Concentration‐dependent conformation transition of regenerated silk fibroin induced by graphene oxide nanosheets incorporation

Regenerated silk fibroin (RSF)/graphene oxide (GO) nanocomposite has been substantially investigated due to its significant multifunctional potential. Here, in combination of micromorphology, crystalline conformation, dynamic mechanical property characterization, and Fourier self‐deconvolution (FSD) quantitative analysis, we investigated the RSF molecular chains conformation transition induced by GO nanosheet incorporation, and its influence on the structural and mechanical properties of solution casted RSF/GO composite films. The GO nanosheet promoted the silk fibroin molecular chains conformation transition from random coil to β‐sheet structure, and a correlation between β‐sheet structure fraction and GO concentration was revealed. The β‐sheet structure fraction increases further improved the dynamic mechanical property of composite films. Moreover, based on nucleation‐dependent aggregation of silk fibroin molecular chains, a mechanism considering the competition effect between GO concentration and its total surface area was proposed to explain the observed concentration‐dependent conformation transition phenomenon. The study improves our understanding on silk fibroin conformation transition process in RSF/GO composite and would provide a valuable reference for the rational design of bioinspired multifunctional materials with enhanced mechanical properties. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1506–1515     

Physical properties and structure of silk. II. Dynamic mechanical and dielectric properties of silk fibroin

Dynamic mechanical and dielectric properties of amorphous regenerated films of silk fibroin were studied as a function of temperature. A mechanical loss tangent peak at about 175°C may be due to the segmental motion of the main chains in the amorphous silk fibroin film. The dynamic modulus of the amorphous silk fibroin increased at 185°C due to the crystallization of the silk fibroin. Dielectric loss tangent peaks were observed at about ?40°C and 175°C at 1 kHz. The former is ascribed to the local motion of the amorphous silk fibroin with absorbed water, while the latter seems to originate from the segmental motion of the main chains and the crystallization of silk fibroin.     

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.     

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     

Influence of the draw ratio on the tensile and fracture behavior of NMMO regenerated silk fibers

The tensile properties and fracture surfaces of N‐methylmorpholine‐N‐oxide (NMMO) regenerated silk fibroin fibers produced with a range of draw ratios has been characterized and related to their microstructure with data obtained from Raman spectroscopy and birefringence measurements. The spinning process allows control of two different draw ratios, coagulation, and postspinning, and it has been found that the microstructure and the properties of the fibers can be modified by the proper combination of both draw ratios. NMMO regenerated silk fibroin fibers subjected to postspinning drawing yield tensile properties comparable to other regenerated fibers and strain at breaking comparable to natural Bombyx mori silk fibers. Tensile strength; however, is still significantly lower than that of natural fibers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2568–2579, 2007     

In situ measurement of the thermal expansion behavior of benzocyclobutene films

In situ measurement techniques suitable for determination of the coefficient of thermal expansion (CTE) in thin, spin‐cast polymer films in both the in‐plane and through‐plane directions are presented. An examination of the thermal expansion behavior of cyclotene thin films has been performed. In particular, the effect of film thickness on the in‐plane and through‐plane CTE and in‐plane Young's modulus of spin‐coated cyclotene films was examined. It is shown that the mechanical response of in situ cyclotene films can be adequately described by isotropic film properties. It was also demonstrated that there is no thickness dependence on the free‐standing mechanical properties or on the resulting through‐plane thermal strain in an in situ film. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 311–321, 1999     

Influence of graphene oxide incorporation and chemical cross‐linking on structure and mechanical properties of layer‐by‐layer assembled poly(Vinyl alcohol)‐Laponite free‐standing films

Functional fillers in multilayered films provide opportunity in tailoring the mechanical properties through chemical cross‐linking. In this study, Laponite‐graphene oxide co‐dispersion was used to incorporate graphene oxide (GO) easily into polyvinyl alcohol (PVA)/Laponite layer‐by‐layer (LBL) films. The LBL films were found to be uniform and the layer thickness increased linearly with number of depositions. The process was extended to a large number of depositions to investigate the macroscopic mechanical properties of the free‐standing films. The LBL films showed remarkable improvements in mechanical properties as compared to neat PVA film. The GO‐incorporated LBL films displayed higher enhancements in the tensile strength, ductility, and toughness as compared to that of PVA/Laponite LBL films, upon chemical cross‐linking. This suggests the advantageous effects of GO incorporation. Interestingly, cross‐linking of LBL films for longer time period (>1 h) and higher temperature (～80 °C) was not found to be much beneficial. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2377–2387     

