Polymer Characterisation (Surface Tension) PDF

Polymer characterisation (Surface tension) Tue, 10/31 17:22PM · 11mins Transcript 00:00 Hi, today's lecture is about surface tension, a fascinating subject. I've put there a couple of videos that are very interesting videos, fascinating subject. I've put there a couple of videos that are very intere...

Polymer characterisation (Surface tension) Tue, 10/31 17:22PM · 11mins Transcript 00:00 Hi, today's lecture is about surface tension, a fascinating subject. I've put there a couple of videos that are very interesting videos, fascinating subject. I've put there a couple of videos that are very interesting videos for you to watch, and there are many of these on the internet, so feel free to search and find out more about this interesting property. 00:35 So the definition of surface tension is the tendons of liquid surfaces to shrink into the minimal surface area possible. Water is a very nice example of this phenomenon. If you see a droplet of water, why it takes into this shape here as it falls from a tap, for example. 00:56 The force, the attraction force between the molecules is something very interesting, particularly in relation to water, because the effect of hydrogen bonding, so the interactions between the negatively charged oxygen and the positively charged hydrogen forms what we call a network of forces that they will interact, the molecules will like each other, and the tendons is really to shrink into this minimal surface area possible due to those interactions. 01:27 So surface tension is represented by the symbol gamma, which is this letter here, and is measured in force per unit length. So it's normally Newton divided by meter. This is the SI unit. When we look at surface tension of liquids, for example, water, as I said before, due to those interactions, hydrogen interactions, water has a very high surface tension, a value of 72.8 millinewtons per meter at 20 degrees centigrade. 01:59 Here as well, the value of the surface tension is 72.8 millinewtons per meter at 20 degrees centigrade. temperature, temperature is quite important. Like for density, when we discussed in previous lectures, all these measurements are done at a room temperature around 20 degrees celsius. 02:11 If you compare water with other liquids, for example, these bare fluoropoly ethers, as the chemical structure here illustrates, these kind of compounds have a very, very low surface tension compared to water. 02:25 And they are quite interesting materials as well from the opposite reasons. They wet the surface so well that it's quite difficult to get rid of them, so you can only remove these kinds of compounds with similar compounds, so normally similar compounds like each other. 02:41 So they can only be removed from a surface with similar pure fluororeganic solvents in order to clean the surface. So that's very low surface tension, the materials tend to stick to the surfaces. Thank you. 02:56 This is just a table of values. I like to give you this comparative table to compare between the different compounds. So water is the top of the pile. This is in millinewtons, em is here, it's millinewtons per meter. 03:13 So going from almost 73 down to nearly 28 for a very hydroscopic compound, no sorry, a very hydrophobic, so this is non polar compound compared to water, which is very polar compound. So you see the difference in behavior, which is again linked with their chemical structure. 03:35 Powered by Notta.ai So remember what I said about the density before, always look at the chemical structure of a compound to give you some idea about their properties, their physical properties. How do you measure surface tension? 03:48 We have different machines. I've just illustrated here one of the tensiometers that we used to have in the lab. I think we still do, but the principle here is you have a tensiometer and a balance. So perhaps the next slide tells you a little bit in detail, this step -by -step of this kind of measurement. 04:07 Basically you have a platinum wire. This is just a ring surface, a ring shape wire that you dip into the solution. This is a petri dish with a liquid that you want to measure the surface tension. And it's the surface tension in relation to this metal, which is a platinum wire. 04:25 And then if you go from one to eight in this sequence here, you dip that into the water and then submerge that and you start to pull it out by applying a constant force and a known force. And then you can measure the force that requires for you to actually remove this ring from the solution as a function of time. 04:44 So this technique is well established for many liquids and it can be readily applicable or applied to polymers that are liquids or polymer solutions. It's a nice way. So you measure, first of all, you dip your ring into the solution. 05:01 So it's almost like a negative force here. But as you start pulling out and when you remove from the surface, obviously the force drops. So the area of this curve is related to the surface tension of that material. 