Effect of Molecular Stiffness on the Physical Aging of Polymers

• Published in: Macromolecules Vol. 53; no. 18; pp. 7684 - 7690
• Main Authors: Frieberg, Bradley R, Glynos, Emmanouil, Sakellariou, Georgios, Tyagi, Madhusudan, Green, Peter F
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.09.2020
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.0c01331

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 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 N arm; the dependence of β 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 ⟨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.