DSC - DIFFERENTIAL SCANNING CALORIMETRY

►THEORY

►EXPERIMENTAL METHOD

►RESULTS

►ANALYSING THE RESULTS

►THEORY

    Differential scanning calorimetry is a technique we use to study what happens to polymers when they're heated.

    We use it to study what we call the thermal transitions of a polymer. The thermal transitions are the changes that take place in a polymer when you heat it. We can differentiate between:

• First order transition: a thermal transition that involves both a latent heat and a change in the heat capacity of the material. Transitions like melting and crystallization.

• Second order transition: a thermal transition that involves a change in heat capacity, but does not have a latent heat. The glass transition is a second order transition.

    What we do in DSC is to heat a polymer in a device that looks something like this:

    In the most popular DSC design, two pans sit on a pair of identically positioned platforms connected to a furnace by a common heat flow path. In one pan, you put your polymer sample. The other one is the reference pan. You leave it empty. You then tell the nifty computer to turn on the furnace. So the computer turns on the furnace, and tells it to heat the two pans at a specific rate, usually something like 10^oC per minute. The computer makes absolutely sure that the heating rate stays exactly the same throughout the experiment. But more importantly, it makes sure that the two separate pans heat at the same rate as each other.

    Having the polymer in the sample pan means that it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan. Just how much more heat is what we measure in a DSC experiment.

    We make a plot as the temperature increases. On the x-axis we plot the temperature. On the y-axis we plot the difference in heat flow between the sample and reference. A whole plot will often look something like this:

     Of course, not everything you see here will be on every DSC plot. But we are going to analyze this one because from this complete graph, we can learn a lot of concepts: 

• HEAT CAPACITY

    If we observe the first part of the plot beginning on the left, for low temperatures, we can see this:

    When you put a certain amount of heat into something, its temperature will go up by a certain amount, and the amount of heat it takes to get a certain temperature increase is called the heat capacity, or C_p.

    We get the heat capacity by dividing the heat flow by the heating rate. And we end up with heat supplied, divided by the temperature increase.

• THE GLASS TRANSITION TEMPERATURE

     When we heat the polymer a little more after a certain temperature, our plot will shift downward suddenly,

    This means heat is being absorbed by the sample and that we have an increase in its heat capacity. This happens because the polymer has just gone through the glass transition. Polymers have a higher heat capacity above the glass transition temperature than they do below it. Because of this change in heat capacity that occurs at the glass transition, we can use DSC to measure a polymer's glass transition temperature. We usually just take the middle of the incline to be the Tg.

    If you want to know more about the glass transition: http://www.psrc.usm.edu/macrog/tg.htm 

• CRISTALLIZATION

    Above the glass transition, the polymers have a lot of mobility and never stay in one position for very long. When they reach the right temperature, they will have gained enough energy to move into very ordered arrangements, which we call crystals.

    When polymers fall into these crystalline arrangements, they give off heat; for this reason, we call crystallization an exothermic transition.

    The peak in the plot tells us that the polymer can in fact crystallize. The temperature at the highest point is the polymer's crystallization temperature, or T_c and the area of the peak is the latent energy of crystallization for the polymer. 

• MELTING

    If we keep heating our polymer past its T_c, eventually we'll reach another thermal transition, one called melting. When we reach the polymer's melting temperature, or T_m, those polymer crystals begin to fall apart, that is they melt. There is a latent heat of melting as well as a latent heat of crystallization. When the polymer crystals melt, they must absorb heat in order to do so, for this reason, because we have to add energy to the polymer to make it melt, we call melting an endothermic transition. This extra heat flow during melting shows up as a large dip in our DSC plot as heat is absorbed by the polymer.

    We can measure the heat of melting by measuring the area of this dip and we usually take the temperature at the apex of the dip to be the point where the polymer is completely melted.  

• HOW MUCH CRYSTALLINITY?

    DSC can also tell us how much of a polymer is crystalline and how much is amorphous.

 CONCLUSIONS

If you remember the first complete plot, we can conclude and clarify some facts:

• The crystallization peak and the melting dip will only show up for polymers that can form crystals. Completely amorphous polymers won't show any crystallization, or any melting either.

• For the glass transition, there is no peak, and there's no dip, either. The only thing we do see at the glass transition temperature is a change in the heat capacity of the polymer. This is because there is no latent heat given off, or absorbed, by the polymer, like happens in melting and crystallization.

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►EXPERIMENTAL METHOD 

1. First of all, we are going to weigh each sample. For achieve this purpose we use a pan. This pan is composed of other two pans, one made of plastic and the other made of aluminium. The aluminium one is very small like the size of the sample's grain and it is placed inside of the plastic pan. 

