The overall objective of this PhD thesis is to contribute to a greater understanding of the mechanisms involved in the generation of low density and low cell size polymeric foams. Specifically, this means to understand what parameters contribute to an increase in nucleation density and a decrease in cell size. Polymeric foams with low cell size and density serve a lot of different applications such as lightweight insulation, filtration, catalysis as well as tissue engineering. A decrease in density and cell size below 70 nm leads to an increase in mechanical properties and a decrease in thermal conductivity compared to conventional foams with cell sizes in the micrometer range. Below a cell size of 40 nm, optical transparency can be achieved.
Such nanocellular foams are produced using high-pressure infiltration of CO2 in an autoclave followed by rapid depressurisation. This makes it difficult to observe the process experimentally and determine mechanisms and parameters influencing nucleation and growth. In order to obtain an insight into nucleation and growth mechanisms that are potentially relevant for nanofoam formation, a novel solid-state foaming process employing PMMA and methanol was developed. The process is partly analogous to the CO2-based nanofoam development process, but experimentally more accessible. This foam and its development were characterized and important mechanisms in the process for nucleation and void development were identified. The most important finding was that methanol-desorption-induced stresses develop and gradually expand post-critical nuclei into a foamable state. This desorption-induced stress-development and consequent nuclei expansion was identified as a necessary parameter to achieve foam formation within the PMMA and methanol system. It was shown that externally applied stresses also contribute to nuclei expansion towards a foamable state. Furthermore, the expansional effect of externally applied stresses superimposes with the expansional effect on nuclei development of desorption-induced stresses. Altogether, desorption-induced stress development could be identified as a so far unrecognized mechanism that influences foam development. Furthermore, it was observed that an increase in sample (methanol-charged PMMA in unfoamed state) temperature following a freeze is also a source of stress resulting from density differences. Since the nano-foam production process comprises desorption as well as a temperature increase following a freeze, the hereby resulting stresses are potentially relevant in the nano-foam production process as well. The implications of this work for the production of nanofoams is therefore the identification of production-accompanied stress development as a foam properties influencing magnitude.