[MUSIC] So this lecture is about polymers and fibres. Before we can understand how polymers and fibres can be used in forensic science, we first need to know a bit about the chemistry of polymers and fibres. So what is a polymer? Well, if you take a small molecule and then you link it to another one of the same small molecule, and then to another one, and then to another one, and then to another one and so on, you end up with a long chain molecule made up of those original single molecules linked together. So, those original single molecules are known as the monomer and this long chain molecule is known as the polymer. Poly of course comes from the Greek for many. Now, not all polymers consist of same monomer over and over and over again. We can mix up different monomers. For instance, if you have two different monomers, then when you link them up to make a polymer, you can get what we call a co-polymer, where the two have been combined to make a single long, long molecule. The main thing to understand is that a polymer is made up of long chains of these individual monomers. So here's an example of a very common polymer that we all encounter everyday in our lives, and that's polyethylene, which is better known as polythene. So polythene is the polymer derived from the gas ethylene, so every year millions of tons of ethylene are polymerized to make polythene for all sorts of uses including of course, things like shopping bags. We're also familiar with polymers like polypropylene and polystyrene. Polystyrene, of course, is the material that's used to make styrofoam packaging, and these are polymers from the corresponding monomers, propylene and styrene, and its done in a quite similar way to the polymerization of ethylene. Now polyethylene, polypropylene, polystyrene are widely used to make all sorts of things but they're not actually good for making fibres, so people don't make fibres out of these particular polymers. Here's a polymer that is excellent for making fibres. It's nylon. And nylon is a co-polymer, where you take two quite different compounds and induce them to form a chain. So nylon-6,6, there are different forms of nylon, but nylon-6,6 comes from adipic acid polymerised with diaminohexane, and if you count up the atoms you'll realize that molecules of water are being produced as a by-product. It's nylon-6,6 because adipic acid has six carbon atoms and diaminohexane has six carbon atoms. And you can see it forms a polymer where you have alternating adipic acid and diaminohexane units. And nylon, as I mentioned, is an excellent polymer for making fibres and probably at this very moment, all of us are wearing something containing some nylon. So, suppose we have a fibre found at a crime scene. What could you do with that fibre to identify it? Well, the simplest thing you can do is just to look at it under the microscope, and this may help you determine the type of fibre. For instance, whether it's wool or nylon or cotton or some other. Now because we can consider fibres as organic chemicals, we can also use on them the spectroscopic techniques that we studied in an earlier lecture. So you will use spectroscopy on the fibres to determine their chemical composition. Microscopy, you can also look at physical features such as the shape of a fibre. Now, some fibres, specifically the synthetic fibres, the shape is characteristic of the manufacturing, which we'll talk about in a moment. Now, most fibres are going to be colourless. Most synthetic fibres will originally be colourless, but we like to have coloured fibres in our clothes, so a dye has to be added to the synthetic fibres to give them those colours. So, not only can we do spectroscopic analysis to determine the chemical nature of the fibre, we can also do spectroscopic analysis to determine the chemical nature of the dye, and this, of course, will be another step towards individualization of that fibre. The manufacturers also add other chemicals to the fibres to make them attractive for use in clothing. For instance, one kind of chemical that's added are called delustrants, and these are to make the fibres less shiny. So again, we can use chemical spectroscopic methods to identify the delustrant. And the more different chemical components of the fibre that we can analyze, then the better we can get towards individualization of that fibre. Obviously there's going to be a difference between natural fibres and synthetic fibres. Natural fibres coming from plants and animals; synthetic fibres coming from the chemical industry. So, let's take a quick look at making synthetic polymers and synthetic fibres. Synthetic polymer fibres are typically made by a process called extrusion. So on one side of a barrier, there is a polymer either in it's molten state, or in solution, so it's a liquid form. The barrier is permeable, there are holes in it. So when the liquid form of the polymer is forced through the barrier into a new medium where it solidifies, as it goes through these holes, it will form fibres, and this is called extrusion. And synthetic fibres are typically made by extrusion processes. So clearly, the shape of the fibre and the size of the fibre are going to depend on the hole that the material was forced through when the fibre was formed. This will vary from manufacturer to manufacturer. So if you have a synthetic fibre, you can look at it under the microscope and the shape of the fibre will help you identify which manufacturer, and perhaps, which batch of material when it was made. I mentioned that we can also identify the chemicals that make up the fibres using spectroscopic methods, and a very good method for doing this is from the infrared spectrum. Now, the normal infrared spectrum