. Both briefringence (Δn) and isotropic refractive index (n_iso) of tussah silk fibers decreased by the reaction with epoxides, although with different rate and extent, confirming the decrease of average molecular orientation. The extent of decrease of strength and elongation depends on the kind of epoxide and on the weight gain value. Epoxide-treated tussah silk fibers did not show significant changes of surface characteristics as the weight gain values attained up to 60%.     

Chemical modification of tussah silk fabrics with ethyleneglycol diglycidyl ether by a pad-batch method

Tussah silk fabrics have been chemically modified by pad-batch treatment with ethyleneglycol diglycidyl ether (EDGE). The maximum amount of weight gain attained was 5–6%. Epoxide adducts were formed with tyrosine and basic amino acid residues. The moisture regain of the fabric with 3% weight gain decreased slightly compared with that of the untreated sample. The crease recovery in the wet state improved significantly, while in dry state it remained unchanged. Mechanical and optical properties, and X-ray crystallinity did not exhibit noticeable changes. The position of the major DSC endothermic transition at 358d?C remained unchanged, regardless of the epoxide modification. Similar comments apply to the TMA contraction peak occurring at about 359d?C. The storage and loss modulus curves of EDGE-treated tussah silk fibres exhibited some changes, the most relevant consisting in a shift to higher temperature of the loss modulus peak.     

Physical,chemical, and dyeing properties of Bombyx mori silks grafted by 2‐hydroxyethyl methacrylate and methyl methacrylate

To obtain silk weight gain and to improve silk properties, Bombyx mori silks were grafted with either 2‐hydroxyethyl methacrylate (HEMA) or methyl methacrylate (MMA). The moisture regain of the HEMA‐grafted and MMA‐grafted silks depended on the hydrophilicity of the used monomers. The acid and alkaline resistances of the HEMA‐grafted and MMA‐grafted silks were clearly improved. Both commercial synthetic dyes, that is, acid and reactive dyes, and a natural dye extracted from turmeric, with potassium aluminum sulfate as a mordant, were used in this study. The results suggested that the dye uptake increased in the presence of poly(2‐hydroxyethyl methacrylate) or poly(methyl methacrylate) in the silk fibroin structures when acid and curcumin dyes were used. The washfastness level of the HEMA‐grafted silk dyed by acid and reactive dyes was similar to that of the degummed silk. However, the colorfastness to washing of the MMA‐grafted silk dyed by an acid dye was improved when the polymer add‐on concentration was 65%. In addition, the washfastness for both grafted silks was improved when they were dyed with natural curcumin dyestuff. The acid and alkaline perspiration fastness properties remained unchanged for the HEMA‐grafted and MMA‐grafted silks when acid, reactive, and curcumin dyes were applied. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Changes in the fine structure of silk fibroin fibers following gamma irradiation

The physicochemical changes of silk fibers irradiated with γ-rays was studied in relation to the amount of absorbed does in the range 0–21 Mrad. The yellowing index (b/L) suddenly increased at low dose for both raw and degummed silk fibers. An equilibrium value was attained from 10 Mrad upward. The tensile properties were significantly affected by exposure to γ-rays. Both strength and elongation at break decreased at almost the same rate and extent, attaining a final value that was one-half of the untreated control. The birefringence and isotropic refractive index of exposed silk fibers decreased, the effect being larger in the low dose range, suggesting a decrease of crystallinity and molecular orientation. X-ray diffraction curves, however, demonstrated that the crystalline structure remained unchanged even after exposure of the highest γ-ray dose. The thermal behavior evaluated by DSC and TMA measurements showed that the γ-irradiation induced a slight decrease of thermal stability in irradiated silk fibers, this effect being detectable only at 21 Mrad of the absorbed dose. The dynamic viscoelastic behavior suggested that the thermal movement of the fibroin molecules in the amorphous and crystalline regions increased with increasing absorbed dose, attributing to the physicochemical modifications induced by the ionizing radiations. © 1994 John Wiley & Sons, Inc.     

Characterization of methacrylonitrile-grafted silk fibers

The structure and physical properties of silk fibroin fibers graft-polymerized with methacrylonitrile (MAN) were analyzed in relation to the weight gain on the basis of the results of tensile properties as well as of thermal analysis and X-ray diffractometry. The solubility of the specimen in NaOH solution and the moisture absorption decreased slightly with the duration of the MAN treatment. However, the polymerizing treatment with MAN did not affect significantly the tensile properties, i.e., strength and elongation at break of the original fibers. The position of the endothermic peak attributed to the thermal decomposition of the silk fibroin shifted to higher temperatures when the weight gain exceeded 25%, and a constant value at 328°C was obtained above a weight gain of 40%. Wide-range X-ray diffraction diagram of silk fibers with a weight gain of 10% showed diffraction maxima at the equator corresponding to the molecule oriented crystal structure of the silk fiber, in addition to the spots on a series of hyperbolic arcs arranged symmetrically at about the equator, which are associated with the crystalline form of the MAN polymer copolymerized in the specimen. Crystalline structure of the silk fiber remained unchanged essentially regardless of MAN treatment.     

