Measuring the Mechanical Properties of Individual Polymer Nanofibers

Abstract

Current nanofiber modeling possesses limited ability to characterize fiber/support structure and predict movement under flow. The increased viscosity in liquid applications, relative to traditional air applications, can lead to fine fiber blowout as the fiber size decreases. Currently, this is solved by [redacted to protect potential IP]. Opportunities for Donaldson exist to develop a nanoindentation test method, that with the application of the Oliver-Pharr method, can be used to determine the mechanical properties of individual nanofibers. This knowledge will aid in the development of nanofibers that maintain integrity in liquid applications. In this project, we are focused on determining if nanoindentation can be applied to nanofibers that Donaldson currently manufactures, specifically electrospun polyamide (PA) fibers with and without a cross-linking agent. The main objective of the project is to evaluate nanoindentation as a measurement technique for fibers by comparison to tensile testing and thin films of similar dimensions. Then apply the previously determined test method to other nanofiber materials, specifically polyether sulfone (PES) and polyvinylidene difluoride (PVDF), to determine its general applicability. This project was executed in two cycles, described below: Cycle 1: Determined a method to generate aligned electrospun PA fibers for nanoindentation and tensile testing. Measurement of the same material in both stress states enables the conversion of the indentation modulus to a Young’s modulus for use as a modeling input. The resolution of nanoindentation proved sufficient to differentiate between a crosslinked and non-crosslinked material. The surface curvature of fibers larger than a micron proved to have a negligible reduction in contact area and indentation modulus compared to a flat geometry. This cycle increased both technology readiness level (TRL) and technology understanding level (TUL) from 1 to 2. Cycle 2: Confirmed that the indentation modulus of PA fibers increases as the fiber diameter decreases, with a ~3x increase at a diameter of 250 nm compared to a large fiber, which agrees with the literature for other polymer nanofibers. The application of the curvature correction model was found to be unnecessary at diameters greater than 300 nm since the indentation depth scales with fiber diameter. Evaluation of the thermally activated crosslinker currently used in application found that the indentation modulus at decreased concentration had no effect on the modulus, while the current concentration offers ~30% improvement. A UV cured crosslinker offered no improvement in mechanical properties, self-crosslinking to form a surface layer with a decreased indentation modulus. The procedure developed in Cycle 1 was successfully applied to PES and PVDF fibers and is thus ready to be used as a test method for modeling inputs. This cycle increased the TRL level from 2 to 3 and the TUL level stayed at 2.