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Fabrication and characterization of silk fibroin powder/polyurethane fibrous membrane.

INTRODUCTION

Considerable efforts to develop scaffolds for tissue engineering have been attempted using biodegradable and biocompatible polymers over the last decade. Principally, the scaffold should be designed by mimicking the structure and biological function of native extracellular matrix (ECM) proteins, which provide mechanical support and regulates cell activities, incorporating physical, chemical, and biological cues to guide cells into functional tissues via cell migration, adhesion, and differentiation. The native ECM is a molecular complex made up of proteins and polysaccharides (1). Silk proteins are one of the candidate materials for biomedical applications, because it has several distinctive biological properties including good biocompatibility, good oxygen and water vapor permeability, biodegradability, and minimal inflammatory reaction (2-4).

Some researchers have investigated the effects of the silk fibroin (SF) on the culture of fibroblasts and osteoblasts and concluded that SF has positive effects on cell adhesion, viability, growth, and differentiated functions (5-9). SF is available in various forms, such as fiber, film, and powder. Silk film obtained from dissolution with suitable solvent is widely used in clinical application. Nevertheless, the kind of film is very brittle and easily soluble in water, which limits its applications. Silk fiber was easily decomposed in the process of spinning under a high temperature, and the inherent microstructure of SF was damaged inevitably. Therefore, the protein biofibers lost the original properties of the protein. The improved methods by blend the solution of SF with other natural or synthetic polymers have been reported (10-13). In the same time, the microstructure of SF was damaged also. Another alternative approach to overcome the above drawbacks is the utilization in the powder form of SF. This is a novel form, which keeps the original natural properties of silk without damaging its microstructure (14), (15). SF was produced as one of the useful physical forms of SF protein which had some special properties compared with the fiber, silk films, and silk/natural or synthetic polymer blend film for biomaterial applications (16), (17).

The native ECM is in the nanoscale fibrous network structure (18). The nanofibrous scaffolds can improve the regeneration of tissues in vitro, including bone, cartilage, cardiovascular tissue, nerve, and bladder, and minimize the scars in the regenerated tissues as human cells can attach and organize well around the fibers with diameters smaller than those of the cells (19). The nanoscale fibrous network structure can be obtained by electrospinning, which is a promising technique currently being explored for use in the development of biomaterial scaffolds for tissue-engineering applications. In addition, the nanofibrous scaffolds may need to include provisions for mechanical support appropriate to the level of functional tissue development. Polyurethane (PU) is one of the synthetic candidate biomaterials because it is strong, flexible, resistant to tear, and compatible to blood (20). The blend casting film of PU and SF powder had researched widely (21), (22), however, the blend fibrous membrane of PU and SF powder fabricated by electrospinning, which is an interesting way to combine the natural characteristics of SF, good mechanical properties of PU with nanoscale fibrous network structure, did not been investigated.

In this work, the blend fibrous membranes with the different mass ratio of PU to SF were fabricated by electrospinning. We interpreted the results of observations made by field-emission scanning electron microscope (FESEM), analyzed the surface microstructure of the blend fiber and the crystallinity and molecular orientation of PU in fibers, and tested the surface wettabilities and the mechanical properties of the blend fibrous membranes.

EXPERIMENTAL PART

Materials

A segmented PU with the type of Pellethane 2363-80A was provided by Dow Chemical Company. N, N-dimethylformamide (DMF), a kind of solvent, was supplied by Tianjin 'Tianda Chemical Reagent. SF was obtained by heating silk in a carbonate sodium solution for 3 h and was then ground to SF powder (23).

Electrospirming

To prepare SF/PU blend solution, the SF and PU were thoroughly dispersed in DMF. The mass ratio of PU to DMF in solution concentration was fixed at 10 wt%. A series of the SF/PU solutions were coded as PU, SF/PU 1, SF/PU3, and SF/PUS, corresponding to the SF/PU mass ratio of 0/10, 1/9, 3/7, and 5/5, respectively. It was followed by degassing in vacuum to obtain an electrospinning solution. A schematic diagram of electrospinning process is shown in Fig. 1. The electrospinning apparatus consisted of a 5 ml syringe that connected to a syringe pump. The positive lead from a high voltage supply was connected via an alligator clip to the external surface of needle. The SF/PU solution was electrospun onto a rotating stainless steel drum to obtain a SF/PU fibrous membrane. The solution flow rates were controlled by the syringe pump (0.2 ml/h). The applied voltage was 20 kV, and the tip-collector distance was 10 cm. All electrospinning were carried out at room temperature.

