The influence of the UV-initiated RAFT-polymerization on the structures and the properties of polymer networks

In this work the influence of the Reversible Addition-Fragmentation chain Transfer (RAFT)-polymerization on the microstructures and the resulting properties of polymer networks was investigated. Elastomeric, statistical poly(butylacrylat-co-1,4-butanedioldiacrylate, BA‑co‑BDDA)- and poly(ethylacrylat-co-1,4-butanedioldiacrylate, EA-co-BDDA) networks were synthesized via UV-initiated RAFT polymerization. The amount of the crosslinker BDDA as well as the amount of the RAFT agent S-ethyl-propane-2-ylonate-S'-hexyl-trithiocarbonate (EPHT) were varied. Compared to their counterparts made from UV-initiated, conventional radical polymerization the RAFT networks were much softer as tensile tests revealed. They featured lesser elasticity moduli and with an amount of BDDA under 1 mol% greater elongations at break. A similar tendency was shown in swelling experiments conducted in acetone. The RAFT networks with an amount under 1 mol% BDDA featured greater volume degrees of swelling than the corresponding conventional ones. During these experiments it was found that the lower strand densities of the RAFT networks were caused by a part of the sol-fraction which is not added to the overall network as well as the reduction of nanogel domains in the RAFT networks, respectively. Both are a result of the modified mechanism of network building during the RAFT polymerization. The modified microstructures of the RAFT networks had also an enormous effect on the tackiness of the networks. The greater tackiness of the RAFT networks were investigated via rolling ball- as well as probe tack tests, respectively. Using Atomic Force Microscopy (AFM) it was found that this is a consequence of the modified surface structures as well as the mechanical properties of the surfaces. Additionally to the swelling experiments special Slice Selective Excitation (SSE)-Nuclear Magnetic Resonance (NMR)-measurements were conducted. In these experiments it was found that the RAFT networks exhibit a more homogenous distribution of the strand densities than the conventional networks. On the basis of isothermal Differential Scanning Calorimetry (DSC)-measurements it was found that the conventional radical polymerizations proceeded much faster than the RAFT polymerizations which additionally showed a lesser self-acceleration. The kinetics of the crosslinking RAFT polymerizations were more or less similar to the kinetics of none-crosslinking RAFT polymerizations while the kinetics of crosslinking and none-crosslinking conventional radical polymerization showed a great deviation. Furthermore it was found that the RAFT polymerizations featured an increasing retardation with increasing amount of the RAFT agent. This was caused by the absorption of light at the wavelength of 366 nm by the RAFT agent EPHT. That this retardation was not the cause for the modified microstructures of the RAFT networks was shown in tensile tests of poly(BA‑co‑BDDA) networks in which the thiocarbonylthio compound S‑S'‑diethyltrithiocarbonate (DET) was added. At 366 nm the optical absorption of DET is similar to EPHT but due to its molecular structure DET is not able to act as a RAFT agent. For that reason, the mechanical properties of the networks synthesized in the presence of DET were similar to networks made by conventional radical polymerization