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摘要： 文章制备了具有海绵结构的聚氨酯（PU）/聚乙烯醇缩丁醛（PVB）三维（3D）纳米纤维多孔结构。通过SEM、XRD和FTIR对其形貌和结构进行表征，并通过拉伸和压缩实验对其力学性能进行研究。SEM结果显示，不同PU和PVB质量比的三维结构在纤维形态和纤维间交联方面存在差异。XRD和FTIR结果证明了3D纳米纤维海绵中PU和PVB聚合物分子的存在，并明确了PU分子和PVB分子间存在交联。实验表明，当PU和PVB质量比为7︰3时，3D纳米纤维海绵中纤维形貌好，结构稳定，断裂强力为2.2 MPa，断裂伸长为175.5%。该多孔纤维轻质海绵具有优异的压缩回复性能，可应用于不同的领域。
关键词：
聚氨酯;聚乙烯醇缩丁醛;三维结构;多孔海绵;力学性能
中图分类号： TS102.5
文献标志码： A
文章编号： 1001-7003（2021）10-0036-08
引用页码： 101107
DOI： 10.3969/j.issn.1001-7003.2021.10.007（篇序）
Preparation of 3D polyurethane/polyvinylbutyral nanofiber sponge
CHADYAGONDO Tavonga Trevor， CHEN Qiang， SHI Jingya， LI Ni
（College of Textile Science and Engineering（International Institute of Silk）， Zhejiang Sci-Tech University， Hangzhou 310018， China）
Abstract：
In this study， a polyurethane（PU） and polyvinyl butyral（PVB） three-dimensional（3D） nanofiber porous sponge structure was prepared. Its morphology and structure were characterized by SEM， XRD and FTIR， and the mechanical properties were investigated by tensile and compressing tests. SEM results revealed that for the three-dimensional structure of different PU︰PVB ratios， there existed differences in fibre morphology and the interfibrous crosslinking. As proved by XRD and FTIR results， PU and PVB polymeric molecules existed in the 3D nanofiber sponge， and it was clarified that there exited crosslinking between PU molecules and PVB molecules. Experiments showed that when the mass ratio of PU and PVB was 7︰3， the fiber morphology of the 3D nanofiber sponge was good， the structure was stable， a breaking strength of 2.2 MPa， and a breaking elongation of 175.5%. The porous fiber lightweight sponge has excellent compression and resilience performance， and it can be applied in different fields.
Key words：
polyurethane; polyvinyl butyral; three-dimensional structure; porous sponge; mechanical property
收稿日期： 2021-04-06;
修回日期： 2021-09-14
基金項目：
作者简介： Tavonga Trevor Chadyagondo（1990），男，硕士研究生，研究方向为3D纳米纤维多孔结构。通信作者：李妮，副教授，[email protected]。
Electrospinning has attracted extensive attention due to its production of nanofibrous mats[1] with controllable and feasible mechanical[2] or biological properties[3-4] and its ability to change the combination of polymers to fabricate specific fibers[5]. Meanwhile， in view of some advantages of electrospun nanofibrous mats， such as large surface-to-volume ratios[6]， the diversity of available materials， and the ease of surface modifications[7-8]， they have been used as the substrates for various purposes[4-6]. A sponge-like three-dimensional network structure， characterized by reversible compressibility[9]， high porosity[9-10]， low density， flexibility and other attractive properties， shows promising application prospect as filters[9-11]， thermal insulation materials and drug delivery[3，4，9，12-13].Several papers have reported the practical approaches for producing lightweight sponge polymeric materials[14-16]. Ding Bin reported the construction of mechanically robustfibrous networks from Polyacrylonitrile（PAN） and SiO2unbonded aerogels by heating to form bonded 3Dfibrous networks， which also disclosed problems of heat shrinkage and the relaxation of orientedmolecular chains[17]. Other information explained the shortcomings of the brittleness of traditionalcolloidal aerogels[18]. Conclusions were drawn based on inherentfragility of aerogel monoliths， limiting their applications in which they were not affected by any load[19]. The mesoporoussilica structure of an aerogel was cross-linked by the reaction of di-isocyanates withsilanols on the surfaceofwet gels before supercriticaldrying[20]， thereby developing a silica sponge material.
Polyurethane（PU） is a thermoplastic elastomer withexcellent performance of wear resistance，flexibility， hydrolyticstability， stretchability， and workability at low cost[21-24]. In recentyears， PU has been studied in some literaturesin relation to its production for electrospun waterproof andbreathable functions[8，22，25-26]. Polyvinyl butyral（PVB） has been often adopted as the crosslinker/binder because of itswell-knowngood adhesion， film-forming and excellent flexibility， as well as outstanding UV resistance[27-28].
