Abstract
Well-defined three-arm and four-arm star polymers designed via a Z-group approach carrying trithiocarbonate functionalities at the core are prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization featuring molecular weights of Mn,SEC = 156 kDa, Đ = 1.16 (3-arm) and Mn,SEC = 162 kDa, Đ = 1.15 (4-arm) based on multi-angle laser light scattering (MALLS) detection, respectively. The star-shaped polystyrenes are subjected (in bulk) to thermal stress in the temperature range between 140 and 200 °C from 10 minutes up to 96 h. The thermally treated 3-arm and 4-arm star polymers are analyzed via size exclusion chromatography (SEC) to quantify the degradation process at variable temperatures as a function of time under an argon atmosphere. Cleavage rate coefficients of the star polymers are deduced as a function of temperature, resulting in activation parameters for the cleavage process, i.e. Ea = 131 kJ mol−1; A = 3.93 × 1011 s−1 (Mn,SEC = 156 kDa, Đ = 1.16, 3-arm star) and Ea, = 134 kJ mol−1; A = 9.13 × 1011 s−1 (Mn,SEC = 162 kDa, Đ = 1.15, 4-arm star), respectively. Processing of the star-shaped polymers is mimicked via a small scale counter rotating twin screw extrusion to achieve nonlinear shear and elongation flow under pressure. Furthermore, a rheological assessment via the linear shear deformation region (small amplitude oscillatory shear, SAOS) allows for a correlation of the processing conditions with the thermal degradation properties of the star polymers in the melt. Zero shear viscosity (η0) as a criterion of the degradation process is measured in the rheometer and correlated to the weight-average molecular weight, Mw.
Highlights
Molecular and mechanistic knowledge of the fundamental processes occurring in polymeric materials carrying functional groups under thermal and mechanical stress is highly relevant for enabling their large scale processing via e.g. extrusion and injection molding as well as techniques involving the shaping and manufacturing of polymer based products
Z-type trithiocarbonate functional star-shaped polystyrenes with 3-arm and 4-arm featuring a Z-group design have been prepared via reversible addition fragmentation chain transfer (RAFT) polymerization with high molecular weight and relatively low polydispersity
The kinetic analysis allows for a comparison between the thermal stability of Z-group trithiocarbonate RAFT star polystyrenes as a function of time and their extrusion processes under thermal and mechanical stress.[29]
Summary
Mostly anionic polymerizations have been widely utilized to generate complex macromolecular architectures with controlled molecular weights, well-de ned chain ends, low polydispersity and various compositions.[10,11,12] there has been rapid growth in the area of controlled/living radical polymerizations (LRP) in the last two decades due to the variety of applicable monomers and the more tolerant experimental conditions than living ionic polymerization routes.[13]. In the present study – a signi cant extension of our previous work on linear polymer into branched polymer melt29 – highly pure and well-de ned star-shaped polymers were prepared via RAFT polymerization via the Z-group approach. The stability of such star-shaped polymer is investigated under thermal and mechanical stress under an inert atmosphere (nitrogen or argon) to understand their cleavage kinetics as well as rheological behavior in order to provide guidelines for their potential thermal processing. The precipitate was ltered off and dried under vacuum to afford the polymers as white powders
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