University of Toronto,
Keywords: micro-/nano-cellular structures, micro-/nano-structuring, supercritical fluid
Summary:Most new polymeric products contain two or more polymers and/or functional additives resulting in desired properties contributed from each component. Recently, our group is working on creating coextruded micro-/nano-layered foams and hierarchically porous structures to tune morphologies and properties. we present a fundamental and experimental investigation of cell nucleation and growth mechanisms in advanced Micro-/Nano-Layered (MNL) polymeric structures with alternating film and foam layers. Foams can be prepared from any type of plastic by introducing a gas or supercritical fluid (SCF) within the polymer matrix. The applications of microcellular plastics containing billions of tiny bubbles less than 10 microns in size have broadened due to the lightweight characteristics, excellent strength-to-weight ratios, superior insulating abilities, energy absorbing performances, and the comfort features associated with plastic foams, as well as their cost-effectiveness and cost-to-performance ratios. We found that the cell nucleation and growth phenomenon in MNL systems are governed by the synergy of two categories of parameters: morphological parameters (i.e., film and foam layer thicknesses and the number of layer interfaces) and material parameters (i.e., material stiffness and compatibility with neighboring layers). The presence of adjacent film layers can significantly increase cell density through three mechanisms: promoting heterogeneous cell nucleation, preventing cell deterioration, and confining cell growth. The influence of film layers varied in different layer thickness regions and interface densities, where stiffer and more compatible film layers produced higher cell densities. In addition, hierarchically porous (HP) structures were developed to provide an effective alternative to develop lightweight thermal insulation materials, which play an important role in efficient energy management. Unlike elaborate synthetic methods, achieving HP features via SCF techniques is still at an early stage. In this study, a facile and effective approach is reported to develop tailored HP foams via SCF foaming of binary blends with nanoscale phase separation patterns. The resultant cyclic olefin copolymer (COC) blend foams are extremely lightweight (50 kg m-3) and have excellent thermal conductivity of 25.8 mW m-1K-1, which is on par with the thermal conductivity of air. Through the SCF foaming process, tunable HP structures can be created from COC blends with nanoscale phase patterns (phase size from ~100 to 240 nm).