|
|
|
|
|
|
|
|
|
|
Strong, Tough Materials Found in Nature and Mechanical, Chemical & Biological Engineering Methods Employed to Emulate these Materials |
|
|
|
Deformation Upon Heating,
with Nitin Kumar and Gareth McKinley |
|
To demonstrate the thermomechanical integrity of these nanocomposites, two polyurethane films containing 0 and 20 wt% Laponite were held in improvised grips under a constant load equal to 1.08 N (initial stress of 1.75 MPa) and heated from 45°C to 125°C at ~1°C/min. Photographs were captured as the experimental set-up underwent heating in an oven with a transparent window. The pure polyurethane (left) and 20 wt% Laponite reinforced polyurethane (right) films were colored black for clarity. The pure polyurethane exhibits little resistance to heat distortion, deforming significantly with increase in temperature, reaching the maximum allowable extension at 95°C, and finally breaking at 120°C. The nanocomposite containing 20 wt% Laponite resists heat distortion and does not show visually any deformation upon heating; however, at 125°C the grips slipped from the sample. Initial sample length: 46 mm. Click on the picture for the video (17.5MB). |
|
Recovery of Deformation Upon Heating a Shape Memory Polyurethane Nanocomposite,
with Gareth McKinley |
|
Shape memory is the ability of a material upon deformation to maintain the deformed shape, but upon external stimulus to recover the original, un-deformed shape. The thermo-responsive shape memory polyurethane shown reinforced with 4 wt% Laponite was stretched 100% at room temperature. This shape was maintained even when a 0.14 N load (~0.25 MPa) was applied to its end. However, upon application of heat with a hot air gun to a temperature above 55°C the polyurethane recovers its original, un-stretched shape. Click on the picture for the video (6.1MB). |
|
Slow Solution Casting Set-up,
with Nitin Kumar and Gareth McKinley |
The picture on the left shows a teflon casting dish filled with red dye (4 cm x 4 cm x 6 cm inner dimensions). This dish is filled with a polyurethane/nanoparticle/solvent solution and then placed in an oven (middle image). The oven has two tubes connected to it (top left). One tube carries the entering nitrogen purge flow while the other tube carries the exiting nitrogen and solvent vapor. This set-up which includes a nitrogen flow-meter allows the annealing temperature of the material and the solvent evaporation rate to be controlled. After complete solvent removal the resultant polyurethane nanocomposite film (~0.1 mm thick) looks like that shown in the picture on the right: smooth, continuous and easily cut into strips that can be thermally and mechanically characterized. The physical properties of the films were observed to depend on the solvent evaporation rate. The mechanical properties of films prepared at higher evaporation rates repeatedly proved to be inferior. Consequently, a constant slow evaporation rate is maintained for all samples. |
|
Resin Spinning Set-up,
with Nitin Kumar and Gareth McKinley |
|
A liquid mixture of polymer, resin, and solvent are forced through the small tip of a syringe. The ejected fiber is then attached to a spool and wound up. By changing the distance between the spool and the syringe tip as well as the flow rate of the mixture and the angular velocity of the spool, the spinnibility of a mixture and the fiber's resultant mechanical properties can be changed. The shear along the syringe tip, as well as the stretching of the ejected fiber, and the evaporation of solvent when exposed to ambient air aids in molecular orientation. Click on the picture for the video (7.5MB). |
|
Continuous Spinning of a Fiber,
with Nitin Kumar and Gareth McKinley |
|
Conventional "wet-spinning" of silk relies upon low viscosity solvents, high strain rates, and "quenches" to capture molecular orientation. Resin-spinning,
however, uses volatile solvent pairs and high extensional viscosity liquids to create an oriented fiber by retarding resin dynamics and increasing relaxation time.
Ultimately, via resin spinning our group is able to spin one continuous fiber of uniform diameter for a significant length of time. The movie, capturing
just a snippet of spinning time, shows the mixture exiting the syringe tip with good uniformity.
Click on the picture for the video (1.8MB).
|
|
Resultant Spun Fiber,
with Nitin Kumar and Gareth McKinley |
The picture on the left shows one continuous fiber, approximately 8µm in diameter, wound around a large sewing spool, approximately 3cm in diameter and 5.5cm tall. The glossy horizontal sheen is all that the digital camera can capture of the single thread. The picture on the right shows a segment of a 24µm-fiber under cross-polarization. The fiber appears to be highly oriented, mimicking the structure of spider silk. |