Observation of boson peaks by inelastic neutron scattering in polyolefins

Inelastic neutron scattering was used to probe the nature of the boson peak in atactic polypropylene (aPP), head-to-head polypropylene (hhPP), polyiso butylene (PIB) and a 1/1 mass ratio hhPP/PIB blend. Atactic polypropylene i s among the most "fragile" of glass formers and was found to have a shoulde r rather than the distinctive peak exhibited by the other three polymers. T his difference is already apparent at 15 K where relaxations are not expect ed to occur. The results suggest that the fragility of hhPP is intermediate between aPP and PIB. Within this group of polymers which have similar chem ical structures the position of the boson peak appears to correlate with th e glass transition temperature and the cohesive energy density. The possibi lity of a correlation with chain stiffness as expressed by the characterist ic ratio is also discussed. (C) 1999 American Institute of Physics. [S0021- 9606(99)50128-1].