is recorded on a fairly large amount of material, but a fibre found at a crime scene would be just a single fibre, so we have to use what is called a IR microscope. And this combines a microscope, that is so you can see the fibre, and the IR spectrometer. And this combination of the IR microscope allows the analysis of the chemical constituents of the fibre. So this is the infrared spectrometer that I use for lecture demonstrations. Most infrared spectrometers are very sensitive machines and you can't move them, but this one's very robust and I can pick it up and carry it across campus and use it for lecture demonstrations. It's not the kind of machine that a forensic scientist would use for fibre analysis, and that's because it's just an ordinary infrared. It's not an IR microscope. So we're not going to be using the tiny little fibre samples that might be collected at the crime scene. We're going to be using some slightly bigger pieces of string. So I have two samples here. One of these is genuine wool, one of these is synthetic wool, and an ordinary person just by inspection really wouldn't be sure which one was which. But we're going to use the infrared spectrometer to prove the identity of these two. This machine here works by reflectance. The IR beam is generated right here, and it reflects off this little window right here. And so what we're going to do is press the sample against that little window. So, this is the IR spectrum from some of that first sample and an important band to look at is this one here. We can see a strong absorbance at 1624 wavenumbers and we know that this kind of absorbance is characteristic of the stretching of the carbon oxygen bond in a protein. Now wool, of course, is a protein. Wool is made of protein. We can conclude that this sample is genuine wool. Now let's analyze the second sample. So if we look at the IR spectrum of the second sample, which is the green trace on the screen, we can see there's a signal at that absorbance, but it's shifted. We can now see that the carbon oxygen stretching absorbance is coming at 1729. This is not where you expect to see this absorbance when you're dealing with a protein. So clearly, this second sample is not real wool and it probably is polyester. And if we look a little bit further at the spectrum, we can see there's another band just here at 2241, and what this is telling us is that this fibre sample is probably some kind of blend of different polymers, and this second absorbance here at 2241 is probably a nitrile polymer. Our infrared spectrometer is not limited to analyzing fibres. We can analyze just about anything that we can put into that sample holder. So here, I have two dollars, two Singapore dollars. Now, money is normally made of paper but this is not, this is a polymer note and we can analyze this using infrared. This is the spectrum of the Singapore $2 note, and you can see that it shows an absorbance at 1714 wavenumbers, which is quite similar to the polyester. So these bank notes are not made of paper, they're made of polymer, and it's telling us that at least one of the polymers in this bank note is quite similar to that second fibre sample. So this is the Singapore $50 note. So let's use infrared spectroscopy to find out what this is made of. So if I put the $50 note, so the red trace is the spectrum of the $50 note and you can see that there's almost no absorbance in the region about 1700 wavenumbers, but there's a big absorbance in the region of about 3200 to 3300 wavenumbers. This is because our $50 is still made of paper. Paper is made of cellulose, it doesn't have any C double bond O in its molecule, so there's no absorbance in the same area where we saw the polyester absorbing. But cellulose has lots of hydroxy groups with O bonds, and these absorb in that region above 3000 wavenumbers, and that's what's responsible for this absorbance that's indicated at about 3200. So using the infrared spectroscopy, we can very quickly tell what these different things are all made of. Here's one example of a case where the IR spectrum of a fibre prove to be very important. And this is the case of a U.S. Army officer called Jeffrey MacDonald who was stationed at Fort Bragg, North Carolina. And he was convicted of murdering his wife and daughters. And they were killed by being beaten and stabbed in the night, in their own home. Now, MacDonald's defense lawyers came up with a defense that the murders were not done by MacDonald himself but they were done by a group of drug addicts who had broken into the house. Problem was, at the original trial, there was no evidence that anyone else other than the family had been in the house. So the defense here is trying to raise reasonable doubt, but without evidence that there were other intruders, this was not considered to be reasonable and MacDonald was found guilty. However, the defense did find additional evidence which they used at the appeal. And that evidence was a fibre from the crime scene which the defense said did not match any of the clothes of the victims, and therefore was evidence of an intruder. Well, the way to confirm that this fibre did not match any of the clothes was by doing IR spectroscopy of all the clothes. This was done by the F.B.I., and here is one result. The lower trace shows the infrared absorbance spectrum of this fibre which the defense is claiming is from the real attackers. The upper trace is the infrared spectrum of fibres in a fall belonging to one of the victims, Mrs. MacDonald. And you can see that the two traces matched very closely, so this is not evidence of any intruders, because it could be just from the item of clothing belonging to Mrs. MacDonald. The appeal was not successful.
