Effect of Molecular Stiffness on the Physical Aging of Polymers

Upon quenching a polymer melt to a temperature T below its glass transition temperature T-g, structural relaxations, physical aging, enable the material to return to equilibrium. Whereas the physical aging rate beta is independent of chain length for linear chain polymers, in the case of star-shaped polymers, beta depends on the functionality f (number of arms per molecule) and the degree of polymerization per arm N-arm; the dependence of beta on f, and on N-arm, is suppressed for large N-arm. Incoherent elastic neutron scattering measurements of star-shaped polystyrenes reveal that the mean square atomic vibrations decrease with increasing f. Consequently, the harmonic force constants kappa proportional to 1/, a measure of the local macromolecular stiffness, increase with decreasing f. This connection between the decrease of the aging rates and the increase of the molecular stiffness with increasing f is reported here for the first time, providing a rationale for understanding the aging of macromolecules of varying topologies.