FTIR is one of the most commonly reported characterisation techniques in polymer composites research, yet the FTIR section is also one of the most poorly written. Most researchers list peaks and assign functional groups correctly, but stop there. The result is a section that reads like a table caption, not a scientific interpretation.
This guide explains what FTIR is actually telling you in a composites study, how to read spectral changes as filler loading increases, what chemical interactions to look for, and how to write an FTIR section that satisfies peer reviewers at journals like Polymer Composites, Polymer Testing, and Industrial Crops and Products.
What FTIR Is Measuring in a Composite System
Fourier Transform Infrared Spectroscopy measures how a material absorbs infrared radiation at different wavelengths. Different chemical bonds absorb at characteristic wavenumbers (measured in cm−1). When you record an FTIR spectrum, the peaks correspond to the vibrational modes of specific functional groups within the material.
In a polymer composite study, you are not just identifying what functional groups are present. You are doing three things:
First, you are confirming the chemical identity of your filler material and your matrix. This establishes that you are working with what you say you are working with. Second, you are tracking how the spectrum of the composite changes as filler concentration increases. Shifts in peak position, changes in peak intensity, and the appearance or disappearance of peaks are all meaningful. Third, you are using those spectral changes to make an argument about the nature of the interaction between the filler and the matrix.
Reference Peak Table: Natural Filler and Polymer Systems
The table below covers the key absorption bands commonly encountered in bio-filler reinforced polymer composite research. Use this as a working reference for peak assignment.
| Wavenumber (cm−1) | Vibration Mode | Functional Group | Origin in Composite |
|---|---|---|---|
| 3300–3500 | O–H stretching | Hydroxyl (–OH) | Cellulose, hemicellulose, moisture in natural filler |
| 2900–3000 | C–H stretching | –CH⊂2; / –CH⊂3; | Aliphatic chains in both filler and polymer matrix |
| 1720–1730 | C=O stretching | Ester carbonyl | Polyester matrix (diagnostic band) |
| 1600–1650 | C=C stretching | Aromatic ring | Lignin in filler; aromatic units in polymer backbone |
| ~1430 | C–H bending | –CH⊂2; | Cellulose crystallinity band |
| ~1250 | C–O–C asymmetric stretch | Ether/ester linkage | Lignin and hemicellulose in filler; ester bonds in polyester |
| 1000–1100 | C–O–C / C–O stretching | Glycosidic ether | Cellulose (dominant band; intensity grows with filler loading) |
| ~830 | C–H out-of-plane | Aromatic C–H | Para-substituted rings in epoxy backbone |
| 500–700 | Skeletal vibrations | Aromatic deformation | Lignin in filler; polyester aromatic units |
Reading Spectral Changes as Filler Loading Increases
In a series of composite spectra recorded at 0%, 5%, 10%, 15%, and 20% filler loading, you will typically observe four types of spectral change. Each carries a different scientific meaning.
Change Type 1
Intensity Increase at Filler-Specific Peaks
If a peak associated with the filler material grows in intensity as loading increases, this confirms that the filler is being incorporated into the matrix in proportion to the amount added. The hydroxyl band around 3300–3500 cm−1 and the cellulose C–O–C band at 1000–1100 cm−1 are typical examples in natural filler systems. This is confirmatory evidence that the composite is forming as intended.
Change Type 2
Peak Shifting
If a peak in the composite spectrum appears at a slightly different wavenumber compared to the same peak in the pure filler or pure matrix spectrum, this suggests a chemical interaction. For example, if the O–H stretching band in the filler appears at 3420 cm−1 but shifts to 3400 cm−1 in the composite, the hydroxyl group is participating in hydrogen bonding with the matrix. Peak shifts of even 10–20 cm−1 are considered meaningful in FTIR interpretation.
Change Type 3
Broadening of Peaks
A peak that becomes broader in the composite compared to the pure component suggests increased disorder or the presence of multiple similar interactions overlapping. Hydroxyl bands particularly tend to broaden when hydrogen bonding is occurring at multiple sites. Broadening is often associated with increased compatibility or interaction between the filler surface and the polymer chains.
