Hi,
I took some FTIR (ATR) readings of a polymer composite (epoxy+ fumed silica+ ceramic filler) we are working on. The peaks seem to decrease in intensity with increase in ceramic and filler content. I am new to FTIR but based on what I have read decrease in peak intensity corresponds to reduction of bonds which absorb that wavelength if so doesn't that mean these fillers are impeding growth of the polymer chain bonds? But I had a doubt that wouldn't these opaque particles being in higher concentration block the IR and reduce transmittance and then the decrease in peaks just means the IR was not able to penetrate the sample due to presence of these particles and not actual decrease in polymer chain formation. Can someone explain this to me.The attached graph is transmitance % vs wavelength Thank you.
Dear Alex,
Adding anything to the optical path of a beam of light will reduce its power by a certain amount. I can't say the specifics for your case, but there will always be a non-zero absorption which you could quantify using Beer-Lambert law if you want (A=I/I0, is proportional to the object length, please check Mark Fox's Optical properties of solids for more info). In your case, to get a better FT-IR characterization of your samples, you would need to run a background measurement of a pure ceramic sample with some thickness, and run a measurement of your sample embedded in a ceramic with the same thickness as the background sample. In that way, you will account for the optical changes due to the thickness of the ceramic.
I hope that helps answer your question.
I forgot to add the correct definition for the absorption A, it should be A = -ln(I/I0).
Peak intensity depends on optical path length, and absorption coefficients. In transmission measurements the optical path length depends on sample thickness and concentration of analyte. However, on ATR-FTIR the spectra is subject to additional sources of distortion, in an ATR setup the effective optical path-length is generally independent on sample thickness, and it depends on the penetration depth, which is dependent, for example, on refractive index and the quality of the contact between sample and crystal (pressure applied). In other words, ATR is a great technique for qualitative analysis, i.e. identifying peaks, but it is hard to perform quantitative analysis, specially in solid samples.
In your case, I am not sure if the filling content changes the absorption but I am sure it changes the refractive index of the sample, and thus the penetration depth.
Adding to my previous comment, I see your spectra is very noisy. I suggest you use a lower resolution and more scans to average. This depend on the sharpness of the features expected, but a good starting point is 4cm-1 resolution and 32 scans. A big part of the problem is that the reflection is to high (meaning absorption from the sample is too low). I suggest you add sufficient material over all the crystal surface and try increasing the pressure for a better contact with the crystal. Apply consistent pressure, and do multi-sample analysis to verify reproducibility of the results.
First of all you should clarify your operating conditions to avoid the unacceptable noise which has been observed your provided spectra. Then it could be much more benificial if you consider a comparative study among all the considered constituent / ingredient which you were incorporated in the polymeric composites individually for better understanding the actual behaviour of your final fabricated composites.
It is a common beginner's mistake to see results like this and think that the chemical bonds change in composites with different filler proportions. Your composites, depending on the proportion of fillers, have different hardnesses. Those with more fillers are harder and more rigid, so the contact with the crystal is less intimate, which causes weaker signals, i.e. a worse signal-to-noise ratio.
Certainly, differences in readings occur because the ceramic materials are suspended materials and are not part of the structural structure of the composition (heterogeneous) or the occurrence of discontinuous phases.
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