1H pulsed NMR study of bombyx mori silk fibroin: Dynamics of fibroin and of absorbed water

The dynamical behavior of the Bombyx mori silk fibroin chain and of absorbed water in silk fiber, film, and powder has been studied by ¹H pulsed nuclear magnetic resonance (NMR). Segmental motions do not occur and only the rapid rotation of the methyl groups of alanine residues is observed from ?120 to 130°C. This is independent of the conformation or form of the silk fibroin samples. Magnetization of dry silk fibroin by the solid-echo method shows a single Gaussian decay, while two components are observed in the solid-echo signals of films containing 6–10 w/w% water. An immobile component with a T₂ value of 11 μs is attributed to silk fibroin, and the mobile component to bound water. The T₂ of the latter varies from 50 to 200 μs, depending on the sample. The dynamical behavior of water trapped in the film is discussed on the basis of these T₂ values.     

甘油对丝素蛋白膜水溶性和结构的影响

探讨了甘油的加入对丝素蛋白溶解过程的结晶结构及结晶度的影响. 以甘油为添加剂用流延法于室温制备一系列丝素共混膜, 测试了其含水率、溶失率及结构和机械性能. 结果表明, 随着甘油加入量的增加, 丝素蛋白的溶失率逐渐降低; 当甘油/丝素质量分数超过10%时, 共混膜呈现水不溶性. 当甘油加入量较少时, 丝素蛋白呈现少量的Silk Ⅱ结晶, 而Silk Ⅰ结构不明显. 随着甘油含量的不断增加, Silk Ⅰ结晶逐渐增加而Silk Ⅱ结晶逐渐减少. 当甘油加入量达到不溶点(10%)时, 丝素蛋白主要转变为Silk Ⅰ结晶, 而几乎没有Silk Ⅱ结晶. 甘油的加入可使共混膜的柔韧性显著提高, 并促使丝素蛋白结晶度提高以及促使丝素蛋白向Silk Ⅰ结晶转变, 从而降低丝素蛋白膜的水溶性.     

Thickness dependence of thermally induced changes in surface and bulk properties of Nafion® nanofilms

Thermally induced changes in surface wettability, dewetting behavior, and proton transport of “self‐assembled” nanothin Nafion^® films (4–300 nm) on SiO₂ substrate is reported. Thermal annealing induces switching of the surface wettability of 55 nm and thinner films from hydrophilic to super‐hydrophobic. Thickness dependence of this behavior is observed with higher annealing temperature required for lower thickness films, indicating highly restrictive mobility of Nafion^® ionomer as film thickness decreases. Dewetting is only observed for 4‐nm thin film. Significant suppression in proton conductivity upon thermal annealing was noted. Similarly, two other bulk properties, water uptake and swelling, were found to decrease upon annealing. This work reports a systematic examination of the thickness dependence of thermally induced changes in both surface and bulk properties of ultra‐thin Nafion^®. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1267–1277     

原位化学氧化聚合制备聚苯胺/丝素复合导电膜

采用原位化学氧化聚合方法在蚕丝丝素蛋白膜表面生长聚苯胺,制备得到表面均匀覆盖导电聚合物的复合导电丝素膜,其电导率约为3×10^-2S/cm。纤维表面与导电聚合物的相互作用改善了原丝素膜的耐热性能,但并未降低其力学性能。     

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.     

