Microstructural PALS study of regulated dimethacrylates: Thiol‐ versus β‐allyl sulfone‐based networks

Abstract

Radical photocuring of multifunctional (meth)acrylates is lacking control over the irregular chain growth process yielding highly crosslinked, inhomogeneous networks. Chain transfer agents (CTAs, e.g., thiols or β‐allyl sulfones) have been widely used to modify this curing process, thus reducing shrinkage stress and increasing the toughness of the formed photopolymers. Resulting photopolymer networks exhibit higher bulk density, lower crosslinking density, and narrow glass transitions. Consequently, a more homogeneous network structure was postulated for those networks. Whereas macroscopic properties of the modified final materials have already been studied, herein the microstructural arrangement of such modified networks has also been evaluated with the help of positron annihilation lifetime spectroscopy (PALS). A more homogenous network structure with a decreased average free‐volume void size was confirmed for CTA‐based dimethacrylate networks. A sharper distribution of the ortho‐positronium (o‐Ps) lifetime, mainly for the β‐allyl sulfone‐based photopolymers, hints toward a more regulated network structure. Moreover, the combination of PALS, DMTA, density and swelling experiments elucidates relations between void formation, crosslinking density and macroscopic characteristics such as shrinkage stress and mechanical properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2476–2484

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

“…It can be concluded that the higher T g for the poly2M‐co‐DAS network indicates a higher rigidity of chains compared with poly2M‐co‐EDDT and the smaller ∆ T g hints toward a more homogeneous network architecture. These results are consistent with our previously reported findings and the published literature…”

“…This indicates the more restricted mobility of chain constituents in poly2M‐co‐DAS within the rigid state. This is probably due to the previously reported tight network structure, which was also confirmed by the higher bulk density, packing efficiency, and the smallest free‐volume voids determined at room temperature below T g . On the other hand, above T g TMA , the higher values of CVTE 5.43–5.49 × 10 −4 K −1 for samples containing β‐allyl sulfone (poly2M‐co‐DAS) and dithiol (poly2M‐co‐EDDT) spacers reflect the flexibility of the crosslinked structure compared to poly2M, which is confirmed by their similar thermal expansion behavior in the rubbery state.…”

“…Arrhenius plots of the relaxation temperatures (poly2M: ■ solid; poly2M‐co‐EDDT: ● dash; and poly2M‐co‐DAS: ▼ dot). The inset shows the tan δ plots determined at 1 Hz …”

“…As expected, the activation energy of the glass transition is the highest for poly2M due to the highest crosslink density. The materials with regulated network architecture show comparable activation energies of the glass transition, but their ranking does not follow the crosslink density (crosslink density is higher for poly2M‐co‐EDDT than for poly2M‐co‐DAS).…”

“…However, thiol‐ene networks with incorporation of multifunctional thiols have not been reported. In our pilot PALS study we investigated modified dimethacrylate networks at room temperature . The inhomogeneous network structure of a pure dimethacrylate‐based photopolymer (poly2M) exhibits larger and less uniform free‐volume voids than CTA‐based networks (i.e., β‐allyl sulfone‐ and thiol‐based networks).…”