Polymeric fibers of reduced dimensionality have been shown to exhibit significantly enhanced mechanical properties at the nanoscale. While it is well known through computational studies that the mechanical properties (Young’s modulus, Tensile strength) tend to increase abruptly below a certain nanofiber diameter value, various experimental challenges at the nanoscale, have prevented detailed verification of such behavior. The key challenges in experimental methods of measuring individual fiber properties include (i) repeatability of fiber diameter, (ii) control on fiber diameter (nano-micro), (iii) control on fiber deposition at precise locations, (iv) establishment of boundary conditions, and (v) fragility of nanofibers.  To overcome these challenges, it is common to use mats of fibers for mechanical testing and then deduce individual fiber properties. However, such strategies often report significant differences (sometimes an order of magnitude) in the mechanical properties of whole mats and individual fibers. Here, employing cantilever mechanics, we report a new experimental method to measure the mechanical properties of individual nanofibers. We utilize our previously reported non-electrospinning Spinneret based Tunable Engineered Parameters (STEP) technique to deposit a parallel array of nanofibers with consistent diameters on a hollow scaffold base. The deposited nanofibers are stretched at various strain rates (0.01-1 s^-1^). Fusing fibers to scaffold ensures maintenance of fixed-fixed boundary conditions.  The force associated with nanofiber stretching leads to the cantilever deflection, which is accurately measured by a high precision laser positioned underneath the cantilever. Cantilever deflections can be directly converted into force values using the cantilever beam equation. Additionally, due to the parallel arrangement of the fiber array and the near-identical diameters of individual fibers, we can treat fibers as springs in parallel, thus allowing us to estimate the tension in each fiber. Thus, by knowing the temporal evolution of the fiber stretching forces, initial fiber diameter and the fiber stretching rate, we can precisely characterize the stress-strain behavior of individual nanofibers. Our method, have enabled us to characterize micro/nano fibers from i) polystyrene (fiber diameters \~ 200-800 nm, molecular weight: 2,000,000 g/mol) and ii) PLGA (fiber diameters \~ 1-5 μm, molecular weight: 1,300,000 g/mol). In general, our results demonstrate that the various mechanical properties are fiber diameter dependent with the Young’s modulus and tensile strength increasing with a decrease in the fiber diameter. Maximum strain at fiber failure was observed to increase with fiber diameter for both polystyrene (\~30-60%) and PLGA (\~150-300%). Altogether, eliminating the challenging steps required for establishment of fixed-fixed boundary conditions reported in other characterization approaches, makes our method easy-to–implement, while still maintaining a high degree of repeatability. Our current and future efforts are aimed at using the platform to characterize \~100 nm fibers and extend the technique to test other polymeric systems including biopolymers, conductive polymers and composite fibers.  We expect that our results using different polymers of a range of molecular weights will increase the application design space as well as will provide the experimental data to aid theoretical modeling of polymer structure-property relationships at the nanoscale.  


Integrating Parallel Springs With Cantilever Physics for Characterizing Mechanical Properties of Individual Polymeric Nanofibers

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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