In this paper， electrospinning and freeze-drying of dispersions of short electrospun fibers were employed to fabricate ultra-light PU/PVB sponges. The morphology and structure were examined， the tensile and compression properties of PU/PVB sponges were evaluated systematically in this study， aiming to prepare a kind of 3D nanofiber sponge with stable structure.
1 實 验
1 Experiment
1.1 材 料
1.1 Materials
PU（Mw=120，000 g/mol） and PVB（Mw=40 000-70 000 g/mol） were purchased from Shanghai Macklin Biochemical Co.， Ltd and used without further purification. Dimethylformamide（DMF） and ethanol were bought from Hangzhou Gaojing Fine Chemical Industry Co.， Ltd.
1.2 PU/PVB纳米纤维膜
1.2 Electrospinning of PU/PVB nanofiber mat
Several PU/PVB blends with mass ratiosranging from 8︰2， 7︰3， 6︰4 were prepared and weighed on a Mettler Toledo AL204 Balance. Then homogeneous 12% PU/PVB solution was prepared by dissolving 2.4 g of PU and PVB in DMF and stirredconstantly with amagnetic stirrer（IKA C-MAG HS7） at standard room temperature for 24 hours. In order to quantify the changes in viscosity with the alternating ratios of PU︰PVB， a rotational viscometer（NDJ-9S） was used to couple with a disc spindle to measure the dynamic viscosity. During electrospinning， the solution flow rate from the syringe pumps（KDS100） was 1 mL/hr. The applied voltage was 15 kV. Electrospinning was performed on a rotating aluminium collector plate for 10 hrs using dual needles. The distance from the tip of the needle to the rotating collector was 15 cm. The collected electrospun matwith the aluminium foil were put into a vacuum dryer at standard room temperature for at least 30 hours. 1.3 三维PU/PVB纳米纤维海绵
1.3 Fabrication of 3D PU/PVB nanofiber sponge
The prepared PU/PVB nanofiber mats were cut into short pieces and deposited into ethanol， and then homogenised in a high speed FJ200-SH dispersing homogeniser at a rotation speed of about 15 000 r/min for 30 min. Different short fibre dispersions were prepared by controlling the weight ratio of fibre nonwoven in the dispersion and the volume of the dispersion solvent. Then the fibre dispersion was centrifuged（Anke DZ-267-32）， put into a mould and frozen at 0 ℃， and then was dried using a freeze-drier（Labonco RS232） for 48 h under a vacuum of 0.35 mbar. These steps were shown in Fig.1. The mass of the sponge was measured by a precise balance， and the volume of the sponge was calculated from the relative dimension of the fibrous 3D sponge. The density of the rectangular sponge obtained was determined by dividing the mass with the volume.
1.4 表 征
1.4 Characterization
To investigate the morphology and structure of PU/PVB nanofiber mat and 3D PU/PVB nanofiber sponge， the membranes were observed using a Scanning Electron Microscope（SEM， vltra55， Germany）. The microstructure of the mat and nano-sponge were evaluated using X-ray diffraction（XRD， ARL X’TRA， SWISS） and Fourier Transform Infrared spectroscopy（FTIR， AVATAR5700， USA）. To determine the mechanical properties of the 3D sponge， universaltensile and compression tests were performed in the paper. The tensile tests were conducted by cutting out a 4 cm by 1.5 cm strips， which were then clamped onto the test apparatus. A force was applied to the
specimen by separating the testing machine crossheads. Data from the test were used to determine tensile strength and elongation of elasticity. The compression of 3D nano-sponge was investigated by a self-assembly experiment.
2 結果与分析
2 Results and analysis
2.1 PU/PVB纳米纤维膜
2.1 PU/PVB nanofibre mats
The general morphologies of PU/PVB nanofiber mats with mass ratios of PU/PVB blends ranging from 8︰2， 7︰3， 6︰4 were shown in Fig.2.
The morphology of the fibre mats did not change much when varying the PVB/PU massratio 8︰2 to 7︰3. However， when the ratio reached 6︰4， the nanofiber cluttering style changed significantly， as shown in Fig.2（c）. At this time， nanofibers were inhomogeneous lengthwise and they adhered together. Studies have shown that among all parameters， the viscosity of solution had a significant impact on fibre formation during spinning[29-30]. Increasing the PVB content while keeping the spinning solution at 12% would affect the spinnability of the solution because the viscosity of the solution dropped significantly（Tab.1）. At this time， the fibre diameter reduced with the increase of PVB， which could be explained by the reduction in viscosity of the solutions[30]. In general， the reduction of viscosity results in thinner fibres though it is very difficult to then eliminate the possibility of beading when the ratio of PU to PVB is 6︰4. Meanwhile， it was found that the obtained membranes changed from soft handle to a moderately rough at higher proportion of PVB， and the 40% PVB had the toughest handle.
