Crystalline polymers Longitudinal Acoustical Modes in crystalline polymers and block co- polymers are studied by Raman spectroscopy and Small Angle X-ray Diffraction. The LAM frequencies are modelled by treating the chain molecules as elastic rods perturbed by end masses and forces. The effects of polymer chain end groups, chain length and temperature on LAM frequencies are described mathematically, allowing for the description of the stacking morphology of the lamellar structures.