I have run a DSC experiment on a copolymer of polypropylene and polyethylene, resulting in the following plot.
In order to calculate the heat of fusion, I take the two starred points surrounding what I assume is my melting peak. I shift the peak so those two points are located at zero, convert to Watts, divide by my heating rate and the mass of my sample. I integrate the resulting line in MATLAB, and find a heat of fusion of 31.2 J/kg, which does not match well with the value I expected for polypropylene or polyethylene (~1 order of magnitude too low).
I was wondering where the error in my calculations might be. Was my selection for points surrounding the peak inaccurate? Or perhaps I shouldn't have vertically shifted the curve.
Thanks for your help!
Basically, you need to find the baseline. There are tons of different methods, ranging from straight lines to more complex assumptions.
I would suggest you to try get an evaluation from the DSC-measurement program you used. Obviously you need to throw the first segment away. I think that segments 3 and 4 should give you some good data once you set a proper baseline and integrated the data properly.
Dear Rich,
An order of magnitud is to large to be attributed to the base line or optimal selection of the points (which are very good taken in your graph). I suggest to analyze the composition of the sample which are is more susceptible to induce large variations of L.
Best regards,
Pedro
What DSC are you using? Most systems have the ability to calculate the heat of fusion for you. The 100% heat of fusion of PE is 293 J/g and that of PP is 207 J/g. Based on your calculations of 0.312 J/g this would mean that your samples are nearly completely amorphous which isnt true from the large melting points you observe. Also did you heat the samples any higher as the melting points of the individual polymers are over 100 C so you may not have melted the whole sample in this analysis.
To be sure to always set the same limits for each of your sample, try to use the derivative of the heat flow as an indication of the variation of the heat flow.
I attached an example for a PP/PE copolymer (note that this sample was heated at 220°C, I shorten the range only for representation purpose so maybe Catherine has a point).
However, what DSC are you using? The software of the DSC usually come with an analysis sofware that already gives you the heat of fusion in (W/g) and also offers different methods to evaluate the area of the melting peak. Your calculation sounds good to me.
If your copolymer is a TPE, the crystallinity can vary a lot compared to the one of PP or PE alone. It depends on the chemistry of the copolymer is this a block copolymer for example (see the paper attached)? To my knowledge, there are some producers that call their product TPE but the are a short number of real TPE on the market...
Steven... You are very lucky to get such invaluable responses from Florian, Grima, Catherine and violette describing the objectives and give excellent evaluation of the situation. Whatever new technologies are available that the manufacturers of instruments may present, the practical evaluation method stands the best. I used to first evaluate the real melting point from a M.P. apparatus where you can exactly determine the range over which the melting occurs. Second I run a standard material in my DSC that have a well known enthalpy of phase change (including melting) and record the melting peak for comparison with what the stored data of your instruments. Thus, NOW and only NOW you are aware of the performance of the system and obtain your enthalpy with the highest possible accuracy... Good luck
Thanks for the answers! I was trying to get boil down what the DSC program would do but, your remarks have been very informative.
Violette: The polymer is marketed as a TPE, but I think you're right about it being a bit of a misnomer...
it is better to use related software for calculation of heat fusion. then knowing the heat fusion for a 100 % crystaline polymer, you can easily calculate the degree of crystalinity. see file:///C:/Users/User/Downloads/materials-10-00442.pdf.
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