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Model Branched Polystyrenes. From Synthesis to Rheology and Correlated Foaming PropertiesThursday (08.10.2020) 15:50 - 16:10 Room 1 Part of:
The rheology of branched polymer melts are of special interest to academia and industry. Especially in foaming process where nucleation, shear and elongational flow occurs, the strain hardening and dynamic dilution effects of the branches have a distinct influence on the manufacturing process and consequently on the final properties of the product. Investigating the rheological and foaming properties and correlating it to molecular topological parameters like length of the main and side chains as well as the degree of branching requires controlled synthesized and well characterized model polymers. A series of comb-PS from loosely grafted to bottlebrush-like structures were synthesized by the combination of anionic polymerization and grafting-onto method. These well-characterized comb PS had same backbone, similar branch length but different branching density. Shear rheological and extensional properties of these combs were investigated and correlated with the molecular structure. In the current work, the samples were foamed using a batch foaming setup at constant saturation pressure and pressure release rate conditions with supercritical CO2 as the blowing agent. Shear and extensional rheological data in the framework of the zero shear viscosity (η_0) and strain hardening factor (SHF) as a function of number of side chains was used to define a rheological fingerprint and correlate them to foam characteristics of well-defined model polymers. This research revealed that all comb-PS with the same length of sidechains had similar cell density and cell size. However, there was an optimum number of branches per backbone resulted in a maximum volume expansion ratio (V.E.R.) at minimum zero shear viscosity with a maximum SHF > 200 for Hencky strains below ε_H=4, which is tremendously high.
Such a high strain hardening shows of great fundamental and technical importance in extensional processes and could be directly correlated to the foaming results. This allows a better understanding of the important parameters of molecular dynamics during the foaming process and synthetic optimization of the topological structure towards micro- and nanoporous foams.