Physical properties and structure of silk. V. Thermal behavior of silk fibroin in the random-coil conformation

The thermal behavior of films of amorphous silk fibroin in the random-coil conformation has been investigated in the temperature range 25–220°C by differential scanning calorimetry (DSC), thermal expansion, dynamic mechanical measurements, x-ray diffraction, and infrared spectroscopy. As the temperature is raised, water is lost up to about 100°C. Intramolecular and intermolecular hydrogen bonds are broken between 150 and 180°C. The glass transition is observed at 173°C by DSC. The random-coil→β-form transition accompanied by reformation of hydrogen bonds takes place above 180°C. Thermally induced crystallization to the β-form crystals starts at about 190°C.     

Physical and chemical properties of tussah silk fibroin films

Physical and chemical structure, as well as thermal behavior of solution-cast regenerated films, prepared from tussah (Antheraea pernyi) silk fibroin, were compared with those of solution-cast native films, in order to ascertain whether treatment (degumming, dissolution) used for preparation affected their properties. Regenerated fibroin films exhibited a higher thermal stability than native ones, as shown by differential scanning calorimetry, thermomechanical analysis, and dynamic mechanical behavior. Glass transition temperature and other relevant thermal transitions of the regenerated silk specimen shifted to higher temperatures compared with those of native specimen. Molecular conformation and crystalline structure did not show significant differences between the two kinds of silk films. Amino acid composition and molecular weight, however, distribution changed markedly after dissolving tussah silk fibroin fiber in concentrated LiSCN in polypeptide size was the main features for the regenerated silk fibroin. © 1994 John Wiley & Sons, Inc.     

Structural characterization and mechanical properties of electrospun silk fibroin nanofiber mats

The regenerated silk fibroin dissolved in formic acid was electrospun into nanofiber mats. Structural characteristics of the spun as received and methanol and ethanol treated fibers were examined using the Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. Mechanical properties and air permeability of the electrospun mats were also studied. IR spectroscopy and X-ray diffractometry showed random coil conformation and amorphous structure for as-spun fibers while typical FTIR spectra and X-ray diffractograms of β-sheet crystalline structure were recorded for the methanol and ethanol treated fibers. The mechanical properties of the mats were found to be dependent on fiber diameter. The mats containing fibers with smaller diameter had higher tensile strength but lower breaking strain. Methanol and ethanol treatment enhanced tensile strengths of the mats at the expenses of their breaking strain. Air permeability and pore size of the mats are strongly associated with diameter of the electrospun fibers.     

Physical properties and phase separation structure of Antheraea pernyi/Bombyx mori silk fibroin blend films

The physical structure and compatibility of solution-cast Antheraea pernyi/Bombyx mori silk fibroin blend films were stuided by differential scanning calorimetry (DSC), thermomechanical (TMA) and thermogravimetric (TGA) analysis, dynamic viscoelastic measurement, infrared spectroscopy, and x-ray diffractometry. The DSC curves of the blend films showed independent endotherms at 280 and 358°C, corresponding to the thermal decomposition of B. mori and A. pernyi silk fibroins with random coil conformation. The intensity was roughly proportionate to the amount of each component in the blend. The thermal behavior corresponding to the conformational transitions induced by heating on A. pernyi and B. mori silk fibroins overlapped in the temperature range 190–230°C. Thermal expansion and contraction properties, as well as weight retention behavior of the blend films were intermediate between the pure components, as shown by the TMA and TGA curves. The onset temperature of the storage modulus curve decreased markedly, approaching that of B. mori silk fibroin film when the amount of this component in the blend increased. The loss modulus curve of the blend films showed two peaks at ca. 190 and 210°C, the former corresponding to B. mori, and the latter to A. pernyi silk fibroin. Infrared spectra of the blends exhibited absorption bands characteristic of the pure components overlapping in the spectral region 2000–400 cm^?1. The x-ray diffraction peaks at 23 and 21.5°, attributed to the crystalline spacings of A. pernyi and B. mori fibroins, respectively, overlapped in the diffraction curves of the blends, while the peak at 11.4°, of A. pernyi, increased as the content of this fibroin in the blend increased. The degree of crystallinity, calculated from the x-ray diffraction curves, diminished as the amount of B. mori silk fibroin decreased. A low degree of compatibility exists between the two fibroins when they are cast from aqueous solution in the experimental conditions adopted in this work. © 1994 John Wiley & Sons, Inc.     

