Silk I structure formation through silk fibroin self-assembly

Regenerated silk fibroin films are normally produced by increasing the Silk II structure (β-sheet content). In the present study, silk fibroin films were prepared by controlling the environmental temperature and humidity, resulting in the formation of silk films with a predominant Silk I structure instead of Silk II structure. Wide angle X-ray diffraction indicated that when the relative humidity was 55%, the silk films prepared were mainly composed of Silk I structure, whereas silk films formed on other relative humidity had a higher Silk II structure. Fourier transform infrared analysis (FTIR) results also conformed that the secondary structure of silk fibroin can be controlled by changing the humidity of the films formed process. Thermal analysis results revealed Silk I structure was a stable crystal, and the degradation peak increased to 320°C, indicating a greater thermal stability of these films formed under the 55% relative humidity conditions. Atomic force microscopy (AFM) results depicted silk fibroin in the fresh solution had many nanospheres existing with 20–50 nm diameters and mainly maintained a random coil structure without specific nanostructures. At the same time, it also illustrated the self-assembly process of silk fibroin in the aqueous solution without any human intervention. In addition, this present study also provided additional support for self-assembly mechanism of silk fibroin films formation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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     

Fe-D72树脂催化苯酚羟基化反应实验研究

利用D72离子交换树脂作为载体与铁离子进行离子交换制备Fe-D72树脂催化剂,将其运用到苯酚羟基化反应中。通过单因素法研究催化剂的含铁量、反应温度、反应时间、催化剂用量、苯酚（PH）/过氧化氢（H₂O₂）物质的量之比对苯酚羟基化反应的影响;采用L₉（3⁴）正交表设计正交试验,结果表明：n（PH）/n（H₂O₂）和反应温度对苯二酚收率的影响最大;在考察的正交范围内,n（PH）/n（H₂O₂）=1,反应温度为70 ℃,反应时间为1 h,催化剂量为0.1 g时催化效果最好,此时苯酚转化率为42.4%,苯二酚选择性为94.1%,苯二酚收率为39.8%。Fe-D72催化剂连续重复利用4次,催化稳定性较好。     

Structure and physical properties of silk fibroin/polyacrylamide blend films

This article deals with the characterization of blend films obtained by mixing silk fibroin (SF) and polyacrylamide (PAAm). The DSC curves of SF/PAAm blend films showed overlapping of the main thermal transitions characteristic of the individual polymers. The exothermic peak at 218°C, assigned to the β‐sheet crystallization of silk fibroin, slightly shifted to a lower temperature by blending. The weight‐retention properties (TG) of the blend films were intermediate between those of the two constituents. The TMA response was indicative of a higher thermal stability of the blend films, even at low PAAm content (≤25%), the final breaking occurring at about 300°C (100°C higher than pure SF film). The peak of dynamic loss modulus of silk fibroin at 193°C gradually shifted to lower temperature in the blend films, suggesting an enhancement of the molecular motion of the fibroin chains induced by the presence of PAAm. Changes in the NH stretching region of silk fibroin were detected by FTIR analysis of blend films. These are attributable to disturbance of the hydrogen bond pattern of silk fibroin and formation of new hydrogen bonds with PAAm. The values of strength and elongation at break of blend films slightly improved at 20–25% PAAm content. A sea–island structure was observed by examining the air surface of the blend films by scanning electron microscopy. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1563–1571, 1999     

Preparation of non-woven mats from all-aqueous silk fibroin solution with electrospinning method

In the present study, we successfully prepared non-woven mats from stable regenerated silk fibroin aqueous solution at high concentration. Scanning electronic microscope (SEM) was used to observe the morphology of the fibers. The structure of the fibers was characterized using Fourier transform infrared (FTIR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The mechanical tests were also performed. In the as-spun fibers, silk fibroin was present in a random coil conformation, the stress and strain at break were 0.82 MPa and 0.76%, respectively, while after methanol treatment, the silk fibroin was transformed into a β-sheet-containing structure, the stress and strain at break increased to 1.49 MPa and 1.63%, respectively. This study provided an option for the electrospinning of silk fibroin without using organic solvent or blending with any other polymers, which may be important in tissue engineering scaffold preparation.     

