Investigating the Free-Volume Characteristics of Regulated Dimethacrylate Networks Below and Above Glass Transition Temperature

Abstract

Studying the relationship between microstructural interactions and resulting macroscopic material properties gives a more elaborate understanding on final material performance. The microstructural arrangement with characteristic length, relaxation, and thermal expansion behavior of regulated dimethacrylate networks is studied by differential scanning calorimetry, dynamic mechanical analysis, thermomechanical analysis, and positron annihilation lifetime spectroscopy at a broad temperature range (250–380 K). The length scale ξ (Tg) of cooperative segmental rearrangement is decreased with increasing crosslinking and degree of freedom in regulated chain transfer agent (CTA)-based networks. The macro and microscopic thermal expansion coefficients are determined in the glassy and rubbery state for three polymer systems—a dimethacrylate-based reference (poly2M) and regulated photopolymers with CTAs (i.e., thiol, β-allyl sulfone). The influence of CTAs on the free volume characteristics (i.e., free volume void size, the specific occupied volume V0, free volume number density per unit mass N′, and free volume fraction f) at the glass transition temperature Tg is correlated with the cooperative length, activation energy Ea,α, and mechanically effective crosslinking density. A more complex understanding of the revealed relations contributes to a more elaborate explanation of final material performance.