Synthesis of bisphenol A‐free oligomeric phthalonitrile resins with sulfone and sulfone‐ketone containing backbones

Two new oligomeric sulfone and sulfone‐ketone containing phthalonitrile (PN) resins with excellent processability have been developed. The PN monomers were prepared from the reaction of an excess amount of bisphenol S with 4‐(chlorophenyl)sulfone or 4,4‐dichlorobenzophenone in the presence of a base in a solvent mixture (dimethylsulfoxide/toluene), followed by end‐capping with 4‐nitro‐PN in a two‐step, one‐pot reaction. These PN resins exhibited good viscosities and cure times for molding into various shapes. After being thermally cured to yield crosslinked polymers, these polymers demonstrated superb mechanical properties, thermo‐oxidative stability, and maintained good dielectric properties. Published 2016. 1 J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1639–1646     

Influence of the film‐forming process on the nanostructuration of Pebax®/1‐ethyl‐3‐methylimidazolium triflate ionic liquid: Consequences on the thermal,mechanical, gas,and water transport properties

1‐Ethyl‐3‐methylimidazolium triflate ionic liquid (IL) was incorporated in Pebax® MV 3000 copolymer through solvent cast (SC) or melt blending (MB) for composition from 0 to 30 wt % IL. The morphology was investigated by small angle neutron scattering, transmission electron microscopy, and differential scanning calorimetry. The SC copolymer film exhibited a lower mean correlation distance (D = 87 Å) and smoother transition between the rigid and soft phases in comparison with the MB film (D = 103 Å). By dissolving in the copolymer soft phase, IL acted as a plasticizer, impeded soft segments crystallization and led to linear increase of D. The differences observed in morphology as a function of the film process impacted the mechanical and gas transport properties: below 20 wt % IL, all SC films sustained thermomechanical properties up to 120 °C and exhibited lower permeability than MB films. IL adding made permeability decrease up to 60%, depending on the gas nature and IL amount. Hydration of the films was investigated by sorption and SANS analyses. The impact of water uptake on swelling was similar for all membranes whereas water diffusion depended on the film morphologies and IL amount. Interesting mechanical and transport properties were obtained for IL content up to 20 wt %. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 778–788     

Structure and properties of composites compression‐molded from silk fibroin powder and waterborne polyurethane

The degummed silk filament was pulverized with a home‐made machine to obtain the silk fibroin (SF) powder with the diameter of around 3 µm. The resulting SF powder was blended with waterborne polyurethane (WPU) aqueous dispersion, and then was dried and compression‐molded to prepare novel blended materials with improved miscibility and mechanical properties. WPU acted as a plasticizer and one of the components for the blends during the compression‐molded process. The structure, morphology, and properties of the blended films were investigated. The results indicated that β‐sheet of SF existed in the blended films. The SEM images showed that the cross‐section of the blended films exhibited an overall homogeneous morphology. Furthermore, the transmission electron microscope observation exhibited that some sphere‐like SF particles were well dispersed in the WPU matrix. The hydrogen bond interaction between SF and WPU in the blended films led to an increase of the glass transition temperature for the soft segment of WPU in the blended films. The blended films showed an improved Young's modulus and tensile strength from 1.2 to 288.9 MPa and 0.3 to 16.5 MPa, respectively, with the increasing of SF up to a content of 70 wt%. The hydrogen‐bonding interactions existing in SF and WPU and compression molding method played the important role in improving the miscibility and mechanical properties of the blended films. Copyright © 2011 John Wiley & Sons, Ltd.     

天然蚕丝丝素蛋白的乳化活性

采用不同油相制备了系列丝素蛋白乳液, 研究了丝素蛋白浓度、 油相体积分数和油相极性对丝素蛋白的乳化活性指数、 丝素蛋白乳液的稳定性和类型及乳液液滴的微观形态、 粒径与zeta电位的影响, 探讨了丝素蛋白的乳化活性和乳液稳定机制. 结果表明, 丝素蛋白具有两亲性和表面活性, 可在油水界面富集并形成稳定的黏弹性保护膜; 丝素蛋白的乳化活性随其浓度的增大而减小, 随油相体积分数的增大而增大; 丝素蛋白浓度和油相体积分数的增加可提高稳定乳液体积分数.     

