Fabrication of CA/TPU Helical Nanofibers and its Mechanism Analysis
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To explore the mechanism of cellulose acetate (CA)/thermoplastic polyurethane (TPU) on the fabrication of helical nanofibers, a series of experiments were conducted to find the optimum spinning conditions. The experimental results show that the CA (14 wt%, DMAc/acetone, 1/2 volume ratio)/TPU2 (18 wt%, DMAc/acetone, 3/1 volume ratio) system can fabricate helical nanofibers effectively via co-electrospinning. We focus on the interfacial interaction between the polymer components induced by the polymer structure and intrinsic properties, including solution properties, hydrogen bonding, and miscibility behavior of the two solutions. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) are employed to investigate the interfacial interaction between the two phases of the polymer system. The analysis results provide the explanation of the experimental results that the CA/TPU system has the potential for producing helical nanofibers effectively. This study based on the interfacial interaction between polymer components provides an insight into the mechanism of CA/TPU helical fiber formation and introduces a richer choice of materials for the application of helical fibers.
KeywordsFabrication Interfacial interaction Helical nanofibers Mechanism
Differential scanning calorimetry
Poly(ethylene glycol terephthalate)
Poly(ethylene propanediol terephthalate)
TPU dissolved in DMAc/THF, 3/1 volume ratio
TPU dissolved in DMAc/Acetone, 3/1 volume ratio
Fourier transform infrared spectroscopy
Helical structures with broad spectrum of applications in the fields of nanoscale sensors, filtration materials, oil sorbents, solar cells, and so on [1, 2] have attracted extensive attention due to their large surface-area-to-volume ratio and high porosity. The introduction of helical structure into micro/nanofibers can improve fiber resilience and flexibility, and this three-dimensional (3D) structure of the helices can provide the fiber mat larger porosity . Helical structures can be found in many natural systems such as plant tendrils and fine wool, which are regarded as the consequence of different shrinkages (or extensions) and results in forced winding of the structure . Zhang et al.  focused on the formation, structure, and function of the most common chiral nanoarchitectures and explored how the molecules can form hierarchical chiral nanoarchitectures. The mechanism of such an asymmetric deformation should also be used for generating fiber curvature. Co-electrospinning, compared with the other methods, such as chemical vapor deposition , sol–gel , and hydrothermal , is a simple and efficient method for generating composite fibers with kinds of morphologies at the micro- and nanoscales.
With the aid of co-electrospinning technique, several researchers successfully prepared three-dimensional helical nanofibers from two component solutions. Lin et al.  obtained nanoscale biomimetic wool fibers by electrospinning PAN and TPU using a side-by-side co-electrospinning arrangement. Chen et al.  utilized three kinds of co-electrospinning spinnerets to produce nanosprings from PU and Nomex. Using side-by-side electrospinning, Zhang et al.  reported the generation of fibers with curled and helical morphologies from poly(ethylene glycol terephthalate) (HSPET) and poly(ethylene propanediol terephthalate) (PTT). In the above researches, the helical nanofibers obtained are described as three-dimensional and spring-like structures with nano- to microscale helix diameters. The authors attributed the generation of helical fibers to the fact that the two components involved in co-electrospinning display different shrinkages after electrospinning. But there is no detailed analysis and explanation of the formation mechanism of helical fibers. Based on the concept that an elastomeric and a stiff polymer in co-electrospinning may introduce longitudinal stress and result in coiled shapes of the bicomponent fibers, our previous studies  reported the fabrication of helical nanofibers via co-electrospinning. We compared three component systems, Nomex/TPU, PAN/TPU, and PS/TPU, which represent three kinds of polymer composition arrangements in co-electrospinning, and explored the role of polymer chain rigidity, miscibility, and hydrogen bonding on the formation of helical fibers. It has been experimentally verified that Nomex/TPU system can form fine helical fibers. However, Nomex is a non-hydrophilic polymer, which limited its application in biological tissue and adsorption filtration .