[FIGURE 1 OMITTED]

Characterizations of the SFIPU Fibrous Membranes

The morphologies of the membranes were observed by a Sirion 11 type FE-SEM.

The Fourier transform infrared (FT1R) spectrum was obtained by means of Germanic VERTEX 70 infrared spectrophotometer with the reflection absorption spectroscopy technology and scanning range from 4000 to 650 [cm.sup.-1].

In the X-ray diffraction (XRD) analysis, the sample was fixed onto a stub and placed within the chamber of Analytical X-ray powder diffractometer (Japanese Dmax-rA, wavelength 1.54 A, Cu Ka radiation). The sample was then scanned in the range from 5[degrees] to 50[degrees] in step of 5[degrees]/min.

Contact angle measurements were carried out by (FM40 Easy Drop, KRUSS GmbH, Germany). The measurement used distilled water as the reference liquid and was automatically dropped onto the electrospun membranes. To confirm the uniform structure of electrospun membranes, the contact angle was measured three times from different positions, and an average value was calculated by statistical method.

Thermogravimetric (TG) analyses were performed on the tester of METTLER TG50, Nitrogen protected, at a heating rate of 10[degrees]C/min.

Dynamic mechanical thermal (DMT) analysis was performed on a dynamic mechanical thermal analyzer (DMAQ800, TA Instruments, Newcastle, DE) in tensile mode at a frequency of 1 Hz. The specimen was a rectangular strip (10 mm x 5 mm X 0.2 mm), and the test temperature was from -100 to 100[degrees]C with a heating rate of 5[degrees]C/min.

Mechanical properties were tested on an Instron 5566 Universal Testing Machine, at a gauge length of 20 mm and strain rate of 50 mm/min at room temperature, and the width of the samples was 5 mm. An average value of at least three replicates for each sample was taken.

RESULTS AND 1SCUSSDON

Morphological Characterizations

Figure 2 shows the SEM images of the composite fibrous membranes with different SF/PU mass ratio (0/10, 1/9, 3/7, and 5/5) under the same electrospinning condition. The results indicated that the mass ratio of SF to PU played an important role in determining the membrane properties. The SF was uniformly distributed in the composite fibers without apparent agglomeration. The fibrous membranes obtained from the PU solution without SF exhibited serious "beads on string" morphology. Furthermore, these beads fused together and solidified into a larger molten structure. With the increase of SF in the solution, the number of beads in the fibers decreased. When the mass ratio of SF to PU was 1/9, there were lots of the beads in the fibrous membrane. The mass ratio of SF to PU being 3/7, a little bead in the fiber was found, and to the mass ratio SF to PU being 5/5, the fibers with the even diameter were obtained.

[FIGURE 2 OMITTED]

It is known that the SF does not dissolve in DMF. The SF only increased the solid content in the solution. It was noteworthy that the sphere-like particles of SF had an average size of 41.0 [+ or -] 7.2 nm on the basis of statistic analysis and the onion-like aggregate particles had an average size of around 500 nm X 500 nm (22). The surface and lacuna of SF aggregate particles would adsorb lots of DMF to increase the viscosity of the blend solution. The viscosity of the SF/PU blend solution with mass ratio of 0/10, 1/9, 3/7, and 5/5 was 15.49, 16.65, 17.48, and 18.28 Pa-s (Shanghai Precision Instrument Co., NDJ-8S), respectively. There were sufficient molecular chain entanglements in the solution to prevent the breakup of the electrically driven jet and to allow the electrostatic stresses to further elongate the jet and draw it into fibers (24). Fong et al. (24) showed that the viscosity plays a major role in the formation of beads. They indicated that high viscosity tended to facilitate the formation of fibers without beads. In the other hand, with the increase in SF content in the solution, the PU solution flow rates decreased. It was facile for the small jet to be elongated well to decrease the number of beads.

