How Temperature Influences the Young’s Modulus of Electrospun Polyurethane Nanofibers

Abstract

Electrospun polyurethane (PU) nanofibers were fabricated and examined using atomic force microscopy (AFM) to quantify their mechanical behavior. A three‑point bending test, performed on individual fibers, revealed a Young’s modulus of approximately 25 GPa for ~150 nm diameter fibers at room temperature. As the diameter shrank, the modulus rose, attributed to surface‑tension effects. Raising the temperature produced a linear decline in modulus while the fibrous morphology remained intact.

Background

One‑dimensional (1D) nanomaterials attract attention because of their distinctive properties and diverse applications. While several fabrication routes exist, electrospinning stands out for its low cost, scalability, and ability to produce sub‑micron fibers via electrostatic jet thinning.

Polyurethane combines soft and hard segments linked by urethane bonds; the soft chains provide flexibility, and the hard chains confer strength. Its tunable hardness makes PU a popular choice across industries. Electrospun PU fibers show promise in high‑performance air filters, protective textiles, wound dressings, and sensors. However, their mechanical properties are poorly understood, largely because testing at the nanoscale is challenging. AFM has emerged as a practical tool for probing the elasticity of single 1D nanostructures, and a straightforward AFM‑based three‑point bending method has been developed to measure Young’s modulus with high precision.

In this study, we prepared electrospun PU nanofibers and used AFM three‑point bending to investigate how temperature influences their Young’s modulus.

Methods

Material Preparation

Polyurethane elastomer (Elastollan® 1180A10, BASF) was dissolved in a 1:1 mixture of N,N‑dimethylformamide (DMF) and tetrahydrofuran (THF; Tianjin Hengxing Chemical Reagent Co., Ltd.). The solution was stirred vigorously at room temperature for 12 h to ensure complete dissolution. Electrospinning was carried out with a commercial system (Beijing Ucalery Technology Development Co., Ltd.). A high voltage of 9–10 kV was applied across a 13 cm gap between the spinneret and a grounded rotating mandrel. Fibers were collected, dried under vacuum overnight to remove residual solvents, and stored in a desiccator before testing.

Physical Characterization and Testing

Scanning electron microscopy (SEM, JSM‑6610LV) was used to assess fiber morphology. Thermogravimetric and differential scanning calorimetry (TG/DSC) were performed on a TA Instruments SDT Q600 under argon. The macroscopic modulus of the electrospun membrane was measured with an Instron 5943 universal testing machine. For nanomechanical testing, fibers were deposited onto a silicon template with 2 µm wide, 3 µm deep trenches; a 50 nm diameter spherical AFM probe (Bruker Nano) applied force at the trench center. Cantilever spring constants were calibrated by thermal tuning, and deflection sensitivity was set on a sapphire reference. Five independent measurements per fiber were averaged. Finite element analysis (ANSYS 15.0) evaluated tip penetration, confirming that deformation of the fiber surface is negligible.

Results and Discussion

SEM images show a random network of fibers ranging from a few hundred nanometers to several micrometers. AFM cross‑sections confirm uniform lateral profiles, with an average diameter of ~300 nm. TG/DSC curves display two degradation stages: a minor weight loss between 100–200 °C (water and volatiles) and a pronounced loss at ~300 °C (polymer decomposition). FTIR spectra display characteristic PU peaks at 3320, 2960, 1710, 1530, 1220, 1110, and 777 cm⁻¹, confirming the chemical integrity of the fibers.

In the three‑point bending setup (Fig. 3), the Young’s modulus (E) is calculated using:

E = FL³ / (192 dI), with F the midpoint force, L the suspended length, d the deflection, and I = πr⁴/4. The test satisfies all assumptions (fixed ends, L≫r, small d). FEM results indicate tip penetration <10 % of the deformation, validating the linear elastic model.

The modulus versus diameter plot (Fig. 4a) reveals a clear size dependence: 25 GPa at 150 nm, dropping to ~5 GPa for fibers >300 nm. This trend aligns with reports for other polymer nanofibers and can be attributed to surface tension. Using the surface‑tension model, the intrinsic modulus of the PU nanofiber is ≈5.0 GPa, higher than bulk PU due to chain orientation during electrospinning. The electrospun membrane itself exhibits a low modulus (~0.9 MPa) because of its high porosity.

Temperature‑dependent measurements (Fig. 5a) show a linear decrease in modulus from 25 °C to 60 °C for a 155 nm fiber. AFM imaging at 60 °C confirms that the fiber remains intact, with only a slight diameter increase (200→214 nm). These findings suggest that PU nanofibers maintain dimensional stability at moderate temperatures and could be useful in nanosensors and temperature‑responsive devices.

Durability tests (Fig. 6) involved 50 bending cycles on a 215 nm fiber. The modulus remained essentially unchanged, indicating robust mechanical resilience.

Conclusions

We measured the Young’s modulus of electrospun PU nanofibers using AFM‑based three‑point bending. The modulus rises sharply as diameter decreases, a behavior governed by surface tension. Temperature induces a linear decline in modulus between 25 °C and 60 °C, yet the fibers preserve their morphology and withstand repeated bending without significant loss of stiffness.

Abbreviations

1D

One‑dimensional

AFM

Atomic force microscopy

DMF

N,N‑dimethylformamide

PU

Polyurethane

SEM

Scanning electron microscopy

TGA/DSC

Thermogravimetric differential scanning calorimetry

THF

Tetrahydrofuran