Influence of Microphase Morphology and Long-Range Ordering on Foaming Behavior of PE-b-PEO Diblock Copolymers

Abstract

In this study, spherical, lamellar, and aligned lamellar types of polyethylene-b-poly(ethylene oxide) (PE-b-PEO) diblock copolymers were prepared and comparatively foamed with CO 2 to understand the influence of microphase morphology and long-range ordering on foaming behavior. The experimentally measured melting point, CO 2 solubility, and interfacial tension in the presence of CO 2 indicated the potential of CO 2 -philic PEO block acting as bubble nuclei. Scanning electron microscopy results revealed that lamellar PE-b-PEO produced open cells all the way, whereas spherical PE-b-PEO created closed cells at low temperature and mesh-like openings at high temperature. Continuous PEO microdomains directly provide numerous potential opening channels and therefore induce the bursting of cells. The drastic biaxial stretching of cell walls at high temperature turns the embedded discrete PEO spheres into those channels responsible for the distinct mesh-like perforations. As for the flow aligned lamellar PE-b-PEO, submicron open cells were fabricated with a regular multilayered pattern. Long-range ordering not only suppresses the undesired coalescences but also preserves the self-assembled pattern. Foaming mechanisms, mainly focused on the bubble growth, were finally uncovered for different morphological PE-b-PEO block copolymers, complete understanding of which will be of importance in the block copolymer design to meet the requirement on foam structures.

Control of the Cell Structure of UV-Induced Chemically Blown Nanocellular Foams by Self-Assembled Block Copolymer Morphology

“…The use of self-assembled block copolymers as a template in creating nanocellular foams has been studied extensively for physical foaming. In physical foaming, one side of the block copolymer chain is designed to be CO 2 -philic, e.g., poly­(perfluoro­octylethyl methacrylate), ,,,, poly­(1,1,2,2-tetrahydro­perfluorodecyl acrylate), and poly­(ethylene glycol). ,, During the foaming process, supercritical CO 2 is selectively dissolved into the self-assembled CO 2 -philic domains under a high temperature and a high pressure, and it is expanded by releasing the physical constraints. The cell size can be controlled by adjusting some process parameters, such as gas saturation pressure and foaming temperature.…”

“…The cell size can be controlled by adjusting some process parameters, such as gas saturation pressure and foaming temperature. However, during the gas saturation and foaming processes, the self-assembled morphology may be reconstructed and/or significantly softened due to the supercritical CO 2 . ,,, Therefore, it remains challenging to maintain the framework of a self-assembled template and enhance the ordering of cells through the physical foaming process.…”

“…Using self-assembled block copolymers with CO 2 -philic blocks is another strategy to control the cell structure. During the gas saturation process, the gas will selectively swell the CO 2 -philic blocks. − This property enables the nucleation to be controlled in the CO 2 -philic domains and increases the number density of bubble (cells) during the depressurization process. Using this method, Li et al demonstrated the production of highly dense nanocellular foams with cell sizes and densities of 15–25 nm and (6–15) × 10 10 cells/cm 3 , respectively.…”