2.2 納米纤维海绵
2.2 Fibrous nano-sponge
After building a PVB/PU short nanofiber dispersion in ethanol and setting it into a fibrous sponge via freeze drying， a self-assembled typical low weight nano-sponge was formed with significant pores， as illustrated by the images in Fig.3 and Fig.4， indicating the porosity of the sponges. PVB played a significant role in connecting short fibres with rarely short pieces of undispersed fibrous mats[31]. The optical photographand electron micrographsclearly showed points where bonds were formed between fibres and mats， forming a 3D nano-sponge complex structure. The self-assembling of the fibres was proved by the random distribution of the pores of different sizes across the nano-sponge structure. During the preparation of the 3D sponge， it was found out that when PVB content was less than 20%（PU︰PVB>8︰2）， the 3D sponge with stable structure could not be formed， and short nanofibers or pieces were still separated with each other after freeze drying. At this time， it was concluded that the loose structure was due to the absence of PVB and the fact that short nanofibers or pieces could not be bound together. However， when PVB content was higher than 40%（PU︰PVB<6︰4）， it was found that the nanofibers adhered together more significantly， while the characteristics of sponge porosity were weakened notably.
A significant property elaborated by the presence of PVB was that the PU/PVB sponge could resist the disintegration from manual handle， and can in fact stretch without breaking and show impressive recovery rates even compression strength. Due to the uniform fibre diameter and desirable pore structure， PVB/PUnanofibre matswith a PU and PVB ratio of 7︰3 were used in the following XRD and FTIR discussions.
During the preparation of 3D nano sponge， it was also found that the use of ethanol enhanced the bonding abilityof the PVB to the PU fibre structures， and yet the PU maintained the structural stability of the sponge， also highly promoted the elastic and compressional properties of the 3D nano-sponge. The 3D fibre showed a clear and strong tendency to bond across different sections due to PVB "crystallisation" during freeze drying. In this case， the the bond strength was strong enough to ignore heating， which nevertheless could be used to further improve the mechanical strength of PVBs at the glass transition temperature. 2.3 XRD分析
2.3 XRD analysis
X-ray diffraction（XRD） characterization provides information about structural parameters， such as crystallinity， strain， and crystal defects， as well as many other parameters[32]. Fig.5 shows the curving graph of PU and PU/PVB fibrous material and the different PVB to PU peaks. The graph had a sharp peak， indicating a diffraction at an angle approximately 2θ=20°.
The peak at 20° corresponded to the regular interplanar spacing corresponding to aromatic rings hard segment listed in the International Centre for Diffraction Data（ICDD）. There was a shift in the position of the peak at 20°， indicating a decrease in the chain spacing. These effects indicated an increase in the crosslink that promoted the reduction of the d-spacing and the shift of the peak at 20°[33]. These peak shifts suggested there were some cross-linking patterns induced by the presence of PVB because of the general aromatic rings in PU diffract around 19°.
PVB is completely amorphous in nature[34]， which has been proved by the X-ray diffraction pattern of PU/PVB obtained and shown in Fig.5 above. This X-ray diffraction patternexhibited the characteristics of amorphous PVB. In the XRD pattern of pure PU， there was a crystalline peak around 45°， but more peaks appeared as broadened peaks at approximately 43°， indicating the presence of highly amorphous clay structure in PVB.
2.4 FTIR結果
2.4 FTIR results
Fig.6 showed a full representation of the FTIR spectrum for PU， PVB and PU/PVB fibrous materials used in this research paper. The spectrum showed that the absorption bands and stretching vibrations were consistent with those of PU and PVB functional groups available in literatures[33-36]. These peaks were described and explained as follows.