Therefore, in this article, based on the previous research, we further discuss the CA/TPU co-electrospinning conditions and analyze its mechanism of helical fiber formation. We prepare the composite helical nanofibers with CA, the rigid component and TPU, and the elastomeric component by co-electrospinning technique. In the experimental part, we conducted single-spinning experiments of CA and TPU, respectively. Different CA solution concentration and solvent systems (volume ratio of DMAc to acetone) were applied to find the processing conditions of fine CA fibers. And in the TPU spinning system, we tried two solvent systems, TPU1 (DMAc/THF, 3/1 volume ratio) and TPU2 (DMAc/acetone, 3/1 volume ratio), which enable lower interfacial tension with CA solution. Then, CA with different LiCl concentration and TPU of different solvent systems were conducted to do co-electrospinning experiments, respectively. In the discussion section, we focus on the interfacial interaction between CA and TPU components induced by different polymer structure and intrinsic properties, including solution properties, miscibility, and hydrogen bonding of the two solutions. Thermal and spectroscopic techniques including DSC and FTIR are utilized to study the interaction behavior of the CA/TPU pair. This study provides the insight into the CA/TPU helical fiber formation and introduces a richer choice of materials for the application of helical fibers.
Cellulose acetate (CA, white powder, MW = 100 W g/mol) was purchased from Acros Organics. Thermoplastic polyurethane (TPU, Desmopan DP 2590A) was from Bayer Materials Science. N, N-dimethylacetamide (DMAc, 0.938–0.942 g/ml at 20 °C, surface tension 25.3 dyne/cm, vapor pressure 0.17 kPa (20 °C)), acetone (0.788 g/ml at 20 °C, surface tension 18.8 dyne/cm, vapor pressure 24.64 kPa (20 °C)), tetrahydrofuran (THF, 0.887–0.889 g/ml at 20 °C, surface tension 28.8 dyne/cm, vapor pressure 18.9 kPa (20 °C)), and lithium chloride anhydrous (LiCl, Mw = 42.39 g/mol) were all purchased from Shanghai Chemical Reagents Co., Ltd., China. All of these materials were used without further purification. All experiments were performed at about 25 °C and 40%~ 50% RH.
The morphology of the resultant core-shell fibers were observed under a Scanning Electron Microscope (SEM) (JSM-5600LV, Japan) after gold coating.
The glass transition temperatures of the blends were performed using a DSC from DSC-4000 in a nitrogen atmosphere with temperature. The measurement was made using 5–10 mg sample on a DSC sample cell after the sample was quickly cooled to − 80 °C from the melt of the first scan. The glass transition temperature was obtained as the inflection point of the jump heat capacity with scan rate of 10 °C/min and temperature range of − 80~300 °C.
Infrared spectra were recorded on a Bruker Vector 33 FTIR spectrophotometer, and 32 scans were collected with a spectral resolution 1 cm−1. The film used in this study was sufficiently thin to obey the Beer-Lambert law. IR spectra recorded at elevated temperatures were obtained by using a cell mounted inside the temperature-controlled compartment of the spectrometer.
Properties of different electrospinning solutions
Amount of LiCl
DMAc: acetone = 1:2
DMAc: THF = 3:1
DMAc: acetone = 3:1
To explore the mechanism of CA/TPU helical fibers and the role of solvent effects, we designed two part experiments: the first part was carried out to select the suitable single spinning parameters, and in the second part, the combinatorial experiment: two systems of polymer composition, CA/TPU1 and CA/TPU2, were studied.
In the next part, we will co-spin CA added with different content of LiCl and TPU (included TPU1 and TPU2), respectively. So, two component systems, CA/TPU1 and CA/TPU2, were chosen in the co-electrospinning. Although the single-spinning TPU results are not satisfactory, as the core layer of the co-spinning, it will show another situation.
We have tried various processing conditions for the two component systems, and the experiments show the similar results that the CA/TPU2 fibers can fabricate helical structures more effectively comparing with the CA/TPU1 system. Only a few fibers show helical structures in the CA/TPU1 fiber web. These experiments demonstrate that the LiCl concentration and solvent systems play a crucial role in the generation of helical fibers. In this study, we furtherly analyze the experimental results through the below three aspects to explain the mechanism of the helical fiber formation.
Results and Discussion
In this paper, we try to explore the CA/TPU helical fiber spinning mechanism and discuss how the solution properties, miscibility, and hydrogen bonding of the two solutions affect the morphology of the resultant fibers.