Changes in physical properties of methacrylonitrile (MAN)-grafted silk fibers

Changes in physical properties of silk fibers, grafted with methacrylonitrile (MAN), were investigated as a function of the weight gain. The weight gain increased steadily during the first 60 min of reaction and gradually attained an equilibrium value (60%) after about 4 h. The initial tensile resistance of silk fibers decreased by MAN grafting. The crystalline structure of silk fibers remained unchanged, regardless of MAN grafting, however. a minor and broad peak appeared in the X-ray diffraction curves of MAN-grafted silk fibers with a weight gain of 60%, corresponding to the unoriented MAN polymer attached inside the fibers. Molecular orientation of silk fibroin chains in the crystalline regions, evaluated from X-ray diffraction curves, did not change significantly, while both birefringence and isotropic refractive index decreased as the weight gain increased, implying that MAN polymer attached preferentially to the amorphous and not to the crystalline regions. Dynamic vis-coelastic measurements showed that the position at which the E′ value began to decrease shifted to a lower temperature as the weight gain increased. These findings suggest that the thermal movement of silk fibroin molecules was accelerated by the presence of the poly-MAN chains attached to the amorphous regions of silk fibroin fibers. © 1993 John Wiley & Sons, Inc.     

Studies on Indian silk. I. Macrocharacterization and analysis of amino acid composition

This first in a series of articles characterized the different varieties of Indian silk for their macrostructural parameters, such as filament length, degumming loss, denier, cross section, moisture regain, and intrinsic viscosity, for example. The results of amino acid analysis using a reverse‐phase technique were also reported. Five Indian silk varieties—two mulberry (bivoltine and crossbreed) and three nonmulberry (tasar, muga, and eri)—were investigated. The differences existing between the different varieties and the extent of lengthwise variations within a cocoon in the dimensional and macrostructural parameters were discussed. It was observed that denier of the filament decreases considerably from the outer to the inner layers, whereas density showed an increasing trend in all the varieties. Both the mulberry silks demonstrated lower moisture regain. Electron micrographs of all the nonmulberry varieties showed microvoids in their cross section. Fraction studies showed the development of mushroom structure on the tips. In both types of mulberry silk, glycine, alanine, and serine constitute about 82% of the amino acids present. On the other hand, in nonmulberry silks, these constitute about 73% with a high proportion of alanine. The nonmulberry varieties showed a substantial proportion of amino acids with bulky side groups. Similarly, the higher hydrophilic to hydrophobic amino acid ratio (9.06–9.85) for nonmulberry silks, compared against that of the mulberry varieties (5.29–6.22), was shown to be responsible for the higher moisture content of nonmulberry silks. Cystine and methionine were present in all the varieties. The higher intrinsic viscosity of nonmulberry varieties suggested their higher molecular weight. Through amino acid analysis, it was shown that there is no difference in chemical architecture between the outer and the inner layers of cocoons. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1080–1097, 2004     

Acylation of silk and wool with acid anhydrides and preparation of water-repellent fibers

Silk and wool fibers were acylated with two acid anhydrides, dodecenylsuccinic anhydride (DDSA) and octadecenylsuccinic anhydride (ODSA), at 75°C with N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) as the solvent, the latter of which allowed higher weight gains to be reached. The weight gain and acyl content of wool was always higher than that of silk. Tensile properties of silk remained unchanged regardless of weight gain, whereas wool displayed a noticeably higher extensibility at high weight gain. Fine structural changes of acylated wool were detected by DSC analysis. Moisture regain and water retention of acylated silk and wool decreased significantly, whereas water repellency increased. SEM analysis showed the presence of foreign material firmly adherent to the surface of both silk and wool, whose amount increased with increasing weight gain. These deposits were attributed to the presence of the modifying agents at the fiber surface on the basis of the characteristic IR bands. The possible application of silk and wool fibers acylated with DDSA or ODSA for the preparation of water-repellent textile materials is discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2832–2841, 2001     