At the absorption peak between 800 cm-1 to 1 500 cm-1， there was a peak for stretching vibration of C—O—C around 1 138 cm-1 in PVB（Fig.7）， which also had peaks at 1 390 cm-1 and 1 635 cm-1， indicating the band stretching for O—H and CO respectively. Another band could be observed around 1 120 cm-1， while another was seen near 1 231 cm-1， indicating that the presence of C—N and C—H bands was consistent with the PU spectrum. Furthermore， there was an absorption band at 1 721 cm-1， representing the CO stretch in PU. The curves in Fig.6 strongly showed the presence of asymmetrical and symmetrical stretching vibration of C—H bonds at around 2 866 cm-1， 2 964 cm-1 and 2 983 cm-1. These were important absorption bands because their changing intensity which explained the crosslinking physical bonding of the overall 3D structure. Finally， there were two peaks as shown in Fig.8 at between 3 300 cm-1 to 3 450 cm-1， showing the stretching vibrations of N—H and O—H functional groups in PVB and PU. The peak intensities varied with the change in content of PU and PVB during the construction of the 3D edifice due to the presence of the polarity of functional groups at such band stretches in the PU， PVB and or PU/PVB polymers[33-36]. A notable manifestation of peak differences on the curves signaling interaction between PVB and PU could be seen in Fig.8， which differed in intensity ranging from 2 866 cm-1 to 2 983 cm-1. Despite the presence of the peaks at this range on the PVB spectrum， there was an interchanged peak size for asymmetric and symmetric C—H vibrations， which was also a prominent difference. PU/PVB membrane showed longer peaks while PU/PVB 3D model and PU membrane manifested shorter ones respectively. The peaks for PU/PVB（both membrane and 3D model） were analyzed， and the spectrum showed that the concentration of the C—H stretching vibration increased significantly， as shown by the increase in spectra intensity between 2 800 cm-1-3 000 cm-1 when compared to the PU membrane. The increase in the intensity proved that both PU and PVB existed in the fibrous membrane and 3D sponge. PU was the backbone of the structure， while PVB mainly functioned as a crosslinking agent.
The transformation of the PU/PVB membrane into a 3D structure with ethanol as a dispersing agent reduced the amount of PVB within the fibrous matrix， and yet controlled the dispersion. The subsequent washing and freeze dry method ensured that the dissolved PVB became a crosslinking agent when dried. The spectrum for PU/PVB 3D sponge showed that the peaks in the range of 2 800 cm-1-3 000 cm-1 were still in a range comparable to those of the PU/PVB membrane， showing noteworthy difference in peak intensity. There was a decrease in intensity when comparing fibrous 3D sponge to the fibrous membrane， yet still above the peak intensity of PU only membrane， suggesting that some PVB was lost along with the ethanol solvent during the washing process， but the remaining PVB was essential for the new strongly crosslinked fibre sponge. In contrast， the curves at 3 300 cm-1 to 3 450 cm-1 changed their shape by broadening or rather increasing the area under the curve in this case. This attribute was caused by the formation of strong hydrogen bonds between N—H and O—H molecules present in both PU and PVB polymers. The change in shape also explained a weakened polarity there in came due to the interactions between functional groups.
2.5 拉伸性能
2.5 Stretching performance
In a bid to understand how the sponge would react to different external forces in the form of stress and strain， PVB/PU nano-sponges with different mass ratios of PVB and PU were subjected to these forces on a tensiometer. The samples resembled strength as would be expected from their nature just as reported in other papers for aerogels and pure PVB and pure PU[17]. The main difference was that the depiction of strengths altered due to the presence of different mass ratios of PVB and PU used for sponge moulding. The sponge with a PU/PVB ratio of 7︰3 exhibited the highest yield point to stress， as shown in Tab.2. A measurement of over 2 MPa for this model was recorded as it showed different deformations along its length. It exhibited a significant break above 100% strain， which would be consistent with the reported resistance of either PU or PVB electrospun membrane[11，23，27]. However， the breaking points of the models was less than the fibre mats[7]， which was attributed to the presence of many randomly entangled short fibres in the sponge structure. The intensified test for 40% PVB model showedthat the plastic deformation indentations were less significant， while the yield stress was lower， which was attributed to the decreased PU and strengthened bonding between fibres.
2.6 壓缩实验
2.6 Compression test
A manual compression recovery test was performed on the 3D fibrous sponges to show their recovery capabilities after being compressed by a force（Fig.9）. The sample in Fig.9 had a weight of 0.076 5 g and an approximated volume of above 0.5 cm-3. Fig.9 showed that as soon as the fore was removed from the compressed sponge， it immediately recovered to original height. This phenomenon of instantaneous recovery to approximately the initial length could be observed even after repeated compressions. compared to other organic and inorganic building blocks for 3D sponge materials， the use of electrospun nanofibrous materials played a major role because of a better distribution of stress and strain on nanosized fibres.