With the increase in SF content, the fibers with even diameter and without the beads were obtained by electrospinning. It indicated that SF had good compatibility with PU in the solvent of DMF and SF was dispersed in DMF as an individual sphere in a stable state, and almost no agglomerations occurred. It might be relative to the surface electrical resistances of SF due to that the surface of the SF particles had some negative charges (22). The SF enwrapped in PU fibers and clotted onto fibers were like a knot of fiber (Fig. 2h, the arrows point). The SF was uniformly distributed in the fiber, which was contributed to the stable blend solution and the rapid solidification of the solution during the electrospinning process. It was different from the casting film that there were some cavities between SF and PU inside the film (21).

FTIR Spectroscopy

Figure 3 shows the FTIR of the samples. The pure PU had been extensively studied using FTIR (25-27). There were several absorption bands in the curve. The band near 3325 [cm.sup.-1] was due to the stretching vibration of OH groups; The bands near 2943, 2855, and 2798 [cm.sup.-1] were assigned to the asymmetric, symmetric, and CH2 stretch vibration, respectively; the band near 1702 [cm.sup.-1] was explained by the overlap of the two C=0 absorption of ester and carbamate; the band near 1530 [cm.sup.-1] was due to the amide II from the reaction of OH and NCO groups; the band near 1224 [cm.sup.-1] was attributed to the stretching vibration of CO and the distorted vibration of urethano in PU; and the band at (1110-950 [cm.sup.-1]) was explained by the asymmetric C--O--C stretching (fatty ether). The FTIR of the SF shows the typical SF spectrum with adsorption bands at 1646, 1518, 1234, and 646 [cm.sup.-1] assigned to amide I (C=O stretching vibration), amide 11 (C--N stretching and N--H distortion vibration), amide HI (C--N stretching and N--H deformation vibration), and amide V (C=0 bending vibration), respectively (28). The bands in the range 800-1200 [cm.sup.-1] are characteristics of a specific polypeptide with respect to amino acid linkages. The two weak bands at 1014 and 976 [cm.sup.-1] are attributed to the glycine-glycine linkage and alanine-alanine linkage, respectively (21).

[FIGURE 3 OMITTED]

The spectrums of SF/PU fibrous membranes were very similar to that of the pure PU. This indicated that the interfacial bonding between SF and PU was weak. It was worth noting that some FT1R peaks of SF/PU fibrous membranes gradually shifted when the blend composition was varied. The small magnitude shifts and intensity change of the diffraction peak provided the information about the nature of the specific intermolecular interaction and composition in materials. It was found that the absorption peak emerged at a wavenumber of 1655 [cm.sup.-1] (amide I), assigned to the random coil conformation of the SF in the SF/PU (1/9) fibrous membranes. The infra-red frequencies of silk I were very similar to those of the random coil (29). As the SF content increased to 5/5 (SF/PU), the absorption peak shifted to the wavenumber of 1618 [cm.sup.-1] (amide II), corresponding to the [beta]-sheet conformation of SF. The change from the random coil conformation to the [beta]-sheet conformation for the SF indicated that the fraction of the SF powder in the surface of the SF/PU fibrous membranes increased, and the SF powders were not completely enwrapped in PU fiber, as shown in Fig. 2h. The formation of the [beta]-sheet conformation of SF was relative to the electrospining solution concentration and the electrospining process. As previous mentioned, with the increase in the SF content, the viscosity of the blend solution increased. There were sufficient molecular chain entanglements in the solution to overcome the shear stress in electrospinning process to form fibers. The SF stood the increasingly high-speed shear stress, due to that the SF powder was distributed in the molecular chain entanglements network, accompanying the rapid solidification of PU. So, the [beta]-sheet conformation SF was exposed in the surface of the fibers.