Physical properties and dyeability of NaOH-treated silk fibers

Domestic (Bombyx mori) and wild (tussah, Antheraea pernyi) silk fabrics were treated with diluted NaOH solutions by the pad/batch method. The equillbrium moisture regain of tussah silk fibers increased steadily with alkaline treatment, while that of B. mori did not change. B. mori tensile strenght and elongation at break were slightly impaired. The average molecular orientation and crystallinity of both kinds of silk remained unchanged. Differential scanning calorimetry (DSC) and thermomechanical analysis(TMA) showed that the thermal behavior of B. mori silk was almost unaffected, while that of tussah exhibited slight changes in the temperature range 250–300°C. By dynamic mechanical measurements (DMA) it was elucidated that both storage and loss moduli of B. mori silk fibers decreased following NaOH treatment. On the other hand, tussah silk exhibited a noticeable upward shift of the major loss peak. Alkali-treated tussah silk fibers, dyed with an acid dyestuff, attained a lower degree of dye-bath exhaustion. © 1995 John Wiley & Sons, Inc.     

Chemical modification of Bombyx mori silk with calcium‐salt treatment and subsequent glycerin triglycidyl ether crosslinking

In this article, we propose a new modification method for obtaining porous silk fibers with excellent wet elastic resilience and flexibility. Bombyx mori silks were modified by calcium‐salt treatment and subsequent epoxy crosslinking with glycerin triglycidyl ether. The effects of temperature, time, and catalyst (sodium carbonate) on the crosslinking reaction of the silk fibers were investigated, and the best conditions of reaction were determined as a temperature of 120°C, a crosslinking agent concentration of 7%, and immersion for 1 h with 2% Na₂CO₃ solution before the crosslinking reaction. The change in the structure and the physical properties of the silk fibers after calcium‐salt treatment and epoxy crosslinking was studied. Separating behavior of the microfibers occurred on the surface of the silk fiber after calcium‐salt treatment, and a porous structure formed in the interior of the silk. This porous structure of the silk was enlarged by subsequent epoxy crosslinking, and accordingly, the moisture conduction of the silk fibers improved remarkably. The breaking strength, breaking elongation, and wet elastic resilience of the silk fibers increased evidently after modification, and the modified silks exhibited a better flexibility. The conformation of silk fibroin fibers changed from β sheet to random coil after calcium‐salt treatment, whereas the β‐sheet content in the silk fibers increased after subsequent epoxy crosslinking. The significant reductions in the crystallinity and crystalline sizes in the silk fibers after the crosslinking reaction indicated that the crosslinking reaction occurred within the crystalline region because the calcium‐salt treatment increased the reaction accessibility. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Comparison of Silks from Pseudoips prasinana and Bombyx mori Shows Molecular Convergence in Fibroin Heavy Chains but Large Differences in Other Silk Components

Many lepidopteran larvae produce silk feeding shelters and cocoons to protect themselves and the developing pupa. As caterpillars evolved, the quality of the silk, shape of the cocoon, and techniques in forming and leaving the cocoon underwent a number of changes. The silk of Pseudoips prasinana has previously been studied using X-ray analysis and classified in the same category as that of Bombyx mori, suggesting that silks of both species have similar properties despite their considerable phylogenetic distance. In the present study, we examined P. prasinana silk using ‘omics’ technology, including silk gland RNA sequencing (RNA-seq) and a mass spectrometry-based proteomic analysis of cocoon proteins. We found that although the central repetitive amino acid sequences encoding crystalline domains of fibroin heavy chain molecules are almost identical in both species, the resulting fibers exhibit quite different mechanical properties. Our results suggest that these differences are most probably due to the higher content of fibrohexamerin and fibrohexamerin-like molecules in P. prasinana silk. Furthermore, we show that whilst P. prasinana cocoons are predominantly made of silk similar to that of other Lepidoptera, they also contain a second, minor silk type, which is present only at the escape valve.     

Dyeability of silk fabrics modified with dibasic acid anhydrides

Silk fabrics were chemically modified with different kinds of dibasic acid anhydrides. Samples with different acyl contents, ranging from 24 to 107 mol/10⁵g, were obtained by reaction with succinic (S), glutaric (G), phthalic (PA), and 0-sulfobenzoic (OSBA) anhydrides. Dyeability with a cationic dye (Rhodamine B, C.I. Basic Violet 10) was determined by measuring dye uptake and K/S values. Following acylation, the absorption of basic dye was significantly enhanced for all samples. The maximum dye uptake was attained by OSBA-treated silk fabrics, while samples treated with S. G. and PA did not differ significantly. Dye uptake increased regardless of the content, at least in the range examined. Reflectance measurements and K/S values confirmed the trend evidenced by dye uptake results. © 1994 John Wiley & Sons, Inc.