Shape-Tunable Polymer Nanofibrillar Structures by Oblique Electron Beam Irradiation

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Original entry: William Bonificio, AP 225, Fall 2009

Information

Shape-Tunable Polymer Nanofibrillar Structures by Oblique Electron Beam Irradiation Tae-il Kim, Changhyun Pang, Kahp Y. Suh Langmuir 2009 25 (16), 8879-8882

Soft matter keywords

Shape tuning, nanofibrils, adhesion strength, velcro

Summary

The researchers in this experiment were able to tune the shape of polymer nanofibrils using a very accessible and easy method. First they produced a polyurethane acrylate (PUA) nanofibril structure. Then, by bombarding the polymer with electrons at a shallow angle, the nanofibril structure begane bending over forming small hooks. The researchers forsee applications for adhesion as the stuctures resemble gecko setae, or 'nanovelcro'.

Soft matter discussion

The production of a the nanofibril structures beginning with a silicon master. As shown, the area of surface of the nanofibril that is bombarded with electrons effects the overall final structure of either stooped fibrils and crispated fibrils.
The top two photos show the silicon template used to create the PUA nanofibrils. Below that is finished PUA nanofibrils. Below that show the two shape tuned nanofibrils, the left is the stooped fibril structure, while the right is the crispated fibril structure.


There have been a few attempts carried out in the last few years to tunably shape polymeric nanofibril structures - including here at Harvard. In fact, Aizenberg has used hydrogels to control microscale features. The researchers on this project attempt to control nanoscale features by bombarding the polymer with electrons from a standard E-beam.

First, a vertical array of polyurethane acrylate (PUA) fibrils were produced using standard replica molding techniques using a silicon master. The exact size of the nanofibrils is 100nm diameter, and 1um depth. Then field emission electron spectroscopy with varying tilting angles, voltages, and exposure times was used to irradiate the polymer matrix. The most successful results occurred when using a voltage of 20.0kV, and 5s exposure.

The electron radiation gave rise to asymmetric shrinking of the polymers that resulted from the decomposition of the CO bond. Two types of structures were created, stooped, which shows less bend, at relatively high tilting angles, and crispated, which show more hook like, at relatively low tilting angles.

The area of nanofibril that is irradiated is given by the formula below

<math>h_{exposed}=s\cot\theta</math>

where <math>h_{exposed}</math> is the expsosed height to radiation of the nanofibril, s is the space width between neighboring fibrils, and <math>\theta</math> is the incident angle of the elctron beam.

Finally, the researchers looked into the adhesion properties of the nanofibril structure. The result was an adhesion of 3,5,9.5, and 5.5 <math>Ncm^{-2}</math> for flat, vertical, stooped and crispated nanofibrils respectively.