We have studied the Raman spectrum of β-carotene using excitation wavelengths from visible (514.5 nm) to near-infrared (1064 nm). Qualitatively, the spectrum appears independent of the choice of excitation wavelength, although this is not true in detail; in particular, the ratio of the intensities of the νC-C and νC=C fundamentals to their overtones and combinations varies with the excitation wavelength. The inelastic neutron scattering spectrum shows no evidence for unusual dynamics in this system, and the UV/vis/NIR spectrum shows that there are no allowed electronic transitions from the ground state beyond ∼700 nm. These observations provide strong evidence for the existence of a different mechanism for the enhancement of the bands. Indirect evidence suggests that, with these longer excitation wavelengths, the intensities of the νC-C and νC=C peaks are at least two orders of magnitude smaller than those from resonance enhancement with the use of excitation into the visible absorption bands. The implications for the characterization of polyenes in polymers are discussed.
Raman spectroscopy was used to study strain-induced molecular stress in cold-drawn
polyethylenes, which were being used as a model system for fibrils present in the crazes formed during
environmental stress crack resistance (ESCR) tests. The molecular stress was measured at 240 K in
order to minimize relaxation phenomena. Molecular stress was related to macroscopic strain and, by
correcting for differences in E-moduli, to true stress. In this paper, the measured molecular stress is
related to ESCR values and sample characteristics. It was observed that good ESCR materials showed
a lower molecular stress than worse ESCR materials at the same macroscopic strain level. It was also
observed that the molecular weight has a major effect on the observed molecular stress per macroscopic
strain (molecular stress per macroscopic strain decreases with increasing M
w), whereas the effect of chain
branching is smaller (molecular stress per macroscopic strain decreases with chain branching).
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