05:15 Now, if we're moving to solids, how do you measure the surface energy of solids when a liquid is deposited or is placed on the surface of this solid? So let's imagine now if you have a solid pellet made from a polymer and you want to measure the surface tension between this solid material, which is in this case is a polymer in relation to a liquid that is being dropped into the surface. 05:41 These measurements are done via another technique that's called the contact angle measurement. So basically, you. you drop a liquid into the solid material and you measure the contact angle. So polymers can have a wide range of surface energies and a range of liquid polarities required to measure them, such as a non -polar, a polar liquid, and the polar or hydroxyl hydrogen bonding liquid, such as water that I mentioned before. 06:09 So if you have a polymer in you, you want to see how the contact angle or the energy between this polymeric material and the liquid, this is the way you would do it. And this is just what I described. 06:24 You drop a liquid and this liquid could be a polar, non -polar, or a liquid that has got a strong hydrogen bonding capability. And if the liquid wets the surface, the contact angle, which you hear I'm calling J, it will be almost close to zero, so that the liquid is spread. 06:45 So we say that the liquid wets that surface very well, so the contact angle is zero. If the liquid wets the surface less well, so the contact angle will be anything between zero and 90 degrees. But if the liquid does not wet the surface, in actual fact, this droplet will shrink away from the solid surface and the contact angle is much greater than 90 degrees. 07:06 So these are the three scenarios that you can have when you drop a liquid into a solid surface, and I'm considering here this solid surface to be a polymer material. So this is what we have. We have this equation that is the Young's Equations, that you have the surface tension, the gamma. 07:26 Powered by Notta.ai SV is the interface between the solid surface and here I just put a V for the vapors, S for solid and L for liquid. So gamma SL is the interface between solid and liquid. Gamma SV is a solid and vapor, and gamma LV is liquid. 07:51 and vapor. So this is solid liquid interfacial free energy, solid surface free energy, and liquid surface free energy. So those are the parameters related to this equation. And you have an angle here which I've called theta angle and obviously gamma SV is equal gamma SL plus gamma LV cosine of theta. 08:13 And this is what we are measuring. So you have three vectors here, this vector here, which is the gamma SV is the beta vector plus that vector cosine theta. So this is a very simple relation between the three forces that are happening around this bubble on the surface of the solid. 08:32 Now, again, I always like to give some examples of values related to each material. So polytetrafluor ethylene, this is Teflon, the surface energy between a liquid and a polymer, a solid polymer, in this case is 19 millijoules. 08:49 per meter squared, so per surface area. And then you have graphite, which is about, so this is very small, angles almost, Teflon are quite inert material, most liquid will not wet that surface. But if you compare with graphite, which is also a polymeric type of structure, that is much higher. 09:12 So the the angle there, the contact angle, the surface energy is much higher. So that wets pretty well the surface. So the tensiometer is that how the machine looks like. We have one of these in the lab and they are quite good to screen. 09:31 For example, one of the applications that we use this on the regular basis is when you have an x -cepian that is polymer, and you want, for example, to plasticize this polymer to reduce, for example, the glass transition of the polymer. 09:42 So you have to select which kind of plasticizers will be better. And the good plasticizer will be the one that will wet the surface of that polymer really well. So we will have a spread of the liquid on the top of the polymer. 09:55 So not like this one here, this is kind of a liquid that doesn't like the surface, so it's almost, the contact is very, very minimum there. So there is no, it's not wetting the surface. So and then you can scan, you can select five or six plasticizer and you do this test. 10:11 The one that is spread more on the surface will perhaps be the best, probably will be the best plasticizer for that particular polymer. So it's quite a useful technique to select, for example, the compatibility of plasticizers with a particular z -pint. 10:26 So I hope this gave you some idea and this surface tension is quite important when you're developing suspensions and you want to use surfactants that are a good interface between hydrophobic and hydrophilic regions. 10:41 So that's quite important for the development of suspensions, for example. We stop for now and we carry on. The next couple of lectures will be around the dissolution and solubility of polymers. Bye for now. Powered by Notta.ai