Super-Light Plastic As Strong as Steel

Roland Piquepaille writes "A new composite plastic built layer by layer has been created by engineers at the University of Michigan. This plastic is as strong as steel. It has been built the same way as mother-of-pearl, and shows similar strength. Interestingly, this 300-layer plastic has been built with 'strong' nanosheets of clay and a 'fragile' polymer called polyvinyl alcohol (PVA), commonly used in paints and glue, which acts as 'Velcro' to envelop the nanoparticles. This new plastic could soon be used to design light but strong armors for soldiers or police officers. The researchers also think this material could be used in biomedical sensors and unmanned aircraft."

Comments

Link with pics

[Spy der Mann]

http://www.dailytech.com/Transparent+Plastic+Polymer+is+Strong+as+Steel/article9181.htm

When i saw the title i imagined something more like bulletproof glass, but, as you can see, it's pretty thin.

Re:Link with pics

[kebes]

The technique they are describing is called "Electrostatic Layer-by-Layer Deposition", and the resultant materials are called polyelectrolyte multilayers. Basically you dip a substrate alternately into baths of different polymers, with each step depositing a thin layer of polymer. These materials have been studied for the last decade or so. This group is investigating layering one polyelectrolyte with strong clay platelets (rather than using two polyelectrolytes). Thus they create a "brick and mortar" assembly, where strong (nano-sized) clay platelets are glued together with flexible polymer layers.

The process is good for creating very thin layers, but as you can imagine it's very slow for making thick materials. Each deposition step only adds on the order of a nanometer of material. Hundreds of steps are needed to create films thick enough to actually pick up, bend, and perform mechanical testing.

However some researchers have already investigated switching from the laborious "sequential dipping" technique to a "roll-to-roll" technique. So, instead of dipping a glass slide (or whatever) into vats of liquid one after the other (each time adding a very thin layer), the idea would be to use roll-to-roll technology (like in printing presses) to dip huge sheets of material through various vats at high speed. It's been shown to work (with some difficulties along the way, of course)... so in principle if these materials become sought, there are ways of making them in greater quantities, and thicker than this lab demonstration suggests.

Another unique thing about this "layer-by-layer" method of creating materials is that you can inherently control the composition of the material across the thickness. So you can actually have, for instance, the material's elastic modulus (or dielectric properties, or whatever), vary though the thickness of the material. Maybe you want a sheet of "plexiglass" that is super-strong at its core, but rather soft and rubberlike in its outer layer (so it doesn't hurt when you bang your head against it? Or maybe you want a liquid-like 'healing layer' on the outside to fill in scratches?). This depth-control of the material properties could be quite interesting for many applications where you want a mix of properties.

(Disclosure: Part of my Ph.D. thesis work involved related layer-by-layer materials.)

Re:Link with pics

[Moodie-1]

Could you be referring to xerography? This is the process that photostat machines use and where Xerox got its name.

PVA...

[Alceste]

Dissolves pretty readily in water. I wonder how this is stabilized.

Re:PVA...

[kebes]

It turns out that these kind of materials are not water-soluble, even though both components are, and even though you can easily assemble them from water. It's certainly counter-intuitive, but the assemblies involve electrostatic (charge-charge) links and hydrogen-bonding (like in DNA) links. Even though those kinds of links are inherently water soluble, when you are layering "large" molecules (polymers and nano-platelets count as large in chemistry), then there are so many "sticker groups" that the overall binding is very strong. (There are other more subtle effects, like the entropy of assembly, also at play.) As a result, these materials don't readily dissolve in water.

In the actual scientific paper, they further explain how they "cross-link" the material to make it more stable. Cross-linking is basically chemistry that generates strong covalent bonds between the various molecules. (This is what happens when you make a strong rubber...) They do indeed indicate that the cross-linked materials are more stable against changes in humidity (the un-crosslinked materials swell a bit when exposed to a humid atmosphere; which might be bad for some applications).

Re:HEFTY Eat Your Heart Out!

[kebes]

The dipping procedure is fairly easy to automate, but the technique only adds a very thin layer (think nanometers) for each dipping cycle. The usage of clay platelets in this present work does make the films thicker, but still their 300 layer film is only ~300 microns thick. So it takes awhile to build up enough layers for it to be macroscopically thick and strong. To speed it up, you can use a roll-to-roll process as long as you're trying to create large 'sheets' of material.

I imagine you could produce some pretty interesting seamless objects with this... just smash it on the ground when you're done and shake the broken glass out.