Preparation of insoluble fibroin films without methanol treatment

Insoluble fibroin films were prepared without methanol treatment. A 15 wt % fibroin solution was obtained through concentration, and then, the insoluble film was achieved by the adjustment of the drying temperature and rate. These films were examined through Fourier transform infrared–attenuated total reflection, X‐ray diffraction, and scanning electron microscopy to determine the structure of the silk fibroin. The physical and mechanical properties were investigated to discover the feasibility of using these films as biomedical materials. When fibroin films were dried above 60°C, abundant β‐sheet crystals existed in the fibroin films, and many globule‐containing micelles aggregated and interacted with each other, which resulted in excellent mechanical properties of the regenerated fibroin films in the wet state. Interestingly, the amide III band of the random coil structure in the fibroin films dried at 70 and 80°C was shifted to a lower frequency, 1228 cm^?1, which meant that a partly orientated structure formed. This may have also affected the mechanical properties of the fibroin films. The tensile strength and breaking elongation of the films dried at 70°C were 29.8 MPa and 59.6%, which is distinctly superior to fibroin films treated with methanol. If the drying temperature was raised to 60°C, the stability of the films in water was also excellent. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2168–2173, 2005     

Ni对Cu-Ni/γ-Al_2O_3苯羟基化催化剂的影响

通过程序升温还原方法合成了Cu/γ-Al2O3和Cu-Ni/γ-Al2O3催化剂,使苯直接羟基化制苯酚。该反应过程中,温度和溶剂对Cu-Ni/γ-Al2O3催化剂的反应性能影响进行了探讨。采用H2-TPR、XRD、EDS等表征技术考察了Ni对催化剂结构和性质的影响。结果表明,Ni使催化剂前驱体还原温度增加、活性组分Cu单晶粒度降低、催化剂表面Cu原子数增加;当反应温度为70°C、以水作反应溶剂时,Cu-Ni/γ-Al2O3比Cu/γ-Al2O3催化剂有较高反应活性和选择性,苯转化率为32.4%,苯酚选择性为93.3%,苯酚收率为30.2%。     

Structural characteristics of 2-hydroxyethylmethacrylate (HEMA)/methacrylamide (MAA)-grafted silk fibers

A series of 2-hydroxyethylmethacrylate (HEMA)/methacrylamide (MAA)-grafted silk fibers obtained in various comonomer compositions was prepared and their structural characteristics were studied by X-ray diffractometry, differential scanning calorimetry, and scanning electron microscopy. HEMA/MAA-grafted silk fibers with a graft yield of about 60% obtained in a HEMA/MAA mixture system containing 20% of HEMA and 80% of MAA on a weight basis showed endothermic peaks at 280 and 420°C (shoulder form), which are attributed to the thermal decomposition of the MAA and HEMA polymers, respectively, in addition to the thermal decomposition peak of the silk fibroin fiber which appeared at 323°C. These DSC results suggest that the HEMA/MAA-grafted silk fiber showed a low compatibility in the relation between the silk fibroin molecules and HEMA and/or MAA polymers. The crystalline structure of the HEMA/MAA-grafted silk fiber remained unchanged regardless of the HEMA/MAA grafting ratio even when the graft yield value reached 120%.     

Self‐Expansion Behavior of Silk Fibroin Film

Summary: Silk fibroin cast film was prepared using a ternary solvent system of CaCl₂/CH₃CH₂OH/H₂O (1/2/8 in mole ratio). A drying temperature at casting influenced crystal structure of fibroin. When a drying temperature was set lower than 9 °C, the cast film became amorphous. When a drying temperature was set higher than 40 °C, a fibroin film of silk‐II structure was obtained. In order to produce a fibroin film of silk‐I structure, a preferable temperature range was from 20 to 26 °C. The crystal transformation from random coil structure into silk‐I could be made through exposure of an amorphous film to water vapor. As for the crystal transformation from silk‐I into silk‐II, the treatment with a glycerin solution was effective. In the course of the treatment a film showed self‐thinning and self‐expanding. The expansion ratio exceeded 40% at maximum. The film produced accompanying self‐expansion was ductile in nature.