Permeability and micromechanical properties of silk ionomer microcapsules

We studied the pH-responsive behavior of layer-by-layer (LbL) microcapsules fabricated from silk fibroin chemically modified with different poly amino acid side chains: cationic (silk-poly l-lysine, SF-PL) or anionic (silk-poly-l-glutamic acid, SF-PG). We observed that stable ultrathin shell microcapsules can be assembled with a dramatic increase in swelling, thickness, and microroughness at extremely acidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeable disintegration. These changes are accompanied by dramatic changes in shell permeability with a 2 orders of magnitude increase in the diffusion coefficient. Moreover, the silk ionomer shells undergo remarkable softening with a drop in Young's modulus by more than 1 order of magnitude due to the swelling, stretching, and increase in material porosity. The ability to control permeability and mechanical properties over a wide range for the silk-based microcapsules, with distinguishing stability under harsh environmental conditions, provides an important system for controlled loading and release and applications in bioengineering.     

Synergistic effect of cellulose nanowhiskers reinforcement and dicarboxylic acids crosslinking towards polyvinyl alcohol properties

In the presented work, a significant increase in the tensile strength of the PVA composite material is reported. The obtained best value of 122 MPa and 14.6% swelling shows the excellent synergistic effect of both crosslinker and reinforcement material. The composite films were prepared by simple mechanical dispersion of reinforcement material, bacterial cellulose nanowhiskers (BCNW) into PVA solution followed by crosslinking with diacids, succinic acid (SuA), and adipic acid (AdA). The effect of aliphatic carbon chain length of crosslinker on thermal, mechanical, and water uptake properties is evaluated and discussed in detail. Neat PVA had the strength of 37.3 MPa. With 5% reinforcement of BCNW, that is, without crosslinking, it exhibited 97% increase, that is, 74.5 MPa. With crosslinking of 15 mmol of SuA and AdA PVA films for 2 h had excellent thermal properties with swelling percentage of 19 and 26.6% and tensile strength of 103 and 67 MPa, respectively. The best result obtained was for 5% BCNW–PVA films crosslinked with 15 mmol SuA. These results were explained on the basis of synergetic crosslinking and extended hydrogen bonding between PVA and reinforcement material. The composite films can be used as biological implants, a membrane for pervaporation and filtration system, etc. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2515–2525     

Silk Fibroin Degradation Related to Rheological and Mechanical Properties

Regenerated silk fibroin has been proposed as a material substrate for biomedical, optical, and electronic applications. Preparation of the silk fibroin solution requires extraction (degumming) to remove contaminants, but results in the degradation of the fibroin protein. Here, a mechanism of fibroin degradation is proposed and the molecular weight and polydispersity is characterized as a function of extraction time. Rheological analysis reveals significant changes in the viscosity of samples while mechanical characterization of cast and drawn films shows increased moduli, extensibility, and strength upon drawing. Fifteen minutes extraction time results in degraded fibroin that generates the strongest films. Structural analysis by wide angle X‐ray scattering (WAXS) and Fourier transform infrared spectroscopy (FTIR) indicates molecular alignment in the drawn films and shows that the drawing process converts amorphous films into the crystalline, β‐sheet, secondary structure. Most interesting, by using selected extraction times, films with near‐native crystallinity, alignment, and molecular weight can be achieved; yet maximal mechanical properties for the films from regenerated silk fibroin solutions are found with solutions subjected to some degree of degradation. These results suggest that the regenerated solutions and the film casting and drawing processes introduce more complexity than native spinning processes.

     

新方法制备不溶性丝素薄膜及其性质研究

报道一种制备不溶性丝素蛋白薄膜的新方法.在温和搅拌下,将丝素溶液浓缩至16%,通过调整干燥温度和干燥速率,直接获得不溶于水的丝素薄膜.薄膜结构通过FTIR-ATR和XRD测定,同时对薄膜的机械性能进行研究.结果表明,丝素薄膜在干燥温度高于60℃时,有充足的β折叠结构存在,机械性能良好.当干燥温度为70℃时,材料的拉伸强度和断裂伸长率分别为29.8MPa和59.6%,远远强于经过甲醇处理的丝素薄膜的机械性能.当干燥温度高于60℃时,材料在水中具有优异的稳定性.由于在丝素薄膜的制备中避免了甲醇的使用并使材料具有更好的机械性能,此方法制备的丝素薄膜更为安全,应用会更广泛.