This thesis investigates the structure-process-property relationship in polypropylene (PP) foams prepared using supercritical carbon dioxide. It aims to bridge the gap in research linking foamability to upstream material properties without needing additional downstream processes, focusing on the low- and high-temperature regimes. Initial studies identified crystallization temperature and gas diffusivity as key factors in PP expansion. Building on this work, the thesis explores the foamability of linear and long-chain branched PP through measurable parameters obtained via fundamental characterization. The study confirms that the onset crystallization temperature (T_c-onset) is crucial at low temperatures, while identifying strain hardening ratio (SHR) as a reliable numerical indicator of high-temperature foaming performance. The research then systematically modifies PP’s molecular structure to tune T_c-onset and SHR, thereby controlling foaming behavior in different temperature regimes.

In the low-temperature regime, the introduction of ethylene random comonomers to PP was investigated to tune T_c-onset and enhance foamability. The study showed that while a low T_c-onset is necessary for high expansion at low temperatures, it can delay cell stabilization at high temperatures, compromising its expansion. This finding highlighted the dynamics of the PP expansion profile when systematically varying T_c-onset while maintaining SHR similar. However, it is understood that T_c-onset evaluated under quiescent conditions may be irrelevant in a foaming context. Therefore, this thesis further explores the role of extensional flow-induced crystallization in PP expansion using a unique system. It was found that long-chain branched PP undergoes extensional flow-induced crystallization more intensely and at faster rates, emphasizing its importance in cell stabilization and its complementing relationship with SHR. For high-temperature foaming, ionic modification of PP was shown to present an alternate cost-effective route towards enhancing SHR and, thus, foamability compared to traditional long-chain branching. Finally, through a comprehensive statistical analysis, the research highlighted alternative material parameters that can be adjusted to improve the foamability of PP, while mapping the PP expansion profile according to regions dominated by specific parameters. This approach offers valuable guidelines for industrial product developers to tailor PP resins during development, thereby enhancing foaming performance without additional processing steps.