ACCORDION-TYPE LASER-RAMAN (ALR) OSCILLATIONS IN CRYSTALLINE POLYMERS

If one interprets the ALR absorption lines of crystalline polymers in terms of oscillating elastic rods of length equal to the thickness L of the crystalline lamellae, or the thickness D of the crystalline core of the lamellae, one obtains for the elastic modulus of these rods values which are substantially higher than the axial elastic modulus Ec of the crystal lattice. The effect is particularly conspicous with polymers which in the crystalline state exhibit a helical-chain conformation. The explanation of this effect seems to be the elastic coupling of longitudinal oscillations of chain stems of subsequent lamellae by the intervening amorphous layers. The apparent increase of the ALR oscillation frequency by such coupling is particularly large if the elastic modulus Ea of the amorphous layers is equal or even higher than that of the crystal lattice. The high value of Ea derived from such an analysis is a consequence of the very high ALR frequency (∼0.3 THz), which makes the amorphous layer react as a rigid glass.