Physical characteristics and properties of waterborne polyurethane materials reinforced with silk fibroin powder

Degummed silk filament was pulverized with a home‐made machine to obtain silk fibroin (SF) powder, and the structure, morphology, and particle size of the SF powder were investigated. The individual spherical particles and aggregates with different morphology of silk fibroin coexisted in water. A waterborne polyurethane (WPU) aqueous dispersion was blended with the SF powder to prepare novel blended materials with improved physical properties. The average particle size and zeta potential of the WPU/SF aqueous dispersions were characterized. The result showed that the WPU/SF dispersion with higher SF content exhibited a less negative zeta potential and a larger average particle size. Furthermore, the effect of SF content on the morphology, miscibility, and mechanical properties of the resulting blended films was studied by scanning electron microscopy, wide‐angle X‐ray diffraction, dynamic mechanical thermal analysis, and tensile testing. The films showed an improved Young's modulus and tensile strength from 0.3 to 33.8 MPa, and 0.6 to 5.2 MPa, respectively, with the increasing of SF up to a content of 26 wt %. The negative charges in the periphery and the small particle size made a good effort on dispersing SF powder into the WPU matrix as small aggregates, and the SF powder led to the efficient strengthening of WPU materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 940–950, 2010     

Physical properties and structure of silk. VI. Conformational changes in silk fibroin induced by immersion in water at 2 to 130°c

Silk fibroin films in the random-coil and β-form conformations were immersed in water at temperatures from 2 to 130°C, and conformational changes were followed by x-ray diffraction, infrared spectroscopy and differential scanning calorimetry. On treatment with water below 60°C, the random-coil conformation is converted to the α form and above 70°C to mixtures of the α and β conformations. The β-form content increases as the immersion temperature is raised. The β form is not affected by immersion in water in the temperature range studied.     

Study on porous silk fibroin materials: 3. Influence of repeated freeze–thawing on the structure and properties of porous silk fibroin materials

The influence of repeated freeze–thawing on pore structural characteristics and physical properties of porous silk fibroin materials prepared by freeze drying were studied. It showed that when quick‐frozen silk fibroin solution was repeatedly thawed and frozen before being vacuum dried, thus pore size of prepared porous silk fibroin materials increased from 67 µm to about 120 µm, and pore density decreased from 80 per square millimeter to about 28 per square millimeter; at the same time compression ratio and moisture permeability increased from 22.7% and 230 g/m² hr to about 33.7% and 308 g/m² hr, respectively, tensile strength and dissolvability in hot water decreased from 20.2 N/cm² and 42.7% to about 12.5 N/cm² and 26.1%, respectively. Both the times of repeated thawing and the thawing temperature had a certain influence on the above‐mentioned pore characteristic parameters and physical properties. Copyright © 2001 John Wiley & Sons, Ltd.     

Regenerated protein fibers. II. Viscoelastic behavior of silk fibroin solutions

The dynamic viscoelastic behavior of a concentrated solution of silk fibroin dissolved in the “MU” solvent is measured. The dynamic viscosity η′ and dynamic elasticity G′ increase with increasing concentration of silk fibroin at constant frequency; however, the increasing frequency decreases η′ and G′ at a constant concentration of silk fibroin. When the mixing ratio of C₂H₅OH/H₂O in the “MU” solvent is increased at a constant concentration of LiBr·H₂O, η′ and G′ sharply increase at constant frequency. If the LiBr·H₂O concentration is varied in the “MU” solvents whose ratio of C₂H₅OH/H₂O is kept constant at 100 : 0, both η′ and G′ are greater for LiBr·H₂O concentrations of 50% by weight compared to concentrations of 40% by weight. The dependence of η′ on the temperature of the solution can be predicted by Andrade's viscosity equation. Spinnability improves when the SF concentration is increased. © John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1955–1959, 1997     

Dynamic mechanical and dielectric properties of epoxidized SBS triblock copolymer

The morphological and dynamic properties of epoxidized styrene–butadiene–styrene block copolymers were studied and compared with their parent styrene–butadiene–styrene block copolymer (SBS). Two peaks were observed in the mechanical loss (tan δ) curve which can be attributed to segmental motion of epoxidized polybutadiene (EPPB) and polystyrene. Analysis by DSC thermograms also showed the linear increase of glass transition temperature for EPPB domain with the epoxy group content. Phase separated structures of epoxidized SBS as observed by TEM suggests a considerable degree of mixing occurred between phases after 80 mol % of the double bonds in SBS were epoxidized. The interfacial region displays a third peak and causes much steeper drop in modulus at higher temperature than T_g of EPPB. Parallel dielectric relaxation measurements were also made in the frequency range of 30 Hz–1 KHz as a function of temperature. In each dielectric constant (?′) curve, there is a maximum near the T_g of EPPB determined from the dielectric loss tangent curve. The shift in T_g of EPPB versus epoxy group content was consistent with that measured by the thermal and dynamic mechanic analysis. These findings indicated an 8°C shift in glass transition temperature as the epoxy group content in EPPB increased 10%.     

