The phonon dynamics in hydrogenated stretched polyethylene was investigated using inelastic x-ray scattering. The dispersion curve of longitudinal acoustic modes describing intrachain collective motions was measured in the first Brillouin zone, and compared to inelastic neutron scattering data on deuterated similar samples. This first exploration of the collective dynamics within the first Brillouin zone reveals the presence of an additional low energy component. In spite of its large energy distribution, this feature is seen to slightly disperse with momentum transfer Q. The possible origins of this low energy scattering are discussed; arguments converge towards the collective dynamics of the amorphous regions. (C) 2003 American Institute of Physics.