Molecular Orientation and Anisotropy
Many mechanical properties of polymers are strongly influenced by the molecular orientation that occurs during various shaping processes. Thus, the measurement of this orientation is of particular importance for a better understanding of the molecular mechanism involved in polymer deformation.
By using linearly polarized infrared radiation, the orientation of the functional groups in these polymer systems can be measured due to the fact that absorption is greater for light vibrating along the length of the molecule than it is for light vibrating perpendicular to it. The differences in band intensities between parallel and perpendicular polarized infrared spectra for a polymer sample indicate a high degree of orientation of the polymer chains within the sample while great similarities between parallel and perpendicular polarized infrared spectra for a sample indicate a fairly random orientation with little order. This is the result of the first sample having been drawn or sheared during the manufacturing process while the second sample was not. The parameter used to characterize the degree of molecular orientation of samples is the dicroic ratio R.
While birefringence measures average values over all the components and phases present in a sample, dichroism is specific to particular infrared absorption bands and thus to local molecular segments within
a complex polymer chain.
The RHEONAUT now can accomplish such measurements in situ. Candidate samples are, for example, polymer melts with molecules be in shear flow, causing specific molecular groups be in linear alignment. The mechanical properties and the molecular orientation can be studied at the same time to get a complete description of their dependency, based on true experimental results.
An FTIR spectrometer linked to the HAAKE MARS rheometer by the RHEONAUT module provides both rheological profiles and information about transient structural changes during deformation of polymer melts. The experimental principle is based on the synchronous acquisition of infrared spectra and flow curves during shearing, recovery or stress relaxation of the samples under investigation. The additional requirement is that the polarization of the incident infrared radiation be known and reproducible. Several wire grid polarizers are available in manual and automated versions to set the infrared radiation polarized both parallel and perpendicular to a fixed reference direction of the sample.