Effects of Fiber Density and Strain Rate on the Mechanical Properties of Electrospun Polycaprolactone Nanofiber Mats

Abstract

This study examines the effects of electrospun polycaprolactone (PCL) fiber density and strain rate on nanofiber mat mechanical properties. An automated track collection system was employed to control fiber number per mat and promote uniform individual fiber properties regardless of the duration of collection. Fiber density is correlated to the mechanical properties of the nanofiber mats. Young's modulus was reduced as fiber density increased, from 14,901 MPa for samples electrospun for 30 s (717 fibers +/- 345) to 3,615 MPa for samples electrospun for 40 min (8,310 fibers +/- 1,904). Ultimate tensile strength (UTS) increased with increasing fiber density, where samples electrospun for 30 s resulted in a UTS of 594 MPa while samples electrospun for 40 min demonstrated a UTS of 1,250 MPa. An average toughness of 0.239 GJ/m³ was seen in the 30 s group, whereas a toughness of 0.515 GJ/m³ was observed at 40 min. The ultimate tensile strain for samples electrospun for 30 s was observed to be 0.39 and 0.48 for samples electrospun for 40 min. The relationships between UTS, Young's modulus, toughness, and ultimate tensile strain with increasing fiber density are the result of fiber-fiber interactions which leads to network mesh interactions.

Keywords: electrospinning; material testing; mechanical properties; nanomaterials; polymer.

Figure 1

(A) Automated adjustable track system capable and collecting and post-drawing thousands of electrospun nanofibers individually. (B) A diagram illustrating the collection and extension of electrospun nanofibers using the revolving automated tracks. (C) A representation of the macromolecular structure of a polymer chain at the top of the automated tracks (1), in the middle of the tracks (2), and at the bottom of the tracks as well as in the collecting rack (3).

Figure 2

(Top) Representative electrospun PCL nanofibers by electrospinning duration captured at 1,500x magnification. (A) PCL nanofibers adhered to collection tray. (B) SEM image of electrospun fibers at 8,000x magnification specifically used for measuring fiber diameters. (C) Close up of 10 × 10 mm plastic window squares containing PCL nanofibers. (D) Graph depicting fiber junction counts to total number of fibers. (E) SEM image of electrospun PCL nanofibers captured at 8,000x magnification with red circles used to quantify fiber junctions.

Figure 3

Diameter distributions by each of the electrospinning durations.

Figure 4

(A) Initial tensile testing setup performed on a Shimadzu EZ-SX mechanical tester. (B) Elongation of electrospun PCL nanofibers during testing. (C) Failure of electrospun PCL nanofibers at the conclusion of tensile testing. (D) Representative stress-strain curves by strain rate (mm/min.) for individual fiber mat samples containing 4,753–4,976 fibers. (E) Enlarged linear regions of the stress-strain curves by fiber number where Young's modulus values were selected. (F) Stress-strain curves by fiber number at a 5 mm/min. strain rate (individual representative samples).

Figure 5

(A) Young's modulus (MPa) by fiber number at strain rates of 0.5 mm/min. (blue diamond), 5 mm/min. (orange square), and 50 mm/min. (gray triangle). (B) Ultimate tensile strength (MPa) by fiber number at strain rates of 0.5 mm/min. (blue diamond), 5 mm/min. (orange square), and 50 mm/min. (gray triangle). (C) Toughness (J/N³) by fiber number at strain rates of 0.5 mm/min. (blue diamond), 5 mm/min. (orange square), and 50 mm/min. (gray triangle). (D) Ultimate tensile strain by fiber number at strain rates of 0.5 mm/min. (blue diamond), 5 mm/min. (orange square), and 50 mm/min. (gray triangle).