Effect of Molecular Stiffness on the Physical Aging of Polymers
Bradley Frieberg, Emmanouil Glynos, Γεώργιος Σακελλαρίου, Madhusudan Tyagi, Peter F. Green
Abstract
Upon quenching a polymer melt to a temperature T below its glass transition temperature Tg, structural relaxations, physical aging, enable the material to return to equilibrium. Whereas the physical aging rate β is independent of chain length for linear chain polymers, in the case of star-shaped polymers, β depends on the functionality f (number of arms per molecule) and the degree of polymerization per arm Narm; the dependence of β on f, and on Narm, is suppressed for large Narm. Incoherent elastic neutron scattering measurements of star-shaped polystyrenes reveal that the mean square atomic vibrations ⟨μ2⟩ decrease with increasing f. Consequently, the harmonic force constants κ ∝ 1/⟨μ2⟩, 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.