Controlling Secondary Surface Morphology of Electrospun PVDF Nanofibers via Solvent Choice and Relative Humidity

Abstract

We present a straightforward, reproducible protocol for fabricating polyvinylidene fluoride (PVDF) nanofibers with tailored secondary morphologies—macro‑porous, rough, grooved, and internally porous—using single or binary solvent systems and controlled relative humidity. Systematic studies reveal that water vapor, acting as a non‑solvent, is essential for inducing phase separation when the relative humidity is within a specific window. The rate of solvent evaporation (acetone, DMF, or mixtures) and the interdiffusion of water dictate the emergence of surface pores, interior voids, and textural features. Nitrogen adsorption measurements show that macro‑porous fibers (>300 nm) achieve a specific surface area of 23.31 ± 4.30 m² g⁻¹ and a pore volume of 0.0695 ± 0.007 cm³ g⁻¹, translating into exceptional oil‑absorption capacities of 50.58 ± 5.47 g g⁻¹ (silicone oil), 37.74 ± 4.33 g g⁻¹ (motor oil), and 23.96 ± 2.68 g g⁻¹ (olive oil). These findings provide a practical roadmap for engineering electrospun PVDF nanofibers with customized surface architectures for advanced filtration, sensing, and energy‑harvesting applications.

Background

Electrospinning harnesses electrostatic forces to stretch polymer solutions into fibers ranging from nanometers to micrometers in diameter. By tuning process parameters, a rich library of morphologies—beaded, porous, grooved, hollow, core–sheath, and more—has been realized. Electrospun nanofibers excel in high surface‑area, flexibility, and structural diversity, making them ideal for energy harvesting, sensors, filtration, biomedical devices, and self‑cleaning surfaces.

Secondary surface features—porous, rough, or grooved—can dramatically enhance functional performance. Porous fibers boost catalytic and filtration efficiency; rough surfaces increase triboelectric output; grooved architectures favor tissue engineering and superhydrophobicity. However, systematic control of these features, especially in PVDF, remains underexplored.

Methods

Materials

PVDF (Mn = 275 kDa) from Sigma‑Aldrich; acetone (ACE) and N,N‑dimethylformamide (DMF) from Shanghai Chemical Reagents Co., Ltd. All reagents were used as received.

Electrospinning Setup

Solutions of 18 % ACE/​PVDF, 35 % DMF/​PVDF, and 25 % ACE/DMF PVDF (ratio 4:1 to 1:4) were prepared. A 21 G needle fed a syringe pump (1.5 mL h⁻¹) under 18 kV. Fibers were collected on a 40 cm × 20 cm drum at 2 rpm, 18 cm from the needle. Experiments were conducted at 20 °C with relative humidity set to 5 %, 25 %, 45 %, or 65 % using a humidifier/dehumidifier.

Ternary Phase Diagram

Cloud points were obtained by titrating PVDF solutions with deionized water at 65 % RH. The binodal curve was plotted to identify solvent–water–polymer coexistence.

Characterization

Fiber morphology and cross‑sections were examined by FE‑SEM (Hitachi S‑4800) after gold coating. Diameters were measured via image analysis. Nitrogen adsorption–desorption isotherms (JW‑BK132F) provided surface area, pore size, and volume data. Oil absorption was assessed by immersing 0.3 g of dry fiber in a 1:1 water–oil mixture for 1 h, draining for 40 min, and calculating capacity with Q = (m₀ – m₁)/m₁.

Results and Discussion

ACE‑Based Fibers

At 5 % RH, smooth, solid‑interior fibers formed—no phase separation. Increasing RH to 25–65 % induced macro‑porosity on the surface. Cross‑sections revealed solid interiors at 25 % RH (TIPS) and interior pores at 45–65 % RH (co‑occurrence of TIPS and VIPS). Macro‑pore size grew from ~50 nm (25 % RH) to ~400 nm (65 % RH). Fiber diameter increased from 0.77 µm to 1.81 µm with rising RH.

DMF‑Based Fibers

5 % RH produced smooth fibers; 25–65 % RH yielded rough surfaces and interior pores via VIPS. The higher vapor pressure of water relative to DMF promotes water‑induced phase separation, forming a PVDF sheath that traps DMF and generates pores. Diameter increased from 0.8 µm to 1.79 µm across the RH range.

ACE/DMF Mixture Fibers

Using 25 % PVDF with varying ACE/DMF ratios, a spectrum of morphologies emerged: smooth (5 % RH), pillar‑shallow grooved (25 % RH), pillar‑longitudinal grooved (45–65 % RH). At 25–65 % RH, all fibers displayed interior pores; grooved textures resulted from wrinkle‑based elongation after early glassy skin formation. Increasing RH broadened groove width and depth. Fiber diameters varied with solvent ratio: 1:1 ratio yielded the thinnest fibers (~0.6 µm) at 5 % RH, expanding to ~2 µm at 65 % RH.

Phase Behavior

A ternary phase diagram at 65 % RH shows two zones separated by a binodal curve. The jet starts in a homogeneous zone (I); as ACE evaporates rapidly, the composition shifts into zone II where multiphase separation occurs (TIPS and VIPS). The trajectory depends on solvent volatility.

Surface Area, Porosity, and Oil Absorption

Macro‑porous fibers achieved the highest specific surface area (23.31 ± 4.30 m² g⁻¹) and pore volume (0.0695 ± 0.007 cm³ g⁻¹). Grooved and rough fibers displayed lower values (10.26 ± 2.19 m² g⁻¹, 4.81 ± 0.58 m² g⁻¹). Mesopores (2–50 nm) were common; macro‑pores (>100 nm) were exclusive to macro‑porous fibers, explaining their superior oil uptake. Oil absorption tests confirmed macro‑porous fibers absorbed 50.58 ± 5.47 g g⁻¹ of silicone oil, 37.74 ± 4.33 g g⁻¹ of motor oil, and 23.96 ± 2.68 g g⁻¹ of olive oil—over twice the capacity of grooved or rough fibers.

Conclusions

We established a reliable, tunable strategy for generating macro‑porous, rough, and grooved PVDF nanofibers with internal porosity by adjusting solvent composition and ambient humidity. Key insights: no phase separation at 5 % RH yields smooth fibers; TIPS dominates at 25 % RH (ACE); VIPS governs rough fibers (DMF) and grooved textures (ACE/DMF). These structural controls directly translate into enhanced surface area and oil‑absorption performance, offering a practical guide for designing high‑functionality electrospun materials.