Fourier Transformed Infrared Spectroscopy (FTIR spectroscopy): For Characterization of an Aramid and its Blends


Fourier Transform Infrared Spectroscopy (FTIR spectroscopy) is a technique that is used to identify organic/inorganic compounds in a material by their bonding characteristics associated with that compound.

FTIR spectroscopy works on the basis of the absorption of infra-red light by molecules. The absorption of light is associated with the vibrational motion of bonds present between molecules such as bending (scissoring, rocking, wagging and twisting) and stretching (symmetrical, asymmetrical) at a specific frequency, which separates the different molecules when IR fall. The frequency range for FTIR spectroscopy is measured in wavenumbers from 4000-400 cm-1. The sample for analysis can be in film or powder form.

FTIR spectroscopy - Vibrational motion of bonds between molecules
Vibrational Motion of bonds present between molecules

During analysis in Fourier transform infrared Spectroscopy, the background emission spectrum is recorded followed by the emission spectrum of material. The sample’s absorption spectrum is obtained by taking the ratio between the emission spectra of the sample to the background emission spectra. This absorption spectrum is then inverted by taking its inverse to obtain the transmittance spectrum for compositional analysis.

To understand the mechanics and priniciple of FTIR, you see FTIR on wikipedia.

The presence of functional groups is associated with the presence or absence of absorption of the bands in the compound.

  • If oxygen is present the group may be –OH, C = O, COOR, –COOH, anhydrate etc.
  •  An absorption band between 3600-3200 is may be due to –O –H.
  •  NH group give only one band. It can be differentiated from O –H as the extent of hydrogen bonding is stronger in –OH compounds and causes more lowering in the wave number.

Regions of IR Spectra (FTIR Spectroscopy)

In general, the FTIR spectrum is split into four distinct regions for interpretation:

  • 4000 – 2500 cm -1: Absorption of single bonds formed by hydrogen and other elements such as O – H, N – H, C – H 2500 – 2000 cm -1
  • Absorption of triple bonds such as C≡C, C≡ N 2000 – 1500 cm -1
  • Absorption of double bonds such as C=C, C=O 1500 – 650 cm -1: This region often comprises of many different, complicated bands. This part of the spectrum is unique to each compound and is termed as fingerprint region.

Finger print region (Fourier Transform infrared spectroscopy)

For every compound a very complicated series of absorptions occur between wavenumbers 500 to 1500 due to a different type of bending and stretching within the molecule. Each compound has a unique set of troughs within this region which is useful to identify the molecule. Finger print region can further divided into three regions;1500 – 1350 cm ‾ ¹ , 1350 – 1000 cm ‾ ¹ and Below 1000 cm ‾¹.

Fourier transform infrared spectroscopy
Different regions of IR Spectra

FTIR spectroscopy analysis of Aramid

For compositional analysis of pure aramid, hyperbranched polyamide ester and the blends FTIR spectroscopy was done. It revealed the presence of different elements and functional groups in all three materials. In order to study in detail about aramids and their synthesis methods, Follow here. The most important aramids are Twaron fibers and KEVLAR Fibers. Applications of Aramids can be found here.

The process of polyimide is similar to Aramid’s characterization. There is a very slight difference between Polyamide and Polyimide, although both are equally important from a commercial point of view.

The FTIR spectrum of aramid is shown in Fig. The presence of a peak at 3246 cm-1 confirmed N-H bond in the structure while the band at 3052 cm-1 corresponds to aromatic rings C-H bond stretching vibrations. The peak at 1650 cm-1 showed the clusters of C=O groups and signals at 1602 and 1528 cm-1 correspond to C=C stretching of an aromatic ring. The presence of C-O-C in the structure was confirmed by a peak at 1310 cm-1 (Table).

FTIR spectroscopy peaks for pure aramid
FTIR spectroscopy peaks of Pure aramids
FTIR spectrum of aramid
FTIR spectrum of Aramid

FTIR spectrum of hyperbranched polyamide ester as shown in Fig. revealed the peaks of different bonds stretching needed to be there in the structure during FTIR spectroscopy. The peaks of HBPAE are broader in the FTIR spectrum than aramid peaks due to the presence of hydrogen bonding in the structure.

FTIR spectrum of hyperbranched polyester
IR spectrum of Hyperbranched polyamide ester

The peak at 3302 cm-1 confirmed N-H bond and at 3160 cm-1 C=C aromatic bond stretching was observed. The presence of ester C=O linkage in the structure was revealed by the peak observed at 1645 cm-1 while amide C=O bond was observed by the peak shown at 1680 cm-1.  A number of peaks in the finger print region from 1500 to 500 cm-1 showed presence of some bond stretch and bending vibrations.

The FTIR spectrum of a blend (Fig.) confirmed the presence of different bonds formed by the blending of the aramid and HBPAE. The broadening of peaks in the spectra was observed due to an increase in hydrogen bonging by the reaction of aramid and hyperbranched polymer. N-H bond stretching was observed by the peak at 3249 cm-1 and peak at 3055 cm-1 correspond to C-H bond stretching of aromatic rings. The presence of amide linkage and ester bond was confirmed by peaks observed at 1645 cm-1 and 1628 cm-1, respectively.

FTIR spectrum of blend
IR spectrum of blend

Presence of C-N linkage formed by reaction of aramid and HBPAE was confirmed by peak observed at 1312 cm-1. A sharp peak at 1011 cm-1 confirmed C-O-C bond stretching while peaks observed from 682 to 830 cm-1 correspond to C-H out of plane bending.