Vibrational spectroscopy of linear and branched long alkanes.

In this work we studied monodisperse long chain alkanes as model compounds for semi crystalline polymers, using vibrational spectroscopy. The interest in vibrational spectroscopy is due to its sensitivity to both the crystalline structure and conformational disorder. We used the following vibrational techniques: Fourier transform infrared transmission, Fourier transform infrared - attenuated total reflection (FTIR-ATR), Raman spectroscopy and Inelastic neutron scattering (INS) spectroscopy. In addition, small angle X-ray scattering (SAXS) was used where necessary. In this study FTIR-ATR and INS techniques were not successful in the characterisation of long chain alkanes. A Raman spectroscopic method, based on conformationally sensitive vibrations, was established using a short chain n-alkane to identify solid-solid phase transitions and was applied to long chain alkanes. Using this method a transition to a non-integer form (NIF) form was identified in two 'Y-shaped' long chain alkane samples at similar temperatures as SAXS. One of these samples is partially deuterated (with deuterium end caps) while the other one is its hydrogenated analogue. However, the end-deuterated sample is not 100% end-deuterated due to a fault in the synthesis. Therefore, here we developed a FTIR-transmission method to determine the isotopic ratio in these partially deuterated alkane samples. Our calculations based on this method showed that the end deuterated 'Y-shaped' alkane only consists of 79.9% of the expected deuterium(D)/hydrogen(H) ratio. A SAXS study on a linear end-deuterated alkane showed that the chains achieve a maximum tilt of 35°, which is the most common tilt angle also for polyethylene. Combining these results with those from previous studies on the same sample, a relationship between chain perfecting and the chain tilt was established. It was found that the ordering of the crystal structure induces chain tilting in order to accommodate the incoming chain ends to the crystalline layer. The increase in the domain size during this process was found to be from 16 to more than 50 chains per domain. The attempt to explain the observed shift of the SAXS spacings in the tilting experiment, away from the positions predicted by Bragg's equation by using a paracrystalline model failed. Hence, we suggest a model with an asymmetric thickness distribution will be more appropriate in this regard. The simulated CD[2] bending vibration mode splitting of the infrared spectrum of the random fold model of this end-deuterated alkane is 9.0 cm[-1] whereas a maximum splitting of 9.1 cm[-1] is expected for this mode for a regular fold model of the same alkane. Therefore a vibrational spectroscopic differentiation between these two models based on the experimental CD[2] bending splitting is highly unlikely.