The apparent self‐expansion percentage as a function of initial thickness of the film. The ductility of the film was classified into four stages from the observation of recovery behavior after folding: ?, very soft; ?, soft; ?, middle; ?, hard (see Figure 5 ).     

Copolymer‐induced silk‐based hydrogel with porous and nanofibrous structure

A novel physical blend method was developed to accelerate the self‐assembly process of silk fibroin (SF) solution into porous and nanofibrous hydrogel by temperature‐sensitive copolymer. Silk‐based hydrogel was firstly achieved through blending SF solution with copolymer aqueous solution and then removed the copolymer from blend solution by heat treatment (50°C) after 24 h hydrogelation. Copolymer molecules would interact with SF molecules resulting in reduction of copolymer micelles, which further affect the hydrogelation of SF solutions. Copolymers could be separated from blend solution by heat treatment under an acceptable temperature (50°C), especially the copolymer₂. Fourier transform infrared (FTIR) and X‐ray diffraction showed the blending of copolymer significantly accelerated the self‐assembly of SF into physically crosslinked β‐sheet crystals at room temperature which led to the sol‐gel transition. Results from DTA and X‐ray diffraction showed that the effect of copolymer on crystalline structure of SF in silk‐based hydrogel was very weak. SF molecules transformed from distributed globular nanoparticles to nanofilaments clustered during hydrogelation, resulting in the porous and nanofibrous structure of silk‐based hydrogel. Furthermore, silk‐based hydrogel was prepared in aqueous solution avoiding organic solvents and harsh processing conditions, suggesting that this silk‐based hydrogel could be a potential candidate scaffold for biomedical applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Reactive dyeing of silk using commercial acid dyes based on a three‐component Mannich‐type reaction

Acid dyes are employed for commercially dyeing silk, which results in ionic bonds between the silk fibroin and the dye. This generally leads to low wet fastness properties for dyed silk fabrics. In this work, three commercial acid dyes with aromatic primary amine structures were selected to dye silk using a Mannich‐type reaction, resulting in improved wet fastness of dyed silk by forming covalent bonds between silk fibroin and dye. The Mannich‐type reactive dyeing was applied to silk fabrics at both 30 and 90°C in trials. Dyeing at 90°C can shorten the dyeing time compared with dyeing at 30°C, even although dye exhaustion and relative fixation at 90°C were a little lower. The dyeing process was optimised when the dyeing temperature was 90°C, dyebath pH 4, dye‐to‐formaldehyde ratio 1:30 and holding dyeing time 60 minutes. The results showed that the dye exhaustion on silk fabrics for the three aromatic primary amine‐containing acid dyes exceeded 94% and their relative fixation was over 80%. Their washing and rubbing fastness reached grade 4 or higher. Hence, the colour fastness properties of dyed silk fabrics using the Mannich‐type reactive dyeing method is superior to the conventional acid dyeing method using the same aromatic primary amine‐containing acid dyes. The Mannich‐type reactive dyeing for silk fabrics at 90°C can be developed into a novel and rapid reactive dyeing method, promising an effective dyeing process with excellent colour fastness.     