With the increase in the SF content, the band near 3325 [cm.sup.-1] assigned to the stretching vibration of OH groups was shifted to 3329, 3327, 3322, and 3282 [cm.sup.-1] in the SF/PU fibrous membranes, and the bands near 2943 [cm.sup.-1] assigned to the asymmetric vibration of [CH.sub.2] were shifted 2938, 2938, 2939, and 2942 [cm.sup.-1]. FTIR is a useful tool for understanding the physical or chemical environment of a group in a molecule. The spectral shifts of small magnitude (<10 [cm.sup.-1]) gave the information about the nature of the specific intermolecular interaction in materials (30). The change in the intensity and position of the group FTIR peaks corresponded to the change in the fiber morphology from the beaded fibers to the fibers with even diameter. The environment of the molecules and their aggregates in the medium can be changed under the electrics field (31). The role of electrospinning is mainly inducing disentanglement and parallel packing, to facilitate interchain registration.

As the electrospining solution had a low viscosity, the jet broke into droplets and the electrospinning turned into the electrospraying. The molecule chains of PU were not stretched. PU macromolecules were only in random-coil conformation. With the increase in the SF content, the viscosity of the blend solution increased. The jet were stretched significantly to facilitate interchain registration, namely the molecular orientation of PU along the fiber axis took place. Then, the nature of the specific intermolecular interaction had changed. So, the intensity and position of the group FTIR peaks had changed.

X-Ray Diffraction

XRD analysis is an excellent method to investigate crystalline materials. Through comparison with the XRD patterns, it was possible to investigate and identify the relative significance of the SF content in the solution on the crystal structure and conformation of SF/PU blend fibers. The XRD patterns of SF/PU (1/9, 3/7, and 5/5) blend fibrous membranes, PU and SF/PU (3/7) blend casting membrane are shown in Fig. 4. A broad diffraction hump at near 20[degrees] was observed for all samples, but the area of the diffraction peaks was different. It could be observed that the area of the diffraction peak of the SF/PU fibrous mem-branes increased with the increase in the SF content, thus the crystallinity degree of PU in blend fibrous membranes increased with the increase in the SF content. The SF/PU (3/7) blend casting membrane had a lower diffraction peak area than the other SF/PU blend fibrous membranes. The increase in the diffraction peak area meant the higher crystallinity degree and the higher molecular orientation of PU.

The change in the crystallinity degree of PU could be explained that the presence of the SF promoted the formation of more ordered structure for PU. The hydrogen bond interaction between SF and the hard segment of PU, and the extended chain conformation of the [beta]-sheet structure of SF were contributed to the formation of the crystalline form for PU in the blend fibrous membranes (22). On the other hand, the change in degree of crystallinity and molecular orientation of PU in fibrous membranes ascribed the different spinning process of blend fibers, accompanying the different fiber morphology from the beaded fiber to the fiber with even diameter. As aforementioned, the viscosity of the electrospinning solution was different due to the different mass ratio of SF powder to PU. The effect of the same electric field on the stretch and orientation of PU molecular chain were different. The beaded fiber corresponded to low crystallinity degree and molecular orientation of PU, and the fiber with even diameter corresponded to high crystallinity degree and molecular orientation of PU. Lee et al. and Reneker et al. (32), (33) reported that the crystal structure was developed in PVC and PCL nanofibers through electrospinning process as well as molecular orientation along the fiber axis. Peng et al. (31) discovered that a high voltage could influence the crystals of proteins. Similarly, the crystallinity and molecular orientation of the fibers affected the mechanical properties of the electrospun fibrous membranes. It was worth noting that there was a diffraction peak at 29.2[degrees] and 29.7[degrees] in the XRD diffraction curves of the SF/PU fibrous membranes with blend ratio 3/7 and 5/5, respectively. This corresponded to a [beta]-sheet conformation of SF powder (22). The [3-sheet conformation of SF powder was relative to the stimulation of the electric field and the rapid solidification of PU. This result was in good agreement with that from the FTIR data.