Change Type 4
Preservation of Matrix-Specific Peaks
The fact that the diagnostic peaks of the matrix remain present in the composite spectra confirms that the polymer matrix has not undergone degradation during composite fabrication. For epoxy composites, the preservation of the peak around 830 cm−1 (para-substituted aromatic ring) confirms the epoxy backbone is intact. For polyester, the persistence of the carbonyl peak at 1720–1730 cm−1 serves this role. This is an important negative result that should be stated explicitly.
The Specific Question FTIR Must Answer in Your Paper
In a natural filler-reinforced polymer composite study, the core question FTIR must address is this: is there evidence of chemical interaction or hydrogen bonding between the filler and the matrix, or are the two components merely co-existing without meaningful interfacial interaction?
This matters because interfacial adhesion determines mechanical performance. If FTIR shows no evidence of interaction, it helps explain why mechanical properties decline with filler loading. If FTIR shows peak shifts or new interactions, it supports an argument for better load transfer.
How to Write the FTIR Section
Structure your FTIR discussion in the following sequence. Begin with the filler spectrum, then the pure matrix spectrum, then the composites as a series.
Paragraph 1: Filler Identification
Assign the major peaks in the filler spectrum and connect them to the known chemical components of the material. For a natural filler derived from plant matter, these are cellulose, hemicellulose, and lignin. State which peaks correspond to which component. Do not simply list every peak in the spectrum; select the three to five most diagnostic bands.
Paragraph 2: Matrix Identification
Assign the diagnostic peaks of the matrix. For epoxy, focus on the aromatic C–H band and the ether C–O–C bands. For polyester, the carbonyl peak at 1720–1730 cm−1 is the most important diagnostic band. State that these peaks confirm the chemical integrity of the matrix.
Paragraph 3: Composite Spectra and Changes With Loading
Compare the composite spectra to the individual component spectra. Note which filler peaks appear in the composites, whether their intensity increases with loading, and whether any peak shifts are observed. This is the most scientifically important paragraph of the FTIR section.
Paragraph 4: Interpretation
Draw your conclusion about the nature of the filler-matrix interaction based on the spectral evidence. If peak shifts are present, make the hydrogen bonding argument. If peaks are preserved but show no shift, state that the evidence suggests physical mixing without significant chemical interaction. If matrix peaks show no degradation, state this explicitly as confirmation that the fabrication process did not chemically alter the polymer.
Sample Sentences for Your FTIR Section
These sentence structures are used in published polymer composites research and are appropriate models for your manuscript:
For peak assignment in the filler:
"The broad absorption band observed at 3300–3500 cm−1 is attributed to the O–H stretching vibration of the hydroxyl groups present in the cellulose and hemicellulose components of the bael shell powder."
For confirming filler incorporation in the composite:
"The increasing intensity of the C–O–C stretching band at 1000–1100 cm−1 with increasing filler loading confirms the progressive incorporation of bael shell cellulose into the polymer matrix."
For reporting a peak shift and interpreting it:
"A shift in the O–H stretching band from 3420 cm−1 (bael shell powder) to 3395 cm−1 in the epoxy composites suggests the formation of hydrogen bonds between the hydroxyl groups of the cellulose and the ether oxygen groups of the cured epoxy network."
For confirming matrix integrity:
"The characteristic carbonyl absorption band of the polyester matrix at 1725 cm−1 was retained in all composite formulations, confirming that the ester bonds were not disrupted during the hand lay-up fabrication process."
When You Have the Spectrum but Not the Raw Data File
Journal submission increasingly requires that the actual instrument data file, not a screenshot or photograph of the spectrum, be submitted or be available upon request. If you have recorded FTIR spectra, retain the original instrument output files in the native format of the instrument software. For Perkin Elmer instruments, this is typically a .sp or .spc file. These can be processed in OriginPro to generate publication-quality figures with correct axis labels and appropriate wavenumber ranges.
A screenshot of a spectrum processed only in the instrument software is not acceptable for many Q1 journals. The figure needs to be a vector or high-resolution raster image exported at a minimum of 300 dpi, with axes properly labelled in the correct font matching the rest of your manuscript.