Control of poly(phenylene sulfide) morphology and mechanical behavior from a TIPS system via nucleation density manipulation

Polymer aerogels are highly porous, low-density solids often used in filtration, separation, and insulation applications. Understanding the process-structure-property relationship in these systems guides aerogel processing decisions for tuning the aerogel mechanical response targeting a specific application. In systems developed by a thermally induced phase separation (TIPS) process, aerogel morphology is typically tuned via polymer composition, the strength of the polymer-solvent interactions, and the cooling rate. However, an additional parameter that has been investigated less frequently is the nucleation density of the polymer solution. Control of the nucleation density with specific processing parameters and without the need for nucleating agents allows for the tuning of aerogel morphology from spherulitic to axialitic without changing the initial polymer composition, gelation solvent, or cooling rate. Typically, axialitic morphologies display up to three times the tensile strength as spherulitic morphologies in thin TIPS membranes; however, this morphology-mechanical property relationship has yet to be explored in compression in three-dimensional polymer aerogels. In this work, the nucleation density of PPS TIPS solutions was varied over a range of processing conditions and initial polymer compositions. Aerogel density, porosity, and surface area analysis were conducted on each sample. Scanning electron microscope (SEM) micrographs indicate that the nucleation density can be systematically controlled over the entire explored composition range, and the spherulitic to axialitic transition was observed as nucleation density increased. Image analysis indicates that the dimensions and aspect ratios of axialites change when the nucleation density is varied, thereby changing the network structure of the aerogel. Ultra-small and small angle x-ray scattering (USAXS/SAXS) profiles confirm the change in global morphology as seen by SEM, and additionally allow for further assignment of particular aerogel scattering features. Uniaxial compression testing reveals that nucleation density control affects the mechanical behavior of the aerogels due to changes in the network structure, and also highlights the importance of understanding key processing parameters when investigating the aerogel process-structure-property relationship.