[FIGURE 4 OMITTED]

Water Contact Angle

The surface wettability is an important property of biomaterials which influences the attachment, proliferation, migration, and viability of many different cells (34-36). To clarify the effect of the SF content on surface properties of fibrous membrane, water contact angles were measured and shown in Fig. 5. The contact angles of the SF/PU blend fibrous membranes with blend mass ratio of 0/10, 1/9, 3/7, and 5/5 were 99[degrees], 107[degrees], 129[degrees], and 128[degrees], respectively. The contact angles of SF/PU blend fibrous membranes could be regulated by the SF content in it. There were two factors that affected the hydrophilicity property of the blend fibrous membrane, namely, the surface functional groups and surface microstructure. When the mass ratio of SF to PU increased, the microstructure of the blend fibrous membranes transformed from the molten structures to the fibrous membrane with a little bead and to the fibrous membrane composed of the even diameter fibers (Fig. 2), namely the surface roughness of the blend fibrous membranes increased. The roughness surface prevented water from contacting the solid surface by trapping air bubbles at the water--solid interface. The increase in the contact angles of SF/PU blend fibrous membranes was mainly a result of the increase air trapped in the rough hierarchical micro/nanostructures of membranes (37). The water contact angle of air is commonly regarded to be 180[degrees]. So, the contact angles of the blend fibrous membranes increased gradually. However, the fibrous membrane with the mass ratio of 5/5 had a smaller contact angles than that with the mass ratio of 3/7. It can be explained that the fibrous membrane with the mass ratio of 5/5 had a more fraction of the SF powder exposed in the surface of the SF/PU fibrous membranes than that with the mass ratio of 3/7, which was forenamed. The presence of SF on surface of blend fiber could improve the hydrophilicity (38). In general, hydrophilic surfaces displayed better affinity for cells but lower absorption for proteins than hydrophobic surfaces (36). So, hydrophilic/hydrophobic balance of the substance surface is important for the protein absorption and the further cell attachment activity.

[FIGURE 5 OMITTED]

TG Curves

Figure 6 displays TG curves of different SF/PU fibrous membranes. The weight loss at the first step corresponded to the evaporation of water. The second step meant the thermal decomposition and disintegration of the crystal region in the samples, which theoretical turning temperature point was determined by the method used in the literature (26). The first step weight loss and the initial thermal decomposition temperature in TG curves of SF/PU blend films are summarized in Table I. The first step weight loss increased with the increase in SF content in blend fibrous membranes. This was attributed to the high hydroscopic property of SF. The initial temperature of the thermal decomposition was used to compare the thermal stability of the samples. From Table 1, this could be concluded that the thermal stability of SF/PU fibrous membranes decreased with the increase in SF content in it. This perhaps was due to the relatively low thermal decomposition temperature of the SF (260-280[degrees]C). The gradients of weight loss increased with the decrease in SF content in it, and the curves intersectted at 380[degrees]C, which could be ascribed to the high decomposition rate of SF in the range of 270-350[degrees]C and a low decomposition rate above 350[degrees]C. The SF/PU fibrous membranes had a lower weight loss gradient above 470[degrees]C, corresponding to the slow weight loss process of SF. The remnant weight increased with the increase in SF content, due to the high remnant weight of the SF, as shown in Fig. 6.

[FIGURE 6 OMITTED]

DMT Behavior

DMT analysis is a valuable technique to investigate the mechanical behavior of materials subjected to cyclic stress and to obtain information about the relaxation mechanisms. The dynamics and microstructure of the material is correlated with the microstructure of the material. The dependence of the storage modulus (E') and the loss angle tangent (tan 6) on temperature for SF/PU blend fibrous membranes are shown in Fig. 7. E' of the SF/PU blend fibrous membranes decreased very sharply in the temperature region from--100 to--50[degrees]C. The glass transition temperature of the SF/PU blend fibrous membranes with blend mass ratio of 0/10, 1/9, 3/7, and 5/5 was--46.5[degrees]C,--55.5[degrees]C,--47.2[degrees]C, and--52.8[degrees]C, respectively. In the second half of the curves and above glass transition temperature, E of the SF/PU blend fibrous membranes slowly decreased and was in the order of SF/PU1 > SF/ PU3 > SF/PU5 > PU (Fig. 7a), which was attributed to the different SF/PU fibrous membranes with the different nonwoven structure, as shown in Fig. 2. When the mass ratio of SF to PU was 1/9, there were lots of the beads in the fibrous membrane. The fibers were bonded together through the beads. When the mass ratio mass ratio of SF to PU was 3/7, there was the weak binding between the fibers due to the decrease of the beads. There was very weak binding between the fibers due to the absence of the beads in the SF/PU (5/5) fibrous membrane. The pure PU fibrous membrane was almost a molten structure.
TABLE 1. First step weight loss and initial decomposition
temperature in thermogravimetric curves of SF/PU blend fibrous
membranes.

