A strategic goal of the U.S. Air Force is to be able to deliver munitions to targets anywhere around the globe in less than an hour. This will require very high speeds and novel lightweight and temperature-resistant materials. Nanocomposites are promising emerging materials for structural and functional applications due to unique properties of their nanoscale constituents. However, the currently available nanocomposites based mostly on nanoparticles lack the high strength and stiffness required for structural applications.

The goals of this research were to establish feasibility of manufacturing and evaluate performance of novel continuous polyimide nanofibers and their nanocomposites. The main objectives were to demonstrate feasibility of fabrication of continuous nanofibers from a range of specially synthesized soluble polyimides, characterize their mechanical behavior and properties, and fabricate and characterize polyimide nanofiber-reinforced composites.

A new class of nanoscale reinforcement, i.e. continuous polyimide nanofibers, was explored and developed for the first time. Continuous nanofibers were produced from a range of specially designed and synthesized polyimides (PIs).

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property tradeoff, one can expect it to decrease. By analyzing long (5-10 mm) individual polyacrilonitrile (PAN) nanofibers, it was shown that nanofiber toughness also dramatically improved. Reduction of fiber diameter from 2.8 micrometers to ~100 nanometers resulted in simultaneous increases in elastic modulus from 0.36-48 GPa, true strength from 15-1750 MPa, and toughness from 0.25-605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nanometers. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultra-high ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets.

Several families of soluble polyimides suitable for electrospinning were produced. The focus was on chemical control of solubility that is essential for control of both PI synthesis and subsequent electrospinning of continuous nanofibers from solutions. Control of nanofiber structure formation during electrospinning via altering liquid crystalline state of the solution was also addressed.

Modeling-based approaches to control nanofiber diameter, deposition, and alignment were utilized. Samples of individual nanofilaments and aligned sheets of nanofibers were fabricated. By modifying the conditions of electrospinning, nanofiber diameter could be modulated in a very broad range. New insights into the jet motion and polymer structure formation in the presence of solvent evaporation allowed further refinement of the process. In addition, this new coupled 3D continuum model of the process provided better understanding of macromolecular orientation within the nanofibers.

Variations of the measured strength, modulus, strain at failure, and toughness with diameter of individual as-spun PI nanofibers were plotted and analyzed. The results showed extraordinary increases in strength and modulus as nanofiber diameter decreased. The highest strength and modulus values measured were on par with strengths and moduli of commercial carbon fibers. Such high values of modulus and strength in polymers are usually achieved at the expense of strain at failure. Remarkably, the high strength of the ultrafine PI nanofibers was achieved without statistically noticeable reduction of their failure strain. These unique simultaneous increases in modulus, strength, and strain at failure led to a dramatic increase of toughness. The highest recorded toughness was an order of magnitude higher than toughness of the best existing advanced fibers, and exceeded toughness of spider silk.

This work was done by Yuris Dzenis of the University of Nebraska for the Air Force Office of Scientific Research. AFRL-0239

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2015 issue of Aerospace & Defense Technology Magazine.

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