Study of free volume parameters of polystyrenes of different molecular architecture by positron annihilation lifetime measurement
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Differences of size, content and size distribution of free volumes in linear branched and three-armed polystyrenes, synthesized by radical and anionic processes, were observed by positron annihilation lifetime measurements. The temperature dependence of an average free volume radius was quite similar among polystyrenes of different architectures and molecular weight distributions. The free volume radius increased with temperature, from 0.27 nm (T:∼60 K) to 0.29 nm (T:∼260 K) and 0.30 nm (Tg:363 K), then to 0.35 nm (423 K), showing turning at γ and α transition temperature. The free volume content decreased from 60 K to ∼220 K to ∼300 K showing peculiar minimum at 220 K to 300 K depending on the molecular shape, increased above ∼320 K, upto 340 to ∼360 K. The free volume contents decreased with an increase of molecular weight and by an addition of oligomer or plasticiser, suggesting differences in relaxation time or molecular motion between the edge and middle portions of molecular chain and filling effect of smaller molecules in free volumes, respectively. The apparent free volume fraction showed clear variations atTβ andTg. Size distribution of free volumes suggested more complicated behavior of free volume upon the molecular relaxations and filling effect.
KeywordsOligomer Polystyrene Free Volume Molecular Weight Distribution Molecular Chain
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