3 结 论
3 Conclusions
The combination of polymeric compound for the fabrication of a stable and useful structure by taking advantage of their different properties is an extensive research field among many polymers. This research work provided a versatile 3D nano-sponge， which was prepared with the ratio of PVB to PU of 7︰3， presenting uniform nanofiber and desirable morphology. Meanwhile， the nano-sponge exhibited a high tensile strength of 2.2 MPa and notable extensibility with 175.5% elongation. It also could recover instantly after compression.
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关键词：
聚氨酯;聚乙烯醇缩丁醛;三维结构;多孔海绵;力学性能
中图分类号： TS102.5
文献标志码： A
文章编号： 1001-7003（2021）10-0036-08
引用页码： 101107
DOI： 10.3969/j.issn.1001-7003.2021.10.007（篇序）
Preparation of 3D polyurethane/polyvinylbutyral nanofiber sponge
CHADYAGONDO Tavonga Trevor， CHEN Qiang， SHI Jingya， LI Ni
（College of Textile Science and Engineering（International Institute of Silk）， Zhejiang Sci-Tech University， Hangzhou 310018， China）
Abstract：
In this study， a polyurethane（PU） and polyvinyl butyral（PVB） three-dimensional（3D） nanofiber porous sponge structure was prepared. Its morphology and structure were characterized by SEM， XRD and FTIR， and the mechanical properties were investigated by tensile and compressing tests. SEM results revealed that for the three-dimensional structure of different PU︰PVB ratios， there existed differences in fibre morphology and the interfibrous crosslinking. As proved by XRD and FTIR results， PU and PVB polymeric molecules existed in the 3D nanofiber sponge， and it was clarified that there exited crosslinking between PU molecules and PVB molecules. Experiments showed that when the mass ratio of PU and PVB was 7︰3， the fiber morphology of the 3D nanofiber sponge was good， the structure was stable， a breaking strength of 2.2 MPa， and a breaking elongation of 175.5%. The porous fiber lightweight sponge has excellent compression and resilience performance， and it can be applied in different fields.
Key words：
polyurethane; polyvinyl butyral; three-dimensional structure; porous sponge; mechanical property
收稿日期： 2021-04-06;
修回日期： 2021-09-14
基金項目：
作者简介： Tavonga Trevor Chadyagondo（1990），男，硕士研究生，研究方向为3D纳米纤维多孔结构。通信作者：李妮，副教授，[email protected]。
Electrospinning has attracted extensive attention due to its production of nanofibrous mats[1] with controllable and feasible mechanical[2] or biological properties[3-4] and its ability to change the combination of polymers to fabricate specific fibers[5]. Meanwhile， in view of some advantages of electrospun nanofibrous mats， such as large surface-to-volume ratios[6]， the diversity of available materials， and the ease of surface modifications[7-8]， they have been used as the substrates for various purposes[4-6]. A sponge-like three-dimensional network structure， characterized by reversible compressibility[9]， high porosity[9-10]， low density， flexibility and other attractive properties， shows promising application prospect as filters[9-11]， thermal insulation materials and drug delivery[3，4，9，12-13].Several papers have reported the practical approaches for producing lightweight sponge polymeric materials[14-16]. Ding Bin reported the construction of mechanically robustfibrous networks from Polyacrylonitrile（PAN） and SiO2unbonded aerogels by heating to form bonded 3Dfibrous networks， which also disclosed problems of heat shrinkage and the relaxation of orientedmolecular chains[17]. Other information explained the shortcomings of the brittleness of traditionalcolloidal aerogels[18]. Conclusions were drawn based on inherentfragility of aerogel monoliths， limiting their applications in which they were not affected by any load[19]. The mesoporoussilica structure of an aerogel was cross-linked by the reaction of di-isocyanates withsilanols on the surfaceofwet gels before supercriticaldrying[20]， thereby developing a silica sponge material.
Polyurethane（PU） is a thermoplastic elastomer withexcellent performance of wear resistance，flexibility， hydrolyticstability， stretchability， and workability at low cost[21-24]. In recentyears， PU has been studied in some literaturesin relation to its production for electrospun waterproof andbreathable functions[8，22，25-26]. Polyvinyl butyral（PVB） has been often adopted as the crosslinker/binder because of itswell-knowngood adhesion， film-forming and excellent flexibility， as well as outstanding UV resistance[27-28].
In this paper， electrospinning and freeze-drying of dispersions of short electrospun fibers were employed to fabricate ultra-light PU/PVB sponges. The morphology and structure were examined， the tensile and compression properties of PU/PVB sponges were evaluated systematically in this study， aiming to prepare a kind of 3D nanofiber sponge with stable structure.