Study on porous silk fibroin materials. I. Fine structure of freeze dried silk fibroin

Silk fibroin solution was prepared by dissolving the silk fibroin in triad solvent CaCl₂ · CH₃CH₂OH · H₂O. In this article we tested and analyzed the state of frozen silk fibroin solution and fine structure of freeze dried porous silk fibroin materials. The results indicated that the glass transition temperature of frozen silk fibroin solution ranges from −34 to −20°C, and the initial melting temperature of ice in frozen solution is about −8.5°C. When porous silk fibroin materials are prepared by means of freeze drying, if freezing temperature is below −20°C, the structure of silk fibroin is mainly amorphous with a little silk II crystal structure, and if freezing temperature is above −20°C, quite a lot of silk I crystal structure forms. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2185–2191, 2001     

Production of fine powder from silk by radiation

Silk fine powder was prepared directly from silk fibers irradiated with an accelerated electron beam (EB). Irradiated silk fibers were well pulverized only by physical crushing using a ball mill without any chemical pretreatment. Silk fibroin fibers were irradiated at ambient temperature in the dose range of 250–1 000 kGy. Although unirradiated silk fibers were not pulverized at all, irradiated fibers were easily pulverized and showed a high conversion efficiency from fiber to powder at high irradiation doses. The presence of oxygen in the irradiation atmosphere enhanced pulverization of the silk fibers. The electron microscopic observation showed that the minimum particle size of silk powder obtained from fibers irradiated at 1 000 kGy in oxygen was less than 10 μm. It was found that fibroin powder obtained in this work dissolved remarkably in water, thought unirradiated fibroin fibers were insoluble even in hot water. The soluble fraction was about 60% of fibroin powder for 1 000 kGy irradiation in oxygen.     

Structure and properties of tussah silk fibers graft-copolymerized with methacrylamide and 2-hydroxyethyl methacrylate

Tussah silk fibers were graft-copolymerized with methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA) in aqueous media, using a chemical redox system as an initiator. High weight gain values were obtained with both grafting agents (up to 175%). The extent of homopolymerization was negligible for the MAA grafting system over the entire range of monomer–silk ratios examined, while polymer deposition on the fiber surface occurred when the HEMA–silk ratio exceeded 0.5% (w/w). The moisture content of poly(MAA)-grafted silk fibers was enhanced by grafting. Breaking load, elongation at break, and energy decreased at low weight gain (0–20%) and then remained rather constant. The DSC curves of poly(MAA)-grafted silk showed a new endotherm at about 280°C, due to the melting of poly(MAA) chains. The loss modulus peak of poly(HEMA)-grafted silk fibers broadened and shifted to a lower temperature, showing a tendency to split into two peaks at high weight gain. On the other hand, grafting with poly(MAA) induced a noticeable upward shift of the loss peak. The TMA curves showed that grafting with poly(MAA) resulted in a higher extent of fiber contraction from room temperature to about 250°C. Moreover, the intensity of the final contraction step at about 350°C decreased with increasing weight gain and shifted to a lower temperature. The Raman spectra of grafted fibers were characterized by overlapping of the characteristic lines of both silk fibroin and polymer, the latter showing an intensity proportional to the amount of weight gain. Among the conformationally sensitive vibrational modes of tussah silk fibroin, the amide III range was significantly modified by grafting with both poly(MAA) and poly(HEMA). © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1393–1403, 1998     

Preparation of regenerated Antheraea yamamai silk fibroin film and controlled‐molecular conformation changes by aqueous ethanol treatment

The methods of dissolving Antheraea yamamai (A. yamamai) silk fibroin fibers and preparing the regenerated silk fibroin were studied. The molecular conformations of A. yamamai silk fibroin film treated by different concentration ethanol solution were investigated by XRD, FTIR, and Raman spectra. The results indicated that A. yamamai silk fibroin fibers could be completely dissolved in dense lithium thiocyanate solution at about 35°C for 1 h. The initial regenerated film consisted of α‐helix and random coil components. The aqueous ethanol treatment of the film resulted in significant increase in β‐sheet component and improvement of water resistance of the film. This effect was strongly dependent on ethanol concentration, and 60–80% (w/w) ethanol was most effective. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A new method to prepare porous silk fibroin membranes suitable for tissue scaffolding applications

