Ambient conditions play a critical role in the electrospinning process, directly affecting solvent evaporation rates, fiber morphology, and the formation of defects. Variations in temperature, humidity, and air flow can lead to significant differences in the final nanofiber structure, influencing properties such as diameter, porosity, and mechanical strength. Understanding these effects is essential for producing consistent, high-quality nanofibers for applications ranging from filtration to biomedical engineering.
**Solvent Evaporation Dynamics**
The rate of solvent evaporation during electrospinning is highly sensitive to ambient conditions. Higher temperatures accelerate solvent evaporation, leading to rapid solidification of the polymer jet. For example, polyvinyl alcohol (PVA) nanofibers electrospun at 25°C exhibit smoother surfaces compared to those produced at 35°C, where rapid drying can cause premature solidification and irregularities. Conversely, low temperatures slow evaporation, increasing the time for polymer chain relaxation and often resulting in thicker fibers.
Humidity is another key factor. In environments with high relative humidity (RH), water vapor can condense into the polymer jet, creating porous or bead-like structures. A study on polycaprolactone (PCL) nanofibers demonstrated that increasing RH from 30% to 70% induced pore formation due to moisture-induced phase separation. At very low humidity (<20%), rapid solvent evaporation can cause jet instability, leading to uneven fiber diameters and occasional breakage.
Air flow in the electrospinning chamber also influences solvent evaporation. Uncontrolled air currents disrupt the polymer jet, increasing the likelihood of defects such as branching or non-uniform deposition. In contrast, controlled laminar flow can enhance solvent removal, improving fiber uniformity. For instance, polyacrylonitrile (PAN) nanofibers produced under laminar air flow conditions exhibit fewer defects compared to those spun in turbulent environments.
**Fiber Morphology Variations**
Ambient conditions directly impact fiber diameter and surface texture. Studies on polystyrene (PS) nanofibers revealed that increasing temperature from 20°C to 40°C reduces average fiber diameter by approximately 30%, attributed to faster solvent loss and polymer chain alignment. Similarly, high humidity (>60% RH) often leads to ribbon-like or flattened fibers due to moisture absorption and delayed solidification.
The presence of pores or beads is another common morphological outcome. For example, poly(lactic-co-glycolic acid) (PLGA) nanofibers electrospun at 50% RH exhibit a smooth surface, while those produced at 80% RH develop microporous structures. These pores can be advantageous for drug delivery applications but detrimental for mechanical reinforcement.
**Defect Formation Mechanisms**
Defects such as bead formation, branching, and fiber fusion are strongly influenced by ambient conditions. Beads typically form when solvent evaporation is too slow, causing the polymer jet to break into droplets before solidification. A case study on polyethylene oxide (PEO) nanofibers showed that reducing humidity from 70% to 30% decreased bead density by over 50%.
Branching occurs when secondary jets split from the primary jet, often due to electrostatic instabilities exacerbated by high humidity or temperature fluctuations. For instance, nylon-6 nanofibers electrospun at 25°C and 40% RH exhibit minimal branching, whereas those produced at 30°C and 60% RH show frequent secondary fiber formations.
Fiber fusion happens when partially dried fibers come into contact before complete solvent evaporation, often in high-humidity environments. Research on polyurethane (PU) nanofibers demonstrated that at 75% RH, overlapping fibers frequently fuse, reducing porosity and flexibility.
**Case Studies in Controlled Environments**
1. **Polyvinylidene Fluoride (PVDF) Nanofibers**
- At 25°C and 40% RH, PVDF fibers exhibit uniform diameters (~500 nm) and high crystallinity.
- Increasing humidity to 70% introduces nanopores but reduces piezoelectric properties due to moisture interference.
2. **Silk Fibroin Nanofibers**
- Electrospinning at 20°C and 30% RH produces smooth, continuous fibers ideal for tissue engineering.
- At 80% RH, fibers develop microbeads, compromising mechanical integrity.
3. **Cellulose Acetate Nanofibers**
- Low humidity (<30%) yields thin fibers (~200 nm) with high tensile strength.
- High humidity (>60%) causes fiber adhesion, reducing filtration efficiency.
**Mitigating Ambient-Induced Defects**
Adjusting polymer concentration and solvent selection can counteract ambient effects. For example, using a volatile solvent like chloroform reduces humidity sensitivity, while adding non-solvents like water can induce porosity intentionally. Optimizing voltage and feed rate also helps stabilize the jet under varying conditions.
In summary, ambient conditions are a decisive factor in electrospinning, shaping solvent evaporation kinetics, fiber morphology, and defect formation. By systematically controlling temperature, humidity, and air flow, researchers can tailor nanofiber properties for specific applications while minimizing undesirable structural flaws.