1 實 验
1 Experiment
1.1 材 料
1.1 Materials
PU（Mw=120，000 g/mol） and PVB（Mw=40 000-70 000 g/mol） were purchased from Shanghai Macklin Biochemical Co.， Ltd and used without further purification. Dimethylformamide（DMF） and ethanol were bought from Hangzhou Gaojing Fine Chemical Industry Co.， Ltd.
1.2 PU/PVB纳米纤维膜
1.2 Electrospinning of PU/PVB nanofiber mat
Several PU/PVB blends with mass ratiosranging from 8︰2， 7︰3， 6︰4 were prepared and weighed on a Mettler Toledo AL204 Balance. Then homogeneous 12% PU/PVB solution was prepared by dissolving 2.4 g of PU and PVB in DMF and stirredconstantly with amagnetic stirrer（IKA C-MAG HS7） at standard room temperature for 24 hours. In order to quantify the changes in viscosity with the alternating ratios of PU︰PVB， a rotational viscometer（NDJ-9S） was used to couple with a disc spindle to measure the dynamic viscosity. During electrospinning， the solution flow rate from the syringe pumps（KDS100） was 1 mL/hr. The applied voltage was 15 kV. Electrospinning was performed on a rotating aluminium collector plate for 10 hrs using dual needles. The distance from the tip of the needle to the rotating collector was 15 cm. The collected electrospun matwith the aluminium foil were put into a vacuum dryer at standard room temperature for at least 30 hours. 1.3 三维PU/PVB纳米纤维海绵
1.3 Fabrication of 3D PU/PVB nanofiber sponge
The prepared PU/PVB nanofiber mats were cut into short pieces and deposited into ethanol， and then homogenised in a high speed FJ200-SH dispersing homogeniser at a rotation speed of about 15 000 r/min for 30 min. Different short fibre dispersions were prepared by controlling the weight ratio of fibre nonwoven in the dispersion and the volume of the dispersion solvent. Then the fibre dispersion was centrifuged（Anke DZ-267-32）， put into a mould and frozen at 0 ℃， and then was dried using a freeze-drier（Labonco RS232） for 48 h under a vacuum of 0.35 mbar. These steps were shown in Fig.1. The mass of the sponge was measured by a precise balance， and the volume of the sponge was calculated from the relative dimension of the fibrous 3D sponge. The density of the rectangular sponge obtained was determined by dividing the mass with the volume.
1.4 表 征
1.4 Characterization
To investigate the morphology and structure of PU/PVB nanofiber mat and 3D PU/PVB nanofiber sponge， the membranes were observed using a Scanning Electron Microscope（SEM， vltra55， Germany）. The microstructure of the mat and nano-sponge were evaluated using X-ray diffraction（XRD， ARL X’TRA， SWISS） and Fourier Transform Infrared spectroscopy（FTIR， AVATAR5700， USA）. To determine the mechanical properties of the 3D sponge， universaltensile and compression tests were performed in the paper. The tensile tests were conducted by cutting out a 4 cm by 1.5 cm strips， which were then clamped onto the test apparatus. A force was applied to the
specimen by separating the testing machine crossheads. Data from the test were used to determine tensile strength and elongation of elasticity. The compression of 3D nano-sponge was investigated by a self-assembly experiment.
2 結果与分析
2 Results and analysis
2.1 PU/PVB纳米纤维膜
2.1 PU/PVB nanofibre mats
The general morphologies of PU/PVB nanofiber mats with mass ratios of PU/PVB blends ranging from 8︰2， 7︰3， 6︰4 were shown in Fig.2.
The morphology of the fibre mats did not change much when varying the PVB/PU massratio 8︰2 to 7︰3. However， when the ratio reached 6︰4， the nanofiber cluttering style changed significantly， as shown in Fig.2（c）. At this time， nanofibers were inhomogeneous lengthwise and they adhered together. Studies have shown that among all parameters， the viscosity of solution had a significant impact on fibre formation during spinning[29-30]. Increasing the PVB content while keeping the spinning solution at 12% would affect the spinnability of the solution because the viscosity of the solution dropped significantly（Tab.1）. At this time， the fibre diameter reduced with the increase of PVB， which could be explained by the reduction in viscosity of the solutions[30]. In general， the reduction of viscosity results in thinner fibres though it is very difficult to then eliminate the possibility of beading when the ratio of PU to PVB is 6︰4. Meanwhile， it was found that the obtained membranes changed from soft handle to a moderately rough at higher proportion of PVB， and the 40% PVB had the toughest handle.
