The physical aging of star-shaped macromolecules: role of functionality

The phenomenon of physical aging, structural relaxations that enable the return of a polymer, quenched to a temperature $T_{age}$ below its glass transition temperature $T_g$, to equilibrium, was investigated in a series of star-shaped macromolecules. These macromolecules possessed functionalities that varied from $f=3$ to $f=64$, and their degrees of polymerization per arm $N$ were all comparable ($N\sim100$). The aging of these star-shaped macromolecules is qualitatively similar to that of linear chain polymers, with their aging rates $K$ exhibiting maxima at threshold temperatures $T_{tr}$. The aging rates of the star-shaped molecules, however, are slower than their linear analogs. Moreover, $T_{tr}$ decreased with increasing $f$, and $K$ increased with increasing $f$ for $T_{age} < T_{tr}$. Our results are, in part, rationalized in terms of dynamic percolation models.