Surface properties of silk fibroin films and their interaction with fibroblasts

There is a growing interest in the use of silk as a biomaterial for tissue engineering. Silk threads from Bombyx mori have a fibrous core of fibroin, the protein responsible for biocompatibility and bioactivity, which is surrounded by a family of "gummy" proteins, called sericins, which are almost completely removed during silk degumming. Three different methanol treatments on regenerated fibroin films were used to convert viscous solutions of Silk I to an insoluble crystalline form (Silk II), in an attempt to devise new processing protocols for the creation of a cell guiding fibroin surface. Human fibroblasts (MRC5 line) were used as probes of the cell-biomaterial interaction in the early stages of the process (1 h, 3 h, 6 h and 4 d after seeding). The effect of each treatment on cell adhesion, spreading and distribution was monitored by scanning electron microscopy (SEM) and was correlated to superficial properties (like roughness and crystallinity) and fibroin conformation by means of atomic force microscopy (AFM), used in both topographical and acoustic mode, and attenuated total internal reflection infrared spectroscopy (FTIR-ATR). It was found that traditional methanol treatments where fibroin films were soaked in methanol solution produced roughness patterns that affected only the very early stages of fibroblast adhesion (until 3 h from seeding), while the new treatment proposed could really dialogue with the cells. Its non-homogeneous surface can explain the existence of cells spreading in specific directions and the presence of cell repellent areas even 4 d after seeding.     

Influence of an iodine treatment on the structure and physical properties of Bombyx mori silk fibroin fiber

Silk fibroin (SF) fiber from the Bombyx mori silkworm was treated with a 1.23 N iodine/potassium iodide (I₂–KI) aqueous solution, and the structure and physical properties were investigated to elucidate the effects of the iodine treatment. The SF fiber absorbed polyiodide ions such as I and I by immersion in the I₂–KI solution, and the weight gain of the SF fiber increased with the treatment time; it became saturated at about 20 wt % after 40 h. The results of the weight gain, Fourier transform infrared spectroscopy, and X‐ray diffraction measurements suggested that polyiodide ions mainly entered the amorphous region. Moreover, a new sharp reflection in the meridional direction, corresponding to a period of 7.0 Å, was observed and indicated the possibility of the formation of a mesophase structure of β‐conformation chains. Dynamic viscoelastic measurements showed that the molecular motion of the crystalline regions at about 220 °C was enhanced and shifted to lower temperature by the introduction of polyiodide ions. This indicated that the iodine component weakened the hydrogen bonding between the SF molecules forming the β‐sheet structure and caused molecular motion of the crystal to occur more easily with heating. With heating above 270 °C, the iodine component introduced intermolecular crosslinking to SF, and the melt flow of the sample was inhibited. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3418–3426, 2006     

Dynamic mechanical and dielectric properties of phosphonylated polypentenamers and their hydrogenated derivatives

Dynamic mechanical and dielectric properties of substituted polypentenamers with phosphonate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids, and salts were investigated at two concentrations, 6.5 and 11.1 mole percent. In the unhydrogenated derivatives, one principal relaxation, labeled β, is observed mechanically in the temperature range from ?160 to 100°C. This β relaxation arises from micro-Brownian segmental motion accompanying the glass transition. Its temperature is substantially affected by the substituent concentration while its breadth is affected by the chemical nature of the substituent. An extended “rubbery plateau” region exists in the acid and salt derivatives. The dielectric results generally reinforce the mechanical assignments. In the hydrogenated derivatives, three relaxations labeled α, β, and γ in order of decreasing temperature occur mechanically in this temperature range. The temperatures at which the α and β relaxations occur depend greatly on the chemical nature of the substituents, the substituent concentration, and the thermal history of the sample; while the γ relaxation appears to be independent of these variables. Suggested assignments for the relaxations observed in these polymers, based on the dual glass transition theory of Boyer for semicrystalline polymers, have been proposed. The dielectric results are consistent with the proposed assignments.     

