Novel nanocomposites made of confined polymers

IBM researchers, with collaborators at Stanford University, have combined their expertise in porous materials for microprocessors and thin film mechanical properties to study the fundamental properties of polymers under confinement.

Their challenge was to fill tiny holes called nanopores (1/10,000 the size of a human hair) with long and bulky molecules. The equivalent of fitting a 300 passenger commercial plane into your car garage. While the latter is physically impossible, polymers can deform to adapt to such a significant form factor change. During this process, their intrinsic properties are dramatically modified – which is precisely what our researchers were looking for. But the results weren’t what they expected.

Hybrid nanocomposite art
by students at Stanford University
In their normal state (not confined), polymers' mechanical properties mainly depend upon the number of entanglements (i.e, the number of knots). When you pull on one chain, you pull on many knots such that a tremendous amount of energy has to be dissipated before reaching a breaking point. This is what give polymers their physical strength: from your plastic cup to your suitcase.

Think about trying to pull one noodle from a full plate of cooked pasta. It is almost impossible to pull only one from the bunch. But if you could force all the noodles into a tiny espresso cup, the knots would magically disappear because it is the only way for the entangled pasta to accommodate such a small space. Similarly in polymer physics, polymers' mechanical properties such as resistance to fracture, decrease under confinement due to the loss of entanglements.

For the resulting nanocomposite materials, it was then predicted that their mechanical properties would follow the same trend. Surprisingly, the exact opposite is what IBM and Stanford researchers uncovered. The polymers’ resistance to fracture increased with decreasing entanglements. The results showed that five times more energy was required to break these nanocomposites as initially anticipated.

The discovery has significant implications for the microelectronic industry as microprocessors continue to shrink, and porous materials are widely used in devices such as smartphones, and tablets due to their prominent insulating properties. Its increased strength could help prevent mechanical failures of the next generation of microprocessors when more porous materials, better insulators but more fragile materials, are manufactured. This strategy is currently under development at our Albany research and development center for advanced microelectronics.

Read more about this breakthrough in the Nature Materials article Fundamental limits of material toughening in molecularly confined polymers. 

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