Temperature Effect on the Mechanical Properties of Electrospun PU Nanofibers
- 295 Downloads
Polyurethane (PU) nanofibers were prepared from electrospun method. Atomic force microscopy (AFM) was employed to characterize the mechanical properties of electrospun PU nanofibers. The impact of temperature on the mechanical behavior of PU nanofibers was studied using three-point bending test based on AFM. A Young’s modulus of ~ 25 GPa was obtained for PU nanofibers with diameter at ~ 150 nm at room temperature. With decrease in nanofiber’s diameter, the increasing Young’s modulus can be due to the surface tension effect. The Young’s modulus of the PU nanofiber decreased linearly while the fibrous morphology was maintained with the increase of temperature.
KeywordsPU nanofiber Mechanical property AFM Young’s modulus Three-point bending test
Atomic force microscopy
Scanning electron microscopy
Thermogravimetric differential scanning calorimetry (TG/DSC)
One-dimensional (1D) nanomaterials have been intensively studied due to their unique properties and intriguing applications in many areas [1, 2, 3]. Many synthetic and fabricating methods have already been explored to generate 1D nanostructures in the form of fibers, wires, rods, and tubes from various materials [4, 5]. However, their usefulness is limited by combinations of restricted material ranges, cost, and production rate. Unlike other methods for generating 1D nanostructures, electrospinning has an advantage with its relative low cost and high production rate, which is similar to the commercial processes for producing microscale fibers except for the use of electrostatic repulsions to continuously reduce the diameter of a viscoelastic jet [6, 7].
Polyurethane (PU) is composed of soft and hard segments connected by a urethane linkage, in which the soft segments impart flexibility, whereas the hard segments provide the rigidity and strength [8, 9]. PU materials have been widely used in industry since its hardness can be easily modulated by changing the hard-segment in the structure . Electrospun PU nanofibers have a wide variety of potential applications in high-performance air filters, protective textiles, wound dressing films, and sensors [11, 12]. Understanding the mechanical properties is essential for the application and function of nanomaterials . However, too little attention has been paid to the study of mechanical properties of electrospun nanofibers due to the difficulties in making a nanoscale test. In the past decade, atomic force microscopy (AFM) was employed to characterize the mechanical properties of 1D nanostructure in a straightforward way [14, 15, 16]. A facile AFM-based three-point bending test has been designed to measure the Young’s modulus of a single nanofiber, which involves clamping the 1D nanostructure across a trench by the self-adhesion between the sample and substrate. The midpoint of the suspended 1D nanostructure is subjected to a force applied by the AFM tip, and then, the corresponding deflection at the midpoint is recorded and used to calculate the Young’s modulus. Here, PU nanofibers were prepared from electrospun method. And then three-point bending test was employed to study the effect of temperature on the Young’s modulus of PU nanofibers.
N, N-dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. Polyurethane elastomer (Elastollan® 1180A10) was obtained from BASF. PU was dissolved in the mixture of DMF and THF with a volume ratio of 1:1. The solution was sealed at room temperature for more than 12 h with intensive stirring. A commercially available electrospinning set-up (Beijing Ucalery Technology Development Co., Ltd., China) was used for the fabrication of electrospun PU nanofibers. The distance between the nozzle and a grounded collector was adjusted to 13 cm. A high voltage of 9–10 kV was applied to generate a polymer jet. The resulting fibers were collected on a rotating mandrel, left in vacuum conditions overnight to eliminate solvent residues and then kept in a desiccator for further experimentation.
Physical Characterization and Testing Method
The microstructure and morphology of the as-prepared PU nanofibers were characterized by scanning electron microscopy (SEM, JSM-6610LV, Japan). Thermogravimetric differential scanning calorimetry (TG/DSC) analysis was carried out with a DSC–TGA (SDT Q600, TA Instruments) under argon atmosphere. The macroscopic elastic modulus of electrospun PU membrane was measured by universal testing machine (Instron 5943, USA). The nanomechanical properties of nanofibers were tested by using Multimode 8 AFM (Bruker Nano Inc., USA). First, electrospun PU nanofibers were deposited by using a Si template as collector (purchased from Suzhou RDMICRO Co., Ltd.). The nanofibers suspended on the groove were submitted to AFM test. The width and depth of the groove on the substrate are 2 and 3 μm. The probe is simplified as a sphere with a diameter of 50 nm. The spring constant of the cantilever was measured by thermal tune method. Sensitivity of the cantilever, as the cantilever deflection signal vs. the applied voltage, was calibrated on a sapphire surface. Force curves were recorded to calculate the elastic modulus of a single nanofiber. Each experiment was repeated 5 times, and the results were averaged (arithmetic mean). Finite element simulation was performed to evaluate the degree of tip penetration into the nanofiber surface. The simulation model was established in commercial software package (ANSYS 15.0). The materials of nanofiber, probe, and the substrate are all considered as elastic linear isotropic solids .
Results and Discussion
In summary, the Young’s modulus of a single PU nanofiber prepared from electrospun method was measured by three-point bending test. The increasing Young’s modulus with decreasing diameter can be ascribed to the surface effect. Besides, the Young’s modulus decreases linearly with the increase in temperature in the range of 25 °C~60 °C. PU nanofiber exhibits good durability with no significant degradation in Young’s modulus even after 50 cycles.
The financial support from the National Natural Science Foundation of China (nos. 51002128 and 51401176), Scientific Research Foundation of Hunan Provincial Education Department (no. 17A205), and Natural Science Foundation of Hunan Province (nos. 2018JJ2393 and 2018JJ2394) is greatly acknowledged.
Availability of Data and Materials
We declared that the materials described in the manuscript, including all relevant raw data, will be freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant confidentiality.
JZ, QC, and FX conceived the experiment and carried out the data analysis. XL and YD assisted in the experimental work. All authors contributed to the general discussion and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.