高分子材料加工技术专业英语 Properties of Macromolecular Movements 35.Crystalline polymers.ppt

Crystalline polymers  Vibrational spectroscopy of polymers Exotic Morphologies Molecular vibrations – vibrational spectroscopy Quantum Vibrational Motion Anharmonic Oscillator Types of Vibrations Vibrational spectroscopy Infrared is Rovibrational Spectroscopy The Raman Spectrum Origin of Raman Effect - Classical Raman vs. IR Compare IR and Raman Group frequencies For large molecules - vibrations of some functional groups acts like independent oscillators i.e. always found at ~ same frequencies Factors affecting group frequencies Center of Symmetry (i):determine IR active or Raman active. For IR active vibration an oscillating electric dipole must be generated. For Raman active vibration a change in polarizability of the molecule is produced, which gives rise to induced dipole. Symmetry factor A molecule having a center of symmetry (i) has no permanent dipole moment, so a vibration symmetric to i (sym. mode) does not generate oscillating dipole and therefore it is an IR inactive vibration. But vibration anti-symmetric to i (anti-sym. mode) generates transient oscillating dipole, so it will be IR active vibration and show up in the spectrum. Mutual exclusion rule Symmetric mode vibration usually gives rise to Raman scattering which causes changes in polarizability of molecule, and it is a Raman active vibration, showing up in the Raman spectrum. Mutual exclusion rule : For IR and Raman spectra, some lines missing in one would show up in the other, due to different symmetry requirement for each spectra. Thus, the information from IR data is complementary to that obtained from Raman. General rule of symmetry Symmetry in molecule reduces the normal modes of vibrations and simplifies spectrum. CO2 : sym. stret. = 1340 cm-1 , IR inactive, but Raman active. assym. stret. = 2349 cm-1 , IR active. IR vs. Raman : mutual exclusive. Symmetry : IR shows only assym. stret. Symmetry simplify spectra Mechanical coupling Interaction between