Indeed! You've hit upon one of the main "selling points" of this technique: unlike other coating techniques, it isn't limited to flat surfaces. In fact, you can even coat the insides of objects. For example you can coat the insides of thin capillaries by alternately flowing the two solutions through the capillary. Some companies were also checking whether you could prevent fouling/rusting of pipes by coating their insides with material: coating even huge lengths of pipes becomes easier when all you have to do is flow some solutions through them. (You can even 'fix' a pipe already installed by taking it offline and performing this operation every so once in awhile...)

The ability to coat strange shapes may indeed allow for some neat tricks. Also note that coating glass is easiest, but actually you can layer onto all kinds of surfaces (all that's needed is a bit of surface charge). So you can imagine a sacrificial mold (something that you can burn away at low temperature or dissolve with some other solvent) that you them multilayer to create, as you say, a seamless object of controllable properties.

This looks like something fun to try out.

It's a remarkly simple technique to use. All you need is some water-soluble polymers, a glass microscope slide, and a few beakers! Of course, unless you're really patient (or have a robot or auto-dipper) it takes awhile to get a really thick film!

(Disclosure: Part of my thesis work was on these layer-by-layer materials.)

Re:Strong as Steel?

[SoapDish]

Judging from the description of the "Velcro effect" I'd wager they're talking about ultimate strength. And even then, they may be talking about specific strength, so it could actually require a much larger geometry to achive the same strength as steel.

And yes, yeild strength and ultimate strength are very different quantities when it comes to design (for those that don't know).

The layered construction makes it sound like the material's not isomorphic, and I bet there are different compression and tensile characteristics. Plus, it might not have good high temperature characteristics. Isn't PVA a thermoplastic?

So, of course there will be a lot more research required.

Plus, it's a composite, not a plastic.

Re:I'd been hoping we could get away from plastic

[jtroutman]

This is not the plastic you're thinking of. It's layers of montmorillonite clay, which is naturally occuring (Hydrated Sodium Calcium Aluminum Magnesium Silicate Hydroxide) and polyvinyl alcohol (the glue). Polyvinyl alcohol is derived from vinyl acetate, which in turn is made with ethylene and acetic acid with oxygen and a palladium catalyst. Petroleum is not necessary in any of these steps.

What's important to consider, though, is not what this is currently made from, but that it is a test bed for other materials. Imagine if, instead of using the montmorillonite clay, they used bucky tubes...what about a stronger polymer? This is a proof of concept, not the be-all and end-all application.

Re:Strong as Steel?

[kebes]

If you're interested in the details (and have a subscription to Science), here's the actual paper:
Paul Podsiadlo, Amit K. Kaushik, Ellen M. Arruda, Anthony M. Waas, Bong Sup Shim, Jiadi Xu, Himabindu Nandivada, Benjamin G. Pumplin, Joerg Lahann, Ayyalusamy Ramamoorthy, and Nicholas A. Kotov "Ultrastrong and Stiff Layered Polymer Nanocomposites" Science 5 October 2007: 80-83. DOI: 10.1126/science.1143176.
Blurb:

Deposition of alternating nanoscale layers of clay particles and a polymer yields a transparent composite that is as stiff and strong as steel.

The abstract is:

Nanoscale building blocks are individually exceptionally strong because they are close to ideal, defect-free materials. It is, however, difficult to retain the ideal properties in macroscale composites. Bottom-up assembly of a clay/polymer nanocomposite allowed for the preparation of a homogeneous, optically transparent material with planar orientation of the alumosilicate nanosheets. The stiffness and tensile strength of these multilayer composites are one order of magnitude greater than those of analogous nanocomposites at a processing temperature that is much lower than those of ceramic or polymer materials with similar characteristics. A high level of ordering of the nanoscale building blocks, combined with dense covalent and hydrogen bonding and stiffening of the polymer chains, leads to highly effective load transfer between nanosheets and the polymer.

In response to your questions about actual material response, the paper discusses a variety of metrics for a variety of different preparation conditions. They report that the nano-composite material has an ultimate tensile strength 10 times greater than the pure PVA polymer, up to 480 MPa. They also state that the modulus, E, was 100 times greater than the pure polymer, up to 125 GPa, which they compare to Kevlar (E ~ 80 to 220 GPa).

In terms of energy absorption, they compare the uncrosslinked nano-composite to the crosslinked one. As you might imagine, the crosslinked one was more rigid (and gave rise to the modulus previously mentioned), having a low ultimate strain of 0.33 %. The uncrosslinked one deformed somewhat more (ultimate strain 0.7%), with higher energy absorption potential.

As you note, the comparison of "strong as steel" is not very helpful. But looking at the stress-strain curves, these materials look quite strong. Also, since you can adjust the material properties (optimizing for energy storage versus elastic modulus), they might be great for achieving desired performance for certain niche applications.