Benign Processing of High Performance Polymeric Foams of Poly(arylene ether sulfone)
This work is concerned with the production of high performance polymer foams via a benign foaming process. The first goal of this project was to develop a process and the conditions necessary to produce a low density (>80% density reduction) foam from poly(arylene ether sulfone) (PAES). Water and supercritical carbon dioxide (scCO2) were used as the blowing agents in a one-step batch foaming process. Both water and scCO2 plasticize the PAES, allowing for precise control on both the foam morphology and the foam density. To optimize the foaming conditions, both thermogravimetric analysis and differential scanning calorimetery (DSC) were used to determine the solubility and the reduced glass transition temperature (Tg) due to plasticization of the polymer. It was determined that 2 hours was sufficient time to saturate the PAES with water and scCO2 when subjected to a temperature of 220 oC and 10.3 MPa of pressure. Under these conditions, a combination of 7.5% of water and scCO2 were able to diffuse into the PAES specimen, correlating to ~60 oC reduction in the Tg of the PAES. The combination of water and scCO2 produced foam with up to an 80% reduction in density. The compressive properties, tensile modulus, and impact strength of the foam were measured. The relative compressive properties were slightly lower than the commercially available structural foam made of poly(methacrylimide).
The second objective of the dissertation was to enhance the compressive properties of the PAES foam, without concern for the foam density. Foam was produced over a range of density, by controlling the cell size, in order to optimize the compressive properties. Carbon nanofibers (CNFs) were also added to the PAES matrix prior to foaming to both induce heterogeneous nucleation, which leads to smaller cell size, and to reinforce the cell walls. Dynamic mechanical thermal analysis (DMTA), on saturated CNF-PAES, was used to determine the reduced Tg due to plasticization and establish the temperature for pressure release during foaming. DMTA proved to be more effective than DSC in establishing quantitative results on the reduction in the Tg. The CNF-PAES foam produced had compressive properties up to 1.5 times the compressive properties of the PAES foam.