Four-Angle Polarisation-Resolved Transmission FTIR Mapping for Materials Orientation Analysis

Molecular orientation in polymeric and composite materials can play a significant role in overall mechanical performance and chemical properties. Infrared absorption by specific functional groups occurs preferentially when the electric vector of the probing beam is aligned with the dipole oscillation corresponding to the absorbing frequency, and can therefore be used to gain information on the molecular orientation of selected molecules. The team at the Tokyo Institute of Technology has developed a method whereby the dipole orientation angle (theta) for each pixel of a hyper-spectral image can be determined from Eq. 1, where A(theta 1;2;3;4) are absorbances at the four polarisation azimuths separated by pi/4 [1]. The dipole orientation angle and strength are then plotted as “vectors” over each pixel within the spectral map, enabling the visualisation of molecular orientation. We have applied this method in the analysis of molecular re-orientation in silk fibres [2] and in the study of the effects of additives to poly-lactic acid (PLA) composite materials. Figure 1 shows an example of the orientation vector map of spherulites formed in a PLA solvent cast film, showing the absorption strength (colour) and dipole orientation (vector line) for the C=O absorption at 1759 cm-1. More recently, the method has also been applied to reveal molecular orientation inside the micro-domains of the paracetamol form II, which has a better water solubility and compressibility compared to the commercially used forms I [3], and to analyse polymeric patterns using focal plane array IR detectors [4].