Infrared Linear Dichroism for the Analysis of Molecular Orientation in Polymers and in Polymer Composites

Abstract

The mechanical properties of polymeric materials are strongly affected by molecular orientation occurring under processing conditions. Infrared dichroism is particularly well suited for characterizing polymer chain orientation at a molecular level. The usefulness of this technique has been demonstrated through various applications in homopolymers, semi-crystalline polymers, copolymers, polymer blends, as well as in polymer composites. Determination of molecular orientation can be carried out in the mid- and near-infrared ranges and very small dichroic effects can be detected with the use of a photoelastic modulator. Chain orientation in polymer composites is seen to increase with the filler content in the case of a strong interface between the two phases, making possible a quantification of the degree of bonding between the host polymeric matrix and the incorporated inclusions.

Blown Composite Films of Low-Density/Linear-Low-Density Polyethylene and Silica Aerogel for Transparent Heat Retention Films and Influence of Silica Aerogel on Biaxial Properties

“…A similar observation was found in a previous report, where it was shown that the addition of SA reduced the thermal conductivity by approximately 30% with 1–2 wt.% of SA loading [ 22 ]. Thus, the prepared composite LDPE/LLDPE/SA (0.5–1.5 wt.%) film had a lower thermal conductivity than the neat LDPE/LLDPE blend film, and was suitable for application as a thermally insulating material.…”

“…These findings can be quite convincingly explained by the various material properties that are brought about by adding SA (0.5–1.5 wt.%), either as a consequence of the process being affected by particles or as a result of the variation in SA dispersion in the extrusion blown LDPE/LLDPE blend matrix with an increase in the SA concentration. The corresponding SEM images of the samples support the XRD measurements [ 22 ].…”

“…The XRD peak at 21.6° was ascribed to the 110 reflections of PE [ 22 ]. No obvious peaks of SA appeared in the XRD pattern of the LDPE/LLDPE/SA composite film, indicating that the SA was fully exfoliated.…”

“…Thus, the variation in the crystalline peak (107.21–106.91 °C) for the extrusion blown LDPE/LLDPE/SA (0–1.5%) system may arise from the strong effect of blown extrusion on the crystal degree orientation [ 37 ] and SA aggregates with the increase in the SA concentration as observed through SEM ( Figure 2 A–D). Additionally, the possible influence of the embedded SA particles on the molecular orientation along the flow direction is also reflected in the mechanical behavior of the film in different directions ( Figure 5 and Figure 6 ) [ 22 ].…”

Linearly Polarized Infrared Spectroscopy for the Analysis of Biological Materials

“…The ability to extract information regarding molecular orientation from p-IR data is based on the necessity that for the electric field of the incident light to excite a particular vibrational mode, a component of the electric field vector and vibrational transition dipole moment (TDM) needs to be coaligned. 52 With traditional IR instruments utilizing a globar light source, the incident light is ideally unpolarized, and hence a particular IR-active vibrational mode will absorb at least some of this light, as some component of the incident light will have an electric field aligned with the TDM. Conversely, if the incident beam is linearly polarized, maximal absorption will occur when the angle between the electric field vector and the TDM is 0, assuming the direction of travel of the incident light is orthogonal to the TDM.…”