If it is a direct absorption (transmission) measurement, then the absorbance (intensity) of your peaks are proportional to the concentration and path length through the sample.  See the Beer-Lambert law (http://en.wikipedia.org/wiki/Beer–Lambert_law).

From your spectrum, it looks like all of the peaks are reduced by the same amount in intensity.   That would suggest it's just a reduction in gypsum in your sample.  If the spectral peaks themselves changed shape, or the relative intensities changed, then it would suggest that the additive is adding spectral features, or  modifying the gypsum spectrum.

Thanks for your answer. Can it be related to the amount of samples I used for analysis? I mean in one case I put lots if samples under diamond probe than the other case.

If you are using a diamond probe, it sounds like an ATR measurement.  If so, then the amount of sample shouldn't make a difference, as long as the pressure on the probe for the ATR measurement was the same.  I would guess that the additives simply dilute the amount of gypsum, which is why the gypsum spectrum intensity drops.

Maybe the addition of the additive reduced the concentration of the sample. If this is true, then according to you spectra, the additive is transparent in the infrared region because it did not affect the spectral features of the sample nor added to them.

Miles Weida in my case I dind't use ATR-FTIR and there is a change in the intesity of one of my samples. The "problem" is that the intensity obtained (at 121°C) is even higher than the one for my reference (unheated). I worked with erythritol (sugar alcohol) and prepared the samples under argon and then nitrogen to avoid oxidation. Do you think this change in the peak intensity of the sample at 121°C is only becase I may have used less amount of sample? Thank you!

Maybe the higher signal is just related to the fact that at higher temperature your sample becomes softer, so that in general better contact between ATR crystal and your sample is established.

Helpful discussion..

First of all, the evaluation of IR spectra was right from the start much more advanced than that of Raman spectra. There is a method called dispersion analysis, founded on dispersion theory, which allows you to understand a band in terms of oscillator frequency, damping constant and strength (=amplitude). I guess it was around 1926 when Kramers expanded dispersion theory to incorporate quantum theory. Accordingly, absorption will decrease with increasing temperature, because with increasing temperature more molecules will occupy exited vibrational states. This means that the absorption coefficient will decrease with increasing temperature - so no wonder that the intensities decrease.

Hello everybody. I also have interesting data about loss of intensity of the whole IR spectrum when studying the degradation of a polyester film by ATR-FTIR. For the scientific communities it is clear that ATR-FTIR can only be used with normalization of the spectrum and analysis of the chemical changes/peaks shifts. From my results it appears that the signal intensity could be linked to the surface erosion or roughness. As physicians, do you think it could be possible? I cannot find any paper about this. Regards!

Melanie Salomez I cannot directly answer your question, but ATR adds a number of complications, not only the problem of applying reproducibly pressure to assure intimate contact between sample and ATR crystal. See https://www.researchgate.net/post/ATR_correction-why_would_you_not_use_it

Thank you Thomas for your reply. I have done 10 replicates of FTIR spectra in different zone of my sample so it should take into consideration the variability introduce by the sample and the variability introduced by the pressure between sample and ATR crystal. Overall I have a very good correlation between the average signal intensity and surface degradation parameters.

Through my experiments with this process, the intensity is calculated according to the presence of functional groups in first material, gypsum, and when decrease in intensity that caused by the attachment of additives to the functional groups of gypsum, therefore this process lead to reducing the intensity of absorption..Thank you

Atheer Saieb Naji Al-Azawey I used montmorillonite and after modification, the intensity decreased, I'm keen to know the reason...

regards,

Niloofar Sadeghi There is one thing that has not been mentioned so far, which is the degree of mixing. If two components are mixed micro homogenously, which means that if I take an IR-microscope the sample would give the same spectrum everywhere, then the absorbances of the individual components are approximately additive. If the sample is micro heterogenous, i.e. there is demixing on the microscopic level and I can see parts of the sample consisting only of component 1 or 2, then the intensities are additve (i.e. transmittance or reflectance). This leads to a dramatic band flattening (see attached figure).

The theory behind has originally been developed in another context, Article Modelling IR-spectra of single-phase polycrystalline materia...

Thomas Mayerhöfer thank you for your useful response. according to your explanation, as I understood, I should have expected to see increased intensities at specific areas, but I saw a decrease in patterns.

Niloofar Sadeghi This depends. If you have a micro heterogeneous sample with 5 volume % of a minor component, the absorbances of the bands of your main component may be halved compared to the pure component (!), but due to this band flattening you will not see new bands of these 5 %, just very slight baseline increases...

Niloofar Sadeghi I am following and I have a question. What could be an explanation where after mixing with a 3% wt and the wave number decreases as attached

In spectroscopy, absorbance vs energy has been plotted in FTIR diagrams. The bond energy was represented by kayser (cm^-1). Therefore, more decline in the energy peak indicates a more micro-structure chemical bond.

https://www.sciencedirect.com/science/article/pii/B9780444637765000012

Thanks Thomas Mayerhöfer for your reply about the effect of mixing on peaks in FTIR curves. I have the same problem with the formulations I work on. I prepared tablets by two ways, physical mixture and hot melt extrusion. I noticed a big difference in the IR curve of powders from two ways. I need a reference that has mentioned this clearly, I have read the article you mentioned in your reply but I could not link it to the mixing homogeneity.

