Monodisperse block copolymers: influence of morphology and topology on cell size in polymer foamsWednesday (07.10.2020) 16:20 - 16:40
Foaming of polymers is a very complex procedure and is dependent on various parameters, which include not only the process conditions but also the molecular properties. The latter not only summarize the chemical composition of the polymer, but also the molecular structure and interactions with other components of the system, such as blowing agents. The influence of special parameters, like polymer topology, has already been intensively investigated. Most published studies only consider the above mentioned influencing variables in undefined, polydisperse systems.
Within this presentation several influencing parameters were correlated with the foaming behavior using model systems of different polymer compositions based on polystyrene (PS) and polyisoprene (PI). The samples were foamed using a batch foaming setup at constant saturation pressure (500 bar) and pressure release rate (250 bar/s) with supercritical CO2 as the blowing agent. The most important material parameters for foaming can be divided into two categories:
Phase morphology: Gas solubility and chain mobility are determined by the molecular structure of the polymer. By anionic synthesis of block copolymers and systematic variation of PI content, different phase morphologies (intrinsic heterogeneity) can be generated and the influence of both chain mobility and gas solubility on foaming behavior can be investigated. The blowing agent CO2 has higher solubility in PI compared to pure PS, which results in a lower foam density under the same foaming conditions.
Polymer topology: It is known that long chain branching leads to increased melt strength and induces strain hardening under extensional deformation. However, an increased melt strength during bubble growth, as a biaxial stretching phenomenon, improves the stability of growing cells and prevents coalescence of cells during the foaming process.
The anionic synthesis of clearly defined, monodisperse (Đ < 1.1) linear and branched block copolymers enables the systematic investigation of the influence of the composition and the resulting morphology and allows to improve the basic understanding of the foaming process with regard to nucleation and bubble growth. However, closed-cell foams with cell sizes around 400 nm combined with a cell density of up to 10^13 cell/cm^3 were realized. The aim is not only to investigate the influence of a single parameter, but also to correlate different material parameters systematically.