The selection of polymers for electrospinning is a critical step in producing nanofibers with desired properties for specific applications. The process depends on several key polymer characteristics, including solubility, molecular weight, and viscosity, which directly influence spinnability and the resulting fiber morphology. Both synthetic and natural polymers are used, each offering distinct advantages and challenges.
**Solubility**
A polymer must dissolve in a suitable solvent to form a spinnable solution. The solvent should evaporate during electrospinning to facilitate fiber solidification. Synthetic polymers like polyvinyl alcohol (PVA) dissolve readily in water, while polycaprolactone (PCL) requires organic solvents such as chloroform or dimethylformamide (DMF). Polyacrylonitrile (PAN) is often dissolved in DMF or dimethyl sulfoxide (DMSO) due to its moderate solubility.
Natural polymers, such as collagen and chitosan, present solubility challenges due to their complex structures. Collagen typically requires acidic solvents like acetic acid, while chitosan dissolves in dilute acids due to its cationic nature. Inadequate solubility leads to incomplete dissolution, resulting in solution inhomogeneity and defective fibers.
**Molecular Weight**
Molecular weight significantly impacts chain entanglement, a prerequisite for fiber formation. Polymers with too low molecular weight lack sufficient entanglement, leading to bead formation or electrospraying rather than fiber formation. For instance, PVA with a molecular weight below 30,000 g/mol often produces beads, whereas higher molecular weights (85,000–146,000 g/mol) yield smooth fibers.
Similarly, PCL with molecular weights between 50,000–80,000 g/mol forms continuous fibers, while lower molecular weights result in discontinuous structures. For natural polymers, chitosan with medium molecular weight (190,000–310,000 g/mol) is preferred for electrospinning, as very high molecular weights increase solution viscosity excessively, hindering jet formation.
**Viscosity**
Solution viscosity is determined by polymer concentration and molecular weight. Optimal viscosity ranges between 1–20 poise for most polymers. Too low viscosity causes droplet formation due to insufficient chain entanglement, while excessive viscosity impedes jet elongation, leading to thicker fibers or clogging.
For PVA, concentrations of 8–12 wt% typically yield uniform fibers. PAN solutions at 6–10 wt% in DMF produce fibers with diameters ranging from 200–800 nm. Natural polymers like collagen require precise concentration control; 8–10 wt% in acetic acid often results in consistent fibers, whereas higher concentrations increase fiber diameter and brittleness.
**Synthetic Polymers in Electrospinning**
Polyvinyl alcohol (PVA) is widely used due to its water solubility, biocompatibility, and tunable mechanical properties. It produces smooth fibers with diameters between 100–500 nm under optimized conditions. PVA’s hydroxyl groups allow crosslinking for improved stability in aqueous environments.
Polycaprolactone (PCL) is a biodegradable polyester favored in tissue engineering. Its semi-crystalline nature provides mechanical strength, and fibers typically range from 500 nm to 2 µm in diameter. PCL’s slow degradation rate makes it suitable for long-term implants.
Polyacrylonitrile (PAN) is notable for its high carbon yield, making it a precursor for carbon nanofibers. PAN solutions electrospun at 10–12 wt% yield fibers with diameters of 300–700 nm. Its chemical resistance and thermal stability are advantageous for filtration and energy storage applications.
**Natural Polymers in Electrospinning**
Collagen, a major extracellular matrix component, is ideal for biomedical applications. Electrospun collagen fibers mimic natural tissue architecture, with diameters typically between 100–600 nm. However, collagen’s poor mechanical strength often necessitates crosslinking or blending with synthetic polymers.
Chitosan, derived from chitin, exhibits antimicrobial properties and biocompatibility. Its cationic nature allows interactions with anionic biomolecules, making it useful in wound dressings. Chitosan fibers are challenging to produce alone due to high viscosity, so blending with PEO or PVA is common to improve spinnability.
**Effect of Polymer Properties on Fiber Characteristics**
Fiber diameter is influenced by polymer concentration and molecular weight. Higher molecular weights and concentrations generally increase fiber diameter due to greater chain entanglement and resistance to stretching. For example, increasing PVA concentration from 8 to 12 wt% can double fiber diameter.
Surface morphology depends on solvent volatility and polymer-solvent interactions. Fast-evaporating solvents like chloroform produce porous fibers due to rapid phase separation, while slower-evaporating solvents like DMF yield smoother surfaces.
Mechanical properties are governed by polymer crystallinity and chain alignment. Semi-crystalline polymers like PCL exhibit higher tensile strength than amorphous ones. Fiber alignment during electrospinning further enhances mechanical properties by orienting polymer chains along the fiber axis.
**Conclusion**
The choice of polymer for electrospinning depends on solubility, molecular weight, and viscosity, which collectively determine spinnability and fiber characteristics. Synthetic polymers like PVA, PCL, and PAN offer reproducibility and tunability, while natural polymers such as collagen and chitosan provide biocompatibility but require careful optimization. Understanding these criteria enables the design of nanofibers tailored for specific applications in biomedicine, filtration, and energy storage.