Dear Nour Nashed , the original paper about the size effect in spectra of mixtures is the following:

Article Beyond Beer's Law: Spectral Mixing Rules

It is also explained in this review Article The Bouguer-Beer-Lambert Law: Shining Light on the Obscure

Preprint Wave optics in Infrared Spectroscopy

thanks Thomas Mayerhöfer. that is really helpful.

Is there a solid procedure for analysis of polymeric structures based on their FTIR absorbance data? Will there be a decrease in absorption of IR spectra upon oxidation or degradation of a polymeric network? Can we compare absorbance of two specimens (made from identical materials) and translate as a change in functional group concentrations?

I have tested a large number of epoxy coated steel panels via FTIR after they were under hydrothermal exposure and I saw a quite clear decrease in absorption with increasing exposure time. These panels were immersed in 65°C DI water for a long time and at regular intervals one sample was taken for analysis. The exposure time and temperature are expected to give rise to a degradation process in epoxy coating and I was wondering if I can make these IR analyzes useful.

Thanks

Hossein Zargarnezhad If I understand you correctly, your samples consist of polymer films on a metallic substrate. Before it is possible to conclude anything from the spectra, you first have to correct the resulting spectra from interference effects, which can be very strong (in the last years, these well-known effect has been rediscovered under the name "electric field standing wave effect").

Article The electric field standing wave effect in infrared transfle...

It is possible to remove these effects:Article Removing interference-based effects from the infrared transf...

Once this is achieved, the spectra are useful and one can think of interpreting the changes seen in the spectra.

Thank you so much Thomas Mayerhöfer ! I will try to do this and do the correct according to the suggestion in your paper. Are there some samples for doing this sort of analysis (interpretation of FTIR absorption for individual polymeric samples as a change in polymeric structure)? I'm a bit worried that FTIR might not be a widely accepted quantitative method for analysis.

Hossein Zargarnezhad my pleasure! I am not so deep into IR spectroscopy of Polymers, but there are a number of dedicated books like,

- D. I. Bower, W. F. Maddams - The Vibrational Spectroscopy of Polymers (1989)

- Kuptsov, Zhizhin - Handbook of Fourier transform Raman and infrared spectra of polymers (1998)

- R. Zbinden - Infrared Spectroscopy of High Polymers (1964),

so I am positive that IR is an accepted way of investigating polymers quantitatively.

Melanie Salomez I could not find any literature for your proposed theory. However, I have observed the same pattern with my PET films. Since I don't have a fairly strong background in FTIR, it is hard for me to understand why there is such a correlation.

I have been through the subject and was even thinking to publish something about that but as you said there is not a lot of background in the literature about this subject. Having discussed with FTIR specialists , they were surprised and disagreed also with these observations. But for me there is a correlation, even after background correction and manipulation correction (10 replicates for each measurements!) and the trends is very clear. My hypothesis is that with the surface erosion of the polymer some intensity of the IR beam is lost due to the morphology of the surface. There might be a correlation with the erosion rate and the intensity of the IR beam. I think ATR-FTIR intensity could be a new way to monitor surface erosion.

Melanie Salomez I recently came across something comparable, but for external specular reflection: https://www.sciencedirect.com/science/article/abs/pii/S0022309300001216

I am facing a similar issue as well. I have casted Epoxy with nanofiller. The FTIR showed significantly lower transmittance % ( the max peak is ~ 0.96%) compared to resin ( Max peak ~ 0.4%). I have a question which I would love if someone could answer it . Does change in your film/laminate colour and the colour concentration play a role in transmittance % ?

Gopal Krishna Bhagavatula Can you show us the spectra? It is hard (at least for me), to understand what is going on without the spectra.

Here, I'll suggest to check the thickness of sample as well. In actual, thickness increases with increase in temperature. However in case of polymer, they can't sustain at higher temperature, this may be owing to the weak bonding at higher temperature. This may causes weak vibrations at higher temperature resulting less intense peak in FTIR.

Also as transmittance decreases means absorption increases. Some features in your film absorbing IR frequency.

Check if it works in your case. Check the following article related to temperature and thickness dependent elastic moduli of polymer thin films.

Thank you.

http://www.nanoscalereslett.com/content/6/1/243

Thomas Mayerhöfer

Hi there, I have two different samples, 1- A pure acrylic resin cured for 20hrs at 150°C, 2- The same Acrylic resin blended with 15 weight percent of Triacetin and %5 of Polyethylenglycol, both of which are added as plasticizer, cured under the same circumstance. Before the curing stage, I solved the materials in Acetone and dispersed the blend using magnets on heater-stirrer. The purpose was to enhance the flexibility and reduce the Glass Transition temperature, both of which were occurred. I carried out a FTIR test, in which my wave-numbers are exactly the same, but the transmittan are different, actually, were declined. How can I justify and explain how these transmittance are related to flexibility?? Or even what do these deacreases mean in these spectrums?!

How exactly did you measure your samples - as thin layers on CaF2 (looks like you have divided the experimental spectrum by the spectrum of the substrate)? If so, I would think that the sample with the pure resin had a thinner layer compared to the "plasticized" sample. Concerning how macrosocpic flexibility should alter the spectrum, you need to ask someone who has a better background concerning polymers than I have.

Thomas Mayerhöfer

Thanks for your response.

The thickness was same for both specimens equal to 3 mm. I am sure that the reason is not the thickness.

According to the Beer-Lambert law, can we say that this is occurring because the concentration of the bonds and functional groups has increased? Considering that both of the plasticizers and the resin are made of the same components.

Again, what kind of IR-technique did you apply? Without knowing that it is hard for me to discuss your results.