Polymer Science Seminar
When I was walking down the hall of the 2nd floor of Olney Hall today, I had some thoughts about this excursion. I thought all of it would be over my head and that I would understand none of it at all. Most other excursions were such that I got something I totally didn’t expect. This was, however, exactly what I expected. It was some professor from UMass Amherst who had come to talk to physics students about polymers. I had no idea what he was talking about. It may have been meant for older students, but certainly not for me. Now, I’ll try to do my best to explain what he said. I may be wrong in my transmittal of the message. Don’t blame me for getting it wrong.
He started by introducing himself as Professor Thomas Russell, Professor of Polymer Science at UMass Amherst. The first topic he described was a particular polymer with a silicon substrate, which is part hydrophobic and part hydrophilic. The hydrophobic part, then, would have to follow the hydrophilic part around because they are attached. This also means that anything that attaches to the hydrophobic part must also be hydrophobic, and anything that attaches to the hydrophilic part must be hydrophilic. In this sense the polymer comes to be two separate layers, all the way up to the surface. At the surface, the hydrophobic area has more surface area than the hydrophilic. He had a lot of faculty participation in this demonstration, with him being the hydrophobic part and another faculty member the hydrophilic part. He comments that if you want a polymer to leave the lab, it must be cheap, simple, and must be able to blend with existing technology. He says that because of the simple fact that the hydrophilic part likes the hydrophilic silicon oxide substrate more than the hydrophobic part does, the molecules of the film will orient themselves parallel to the substrate surface. This is because all the hydrophilic parts that attach to a given molecule also do not like the hydrophobic part, and all the hydrophobic parts that attach to a given molecule also do not like the hydrophilic part. Therefore, the film will separate into layers.
He then continued with this example. He said that by subjecting this film to UV radiation, the film changes into a new film. But, there is a problem here. No matter what angle the film is rotated at, the same problem of self-assembly occurs, in that a smooth film is formed. In cases where control of well-organized structures over a large scale is required, self-assembly by itself is no longer enough. In this case, a directed self-assembly is needed, where each nanoscopic element has a defined location. To avoid self-assembly, they took a miscut sapphire substrate, and heated both the film and the sapphire substrate to 1100-1200 degrees Celsius, where the film is unstable. At this high temperature, the film reforms into a stable structure consisting of multiple stable crystal planes that creates a non-smooth film. If I had more knowledge on the basics of how polymer science works, I may have better appreciated this information. Apparently, by subjecting this film to further treatment, these planes will all go the same direction. But isn’t a plane supposed to be directionless (unless of course direction refers to angle)?
At this point, I would like to comment on my thoughts. I have been wondering about why orientation of nanostructures even matters to consumers and ordinary people. After all, they cannot see the nanostructures, so how does the nanostructure matter? The answer is that nanostructured materials are mostly used for scanners or sensors, which will be eventually wired with electronics on the nanoscopic level. As such, nanostructure does matter when everything is on a nanoscopic level.
He then talked about the alternatives to crystallization (This is using a miscut sapphire to avoid self-assembly). Instead of using a miscut sapphire, what Professor Russell did was to first use spin coating on the substrate, coating the substrate with a thin layer of the film. He then subjected it to solvent annealing for 48 hours in a benzene vapor under controlled humidity. This made the ordering in a 2x2 µm2 area nearly perfect. A picture is shown below.
He started by introducing himself as Professor Thomas Russell, Professor of Polymer Science at UMass Amherst. The first topic he described was a particular polymer with a silicon substrate, which is part hydrophobic and part hydrophilic. The hydrophobic part, then, would have to follow the hydrophilic part around because they are attached. This also means that anything that attaches to the hydrophobic part must also be hydrophobic, and anything that attaches to the hydrophilic part must be hydrophilic. In this sense the polymer comes to be two separate layers, all the way up to the surface. At the surface, the hydrophobic area has more surface area than the hydrophilic. He had a lot of faculty participation in this demonstration, with him being the hydrophobic part and another faculty member the hydrophilic part. He comments that if you want a polymer to leave the lab, it must be cheap, simple, and must be able to blend with existing technology. He says that because of the simple fact that the hydrophilic part likes the hydrophilic silicon oxide substrate more than the hydrophobic part does, the molecules of the film will orient themselves parallel to the substrate surface. This is because all the hydrophilic parts that attach to a given molecule also do not like the hydrophobic part, and all the hydrophobic parts that attach to a given molecule also do not like the hydrophilic part. Therefore, the film will separate into layers.
He then continued with this example. He said that by subjecting this film to UV radiation, the film changes into a new film. But, there is a problem here. No matter what angle the film is rotated at, the same problem of self-assembly occurs, in that a smooth film is formed. In cases where control of well-organized structures over a large scale is required, self-assembly by itself is no longer enough. In this case, a directed self-assembly is needed, where each nanoscopic element has a defined location. To avoid self-assembly, they took a miscut sapphire substrate, and heated both the film and the sapphire substrate to 1100-1200 degrees Celsius, where the film is unstable. At this high temperature, the film reforms into a stable structure consisting of multiple stable crystal planes that creates a non-smooth film. If I had more knowledge on the basics of how polymer science works, I may have better appreciated this information. Apparently, by subjecting this film to further treatment, these planes will all go the same direction. But isn’t a plane supposed to be directionless (unless of course direction refers to angle)?
At this point, I would like to comment on my thoughts. I have been wondering about why orientation of nanostructures even matters to consumers and ordinary people. After all, they cannot see the nanostructures, so how does the nanostructure matter? The answer is that nanostructured materials are mostly used for scanners or sensors, which will be eventually wired with electronics on the nanoscopic level. As such, nanostructure does matter when everything is on a nanoscopic level.
He then talked about the alternatives to crystallization (This is using a miscut sapphire to avoid self-assembly). Instead of using a miscut sapphire, what Professor Russell did was to first use spin coating on the substrate, coating the substrate with a thin layer of the film. He then subjected it to solvent annealing for 48 hours in a benzene vapor under controlled humidity. This made the ordering in a 2x2 µm2 area nearly perfect. A picture is shown below.
Furthermore, by using a patterned substrate, such as parallel lines carved onto an oxide layer with features of microscopic size, the lateral order of the film can be improved even further.
The next question that could be asked is what this film is made of. It is made of block copolymers, a class of self-assembling materials. The research that Professor Russell did was on a particular block copolymer called PS-b-PEO, an acronym for polystyrene-block-poly (ethylene oxide). An SFM image of PS-b-PEO on a topographically patterned surface is shown below.
The next question that could be asked is what this film is made of. It is made of block copolymers, a class of self-assembling materials. The research that Professor Russell did was on a particular block copolymer called PS-b-PEO, an acronym for polystyrene-block-poly (ethylene oxide). An SFM image of PS-b-PEO on a topographically patterned surface is shown below.
In short, when I learned of this seminar, I was only told that it was a Polymer Science seminar. I was not told anything about what it was about. During the seminar, though a lot of the higher-level stuff flew over my head, I was able to get a basic understanding of what nanostructured polymers are, what they are used for, and some techniques used to make them. Even with the roadblock though, the seminar was somewhat amusing, and therefore enjoyable, as Professor Russell made us all laugh at his jokes.