How is the elasticity of electrospun polymers affected by solvents?

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A recent study published online in the journal ACS polymers at the end of last year investigated the effects of solvent properties on the elasticity of electrospun polymers. The research has implications for the design of materials for use in fields such as wearable electronics and medical research.

Study: Effects of Solvents on the Elasticity of Electrospinable Polymer Solutions. Image Credit: Kateryna Kon/Shutterstock.com

Ultrafine fibers

Ultrafine fibers have been the subject of recent research in the textile industry and materials science. Studies have been conducted to investigate the use of these fibers in conventional textiles and for smart textile applications in wearable electronics, sensors and devices used for bioengineering. Ultrafine fibers are defined as fibers whose dimensions are less than 5 μm.

However, their current commercial applications are limited to filtration devices and biomedical scaffolds, with a lack of research on their use in advanced applications for consumer products and microelectronics. Research into the use of ultrafine fibers for these areas will increase the number of materials that can be used, such as polymers and additives, and will require a wider range of solvents used in spinning processes as dopants. This will affect the fiber formation processes.

Challenges with ultrafine fiber spinning processes

Large-scale spinning processes used in fiber formation, such as melt spinning, wet spinning, and dry spinning, are affected by large changes in fluid flow properties. However, spinning processes for ultrafine fibers such as electrospinning face unique challenges when forming the fibers. These processes can be significantly affected by small changes in properties such as elasticity, surface tension, conductivity and viscosity.

Due to these challenges, there is an urgent need for research to investigate the relationship between the chemical composition of materials and the processing of multicomponent mixtures. Elucidating this knowledge will aid in the development of smart materials based on ultrafine fibers for use in emerging technologies.

Electrospinning is widely considered to be the forerunner among manufacturing technologies for the production of ultrafine fibers. In this process, the pulling force is provided by electric fields instead of applying mechanical force. However, this process suffers from high cost and low productivity, which limits its current use to high-value products such as cell growth scaffolds and pharmaceuticals. The manufacture of these products comes with complex product specifications and high performance requirements.

Bringing electrospun products to market requires a fundamental understanding of how electrospinning is affected by solution properties. Significant knowledge gaps on how viscoelastic behavior is affected by material properties under conditions similar to those encountered during the electrospinning process. Chief among these is elongational flow and fluid extension, which exist at high levels in the process. Currently, viscoelasticity and surface tension are believed to primarily determine the electrospinnability of polymer solutions.

The study

The authors studied the influence of the surface tension of the solvent on the formation of fibers. Extensional drip-on-solvent (DoS) rheology was used in the study to investigate the effect of solvent surface tension on the formation of ultrathin electrospun polymer fibers.

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Current studies have focused on shear viscosity analysis to understand chain entanglement effects and intermolecular attractions to infer the effect on electrospinnability from viscoelastic effects. In particular, they used zero-shear viscosity analyses. The use of extensional rheology is more effective in predicting this because polymer solutions experience significant elongation and extensional flow during electrospinning.

Additionally, previous studies have not considered solvents with low surface tension or analyzed systems that include multiple polymers and solvents. The authors said this is understandable, since techniques such as capillary rupture extensional rheometry cannot study weakly elastic or low-viscosity solvents due to their lack of sensitivity. DoS extensional rheology deduces that for the formation of smooth fibers a significantly higher elasticity is required by solutions with high surface tension.

Different polymer/solvent systems have been studied by the authors, with high and low molecular masses. Among the results obtained from their analyses, the authors noted that extensional relaxation time increases with polymer concentration, and that increasing surface tension requires higher polymer concentrations and, therefore, elasticity, to overcome Rayleigh instabilities that lead to the formation of droplets and beads to ensure smooth fiber formation. Increased extension speeds can further improve jet stability.

The authors noted that one property of the solvent that they did not investigate in detail was vapor pressure. Indeed, in the region of the stable jet which occurs just after the Taylor cone, the evaporation of the solvent is negligible. They concluded that solvent evaporation is unlikely to cause a significant difference in fiber formation and therefore vapor pressure is outside the scope of their study.

The authors stated that by understanding the relationship between solvent characteristics, viscoelasticity and electrospinnability, the fabrication and design of more complex spinnable solutions will be improved for use in applications such as pharmaceuticals, bio- engineering and portable electronics. Although the authors noted that there are some limitations, their study improves the current knowledge base.

Further reading

Ewaldz, E, Randrup, J & Brettmann, B (2021) Effects of Solvents on the Elasticity of Electrospinable Polymer Solutions [online] ACS Polym. At | pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acspolymersau.1c00041

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