-         Firstly, we weigh the pan.

-         Secondly, we weigh the group: sample + pan.

-         And finally, we obtain the sample´s weigh. 

2. The next step is to shut the polymer's grain in the aluminium pan and apply pressure to close it well.

3. Now, we can put the sample inside DSC device.

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►RESULTS

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     ►ANALYSING THE RESULTS

     We obtained three plots at different temperatures for each material, using the method “Heat-Cool-Heat”:8

-         R2550E-T 192ºC

-         R2550E-T 230ºC             with stabilizer

-         R2550E-T 253ºC

-         FM172-T 192ºC

-         FM172-T 234ºC               without stabilizer

-         FM172-T 257ºC

     First of all, we can notice after looking all the plots obtained that, both materials are not completely amorphous polymers because, the crystallization peaks and the melting dips will only show up for polymers that can form crystals.

     The second fact that we can observe in the graphs is that we see one clear Tc, one clear Tg and one clear Tm. This means these temperatures belongs to one polymer or to a miscible mixture, in other words, these temperatures are due to one phase. In order to interpret the obtained plots, first, we should know the content of the run materials. The components of the materials are the followings:

For further information about these polymers description and information:

·        PP-RC

http://www.tangram.co.uk/TI-Polymer-PP_Random-Copolymer.html

• PE-LLD

http://www.goodfellow.com/csp/active/STATIC/E/Polyethylene_-_Low_Density.HTML

• SEBS

Click here

• AO

Click here

We have found the basic properties of the materials´ components:

For polypropylene and polyethylene,

 ^*If you want to see a picture about  tacticity: Click here

Which polymers are showed in the DSC plots?, what information gives us the graphs?

In the graphs, we can observe several things:

 ► Glass transition, crystallization and melting temperature are around these values:

·        Tg = 40-50ºC,

·        Tc = 90-100ºC

·        Tm = 140-145ºC.

However, the theoretical values found do not fit exactly with the showed ones in the experiment.

► For all the samples, we could think that the melting temperature obtained is the melting temperature of the PP. In spite of the fact that theoretical Tm value is around 162-168ºC, for commercial PP this temperature is lower and this polypropylene used is almost always an isotactic PP sequency. Tm depends on the degree of tacticity. For commercial PP is usual to decrease the melting point, (changing the atactic configuration, copolymerizing or adding rubber phases), because of impact resistance, which at Tm=168ºC is not very high. This could be the reason for using PP-RC (Random Copolymer) since the melting temperature for PP-RC is around 140-145gC.

► Although the melting temperature shown in the plots is the Tm of the PP, the glass transition temperature (Tg) appeared seems not to belong to the PP. This means that it is not a Tg of a pure PP. This Tg belongs to the Tg of a copolymer of PP. Clearly, it is obvious because the main component of both materials is PP-Random Copolymer. PP-RC contains a little bit of PE, around 4-5% of PE.  

► There is a difference between FM172 material and R2550E material graphs. R2550E is composed of PE-LLD apart from others but FM172 material does not contain this kind of polymer. It is easy to see this difference in the plots. There is a small melting dip before melting dip of PP. This is the melting point of PE. PE-LLD (linear low density polyethylene) is always a copolymer and has a linear chain.

There is also another small peak after the big crystallization peak and this is also due to PE. 

 ► Regarding the temperature effect on the different materials, we notice that there is no big differences between them and we cannot observe appreciable changes. At different temperatures, what we could expect is that they could have a degradation in molecular structure or maybe, a change in the degree of crystallinity. The problem is that DSC is not too much sensitive to these events, therefore DSC is not too much sensitive to different temperatures. With DSC experiment, we only can see differences in crystals structure.

    However, is important to know the Tc value for PP. We can notice in the graphs that for high temperatures (T=253ºC or 257ºC) Tc value is lower than for low temperatures (T=192ºC), although in small scales. This means for high temperatures, less of heat resistance, since high value of Tc means more heat resistance. The reason to explain this fact could be that the polymer suffers degradation at high temperatures.

 ► There are no differences with regard to the addition of the stabilizer. We can see these differences in Rheometry experiment.

     To conclude, the most important fact that we can emphasize with regard to the thermal properties of the materials is the crystallization temperature for polyprolylene. This is the most indicative value for our study. To sum up, at high temperatures there is a decrease in heat resistance due to the degradation of the material. Therefore, if the material runs too much warm, it is expected that the bag made of this kind of material would have a deformation due to the crystallization of PP.

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