2.2 納米纤维海绵
2.2 Fibrous nano-sponge
After building a PVB/PU short nanofiber dispersion in ethanol and setting it into a fibrous sponge via freeze drying， a self-assembled typical low weight nano-sponge was formed with significant pores， as illustrated by the images in Fig.3 and Fig.4， indicating the porosity of the sponges. PVB played a significant role in connecting short fibres with rarely short pieces of undispersed fibrous mats[31]. The optical photographand electron micrographsclearly showed points where bonds were formed between fibres and mats， forming a 3D nano-sponge complex structure. The self-assembling of the fibres was proved by the random distribution of the pores of different sizes across the nano-sponge structure. During the preparation of the 3D sponge， it was found out that when PVB content was less than 20%（PU︰PVB>8︰2）， the 3D sponge with stable structure could not be formed， and short nanofibers or pieces were still separated with each other after freeze drying. At this time， it was concluded that the loose structure was due to the absence of PVB and the fact that short nanofibers or pieces could not be bound together. However， when PVB content was higher than 40%（PU︰PVB<6︰4）， it was found that the nanofibers adhered together more significantly， while the characteristics of sponge porosity were weakened notably.
A significant property elaborated by the presence of PVB was that the PU/PVB sponge could resist the disintegration from manual handle， and can in fact stretch without breaking and show impressive recovery rates even compression strength. Due to the uniform fibre diameter and desirable pore structure， PVB/PUnanofibre matswith a PU and PVB ratio of 7︰3 were used in the following XRD and FTIR discussions.
During the preparation of 3D nano sponge， it was also found that the use of ethanol enhanced the bonding abilityof the PVB to the PU fibre structures， and yet the PU maintained the structural stability of the sponge， also highly promoted the elastic and compressional properties of the 3D nano-sponge. The 3D fibre showed a clear and strong tendency to bond across different sections due to PVB "crystallisation" during freeze drying. In this case， the the bond strength was strong enough to ignore heating， which nevertheless could be used to further improve the mechanical strength of PVBs at the glass transition temperature. 2.3 XRD分析
2.3 XRD analysis
X-ray diffraction（XRD） characterization provides information about structural parameters， such as crystallinity， strain， and crystal defects， as well as many other parameters[32]. Fig.5 shows the curving graph of PU and PU/PVB fibrous material and the different PVB to PU peaks. The graph had a sharp peak， indicating a diffraction at an angle approximately 2θ=20°.
The peak at 20° corresponded to the regular interplanar spacing corresponding to aromatic rings hard segment listed in the International Centre for Diffraction Data（ICDD）. There was a shift in the position of the peak at 20°， indicating a decrease in the chain spacing. These effects indicated an increase in the crosslink that promoted the reduction of the d-spacing and the shift of the peak at 20°[33]. These peak shifts suggested there were some cross-linking patterns induced by the presence of PVB because of the general aromatic rings in PU diffract around 19°.
PVB is completely amorphous in nature[34]， which has been proved by the X-ray diffraction pattern of PU/PVB obtained and shown in Fig.5 above. This X-ray diffraction patternexhibited the characteristics of amorphous PVB. In the XRD pattern of pure PU， there was a crystalline peak around 45°， but more peaks appeared as broadened peaks at approximately 43°， indicating the presence of highly amorphous clay structure in PVB.
2.4 FTIR結果
2.4 FTIR results
Fig.6 showed a full representation of the FTIR spectrum for PU， PVB and PU/PVB fibrous materials used in this research paper. The spectrum showed that the absorption bands and stretching vibrations were consistent with those of PU and PVB functional groups available in literatures[33-36]. These peaks were described and explained as follows.
