The Fabrication and Uses of Nano-Polymers
By Pushpak Jha
This summer I worked with Professor Feng Hua and his graduate student, Mary, to develop an
ultra thin nano-polymer and a bend sensor. The polymer we mostly worked with was made of the two
chemicals PAH, poly(allylamine hydrochloride) and PAA, poly(aspartic acid). The reason we used these
two chemicals was because after fabricating the nano-polymer we could cross-link the bonds holding the
PAH and the PAA together, making the polymer more stable and durable. Another property of the
polymer is that it is made of a porous fiber network. After looking at the polymer under a scanning
electron microscope we saw that the structure contained many fibers bound together. Because of this
space, Professor Hua believed that the entire polymer could be compressed and then would return to a
normal state after the stress was removed. In essence the fibers act like springs allowing the polymer to be
stretched and compressed without sustaining damage.
The actual fabrication of the polymer is a tedious process. First we took a clean glass microscope
slide (usually we broke it into two equal pieces) and we would heat it on a hot plate. After heating it we
put it on a spin-coater and spin coated a ‘primer’ chemical onto one side of the glass. After that was done
we spin-coated a sacrificial material onto the same side, we used a negative photo resistive chemical.
After the spin-coater was finished we would heat the glass with the chemicals on it to cure the photo
resistive chemical and made it hard. In some cases we added multiple layers of the photo resistive
material to make a thicker sacrificial layer. After adding the sacrificial layers we grew the polymer itself.
Basically we would have two plastic bowls, one with PAH and one with PAA. The glass template would
first go into the PAH for 10 minutes and would then be cleaned. Then it would go into the PAA for 10
minutes and then be cleaned. After that it would go back into the PAH and so on and so forth. During the
summer we made many templates ranging from 10 to 40 such PAH/PAA bi-layers. The PAH and PAA
bond to each other during these steps by electrostatic bonds, because one is positive and the other is
negative. After growing the desired number of bi-layers we heated the template for about two hours to
cross-link the bonds holding the PAH and PAA together. This cross-linking process bonds the PAH and
PAA together chemically, which is much stronger than the electrostatic bonds. In the final step we would
dissolve the sacrificial material by putting the template into acetone. With the sacrificial layer dissolved
nothing held the polymer to the glass and we had a free-standing ultra thin PAH/PAA nano-polymer, see
Figure 1.
Figure 1. Free-standing polymer floating around.
Class: 2011
Major: Electrical Engineering
Advisor: Professor Feng Hua
Program: Honor’s
As is aforementioned, the polymer is made of a porous fiber network. After growing the polymer,
to make the fiber network we would put our template into an acidic solution with a pH of 2.4 for one
minute. However the surface would then be covered with holes and we had to reduce the size of these
holes by soaking the template in water usually overnight, this made the holes smaller than one micron.
The reason we wanted the holes to be smaller than one micron is because we started coating the polymer
in a clay solution. After growing our PAH/PAA bi-layers we grew two PODA [poly(octadecyl acrylate)]
/PSS[poly(styrenesulfonate)] bi-layers and then between 3 to 5 PODA/Clay bi-layers. The clay particles
are one micron in width but only 1 nano-meter thick. These clay particles cover up the holes so that when
we evaporate metal onto the polymer, the metal doesn’t go inside the fiber network see Figure 2.
Figure 2. (a) Shows the fiber network. (b) Close up of a hole after it has been shrunk. (c) Clay covering
the hole.
Most of my work this summer was on developing a bend sensor. This process was slightly
different and didn’t require a free-standing nano-polymer. To being the process we cut a piece of a
conductive ITO film which is basically a piece of plastic covered with a conductive material. We would
then heat it, spin-coat a primer and then spin-coat positive photo resistive material onto the side that had
the conductive material. We then covered it with a mask, exposed it to UV light, and put it in a developer.
After it was developed, the template was covered with the photo resistive material except in the middle
where our mask did not block the UV light. This effectively divided the template into two parts, one on
the right and one on the left. Also, the mask we made also had a small “channel” coming out of one side
but not touching the other. This channel is actually the bottom electrode of the bend sensor, see Figure 3.
Figure 3. The basic design of the bend sensor.
Class: 2011
Major: Electrical Engineering
Advisor: Professor Feng Hua
Program: Honor’s
Next we put the template for just a second or two into a strong acid to dissolve the conductive
material that was present on our ITO film. This acid only dissolved that material where the photo resistive
material was not covering the template. Next we put the whole template in acetone to get rid of the photo
resistive material still present so we could add a 2nd pattern. In the second pattern we once again spincoated photo resistive material and exposed it to UV light under a mask. After developing this, the
template was covered with photo resistive material everywhere except in a small rectangle that covered
part of our “channel” and extended towards the other side, but did not touch it, see Figure 4.
Figure 4. 1) Area for the bottom electrode to connect to wires. 2) Connects wires to the top electrode.
3) The “channel” which is actually the bottom electrode. 4) The rectangle didn’t show up in the picture,
but it has been drawn in here.
Once this was done we grew our nano-polymer layers onto this template. We grew 12 PAH/PAA
bi-layers, then acid treated it, and covered it with 3 layers of clay. In one of the last steps, we put the
template in a machine which vaporized a very thin (200 nm) layer of aluminum onto the surface of our
template. After this we put our template in acetone, which would dissolve all the photo resistive material
and most of that aluminum also came off. The only place the aluminum was left was on top of where we
made the rectangle in the 2nd patterning, see Figure 5.
Figure 5. 1) Area for the bottom electrode to connect to wires. 2) Connects wires to the top electrode.
3) The “channel” which actually forms the bottom electrode. 4) The only place metal is left after the first
vaporization, this metal layer forms the top electrode of our sensor.
Class: 2011
Major: Electrical Engineering
Advisor: Professor Feng Hua
Program: Honor’s
After this we did a 2nd vaporization to connect the metallic top electrode to the other side of the
template where we would more easily connect wires. Lastly we attached wires to both sides of our
template, and then our bend sensor was ready for testing, see Figure 6.
Figure 6. 1) The 2nd vaporization connected the recantangle to the other side of the template. 2) One of
the two wires that connects to the sensor.
The theory behind how the bend sensor works is also very interesting. The two electrodes, one
being the “channel” of the ITO film and the other being the vaporized metal, act like two sides of a
capacitor. These two sides are then insulated by the layers of the nano-polymer. According to Newtonian
physics, this means that no current can flow between these two electrodes since it’s insulated. However
because of a quantum mechanical effect called quantum tunneling, when the insulation material is made
very thin some current does flow. The thinner that insulation, the more current flows between the
electrodes. As you bend the sensor, the nano-polymer stretches and become thinner. Since it becomes
thinner more current flows between the electrodes and you can measure this change. The more you bend
it, the more you stretch the polymer, and the more the current changes. This is how our sensor “senses”
when it’s being bent.
The bend sensors we have made so far work fairly well, however the more sensitive we
can make them the better. Currently the bend sensors are less sensitive the less they are being bent, and
when being bent a lot they become very sensitive. For the rest of the summer we will primarily try to
optimize the bend sensors we have been making.
Class: 2011
Major: Electrical Engineering
Advisor: Professor Feng Hua
Program: Honor’s