Electrospinning process analysis: the relation of process parameters to fiber diameter and process dynamics for closed-loop control design
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The electrospinning process can produce submicron fibers for a variety of applications since a wide range of polymers can be used. For many advanced applications, achieving the desired fiber diameter, maximizing productivity, and achieving high yield are important production objectives. This thesis addresses several important areas needed to develop a general electrospinning control approach that aids in achieving those objectives including: developing a correlation between process conditions to fiber diameter, developing a method to determine an operating regime that meets manufacture objective, identification of process dynamics needed to develop a realtime control system, and insight into the role that relative humidity (RH) has on the process physics. A primary requirement for developing a general electrospinning control approach is to develop a correlation that maps measurable process parameters to the resulting fiber diameter, since electrospun nanofibers can not be observed in real time. Building on Yans identification of a fiber diameter correlation based on measuring the processes straight jet diameter and the ambient relative humidity for Polyethylene oxide (PEO)/water solutions, this thesis extends that work for a different polymer (Poly(vinylpyrrolidone) (PVP)) using three non-aqueous solvents. A correlation of process measurements to fiber diameter is developed, which is useful for both determining the operating regime (specification of voltage, flow rate and RH that meets the manufacturing objectives) as well as real-time control system design. An interpretation of experimental results also provides insight into the multiple roles that RH has on the electrospinning process physics in terms of the effective electric field that determines the upper jet geometry and current; the total solvent evaporation rate; and the impact of the electric stretching forces in the bending region A generalized operating regime determination strategy is presented that obtains the desired fiber diameter with maximum production rate. In addition, the process dynamics characteristics and how they scale with different operating conditions are identified, which is needed to develop real-time control algorithm. The process dynamics of PVP/alcohol solutions are shown to be similar to those of PEO/water solutions, and are determined by the Taylor cone volume dynamics.
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