Tunability of Free Volume and Viscoelastic Damping of Thiol–Ene Networks Deep in the Glassy State

Model thiol-click glassy networks with low shrinkage and sharp glass transition temperatures are studied down to cryogenic temperatures to relate viscoelastic damping to changes in microscopic hole volume. Networks synthesized by polymerization of divinyl sulfone with pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), trimethylolpropane tris(3-mercaptopropionate) (TMPTMP), and pentaerythritol tetrakis(2-mercaptoacetate) (PETT) were studied to ∼50 K using positron annihilation and thermal analysis methods. Across the glass transition temperature, overall volume expansivity is dominated by expansion of microscopic holes. Beneath Tg, holes contract upon cooling and then become static with negligible expansivity at temperatures well above 0 K. A trade-off between complete rigidity of static free volume and viscoelastic damping ability below Tg is discussed. All samples show moderate damping (tan δ > 0.05), and one sample exhibits a prominent β-transition in its viscoelastic loss spectra. These results in...