A new method to prepare porous silk fibroin (SF) membranes without dialysis is proposed. Silk fibers were degummed to remove sericin and the resultant fibroin was dissolved in a CaCl₂‐CH₃CH₂OH‐H₂O ternary solvent. Rather than undergoing dialysis, a fibroin salty solution was diluted in water and then submitted to a mechanical agitation that led to a phase separation through foam formation on the solution surface. This foam was continually collected and then compacted between plates to remove the excess of water. The membranes presented large pores with diameters of greater than 100 μm (as shown by scanning electron microscopy ‐ SEM), porosity of 68% and water content of 91% w/w. X‐ray diffraction (XRD) and infrared spectroscopy (FTIR‐ATR) indicated that the membranes present SF in a β‐sheet structure even before the ethanol treatment. A typical elastic deformation profile and degradation under temperature were observed using calorimetric analysis (DSC), thermal gravimetric analysis (TGA) and mechanical tests. As indicated by the in vitro cytotoxicity tests, these membranes present potential for use as scaffolds. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

Preparation and characterization of low molecular weight silk fibroin by high‐temperature and high‐pressure method

A low molecular weight silk fibroin powder (LMSF) was prepared through high temperature (200°C) and high pressure (20 kgf/cm²), without any addition of chemicals. The carbonized adducts produced during this process were then removed by treatment with activated charcoal. The yield of LMSF by this preparation method was over 60% after the removal of carbonized adducts by using activated charcoal. Amino acid analysis showed an observable decrease in contents of serine and tyrosine in LMSF prepared by this method, as compared to those prepared by neutral salt. The molecular weight of this LMSF was also observably decreased with an increase in the reaction time. From the measurements of differential scanning calorimeter (DSC) and thermal gravimetric analyzer (TGA), thermal properties of LMSF through high temperature and high pressure were also decreased as compared to those produced by neutral salts. In addition, wide‐angle X‐ray diffraction (WAXD) patterns showed that the crystallinity of LMSF differed from that of the original silk fibroin. It can be said that the preparation method of LMSF in this study is a simple, economical, and environmentally compatible process with many advantages. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2890–2895, 2002     

Synthesis and characterization of 4,4′‐diaminodiphenyl methane‐based benzoxazines and their polymers

A series of diamine‐based benzoxazine precursors have been prepared using 4,4′‐diaminodiphenyl methane, formaldehyde, and different phenol derivatives including phenol, p‐cresol, and 2‐naphthol. Their chemical structures were identified by FTIR, ¹H NMR, and elemental analysis. The curing reactions of those precursors were monitored by FTIR and DSC. The obtained materials exhibited higher glass transition temperature and char yields than the corresponding bisphenol‐A based polybenzoxazines. The polybenzoxazine prepared from phenol showed the highest char yields of 65% and thermal stability with 5 and 10% weight‐loss temperatures at 346 and 432°C, respectively. The polybenzoxazine prepared from 2‐naphthol exhibited the highest glass transition temperature at 244°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Properties of poly(styrene)-“grafted” silk fibers and molecular weight of poly(styrene)

Poly(styrene) was grafted onto silk fibers in an aqueous medium, using ammonium persulphate initiator. Add-ons of up to 100% were achieved. The add-on increased linearly with the monomer concentration of the grafting system, all other factors remaining constant. The yield of the reaction attained about 80% and the extent of homopolymerization was negligible. The equilibrium moisture regain decreased noticeably with increasing add-on. Breaking load showed a tendency to increase in the 0–55% range of poly(St) content, while elongation at break and work of rupture decreased. The DSC endothermal peak at 315°C slightly shifted towards higher temperature by graft—copolymerization, and a new endothermal transition appeared beyond 400°C. TG measurements showed an increase of weight retention beyond 300°C for poly(St)-“grafted” silk fibers. The dynamic mechanical behavior was characterized by a shift to lower temperature of the loss modulus peak. The transverse dimension of the fibers increased with increasing add-on. The polymeric residue remaining after alkaline dissolution of silk fibroin showed a porous texture with a sponge-like morphology. The molecular weight of poly(St) removed from silk increased up to 120 kDa in the 0–55% add on range and then remained constant. The “Poly(St)/Silk” molar ratio increased linearly over the add-on range examined. © 1996 John Wiley & Sons, Inc.