Microcellular foam is a polymeric foam with bubble sizes of 10 microns or less that is produced by saturating a polymer with gas and then utilizing the thermodynamic instabilities that result when the polymer is heated and the pressure is reduced to nucleate the cells. A model for the nucleation of microcellular foam in amorphous polymers with additives has been developed. The nucleation process depends on the solubility, concentration, and interfacial energy of any additives present. At very low levels, additives in solution act to increase the free volume of the polymer, resulting in homogeneous nucleation within the free volume Well above the solubility limit, heterogeneous nucleation dominates, as it lowers the activation energy for nucleation to levels below that for homogeneous nucleation. In the vicinity of the solubility limit of the additive, these two nucleation mechanisms compete. The polystyrene‐zinc stearate system has been chosen for experimental evaluation.

This work examines the effect of microstructure upon microcantilever bending stiffness. An existing beam theory model, based upon an isotropic Hooke's law constitutive relationship, is compared to a model based upon a micropolar elasticity constitutive model. The micropolar approach introduces a bending stiffness relation which is a function of any two independent elastic constants of the Hooke's law model (e.g., the elastic modulus and the Poisson's ratio), and an additional material constant (called γ). A consequence of the additional material constant is the prediction of an increased bending stiffness as the cantilever thickness decreases—a stiffening due to the material microstructure which becomes measurable at micron-order thicknesses. Polypropylene microcantilevers, which have a non-homogeneous microstructure due to their semi-crystalline nature, were fabricated via injection molding. A nanoindenter was used to measure their stiffness. The nanoindenter-determined stiffness values, which include the effect of the additional micropolar material constant, are compared to stiffness values obtained from beam theory. The nanoindenter stiffness values are seen to be at least four times larger than the beam theory stiffness predictions. This stiffening effect has relevance in future MEMS applications which employ materials with non-homogeneous microstructures instead of the conventional MEMS materials (e.g., silicon, silicon nitride), which have a very uniform microstructure.

Microcellular polymer foams exhibit greatly improved mechanical properties as compared to standard foams due to the formers' small bubble size. Microcellular foams have bubbles with diameters on the order of 10 microns, volume reductions of 30 to 40 percent, and six or seven times the impact strength of solid parts. They are produced through the use of thermodynamic instabilities without the use of foaming agents. This method leads to a very uniform cell size throughout a part's cross section. A theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubbles nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation. In the vicinity of the solubility limit, the two mechanisms compete.

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Experiments were performed to validate the model for the nucleation of microcellular foams in amorphous thermoplastic polymers. The polystyrene‐zinc stearate system was chosen as the model system. Other additives such as stearic acid and carbon black were also investigated. Molecular weight and orientation effects were studied. Nitrogen and carbon dioxide were used to produce the microcellular bubbles. Results show that amounts of soluble additives at levels just below their solubility limit and high gas saturation pressures yield the most acceptable foams—ones with a large number of uniform small bubbles. In this region, the bubble number is sensitive to both the gas saturation pressure and the concentration of solutes. Increasing the concentration of soluble additives above the solubility limit has little effect on bubble number and almost eliminates the dependence on saturation pressure. Molecular weight and orientation had no effect on the number of bubbles produced. Similarly, carbon black, which is insoluble in and which bonds well to polystyrene, produced no effect on bubble numbers. The agreement between theoretical predictions and experimental results is reasonably good.