Sample  First step     Initial decomposition
        weight lose  temperature ([degrees]C)
           (wt%)

PU              9.5                       275
SF/PU5          3.5                       279
SF/PU3          2.5                       282
SF/PU1          0.5                       298


[FIGURE 7 OMITTED]

The internal consumption factor (tan 6) expresses the relative intensity of the energy storage and loss ability of membranes. In our experience, the affecting factors of tan 6 included the mass ratio of SF to PU and the molecule orientation character of PU. There was a decrease trend of the loss peak with the increase in the SF content in it (Fig. 7b). This perhaps was due to the SF, which had inhibited the crystal of the PU during the electrspinning process. This increased the freedom of the PU macromolecule chains in SF/PU blend fibers, and decreased the tan (5 peak to some degree. In the other hand, with the increase in SF content, the loss angle tangent peak in the blend membranes except the SF/PU 1 shifted to a lower temperature, namely to decrease the glass transition temperature. The glass transition temperature was relative to the phase separation between soft and hard segments of PU and the damping effect between the soft segment molecule chains. When the phase separation between soft and hard segments of PU increased, the glass transition temperature shifted to a lower temperature (22). The molecule chains of random coil configuration had a stronger damping effect than that of extending orientation configu-ration. The glass transition temperature increased with the increase in the damping effect. With the increase in the SF content, there was an increasing extending orientation PU macromolecule chains in SF/PU blend fibers, as shown in Fig. 2. Consequentially, the damping effect between the molecule chains decreased increasingly, and the phase separation between soft and hard segments of PU increased increasingly. Meanwhile, the interaction between PU molecule chains decreased increasingly also due to the SF distributed evenly in the blend fibers. This led to the decrease of glass transition temperature. The SF/PU1 had the lowest glass transition temperature among the SF/PU blend fibrous membranes. This was relative to the membrane structure composed of the fibers with the beads, as shown in Fig. 2c. The fibers between beads were extended well to obtain a high extending orientation PU macromolecule chains. The fibers was the major component of the membrane, thus The SF/PU 1 had the lowest glass transition temperature.

Mechanical Properties

The mechanical properties of the blend fibrous membranes could provide important information about their internal structure (39). The mechanical behavior of the blend membranes with various SF content was investigated by tensile testing at room temperature. The corresponding tensile properties, such as stress at peak, Young's modulus, and strain at peak, are summarized in Table 2. It was noteworthy that the effect of SF content on mechanical properties of the blend fibrous membranes was different from that of the casting membranes (22). There was an obvious decrease in the stress at peak and Young's modulus with the increase in the SF content in it. The blend fibrous membrane with the mass ratio of SF to PU (5/5) had a stress at peak and Young's modulus equal to one-half of that with the mass ratio of SF to PU (3/7). When the mass ratio of SF to PU increased from 3/ 7 to 5/5, there was almost no change in the stress at peak, Young's modulus and strain at peak. The stress at peak and Young's modulus of the blend fibrous membrane with the mass ratio of SF to PU (1/9) were about two times and eight times higher than that of the pure PU membrane, respectively. However, strain at peak was about 26% that of the pure PU membrane. The strain at peak of the blend fibrous membrane with the increase in the SF content was in order of 146.58%, 209.62%, and 185.26%.
TABLE 2. Mechanical properties of SF/PU blend librous membranes
obtained from tensile tests.

