A laser Raman study of the stress dependence of vibrational frequencies of a monocrystalline polydiacetylene

Abstract

The applicaiton of a tensile stress to a monocrystalline fiber of a polydiacetylene, an extended-chain polymer, causes the Raman-observed C≡C and C=C in-chain vibrational frequencies to decrease linearly with elongation. The polymer studied, poly-HDU, results from the solid state polymerization of R–C≡C–C≡C–R, where R=–CH2OCONHC6H5. This experimental relationship between bond frequency shifts and deformation is analyzed by calculating bond force constant changes as a function of macroscopic stress. It is shown that possible effects due to stress-induced changes in electron delocalization in this fully conjugated polymer are negligible and the principal contributions to the bond frequency shifts are due to bond anharmonicity. Models which assume that the entire effect is due to bond anharmonicity produce calculated dependencies of ν (C=C) and ν (C≡C) upon fractional elongation, which are in good agreement with that observed.