Date of Award

Fall 12-2022

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

James Rawlins

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Sergei Nazarenko

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Xiaodan Gu

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Andrea Browning

Abstract

This dissertation attempts to identify and address the problems associated with high-rate thermoplastic composites. Herein, three primary research thrusts were investigated; the first two chapters focus on the melt-state degradation mechanism of PEKK copolymers and its role on downstream crystallization and thermomechanical properties, the third chapter examines the role of graphene nanoplatelets (GNP) on the crystallization and melting behavior of PEKK 70/30, and the final chapter focused on radio frequency and induction heating of PEKK nanocomposites.

The melt-state degradation mechanism was identified using a combination of x-ray photoelectron spectroscopy (XPS), molecular dynamics (MD), and quantum mechanics (QM) simulations. Melt-state degradation resulted in the chain scission of ether linkages, generating radical species, and forming branches. The role of branching on crystallization, rheological properties, and thermomechanical properties was established. As PEKK copolymer T/I ratio decreased, the rate of melt-state degradation and char yield increased.

The inclusion of GNP in PEKK 70/30 was found to accelerate crystallization. GNP acted as an effective nucleating agent, nucleating at lower supercooling than neat PEKK 70/30. GNP increased peak crystallization temperature and degree of crystallinity. The Avrami model was used to quantify the influence of GNP on the half time of crystallization, Avrami exponent, and kinetic rate constant. The Vyazovkin method was used to generate activation energy maps for crystallization of neat PEKK 70/30 and PEKK/GNP. Through-crystallization rheology experiments were developed to measure crystallization in situ for both non-isothermal and isothermal crystallization conditions.

Finally, the potential of a series of nanomaterials to rapidly heat in the presence of electric fields were investigated. PEKK/carbon nanotube (CNT) blends were placed in a radio frequency (RF) electric field with a frequency of 143 MHz. PEKK/CNT heated at a rate of 17 °C/s, however these frequencies were found to be impractical for induction heating. Next, PEKK/GNP films were compression molded to the surface of a PEKK composite laminate and placed under an induction coil with an electric field frequency of approximately 290 kHz. Nanocomposite films on the surface of PEKK composite laminates showed no improvement in heating performance. Finally, continuous carbon nanotubes (CCN) sheets were embedded within a PEKK laminate and subjected to the same induction heating experiments. Laminates containing CCN susceptors significantly increased induction heating rates and maximum temperatures, while reducing total cycle time compared to PEKK laminates without CCN.

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