Date of Award
12-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
School
Polymer Science and Engineering
Committee Chair
Jeffrey Wiggins
Committee Chair School
Polymer Science and Engineering
Committee Member 2
Sergei Nazarenko
Committee Member 2 School
Polymer Science and Engineering
Committee Member 3
Xiaodan Gu
Committee Member 3 School
Polymer Science and Engineering
Committee Member 4
Zhe Qiang
Committee Member 4 School
Polymer Science and Engineering
Committee Member 5
Boran Ma
Committee Member 5 School
Polymer Science and Engineering
Abstract
This dissertation aims to understand how thermoplastic composites can be processed at faster manufacturing rates while identifying how processing variables influence crystallization, morphology, and mechanical properties. Throughout this dissertation, the influence of nano-scale additives, carbon fibers, rapid cooling, shear, and molecular weight on polymer physics, crystallization, morphology, and mechanics is investigated.
The inclusion of carbon nanotubes and graphene nanoplatelets accelerated crystallization, increased nucleation density, reduced lamellar thickness, and shifted governing Hoffman-Lauritzen growth regimes in PEKK. Velisaris-Seferis modelling identified a surface-induced nucleation mechanism in PEKK nanocomposites, and fast scanning calorimetry allowed for the study of rapid cooling rates, helping to identify the temperature leading to the fastest crystallization in PEKK.
Flow-induced crystallization experiments revealed that when shear rates exceeded characteristic polymer relaxation times, crystallization accelerated and morphology gradients developed: ranging from spherulitic to highly oriented fibrillar structures in neat PEKK. In slowly crystallizing PEKK copolymers, shear also increased the degree of crystallinity. Reducing molecular weights shortened relaxation times, diminishing the effect of shear. However, while CNT addition caused longer relaxation times, crystallization was dominated by heterogeneous nucleation, rendering PEKK nanocomposite crystallization and morphology insensitive to flow.
Carbon fiber reinforcement also influenced crystallization behavior. AS4D carbon fibers accelerated PEKK crystallization and promoted transcrystalline growth. Manufacturing studies demonstrated that higher consolidation / crystallization temperatures enhanced matrix-driven properties such as interlaminar shear strength, while global laminate properties like compressive strength were unaffected. This was attributed either to increased transcrystallinity or, likely, increased degree of healing due to greater interlaminar diffusion between plies at elevated temperatures.
Finally, the feasibility of carbon–carbon composites (CCCs) derived from thermoplastic polymer precursors was demonstrated. With the inclusion of a critical amount of carbon nanotubes, PEKK retained dimensional stability through carbonization without the need for crosslinking or thermo-oxidative stabilization. This was attributed to the rigidity of PEKK and the high aspect ratio of the carbon nanotubes, leading to rheological percolation and showing a promising route toward advanced high-temperature materials.
ORCID ID
0009-0003-0484-5684
Copyright
Nicholas Ryan Enos, 2025
Recommended Citation
Enos, Nicholas Ryan, "Processing-Driven Crystallization, Morphology, and Mechanics in PEKK Composites" (2025). Dissertations. 2432.
https://aquila.usm.edu/dissertations/2432