Continuous gradient temperature Raman spectroscopy and differential scanning calorimetry of N-3DPA and DHA from -100 to 10°C.

Docosahexaenoic acid (DHA, 22:6n-3) is exclusively utilized in fast signal processing tissues such as retinal, neural and cardiac. N-3 docosapentaenoic acid (n-3DPA, 22:5n-3), with just one less double bond, is also found in the marine food chain yet cannot substitute for DHA. Gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we apply GTRS and both conventional and modulated DSC to n-3DPA and DHA from -100 to 20°C. Three-dimensional data arrays with 0.2°C increments and first derivatives allowed complete assignment of solid, liquid and transition state vibrational modes. Melting temperatures n-3DPA (-45°C) and DHA (-46°C) are similar and show evidence for solid-state phase transitions not seen in n-6DPA (-27°C melt). The C6H2 site is an elastic marker for temperature perturbation of all three lipids, each of which has a distinct three dimensional structure. N-3 DPA shows the spectroscopic signature of saturated fatty acids from C1 to C6. DHA does not have three aliphatic carbons in sequence; n-6DPA does but they occur at the methyl end, and do not yield the characteristic signal. DHA appears to have uniform twisting from C6H2 to C12H2 to C18H2 whereas n-6DPA bends from C12 to C18, centered at C15H2. For n-3DPA, twisting is centered at C6H2 adjacent to the C2-C3-C4-C5 aliphatic moiety. These molecular sites are the most elastic in the solid phase and during premelting.

Dynamic infrared linear dichroism study of the temperature-dependent, viscoelastic behavior of a poly(ester urethane).

In the study reported here, of the poly(ester urethane), Estane 5703, simultaneous dynamic mechanical analysis (DMA) and dynamic infrared linear dichroism (DIRLD) measurements have been carried out at continuously variable temperatures from -50 to +30 degrees C. Multivariate curve resolution-alternating least squares (MCR-ALS) analysis of the spectral data has been correlated with the thermo-mechanical properties. Spectral changes, analyzed as a function of temperature, are compared with the storage and loss moduli to provide insight into viscoelastic behavior at the molecular level. In addition, the data for pure Estane have been compared to those for plasticized Estane samples, which contain 10 and 30% plasticizer by weight. These comparisons show a strong and consistent correlation between the macroscopic rheological properties and the microscopic (molecular, inter-molecular, and sub-molecular) responses of this block co-polymer.