Theoretical investigation of polymer molecular structure influence on dielectric properties and mechanical properties

Triboelectric nanogenerator (TENG) technologies have explosive development in the field of energy harvesting and self-powered sensing. As the key element of triboelectric devices, dielectric polymers have obtained much attention in recent years. The dielectric properties of polymer determine the output performance of TENG. In this paper, we take silicone rubber as an example of dielectric polymers, to study the properties of molecular structure influence on the dielectric properties and mechanical properties by the molecular dynamics simulation method. The free volume fraction, dielectric constant, and mechanical properties of silicone rubbers with different branch chains were calculated. The dielectric constant is highly related to the free volume distribution and the dipole moments of silicone rubbers with different amounts of branch chains. For fewer branch chains silicone rubber, the free volume distribution contributes most to the dielectric constant; for more branch chains silicone rubber, the dipole moment dominates the dielectric constant. Therefore, the silicone rubber ratio has a great influence on the dielectric constant of silicone rubber. With the increase of temperature, the dielectric constant of 2-chain silicone rubber increases at first and then decreases, and the maximum value is obtained near 300 K. Therefore, it is necessary to control the temperature when silicone rubber is used as a dielectric material. This work can be a guide for improving the dielectric properties of silicone rubber, and it provides a new approach to the optimal design of high-performance triboelectric nanogenerators.     

Dynamic mechanical properties of plasticized polystyrene-based ionomers. II. Melt rheology

A high molecular weight styrene ionomer containing 5 mol% sodium methacrylate was blended with a styrene oligomer (MW 800) and investigated by dynamic mechanical techniques. The focus of the study is on the dynamic melt rheology of these materials, whose ratios by weight of ionomer to oligomer are 60/40, 40/60, and 25/75. The glass-transition temperature and ionic transition are first characterized by torsion pendulum measurements as a function of temperature. It appears that a maximum level of plasticization is achieved for the ionic regions, the extent depending on sample history. Time–temperature superposition is obeyed by the blend of 60 wt% ionomer, but not by the other two blends. Relaxations due to the ionic regions are clearly evident in the relaxation spectra of all three blends. Above a particular temperature, the 25 wt% blend indicates an Arrhenius type of dependence.     

Electrospun core–sheath fibers for integrating the biocompatibility of silk fibroin and the mechanical properties of PLCL

In the process of preparing core–sheath fibers via coaxial electrospinning, the relative evaporation rates of core and sheath solvents play a key role in the formation of the core–sheath structure of the fiber. Both silk fibroin (SF) and poly(lactide‐co‐ε‐caprolactone) (PLCL) have good biocompatibility and biodegradability. SF has better cell affinity than PLCL, whereas PLCL has higher breaking strength and elongation than SF. In this work, hexafluoroisopropanol (HFIP)‐formic acid (volume ratio 8:2), HFIP and HFIP–dichloromethane (volume ratio 8:2) were used to dissolve PLCL as the core solutions, and HFIP was used to dissolve SF as the sheath solution. Then, core–sheath structured SF/PLCL (C‐SF/PLCL) fibers were prepared by coaxial electrospinning with the core and sheath solutions. Transmission electron microscopy images indicated the existence of the core–shell structure of the fibers, and energy dispersive X‐ray analysis results revealed that the fiber mat with the greatest content of core–shell structure fibers was obtained when the core solvent was HFIP–dichloromethane (volume ratio 8:2). Tensile tests showed that the C‐SF/PLCL fiber mat displayed improved tensile properties, with strength and elongation that were significantly higher than those of the pure SF mat. The C‐SF/PLCL fiber mat was further investigated as a scaffold for culturing EA.hy926 cells, and the results showed that the fiber mat permitted cellular adhesion, proliferation and spreading in a manner similar to that of the pure SF fiber mat. These results indicated that the coaxial electrospun SF/PLCL fiber mat could be considered a promising candidate for tissue engineering scaffolds for blood vessels. Copyright © 2014 John Wiley & Sons, Ltd.     

Fabrication of silk fibroin film with properties of thermal insulation and temperature monitoring

Silk fibroin exhibits excellent mechanical properties, good biocompatibility, and biodegradability combined with benign processing conditions, attracting considerable research interest for the application as biomedical materials. Among the diverse forms of sponges, hydrogels, films, and mats manufactured from silk fibroin, films are especially appealing due to the high water and oxygen permeability, good cell attachment, and low immunogenicity. Fabrication of silk fibroin films with novel properties has been successfully developed simply by incorporating various functional components into it. In the present study, the properties of thermal insulation and temperature monitoring for the silk fibroin film are demonstrated for the first time through the incorporation of thermochromic microcapsules within it. Moreover, the silk fibroin film is also endowed with improved mechanical properties in terms of tension strength and elongation at break because of the reinforcing effect of thermochromic microcapsules. The silk fibroin film fabricated with novel features in this study can be a good candidate for the application of wound dressings, tissue engineering scaffolds, and bio‐related devices in the future. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1846–1852