Sample  Stress at peak  Young's Modulus  Strain At peak
            (MPa)            (MPa)             (%)

SF/PU5   4.57 [+ or -]    4.85 [+ or -]  146.58 [+ or -]
                  0.57             0.51              9.7
SF/PU3   9.60 [+ or -]    8.05 [+ or -]  209.62 [+ or -]
                  0.83             0.67             8.01
SF/PU1  10.46 [+ or -]    8.17 [+ or -]  185.26 [+ or -]
                  0.91             0.55             3.21
PU      22.82 [+ or -]   67.28 [+ or -]   48.82 [+ or -]
                  0.41             5.74             5.65


The change in the mechanical properties of the blend fibrous membranes could not be explained by the interface structure of SF and PU in the casting membranes (25). The mechanical properties of fibrous membrane were a reaction of the internal fibrous structure of the membranes. The electrospinning solution with the different mass ratio of SF to PU had a different electrospinning property, to obtain the different structure fibrous membranes, as shown in Fig. 2. With the increase in the SF content, the beaded fibers decreased, to decrease the point-bonded junctions between the fibers. So, there were less binding points in the fibrous membranes to hinder the slip of PU fibers by providing local physical or frictional entanglements to decrease the Young's modulus and stress at peak. At the same time, for the slip of blend fibers, the blend fibrous membrane had a higher strain at peak than the pure PU fibrous membrane. The strain at peak of pure PU fibrous membrane was only 48.82%. This probably was due to the molten structures composed of the beaded fibers. Some gaps could be observed at the interface between the bead agglomerations. These gaps caused the loss in tensile properties of the blend membrane.

CONCLUSIONS

The blend fibrous membranes with the different mass ratio of SF to PU were fabricated by electrospinning. The SF was uniformly distributed in the blend fibers without apparent agglomeration. The effects of the mass ratio of SF to PU on the properties of blend fibrous membranes were investigated carefully. The mass ratio of SF to PU played an important role in influencing the morphology of the blend fiber, and the optimum mass ratio was 5:5. FTIR spectra showed that the fraction of SF in the surface of the SF/PU blend fibers increased with the increase in the SF content in it, and the molecular orientation of PU along the fiber axis also increased. XRD analysis indicated that the high crystallinity degree and molecular orientation of PU were obtained in blend fibers with the high SF content, and there was a [beta]-sheet conformation of SF in surface of blend fibers. Meanwhile, the SF content regulated the hydrophilicity property of the blend membrane. TO-tested results indicated that the thermal stability of the blend membrane declined with the increase in SF content. Additionally, the dynamic storage modulus of SF/PU blend membranes decreased and the phase separation between soft and hard segments of PU increased with the increase in the mass ratio of SF to PU in it. Similarly, the stress at peak and Young's modulus of the blend fibrous membrane decreased gradually; the strain at peak first increased and then decreased.

ACKNOWLEDGMENTS

The authors are grateful to Analytical and Testing Center of Huazhong University of Science and Technology.

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Correspondence to: Weilin Xu; e-mail: weilin-xu@hotmail.com Contract grant sponsor: Nature Science Foundation of Hubei province; contract grant number: 2009CDB170; contract grant sponsor: Education Commission of Hubei Province; contract grant number: Q20101612; contract grant sponsor: Science and Technology Bureau of Wuhan City; contract grant number: 201161038340-03; contract grant sponsor: The 973 project of China; contract grant number: 2012CB722701.

Zikui Bai, (1) Weilin Xu, (1) Jie Xu, (1) Hongjun Yang, (1) Shili Xiao, (1) Xin Liu, (1) Guijie Liang, (1) Libo Chen (2)

(1) Key Lab for Green Processing and Functionalization of New Textile Materials, Ministry of Education, Wuhan Textile University, Wuhan 430073, People's Republic of China

(2) Department of Surgery, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, People's Republic of China

DOI 10.1002/pen.23150
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Author:Bai, Zikui; Xu, Weilin; Xu, Jie; Yang, Hongjun; Xiao, Shili; Liu, Xin; Liang, Guijie; Chen, Libo
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Sep 1, 2012
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