At the absorption peak between 800 cm-1 to 1 500 cm-1， there was a peak for stretching vibration of C—O—C around 1 138 cm-1 in PVB（Fig.7）， which also had peaks at 1 390 cm-1 and 1 635 cm-1， indicating the band stretching for O—H and CO respectively. Another band could be observed around 1 120 cm-1， while another was seen near 1 231 cm-1， indicating that the presence of C—N and C—H bands was consistent with the PU spectrum. Furthermore， there was an absorption band at 1 721 cm-1， representing the CO stretch in PU. The curves in Fig.6 strongly showed the presence of asymmetrical and symmetrical stretching vibration of C—H bonds at around 2 866 cm-1， 2 964 cm-1 and 2 983 cm-1. These were important absorption bands because their changing intensity which explained the crosslinking physical bonding of the overall 3D structure. Finally， there were two peaks as shown in Fig.8 at between 3 300 cm-1 to 3 450 cm-1， showing the stretching vibrations of N—H and O—H functional groups in PVB and PU. The peak intensities varied with the change in content of PU and PVB during the construction of the 3D edifice due to the presence of the polarity of functional groups at such band stretches in the PU， PVB and or PU/PVB polymers[33-36]. A notable manifestation of peak differences on the curves signaling interaction between PVB and PU could be seen in Fig.8， which differed in intensity ranging from 2 866 cm-1 to 2 983 cm-1. Despite the presence of the peaks at this range on the PVB spectrum， there was an interchanged peak size for asymmetric and symmetric C—H vibrations， which was also a prominent difference. PU/PVB membrane showed longer peaks while PU/PVB 3D model and PU membrane manifested shorter ones respectively. The peaks for PU/PVB（both membrane and 3D model） were analyzed， and the spectrum showed that the concentration of the C—H stretching vibration increased significantly， as shown by the increase in spectra intensity between 2 800 cm-1-3 000 cm-1 when compared to the PU membrane. The increase in the intensity proved that both PU and PVB existed in the fibrous membrane and 3D sponge. PU was the backbone of the structure， while PVB mainly functioned as a crosslinking agent.
The transformation of the PU/PVB membrane into a 3D structure with ethanol as a dispersing agent reduced the amount of PVB within the fibrous matrix， and yet controlled the dispersion. The subsequent washing and freeze dry method ensured that the dissolved PVB became a crosslinking agent when dried. The spectrum for PU/PVB 3D sponge showed that the peaks in the range of 2 800 cm-1-3 000 cm-1 were still in a range comparable to those of the PU/PVB membrane， showing noteworthy difference in peak intensity. There was a decrease in intensity when comparing fibrous 3D sponge to the fibrous membrane， yet still above the peak intensity of PU only membrane， suggesting that some PVB was lost along with the ethanol solvent during the washing process， but the remaining PVB was essential for the new strongly crosslinked fibre sponge. In contrast， the curves at 3 300 cm-1 to 3 450 cm-1 changed their shape by broadening or rather increasing the area under the curve in this case. This attribute was caused by the formation of strong hydrogen bonds between N—H and O—H molecules present in both PU and PVB polymers. The change in shape also explained a weakened polarity there in came due to the interactions between functional groups.
2.5 拉伸性能
2.5 Stretching performance
In a bid to understand how the sponge would react to different external forces in the form of stress and strain， PVB/PU nano-sponges with different mass ratios of PVB and PU were subjected to these forces on a tensiometer. The samples resembled strength as would be expected from their nature just as reported in other papers for aerogels and pure PVB and pure PU[17]. The main difference was that the depiction of strengths altered due to the presence of different mass ratios of PVB and PU used for sponge moulding. The sponge with a PU/PVB ratio of 7︰3 exhibited the highest yield point to stress， as shown in Tab.2. A measurement of over 2 MPa for this model was recorded as it showed different deformations along its length. It exhibited a significant break above 100% strain， which would be consistent with the reported resistance of either PU or PVB electrospun membrane[11，23，27]. However， the breaking points of the models was less than the fibre mats[7]， which was attributed to the presence of many randomly entangled short fibres in the sponge structure. The intensified test for 40% PVB model showedthat the plastic deformation indentations were less significant， while the yield stress was lower， which was attributed to the decreased PU and strengthened bonding between fibres.
2.6 壓缩实验
2.6 Compression test
A manual compression recovery test was performed on the 3D fibrous sponges to show their recovery capabilities after being compressed by a force（Fig.9）. The sample in Fig.9 had a weight of 0.076 5 g and an approximated volume of above 0.5 cm-3. Fig.9 showed that as soon as the fore was removed from the compressed sponge， it immediately recovered to original height. This phenomenon of instantaneous recovery to approximately the initial length could be observed even after repeated compressions. compared to other organic and inorganic building blocks for 3D sponge materials， the use of electrospun nanofibrous materials played a major role because of a better distribution of stress and strain on nanosized fibres.
3 结 论
3 Conclusions
The combination of polymeric compound for the fabrication of a stable and useful structure by taking advantage of their different properties is an extensive research field among many polymers. This research work provided a versatile 3D nano-sponge， which was prepared with the ratio of PVB to PU of 7︰3， presenting uniform nanofiber and desirable morphology. Meanwhile， the nano-sponge exhibited a high tensile strength of 2.2 MPa and notable extensibility with 175.5% elongation. It also could recover instantly after compression.
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