Mechanism of CA/TPU Helical Fibers
As we all know, the solution parameters of co-spinning include the solution viscosity, solvent vapor pressure, interfacial tension, and solution conductivity. As shown in Fig. 2, when we change the solvent THF with acetone in TPU, the fiber adhesion phenomenon is reduced. It should be noted that the solvents used by different kinds of TPU are very important. The solution properties are shown in Table 1. As been shown, the solvents of TPU1 are DMAc and THF (3/1 volume ratio), while the solvents of TPU2 are DMAc and acetone (3/1 volume ratio), which result to the different solution properties. As we can see, the surface tension of TPU1 is about 34.45 N m− 1, while the TPU2 is about 25.34 N m− 1, which is much bigger than the TPU2. The surface tension of THF is 28.8 dyne/cm and the vapor pressure is about 18.9(20 °C), while the surface tension of acetone is 18.8 dyne/cm and the vapor pressure is about 24.64 (20 °C). If the solution vapor pressure is too high, then the solvent will evaporate too quickly and the solution will not be able to make a Taylor cone, while if it is too low, then the fibers will reach the collection plate wet and will merge to form a film. In coaxial spinning, it is usually advantageous to use solvents (or solvent mixtures) with different vapor pressures to avoid fiber collapse .
Besides, the solution miscibility between the core and shell is another important factor. As shown in the literature , when used the same solvent in the core and shell solution, it enables lower interfacial tension, which is important for the polymer not to precipitate at the fluid interface near the nozzle. As been shown in Table 1, the solvents of CA solution are DMAc and acetone (1/2 volume ratio), which are similar with the solvent of TPU2 and resulted similar interfacial tension between the CA/TPU2 solution interfaces. It also explains the results that the CA/TPU2 fibers can fabricate helical structures more effectively compare with the CA/TPU1 system in Fig. 4. In general, the solvent property will cause a huge change in the spinning solution properties, thus affecting the composite fiber morphology. However, besides the solution property, the polymer material performance also has an important influence on the formation of helical fibers.
Hydrogen Bonding in Blends
In our previous research, we found that not any polymer component with differential rigidity can form helical fibers, for example, PAN/TPU and PS/TPU system cannot form helical fibers, while Nomex/TPU system could. One of the important reasons is that hydrogen bonds between Nomex/TPU systems help to increase the solution interface interaction.
Miscibility Behavior in Blends
It can be predicted that the more significant of the rigidity differential of the two components, the greater potential for the component system to generate helical structures in co-electrospinning due to the greater interfacial stress between the components. By analyzing the miscibility of the CA/TPU systems, we believe that the partial miscible CA/TPU system tends to generate helical structures due to the intensified interfacial interaction attributed to hydrogen bonding.
The experimental results show that the CA/TPU2 system could form helical nanofibers effectively because the TPU2 solution enables lower interfacial tension with CA solution. Based on the interfacial interaction induced by the polymer structure and intrinsic properties, we explore the mechanism of CA/TPU helical structures from the three aspects: solution properties, hydrogen bonding, and miscibility behavior of the two solutions. When the solutions are charged, an attractive force between the chloride-ions contained in CA molecules and the free charges on the solution surface lead to a longitudinal interfacial interaction in the CA/TPU system. The large rigidity differential of polymer chains of CA and TPU leads to a large interfacial interaction between them. At the same time, the hydrogen bonds between the polymer chains help to obtain a partial miscible blend of the CA and TPU and consequently increase the interfacial interaction between these two components. This study provides an insight into the mechanism of CA/TPU helical fiber formation and introduces a richer choice of materials for the application of helical fibers.
The authors thank prof. Yongchun Zeng for proofreading the manuscript. The authors thank the State Key Laboratory Base of Novel Functional Materials and Preparation Science shared Facilities Center for enabling the use of testing equipment.
This work was financially supported by the Natural Science Foundation of Zhejiang Province (LQ18B070002), the National Natural Science Foundation of China (21471086), and the China Scholarship Council and the K. C. Wong Magna Fund in Ningbo University.
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
HW designed the experiments and wrote the paper. SZ performed the experiments and analyzed the data. LH commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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